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A minority of cancers have breast cancer gene (BRCA) mutations that confer sensitivity to poly (ADP-ribose) polymerase (PARP) inhibitors (PARPis), but the role for PARPis in BRCA-proficient cancers is not well established. This suggests the need for novel combination therapies to expand the use of these drugs. Recent reports that low doses of DNA methyltransferase inhibitors (DNMTis) plus PARPis enhance PARPi efficacy in BRCA-proficient AML subtypes, breast, and ovarian cancer open up the possibility that this strategy may apply to other sporadic cancers. We identify a key mechanistic aspect of this combination therapy in nonsmall cell lung cancer (NSCLC): that the DNMTi component creates a BRCAness phenotype through downregulating expression of key homologous recombination and nonhomologous end-joining (NHEJ) genes. Importantly, from a translational perspective, the above changes in DNA repair processes allow our combinatorial PARPi and DNMTi therapy to robustly sensitize NSCLC cells to ionizing radiation in vitro and in vivo. Our combinatorial approach introduces a biomarker strategy and a potential therapy paradigm for treating BRCA-proficient cancers like NSCLC.

Among patients undergoing stem-cell transplantation from matched related donors, cyclophosphamide plus cyclosporin led to significantly longer GVHD-free, relapse-free survival than standard prophylaxis.

Homologous recombination and DNA repair are important for genome maintenance. Genetic variations in essential homologous recombination genes, including BRCA1 and BRCA2 results in homologous recombination deficiency (HRD) and can be a target for therapeutic strategies including poly (ADP-ribose) polymerase inhibitors (PARPi). However, response is limited in patients who are not HRD, highlighting the need for reliable and robust HRD testing. This manuscript will review BRCA1/2 function and homologous recombination proficiency in respect to breast and ovarian cancer. The current standard testing methods for HRD will be discussed as well as trials leading to approval of PARPi’s. Finally, standard of care treatment and synthetic lethality will be reviewed.

Structural chromosomal rearrangements result from different mechanisms of formation, usually related to certain genomic architectural features that may lead to genetic instability. Most of these rearrangements arise from recombination, repair, or replication mechanisms that occur after a double-strand break or the stalling/breakage of a replication fork. Here, we review the mechanisms of formation of structural rearrangements, highlighting their main features and differences. The most important mechanisms of constitutional chromosomal alterations are discussed, including Non-Allelic Homologous Recombination (NAHR), Non-Homologous End-Joining (NHEJ), Fork Stalling and Template Switching (FoSTeS), and Microhomology-Mediated Break-Induced Replication (MMBIR). Their involvement in chromoanagenesis and in the formation of complex chromosomal rearrangements, inverted duplications associated with terminal deletions, and ring chromosomes is also outlined. We reinforce the importance of high-resolution analysis to determine the DNA sequence at, and near, their breakpoints in order to infer the mechanisms of formation of structural rearrangements and to reveal how cells respond to DNA damage and repair broken ends.

Germline mutations in BRCA1/2 predispose individuals to breast cancer (termed germline-mutated BRCA1/2 breast cancer, gBRCA-BC) by impairing homologous recombination (HR) and causing genomic instability. HR also repairs DNA lesions caused by platinum agents and PARP inhibitors. Triple-negative breast cancers (TNBCs) harbor subpopulations with BRCA1/2 mutations, hypothesized to be especially platinum-sensitive. Cancers in putative ‘BRCAness’ subgroups—tumors with BRCA1 methylation; low levels of BRCA1 mRNA ( BRCA1 mRNA-low); or mutational signatures for HR deficiency and those with basal phenotypes—may also be sensitive to platinum. We assessed the efficacy of carboplatin and another mechanistically distinct therapy, docetaxel, in a phase 3 trial in subjects with unselected advanced TNBC. A prespecified protocol enabled biomarker–treatment interaction analyses in gBRCA-BC and BRCAness subgroups. The primary endpoint was objective response rate (ORR). In the unselected population (376 subjects; 188 carboplatin, 188 docetaxel), carboplatin was not more active than docetaxel (ORR, 31.4% versus 34.0%, respectively; P  = 0.66). In contrast, in subjects with gBRCA-BC, carboplatin had double the ORR of docetaxel (68% versus 33%, respectively; biomarker, treatment interaction  P  = 0.01). Such benefit was not observed for subjects with BRCA1 methylation, BRCA1 mRNA-low tumors or a high score in a Myriad HRD assay. Significant interaction between treatment and the basal-like subtype was driven by high docetaxel response in the nonbasal subgroup. We conclude that patients with advanced TNBC benefit from characterization of BRCA1/2 mutations, but not BRCA1 methylation or Myriad HRD analyses, to inform choices on platinum-based chemotherapy. Additionally, gene expression analysis of basal-like cancers may also influence treatment selection. The phase 3 TNT Trial in subjects with triple-negative breast cancer supports the superiority of carboplatin over docetaxel in BRCA1/2 -mutated tumors and a greater response to taxanes in the nonbasal subtype.

