Targeting BRCA2 deficiency with GSK3 inhibitors

BRCA1 and BRCA2 tumour suppressors are essential for homologous recombination-mediated double-strand break repair and replication fork protection. Therefore, BRCA1 and BRCA2 are essential for cell viability. Deactivating mutations in one of these genes lead to genome instability and are associated with breast and ovarian cancer. We thus performed a chemical library screen and identified glycogen synthase kinase 3 (GSK3) inhibitors as a potent tool to kill BRCA2-deficient cells. One of the best-characterised cellular functions of serine/threonine kinase GSK3 is its role in the WNT pathway. Here, it acts as a regulator of β-catenin, with the inactivation of GSK3 leading to the stabilisation of β-catenin. In this study, we show that inhibition of GSK3 reduces the viability of human BRCA2-deficient cells. In addition, GSK3 inhibition leads to replication stress, DNA damage and genome instability in the context of BRCA2 deficiency. GSK3 inhibition sensitivity can be reversed by depleting β-catenin. Furthermore, GSK3 inhibition-induced replication stress and checkpoint activation is reduced when β-catenin is depleted. BRCA1-deficient cells that have lost 53BP1 are able to overcome PARP inhibitor olaparib sensitivity by reactivating homologous recombination. Interestingly, we find that olaparib-resistant Brca1-/- , 53BP1-deficient cells are sensitive to GSK3 inhibition, whilst Brca1+/+ cells remain unaffected. In summary, these results demonstrate that GSK3 inhibitors may be used to target HR deficiency, including cells that have acquired resistance against currently used therapies.</p

Targeting BRCA2 deficiency with GSK3 inhibitors

BRCA1 and BRCA2 tumour suppressors are essential for homologous recombination-mediated double-strand break repair and replication fork protection. Therefore, BRCA1 and BRCA2 are essential for cell viability. Deactivating mutations in one of these genes lead to genome instability and are associated with breast and ovarian cancer. We thus performed a chemical library screen and identified glycogen synthase kinase 3 (GSK3) inhibitors as a potent tool to kill BRCA2-deficient cells. One of the best-characterised cellular functions of serine/threonine kinase GSK3 is its role in the WNT pathway. Here, it acts as a regulator of β-catenin, with the inactivation of GSK3 leading to the stabilisation of β-catenin. In this study, we show that inhibition of GSK3 reduces the viability of human BRCA2-deficient cells. In addition, GSK3 inhibition leads to replication stress, DNA damage and genome instability in the context of BRCA2 deficiency. GSK3 inhibition sensitivity can be reversed by depleting β-catenin. Furthermore, GSK3 inhibition-induced replication stress and checkpoint activation is reduced when β-catenin is depleted. BRCA1-deficient cells that have lost 53BP1 are able to overcome PARP inhibitor olaparib sensitivity by reactivating homologous recombination. Interestingly, we find that olaparib-resistant Brca1-/- , 53BP1-deficient cells are sensitive to GSK3 inhibition, whilst Brca1+/+ cells remain unaffected. In summary, these results demonstrate that GSK3 inhibitors may be used to target HR deficiency, including cells that have acquired resistance against currently used therapies.</p

A transcription-based mechanism for oncogenic β-catenin-induced lethality in BRCA1/2-deficient cells

BRCA1 or BRCA2 germline mutations predispose to breast, ovarian and other cancers. High-throughput sequencing of tumour genomes revealed that oncogene amplification and BRCA1/2 mutations are mutually exclusive in cancer, however the molecular mechanism underlying this incompatibility remains unknown. Here, we report that activation of β-catenin, an oncogene of the WNT signalling pathway, inhibits proliferation of BRCA1/2-deficient cells. RNA-seq analyses revealed β-catenin-induced discrete transcriptome alterations in BRCA2-deficient cells, including suppression of CDKN1A gene encoding the CDK inhibitor p21. This accelerates G1/S transition, triggering illegitimate origin firing and DNA damage. In addition, β-catenin activation accelerates replication fork progression in BRCA2-deficient cells, which is critically dependent on p21 downregulation. Importantly, we find that upregulated p21 expression is essential for the survival of BRCA2-deficient cells and tumours. Thus, our work demonstrates that β-catenin toxicity in cancer cells with compromised BRCA1/2 function is driven by transcriptional alterations that cause aberrant replication and inflict DNA damage.European Research Council ERC2014 AdG669898 TARLOOPSwiss National Science Foundation 182487European Union 722729, 88604

BRCA

Maintenance of genome integrity requires the functional interplay between Fanconi anemia (FA) and homologous recombination (HR) repair pathways. Endogenous acetaldehyde, a product of cellular metabolism, is a potent source of DNA damage, particularly toxic to cells and mice lacking the FA protein FANCD2. Here, we investigate whether HR-compromised cells are sensitive to acetaldehyde, similarly to FANCD2-deficient cells. We demonstrate that inactivation of HR factors BRCA1, BRCA2, or RAD51 hypersensitizes cells to acetaldehyde treatment, in spite of the FA pathway being functional. Aldehyde dehydrogenases (ALDHs) play key roles in endogenous acetaldehyde detoxification, and their chemical inhibition leads to cellular acetaldehyde accumulation. We find that disulfiram (Antabuse), an ALDH2 inhibitor in widespread clinical use for the treatment of alcoholism, selectively eliminates BRCA1/2-deficient cells. Consistently, Aldh2 gene inactivation suppresses proliferation of HR-deficient mouse embryonic fibroblasts (MEFs) and human fibroblasts. Hypersensitivity of cells lacking BRCA2 to acetaldehyde stems from accumulation of toxic replication-associated DNA damage, leading to checkpoint activation, G2/M arrest, and cell death. Acetaldehyde-arrested replication forks require BRCA2 and FANCD2 for protection against MRE11-dependent degradation. Importantly, acetaldehyde specifically inhibits in vivo the growth of BRCA1/2-deficient tumors and ex vivo in patient-derived tumor xenograft cells (PDTCs), including those that are resistant to poly (ADP-ribose) polymerase (PARP) inhibitors. The work presented here therefore identifies acetaldehyde metabolism as a potential therapeutic target for the selective elimination of BRCA1/2-deficient cells and tumors