The molecular genetics of male infertility

Molecular Human Reproduction vol.3 no.5 pp. 419–430, 1997 The molecular genetics of male infertility D.Meschede1 and J.Horst Institute of Human Gene...
1 downloads 2 Views 190KB Size
Molecular Human Reproduction vol.3 no.5 pp. 419–430, 1997

The molecular genetics of male infertility

D.Meschede1 and J.Horst Institute of Human Genetics of the University, Vesaliusweg 12–14, D-48149 Mu¨nster, Mu¨nster, Germany 1To

whom correspondence should be addressed

The important role of genetic abnormalities in the causation of human male infertility is increasingly recognized. While much remains to be learned in this fast moving field, considerable progress has been achieved over the past years both in the clinical delineation of genetic forms of male infertility and in the characterization of the responsible genes and their mutations. We review the current state of knowledge on monogenic disorders where male infertility is a major and regular feature. Clinical and molecular details are given on a total of seventeen such entities. We restrict our survey to disorders that may actually come to the clinical attention of the reproductive medicine specialist. Key words: clinical genetics/male infertility/molecular genetics/review

Introduction Impaired fertility of the male partner is causative or contributary in up to two thirds of all couples unable to conceive spontaneously (Bhasin et al., 1994). Despite enormous progress in the understanding of human reproductive physiology, the underlying cause of male subfertility can often not be elucidated (Nieschlag, 1996). Even when a varicocele, a genital infection or antisperm antibodies are detected in an infertile man, the explanatory value of such a finding is often uncertain. Therefore, ample space remains for speculation about yet uncharacterized aetiologies of male infertility. Much consideration has recently been given to genetic factors, which might constitute a major source of reproductive pathology in the human male (Lilford et al., 1994). Genetics in general, as well as its clinical offshoot medical genetics, have gone through a stormy development in the past decades (McKusick, 1993). The progress achieved in elucidating the fine structure of the human genome is most impressive and occurs at an ever increasing pace. For the non-specialist, it has become virtually impossible to keep abreast of the torrential inflow of new genetic data. In reproductive medicine the application of modern DNA technology has already yielded a rich harvest. A considerable number of genes are now known that have an essential function in human reproduction and which, when deleted or mutated, can cause pathology in the male reproductive system. With an eye on clinical relevance we review here disease entities that feature male infertility and have an established genetic basis. Primarily chromosomal causes of male infertility (such as Klinefelter syndrome, translocations, etc) are not considered. This topic has been reviewed elsewhere (Lange et al., 1990; De Braekeleer and Dao, 1991; Chandley, 1995). We have also excluded from our survey genetic fertility disturbances that are unlikely to ever come to the attention of the reproductive medicine specialist. This applies to the large number of genetic diseases which compromise viability, physical or mental com© European Society for Human Reproduction and Embryology

petence to such a degree that reproduction is generally out of consideration for the affected subjects. Neither will we consider the common ‘multifactorial’ disorders with a possible negative impact on male fertility, e.g. cryptorchidism or insulin-dependent diabetes mellitus. We include in this review genetic forms of male infertility both with and without an accompanying defect in virilization or pubertal development. However, we will only consider conditions where the gender assignment is unequivocally male. This excludes severe disturbances of male sex determination and differentiation, which have been discussed elsewhere (Berkovitz, 1992; McGillivray, 1992; Katz and Imperato-McGinley, 1994; Meschede et al., 1996a; Ramkisson and Goodfellow, 1996). For a disorder to be included here the responsible gene must be cloned or mapped or at least the mode of inheritance has to be established (Tables I and II). Most of the diseases dealt with are rare, some of them exceedingly so. However, as a group they may contribute significantly to the total caseload of male infertility. Probably many of these disorders are underdiagnosed, and it is our wish to enhance the awareness of them. Not only is it more satisfactory to come to an aetiologically-founded rather than a purely descriptive diagnosis; the patient may actually reap significant benefits from the recognition that a specific genetic condition is present. Coping psychologically with infertility can be facilitated by a specific diagnosis, at-risk relatives may be recognized early, and if treatment of the fertility problem is considered, genetic risks for the offspring can be correctly evaluated and dealt with (Meschede et al., 1995b).

Conditions without a defect in virilization Microdeletions of the long arm of the Y chromosome In 1976, Tiepolo and Zuffardi published a landmark study which demonstrated that deletions in the long arm of the Y 419

D.Meschede and J.Horst

Table I. Genes (in alphabetical order) known to be involved in the causation of human male infertility Gene

Chromosomal location

Function of gene product

Disorder caused by dysfunction of gene

ALDP AMH AMH-RII AR

Xq28 19p13 12q13 Xq11-12

peroxisomal membrane protein anti-Mu¨llerian hormone AMH receptor type II androgen receptor

CFTR DAX-1

7q31.2 Xp21

cAMP-regulated chloride channel transcriptional regulator

DAZ (RBM/YRRM?) DM FDG1 KAL-X (5 KALIG-1 5 ADMLX) LHβ mitochondrial DNA (various genes) p57KIP2 (IGF2?, H19?) SNRPN (?) unknown unknown unknown

Yq11.23 19q13.3 Xp11.21 Xp22.3

RNA binding protein protein kinase nucleotide exchange factor (?) neuronal pathfinding molecule

19q13.32 —

hormone subunit mitochondrial energy metabolism (oxidative phosphorylation) cell cycle regulator nuclear ribonucleoprotein unknown unknown unknown

adrenomyeloneuropathy persistent Mu¨llerian duct syndrome persistent Mu¨llerian duct syndrome infertile male syndrome (mild androgen resistance) and X-linked spinal and bulbar muscular atrophy cystic fibrosis, CBAVD, CUAVD congenital adrenal hypoplasia with hypogonadotrophic hypogonadism azoospermia or severe oligozoospermia myotonic dystrophy Aarskog–Scott syndrome Kallmann syndrome, Xp22 contiguous gene syndrome pubertal failure and infertility Kearns–Sayre syndrome

11p15.5 15q11-13 3q, 11q, 15q, 16q 12q22-qter 21q22.3

Beckwith–Wiedemann syndrome Prader–Willi syndrome Bardet–Biedl syndrome Noonan syndrome polyglandular failure syndrome type I

CBAVD/CUAVD 5 congenital bilateral/unilateral absence of the vas deferens.

Table II. Disorders causing male infertility with an established mode of inheritance, but unknown gene localization Disorder

Mode of inheritance

9 1 0 sperm axoneme defect Globozoospermia Primary ciliary dyskinesia and Kartagener syndrome

autosomal recessive (?) autosomal recessive (?) autosomal recessive

chromosome can cause spermatogenic failure in man. This finding prompted the hypothesis of a male-specific fertility gene in Yq11.23, the terminal segment of the euchromatic (genetically active) portion of the Y chromosome long arm. Baptized the ‘azoospermia factor’ (AZF) this hypothetical entity has been the subject of an intensive gene hunt over the past several years. A gene termed DAZ (deleted in azoospermia) currently appears as the strongest candidate for representing the long-searched-for azoospermia factor on Yq. DAZ has a testis-specific pattern of expression and encodes an RNAbinding protein of yet undetermined function (Reijo et al., 1995). Recently, the genomic structure of this gene has been elucidated (Saxena et al., 1996). It encompasses at least 16 exons that are scattered over 42 kb of genomic DNA. The DAZ sequence is characterized by a high degree of repetitivity, explained by its complex evolutionary history (Saxena et al., 1996). At least two and possibly more copies of DAZ with .99% sequence identity are present on the Y chromosome. RBM (RNA-binding motif; previously YRRM 5 Y-located RNA recognition motif) is another multiple-member family of Y-specific genes. Up to 30 copies, some active and some pseudogenes, are distributed over the euchromatic portions of the Y chromosome including the short arm (Schempp et al., 1995). RBM genes are specifically expressed in the germ cells of the mature testis (Chandley and Cooke, 1994), but the functional importance of their gene products for spermatogen420

esis remains to be determined. The RBM family has been suggested as the prime candidate for the azoospermia factor (Ma et al., 1993; Chandley and Cooke, 1994), but so far the case for DAZ appears stronger. Several groups have shown that some infertile men carry submicroscopic deletions in Yq that are not present in their fathers’ or brothers’ Y chromosomes (Vogt et al., 1992; Ma et al., 1993; Kobayashi et al., 1994; Reijo et al., 1995; Najmabadi et al. 1996; Qureshi et al., 1996; Reijo et al., 1996; Stuppia et al., 1996; Vogt et al., 1996; Simoni et al., 1997). It is widely believed that these de-novo microdeletions actually cause the azoospermia or oligozoospermia phenotypes observed, and they will most likely be passed on to the sons of these infertile men if intracytoplasmic sperm injection (ICSI) treatment is carried out (Chandley and Hargreave, 1996). The rate of reported microdeletions ranges from 3– 18% (Table III). The variability is most likely explained by different selection criteria for the study subjects. Most, but not all, Yq microdeletions described so far include the DAZ gene cluster (Qureshi et al., 1996; Vogt et al., 1996). This has led Vogt et al. (1996) to postulate the existence of three nonoverlapping regions containing different azoospermia factors, termed AZFa, AZFb and AZFc. The latter coincides with the DAZ locus, but no candidate genes for AZFa and AZFb have been brought forward yet. Vogt (1996) also postulated that the different categories of Yq microdeletions might be specifically associated with particular types of testicular histopathology. Studies by other groups, however, have not substantiated this claim (Reijo et al., 1995; Qureshi et al., 1996). The AZF/DAZ gene story has recently taken another surprising twist when an autosomal homologue of DAZ was discovered on the short arm of chromosome 3. It was called DAZH, SPGYLA or DAZLA respectively (Saxena et al., 1996; Shan et al., 1996; Yen et al., 1996). Similar to the Y chromosomal DAZ genes, DAZH is expressed in adult testis but not in