We hypothesized that IV busulfan (Bu) dosing could be safely intensified through pharmacokinetic (PK-) dose guidance to minimize the inter-patient variability in systemic exposure (SE) associated with body-sized dosing, and that this should improve outcome of AML/MDS patients undergoing allogeneic stem cell transplantation. To test this hypothesis, we treated 218 patients (median age 50.7 years, male/female 50/50%) with fludarabine 40 mg/m 2 once daily x4, each dose followed by IV Bu, randomized to 130 mg/m 2 ( N =107) or PK-guided to average daily SE, AUC of 6000 μ M  min ( N =111), stratified for remission status and allo-grafting from HLA-matched donors. Toxicity and GvHD rates in the groups were similar; the risk of relapse or treatment-related mortality remained higher in the fixed-dose group throughout the 80-month observation period. Further, PK-guidance yielded safer disease control, leading to improved overall and PFS, most prominently in MDS patients and in AML patients not in remission at allogeneic stem cell transplantation. We conclude that AML/MDS patients receiving pretransplant conditioning treatment with our 4-day regimen may benefit significantly from PK-guided Bu dosing. This could be considered an alternative to fixed-dose delivery since it provides the benefit of precise dose delivery to a predetermined SE without increasing risk(s) of serious toxicity and/or GvHD.

Findings of retrospective studies suggest that sorafenib maintenance post-transplantation might reduce relapse in patients with FLT3 internal tandem duplication (FLT3-ITD) acute myeloid leukaemia undergoing allogeneic haematopoietic stem-cell transplantation. We investigated the efficacy and tolerability of sorafenib maintenance post-transplantation in this population. We did an open-label, randomised phase 3 trial at seven hospitals in China. Eligible patients (aged 18–60 years) had FLT3-ITD acute myeloid leukaemia, were undergoing allogeneic haematopoietic stem-cell transplantation, had an Eastern Cooperative Oncology Group performance status of 0–2, had composite complete remission before and after transplantation, and had haematopoietic recovery within 60 days post-transplantation. Patients were randomly assigned (1:1) to sorafenib maintenance (400 mg orally twice daily) or non-maintenance (control) at 30–60 days post-transplantation. Randomisation was done with permuted blocks (block size four) and implemented through an interactive web-based randomisation system. The primary endpoint was the 1-year cumulative incidence of relapse in the intention-to-treat population. This trial is registered with ClinicalTrials.gov, NCT02474290; the trial is complete. Between June 20, 2015, and July 21, 2018, 202 patients were enrolled and randomly assigned to sorafenib maintenance (n=100) or control (n=102). Median follow-up post-transplantation was 21·3 months (IQR 15·0–37·0). The 1-year cumulative incidence of relapse was 7·0% (95% CI 3·1–13·1) in the sorafenib group and 24·5% (16·6–33·2) in the control group (hazard ratio 0·25, 95% CI 0·11–0·57; p=0·0010). Within 210 days post-transplantation, the most common grade 3 and 4 adverse events were infections (25 [25%] of 100 patients in the sorafenib group vs 24 [24%] of 102 in the control group), acute graft-versus-host-disease (GVHD; 23 [23%] of 100 vs 21 [21%] of 102), chronic GVHD (18 [18%] of 99 vs 17 [17%] of 99), and haematological toxicity (15 [15%] of 100 vs seven [7%] of 102). There were no treatment-related deaths. Sorafenib maintenance post-transplantation can reduce relapse and is well tolerated in patients with FLT3-ITD acute myeloid leukaemia undergoing allogeneic haematopoietic stem-cell transplantation. This strategy could be a suitable therapeutic option for patients with FLT3-ITD acute myeloid leukaemia. None.

