The role of sperm DNA damage in the origin of infertility associated with Y chromosome long arm deletions in the mouse model

Abstract

In mouse and man Y chromosome deletions are frequently associated with spermatogenic defects. Mice with severe non-pairing Y chromosome long arm (NPYq) deficiencies are infertile in vivo and in vitro. We have previously shown that sperm from these males, although having grossly malformed heads, were able to fertilize oocytes via intracytoplasmic sperm injections (ICSI) and yield live offspring. However, in continuing ICSI trials we noted a reduced efficiency when cryopreserved sperm were used and with epididymal sperm as compared to testicular sperm. Our initial study tested if NPYq deficiency is associated with sperm DNA damage-a known cause of poor ICSI results. We observed that epididymal sperm from mice with severe NPYq deficiency are impaired in oocyte activation ability, and have an increased incidence of oocyte arrest and paternal chromosome breaks. Comet assays revealed increased DNA damage in both epididymal and testicular sperm, and transmission electron microscopy showed sperm having impaired membrane integrity and abnormal chromatin condensation. We therefore concluded that the increased DNA damage associated with NPYq deficiency might be a consequence of disturbed chromatin remodeling taking place during spermiogenesis. There are four distinct multi-copy genes found in the long arm of the Y chromosome. One of them, Sly, is known to control the expression of sex chromosome genes after meiosis; Sly deficiency results in a remarkable upregulation of sex chromosome genes. Sly deficiency has also been shown to be the underlying cause of sperm head anomalies and infertility associated with NPYq gene loss. We therefore hypothesized that Sly is our target gene. To test this, we examined mice with transgenically (RNAi) silenced Sly. Our analysis of Sly-deficient mice demonstrated similar 'sperm DNA damage' phenotype. This confirmed that lack of Sly is responsible for the sperm DNA damage/chromatin packaging defects observed in mice with NPYq deletions. This project provides the first evidence of DNA damage in sperm from mice with NPYq deficiencies and that the multi-copy NPYq-encoded Sly gene plays a key role in processes regulating chromatin remodeling and thus maintaining DNA integrity in sperm.