Synthetic Lethal Interactions of RECQ Helicases

Abstract

DNA helicases have risen to the forefront as genome caretakers. Their prominent roles in chromosomal stability are demonstrated by the linkage of mutations in helicase genes to hereditary disorders with defects in DNA repair, the replication stress response, and/or transcriptional activation. Conversely, accumulating evidence suggests that DNA helicases in cancer cells have a network of pathway interactions such that codeficiency of some helicases and their genetically interacting proteins results in synthetic lethality (SL). Such genetic interactions may potentially be exploited for cancer therapies. We discuss the roles of RECQ DNA helicases in cancer, emphasizing some of the more recent developments in SL.

Keywords: Bloom’s syndrome; RECQ; Rothmund–Thomson Syndrome; Werner syndrome; cancer; genetic disease; genomic stability; helicase; synthetic lethality.

Figure 1.. Involvement of DNA helicases to promote cancer cell growth and tumor development.

A. Helicases involved in DNA repair and fork stabilization enable cancer cells to cope with elevated replication stress induced upon oncogene activation. Oncogene-induced origin activation results in replication stalling/collapse due to increased inter-origin distance, reduced dNTP pool, increased transcription-replication interference, and head-to-tail fork collisions [106]. ATR-Chk1 pathway activation upon oncogene-induced replication stress promotes cancer cell fitness to evade the DNA damage response (DDR) barrier (e.g., TP53 loss) [107]. ATR phosphorylation of RecQs (e.g., WRN) stabilizes forks, prevents DSB formation, and ensures replication resumption, thereby enabling cancer cell proliferation [35]. B. Cancer cells gain replicative immortality by activating telomere maintenance mechanisms (TMM) including telomerase expression or ALT. In ALT-positive cancer cells, WRN and BLM helicases dissemble HR intermediates, thereby promoting telomere lengthening [–110]. FANCM cooperates with BLM/BRCA1 to mitigate replication stress at ALT telomeres by resolving non-canonical DNA structures [23]. C. Helicases confer a proliferative advantage to cancer cells by resolving non-canonical DNA structures. Cancer-related genes that undergo somatic copy number amplification (e.g. c-MYC) are characterized by abundant predicted G-quadruplex (G4) forming sequences in their promoter regions [111]. Because promoter G4 stabilization may repress transcription [112]. G4-resolving helicases (e.g., WRN) promote oncogene expression and increase cancer cell proliferation [64]. However, in some cases increased G4 formation is correlated with elevated transcriptional activity [111], suggesting G4 resolution in regulatory regions may repress transcription [59]. D. Helicases enable cancer cells to gain replicative immortality, leading to tumorigenesis.

Figure 2.. Proposed models illustrating how WRN helicase resolves non-B DNA secondary structures at microsatellite repeats and facilitate DNA synthesis.

A. Microsatellite-associated non-canonical DNA structures potentially resolved by WRN helicase activity. The microsatellite repeat sequences can adopt various non-B DNA conformations during DNA replication. B and C. Using its helicase activity, WRN resolves secondary structures formed in either the leading (B) or lagging (C) strand and allows smooth DNA synthesis through repeat regions. D. WRN helicase facilitates restart of stalled replication at DNA secondary structures. A replication fork encounters a non-B DNA structure of repeat sequences and regresses to form a four-way “chicken-foot” like structure. Using its 3′-5′ helicase activity, WRN partially unwinds the duplex region of the “middle toe” of the regressed chicken-foot regressed fork, thereby facilitating 5′-3′ nucleolytic processing by DNA2 nuclease. The partially single-stranded regressed fork is reset to an active fork by HR or reverse branch-migration which allows DNA synthesis to continue past the DNA secondary structure element. E. During fork regression caused by DNA secondary structure, the nascently synthesized DNA strand with repeat sequence elements could fold back on itself and form a non-canonical structure within the regressed arm of the reversed fork. WRN helicase may resolve the regressed strand secondary structure to allow DNA2-mediated end resection and fork restart. Repeats sequences are shown in purple.