EXPRESSION OF THE HPRT GENE DURING SPERMATOGENESIS - IMPLICATIONS FORSEX-CHROMOSOME INACTIVATION

The goal of this study was to determine the developmental pattern of e xpression of the X-linked gene for hypoxanthine phosphoribosyltransfer ase (Hprt) during spermatogenesis and the relevance of this expression to X-chromosome inactivation during meiotic prophase. The results dem onstrated that HPRT activity is maintained in mouse spermatogenic cell s throughout development in spite of X-chromosome inactivation; howeve r, specific activities of HPRT in meiotic and postmeiotic germ cells w ere significantly lower than in premeiotic ones. Maintenance of Hprt t ranscripts all stages was also demonstrated. Interestingly, the highes t level of Hprt transcripts was found in leptotene/zygotene spermatocy tes, suggesting a hyperactivation of the Hprt gene and/or stabilizatio n of Hprt transcripts in these cells. Hprt transcripts were present at very low levels in pachytene spermatocytes, and at slightly elevated levels in round spermatids. It was also found that the relative abunda nce of Hprt transcripts in the somatic cells of germ-cell-deficient te stes was much greater than that in meiotic and postmeiotic germ cells, even though their activities of HPRT were similar. Examination of the translational status of Hprt transcripts in testicular cells revealed that while most of the transcript was translationally active in somat ic cells of testes, less than half of the transcript was on polysomes in pachytene spermatocytes and round spermatids. Since no functional a utosomal Hprt gene exists in the mouse, these data suggest that the ge rm cell maintains both transcript and protein product of the Hprt gene in spite of apparent X-chromosome inactivation.