Persistent double strand break accumulation does not precede cell death in an Olaparib-sensitive BRCA-deficient colorectal cancer cell model

Abstract

The poly (adenosine diphosphate (ADP)-ribosyl) polymerase inhibitors (PARPi) selectively kill cancer cells with BRCA1 or BRCA2 (BRCA)-mutations. It has been proposed that cell death induction after PARPi depends on unrepaired double strand breaks (DSBs) that accumulate due to the homologous recombination deficiency of BRCA-mutated cells. Such accumulation of DSBs is inferred mainly from the high levels of DNA damage markers like phosphorylated histone H2AX. Herein, we developed a model of isogenic cell lines to show that depletion of BRCA causes PARPi-triggered cell death, replication stress (phosphorylated-H2AX and 53BP1 foci), and genomic instability. However, persistent DSBs accumulation was not detected under the same experimental conditions. Hence, at least in this cellular model, the trigger for cell death in PARPi-treated BRCA-depleted samples is not the accumulation of unrepaired DSBs. Instead, cell death better correlates with a rapid and aberrant resolution of DSBs by error-prone pathways that leads to severe chromosomic aberrations. Therefore, our results suggest that in PARPi-treated BRCA-deficient cells, chromosome aberrations may dually trigger both genomic instability and cell death.Despite their current success in the clinical setting, the mechanism of action of PARPi has only been partially elucidated. While it is accepted that double-strand breaks

BRCA mutations and gastrointestinal cancers: When to expect the unexpected?

“…It has already been explained how, in cells that harbour HR system mutations leading to defective DNA double-strand break repair, as in BRCAm patients, genome stability is maintained through the activity of BER and PARP enzymes. However, Paviolo et al [ 39 ] demonstrated that in PARPi-treated BRCA-defective CRC samples, even though DSBs are generated, they are not directly responsible for cell death. Indeed, more than DSBs, it was proven that rapid and anomalous repair of these defects, through means of DNA repair mechanisms different from HRD, leads to severe chromosomic aberrations which are the cause of cell death.…”

Fanconi Anemia Pathway in Colorectal Cancer: A Novel Opportunity for Diagnosis, Prognosis and Therapy

“…Several studies propose that PARPIs prompt cell death by accumulating unrepaired DSBs due to the HR deficiency in BRCA -mutated cells. Even though BRCA mutations are infrequent in CRC, the PARPIs are undergoing several clinical trial investigations as single agents or combined with radiation or chemotherapeutic agents [ 138 , 139 , 140 , 141 , 142 , 143 , 144 ].…”

“…In this regard, Paviolo et al [ 140 ] have shown that the treatment of BRCA-deficient CRC cell lines with olaparib may lead to genomic instability and cell death [ 140 ]. Moreover, Reisländer et al [ 144 ] have revealed that the treatment of BRCA1- or BRCA2-deficient CRC cell lines with olaparib boosted the up-regulation of innate immune response genes upon intrinsically high levels of DNA damage [ 144 ].…”

Therapeutic opportunities for PLK1 inhibitors: Spotlight on BRCA1-deficiency and triple negative breast cancers

“…The emergence of PLK1 inhibition as a strategy to treat TNBC and BRCA1-deificient cancers opens the discussion of whether such strategy can be complementary and/or alternative to PARPi. The available evidence indicates that PARPi and PLK1i work through different molecular mechanisms [45,[110][111][112].For instance, PLK1 inhibition shows only mild synthetic lethal activity with BRCA2-deficient cells [45], while PARP inhibition induces synthetic lethality both with BRCA1 and BRCA2, as well as with other related proteins that trigger homology-directed repair deficiencies [113]. In addition, while some TNBCs display sensitivity to PARP inhibition [114], this response does not seems to be selective for all types of TNBCs, in contrast to the more general link found between this cancer subtype and PLK1i sensitivity [45,[84][85][86].…”