The molecular genetics of male infertility

Table III. Y chromosomal microdeletions in azoospermic and oligozoospermic men azoo or oligo (n)a,b Kobayashi et al. (1994) Reijo et al. (1995) Najmabadi et al. (1996) Qureshi et al. (1996) Stuppia et al. (1996) Reijo et al. (1996) Vogt et al. (1996) Simoni et al. (1997)

63 – – – – 370 –

azoo (n)a

oligo (n)a

azoo or oligo 1 deletion (n)b,c

azoo 1 deletion (n)c

oligo 1 deletion (n)c

prevalence of deletions (%)

– 89 50 51 19 – – 74

– – 10 47 14 35 – 94

10 – – – – – – –

– 12 10 4 4 – 7 3

– – 1 4 2 2 6 2

16 13 18 8 18 6 4 3

aTotal number of studied subjects with azoospermia (azoo) or oligozoospermia (oligo). bIn these studies the number of oligo- versus azoospermic men is not specified. cNumber

of patients with Y chromosomal microdeletions and azoospermia (azoo) or oligozoospermia (oligo).

somatic tissues. Expression has also been reported in the adult ovary (Saxena et al., 1996), a finding not confirmed in two other reports (Shan et al., 1996; Yen et al., 1996). DAZH has been discussed as a candidate gene for autosomal recessive forms of male infertility (Yen et al., 1996). Another interesting hypothesis is that an intact DAZH gene might be able to partially compensate for the loss of DAZ on the Y chromosome. This would provide an explanation for the substantial phenotypic variability among men with Y chromosomal microdeletions (Saxena et al., 1996). No mutations or deletions have been reported at the DAZH locus on 3p24 so far.

Congenital absence of the vas deferens and cystic fibrosis Congenital bilateral absence of the vas deferens (CBAVD) is an important cause of obstructive azoospermia in otherwise healthy men. It also represents one of the manifestations of the autosomal recessive systemic disease cystic fibrosis (CF) (Kaplan et al., 1968). Congenital unilateral absence of the vas deferens (CUAVD) may be a chance finding and is compatible with normal fertility. However, some men presenting with CUAVD are azoospermic due to an obstruction of the contralateral vas at the inguinal or pelvic level (Mickle et al., 1995). Cystic fibrosis, most cases of CBAVD, and some cases of CUAVD are caused by mutations in the CFTR (cystic fibrosis transmembrane conductance regulator) gene. CBAVD and CUAVD can be considered as minimal variants of CF confined to the male genital tract. The genotype-phenotype correlation is complex and cannot be worked out here in full detail. In general terms, mutations in the CFTR gene are divided into severe and mild ones (Kerem and Kerem, 1996). If an individual is homozygous for a severe mutation, full-blown cystic fibrosis with progressive lung disease and pancreatic insufficiency will result. Homozygosity for a mild mutation or compound heterozygosity for a severe and a mild mutation usually has less dire consequences; the patient will either present with bronchopulmonary disease that takes a milder and less progressive course than classical CF (Strong et al., 1991), or with a forme fruste of cystic fibrosis such as CBAVD. CFTR is expressed by a large gene comprising 27 exons scattered over .230 000 bp of genomic DNA. The mutational spectrum is highly heterogenous; at the time of writing .500

different CFTR mutations have been reported (Kerem and Kerem, 1996), many of them encountered in only a single patient or family. If men with CBAVD are tested for mutations in the CFTR gene the yield of positive findings depends on the molecular screening strategy employed. The more extensive the search the higher the detection rate. In most published series 60–90% of CBAVD patients are reported as mutationpositive, but in less than half of them two mutations are found (Table IV). R117H and the T5 allele are mild mutations that have a high prevalence specifically among men with CBAVD (Kiesewetter et al., 1993; Chillo`n et al., 1995; Costes et al., 1995). No convincing evidence has been brought forward that the presence of a single CFTR mutation (i.e. simple heterozygosity) has any phenotypic consequences. Therefore, it appears as though all men with CFTR gene-associated CBAVD actually carry two mutations, one of them often going undetected with the current screening technology. For further details the reader is referred to the comprehensive review by De Braekeleer and Fe´rec (1996). CBAVD may be genetically heterogenous. Rave-Harel et al. (1995) have presented evidence which suggests that a minority of CBAVD cases might be totally unrelated to CFTR gene pathology. This is also true for congenital unilateral absence of the vas deferens. When CUAVD is combined with an extrascrotal obstruction of the contralateral vas, CFTR gene mutations are highly prevalent. In contrast, no such mutations were found when the contralateral vas was patent and anatomically normal (Mickle et al., 1995).

Primary ciliary dyskinesia (including Kartagener syndrome) and other ultrastructural sperm defects Primary ciliary dyskinesia (PCD) is a summary term for a group of closely related disorders caused by the immotility or dysmotility of ciliary structures, i.e. airway cilia and sperm axonemes (Carson and Collier, 1988). Chronic airway disease with rhinitis, sinusitis, bronchitis, bronchiectasis, and middle ear infections is the most constant feature. Minor and major malformations, especially of the heart, may occur in up to one third of patients with PCD (Engesaeth et al., 1993). Affected males are usually infertile due to immotility of spermatozoa (Eliasson et al., 1977), but this is not an obligatory finding (Jonsson et al., 1982). Various ultrastructural defects of ciliary structures have been described including absence of dynein 421

D.Meschede and J.Horst

Table IV. Mutations in the CFTR (cystic fibrosis) gene in men with congenital bilateral absence of the vas deferens. Figures in parentheses are percentages

Maddalena and Sherins, 1992 Gervais et al., 1993 Osborne et al., 1993 Patrizio et al., 1993 Williams et al., 1993 Culard et al., 1994 Oates and Amos, 1994 Casals et al., 1995 Costes et al., 1995 Mercier et al., 1995 Chillo`n et al., 1995 Silber et al., 1995 Authors’ series, 1997b Σc aIn

Patients (n)

Mutation-positive

2 mutations detecteda

1 mutation detecteda

No mutation detected

18 23 26 64 35 12 49 30 45 67 102 52 22 545

10 12 10 41 20 8 40 22 40 44 80 37 16 380

1 3 2 6 5 2 9 3 15 16 54 30 10 156 (29)

9 9 8 35 15 6 31 19 25 28 26 7 6 224 (41)

8 11 16 23 15 4 9 8 5 23 22 15 6 165 (30)

(56) (52) (38) (64) (57) (66) (82) (73) (89) (66) (78) (70) (73) (70)

some studies the T5 allele (see text) is included as a mutation, in some not.

bMeschede et al. (1995a) and unpublished data. cThe patient cohorts from some studies may overlap.

arms (Eliasson et al., 1977), defective radial spokes (Sturgess et al., 1979), and random ciliary orientation (Rutland and de Iongh, 1990). The combination of bronchiectasis, sinusitis and sperm immotility with situs inversus is called Kartagener syndrome. Defects in the determination of left–right asymmetry such as complete or partial situs inversus are also encountered without underlying ciliary dysfunction (Penman Splitt et al., 1996). Some overlap may exist between this type of anomaly, termed laterality sequence or Ivemark syndrome, on one hand and PCD and Kartagener syndrome on the other hand. PCD, which is a genetic disorder with autosomal recessive inheritance, must also be differentiated from acquired forms of ciliary dysfunction (Wilton et al., 1985; Carson and Collier, 1988). An autosomal dominant or X-linked form of Kartagener syndrome may exist, but only a single familiy with this inheritance pattern has been observed so far (Narayan et al., 1994). No mapping data are available yet for the gene or genes responsible for PCD/Kartagener syndrome. A considerable number of other monomorphic human sperm defects have been described. Most appear to be exceedingly rare, and they may only be detectable through electron microscopy (Zamboni, 1987). For the ‘9 1 09 axoneme defect (Neugebauer et al., 1990) and globozoospermia (round head defect; Schill, 1991) evidence from family studies is accumulating that these are genetically determined conditions. The mode of inheritance most likely is autosomal recessive or X-linked (Nistal et al., 1978; Afzelius and Eliasson, 1979, Flo¨rkeGerloff et al., 1983). No mapping data for the responsible genes are available yet.

Persistent Mu¨llerian duct syndrome The regression of the Mu¨llerian ducts in early embryonic life is mediated through anti-Mu¨llerian hormone (AMH), also called Mu¨llerian inhibiting substance (MIS). This hormone exerts its biologic effect through two specific receptors (AMHRI and AMH-RII). The genes for the hormone and the type II receptor have been mapped to 19p13 and 12q13 respectively, and cloned. Mutations in either gene can cause the persistent Mu¨llerian duct syndrome (PMDS), an autosomal recessive 422

condition. Clinically, this disorder is characterized by the presence of Fallopian tubes and a uterus in an otherwise normally virilized man. The Mu¨llerian structures are often located in an inguinal hernia. Testicular function and fertility can be normal, but many patients have inguinal or even abdominal testes as the gonads may be attached to the Fallopian tubes (Josso et al., 1991). Cryptorchidism is probably the mechanism leading to spermatogenic impairment in patients with PMDS (Josso et al., 1983). Also, the testes are prone to torsion, which may lead to secondary gonadal atrophy. Interestingly, some patients with PMDS have concomitant malformations of the epididymides and the proximal vasa deferentia (Imbeaud et al., 1996). In 85% of PMDS families the molecular basis of the disorder can be elucidated by analysis of the AMH and AMR-RII genes (Imbeaud et al., 1994, 1995, 1996). While affected families of Arabian and Mediterranean origin tend to have mutations in the hormone gene, patients from the northern parts of Europe mostly display AMR-RII gene mutations. Here, a specific 27 bp deletion (∆6331–6357) located in exon 10 predominates. Measuring the serum concentration of AMH in prepubertal individuals with PMDS allows to direct the search for mutations; patients with mutations in the AMH gene have very low hormone levels in their blood, while patients carrying AMH-RII mutations have normal or supranormal AMH serum concentrations (Imbeaud et al., 1996).

