The Presence of a truncated DNA base excision repair pathway in human spermatozoa, mediated by OGG1 (#53)
DNA damage in the male germ line is associated with poor fertilisation rates, impaired pre-implantation embryo development, an increased incidence of miscarriage and significant morbidity in the offspring, including childhood cancer. The replacement of histones with protamines during spermatogenesis is theoretically designed to protect the paternal genome from oxidative insults, however, such high levels of compaction also place limits on the capacity of sperm chromatin to mount a DNA repair response. In this study, the ability of human spermatozoa to repair the highly mutagenic oxidative base adduct 8-hydroxy, 2`-deoxyguanosine (8OHdG) was investigated. The primary enzyme responsible for the excision of 8OHdG in somatic cells, 8-oxoguanine DNA glycosylase 1 (OGG1), was detected via Western blot while mass spectrometry indicated that the major isoform of this enzyme present in human spermatozoa is OGG1a. Immunocytochemical analysis revealed that OGG1 was localised to the nucleus as well as the midpiece of the spermatozoon which houses the mitochondrial genome. This glycosylase was found to be biochemically functional, actively cleaving 8OHdG from the DNA and releasing these adducts into the extracellular space upon treatment with H2O2, in a time– and dose- dependent manner (P<0.001). Furthermore, this activity was significantly suppressed in the presence of cadmium (II), a well-recognised inhibitor of OGG1 (P<0.001). Uniquely, this highly specialized cell type did not possess the downstream components of the base excision repair pathway, APE1 and XRCC1, suggesting that the resulting abasic site is repaired to completion by the oocyte following fertilisation, in order to protect the offspring from paternally inherited genetic damage.