Abstract A114: Preclinical evaluation of the PARP inhibitor olaparib in homologous recombination deficient (HRD) MRE11 mutant microsatellite instable (MSI) colorectal cancer
Authors: Knights Charlotte , Chresta Christine , Riches Lucy , Mangena Ramu , Avis Tim , O'Shaughnessy Aisling , Unwin Louise , Dunham Victoria , Jones Louise , Cranston Aaron , Carmichael Jim , Martin Niall , O'Connor Mark , Lau Alan 
Published: June 2012

Abstract
Background: The oral PARP inhibitor olaparib (AZD2281, KU-0059436) has shown anti-tumor activity in patients with hereditary BRCA mutations and provides positive evidence for synthetic lethal approaches for the treatment of cancers with DNA repair defects. The BRCA genes are regulators of homologous recombination (HR) DNA repair pathway but deficiency in other mediators of this pathway, such as the MRE11-NBS1-Rad50 (MRN) complex have also been demonstrated to be synthetically lethal with PARP inhibition in pre-clinical models. Cells deficient in HR DNA repair or signalling pathways are potentially sensitive to PARP inhibition and have been termed homologous recombination deficient (HRD). Examples of HRD include inactivating MRE11 mutations which have been linked to mismatch repair (MMR) deficient, microsatellite instable (MSI) colorectal and endometrial cancers. Previous studies have indicated a high frequency (up to 87% of MSI colorectal cancers) of −1/−2 frameshifts within the poly(T)11 microsatellite repeat sequence and a corresponding reduction in expression. We have therefore undertaken a pre-clinical study to determine whether MRE11-deficient colorectal cancer (CRC) respond to olaparib treatment.
Methods: CRC cell line panels with known MSI status as well as isogenic matched cell lines models of MRE11 and MMR-deficiency were assessed for their response to olaparib by both MTS viability and clonogenic survival assays. Quantitative RT-PCR mRNA and protein expression analysis of MRE11 and DNA damage response genes were obtained for each cell line and HRD status determined. Expression of Pgp (ABCB1) multi-drug transporter, a pre-clinical resistance factor for olaparib and whose over-expression is associated with CRC, was also determined.
Results: MTS viability assays showed a limited range of sensitivities between CRC cell lines and only modest correlation with molecular profiles. Analysis using sensitive long term clonogenic survival assays showed a wider range of sensitivities and revealed that HRD (MRE11 mutant) and MSI CRC cells were sensitive to olaparib treatment while microsatellite stable (MSS) and non-HRD (MRE11 wild-type) cells remained insensitive. Using isogenic cell line models we confirmed that loss of MRE11 expression can confer sensitivity to olaparib whereas MMR-deficiency did not. CRC cells which were HRD but overexpressed Pgp were more resistant to olaparib but could be re-sensitized to treatment by co-administration of the Pgp inhibitors or through the use of alternative non-Pgp substrate PARP inhibitors.
Conclusions: Here we demonstrate that CRC cell lines with HRD due to MRE11 mutation and loss of protein expression are highly sensitive to the PARP inhibitor olaparib. These data provide evidence that patients with MSI and MRE11-deficient colorectal tumors could be suitable for treatment with olaparib.
Citation Information: Mol Cancer Ther 2009;8(12 Suppl):A114.