Aarskog–Scott syndrome This is an X-linked recessive disorder characterized by genital, facial and skeletal abnormalities (Porteous and Goudie, 1991). Cryptorchidism is common, and subfertility in affected men is thought to be prevalent. There may be a specific defect of the sperm acrosome associated with Aarskog–Scott syndrome (Meschede et al., 1996b). The most prominent facial features in Aarskog–Scott syndrome include hypertelorism, eyelid ptosis, abnormalities of auricular shape, and a broad nasal bridge. Most patients are short. Mild cutaneous syndactyly and short broad hands are fairly typical (Porteous and Goudie, 1991). The disorder may be genetically heterogenous as some family

The molecular genetics of male infertility

reports suggest an autosomal dominant or autosomal recessive mode of inheritance (Grier et al., 1981; Guion-Almeida and Richieri-Costa, 1992). The FGD1 gene responsible for the Xlinked form has been mapped to Xp11.21 (Glover et al., 1993) and was recently cloned. Mutations in FGD1 segregate with the clinical phenotype (Pasteris et al., 1994). FDG1 encodes a protein with strong homology to Rho/Rac guanine nucleotide exchange factors. It may be involved in growth regulation and signal transduction.

Conditions with disturbed virilization Kallmann syndrome This disorder features the characteristic combination of hypogonadotrophic hypogonadism with anosmia (Meschede et al., 1994). The primary endocrine abnormality is defective hypothalamic secretion of gonadotrophin-releasing hormone (GnRH). Serum concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH) and testosterone are low, and spermatogenesis does not unfold. Most patients present for the absence of pubertal development. While hypogonadism and anosmia are the hallmarks of the disorder, some other abnormalities may occasionally be observed such as unilateral renal aplasia, mirror movements of the extremities, and pes cavus. Inheritance of the Kallmann syndrome is X-linked recessive in most families. However, many cases are sporadic, and there may be autosomal dominant and autosomal recessive familial forms of the disorder. The gene for the X-linked form was mapped to Xp22.3, spans 200 000 bp genomic DNA, and probably encodes a neuronal pathfinding molecule (Franco et al., 1991). Many different mutations in this gene, variably termed as KAL-X, KALIG-1 or ADMLX, were described in individuals with Kallmann syndrome. The whole gene can be deleted (Hardelin et al., 1993a), but more often point mutations are found (Hardelin et al., 1993b). Kallmann syndrome may occur as a component of a complex disorder designated as Xp22 contiguous gene syndrome (Ballabio et al., 1989). The other abnormalities encountered in this subgroup of patients include short stature, mental retardation, chondrodysplasia punctata, and ichthyosis due to steroid sulfatase deficiency. Congenital adrenal hypoplasia with hypogonadotrophic hypogonadism Congenital adrenal hypoplasia with hypogonadotrophic hypogonadism (CAH/HH) is an X-linked recessive disorder that usually presents in infancy or childhood with signs of adrenal insufficiency such as growth retardation, poor feeding, vomiting, weight loss, muscular weakness and lethargy (Kletter et al., 1991). Pubertal development does not occur at the expected age, and subnormal concentrations of testosterone, LH and FSH are found (Hay et al., 1981). Cryptorchidism is a common abnormality in such patients. Whether the hypogonadotrophic hypogonadism has a primary hypothalamic or pituitary basis is a still unsettled question (Kletter et al., 1991). While pubertal development can be induced by exogenous gonadotrophins, little is known about the fertility of successfully treated patients. Congenital adrenal hypoplasia

with hypogonadotrophic hypogonadism has been mapped to the short arm of the X chromosome (Xp21). The DAX-1 gene responsible for the disorder was recently cloned and shown to encode a 470 amino acid protein that belongs to the nuclear hormone receptor superfamily and functions as a transcriptional regulator (Zanaria et al., 1994). Highly specific expression of DAX-1 was demonstrated in human adrenal and testicular tissues. Deletions or point mutations of the DAX-1 gene are found in the majority of patients with CAH/HH (Muscatelli et al., 1994). Large deletions may encompass other genes that lie close to DAX-1. This explains why in some cases CAH/ HH is associated with glycerol kinase deficiency, Duchenne muscular dystrophy, short stature, and psychomotor retardation (Wise et al., 1987).

Hypogonadism due to mutation in LHβ subunit gene LH is a heterodimer consisting of an α and a β subunit. The gene encoding the latter is located on 19q13.32 in immediate proximity to a cluster of genes and pseudogenes for the human chorionic gonadotrophin (HCG) β subunit. If a mutation is present in both alleles of the LHβ gene this can lead to the production of biologically inactive LH which may still be immunoreactive in standard hormone assays. Weiss et al. (1992) reported on an adolescent with pubertal delay, increased serum concentrations of LH, normal FSH, and low testosterone. He was demonstrated to be homozygous for a missense mutation in codon 54 of the LHβ gene. Treatment with exogenous chorionic gonadotrophin induced puberty and sperm production, but he was still unable to induce a pregnancy. The proband had three infertile maternal uncles heterozygous for the LHβ gene mutation, but it remained unclear whether any causal relationship between the mutation and the fertility problem in the heterozygous relatives existed. Infertile male syndrome Resistance to the biological action of androgens is caused by androgen receptor dysfunction and comes to clinical attention through various degrees of undervirilization or even sex reversal (McPhaul et al., 1993; MacLean et al., 1995; Meschede et al., 1996a). Among the disorders with androgen resistance the complete form of testicular feminization is the most severe, the undervirilized fertile male syndrome the mildest, and incomplete testicular feminization, Reifenstein syndrome, and the infertile male syndrome (IMS) range in between. While testicular feminization and the Reifenstein syndrome are characterized by sex reversal or intersex genitalia, respectively, patients with the infertile male syndrome (IMS) have a normal male phenotype and primarily present with infertility due to defective spermatogenesis. Some early reports suggested a high prevalence of IMS among men with abnormal sperm counts (Aiman and Griffin, 1982; Morrow et al., 1987), but little confirmatory evidence for this has been brought forward since. In addition to spermatogenic impairment IMS may present with mild somatic signs of deficient androgen action such as gynecomastia or scant beard growth. The disorder is inherited in an X-linked recessive fashion. Mutations in the androgen receptor gene, located on the long arm of the X-chromosome (Xq11-12), are the molecular basis of IMS 423

D.Meschede and J.Horst

and other androgen resistance states. The mutational spectrum in this gene is highly heterogenous (McPhaul et al., 1993). Point mutations predominate over deletions, and the latter may either comprise the whole gene or single exons. Correlating genotype with phenotype in the androgen resistance states has met with limited success so far (for a detailed review see MacLean et al., 1995). In broad terms, there is a tendency for deletions, nonsense mutations and splice mutations to impair the receptor’s function or abundance to such a degree that one of the more severe phenotypes will result.

X-linked spinal and bulbar muscular atrophy A specific type of mutation of the androgen receptor gene causes a disease that has little clinical similiarity with the androgen resistance states (see preceding section). This disorder is called X-linked spinal and bulbar muscular atrophy (SBMA) or Kennedy disease. Onset of clinical signs is after the age of 20, with muscular weakness, cramps and fasciculations (Harding et al., 1982). In most cases the disease runs a relatively benign course with normal life expectancy, but significant physical handicaps may result. Atrophy and weakness usually start in the proximal leg muscles and may later spread to the upper extremities and the face. If the laryngeal and pharyngeal muscles get involved dysarthria and dysphagia result. The male reproductive system is affected in the majority of patients (Harding et al., 1982; Arbizu et al., 1983). Secondary testicular atrophy with consecutive infertility and in some cases subnormal serum testosterone develop, and gynecomastia is present in many affected subjects. Most men with SBMA already have children when the disease process affects the testes to such an extent as to become clinically obvious. X-linked spinal and bulbar muscular atrophy belongs to a group of human disorders caused by the expansion of a trinucleotide repeat sequence, a type of molecular abnormality also known as dynamic mutation (Willems, 1994). The human genome is densely strewn with repetitive DNA elements of various lengths. One subclass consists of tandemly arranged trinucleotide repeats, for example (CAG)n, (CGG)n, or (CTG)n. Repetitive sequences of this kind are found within or in close proximity to several genes with an essential function in the human nervous system. They usually represent a polymorphic molecular trait, meaning that within a certain limit the exact copy number of the respective trinucleotide codon varies from person to person without any phenotypic consequences. However, once the repetitive sequence expands beyond a certain size the gene’s function gets compromized and may ultimately be completely abolished. Another remarkable feature of this type of mutation is its potentially dynamic behaviour in meiosis (Figure 1). When passed on from parent to child triplet repeat sequences may expand, sometimes dramatically so as in myotonic dystrophy (see below). This often results in a more severe clinical phenotype and an earlier disease onset in the offspring. Sometimes, however, an already expanded repeat may spontaneously revert to a normal size, a phenomenon known as reverse mutation. The repeat sequence responsible for Kennedy disease is located in exon 1 of the androgen receptor gene and codes for a polyglutamine stretch in the receptor protein (La Spada et al., 424

Figure 1. Schematic drawing of a gene carrying a dynamic mutation. The gene contains an unstable triplet repeat sequence (see text) indicated by the black area. When passed on to the next generation the repeat sequence may undergo meiotic expansion (e.g. from generation 1 to 2), but not necessarily so (generation 2 to 3). Very large expansions as present in generations 4 and 5 may lead to a functional inactivation of the gene, resulting in a clinical disorder.

1991). Normal subjects have 15–31 CAGs, whereas individuals affected with SBMA have 40 such triplets or more (La Spada et al., 1992). Measuring the repeat size is therefore a valuable molecular tool to confirm or exclude the diagnosis of Kennedy disease. It has been demonstrated that a receptor protein with an abnormally long polyglutamine stretch does not effectively activate certain androgen-dependent genes (Mhatre et al., 1993). The instability of the (CAG)n sequence in meiosis is only moderate. While both expansions and contractions are observed, they mostly do not result in the gain or loss of more than one to three repeat units (La Spada et al., 1992; Zhang et al., 1995).

