40. Cytoplasmic localisation of β catenin is a marker of poor outcome in breast cancer patients

Abstract

β catenin is involved in cell adhesion via catenin-cadherin complexes and as a transcriptional regulator in the Wnt signaling pathway. Its deregulation is important in the genesis of a number of human malignancies, particularly colorectal cancer. A range of studies have been undertaken in breast cancer but there is a lack of consensus on associations reported between β catenin expression, clinicopathological parameters and disease outcome. We undertook an immunohistochemical study measuring the levels and subcellular localisation of β catenin in 292 invasive ductal breast cancers with known treatment and outcome. Forty percent of tumours were >20mm, 45% were grade >2, 43% were lymph node positive, 68% were ER positive, 57% were PR positive and 18% were HER2 FISH positive. The median age was 54 years and patients were treated with endocrine therapy (49%), chemotherapy (38%) or both (24%). Cases were prospectively followed-up for a median of 64 months and outcome events measured were: recurrence (local or distant) (25%), metastasis (23%) and all deaths were recorded but only breast cancer-related deaths (18%) were considered for survival analyses. β catenin expression was found predominantly in the membrane and cytoplasm of cells, with only 3 cases showing detectable nuclear expression, hence we described a histoscore value for the cytoplasmic and the membrane expression independently, and then derived a third histoscore which represents both categories of expression in one continuous variable (membrane minus cytoplasmic histoscore) as a measure of subcellular localisation, the ‘membrane to cytoplasmic score' (MTC). When this continuous value is positive the tissue shows predominantly membranous expression and when the value is negative the tissue shows mainly cytoplasmic expression. No association with outcome was observed for cytoplasmic or membrane expression alone, however a shift from membrane to cytoplasm expression calculated using the MTC score was associated with worse outcome in univariate analysis (<i>p</i>=0.004), and approached significance in a multivariate analysis model that included lymph node, PR and HER2 status (<i>p</i>=0.054). The MTC score was then used for further statistical analyses due to the importance of both subcellular location and levels of expression of β catenin. An association was identified between a shift to high cytoplasmic expression (low MTC score) and high tumour grade (<i>p</i>=0.004), positive Ki67(<i>p</i>=0.0001), negative ER (<i>p</i>=0.005), positive HER2(<i>p</i>=0.01) status and an active PI3K pathway (<i>p</i>=0.005), measured as PIK3CA mutations (<i>p</i>=0.05) or PTEN loss (<i>p</i>=0.05). A shift to low cytoplasmic expression (high MTC score) was associated with the Luminal A subtype (<i>p</i>=0.004). A shift in subcellular localisation of β catenin is associated with alterations in cell cycle regulatory proteins and signalling transduction pathways, in clinical breast cancer samples. Furthermore, cytoplasmic accumulation of β catenin is associated with a poor prognosis in invasive ductal carcinoma. It is possible that in a larger cohort, independence may be established and further efforts are needed to validate these findings in other breast cancer cohorts.