The presence of a truncated base excision repair pathway in human spermatozoa, Mediated by OGG1

Abstract

DNA repair has long been considered impossible in human spermatozoa due to the high level of DNA compaction observed in these cells. However, detailed examination of the base excision repair pathway in human spermatozoa has revealed the presence of an enzyme critical to this pathway, OGG1. This glycosylase was associated with the sperm nucleus and mitochondria and could actively excise 8-hydrdoxy, 2′-deoxyguanosine, releasing this adduct into the extracellular space. This activity was significantly reduced in the presence of cadmium (II), a recognized inhibitor of OGG1, in a time- and dose- dependent manner (P<0.001). Remarkably, spermatozoa do not possess the downstream components of the base excision repair pathway, APE1 and XRCC1. The absence of these proteins was particularly significant, as APE1 is required to create a 3′-hydroxyl (3′-OH) terminus at the apurinic site created by OGG1, which would be recognized by the TUNEL assay. As a result, TUNEL was unable to detect oxidatively induced DNA damage in spermatozoa following exposure to hydrogen peroxide. In the same cells, intracellular and extracellular 8OHdG could be clearly detected in a manner that was highly correlated with the outcome of SCSA (Sperm Chromatin Structure Assay). However, incubation of these cells for 48 hours revealed a time-dependent increase in TUNEL positivity, suggesting the perimortem activation of a nuclease. These results emphasize the limited capacity of mature spermatozoa to mount a DNA repair response to oxidative stress, and highlight the importance of such mechanisms in the oocyte in order to protect the embryo from paternally mediated genetic damage.