Distinct roles of BRCA2 in replication fork protection in response to hydroxyurea and DNA interstrand crosslinks

Abstract

DNA interstrand crosslinks (ICLs) are a form of DNA damage that requires the interplay of a number of repair proteins including those of the Fanconi anemia (FA) and the homologous recombination (HR) pathways. Pathogenic variants in the essential gene BRCA2/FANCD1, when monoallelic, predispose to breast and ovarian cancer, and when biallelic, results in a severe subtype of Fanconi anemia. BRCA2 function in the FA pathway is attributed to its role as a mediator of the RAD51 recombinase in HR repair of the programmed DNA double strand breaks (DSB). BRCA2 and RAD51 functions are also required to protect stalled replication forks from nucleolytic degradation during response to hydroxyurea (HU). While RAD51 has been shown to be necessary in the early steps of ICL repair to prevent aberrant nuclease resection, the role of BRCA2 in this process has not been described. Here, based on the analysis of BRCA2 DNA binding domain (DBD) mutants discovered in FA patients presenting with atypical FA-like phenotypes, we establish that BRCA2 is necessary for protection of DNA at an ICL. Cells carrying DBD BRCA2 mutations are sensitive to ICL inducing agents but resistant to HU treatment consistent with relatively high HR repair in these cells. BRCA2 function at an ICL protects against DNA2-WRN nuclease-helicase complex and not the MRE11 nuclease implicated in the resection of HU-stalled replication forks. Our results also indicate that unlike the processing at HU-stalled forks, function of the SNF2 translocases (SMARCAL1, ZRANB3, or HLTF), implicated in fork reversal, are not an integral component of the ICL repair, pointing to a different mechanism of fork protection at different DNA lesions.2 Introduction:

RAD51AP1 and RAD54 underpin two distinct RAD51-dependent routes of DNA damage repair via homologous recombination

“…However, in the context of MMC and HU, agents that more immediately lead to stalled and collapsed DNA replication (Rickman et al 2020;Thakar and Moldovan 2021), the presence of RAD51AP1, surprisingly, is more than equally important than the presence of RAD54B. As such, our results also help explain the comparatively mild phenotypes of human cells with RAD51AP1 deficiency (Henson et al 2006;Modesti et al 2007;Wiese et al 2007), as most of the RAD51AP1 activity apparently can be compensated for by RAD54.…”