Abstract PR-10: PARP inhibition induces genomic instability in normal human cells.
Authors: Ito Shuhei 
Published: June 2012

Recent research has focused on poly(ADP-ribose) polymerases (PARPs) as potential chemotherapeutic targets. Tumors which harbor defects in DNA double-strand repair by homologous recombination (HR), including those with mutant BRCA1 and BRCA2, are exquisitely sensitive to PARP inhibitors, agents that block single-strand break repair. This approach to cancer therapy is known as synthetic lethality, given that PARP inhibitors are not toxic to normal cells at doses that kill HR-deficient cells. While initial trials with PARP inhibitors have demonstrated their promise, many questions remain regarding their clinical use. While PARP inhibition in HR-proficient cells is not cytotoxic, little data exists with regard to genotoxicity.

To assess genotoxic effects resulting from PARP inhibition, we quantified frequencies of sister chromatid exchange (SCE), a type of HR with the potential to cause copy number variation at repeats, and chromosomal aberrations. Human non-tumorigenic cell lines (MCF-10A, HMEC-hTERT, EBV-transformed B cells), human mammary tumor cell lines (MCF-7, MDA-MB-468) and primary human T cells obtained from normal donors were tested. Short duration, non-cytotoxic dosing of three PARP inhibitors (olaparib, ABT-888 and BSI-201) currently in clinical trials was used. Olaparib was extensively studied and demonstrated profound induction of SCEs (4.4–9.6 fold) and chromatid-type aberrations (1.7–5.5 fold) in primary human T cells and non-tumorigenic and tumorigenic cell lines. Both olaparib and ABT-888 induced a dose-dependent increase in the frequency of SCEs at non-cytotoxic concentrations (typically greater than 90% cell viability). By contrast, even at cytotoxic doses, SCE frequencies were only slightly higher than the baseline with BSI-201. These results indicate that the SCE frequencies induced by PARP inhibitors are agent- and dose-dependent. A direct correlation between SCE induction and PAR activity inhibition was observed with the tested PARP inhibitors: whereas olaparib and ABT-888 markedly inhibited PARylation (97%), BSI-201 decreased PAR levels more modestly (60%). To assess these differences between agents in a cellular context, BRCA1 and BRCA2 mutant mouse embryonic stem (mES) cells along with wild-type mES cells were treated with olaparib and BSI-201. As expected, olaparib resulted in marked hypersensitivity in these HR-deficient cells as compared to wild type, with greater than a 200-fold increased sensitivity for Brca1-deficient cells. By contrast, BSI-201 resulted in minimally (1.5-fold) increased sensitivity.

In conclusion, we have demonstrated genomic instability arising from short-term, low dose PARP inhibitors exposure of normal human cells. The genotoxic risk from clinical use of PARP inhibitors should be considered, especially for patients with early stage cancers and in prevention or non-oncologic indications.

This work was supported by The Hecksher Foundation for Children (M.E.M.), National Institutes for Health grants P01CA94060 (M.E.M. and M.J.) and the Uehara Memorial Foundation and Sumitomo Life Social Welfare Services Foundation (S.I.).

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr PR-10.