Myotonic dystrophy Like Kennedy disease, myotonic dystrophy is caused by the expansion of an unstable trinucleotide repeat sequence (see preceding paragraph). The clinical severity of this autosomal dominant disorder is highly variable. Many affected individuals never present to their physicians and are only identified through family studies (Roses, 1993). At the other end of the spectrum are children with congenital myotonic dystrophy, a seriously handicapping condition. Intermediate in severity is the adult form of the disorder, which may be encountered in patients presenting with infertility and hypogonadism. Here, the most typical features include a myopathic facies, slowly progressive muscular weakness, mild myotonia, cataracts, and early frontal balding. Intellectual abilities are often in the low range. Puberty in these patients tends to start earlier than in peers (Roses, 1993). Similar to Kennedy disease, clinically relevant gonadal pathology usually develops after the prime reproductive years. Patients then present with testicular atrophy, loss of libido and potency, and infertility (Becker and Hofmann, 1975; Takeda and Ueda, 1977). The late onset of testicular dysfunction, combined with some selection bias favouring the inclusion of minimally affected individuals, probably explains why retrospective epidemiological analyses did not reveal a reduced number of children in affected families (Dao et al., 1992).

The molecular genetics of male infertility

The gene for myotonic dystrophy is located on the long arm of chromosome 19 (19q13.3) and encodes a serine/threonine protein kinase. Its 39 untranslated region contains an unstable triplet repeat motif (Mahadevan et al., 1992) that in normal individuals has 5–35 CTGs. Mutant alleles are larger and have between 50 and several thousand CTG repeat units. Disease severity correlates positively and age of clinical onset negatively with the size of the pathological allele (Tsilfidis et al., 1992; Redman et al., 1993). The age of onset in affected families may decrease from generation to generation, a phenomenon called anticipation (McInnis, 1996). It is explained by the instability of expanded alleles. In other words, expanded alleles have the tendency to expand even further when passed on through the germline. This dynamic behaviour of the mutation adds further complexity to the inheritance pattern of myotonic dystrophy. An expansion to extremely large repeat sizes, as observed in children with the congenital form, usually occurs in female meiosis (Redman et al., 1993), while smaller intergenerational expansions from only mildly expanded alleles (,100 CTGs) may preferentially arise in male meiosis (Brunner et al., 1993a). During spermatogenesis expanded pathological alleles may also undergo spontaneous reversion to a normal size (Brunner et al., 1993b).

Kearns–Sayre syndrome Kearns–Sayre syndrome most prominently involves the neuromuscular and endocrine systems. Onset is before age 20 with progressive external ophthalmoplegia, pigmentary degeneration of the retina, a cardiac conduction defect, and cerebellar ataxia. Pathology of the reproductive system is reported in 20–30% of affected males. Abnormal findings include cryptorchidism, pubertal delay, subnormal testicular volume, and low gonadotrophin levels (Harvey and Barnett, 1992). The prevalence of clinical subfertility in these individuals has not been documented. Other endocrine abnormalities are thyroid disease, short stature with or without growth hormone deficiency, and diabetes mellitus. Kearns–Sayre syndrome belongs to a group of multisystemic disorders caused by mutations in the mitochondrial genome (Johns, 1995). Mitochondria contain multiple copies of a 16 569 bp DNA ring molecule. Its 37 genes act in concert with nuclear genes to create the molecular machinery for oxidative phosphorylation (Shoffner and Wallace, 1995). Deletions of variable size and location within the mitochondrial genome are found in 85% of patients with Kearns–Sayre syndrome (Moraes et al., 1989). A conceptus’ endowment with mitochondria is almost exclusively derived from the oocyte, while the sperm has contributed none or very few of these organelles (Gyllenstein et al., 1991). This explains why all mitochondrial disorders including Kearns–Sayre syndrome are inherited strictly through the maternal line. Adrenomyeloneuropathy Mutations in the adrenoleukodystrophy protein (ALDP) gene underly three clinically-related, but distinct peroxisomal disorders: adrenoleukodystrophy (ALD), adrenomyoloneuropathy (AMN), and isolated Addison disease (not further discussed here). Of these, adrenoleukodystrophy is the most severe, presenting with progressive neurological deterioration, adrenal

failure, and ultimately death in childhood (Moser et al., 1995). In contrast, the clinical onset of adrenomyeloneuropathy is usually around the age of 30 years. Its clinical hallmarks are progressive paraparesis, sphincter disturbances, peripheral neuropathy, adrenocortical failure, and thin scalp hair. Nearly all affected men have signs of endocrine and/or exocrine testicular failure such as impotence, gynaecomastia, poor androgenization, azoo- or oligospermia, abnormal testicular histology, low ejaculate volume, low testosterone and elevated LH and FSH (Powers and Schaumburg, 1981; Assies et al., 1995). Elevated plasma and tissue values of very long chain fatty acids are an important biochemical hallmark of adrenomyeloneuropathy. The ALDP gene is located on the terminal long arm of the X chromosome (Xq28) and encodes a peroxisomal membrane protein with putative transporter function (Mosser et al., 1993). The yield of positive findings is high in ALD and AMN patients if the coding region of ALDP is screened for mutations. Among the mutations reported so far small alterations such as one-base changes and small deletions predominate (Braun et al., 1995; Ligtenberg et al., 1995).

Polyglandular failure syndrome types I and II The polyglandular failure syndrome type I is a rare autosomal recessive disorder with multiple endocrine deficiencies, mucocutaneous candidiasis, and signs of ectodermal dysplasia (nail and enamel dystrophy, alopecia, vitiligo). The responsible gene was recently mapped to chromosome 21q22.3 (Aaltonen et al., 1994), but has not been cloned yet. There is evidence that the polyglandular failure syndrome type I is genetically homogenous, i.e. all cases are caused by mutations at the 21q22.3 locus (Bjo¨rses et al., 1996). The disorder is also called APECED, an acronym for autoimmune polyendocrinopathy, candidiasis, and ectodermal dystrophy (Ahonen, 1985; Ahonen et al., 1990). Among the endocrine manifestations adrenocortical failure, hypoparathyroidism, and hypogonadism are most prevalent. Insulin-dependent diabetes mellitus, parietal-cell atrophy, and hypothyroidism are encountered in only 5–10% of the cases. A large study of Finnish patients showed the prevalence of hypogonadism to be 14% among males and 60% among females with APECED (Ahonen et al., 1990). Male infertility may be due to oligozoospermia, antisperm antibodies, or both (Tsatsoulis and Shalet, 1991). The polyglandular failure syndrome type II (Schmidt syndrome) also features thyroid abnormalities, Addison’s disease, and primary hypogonadism. In contrast to type I, diabetes mellitus is common and hypoparathyroidism uncommon. Occasional familial aggregation has been observed, but most cases are sporadic. It is unclear so far whether the type II syndrome is a genetic disorder. Bardet–Biedl syndrome This autosomal recessive disorder is genetically heterogenous. No candidate gene has been identified yet, but at least four distinct chromosomal regions are known to be linked with the disorder. These are on 3q, 11q, 15q, and 16q (Kwitek-Black et al., 1993; Leppert et al., 1994; Sheffield et al., 1994; Carmi et al., 1995). The clinical picture is quite variable. The most typical features include obesity, retinitis, hexadactyly, renal 425

D.Meschede and J.Horst

disease, cardiac abnormalities, and diabetes (Greene et al., 1989). Mental retardation occurs in ~50% of the patients. Male individuals frequently have hypoplastic genitalia. The issue of fertility has not been systematically studied, but it appears that spermatogenic impairment is the rule in affected men (Toledo et al., 1977; Greene et al., 1989).

Noonan syndrome Noonan syndrome is a congenital disorder with an wide spectrum of clinical severity. Originally delineated in the paediatric age group (Noonan and Lezington, 1968) it is probably underdiagnosed in adults, in particular when the clinical signs are mild and confined to the reproductive system. The full picture of Noonan syndrome is characterized by a webbed or short neck, typical facies (ptosis, hypertelorism, low set ears), pectus anomalies, developmental delay, pulmonary stenosis, and short stature (Mendez and Opitz, 1985; Sharland et al., 1992). Abnormalities of the genital tract occur in approximately half the men with Noonan syndrome and include cryptorchidism, azoo- or oligozoospermia, and delayed pubertal development (Elsawi et al., 1994). Reproductive failure has also been observed in a clinical variant of the disorder, the neurofibromatosis–Noonan syndrome (Meschede et al., 1993). Most cases of Noonan syndrome are sporadic. In ,50% patients, certain or possible signs of the disorder can be detected in one of the parents (Sharland et al., 1993). However, in some families the disorder clearly follows a pattern of autosomal dominant inheritance. A locus for this dominant form was recently mapped to 12q22-qter (Jamieson et al., 1994). No candidate gene in this region has been identified yet. Anomalies of imprinted genes and uniparental disomy Genomic imprinting is a recently discovered phenomenon that underlies some otherwise unexplainable deviations from the rules of Mendelian inheritance. In classical Mendelian genetics it is assumed that the maternally and the paternally inherited copies (alleles) of a gene are equally expressed and therefore can substitute for each other. In contrast, imprinted genes carry a parent-of-origin specific molecular modification that silences the expression of either the maternal or the paternal allele. Therefore, loss of the other (actively expressed) allele will result in the complete or near complete absence of the gene product. In an analogous fashion, overexpression of an imprinted gene may result if that allele which should be transcriptionally silenced has lost its imprint and is thus derepressed. It can be easily deduced that heterozygosity for a point mutation or a deletion in an imprinted gene can have dramatic functional consequences. Over- or underexpression of such genes may alternatively result from uniparental disomy (UPD), another genetic oddity only recently demonstrated in humans (Engel, 1993). While under normal circumstances one of the two homologues of a chromosome pair are derived from each parent (biparental inheritance), in UPD both homologues or at least parts of them are contributed by the same parent (Figure 2). Various mechanisms, not further discussed here, can give rise to this genetic anomaly. Its importance as a mechanism of human disease is increasingly recognized 426

Figure 2. Normally the two homologous partners of a chromosome pair are contributed separately by the oocyte and the sperm (biparental inheritance). Gamete complementation as shown in the lower half of the figure is one of the mechanisms giving rise to uniparental disomy. Here, an aneuploid oocyte disomic for a chromosome is complemented by an aneuploid sperm nullisomic for the same chromosome. The karyotype of the zygote is numerically normal, but both partners of the chromosome pair are of exclusively maternal origin.

