A supernumerary chromosome 20, identified by FISH, in a male with azoospermia—cause or coincidence?

Investigation of azoospermia involves endocrine and genetic tests, including molecular analysis for cystic fibrosis gene mutations and chromosome analysis. Chromosome testing is important as 12% of azoo/ oligo-spermic subjects are karyotypically abnormal [De Braekeleer and Dao, 1991]. Common cytogenetic findings in association with infertility include sex chromosome abnormalities, such as XXY males, and autosomal abnormalities, although the latter are infrequent [Retief et al., 1984;Bourrouillou et al., 1985;Rosenmann et al., 1985; Gabriel-Robez and Rumpler, 1996; PaoloniGiacobino et al., 2000]. Certain unbalanced chromosomal rearrangements may interfere with cell division in the spermatocytes resulting in azoospermia. We report a male patient with azoospermia, where cytogenetic analysis revealed a supernumerary marker chromosome (SMC), which was characterized by FISH to be a chromosome 20 derivative. A 35-year-old azoospermic male was referred for infertility investigation. Endocrine tests and cystic fibrosis mutational analysis were normal. Cytogenetic analysis revealed the presence of a small SMC, in 80% of blood lymphocyte cells analyzed (Fig. 1a). Due to the very small size of the derivative and the fact that the chromosome 20 paint contained centromeric probes, it was difficult to determine how much, if any, euchromatin was present. Chromosome analyses of parental bloods were normal indicating that the marker was de novo. FISH analyses were performed using the alpha satellite multiprobe (Cytocell). The chromosome 20 centromeric probe gave positive results indicating the origin of the marker chromosome. The findings were confirmed using a whole chromosome paint for chromosome 20 (Fig. 1b) (47,XY,þmar.ish der(20)(wcp20þ, D20Z1þ)[80]/46,XY[20]). SMCs present a major genetic counseling problem to clinicians. The use of FISH has determined the identity and composition ofmany of thesemarkers [Crolla, 1998; Crolla et al., 1998] and, thus, has provided more accurate information on which to base the risk of cytogenetically abnormal offspring. However, due to the rarity of these SMCs, no clear karyotype/phenotype correlation has emerged. Ten cases of supernumerary derivative 20 chromosomes have been reported [Callen et al., 1991; Blennow et al., 1993; Batista et al., 1995; Van Langen et al., 1996; Viersbach et al., 1997; Crolla et al., 1998; Austin-Ward et al., 2000; Stankiewicz et al., 2000]. Two patients were detected prenatally and at age 20 months, were phenotypically and developmentally normal [Viersbach et al., 1997]. The eight other cases, detected post-natally (one adult and seven children), showed varying degrees of phenotypic abnormality that may be due to varying amounts of euchromatin in the marker. The adult patient, an 18-year-old female, had primary amenorrhoea and mild learning disability [Stankiewicz et al., 2000]. Although some of the seven children shared common features, such as psychomotor retardation and developmental delay, no consistent phenotype was evident. In our patient, the question therefore remains—is the derivative chromosome 20 the cause of infertility or a coincidental finding? Thefirst case reports ofSMCswere in oligospermicmales [Smith et al., 1965]. Whether they were the cause of the infertility or just chance findings— remains unresolved. Robertsonian and reciprocal translocations have also been found in association with infertility, and there is some evidence to suggest that the frequencies of these translocations are increased in subfertile and infertile males [Chandley, 1979; Retief et al., 1984; Bourrouillou et al., 1985; Rosenmann et al., 1985; Gabriel-Robez and Rumpler, 1996]. It is not known, however, if the frequency of supernumerary chromosomes is raised in the infertile population. SMCs are preferentially maternally transmitted [Buckton et al., 1985], suggesting either a reduced fertility in male carriers or that the marker is excluded in spermatogenesis. Pairing failure at meiosis has been *Correspondence to: Prof. Patrick J. Morrison, Northern Ireland Regional Genetics Centre, Belfast City Hospital Trust, Lisburn Road, Belfast BT9 7AB, Northern Ireland, United Kingdom. E-mail: patrick.morrison@bch.n-i.nhs. uk