Elisabetta Incorvaia

High-fidelity DNA repair are nuclear mechanisms essential to maintain genome integrity. However those mechanisms can be channeled away from DNA stability towards mutations and recombinations. During immunoglobulin locus diversification in B cells, Activation Induced Deaminase (AID) triggers, through DNA deamination of cytosine into uracil, DNA instability. AID-induced lesions can be resolved via error-free or error-prone DNA repair, depending on cellular milieu and local chromatin context. To uncover the regulation of repair pathway choice, AID damages on a DNA plasmid were resolved using cellular extract either via base excision repair (BER: short patch SP-BER or long patch LP-BER) or mismatch repair (MMR). Cell-type origins presented quantitative differences in DNA repair kinetics: a) overall sensitivity, b) non-B cells activating non-canonical MMR first, c) B cells activating SP-BER first, and d) LP-BER only activated in B cells. Analysis of single components known to influence DNA repair, such as chromatin and nucleosome formation, revealed significant changes in DNA repair pathway choice. DNA with nucleosome favourable base-stacking preferred LP-BER, while non-nucleosome stacking DNA preferred SP-BER and MMR. Overall these results provide insight into the cellular context that can influence DNA repair. The use of B cells and cancer cell lines recapitulates in vivo Ig locus diversification, while at the same time provide insight into the mechanisms of tumourgenesis.