Division of Labor by the HELQ, BLM, and FANCM Helicases during Homologous Recombination Repair in Drosophila melanogaster

Abstract

Repair of DNA double-strand breaks by homologous recombination (HR) requires a carefully orchestrated sequence of events involving many proteins. One type of HR, synthesis-dependent strand annealing (SDSA), proceeds via the formation of a displacement loop (D-loop) when RAD51-coated single-stranded DNA invades a homologous template. The 3′ end of the single-stranded DNA is extended by DNA synthesis. In SDSA, the D-loop is then disassembled prior to strand annealing. While many helicases can unwind D-loops in vitro, how their action is choreographed in vivo remains to be determined. To clarify the roles of various DNA helicases during SDSA, we used a double-strand gap repair assay to study the outcomes of homologous recombination repair in Drosophila melanogaster lacking the BLM, HELQ, and FANCM helicases. We found that the absence of any of these three helicases impairs gap repair. In addition, flies lacking both BLM and HELQ or HELQ and FANCM had more severe SDSA defects than the corresponding single mutants. In the absence of BLM, a large percentage of repair events were accompanied by flanking deletions. Strikingly, these deletions were mostly abolished in the blm helq and blm fancm double mutants. Our results suggest that the BLM, HELQ, and FANCM helicases play distinct roles during SDSA, with HELQ and FANCM acting early to promote the formation of recombination intermediates that are then processed by BLM to prevent repair by deletion-prone mechanisms.

CRISPR knockout genome-wide screens identify the HELQ-RAD52 axis in regulating the repair of cisplatin-induced single stranded DNA gaps

“…DNA synthesis extension of the 3’ end ensures replication through the gapped region, and upon re-annealing to the parental strand of the original chromatid, the gap is filled. HELQ activity may also be required at this stage (in addition to or instead of the 3’ end unwinding), since previous studies have shown that HELQ participates in 3’ end extension downstream of D-loop formation, potentially through removing RAD51 from the dsDNA formed at the D-loop, and thus allowing for DNA polymerase engagement (Adelman et al, 2013; Thomas et al, 2022; Ward et al, 2010).…”

“…Indeed, the 3' end formed on the parental strand could invade the sister chromatid allowing repair from this template. Loss of HELQ may interfere with this process, since HELQ was shown to participate in D-loop extension as mentioned above (Adelman et al, 2013;Thomas et al, 2022;Ward et al, 2010). Thus, HELQ-depleted PRIMPOL-overexpressing cells not only accumulate ssDNA gaps due to defective gap filling, but also, upon MRE11-mediated conversion of these gaps into DSBs, are unable to repair these breaks since a critical step in the HR mechanism is compromised.…”

HELQ as a DNA helicase: Its novel role in normal cell function and tumorigenesis (Review)

“…Kras signaling is negatively associated with HELQ expression in patients with CLL, and ulixertinib, a Kras signaling inhibitor, may offer a new therapeutic option for patients with HELQ-low CLL (88,91). Furthermore, topotecan, a topoisomerase I inhibitor, is sensitive in HELQ mutants that induce single-ended DNA DSBs in replicating cells (92). In additionally, a RAD51 inhibitor, Cam833, which disrupts the interaction between RAD51 and BRCA2, synergizes with the poly (ADP-ribose) polymerase (PARP) inhibitors (93).…”

Interaction of human HelQ with DNA polymerase delta halts DNA synthesis and stimulates DNA single-strand annealing

“…It contributes to DNA break repair in metazoans but is absent from yeasts. Loss of HelQ ( HELQ − / − ) in cells predisposes to cancers and infertility ( 24–27 ) and corresponds to increased frequencies of tandem DNA duplications and long-tracts of DNA repair synthesis during homologous recombination ( 28 , 29 ) and reduced repair by synthesis-dependent strand annealing (SDSA) ( 30 ). HelQ also promotes DNA strand annealing in contexts distinct from MMEJ-like DNA repair by its homologue PolQ, a polymerase-helicase that is also found only in metazoans ( 28 ).…”