Hypoxia-regulated carbonic anhydrase IX (CAIX) protein is an independent prognostic indicator in triple negative breast cancer.

BACKGROUND: The effect of extracellular microenvironment (hypoxia and pH) has been regarded as a key hallmark in cancer progression. The study aims to investigate the effects of carbonic anhydrase IX (CAIX), a key hypoxia-inducible marker, in triple-negative breast cancer (TNBC) in correlation with clinicopathological parameters and predicting survival outcomes. METHODS: A total of 323 TNBC cases diagnosed at the Department of Anatomical Pathology, Singapore General Hospital from 2003 to 2013 were used. Immunohistochemical staining (IHC) was performed using CAIX antibody and digital mRNA quantification was performed using NanoString assays. CAIX membranous expression was correlated with clinicopathological parameters using Chi-squared test or Fisher's exact tests. Disease-free survival (DFS) and overall-survival (OS) were estimated using Kaplan-Meier analysis and compared between groups with the log-rank test. RESULTS: Forty percent of TNBCs were observed to express CAIX protein and demonstrated significant association with larger tumour size (P = 0.002), higher histological grade (P < 0.001), and significantly worse disease-free survival (DFS) and overall survival (OS) (after adjustment: HR = 2.99, 95% CI = 1.78-5.02, P < 0.001 and HR = 2.56, 95% CI = 1.41-4.65, P = 0.002, respectively). Gene ontology enrichment analysis revealed six significantly enriched cellular functions (secretion, cellular component disassembly, regulation of protein complex assembly, glycolytic process, cellular macromolecular complex assembly, positive regulation of cellular component biogenesis) associated with genes differentially expressed (CAIX, SETX, WAS, HK2, DDIT4, TUBA4α, ARL1). Three genes (WAS, SETX and DDIT4) were related to DNA repair, indicating that DNA stability may be influenced by hypoxia in TNBC. CONCLUSIONS: Our results demonstrate that CAIX appears to be a significant hypoxia-inducible molecular marker and increased CAIX protein levels are independently associated with poor survival in TNBC. Identification of CAIX-linked seven gene-signature and its relationship with enriched cellular functions further support the implication and influence of hypoxia-mediated CAIX expression in TNBC tumour microenvironment.