Genetics of Male Infertility - Y-Chromosome Microdeletions

Y-Chromosome Microdeletions

Y-chromosome microdeletions are common in about 10-15% of men with azoospermia or severe oligospermia. These microdeletions are too small to be detected by karyotyping. They can be easily identified using polymerase chain reaction.

Most of the microdeletions that cause azoospermia or oligospermia occur in the non-overlapping regions of the long arm of the Y-chromsome. These regions, also called azoospermia factor regions, are responsible for spermatogenesis. The loci are termed AZFa, AZFb and AZFc from proximal to distal Yq (Yq11.21-23 region). Several genes located in AZF regions which are found to be associated with spermatogenesis are viewed as “AZF candidate genes”.

AZFa and candidate genes: The size of the AZFa region is estimated to be 1Mb. The frequency of deletions in AZFa is lower than that of both AZFb and AZFc.

The important candidate genes include USP9Y (ubiquitin- specific protease 9, Y chromosome) and DBY (dead box on the Y).

USP9Y is a single copy gene that functions as a C-terminal ubiquitin hydrolase. It is ubiquitously expressed in a wide variety of tissues.

DBY is ubiquitously expressed in humans. In addition it exhibits transcripts unique to the testis. The protein it encodes is an RNA helicase. DBY is more frequently deleted than USP9Y.

The other candidate genes include UTY (ubiquitous TPR motif on the Y) and TB4Y (thymosin B4-isoform).

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AZFb and candidate genes:

AZFb locus spans about 1.5 Mb of the Y chromosome.

Two important candidate gene families that have been mapped on the AZFb locus are EIF1AY (translation initiation factor 1 A, Y isoform) and RBMY (RNA binding motif on Y).

EIF1AY encodes a ubiquitously expressed translation initiation factor. However it also possesses abundant testis-specific transcripts.

RBMY is a multicopy gene family thought to consist of 30-40 members, some of which are pseudogenes. Numerous RBMY genes have been detected across both arms of the Y chromosome. However, only genes within AZFb, produce detectable levels of the protein. These proteins contain an RNA- binding motif.

AZFc and candidate genes:

AZFc locus spans about 3Mb of the Y chromosome .Deletions of the AZFc locus occur more frequently than AZFa or AZFb locus.

DAZ (deleted in azoospermia) gene is an important candidate gene located in AZFc. This is the most frequently deleted gene in infertile men.

DAZ belongs to a multicopy gene family. Its autosomal copy is found on short arm of chromosome 3 (DAZ- like). It has 16 exons. It is expressed exclusively in testicular tissue. The protein has a regulatory role in RNA metabolism.

Other genes mapped to the AZFc region include CDY1 (chromodomain Y1) and BPY2 (basic protein Y2). Though the functions of these genes have yet to be elucidated, they are found to be expressed uniquely in the testis.