(Ledbetter and Engel, 1995). Uniparental disomy and other abnormalities of imprinted genes on chromosomes 7, 11, 14, and 15 are known or suspected to adversely affect the male reproductive system. The Silver–Russell syndrome, the Beckwith–Wiedemann syndrome, the maternal UPD 14 phenotype, and the Prader–Willi syndrome represent the corresponding clinical phenotypes. Kotzot et al. (1995) have provided evidence that the Silver– Russell syndrome is caused by maternal UPD of chromosome 7 in ~10% of the cases. This disorder is characterized by preand postnatal growth deficiency, a small triangular face and body asymmetry (Marks and Bergeson, 1977; Patton, 1988). Affected males often have cryptorchidism, hypospadias, small testicular volume, and hypergonadotrophic hypogonadism (Marks and Bergeson, 1977; Angehrn et al., 1979). Masculinization may be incomplete in a minority of boys with Silver– Russell syndrome. No candidate gene for this disorder has been identified yet. The Beckwith–Wiedemann syndrome (BWS) is charac-

The molecular genetics of male infertility

terized by pre- and post-natal overgrowth, neonatal hypoglycaemia, abdominal wall defects, renal anomalies, facial nevus flammeus, a large tongue, and ear pits and creases (Elliot and Maher, 1994). Children with this disorder are prone to develop embryonal type tumours, in particular Wilms tumour. Otherwise, the prognosis is good. Affected males frequently have cryptorchidism (Elliot and Maher, 1994), and from family studies there is evidence for reduced fertility not only in affected individuals, but also in clinically unaffected carriers (Moutou et al., 1992). The BWS locus has been mapped to 11p15.5, an imprinted chromosomal region (Weksberg and Squire, 1995). The genetic pathophysiology of BWS is very complex and still incompletely understood. Cytogenetically visible duplications, translocations and inversions of 11p are seen in only 3% of the cases. Apart from alterations in the p57KIP2 gene (see below), incompletely characterized ‘imprinting mutations’ have been described, and in 20% of the sporadic cases there is paternal uniparental disomy of chromosome 11 or parts of it (Henry et al., 1991). Insulin-like growth factor 2 (IGF2), H19, and p57KIP2 have been suggested as candidate genes for BWS (Weksberg et al., 1993; Reik et al., 1995; Brown et al., 1996; Hatada et al., 1996). Under normal circumstances all these genes are uniparentally expressed, either from the maternal (H19, p57KIP2) or the paternal allele (IGF2). p57KIP2 encodes a protein important in the control of the cell cycle and so far is the only gene definitely shown to be mutated in BWS (Hatada et al., 1996). However, expression anomalies of IGF2 and imprinting defects in the IGF2-H19 domain have also been demonstrated in BWS patients (Reik et al., 1995; Brown et al., 1996). Maternal uniparental disomy of chromosome 14 may affect the reproductive system in that it induces precocious puberty in both sexes (Temple et al., 1991; Pentao et al., 1992; Antonarakis et al., 1993; Tomkins et al., 1994). The only adult man reported so far had small testicles, but normal LH, FSH and testosterone concentrations. His fertility status has not been established (Temple et al., 1991). Other consequences of maternal UPD 14 include short stature and possibly impaired psychomotor development, hydrocephalus, and orthopedic abnormalities. There is no evidence for consistent anomalies of the reproductive tract in paternal uniparental disomy of chromosome 14 (Walter et al., 1996). The most prominent features of the Prader–Willi syndrome (PWS) include psychomotor retardation (not an obligate finding), muscular hypotonia, obesity, hyperphagia and short stature (Holm et al., 1993). In males, the genitalia are hypoplastic, pubertal development is delayed and incomplete, and abnormally low serum concentrations of testosterone, LH and FSH are found. The hypogonadism is due to hypothalamic dysfunction, but there may also be a primary testicular disturbance (Jeffcoate et al., 1980). Men with PWS are thought to be infertile, but whether this applies to all patients is unclear. The disorder has been mapped to 15q11–q13, a chromosomal region that contains the imprinted genes SNRPN, PAR1, PAR5, ZNF127, and IPW7. Cytogenetically visible or submicroscopic deletions in this part of chromosome 15 can be demonstrated in ~70% of PWS patients (Knoll et al., 1993), and the deletion always affects the paternally inherited chromosome. Most non-

deletion cases have maternal uniparental disomy of the critical genomic region. In rare cases, PWS is caused by mutations which disturb the imprinting process itself (Dittrich et al., 1996). These mutations always involve the SNRPN (small nuclear ribonucleoprotein N) gene.

Conclusion The progress achieved in characterizing genes that can ablate or severely compromise male fertility is most impressive. Some of the more common gene defects of this kind have been discussed here. However, many patients present with less pronounced forms of subfertility. It will be an important and demanding task to elucidate the role of genetic factors in such milder degrees of fertility impairment or even in the normal interindividual variability of fertility. This avenue of research may also permit a better understanding of the complex interplay between an individual’s genetic constitution and its susceptibility to environmental reproductive toxins.

References Aaltonen, J., Bjo¨rses, P., Sandkuijl, L. et al. (1994) An autosomal locus causing autoimmune disease: autoimmune polyglandular disease type 1 assigned to chromosome 21. Nature Genet., 8, 83–87. Afzelius, B.A. and Eliasson, R. (1979) Flagellar mutants in man: on the heterogeneity of the immotile-cilia syndrome. J. Ultrastruct. Res., 69, 43–52. Ahonen, P. (1985) Autoimmune polyendocrinopathy – candidosis – ectodermal dystrophy (APECED): autosomal recessive inheritance. Clin. Genet., 27, 535–542. Ahonen, P., Mylla¨rniemi, S., Sipila¨, I. and Perheentupa, J. (1990) Clinical variation of autoimmune polyendocrinopathy – candidiasis – ectodermal dystrophy (APECED) in a series of 68 patients. N. Engl. J. Med., 322, 1829–1836. Aiman, J. and Griffin, J.E. (1982) The frequency of androgen receptor deficiency in infertile men. J. Clin. Endocrinol. Metab., 54, 725–732. Angehrn, V., Zachmann, M. and Prader, A. (1979) Silver–Russell syndrome. Observations in 20 patients. Helv. Paediat. Acta, 34, 297–308. Antonarakis, S.E., Blouin, J.-L., Maher, J. et al. (1993) Maternal uniparental disomy for human chromosome 14, due to loss of a chromosome 14 from somatic cells with t(13;14) trisomy 14. Am. J. Hum. Genet., 52, 1145–1152. Arbizu, T., Santamaria, J., Gomez, J.M. et al. (1983) A family with adult spinal and bulbar muscular atrophy, X-linked inheritance and associated testicular failure. J. Neurol. Sci., 59, 371–382. Assies, J., van Geel, B.M., Looren, L.J.G. and Barth, P.G. (1995) Testicular dysfunction in X-linked adrenoleukodystrophy. [Abstr.] J. Endocrinol., 144 (Suppl.), P357. Ballabio, A., Bardoni, B., Carrozzo, R. et al. (1989) Contiguous gene syndromes due to deletions in the distal short arm of the human X chromosome. Proc. Natl. Acad. Sci. USA, 86, 10001–10005. Becker, H. and Hofmann, N. (1975) Endokrinologische und histomorphologische Untersuchungen zur Hodenatrophie bei Dystrophia myotonica. Dtsch. Med. Wsch., 100, 149–152. Berkovitz, G.D. (1992) Abnormalities of gonadal determination and differentiation. Semin. Perinatol., 16, 289–298. Bhasin, S., De Kretser, D.M. and Baker, H.W.G. (1994) Pathophysiology and natural history of male infertility. J. Clin. Endocrinol. Metab., 79, 1525–1529. Bjo¨rses, P., Aaltonen, J., Vikman, A. et al. (1996) Genetic homogeneity of autoimmune polyglandular disease type I. Am. J. Hum. Genet., 59, 879–886. Braun, A., Ambach, H., Kammerer, S. et al. (1995) Mutations in the gene for X-linked adrenoleukodystrophy in patients with different clinical phenotypes. Am. J. Hum. Genet., 56, 854–861. Brown, K.W., Villar, A.J., Bickmore, W. et al. (1996) Imprinting mutation in the Beckwith–Wiedemann syndrome leads to biallelic IGF2 expression through an H19-independent pathway. Hum. Mol. Genet., 5, 2027–2032. Brunner, H.G., Bru¨ggenwirth, H.T., Nillesen, W. et al. (1993a) Influence of

