Tumor suppressor Hypermethylated in Cancer 1 represses expression of cell cycle regulator E2F7 in human primary cells
Hypermethylated in Cancer 1 (HIC1) is an established tumor suppressor, which is frequently inactivated in various cancers. In colorectal carcinoma (CRC), silencing of HIC1 has been recognized as one of the important events in malignant tumor progression. Strikingly, CRC patients with high HIC1 expression have a worse prognosis than patients with relatively low HIC1 mRNA levels. To analyze the function of HIC1, we performed expression profiling of human primary fibroblasts after downregulation of HIC1 by RNA interference. We show that HIC1 deficiency triggers a p53-dependent response and that disruption of the HIC1 gene in human colon cells delays cell cycle progression under serum deficiency conditions. Moreover, treatment with etoposide, a DNA-damaging agent, significantly impairs the proliferation rate and dynamics of damaged DNA repair in HIC1-deficient compared with wild-type cells. One of the genes upregulated in HIC1-depleted cells encodes cell cycle regulator E2F7. E2F7 is an atypical member of the E2F family, which functions primarily as a transcriptional repressor, and its downregulation is essential for proper cell cycle progression and expression of genes involved in DNA repair. We demonstrated that E2F7 is indeed the target of transcriptional repression mediated by HIC1. Moreover, our results suggest that the phenotypic manifestations associated with loss of the HIC1 gene, in particular the changes in cell cycle progression and slowed repair of damaged DNA, are caused by dysregulation of E2F7 expression. Finally, we observed an inverse relationship between HIC1 and E2F7 in a panel of CRC. Importantly, CRC patients who express relatively high levels of E2F7 have a remarkably better prognosis than patients with intermediate or low levels of E2F7 expression.