Sperm DNA Fragmentation: Mechanisms of Origin

Spermatozoa have the task to deliver an intact paternal genome to the oocyte and to support a successful embryo development. The high levels of sperm DNA fragmentation (sDF) found in sub-/infertile men threat human reproduction and health of the offspring. Strategies to prevent the onset of this type of sperm damage are extensively sought.sDF can be induced by factors like lifestyle-related habits, diseases, drugs, aging, infections and exposure to pollutants. At the cell level, all these factors induce sperm DNA breaks by three main mechanisms: apoptosis, impairment of sperm chromatin maturation and oxidative stress. Apoptosis and defects in maturation of sperm chromatin appear to act in the testis and account for DNA breaks found in dead ejaculated spermatozoa, whereas oxidative stress is likely inducing sDF during the transit through the male genital tracts and accounting for DNA breaks observed in viable spermatozoa of the ejaculate. Oxidative stress appears to be also the main mechanism responsible for induction of sDF after ejaculation, during in vitro manipulation of spermatozoa. Whether or not mature spermatozoa are able to trigger a cell death program is not yet clarified. In particular, it is not clear whether apoptotic nucleases or reactive oxygen species are responsible for producing DNA breaks in ejaculated mature spermatozoa. Knowledge of the mechanisms inducing sDF is a valuable starting point to define possible therapeutic options that however are still far to be established.