Molecular identification of HY genes and peptide epitopes

The work on deleted and mutant Y chromosomes which separated Hya/HYA from other Y chromosome genes provided detailed mapping information which has led to the positional cloning of the genes and the identification of several H-Y epitopes in mouse and one in humans. New Y chromosome genes have also been discovered during the course of these studies and it has transpired that different H-Y peptide epitopes are derived from several linked genes, making Hya, like the loci encoding other minor transplantation antigens (see section on Minor transplantation antigens), a gene complex rather than a single gene. The first H-Y epitope to be characterized molecularly was discovered following expression cloning using H-Y-specific T cells to detect the presence of the epitope on cells transfected with DNA containing the relevant gene. The H-Y epitope associated with H-2Kk (H-Y/Kk) was identified as TENSGKDI, the product of one of the 3' exons of the Smcy gene. The human H-Y epitope (H-Y/HLA-B7) is the peptide SPSVDKARAEL, a product of the homologous human gene, SMCY. This was deduced following the alternative approach of peptide elution coupled with microcapillary liquid chromatography and electrospray ionisation mass spectroscopy, and comparing peptide sequence with the known DNA sequence of the SMCY gene. The mouse H-YDk epitope is also a product of the Smcy gene, being derived from one of the 5' exons, but an H-Y/Db epitope, WMHHNMDLI, is encoded by a separate but adjacent gene, Uty. Smcy and Uty have X chromosome homologs, Smcx and Utx, which escape X inactivation; X and Y copies of both genes are ubiquitously transcribed, suggesting that two copies may be needed for function, which is currently unknown, but from their sequence the products could be DNA- or protein-binding proteins, and be involved in transcription. Perhaps unsurprisingly the H-Y epitopes, which are the basis for male-specific transplantation rejection responses, are derived from sequences at which the X and the Y homologs diverge.

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