J. Ross Chapman

Poly(ADP-ribose) polymerase inhibitors (PARPi) exploit homologous recombination deficiency (HRD) in BRCA1/2-mutated cancers to induce synthetic lethality. PARPi also kill cancer cells lacking the DNA damage-responsive kinase ATM, however, inconsistent evidence of HRD and variable clinical responses have obscured the underlying mechanism. Here we define how PARPi induce cytotoxicity in ATM-deficient cells and reveal a critical role for ATM in regulating DNA replication. In the absence of ATM, unrestrained PRIMPOL-dependent repriming at spontaneous oxidative base adducts generates discontinuous daughter strands containing DNA gaps that activate PARP. This defect is sustained by aberrant suppression of replication fork slowing - presumably via fork reversal - by the BRCA1-A complex, whose recruitment to stalled forks is normally counteracted by ATM. The resulting gaps require homologous recombination (HR) for post-replicative repair and underlie the synthetic lethal interaction with PARPi. Suppressing repriming, reducing oxidative stress, or blocking base excision repair alleviates these defects. Collectively, our findings reveal how spontaneous base damage cooperates with replication dysfunction to drive PARPi sensitivity and establish a paradigm of post-replicative repair addiction in ATM-deficient cells. Together, our findings define a mechanistic link between oxidative DNA damage and ATM-dependent replication control, illuminating how oxidative stress may exacerbate genome instability in Ataxia Telangiectasia.

Perturbed DNA replication can lead to incompletely replicated DNA when cells enter mitosis and can interfere with chromosome segregation. Cells therefore require mechanisms to resolve these lesions during mitosis. The CIP2A-TOPBP1 complex is described to tether fragmented DNA molecules during mitosis. Whether CIP2A also functions in processing of incompletely replicated DNA remained unclear. We show that CIP2A-TOPBP1 form large filamentous structures at sites of incomplete DNA replication during mitosis, and that CIP2A-TOPBP1 facilitate the recruitment of SMX tri-nuclease complex members SLX4, MUS81 and XPF-ERCC1. These structures form in proximity to sites of mitotic DNA synthesis, although CIP2A is not required for mitotic DNA synthesis. In addition to its globular and coiled-coil domain, the unstructured C-terminal domain of CIP2A is essential for CIP2A-TOPBP1 filamentous structure formation and recruitment of the SMX complex. BRCA1^-/- and BRCA2^-/- cells have increased mitotic DNA lesions that recruit CIP2A and SLX4. We show that the C-terminal part of CIP2A is required for survival of BRCA2^-/- cells. Moreover, SLX4 is crucial for genome stability in BRCA2^-/- cells. Combined, we demonstrate that CIP2A-TOPBP1 recruits the SMX complex during mitosis, which is required to resolve mitotic DNA lesions, allows faithful chromosome segregation and maintain viability of BRCA2^-/- cells.

ERCC6L2 disease is a recessive bone marrow failure (BMF) syndrome caused by mutations in the SNF2-like putative DNA helicase ERCC6L2. While implicated in DNA replication, double strand break (DSB) repair via non-homologous end joining (NHEJ), and interstrand crosslink (ICL) repair, how ERCC6L2 supports haematopoietic longevity remains unclear. Investigating this in vivo , we find that an Ercc6l2- deficient haematopoietic stem and progenitor cell (HSPC) compartment in mice is unexpectedly resilient. Ercc6l2 loss was also tolerated in mice co-deficient for endogenous formaldehyde detoxification, which precipitates early-onset BMF in models of Fanconi anaemia. Instead, Ercc6l2 -deficient mice display a mild immunodeficiency, arising from defects in immunoglobulin class-switch recombination (CSR), that synergise with shieldin-deficiency, implicating ERCC6L2 and shieldin in distinct repair mechanisms. Furthermore, we demonstrate that ERRC6L2 stimulates chromosome fusions in the context of staggered, but not blunt dysfunctional telomeres. We reconcile ERCC6L2’s NHEJ function through proteomic elucidation of its endogenous interactome and AlphaFold structural modelling to reveal a complex formed of ERCC6L2 and KU that is bridged by the NHEJ accessory factor MRI/CYREN. Consequently, ERCC6L2-MRI inter-dependence characterises CSR. Together, our findings implicate the ERCC6L2-MRI complex as a KU-regulatory DNA translocase coordinating classical-NHEJ at staggered-end DSBs. We suggest that similar staggered-end breaks represent the pathological substrates driving haematopoietic failure in ERCC6L2 disease.

