Targeting ataxia telangiectasia-mutated and Rad3-related kinase (ATR) in PTEN-deficient breast cancers for personalized cancer therapy

Background: Phosphatase and tensin homolog (PTEN) is a multifunctional gene which acts as a tumour suppressor gene and is involved in DND damage response (DDR) mechanisms. PTEN has been found to be mutated in different types of human cancers including breast cancer. Ataxia-telangiectasia mutated (ATM) and RAD3- related (ATR) are involved in DDR and they have roles in cell cycle regulation and apoptosis. Previously ATM inhibition caused synthetic lethality in prostate and colorectal cancer cells with PTEN deficiency. In this study we hypothesize that PTEN plays key roles in breast carcinogenesis and that inhibition of ATR in the context of PTEN deficiency can provide a novel therapeutic approach through a synthetic lethality mechanism. Methods: In this study a large, well-characterised and molecularly annotated series of breast cancer (n=1954) was utilized to evaluate the clinicopathological and biological role of PTEN protein expression assessed using immunohistochemistry (IHC) and tissue microarrays technology. Several breast cancer cell lines (n=4) representing various molecular classes and PTEN status were studied in vitro using functional assays. Cellular consequences of ATR inhibitor (VE-821) treatment were investigated in a panel of PTEN-proficient including MCF7 and MDA-MB-231, and PTEN-deficient including BT-549 and MDA-MB-468 breast cancer cell lines. DNA repair expression profiling, MTS cell-proliferation assay, FACS for cell cycle, γH2AX and FITC-annexin V flow cytometry analysis were performed on PTEN-deficient and PTEN-proficient cells to study the functional consequences of PTEN deficiency on breast cancer cells. Results: PTEN was expressed in the nucleus and cytoplasm of malignant cells. The negative nuclear expression was detected in 62.6%, whilst negative/low cytoplasmic expression was found in 40.2% cases of breast cancer. The negative nuclear PTEN IHC expression was associated with features of aggressive behavior including higher grade, nuclear pleomorphism, higher mitotic index, larger tumour size, oestrogen receptor (ER) negativity, high risk Nottingham prognostic index (NPI≥3.4) and shorter breast cancer specific survival (BCSS) (pvalue <0.05). Interestingly, in tumours with low nuclear PTEN, high ATR and/or high pChk1 (pCHK1Ser345) expression was also linked to poor BCSS (P-values