Crypt-restricted loss and decreased protein expression of cytochrome C oxidase subunit I as potential hypothesis-driven biomarkers of colon cancer risk

Abstract

There is an increasing demand for the development of intermediate biomarkers to assess colon cancer risk. We previously determined that a live cell bioassay, which assesses apoptosis resistance in the nonneoplastic colonic mucosa, detects approximately 50% of patients with colon cancer. A hypothesis-driven biomarker that reflects apoptosis resistance in routine formalin-fixed, paraffin-embedded tissue would be easier to use. Cytochrome c oxidase is a critical enzyme that controls mitochondrial respiration and is central to apoptosis. We did an immunohistochemical study of cytochrome c oxidase subunit I expression in 46 colonic mucosal samples from 16 patients who had undergone a colonic resection. These included five patients without evidence of colonic neoplasia (three normal and two diverticulitis), three patients with tubulovillous adenomas, and eight patients with colonic adenocarcinomas. Analysis of aberrancies in expression of cytochrome c oxidase subunit I showed that, compared with nonneoplasia, the patients with neoplasia had a higher mean incidence of crypts having decreased expression (1.7 versus 22.8, P = 0.03) and a higher mean incidence having crypt-restricted loss (0.6 versus 3.2, P = 0.06). The percentage with segmented loss was low and was similar in the two groups. Combining these results, the mean % normal (i.e., with none of the three types of abnormality) was 96.7 in nonneoplasia versus only 73.2 in patients with neoplasia (P = 0.02). It should be noted that a defect in cytochrome c oxidase subunit I immunostaining was not detected in all biopsy samples from each patient for whom some abnormality was found, indicating a "patchiness" in the cytochrome c oxidase subunit I field defect. As a result of this "patchiness," the increased variability in the incidence of crypt-restricted loss of cytochrome c oxidase subunit I expression was a statistically significant feature of the neoplasia group. Crypt-restricted loss of cytochrome c oxidase subunit I has not been previously reported in colonic mucosa and is presumably the result of a crypt-restricted stem cell mutation. Decreased cytochrome c oxidase subunit I expression also significantly correlated with apoptosis resistance, a factor known to contribute to carcinogenesis. The results suggest, however, that aberrant cytochrome c oxidase subunit I expression may be a better biomarker than loss of apoptosis competence for increased colon cancer risk.