Tumor suppressor p53-binding protein 1 (53BP1) regulates DNA end joining in lymphocytes, diversifying immune antigen receptors. This involves nucleosome-bound 53BP1 at DNA double-stranded breaks (DSBs) recruiting Rap1-interacting factor 1 homolog (RIF1) and shieldin, a poorly understood DNA-binding complex. The 53BP1–RIF1–shieldin axis is pathological in BRCA1-mutated cancers, blocking homologous recombination (HR) and driving illegitimate nonhomologous end joining (NHEJ). However, how this axis regulates DNA end joining and HR suppression remains unresolved. We investigated shieldin and its interplay with the Ctc1–Stn1–Ten1 (CST) complex, which was recently implicated downstream of 53BP1. Immunophenotypically, mice lacking shieldin or CST are equivalent, with class-switch recombination coreliant on both complexes. Ataxia-telangiectasia mutated kinase-dependent DNA damage signaling underpins this cooperation, inducing physical interactions between these complexes that reveal shieldin as a DSB-responsive CST adaptor. Furthermore, DNA polymerase ζ functions downstream of shieldin, establishing DNA fill-in synthesis as the physiological function of shieldin–CST. Lastly, we demonstrate that 53BP1 suppresses HR and promotes NHEJ in BRCA1-deficient mice and cells independently of shieldin. These findings showcase the versatility of the 53BP1 pathway, achieved through the collaboration of chromatin-bound 53BP1 complexes and DNA end-processing effector proteins.

53BP1 regulates DNA end-joining in lymphocytes, diversifying immune antigen receptors. This involves nucleosome-bound 53BP1 at DNA double-stranded breaks (DSBs) recruiting RIF1 and shieldin, a poorly understood DNA-binding complex. The 53BP1-RIF1-shieldin axis is pathological in BRCA1 -mutated cancers, blocking homologous recombination (HR) and driving illegitimate non-homologous end-joining (NHEJ). However, how this axis regulates DNA end-joining and HR suppression remains unresolved. We investigated shieldin and its interplay with CST, a complex recently implicated in 53BP1-dependent activities. Immunophenotypically, mice lacking shieldin or CST are equivalent, with class-switch recombination co-reliant on both complexes. ATM-dependent DNA damage signalling underpins this cooperation, inducing physical interactions between these complexes that reveal shieldin as a DSB-responsive CST adaptor. Furthermore, DNA polymerase ζ functions downstream of shieldin, establishing DNA fill-in synthesis as the physiological function of shieldin-CST. Lastly, 53BP1 suppresses HR and promotes NHEJ in BRCA1-deficient mice and cells independently of shieldin. These findings showcase the resilience of the 53BP1 pathway, achieved through the collaboration of chromatin-bound 53BP1 complexes and DNA end-processing effector proteins.

Understanding the genetic and nongenetic determinants of tumor protein 53 (TP53)-mutation-driven clonal evolution and subsequent transformation is a crucial step toward the design of rational therapeutic strategies. Here we carry out allelic resolution single-cell multi-omic analysis of hematopoietic stem/progenitor cells (HSPCs) from patients with a myeloproliferative neoplasm who transform to TP53-mutant secondary acute myeloid leukemia (sAML). All patients showed dominant TP53 ‘multihit’ HSPC clones at transformation, with a leukemia stem cell transcriptional signature strongly predictive of adverse outcomes in independent cohorts, across both TP53-mutant and wild-type (WT) AML. Through analysis of serial samples, antecedent TP53-heterozygous clones and in vivo perturbations, we demonstrate a hitherto unrecognized effect of chronic inflammation, which suppressed TP53 WT HSPCs while enhancing the fitness advantage of TP53-mutant cells and promoted genetic evolution. Our findings will facilitate the development of risk-stratification, early detection and treatment strategies for TP53-mutant leukemia, and are of broad relevance to other cancer types.

