Reduced/Absent PTEN Expression Is More Common in HER2 Non-Amplified DCIS.

BACKGROUND: Activation of the phosphatidylinositol 3-kinase (PI3K)-AKT pathway is a common event in breast cancer and appears to correlate with specific biologic subsets of disease. Laboratory models demonstrate that PI3K activation is necessary for several aspects of transformation including accelerated proliferation and invasiveness. We hypothesized that this pathway may play a role in the transition from carcinoma in situ to invasive cancer. The aim of this study was to assess whether well characterized modes of PI3K activation (PI3K mutation, PTEN loss) are present in DCIS.METHODS: 97 cases of pure high grade DCIS were identified from the MSKCC breast service database (1999-2003). Archival formalin-fixed paraffin embedded (FFPE) blocks were retrieved and appropriate sections obtained for DNA extraction and immunohistochemistry (IHC). Manual microdissection was performed to isolate pure DCIS lesions for multiplex array (Sequenom®) genotyping for PIK3CA mutations. Following antibody optimization, IHC staining for ER, PR, HER2, PTEN and pAKT were performed on freshly cut sections. HER2 amplification was confirmed by FISH. Stained sections were scored by two reviewers. ER and PR positivity was defined as any positive nuclear staining. PTEN and pAKT expression were scored relative to staining in adjacent normal ductal epithelium. Fisher9s exact test was used for all comparisons.RESULTS: Of 97 cases, 47 (48%) cases were ER positive and 56 (58%) cases were HER2 amplified. Reduced PTEN expression was found in 27/97 (28%) cases and complete loss of PTEN in 8/97 (8%) cases. Overexpression of pAKT was seen in 10/97 (10%) cases. PIK3CA hotspot mutations (H1047R and E545A) were identified in 2/20 (10%) cases studied. Reduced or absent PTEN expression was more common in HER2 non-amplified DCIS as compared to HER2 amplified DCIS (22/41 (54%) vs 13/56 (23%), p=0.003). Conversely, pAKT overexpression was more common in HER2 amplified DCIS, 12% vs 7%, although this did not reach statistical significance.Exploratory analysis using IHC as a surrogate for molecular subtype demonstrated that 34 (35%) of high grade DCIS cases were luminal A, 13 (14%) cases were luminal B, 43 (44%) cases were HER2 subtype, and 7 (7%) were basal subtype. Reduced or absent PTEN expression was seen with similar frequency in HER2 negative subtypes, Luminal A (ER+HER2-) 53% and basal subtype (ER-PR-HER2-) 56%. Among HER2 amplified subtypes, overexpression of pAKT was more common in luminal B than in HER2+ cases (23% vs 10%), although this comparison is limited by small numbers. Overexpression of pAKT was not seen in the basal subtype. The PIK3CA mutations (n=2) identified were both in HER2 subtype (ER- HER2+) cases with normal PTEN expression.CONCLUSIONS: The PI3K pathway is activated in high grade DCIS and the mechanism of activation appears to correlate with molecular subtype. Reduced or complete loss of PTEN expression is more common in HER2 non-amplified high grade DCIS and occurs in both ER positive and ER negative cases. Loss of PTEN expression did not correlate with overexpression of pAKT. Preliminary data suggests that PIK3CA mutations are also an early event in tumorigenesis. Further analyses to delineate the role of this pathway in breast cancer tumorigenesis are ongoing. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 6148.