DSB landscape in mature human spermatozoa

A central tenet of human evolutionary biology is the paternal bias in germline mutations, which is often to observe in different mammalian taxa. During meiosis, intentional DSBs are generated as a prerequisite for an exchange of genetic material in the prophase of meiosis and finally postmeiotic DSBs have been detected during spermatogenesis being implicated in the histone-to-protamine transition, in that histones are largely but not completely replaced by the more basic protamines PRM1 and PRM2. Both cell type and developmental stage influence the balance between different/specific? DSB repair pathways. Thus, differentiated somatic cells often resolve DSBs by NHEJ, whereas embryonic stem cells preferably use HR. Since round spermatids are haploid cells, DSBs cannot be repaired by homology-based mechanisms. Instead, breaks must be repaired by error prone mechanisms such as non-homologous end joining (NHEJ) or microhomology-mediated end joining (MMEJ) most likely involving polymerase Theta (Theta mediated end joining, TMEJ). To disentangle the relative contribution of DSBs and their repair during spermatogenesis separating it from DSB induction/repair during early zygotic reprogramming, we concentrated on mature human sperm heads as a proxy to the male pronucleus before zygotic reprogramming and thus before reconstituting the nucleo-histone profiles. To this end we applied a modified BLISS experimental procedure on DNA of mature human sperm heads that were isolated by differential lysis of human ejaculate samples.