DNA methylation plays critical roles in the development and differentiation of mammalian cells, and its dysregulation has been implicated in oncogenesis. This study was designed to determine whether DNA hypomethylation-associated aberrant gene expression is involved in adult T-cell leukemia (ATL) leukemogenesis. We isolated hypomethylated DNA regions of ATL cells compared with peripheral blood mononuclear cells from a carrier by a methylated CpG-island amplification/representational difference analysis method. The DNA regions identified contained MEL1, CACNA1H, and Nogo receptor genes. Sequencing using sodium bisulfite-treated genomic DNAs revealed the decreased methylated CpG sites, confirming that this method detected hypomethylated DNA regions. Moreover, these hypomethylated genes were aberrantly transcribed.
IntroductionAdult T-cell leukemia (ATL) is a neoplastic disease of CD4 ϩ T lymphocytes that is etiologically associated with human T-cell leukemia virus type I (HTLV-I). [1][2][3][4][5][6] After transmission, HTLV-I increases the number of infected cells through its viral proteins, including Tax. Tax encoded by the pX region between env and the 3Ј long terminal repeat (LTR) is an oncoprotein with pleiotropic actions, 7,8 including transactivation of the nuclear factor kappaB (NFB), serum responsive factor (SRF), and cyclic adenosine monophosphate (cAMP) response elementbinding protein (CREB) pathways; transrepression of genes, such as DNA polymerase , lck, and p18 genes; and functional inactivation of p53, p16, and mitotic arrest-defective 1 (MAD1). 8,9 With these actions, Tax promotes the proliferation and inhibits apoptosis of infected cells, resulting in their clonal expansion and increased proviral load.The cumulative risk of developing ATL after HTLV-I infection was estimated to be 6% in men and 2% in women. 10 In addition, there is a long latent period before the onset of ATL, suggesting that additional factors other than viral proteins are implicated in leukemogenesis. Although somatic changes of genes, such as mutation of p53 11 or deletion of p16, 12 were reported in ATL, they were not so frequent in ATL cells and were predominantly observed in aggressive forms of ATL, such as acute and lymphoma types. This suggested that these genetic changes were implicated in the progression of ATL. In addition, it has been reported that DNA methylation of the p16 gene silenced its expression, indicating that epigenetic changes were also implicated in leukemogenesis. 13 Epigenetic changes include dysregulated DNA methylation in cancer cells, which consists of hypermethylation and hypomethylation of DNA. 14 Hypermethylation is frequently associated with gene silencing when hypermethylation occurs in the promoter region of genes. 15 The same mechanism is considered to regulate the transcriptional silencing of various tumor suppressor genes, including p16, 16 p15, 17 hMLH1, 18 BRCA1, 19 and GSTP1 genes. 20 From the perspective of the whole genome, genome-wide hypomethylation has been reported in cancer cells...