DNA end-joining driven by microhomologies catalyzed by nuclear extracts

Efficiency of nonhomologous DNA end joining varies among somatic tissues, despite similarity in mechanism

Failure to repair DNA double-strand breaks (DSBs) can lead to cell death or cancer. Although nonhomologous end joining (NHEJ) has been studied extensively in mammals, little is known about it in primary tissues. Using oligomeric DNA mimicking endogenous DSBs, NHEJ in cell-free extracts of rat tissues were studied. Results show that efficiency of NHEJ is highest in lungs compared to other somatic tissues. DSBs with compatible and blunt ends joined without modifications, while noncompatible ends joined with minimal alterations in lungs and testes. Thymus exhibited elevated joining, followed by brain and spleen, which could be correlated with NHEJ gene expression. However, NHEJ efficiency was poor in terminally differentiated organs like heart, kidney and liver. Strikingly, NHEJ junctions from these tissues also showed extensive deletions and insertions. Hence, for the first time, we show that despite mode of joining being generally comparable, efficiency of NHEJ varies among primary tissues of mammals.

Homologous recombination-mediated double-strand break repair in mouse testicular extracts and comparison with different germ cell stages

Homologous recombination (HR) is established as a significant contributor to double-strand break (DSB) repair in mammalian somatic cells; however, its role in mammalian germ cells has not been characterized, although being conservative in nature it is anticipated to be the major pathway in germ cells. The germ cell system has inherent limitations by which intact cell approaches are not feasible. The present study, therefore, investigates HR-mediated DSB repair in mouse germ cell extracts by using an in vitro plasmid recombination assay based on functional rescue of a neomycin (neo) gene. A significantly high-fold increase in neo+ (Kan(R)) colonies following incubation of two plasmid substrates (neo delta1 and neo delta2) with testicular extracts demonstrated the extracts' ability to catalyze intermolecular recombination. A significant enhancement in recombinants upon linearization of one of the plasmids suggested the existence of an HR-mediated DSB repair activity. Comparison of the activity at sequential developmental stages, spermatogonia, spermatocytes and spermatids revealed its presence at all the stages; spermatocyte being the most proficient stage. Further, restriction analysis of recombinant plasmids indicated the predominance of gene conversion in enriched spermatocytes (mostly pachytenes), in contrast to gonial and spermatid extracts that showed higher reciprocal exchange. In conclusion, this study demonstrates HR repair activity at all stages of male germ cells, suggesting an important role of HR-mediated DSB repair during mammalian spermatogenesis. Further, the observed preference of gene conversion over reciprocal exchange at spermatocyte stage correlates with the close association of gene conversion with the meiotic recombination program.