PPM1D Overexpression and Gene Amplification in Breast Cancers: A QRT-PCR and Chromogenic In Situ Hybridisation Study
MB Lambros, R Natrajan, FC Geyer, MA Lopez-Garcia, K Savage, M Lacroix-Triki, JS Reis-Filho. ICR, London, United Kingdom; Institut Claudius Regaud, Toulouse, France
Background: PPM1D maps to 17q23.2 and encodes a serine threonine phosphatase, which plays a major role in the regulation of the p53 pathway. Our group has recently demonstrated that PPM1D expression and phosphatase activity are required for the survival of cancer cells harbouring 17q23.2 amplification, suggesting that PPM1D is one of the drivers of this amplicon. In this study we investigated if PPM1D is overexpressed when amplified in breast cancers and the correlations between PPM1D overexpression and amplification with clinicopathological features and survival of breast cancer patients.
Design: A cohort of 245 patients with invasive breast cancer treated with therapeutic surgery followed by adjuvant anthracycline-based chemotherapy was retrieved from the files of the authors' institutions. mRNA was extracted from representative sections of tumours containing >50% of tumour cells. TaqMan quantitative real-time PCR using primers for PPM1D and for two house keeping genes was performed. PPM1D overexpression was defined as the top quartile of expression levels. Chromogenic in situ hybridisation with in house generated probes for PPM1D was performed with observers blinded to PPM1D expression levels and clinicopathological features. Amplification was defined as >5 signals per nucleus or large gene clusters in >50% of cancer cells.
Results: PPM1D overexpression and amplification were found in 25% and 6% of cancers, respectively. A strong direct association between PPM1D mRNA expression levels and PPM1D amplification was found (Mann-Whitney U test, p<0.00001). All cases harbouring PPM1D amplification displayed PPM1D overexpression. PPM1D overexpression was associated with oestrogen receptor expression and topoisomerase IIa expression, and inversely correlated with expression of EGFR, and cytokeratins 5/6 and 17. PPM1D amplification was significantly associated with HER2 overexpression, and HER2, TOP2A and CCND1 amplification. No association between PPM1D overexpression and gene amplification with survival was observed.
Conclusions: PPM1D is consistently overexpressed when amplified, however PPM1D overexpression is more pervasive than gene amplification. PPM1D overexpression and amplification are associated with tumours displaying luminal or HER2 phenotypes. Co-amplification of PPM1D and HER2/ TOP2A and CCND1 are not random events and may suggest the presence of a 'firestorm' genetic profile.
Monday, March 22, 2010 1:00 PM
Poster Session II # 61, Monday Afternoon