Down-regulation of MTUS1 in human colon tumors.

Loss of proliferative control and failure to undergo cellular differentiation are key events during carcinogenesis. We recently identified a new potential tumor suppressor gene named MTUS1 (mitochondrial tumor suppressor 1), down-regulated in undifferentiated tumor cell lines, inhibiting tumor cell proliferation after recombinant over-expression. The aim of this study was to investigate whether MTUS1 is also down-regulated in human tumor tissues, and whether reduced expression of MTUS1 enhances cellular proliferation. Expression of MTUS1 in human colon cancer tissues was compared with corresponding normal colon tissues using Western blot analysis and RT-PCR. Investigation of the DNA sequence and methylation pattern was performed using bisulfite reaction and DNA sequencing. Promotor activity was measured by promoter assays. Silencing of MTUS1 was carried out by siRNA transfection. Proliferation was measured by cell count. MTUS1 expression is significantly down-regulated in colon cancer tissues, compared to the corresponding normal tissues, on protein and mRNA level. No mutations of MTUS1 were detected in the coding sequence or the predicted promoter region in cancer tissues. No difference of CpG methylation, but an altered CpNpG methylation was found in the predicted promoter region. Functional significance of the predicted promoter region was demonstrated by promoter assays. Down-regulation of the MTUS1 expression by siRNA transfection significantly increased cellular proliferation. This study demonstrates a significant down-regulation of the MTUS1 expression in human colon cancer tissues. Since reduced expression of MTUS1 results in increased cellular proliferation, these data suggest that MTUS1 could be involved in the loss of proliferative control in human colon cancer.