Double strand breaks (DSBs) are induced in the DNA following exposure of cells to ionizing radiation (IR) and are highly consequential for genome integrity, requiring highly specialized modes of processing. Erroneous processing of DSBs is a cause of cell death or its transformation to a cancer cell. Four mechanistically distinct pathways have evolved in cells of higher eukaryotes to process DSBs, providing thus multiple options for the damaged cells. The homologous recombination repair (HRR) dependent subway of gene conversion (GC) removes IR-induced DSBs from the genome in an error-free manner. Classical non-homologous end joining (c-NHEJ) removes DSBs with very high speed but is unable to restore the sequence at the generated junction and can catalyze the formation of translocations. Alternative end-joining (alt-EJ) operates on similar principles as c-NHEJ but is slower and more error-prone regarding both sequence preservation and translocation formation. Finally, single strand annealing (SSA) is associated with large deletions and may also form translocations. Thus, the four pathways available for the processing of DSBs are not alternative options producing equivalent outcomes. We discuss the rationale for the evolution of pathways with such divergent properties and fidelities and outline the logic and necessities that govern their engagement. We reason that cells are not free to choose one specific pathway for the processing of a DSB but rather that they engage a pathway by applying the logic of highest fidelity selection, adapted to necessities imposed by the character of the DSB being processed. We introduce DSB clusters as a particularly consequential form of chromatin breakage and review findings suggesting that this form of damage underpins the increased efficacy of high linear energy transfer (LET) radiation modalities. The concepts developed have implications for the protection of humans from radon-induced cancer, as well as the treatment of cancer with radiations of high LET.

DNA double-strand breaks (DSBs) are deleterious lesions that challenge genome integrity. To mitigate this threat, human cells rely on the activity of multiple DNA repair machineries that are tightly regulated throughout the cell cycle 1 . In interphase, DSBs are mainly repaired by non-homologous end joining and homologous recombination 2 . However, these pathways are completely inhibited in mitosis 3 – 5 , leaving the fate of mitotic DSBs unknown. Here we show that DNA polymerase theta 6 (Polθ) repairs mitotic DSBs and thereby maintains genome integrity. In contrast to other DSB repair factors, Polθ function is activated in mitosis upon phosphorylation by Polo-like kinase 1 (PLK1). Phosphorylated Polθ is recruited by a direct interaction with the BRCA1 C-terminal domains of TOPBP1 to mitotic DSBs, where it mediates joining of broken DNA ends. Loss of Polθ leads to defective repair of mitotic DSBs, resulting in a loss of genome integrity. This is further exacerbated in cells that are deficient in homologous recombination, where loss of mitotic DSB repair by Polθ results in cell death. Our results identify mitotic DSB repair as the underlying cause of synthetic lethality between Polθ and homologous recombination. Together, our findings reveal the critical importance of mitotic DSB repair in the maintenance of genome integrity. In mitosis, genome integrity is maintained by DNA polymerase theta-dependent repair of DNA double-strand breaks, which is regulated by Polo-like kinase 1 activity.

Double-strand DNA breaks (DSBs) are continuously induced in cells by endogenously generated free radicals and exogenous genotoxic agents such as ionizing radiation. DSBs activate the kinase activity in sensor proteins such as ATM and DNA-PK, initiating a complex DNA damage response that coordinates various DNA repair pathways to restore genomic integrity. In this study, we report the unexpected finding that homologous chromosomes contact each other at the sites of DSBs induced by either radiation or the endonuclease I-PpoI in human somatic cells. Contact involves short segments of homologous chromosomes and is centered on a DSB in active genes but does not occur at I-PpoI sites in intergenic DNA. I-PpoI-induced contact between homologous genes is abrogated by the transcriptional inhibitors actinomycin D and α-amanitin and requires the kinase activity of ATM but not DNA-PK. Our findings provide documentation of a common transcription-related and ATM kinase-dependent mechanism that induces contact between allelic regions of homologous chromosomes at sites of DSBs in human somatic cells.