Mutational signatures reveal the role of RAD52 in p53-independent p21 driven genomic instability

Background: Genomic instability promotes evolution and heterogeneity of tumors. Unraveling its mechanistic basis is essential to design appropriate therapeutic strategies. In a recent study we reported an unexpected oncogenic property of p21WAF1/Cip1 showing that its chronic expression, in a p53-deficient environment, causes genomic instability by deregulating the replication licensing machinery. Results: Extending on this work we now demonstrate that p21WAF1/Cip1 can further fuel genomic instability by suppressing the repair capacity of low and high fidelity pathways that deal with nucleotide abnormalities. Consequently, fewer single nucleotide substitutions (SNSs) occur, while formation of highly deleterious DNA double-strand breaks (DSBs) is enhanced, crafting a characteristic mutational signature landscape. Guided by the mutational signatures formed, we found at the mechanistic level that the DSBs were repaired by Rad52-dependent Break-Induced Replication (BIR) and Single-Strand Annealing (SSA). Conversely, the error-free synthesis-dependent strand annealing (SDSA) repair route was deficient. Surprisingly, Rad52 was activated transcriptionally in an E2F1-dependent manner, rather than post-translationally as is common for DNA repair factor activation. Conclusions: Our results signify the importance of mutational signatures as guides to disclose the "repair history" leading to genomic instability. In this vein, following this approach we unveiled how chronic p21WAF1/Cip1 expression rewires the repair process, identifying Rad52 as a source of genomic instability and a candidate therapeutic target.