| Abstract
| - Myotonic dystrophy 1 (DM1) is a multi-system disorder characterized by endocrine defects that include testicular and tubular atrophy, oligospermia, Leydig cell hyperproliferation and increased follicle stimulating hormone (FSH) levels. DM1 results from a CTG expansion that causes transcriptional silencing of the flanking SIX5 allele. Loss of Six5 results in male sterility and a progressive decrease in testicular mass with age. We demonstrate a strict requirement of Six5 for both spermatogenic cell survival and spermiogenesis. Leydig cell hyperproliferation and increased intra-testicular testosterone levels are observed in the Six5−/−mice. Although increased FSH levels are observed in the Six5+/−and Six5−/−mice, serum testosterone levels and intra-testicular inhibin alpha and inhibin beta B levels are not altered in the Six5 mutant animals when compared with controls. Significantly, steady-state c-Kit levels are reduced in the Six5−/− testis. Thus, decreased c-Kit levels could contribute to the elevated spermatogenic cell apoptosis and Leydig cell hyperproliferation in the Six5−/− mice. The results support the hypothesis that the reduced SIX5 levels contribute to the male reproductive defects in DM1.
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