The proteins FABP7 and OATP2 are associated with the basal phenotype and patient outcome in human breast cancer

The basal-like or basal phenotype (BP) class of breast cancers have recently attracted attention as a poor prognostic form of breast cancer. However, BP appears to encompass biologically and clinically heterogeneous tumours, resulting in a lack of consensus definition of BP. We analysed 48,000 gene transcripts in 132 invasive breast carcinomas to identify two novel genes (OATP2 and FABP7) significantly associated with BP [defined by cytokeratin (CK)5/6 and/or CK14 positivity]. Using a series of invasive breast carcinoma cases (n = 899), prepared as tissue microarrays, we assessed OATP2 and FABP7 protein expression using immunohistochemistry to investigate associations with clinicopathological variables, patients' outcome and ability to refine BP classification. A total of 7.9 and 15.6% cases were OATP2 and FABP7 positive, respectively. OATP2 was associated with tumours of high histological grade (p < 0.01), ER and PgR negativity (p < 0.01) and shorter breast cancer-specific survival (p = 0.04). FABP7 expression was associated with lower lymph node stage (p < 0.01), ER and PgR negativity (p < 0.01). BP tumours which were FABP7 positive had a significantly longer BCSS (p = 0.05) and disease-free survival (p = 0.01) compared with FABP7 negative basal tumours (p < 0.01). OATP2 positive tumours were associated with adverse survival and increased risk of early recurrence. This study confirms the biological and clinical heterogeneity of the BP in breast cancer. We have identified a novel subgroup of basal tumours showing FABP7 expression that have significantly better clinical outcome. Further studies analysing the role of FABP7 are therefore warranted.