Osteosarcoma cells with genetic signatures of BRCAness are susceptible to the PARP inhibitor talazoparib alone or in combination with chemotherapeutics

We recently discovered mutation signatures reminiscent of BRCA deficiency in the vast majority of a set of primary osteosarcomas (OS). In the current study, we therefore investigated the sensitivity of a panel of OS cell lines to the poly(ADP)-ribose polymerase (PARP) inhibitor talazoparib alone and in combination with several chemotherapeutic drugs (i.e. temozolomide (TMZ), SN-38, doxorubicin, cisplatin, methotrexate (MTX), etoposide/carboplatin). Here, we identified an association between homologous recombination (HR) repair deficiency and the response of OS cell lines to talazoparib. All OS cell lines with molecular features characteristic of BRCA1/2 mutant tumors (so-called “BRCAness”), such as disruptive gains in PTEN or FANCD2 and/or losses of ATM, BAP1, BARD1 or CHEK2, were susceptible to talazoparib-induced reduction of cell viability (i.e. MG63, ZK-58,, SaOS-2 and MNNG-HOS). Consistent with their high sensitivity to talazoparib, MG63 and ZK-58 cells scored positive in a DNA-based measure of genomic instability (i.e. homologous recombination deficiency (HRD)-loss of heterozygosity (LOH) score). In contrast, U2OS cells that carry a heterozygous BRCA2 mutation and therefore most likely have one intact BRCA2 allele left proved to be resistant to talazoparib. Furthermore, we identified TMZ as the most potent chemotherapeutic drug together with talazoparib to synergistically reduce cell viability, as confirmed by calculation of combination index (CI) values, and to suppress long-term clonogenic survival. Mechanistically, talazoparib and TMZ cooperated to induce apoptotic cell death, as demonstrated by activation of BAX and BAK, loss of mitochondrial membrane potential (MMP), caspase activation, DNA fragmentation and caspase-dependent cell death. Genetic silencing of BAX and BAK or pharmacological inhibition of caspases by zVAD.fmk significantly rescued OS cells from talazoparib/TMZ-induced apoptosis. These findings have important implications for the development of novel treatment strategies using PARP inhibitors alone or together with chemotherapy in a subset of OS with features of BRCAness.