427

D.Meschede and J.Horst sex of the transmitting parent as well as of parental allele size on the CTG expansion in myotonic dystrophy (DM). Am. J. Hum. Genet., 53, 1016–1023. Brunner, H.G., Jansen, G., Nillesen, W. et al. (1993b) Reverse muation in myotonic dystrophy. N. Engl. J. Med., 328, 476–480. Carmi, R., Rokhlina, T., Kwitek-Black, A.E. et al. (1995) Use of a DNA pooling strategy to identify a human obesity syndrome locus on chromosome 15. Hum. Mol. Genet., 4, 9–13. Carson, J.L. and Collier, A.M. (1988) Ciliary defects: cell biology and clinical perspectives. Adv. Pediatr., 35, 139–166. Casals, T., Bassas, L., Ruiz-Romero, J. et al. (1995) Extensive analysis of 40 infertile patients with congenital absence of the vas deferens: in 50% of cases only one CFTR allele could be detected. Hum. Genet., 95, 205–211. Chandley, A.C. and Cooke, H.J. (1994) Human male fertility – Y-linked genes and spermatogenesis. Hum. Mol. Genet., 3, 1449–1452. Chandley, A.C. (1995) The genetic basis of male infertility. Reprod. Med. Rev., 4, 1–8. Chandley, A.C. and Hargreave, T.B. (1996) Genetic anomaly and ICSI. Hum. Reprod., 11, 930–932. Chillo´n, M., Casals, T., Mercier, B. et al. (1995) Mutations in the cystic fibrosis gene in patients with congenital absence of the vas deferens. N. Engl. J. Med., 332, 1475–1480. Costes, B., Girodon, E., Ghanem, N. et al. (1995) Frequent occurrence of the CFTR intron 8 (TG)n 5T allele in men with congenital bilateral absence of the vas deferens. Eur. J. Hum. Genet., 3, 285–293. Culard, J.-F., Desgeorges, M., Costa, P. et al. (1994) Analysis of the whole CFTR coding regions and splice junctions in azoospermic men with congenital bilateral aplasia of epididymis or vas deferens. Hum. Genet., 93, 467–470. De Braekeleer, M and Dao, T.-N. (1991) Cytogenetic studies in male infertility: a review. Hum. Reprod., 6, 245–250. De Braekeleer, M. and Fe´rec, C. (1996) Mutations in the cystic fibrosis gene in men with congenital bilateral absence of the vas deferens. Hum. Mol. Reprod., 2, 669–677. Dao, T.-D., Mathieu, J., Bouchard, J.-P. and De Braekeleer, M. (1992) Fertility in myotonic dystrophy in Saguenay-Lac-St-Jean: a historical perspective. Clin. Genet., 42, 234–239. Dittrich, B., Buiting, K., Korn, B. et al. (1996) Imprint switching on human chromosome 15 may involve alternative transcripts of the SNRPN gene. Nature Genet., 14, 163–170. Eliasson, R., Mossberg, B., Camner, P. and Afzelius, B.A. (1977) The immotile-cilia syndrome. A congenital ciliary abnormality as an etiologic factor in chronic airway infections and male sterility. N. Engl. J. Med., 297, 1–6. Elliot, M. and Maher, E.R. (1994) Beckwith–Wiedemann syndrome. J. Med. Genet., 31, 560–564. Elsawi, M.M., Pryor, J.P., Klufio, G. et al. (1994) Genital tract function in men with Noonan syndome. J. Med. Genet., 31, 468–470. Engel, E. (1993) Uniparental disomy revisited: the first twelve years. Am. J. Med. Genet., 46, 670–674. Engesaeth V.G., Warner, J.O. and Bush, A. (1993) New associations of primary ciliary dyskinesia syndrome. Pediatr. Pulmonol., 16, 9–12. Flo¨rke-Gerloff, S., To¨pfer-Petersen, E., Mu¨ller-Esterl, W. et al. (1983) Biochemical and genetic investigation of round-headed spermatozoa in infertile men including two brothers and their father. Andrologia, 16, 187–202. Franco, B., Guioli, S., Pragliola, A. et al. (1991) The gene deleted in Kallmann’s syndrome shares homology with neural cell adhesion and axonal pathfinding molecules. Nature, 353, 529–536. Gervais, R., Dumur, V., Rigot, J.-M. et al. (1993) High frequency of the R117H cystic fibrosis mutation in patients with congenital absence of the vas deferens. N. Engl. J. Med., 328, 446–447. Glover, T.W., Verga, V., Rafael, J. et al. (1993) Translocation breakpoint in Aarskog syndrome maps to Xp11.21 between ALAS2 and DXS323. Hum. Mol. Genet., 2, 1717–1718. Greene, J.S., Parfrey, P.S., Harnett, J.D. et al. (1989) The cardinal manifestations of Bardet–Bield syndrome, a form of Laurence–Moon–Biedl syndrome. N. Engl. J. Med., 321, 1002–1009. Grier, R.E., Farrington, E.H., Kending, R. and Manunes, P. (1981) Autosomal dominant inheritance of the Aarskog syndrome. Am. J. Hum. Genet., 33 (Suppl.), 64A. Guion-Almeida, M.L. and Richieri-Costa, A. (1992) Aarskog syndrome in a Brazilian boy born to consanguineous parents. Am. J. Med. Genet., 43, 808–810.

428

Gyllenstein, U., Wharton, D., Josefsson, A. and Wilson, A.C. (1991) Paternal inheritance of mitochondrial DNA in mice. Nature, 352, 255–257. Hardelin, J.-P., Levilliers, J., Young, J. et al. (1993a) Xp22.3 deletions in isolated familial Kallmann’s syndrome. J. Clin. Endocrinol. Metab., 76, 827–831. Hardelin, J.-P., Levilliers, J., Blanchard, S. et al. (1993b) Heterogeneity in the mutations responsible for X chromosome-linked Kallmann syndrome. Hum. Mol. Genet., 2, 373–377. Harding, A.E., Thomas, P.K., Baraitser, M. et al. (1982) X-linked recessive bulbospinal neuronopathy: a report of ten cases. J. Neurol. Neurosurg. Psych., 45, 1012–1019. Harvey, J.N. and Barnett, D. (1992) Endocrine dysfunction in Kearns–Sayre syndrome. Clin. Endocrinol., 37, 97–104. Hatada, I., Ohashi, H., Fukushima, Y. et al. (1996) An imprinted gene p57KIP2 is mutated in Beckwith–Wiedemann syndrome. Nature Genet., 14, 171–173. Hay, J.D., Smail, P.J. and Forsyth, C.C. (1981) Familial cytomegalic adrenocortical hypoplasia: an X-linked syndrome of pubertal failure. Arch. Dis. Child., 56, 715–721. Henry, I., Bonaiti-Pellie, C., Chehensse, V. et al. (1991) Uniparental paternal disomy in a genetic cancer-predisposing syndrome. Nature, 351, 665–667. Holm, V.A., Cassidy, S.B., Butler, M.G. et al. (1993) Prader–Willi syndrome: consensus diagnostic criteria. Pediatrics, 91, 398–402. Imbeaud, S., Carre-Eusebe, D., Rey, R. et al. (1994) Molecular genetics of the persistent Mu¨llerian duct sydrome: a study of 18 families. Hum. Mol. Genet., 3, 125–131. Imbeaud, S., Faure, E., Lamarre, I. et al. (1995) Insensitivity to anti-Mu¨llerian hormone due to a mutation in the human anti-Mu¨llerian hormone receptor. Nature Genet., 11, 382–388. Imbeaud, S., Belville, C., Messika-Zeitoun, L. et al. (1996) A 27 base-pair deletion of the anti-Mu¨llerian type II receptor gene is the most common cause of the persistent Mu¨llerian duct syndrome. Hum. Mol. Genet., 5, 1269–1277. Jamieson, C.R., van der Burgt, I., Brady, A.F. et al. (1994) Mapping a gene for Noonan syndrome to the long arm of chromosome 12. Nature Genet., 8, 357–360. Jeffcoate, W.J., Laurance, B.M., Edwards, C.R.W. and Besser, G.M. (1980) Endocrine function in the Prader–Willi syndrome. Clin. Endocrinol., 12, 81–89. Johns, D.R. (1995) Mitochondrial DNA and disease. N. Engl. J. Med., 333, 638–644. Jonsson, M.S., McCormick, J.R., Gillies, C.G. and Gondos, B. (1982) Kartagener’s syndrome with motile spermatozoa. N. Engl. J. Med., 307, 1131–1133. Josso, N., Fekete, C., Cachin, O. et al. (1983) Persistence of Mu¨llerian duct in male pseudohermaphroditism, and its relationship to cryptorchidism. Clin. Endocrinol., 19, 247–258. Josso, N., Boussin, L., Knebelmann, B. et al. (1991) Anti-Mu¨llerian hormone and intersex states. Trends Endocrinol. Metab., 2, 227–233. Kaplan, E., Shwachman, H., Perlmutter, A.D. et al. (1968) Reproductive failure in males with cystic fibrosis. N. Engl. J. Med., 279, 65–69. Katz, M.D. and Imperato-McGinley, J. (1994) Fetal hormones in sexual differentiation. Infert. Reprod. Med. Clin. N. Am., 4, 641–670 Kerem, B. and Kerem, E. (1996) The molecular basis for disease variability in cystic fibrosis. Eur. J. Hum. Genet., 4, 65–73. Kiesewetter, S., Macek, M., Davis, C. et al. (1993) A mutation in CFTR produces different phenotypes depending on chromosomal background. Nature Genet., 5, 274–278. Kletter, G.B., Gorski, J.L. and Kelch, R.P. (1991) Congenital adrenal hypoplasia and isolated gonadotropin deficiency. Trends Endocrinol. Metab., 2, 123– 128. Knoll, J.H.M, Wagstaff, J. and Lalande, M. (1993) Cytogenetic and molecular studies in the Prader–Willi and Angelman syndromes: an overview. Am. J. Med. Genet., 46, 2–6. Kobayashi, K., Mizuno, K., Hida, A. et al. (1994) PCR analysis of the Y chromosome long arm in azoospermic patients: evidence for a second locus required for spermatogenesis. Hum. Mol. Genet., 3, 1965–1967. Kotzot, D., Schmitt, S., Bernasconi, F. et al. (1995) Uniparental disomy 7 in Silver–Russell syndrome and primordial growth retardation. Hum. Mol. Genet., 4, 583–587. Kwitek-Black, A.E., Carmi, R., Duyk, G.M. et al. (1993) Linkage of Bardet– Biedl syndrome to chromosome 16q and evidence for genetic heterogeneity. Nature Genet., 5, 392–396. Lange, R., Michelmann, H.W. and Engel, W. (1990) Chromosomale Ursachen der Infertilita¨t beim Mann. Fertilita¨t, 6, 17–28.