The efficacy of poly(ADP)-ribose polymerase 1 inhibition (PARPi) in BRCA1-deficient cells depends on 53BP1 and shieldin, which have been proposed to limit single-stranded DNA at double-strand breaks (DSBs) by blocking resection and/or through CST–Polα–primase-mediated fill-in. We show that primase (like 53BP1–shieldin and CST–Polα) promotes radial chromosome formation in PARPi-treated BRCA1-deficient cells and demonstrate shieldin–CST–Polα–primase-dependent incorporation of BrdU at DSBs. In the absence of 53BP1 or shieldin, radial formation in BRCA1-deficient cells was restored by the tethering of CST near DSBs, arguing that in this context, shieldin acts primarily by recruiting CST. Furthermore, a SHLD1 mutant defective in CST binding (SHLD1Δ) was non-functional in BRCA1-deficient cells and its function was restored after reconnecting SHLD1Δ to CST. Interestingly, at dysfunctional telomeres and at DNA breaks in class switch recombination where CST has been implicated, SHLD1Δ was fully functional, perhaps because these DNA ends carry CST recognition sites that afford SHLD1-independent binding of CST. These data establish that in BRCA1-deficient cells, CST–Polα–primase is the major effector of shieldin-dependent DSB processing.

Protein ubiquitination at sites of DNA double-strand breaks (DSBs) by RNF168 recruits BRCA1 and 53BP1^1,2, which are mediators of the homologous recombination and non-homologous end joining DSB repair pathways, respectively³. Non-homologous end joining relies on 53BP1 binding directly to ubiquitinated lysine 15 on H2A-type histones (H2AK15ub)^4,5 (which is an RNF168-dependent modification⁶), but how RNF168 promotes BRCA1 recruitment and function remains unclear. Here we identify a tandem BRCT-domain-associated ubiquitin-dependent recruitment motif (BUDR) in BRCA1-associated RING domain protein 1 (BARD1) (the obligate partner protein of BRCA1) that, by engaging H2AK15ub, recruits BRCA1 to DSBs. Disruption of the BUDR of BARD1 compromises homologous recombination and renders cells hypersensitive to PARP inhibition and cisplatin. We further show that BARD1 binds nucleosomes through multivalent interactions: coordinated binding of H2AK15ub and unmethylated H4 lysine 20 by its adjacent BUDR and ankyrin repeat domains, respectively, provides high-affinity recognition of DNA lesions in replicated chromatin and promotes the homologous recombination activities of the BRCA1–BARD1 complex. Finally, our genetic epistasis experiments confirm that the need for BARD1 chromatin-binding activities can be entirely relieved upon deletion of RNF168 or 53BP1. Thus, our results demonstrate that by sensing DNA-damage-dependent and post-replication histone post-translation modification states, BRCA1–BARD1 complexes coordinate the antagonization of the 53BP1 pathway with promotion of homologous recombination, establishing a simple paradigm for the governance of the choice of DSB repair pathway.

Genomic instability is a hallmark of cancer, and has a central role in the initiation and development of breast cancer^1,2. The success of poly-ADP ribose polymerase inhibitors in the treatment of breast cancers that are deficient in homologous recombination exemplifies the utility of synthetically lethal genetic interactions in the treatment of breast cancers that are driven by genomic instability³. Given that defects in homologous recombination are present in only a subset of breast cancers, there is a need to identify additional driver mechanisms for genomic instability and targeted strategies to exploit these defects in the treatment of cancer. Here we show that centrosome depletion induces synthetic lethality in cancer cells that contain the 17q23 amplicon, a recurrent copy number aberration that defines about 9% of all primary breast cancer tumours and is associated with high levels of genomic instability^4,5,6. Specifically, inhibition of polo-like kinase 4 (PLK4) using small molecules leads to centrosome depletion, which triggers mitotic catastrophe in cells that exhibit amplicon-directed overexpression of TRIM37. To explain this effect, we identify TRIM37 as a negative regulator of centrosomal pericentriolar material. In 17q23-amplified cells that lack centrosomes, increased levels of TRIM37 block the formation of foci that comprise pericentriolar material—these foci are structures with a microtubule-nucleating capacity that are required for successful cell division in the absence of centrosomes. Finally, we find that the overexpression of TRIM37 causes genomic instability by delaying centrosome maturation and separation at mitotic entry, and thereby increases the frequency of mitotic errors. Collectively, these findings highlight TRIM37-dependent genomic instability as a putative driver event in 17q23-amplified breast cancer and provide a rationale for the use of centrosome-targeting therapeutic agents in treating these cancers.