Abstract
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.
Highlights
OS is the most common primary bone cancer in children and young adults [1]
To investigate whether OS cells are susceptible to PARP1 inhibition, we tested the sensitivity of a panel of OS cell lines (i.e. MG63, ZK-58, SaOS-2, MNNG-HOS and U2OS) towards treatment with the poly(ADP)-ribose polymerase (PARP) inhibitor talazoparib
We recently identified mutation signatures characteristic of BRCA deficiency in OS [3], indicating that OS might be susceptible to PARP inhibitors
Summary
OS is the most common primary bone cancer in children and young adults [1]. There is a high demand for more effective treatment options for OS patients, as the outcome of patients with refractory and/or metastatic disease remains poor with five-year survival rates below 30% [2]. Loss-of-function mutations in BRCA1 or BRCA2 as well as BRCAness have previously been reported to confer sensitivity towards PARP inhibitors, since ineffective HR repair induces a dependency on base excision repair to cope with DNA damage [4]. Blockage of base excision repair using PARP inhibitors causes synthetic lethality in cancers with defects in HR repair, as cells are no longer capable to repair the cumulating DNA damage and undergo cell death [5, 6]. All PARP inhibitors inhibit the catalytic function of PARP1, required for PARylation of damaged DNA, while they differ in their ability to trap PARP1 to DNA, leading to replication fork stalling and cell death [10]. Talazoparib has been reported to harbor the highest PARP1 trapping activity, while its ability to inhibit catalytic PARP1 function is comparable to other inhibitors [10, 11]. We focused our study on the PARP inhibitor talazoparib
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