[Sex chromosomes and male infertility]

Male infertility is a worldwide problem, with a variety of causes including genetic factors. Sex chromosomes are particularly interesting, as males only have a single copy of both chromosomes. The Y chromosome is obviously an area of interest in the study of male-factor infertility because it contains many of the genes that are critical for spermatogenesis and the development of male gonads. Y chromosome microdeletions are the most commonly known genetic causes of spermatogenic failure in males. The azoospermia factor (AZF) region is a particular area on the long arm of the Y chromosome, Yq, where microdeletions occur most frequently. Fourteen Y chromosome genes encoding putatively functional proteins and expressed in the human testis are found to be located in one of the three AZF intervals. The exact role of specific AZF genes in spermatogenesis is largely unknown, for each of the most classical Yq deletions removes multiple genes. The importance of the X chromosome in mammalian spermatogenesis is suggested by its enrichment of germ cell-specific genes expressed in spermatogenesis, such as AR, USP26, TAF7L, TEX11, KAL1, AKAP4, and NXF2. Genes on the X chromosome may be under unique evolutionary pressure due to their hemizygous expression in male. The mutations in the single copy X-linked genes, unlike in autosomal genes, would not be masked by a normal allele. Many researches have been conducted on the relationship between spermatogenesis and the genes on the X chromosome, but the involvement of the X chromosome in male infertility remains less understood and deserves further characterization.