Abstract
Poly(ADP-ribose) polymerases (PARPs) are the first proteins involved in cellular DNA repair pathways to be targeted by specific inhibitors for clinical benefit. Tumors harboring genetic defects in homologous recombination (HR), a DNA double-strand break (DSB) repair pathway, are hypersensitive to PARP inhibitors (PARPi). Early phase clinical trials with PARPi have been promising in patients with advanced BRCA1 or BRCA2-associated breast, ovary and prostate cancer and have led to limited approval for treatment of BRCA-deficient ovary cancer. Unlike HR-defective cells, HR-proficient cells manifest very low cytotoxicity when exposed to PARPi, although they mount a DNA damage response. However, the genotoxic effects on normal human cells when agents including PARPi disturb proficient cellular repair processes have not been substantially investigated. We quantified cytogenetic alterations of human cells, including primary lymphoid cells and non-tumorigenic and tumorigenic epithelial cell lines, exposed to PARPi at clinically relevant doses by both sister chromatid exchange (SCE) assays and chromosome spreading. As expected, both olaparib and veliparib effectively inhibited poly-ADP-ribosylation (PAR), and caused marked hypersensitivity in HR-deficient cells. Significant dose-dependent increases in SCEs were observed in normal and non-tumorigenic cells with minimal residual PAR activity. Clinically relevant doses of the FDA-approved olaparib led to a marked increase of SCEs (5-10-fold) and chromatid aberrations (2-6-fold). Furthermore, olaparib potentiated SCE induction by cisplatin in normal human cells. Our data have important implications for therapies with regard to sustained genotoxicity to normal cells. Genomic instability arising from PARPi warrants consideration, especially if these agents will be used in people with early stage cancers, in prevention strategies or for non-oncologic indications.
Highlights
Poly(ADP-ribose) polymerase 1 (PARP1) and PARP2 are rapidly activated by DNA strand breaks where through target-protein ribosylation they promote the repair of DNA singlestrand breaks (SSBs) and coordinate cellular responses to stress [1,2]
To ascertain whether genotoxicity is incurred in repair proficient human cells with inhibition of Poly(ADP-ribose) polymerases (PARPs) activity, we used a variety of cell types: breast epithelial cell lines, both non-transformed (MCF-10A, HMEC-hTERT) and tumorigenic (MDA-MB-468, MCF-7), an EBVtransformed B lymphocyte cell line (EBV-BL), and primary T cells from two individuals
Multiple sister chromatid exchange (SCE)/chromosome were frequently observed with PARP inhibition, such that chromosomes without an SCE were in a minority (Fig 1C)
Summary
Poly(ADP-ribose) polymerase 1 (PARP1) and PARP2 are rapidly activated by DNA strand breaks where through target-protein ribosylation they promote the repair of DNA singlestrand breaks (SSBs) and coordinate cellular responses to stress [1,2]. PolyADP-ribosylation (PAR) stimulated by DNA strand breaks is primarily mediated by PARP1, while PARP2 is responsible for 10–15% of the total activity [3]. Less potent PARP inhibitors (PARPi), which did not demonstrate synthetic lethality in BRCA-deficient tumor cells, had been shown to increase SCEs 2-fold in normal human cells at high doses [10,11,12]
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