Silencing of imprinted CDKN1C gene expression is associated with loss of CpG and histone H3 lysine 9 methylation at DMR-LIT1 in esophageal cancer

The putative tumor suppressor CDKN1C is an imprinted gene at 11p15.5, a well-known imprinted region often deleted in tumors. The absence of somatic mutations and the frequent diminished expression in tumors would suggest that CDKN1C expression is regulated epigenetically. It has been, however, controversial whether the diminution is caused by imprinting disruption of the CDKN1C/LIT1 domain or by promoter hypermethylation of CDKN1C itself. To clarify this, we investigated the CpG methylation index of the CDKN1C promoter and the differentially methylated region of the LIT1 CpG island (differentially methylated region (DMR)-LIT1), an imprinting control region of the domain, and CDKN1C expression in esophageal cancer cell lines. CDKN1C expression was diminished in 10 of 17 lines and statistically correlated with the loss of methylation at DMR-LIT1 in all but three. However, there was no statistical correlation between CDKN1C promoter MI and CDKN1C expression. Furthermore, loss of CpG methylation was associated with loss of histone H3 lysine 9 (H3K9) methylation at DMR-LIT1. Histone modifications at CDKN1C promoter were not correlated with CDKN1C expression. The data suggested that the diminished CDKN1C expression is associated with the loss of methylation of CpG and H3K9 at DMR-LIT1, not by its own promoter CpG methylation, and is involved in esophageal cancer, implying that DMR-LIT1 epigenetically regulates CDKN1C expression not through histone modifications at CDKN1C promoter, but through that of DMR-LIT1.