Mechanism Of DNA Methylation-mediated GAD1Activation In Carcinogenesis

Glutamate decarboxylase1(GAD1) is up-regulated in neoplastic tissues, but the mechanisms underlying this up-regulation remain unclear. In contrast to the concept that DNA methylation silences gene transcription, we here demonstrate that the human Glutamate decarboxylase1(GAD1) promoter is hypermethylated both in colon and liver cancer with high levels of GAD1. The key locus responsible for GAD1activation has been mapped to a DNA methylation-sensitive CTCF binding site (CTCF-BS3) and a site containing Fuzzy Tandem Repeats (FTRs) for ZNF263binding. We propose that, in normal cells, CTCF binding to CTCF-BS3could act as a bridge to recruit the ZNF263/KAP1/SETDB1repressosome to the GAD1promoter, which is an effect that might be inhibited by DNA methylation in cancer cells.Consequently, the repressosome might be excluded from the promoter, resulting in GAD1reactivation. This is the first time that a mechanism underlying DNA methylation-mediated gene activation in cancer has been revealed. Moreover, up-regulation of GAD1is correlated with mucinous colon cancer and poor survival rate. GAD1knockdown increased cell’s apoptosis induced by low pH. These findings provide new insights on the role of GAD1played in cancer.Recent genome-wide methylation studies have challenged the notion that DNA methylation always negatively regulates gene expression, but the mechanism underlying this phenomenon remains elusive. Here we provide a new model for the DNA methylation-mediated GAD1activation. Although it has well-documented that DNA methylation could prevent CTCF binding, our data provide new function for the intragenic CTCF binding sites in gene transcription. This is the first time to substantially provide that ZNF263could repression gene expression by forming a repressome with KAP1/SETDB1. In addition, our findings also provide new clues for the roles of GAD1played in mucinous colon tumor.