HELB Is a Feedback Inhibitor of DNA End Resection

Abstract

DNA double-strand break repair by homologous recombination is initiated by the formation of 3' single-stranded DNA (ssDNA) overhangs by a process termed end resection. Although much focus has been given to the decision to initiate resection, little is known of the mechanisms that regulate the ongoing formation of ssDNA tails. Here we report that DNA helicase B (HELB) underpins a feedback inhibition mechanism that curtails resection. HELB is recruited to ssDNA by interacting with RPA and uses its 5'-3' ssDNA translocase activity to inhibit EXO1 and BLM-DNA2, the nucleases catalyzing resection. HELB acts independently of 53BP1 and is exported from the nucleus as cells approach S phase, concomitant with the upregulation of resection. Consistent with its role as a resection antagonist, loss of HELB results in PARP inhibitor resistance in BRCA1-deficient tumor cells. We conclude that mammalian DNA end resection triggers its own inhibition via the recruitment of HELB.

BRCA1 Mutation-Specific Responses to 53BP1 Loss-Induced Homologous Recombination and PARP Inhibitor Resistance

“…Mutations or downregulation of proteins that regulate DNA repair pathway choice have been shown to promote HR and PARPi resistance in the absence of BRCA1 activity (Bouwman et al, 2010; Bunting et al, 2010; Tkáč et al, 2016; Xu et al, 2015). Specifically, when 53BP1 is depleted or knocked out, DNA end resection and HR ensue in the absence of BRCA1 activity and result in PARPi resistance (Bouwman et al, 2010; Bunting et al, 2010).…”

Homology-Directed Repair and the Role of BRCA1, BRCA2, and Related Proteins in Genome Integrity and Cancer

“…Loss of other proteins that affect levels of end-resected DNA, such as REV7/MAD2L2 and HELB, similarly confers PARPi resistance to BRCA1-deficient cells (Boersma et al 2015, Patel et al 2011, Tkac et al 2016, Wang et al 2014, Xu et al 2015, Zimmermann & de Lange 2014). These proteins could operate in the same step as 53BP1 or in distinct pathways to control levels of resected DNA.…”

“…Notably, given that ATM contributes independently to HDR in BRCA1-53BP1 double mutant cells (Bunting et al 2010, Chen et al 2017), evidence suggests that HDR-restored BRCA1-deficient tumor cells can be resensitized to PARPi by ATM kinase inhibition (Tkac et al 2016, Xu et al 2015). ATR kinase also has a role in bypassing the requirement for BRCA1 for HDR in PARPi-resistant cells by promoting RAD51 recruitment to damage sites (Yazinski et al 2017).…”

Sharpening the ends for repair: mechanisms and regulation of DNA resection

“…2). Interestingly, Durocher and colleagues have recently proposed another negative feedback mechanism for resection termination in which the recruitment of DNA helicase HELB by RPA to ssDNA inhibits the nuclease activities of Exo1 and BLM-Dna2, although the detailed biochemical mechanism of this inhibition remains to be defined [130]. Another possible mechanism for resection termination is the second end capture during HR, which may prevent further resection by annealing the complimentary strands and formation of double Holliday junction [131][132][133][134].…”