The molecular genetics of male infertility La Spada, A..R., Wilson, E.M., Lubahn, D.B. et al. (1991) Androgen receptor gene mutations in X-linked spinal and bulbar muscular atrophy. Nature, 352, 77–79. La Spada, A.R., Roling, D.B., Harding, A.E. et al. (1992) Meiotic stability and genotype–phenotype correlation of the trinucleotide repeat in X-linked spinal and bulbar muscular atrophy. Nature Genet., 2, 301–304. Ledbetter, D.H. and Engel, E. (1995) Uniparental disomy in humans: development of an imprinting map and its implications for prenatal diagnosis. Hum. Mol. Genet., 4, 1757–1764. Leppert, M., Baird, L., Anderson, K.L. et al. (1994) Bardet–Biedl syndrome is linked to DNA markers on chromosome 11q and is genetically heterogenous. Nature Genet., 7, 108–112. Ligtenberg, M.J.L., Kemp, S., Sarde, C.-O. et al. (1995) Spectrum of mutations in the gene encoding the adrenoleukodystrophy protein. Am. J. Hum. Genet., 56, 44–50. Lilford, R., Jones, A.M., Bishop, D.T. et al. (1994) Case-control study of whether subfertility in men is familial. Br. Med. J., 309, 570–573. Ma, K., Inglis, J.D., Sharkey, A. et al. (1993) A Y chromosome gene family with RNA-binding protein homology: candidates for the azoospermia factor AZF controlling human spermatogenesis. Cell, 75, 1287–1295. Maddalena, A. and Sherins, R.J. (1992) Cystic fibrosis (CFTR) mutations in men with congenital absence of the vas deferens. Am. J. Hum. Genet., 51 (Suppl.), A340. MacLean, H.E., Warne, G.L. and Zajac, J.D. (1995) Defects of androgen receptor function: from sex reversal to motor neurone disease. Mol. Cell. Endocrinol., 112, 133–141. Mahadevan, M.; Tsilfidis, C., Sabourin, L. et al. (1992) Myotonic dystrophy mutation: an unstable CTG repeat in the 39 untranslated region of the gene. Science, 255, 1253–1255. Marks, L.J. and Bergeson, P.S. (1977) The Silver–Russell syndrome. A case with sexual ambiguity and a review of the literature. Am. J. Dis. Child., 131, 447–451. McGillivray, B.C. (1992) Genetic aspects of ambiguous genitalia. Ped. Clin. N. Am., 39, 307–317. McInnis, M.G. (1996) Anticipation: an old idea in new genes. Am. J. Hum. Genet., 59, 973–979. McKusick, V.A. (1993) Medical genetics. A 40-year perspective on the evolution of a medical specialty from a basic science. J. Am. Med. Assoc., 270, 2351–2356. McPhaul, M.J., Marcelli, M., Zoppi, S. et al. (1993) The spectrum of mutations in the androgen receptor gene that causes androgen resistance. J. Clin. Endocrinol. Metab., 76, 17–23. Mendez, H.M.M. and Opitz, J.M. (1985) Noonan syndrome: a review. Am. J. Med. Genet., 21, 493–506. Mercier, B., Verlingue, C., Lissens, W. et al. (1995) Is congenital bilateral absence of vas deferens a primary form of cystic fibrosis? Analyses of the CFTR gene in 67 patients. Am. J. Hum. Genet., 56, 272–277. Meschede, D., Froster, U.G., Gullotta, F. and Nieschlag, E. (1993) Reproductive failure in a patient with neurofibromatosis–Noonan syndrome. Am. J. Med. Genet., 47, 346–351. Meschede, D., Behre, H.M., Nieschlag, E. and Horst, J. (1994) Kallmann– Syndrom. Pathophysiologie und Klinik. Dtsch. Med. Wsch., 119, 1436–1442. Meschede, D., Dworniczak, B., Eigel, A. et al. (1995a) Mutationsanalyse und genetische Beratung bei Ma¨nnern mit kongenitaler Aplasie der Samenleiter. Fertilita¨t, 11, 22–26. Meschede, D., De Geyter, C., Nieschlag, E. and Horst, J. (1995b) Genetic risk in micromanipulative assisted reproduction. Hum. Reprod., 10, 2880–2886. Meschede, D., Behre, H.M. and Nieschlag, E. (1996a) Sto¨rungen im Bereich der Androgenzielorgane. In Nieschlag, E. and Behre, H.M. (eds), Andrologie. Grundlagen und Klinik der reproduktiven Gesundheit des Mannes. SpringerVerlag, Berlin, pp. 219–234. Meschede, D., Rolf, C., Neugebauer, D.C. et al. (1996b) Sperm acrosome defects in a patient with Aarskog syndrome. Am. J. Med. Genet., 66, 340–342. Mhatre, A.N., Trifiro, M.A., Kaufman, M. et al. (1993) Reduced transcriptional regulatory competence of the androgen receptor in X-linked spinal and bulbar muscular atrophy. Nature Genet., 5, 184–188. Mickle, J., Milunsky, A., Amos, J.A. and Oates, R.D. (1995) Congenital unilateral absence of the vas deferens: a heterogenous disorder with two distinct subpopulations based upon aetiology and mutational status of the cystic fibrosis gene. Hum. Reprod., 10, 1728–1735. Moraes, C.T., DiMauro, S., Zeviani, M. et al. (1989) Mitochondrial DNA deletions in progressive external ophthalmoplegia and Kearns–Sayre syndrome. N. Engl. J. Med., 320, 1293–1299.

Morrow, A.F., Gyorki, S., Warne, G.L. et al. (1987) Variable androgen receptor levels in infertile men. J. Clin. Endocrinol. Metab., 64, 1115–1121. Moser, H.W, Smith, K.D. and Moser, A.B. (1995) X-linked adrenoleukodystrophy. In Scriver, C.R., Beaudet, A.L., Sly, W.S. and Valle, D. (eds), The Metabolic and Molecular Bases of Inherited Disease. McGraw-Hill, New York, pp. 2325–2349. Mosser, J., Douar, A.-M., Sarde, C.-O. et al. (1993) Putative X-linked adrenoleukodystrophy gene shares unexpected homology with ABC transporters. Nature, 361, 726–730. Moutou, C., Junien, C., Henry, I. and Bonaiti-Pellie, C. (1992) Beckwith– Wiedemann syndrome: a demonstration of the mechanisms responsible for the excess of transmitting females. J. Med. Genet., 29, 217–220. Muscatelli, F., Strom, T.F., Walker, A.P. et al. (1994) Mutations in the DAX1 gene give rise to both X-linked adrenal hypoplasia congenita and hypogonadotropic hypogonadism. Nature, 372, 672–676. Najmabadi, H., Hunag, V., Yen, P. et al. (1996) Substantial prevalence of microdeletions of the Y-chromosome in infertile men with idiopathic azoospermia and oligozoospermia detected using a sequence-tagged sitebased mapping strategy. J. Clin. Endocrinol. Metab., 81, 1347–1352. Narayan, D., Krishnan, S.N., Upender, M. et al. (1994) Unusual inheritance of primary ciliary dyskinesia (Kartagner’s syndrome). J. Med. Genet., 31, 493–496. Neugebauer, D.-C., Neuwinger, J., Jockenho¨vel, F. and Nieschlag, E. (1990) ‘9 1 09 axoneme in spermatozoa and some nasal cilia of a patient with totally immotile spermatozoa associated with thickened sheath and short midpiece. Hum. Reprod., 5, 981–986. Nieschlag, E. (1996) Nosologie andrologischer Krankheitsbilder. In Nieschlag, E. and Behre, H.M. (eds), Andrologie. Grundlagen und Klinik der reproduktiven Gesundheit des Mannes. Springer-Verlag, Berlin, pp. 85–88. Nistal, M., Herruzo, A. and Sanchez-Corral, F. (1978) Teratozoospermia absoluta de presentacio´n familiar. Espermatozoides microce´falos irregulares sin acrosoma. Andrologia, 10, 234–240. Noonan, J.A. and Lezington, K. (1968) Hypertelorism with Turner phenotype. A new syndrome with associated congenital heart disease. Am. J. Dis. Child., 116, 373–380. Oates, R.D. and Amos, J.A. (1994) The genetic basis of congenital bilateral absence of the vas deferens, and cystic fibrosis. J. Androl., 15, 1–8. Osborne, L.R., Lynch, M., Middleton, P.G. et al. (1993) Nasal epithelial ion transport and genetic analysis of infertile men with congenital bilateral absence of the vas deferens. Hum. Mol. Genet., 2, 1605–1609. Pasteris, N.G., Cadle, M., Logie, L.J. et al. (1994) Isolation and characterization of the faciogenital dysplasia (Aarskog–Scott syndrome) gene: a putative rho/rac guanine nucleotide exchange factor. Cell, 79, 669–678. Patrizio, P., Asch, R.H., Handelin, B. and Silber, S.J. (1993) Aetiology of congenital absence of vas deferens: genetic study of three generations. Hum. Reprod., 8, 215–220. Patton, M.A. (1988) Russell–Silver syndrome. J. Med. Genet., 25, 557–560. Penman Splitt, M., Burn, J., and Goodship, J. (1996) Defects in the determination of left-right asymmetry. J. Med. Genet., 33, 498–503. Pentao, L., Lewis, R.A., Ledbetter, D.H. et al. (1992) Maternal uniparental isodisomy of chromosome 14: association with autosomal recessive rod monochromacy. Am. J. Hum. Genet., 50, 690–699. Porteous, M.E.M. and Goudie, D.R. (1991) Aarskog syndrome. J. Med. Genet., 28, 44–47. Powers, J.M. and Schaumburg, H.H. (1981) The testis in adreno– leukodystrophy. Am. J. Pathol., 102, 90–98. Qureshi, S.J., Ross, A.R., Ma, K. et al. (1996) Polymerase chain reaction sceeening of Y chromosome microdeletions: a first step towards the diagnosis of genetically-determined spermatogenic failure in men. Mol. Hum. Reprod., 2, 775–779. Ramkissoon, Y. and Goodfellow, P. (1996) Early steps in mammalian sex determination. Curr. Opin. Genet. Devel., 6, 316–321. Rave-Harel, N., Madgar, I., Goshen, R. et al. (1995) CFTR haplotype analysis reveals genetic heterogeneity in the etiology of congenital bilateral aplasia of the vas deferens. Am. J. Hum. Genet., 56, 1359–1366. Redman, J.B., Fenwick, R.G., Fu, Y.-H. et al. (1993) Relationship between parental trinucleotide GCT repeat length and severity of myotonic dystrophy in offspring. J. Am. Med. Assoc., 269, 1960–1965 Reijo, R., Lee, T.-Y., Salo, P. et al. (1995) Diverse spermatogenic defects in humans caused by Y chromosome deletions encompassing a novel RNAbinding protein gene. Nature Genet., 10, 383–393. Reijo, R., Alagappan, R.K., Patrizio, P. and Page, D.C. (1996) Severe oligozoospermia resulting from deletions of azoospermia factor gene on Y chromosome. Lancet, 347, 1290–1293.

