LAUSR

Colorectal cancer (CRC) initiates in the large intestine (colon or rectum) and is a leading cause of cancer-related deaths in the United States in both males and females and across all racial and ethnic groups (1). As much as 16% of these patients harbor a cancer susceptibility gene pathogenic variant, such as mutations in the adenomatous polyposis coli gene, the DNA polymerase genes POLE or POLD1 or the base excision repair genes MUTYH or NTHL1 (2). The most prominent of these hereditary CRC syndromes include pathogenic mutations in genes of the mismatch repair (MMR) pathway, including MLH1, MSH2, MSH6, and PMS2 (2). Patients with MMR deficiency are classified with Lynch syndrome (3) and have a high incidence of many cancers in addition to CRC, including cancer of the stomach, endometrium, and ovaries, among others (2, 3). As such, there is a dire need to uncover new therapies that might be selective for MMR-deficient cancers such as CRC. In PNAS, Hao et al. (4) uncover the mechanism that leads to apoptosis for a recently identified targeted therapy approach for MMR-deficient CRC (Fig. 1). An active area of discovery for tumor targeted therapeutic approaches relies on synthetic lethality, whereby treatments are designed to exploit compensatory (synthetic lethal) relationships among biological pathways essential for tumor growth, one of which is uniquely defective in the tumor (5, 6). Such an approach, for example, has been highly effective for the treatment of breast cancer with BRCA1/BRCA2 deficiency or with defects in other homologous recombinant genes, shown to be selectively sensitive to inhibitors of the DNA damage response (DDR) signaling enzyme, PARP1, such as olaparib (7) or talazoparib (8). Recent efforts (6, 9–13) have indicated that MMR-deficient CRC tumor cells are highly sensitive to loss of expression of WRN, a RecQ-family ATPdependent helicase/bifunctional 3′-5′ exonuclease, pointing to a synthetic lethal relationship between WRN and the MMR pathway in CRC. Hao et al. (4) build on their lab’s expertise on the mechanism of p53-dependent cell death, further documenting the significance of the WRN/MMR synthetic lethal relationship. Importantly, they find that the loss of or inhibition of WRN, in MMR-defective cells, triggers DNA damage that leads to p53-dependent and p53-independent PUMA activation that precipitates the onset of mitochondria-mediated apoptosis (Fig. 1). MMR is a post-replicative DNA repair pathway that recognizes and repairs base-base mis-pairs and DNA strand misalignments that arise during DNA replication (14, 15). Such lesions or DNA replication errors are recognized by the MSH2/ MSH6 heterodimer (the MutSα complex) that in turn recruits the MLH1/PMS2 heterodimer (the MutLα complex) (16). The base-base or strand misalignment error is then corrected by excision of the ‘error-containing’ DNA strand followed by gap-filling DNA synthesis, improving the overall fidelity of DNA replication by ~1,000-fold (14, 16, 17). As such, pathogenic defects in MMR lead to elevated mutation rates (14, 18) and genetic variability characterized by microsatellite instability (MSI) (19, 20). Regions of the genome encoding microsatellites or tracts of short (2 to 4 base) tandem repeats are highly unstable when MMR is defective, giving rise to either expansions or contractions of these microsatellites (19, 20). Close to 15% of CRC is classified by high levels of MSI (also called MSI-high), whereas 85% are found to have chromosomal instability but with genetically stable microsatellite regions, defined as microsatellite stable (MSS) (21). WRN loss (via RNA interference or CRISPR/cas9-mediated gene knockout, KO) in MMR-deficient and MSI-high cells (9–13) leads to elevated DNA damage (DNA double-strand breaks, DSBs) and cell death (9), not seen in MSS cells (10). Interestingly, it is the helicase function of WRN that is required for viability of MMR-deficient cells, not the WRN exonuclease activity (9–11). WRN may help resolve abnormal genomic structures, such as long (TA)n repeat expansions (13), that accumulate in MMR-deficient and MSI-high cells (10, 11, 13). Upon loss of WRN in MMR-deficient/MSI-high cells, such genomic structures are likely not resolved, leading to an increase in DNA damage and the activation of the DDR signaling kinases ATM and CHK2. The increase in DSBs upon loss of WRN in MSI-high cells (10, 11, 13), with high prevalence of end-resected breaks (13), is in-line with an increase in ATM/CHK2 activation. Hao et al. (4) show in MSI CRC, but not MSS CRC, that WRN depletion triggers an increase in DNA damage, as measured by phosphorylation (activation) of ATM(Ser1981) and CHK2(Thr68). Further, they find that ATM inhibition suppresses the WRN loss-induced phenotype, highlighting the significance of DNA damage induced apoptosis following WRN inhibition in MMR-deficient CRC cells (Fig. 1). WRN dependency in MMR-deficient/MSI-high cells has been documented in over 60 preclinical models (12), and loss