BRCA deficient cells can be killed without triggering DNA replication stress

Salta

Congreso; LV Reunión Anual de Sociedad Argentina de Investigación Bioquímica y Biología Molecular; 2019

Sociedad Argentina de Investigación Bioquímica y Biología Molecular

The identification of compounds with selective tumor cytotoxicity can improve anticancer therapies. A promising strategy which takes advantage of the frequent impairment of DNA repair pathways in tumor cells is synthetic lethality (SL). For example, loss of the Homologous Recombination Repair (HRR) capacity is frequent in breast and ovarian cancer, and the accumulation of HRR substrates should selectively kill such cancer cells. Such hypothesis has been validated in BRCA1 and BRCA2-deficient cell lines treated with different Poly (ADP-ribose) polymerase inhibitors (PARP-i). Several pharmaceutical companies evaluated PARP-i in more than 400 clinical trials, and to date the FDA has approved 4 PARP-i. However, evidence of acquired resistance to PARPi has prompted the search of other synthetic lethal interactions in HRR-deficient cells. To this end, a cell-based phenotypic screening of several thousands of compounds was carried out using a high-throughput method recently described by us (Carbajosa S et al, Clinical Cancer Research, 2019). The compounds were from different sources, including a library kinase inhibitors (PKIS2), natural products and extracts from plant species from Argentina and a library of 13K natural products a natural products-like compounds from GSK. After the screening, 4 hits were validated in different BRCA-deficient cell lines. The two first hits, Polo like kinase-PLK1 (hit 1) and Rho-associated protein kinases ROCK1/2 (hit 2), were validated using commercially available inhibitors for these two kinases. The other two hits were identified from the other compound sources and will be referred as hit 3 and hit 4. Intriguingly, the mechanisms of cell killing associated with the PARPi, differs mainly from the mechanisms of cell killing triggered by the 4 hits identified in our screening. PARPi-induced cell death in BRCA1 and BRCA2 deficient cells is accompanied by replication stress (revealed by replication stress markers such as γH2AX and 53BP1 nuclear foci) and acute chromosome instability (CIN) revealed by miconuclei accumulation and chromosomic aberrations analysis. In contrast, cell death triggered by the hits 1-4 was not associated with acute replication stress or CIN. Another difference was observed when evaluating the genetic backgrounds affected by the hits. In agreement with the literature, PARPi induce SL in both BRCA1 and BRCA deficient cells. In contrast, hits 1-4 were, in most cases, not effective in both BRCA-deficient backgrounds, but showed SL either on BRCA1 or BRCA2-deficient cells. We conclude that novel type of synthetic lethal interactions may be achieved without increasing CIN, which in-vivo might prevent the acquisition of resistance mechanisms. On the other hand, our data also surprisingly reveal that many novel SL interactions may not broadly apply to all type of HR deficiencies.