429

D.Meschede and J.Horst Reik, W., Brown, K.W., Schneid, H. et al. 1995) Imprinting mutations in the Beckwith–Wiedemann syndrome suggested by an altered imprinting pattern in the IGF2-H19 domain. Hum. Mol. Genet., 4, 2379–2385. Roses, A.D. (1993) Myotonic dystrophy. In Rosenberg, R.N., Prusiner, S.B., DiMauro, S. et al. (eds), The Molecular and Genetic Basis of Neurological Disease. Butterworth-Heinemann, Boston, pp. 633–646. Rutland, J. and De Iongh, R.U. (1990) Random ciliary orientation. A cause of respiratory tract disease. N. Engl. J. Med., 323, 1681–1684. Saxena, R., Brown, L.G., Hawkins, T. et al. (1996) The DAZ gene cluster on the human Y chromosome arose from an autosomal gene that was transposed, repeatedly amplified and pruned. Nature Genet., 14, 292–299. Schempp, W., Binkele, A., Arnemann, J. et al. (1995) Comparative mapping of YRRM- and TSPY-related cosmids in man and hominoid apes. Chromosome Res., 3, 227–234. Schill, W.-B. (1991) Some disturbances of acrosomal development and function in human spermatozoa. Hum. Reprod., 6, 969–978. Shan, Z., Hirschmann, P., Seebacher, T. et al. (1996) A SPGY copy homologous to the mouse gene Dazla and the Drosophila gene boule is autosomal and expressed only in the human male gonad. Hum. Mol. Genet., 5, 2005–2011. Sharland, M., Burch, M., McKenna, W.M. and Patton, M.A. (1992) A clinical study of Noonan syndrome. J. Med. Genet., 67, 178–183. Sharland, M., Morgan, M., Smith, G. et al. (1993) Genetic counselling in Noonan syndrome. Am. J. Med. Genet., 45, 437–440. Sheffield, V.C., Carmi, R., Kwitek-Black, A. et al. (1994) Identification of a Bardet–Biedl syndrome locus on chromosome 3 and evaluation of an efficient approach to homozygosity mapping. Hum. Mol. Genet., 3, 1331– 1335. Shoffner, J.M. and Wallace, D.C. (1995) Oxidative phosphorylation diseases. In Scriver, C.R., Beaudet, A.L., Sly, W.S. and Valle, D. (eds), The Metabolic and Molecular Bases of Inherited Disease. McGraw Hill, New York, pp. 1535–1609. Silber, S.J., Nagy, Z., Liu, J. et al. (1995) The use of epididymal and testicular spermatozoa for intracytoplasmatic sperm injection: the genetic implications for male infertility. Hum. Reprod., 10, 2031–2043. Simoni, M., Gromoll, J., Dworniczak, B. et al. (1997) Screening for deletions of the Y chromosome involving the DAZ (Deleted in Azoospermia) gene in azoospermia and severe oligozoospermia. Fertil. Steril., 67, 542–547. Strong, T.V., Smit, L.S., Turpin, S.V. et al. (1991) Cystic fibrosis gene mutation in two sisters with mild disease and normal sweat electrolyte levels. N. Engl. J. Med., 325, 1630–1634. Stuppia L., Mastroprimiano, G., Calabrese, G. et al. (1996) Microdeletions of interval 6 of the Y chromosome detected by STS–PCR in 6 of 33 patients with idiopathic oligo- or azoospermia. Cytogenet. Cell Genet., 72, 155–158. Sturgess, J.M., Chao, J., Wong, J. et al. (1979) Cilia with defective radial spokes. A cause of human respiratory disease. N. Engl. J. Med., 300, 53–56. Takeda, R. and Ueda, M. (1977) Pituitary–gonadal function in male patients with myotonic dystrophy – serum luteinizing hormone, follicle stimulating hormone and testosterone levels and histological damage of the testis. Acta Endocrinol., 84, 382–389. Temple, I.K., Cockwell, A., Hassold, T. et al. (1991) Maternal uniparental disomy for chromosome 14. J. Med. Genet., 28, 511–514. Tiepolo, L. and Zuffardi, O. (1976) Localization of factors controlling spermatogenesis in the nonfluorescent portion of the human Y chromosome long arm. Hum. Genet., 34, 119–124. Toledo, S.P.A., Medeiros-Neto, G., Knobel, M. and Mattar, E. (1977) Evaluation of the hypothalamic–pituitary–gonadal function in the Bardet– Biedl syndrome. Metabolism, 26, 1277–1291. Tomkins, D.J., Waye, J.S., Whelan, D.T. and Cox, D.W. (1994) Maternal uniparental disomy of chromosome 14 in a boy with t(14q14q) associated with a paternal t(13q14q). Am. J. Hum. Genet., 55 (Suppl.), Abstract 685. Tsatsoulis, A. and Shalet, S.M. (1991) Antisperm antibodies in the polyglandular autoimmune (PGA) syndrome type I: response to cyclical steroid therapy. Clin. Endocrinol., 35, 299–303. Tsilfidis, C., MacKenzie, A.E., Mettler, G. et al. (1992) Correlation between CTG trinucleotide repeat length and frequency of severe congenital myotonic dystrophy. Nature Genet., 1, 192–195. Vogt, P., Chandley, A.C., Hargraeve, T.B. et al. (1992) Microdeletions in interval 6 of the Y chromosome of males with idiopathic sterility point to disruption of AZF, a human spermatogenesis gene. Hum. Genet., 89, 491–496. Vogt, P.H., Edelmann, A., Kirsch, S. et al. (1996) Human Y chromosome azoospermia factors (AZF) mapped to different subregions in Yq11. Hum. Mol. Genet., 5, 933–943.

430

Walter, C.A., Shaffer, L.G., Kaye, C.I. et al. (1996) Short-limb dwarfism and hypertrophic cardiomyopathy in a patient with paternal isodisomy 14: 45,XY,idic(14)(p11). Am. J. Med. Genet., 65, 259–265. Weiss, J., Axelrod, L., Whitcomb, R.W. et al. (1992) Hypogonadism caused by a single amino acid substitution in the β subunit of luteinizing hormone. N. Engl. J. Med., 326, 179–183. Weksberg, R. and Squire, J. (1995) Genomic imprinting in Beckwith– Wiedemann syndrome. In Ohlsson, R., Hall, K. and Ritzen, M. (eds), Genomic Imprinting: Causes and Consequences. Cambridge University Press, Cambridge, pp. 237–251. Weksberg, R., Shen, D.R., Fei, Y.L. et al. (1993) Disruption of insulin-like growth factor 2 imprinting in Beckwith–Wiedemann syndrome. Nature Genet., 5, 143–150. Willems, P.J. (1994) Dynamic mutations hit double figures. Nature Genet., 8, 213–215. Williams, C., Mayall, E.S., Williamson, R. et al. (1993) A report on CF carrier frequency among men with infertility owing to congenital absence of the vas deferens. J. Med. Genet., 30, 973. Wilton, L.J., Teichtahi, H., Temple-Smith, P.D. and de Kretser, D.M. (1985) Structural heterogeneity of the axonemes of respiratory cilia and sperm flagella in normal men. J. Clin. Invest., 75, 825–831. Wise, J.E., Matalon, R., Morgan, A.M. and McCabe, E.R.B. (1987) Phenotypic features of patients with congenital adrenal hypoplasia and glycerol kinase deficiency. Am. J. Dis. Child., 141, 744–747. Yen, P.H., Chai, N.N. and Salido, E.D. (1996) The human autosomal gene DAZLA: testis specificity and a candidate for male infertility. Hum. Mol. Genet., 5, 2013–2017. Zamboni, L. (1987) The ultrastructural pathology of the spermatozoon as a cause of infertility: the role of electron microscopy in the evaluation of semen quality. Fertil. Steril., 48, 711–734. Zanaria, E., Muscatelli, F., Bardoni, B. et al. (1994) An unusual member of the nuclear hormone receptor superfamily responsible for X-linked adrenal hypoplasia congenita. Nature, 374, 635–641. Zhang, L., Fischbeck, K.H. and Arnheim, N. (1995) CAG repeat length variation in sperm from a patient with Kennedy’s disease. Hum. Mol. Genet., 4, 303–305. Received on October 4, 1996; accepted on March 3, 1997