KAMP

Structure and function of the spermatogenesis genes located in AZFa, a region of the human Y chromosome deleted in men with complete germ cell aplasia

Kamp, Christine¹, Kirsch, S¹, Hirschmann, P¹, Ditton, HJ¹, Brede, G², Tyler-Smith, C², Rappold, GA¹, and Vogt, PH¹

¹Institute of Human Genetics, Heidelberg, Germany
²Department of Biochemistry, Oxford, United Kingdom

In mammalian species X and Y, the so-called sex chromosomes, evolved from an extant pair of ordinary autosomes. One of these autosomes was elected to become the Y chromosome most likely because of a male specific selection of SA (Sexually Antagonistic) alleles, i.e., favoured in one sex, but disfavoured in the other. This resulted in a continuous reduction of crossing-over events and an accumulation of Y-specific DNA loci functional for male sex determination and male germ cell development. We focussed our research on a Y region in proximal Yq11 which was mapped by STS content analyses to be essential for male germ cell proliferation (AZFa region; 1). Men with deletions of AZFa suffer from a complete aplasia of germ cells in their testis tubules.

The molecular extension of the AZFa region is not known. We therefore established first a physical restriction map along the AZFa region with the aid of a complete YAC contig and estimated a molecular AZFa extension of at least one 1 megabase (Keil, R et al. in prep.). To analyse the gene content of AZFa we performed systematically organised exon trapping experiments with a series of PAC clones mapped in a contig by Alu-vector PCR, cross hybridizations of Y-specific end fragments and overlapping STS contents. 11 PAC clones were sufficient to cover the complete AZFa region. Multiple exons were isolated in each exon trapping experiment. Sequence analyses and homology searches in the EST and genomic databases identified some of them as exons of the DFFRY gene and DBY gene isolated recently as complete cDNA clones by Lahn and Page (2). Some of them hit other ESTs expressed in multiple tissues, some of them hit no data bank entry. This suggests that the AZFa region contains multiple Y genes expressed not only in testis tissue. This view got support by subsequent analysis of each novel exon clone on RNA-dot-blots and their identification in GeneFinder cDNA pools (Resource Center of German Human Genome project).

References

1. Vogt, PH et al. (1996) Human Y chromosome azoospermia factors mapped to different subregions in Yq11, Hum. Mol. Gen. 5: 933-943.
2. Lahn, BT and Page, DC  (1997) Functional Coherence of the Human Y Chromosome, Science 278: 675-680.