Loss of 5‐hydroxymethylcytosine is accompanied with malignant cellular transformation

Abstract

Dysregulated DNA methylation followed by abnormal gene expression is an epigenetic hallmark in cancer. DNA methylation is catalyzed by DNA methyltransferases, and the aberrant expression or mutations of DNA methyltransferase genes are found in human neoplasm. The enzymes for demethylating 5-methylcytosine were recently identified, and the biological significance of DNA demethylation is a current focus of scientific attention in various research fields. Ten-eleven translocation (TET) proteins have an enzymatic activity for the conversion from 5-methylcytosine to 5-hydroxymethylcytosine (5-hmC), which is an intermediate of DNA demethylation. The loss-of-function mutations of TET2 gene were reported in myeloid malignancies, suggesting that impaired TET-mediated DNA demethylation could play a crucial role in tumorigenesis. It is still unknown, however, whether DNA demethylation is involved in biological properties in solid cancers. Here, we show the loss of 5-hmC in a broad spectrum of solid tumors: for example, a significant reduction of 5-hmC was found in 72.7% of colorectal cancers (CRCs) and 75% of gastric cancers compared to background tissues. TET1 expression was decreased in half of CRCs, and a large part of them was followed by the loss of 5-hmC. These findings suggest that the amount of 5-hmC in tumors is often reduced via various mechanisms, including the downregulation of TET1. Consistently, in the in vitro experiments, the downregulation of TET1 was clearly induced by oncogene-dependent cellular transformation, and loss of 5-hmC was seen in the transformed cells. These results suggest the critical roles of aberrant DNA demethylation for oncogenic processes in solid tissues. (Cancer Sci 2012; 103: 670-676) P atterns of DNA methylation, histone modification and chromatin structure are profoundly altered in human cancers.(1-5) In particular, aberrant promoter hypermethylation leading to inappropriate transcriptional silencing of genes, especially tumor suppressor genes, is often found in various types of human neoplasm, including colorectal and gastric cancers.(6-9) DNA methylation is catalyzed by DNA methyltransferases (DNMTs), and it is reported that the increased level of DNMT1 is correlated with the histological grade or poor prognosis of human cancers. (10)(11)(12) In addition, a recent report demonstrated somatic mutations in the DNMT3A gene from acute myeloid leukemia patients. (13) Global loss of methylated DNA in paternal genome after fertilization suggests active DNA demethylation pathway in mammalian cells, although the molecular mechanism has been unknown for a long time. The recent discovery of ten-eleven translocation (TET) proteins those are capable of converting from 5-methylcytosine to 5-hydroxymethylcytosine (5-hmC) gave a breakthrough to the epigenetic research field. (14)(15)(16)(17)(18)(19) Following studies showed that the activationinduced cytidine deaminase family convert cytosine to uracil and 5-hmC to 5-hydroxymethyluracil, (20,21) and that TET1 mediates further oxidation of 5-h...

The Mammalian de Novo DNA Methyltransferases DNMT3A and DNMT3B Are Also DNA 5-Hydroxymethylcytosine Dehydroxymethylases

“…global DNA hypomethylation complicated by regional hypermethylation (31,32) and up-regulated expression of the DNMTs including DNMT3A and DNMT3B (31,33), are frequently observed. The environment of the cancer cells, in particular the presence of oxidative stresses, likely would facilitate 5-mC oxidation (18,34) as well as the dehydroxymethylase activities of DNMT3A/3B, thus leading to a profound reduction of the 5-hmC level (35)(36)(37) and also causing global hypomethylation of the cancer genomes (31,32). In this aspect, it seems like that the de novo DNMTs serve a protective function when cells are under oxidative stress.…”

Epigenetic regulation mechanisms of microRNA expression

“…However, like 5mC alterations, the 5hmC patterns undergo considerable changes linked to genome instability (70,71) across several forms of human cancers (72). Recent studies revealed that 5hmC is consistently found at significantly reduced levels (more than 50% reduction, p ≤ 0.01) in various solid tumors (73)(74)(75). Recently, using a mouse model, Pan et al (76) studied the role of hydroxymethylation in neurophysiological processes and provided the first evidence that miRNAs can be regulated by hydroxymethylation of their promoters.…”

Hypermethylation of TET1 Promoter Is a New Diagnosic Marker for Breast Cancer Metastasis

“…There are three TET genes in mammalian cells: TET1, TET2 and TET3 (Pastor et al, 2013). Furthermore, deregulation of the three proteins were accompanied by some diseases such as cancers (Kudo et al, 2012;Lian et al, 2012). TET1 act as tumor suppressors regulating cancer development, growth and invasion (Mercher et al, 2012;Ciccarone et al, 2014).…”