Retinoic acid-induced protein 3: identification and characterisation of a novel prognostic colon cancer biomarker.

Aim Of The Study: Validated molecular biomarkers are urgently required in colon cancer (CC) to accurately define prognosis and, ideally, to predict response to therapeutic modalities such as adjuvant chemotherapy. We aimed to identify and characterise a novel membrane-associated protein in CC tissues which may offer diagnostic and, potentially, therapeutic targeting opportunities.

Methods: Label-free mass spectrometric (MS) quantitation was employed to profile matched colon tissues for malignancy-associated proteins. The putative diagnostic utility of a chosen marker was evaluated using immunohistochemistry (IHC) on 367 CC tissue samples contained within the NCI Progression Colon Cancer tissue microarray (TMA) set.

Results: Retinoic acid-induced protein 3 (RAI3) was initially identified as a plasma membrane protein overexpressed in CC. Cancer-associated RAI3 over-expression was confirmed by RAI3 IHC. Although RAI3 IHC expression patterns were variable within neoplastic epithelium, 76% (n=236) of interpretable CC cases (n=312) displayed diffuse cytoplasmic expression. Of note, a sub-set of CC tissues (n=23, 7.4%) displayed very strong cytoplasmic expression, a feature significantly associated with disease recurrence in Dukes' A-C (stage I-III) patients (hazard ratio (HR)=3.076, [95%confidence interval (CI)=1.738-5.445]; p<0.001) when compared to low or negative expression of RAI3. This association retained univariate significance in Dukes' B/stage II patients only (HR=3.494, [95%CI=1.197-10.20]; p<0.022). Significantly, the prognostic capacity of RAI3 was maintained in the stage I-III cohort following multivariate modelling (HR=2.11, [95%CI 1.109-4.017], p=0.023).

Conclusion: RAI3 is a putative prognostic marker that identifies a small subset of CC patients with high recurrence risk. This study demonstrates the potential value of modern proteomic technology in clinically relevant applications.