Hypomethylation of TET2 Target Genes Identifies a Curable Subset of Acute Myeloid Leukemia.

BACKGROUND: Acute myeloid leukemia (AML) is curable in a subset of cases. The DNA methylation regulator TET2 is frequently mutated in AML, and we hypothesized that studying TET2-specific differentially methylated CpGs (tet2-DMCs) improves AML classification. METHODS: We used bisulfite pyrosequencing to analyze the methylation status of four tet2-DMCs (SP140, MCCC1, EHMT1, and MTSS1) in a test group of 94 consecutive patients and a validation group of 92 consecutive patients treated with cytarabine-based chemotherapy. Data were analyzed with hierarchical clustering, Cox proportional hazards regression, and Kaplan-Meier analyses. All statistical tests were two-sided. RESULTS: In the test cohort, hierarchical clustering analysis identified low levels of tet2-DMC methylation in 31 of 94 (33%) cases, and these had markedly longer overall survival (median survival 72+ vs 14 months, P = .002). Similar results were seen in the validation cohort. tet2-DMC-low status was shown to be an independent predictor of overall survival (hazard ratio = 0.29, P = .0002). In The Cancer Genome Atlas (TCGA) dataset where DNA methylation was analyzed by a different platform, tet2-DMC-low methylation was also associated with improved outcome (median survival = 55 vs 15 months, P = .0003) and was a better predictor of survival than mutations in TET2, IDH1, or IDH2, individually or combined. CONCLUSIONS: Low levels of tet2-DMC methylation define a subgroup of AML that is highly curable and cannot be identified solely by genetic and cytogenetic analyses.

TET2 Mutations Affect Non-CpG Island DNA Methylation at Enhancers and Transcription Factor-Binding Sites in Chronic Myelomonocytic Leukemia.

TET2 enzymatically converts 5-methylcytosine to 5-hydroxymethylcytosine as well as other covalently modified cytosines and its mutations are common in myeloid leukemia. However, the exact mechanism and the extent to which TET2 mutations affect DNA methylation remain in question. Here, we report on DNA methylomes in TET2 wild-type (TET2-WT) and mutant (TET2-MT) cases of chronic myelomonocytic leukemia (CMML). We analyzed 85,134 CpG sites [28,114 sites in CpG islands (CGI) and 57,020 in non-CpG islands (NCGI)]. TET2 mutations do not explain genome-wide differences in DNA methylation in CMML, and we found few and inconsistent differences at CGIs between TET2-WT and TET2-MT cases. In contrast, we identified 409 (0.71%) TET2-specific differentially methylated CpGs (tet2-DMCs) in NCGIs, 86% of which were hypermethylated in TET2-MT cases, suggesting a strikingly different biology of the effects of TET2 mutations at CGIs and NCGIs. DNA methylation of tet2-DMCs at promoters and nonpromoters repressed gene expression. Tet2-DMCs showed significant enrichment at hematopoietic-specific enhancers marked by H3K4me1 and at binding sites for the transcription factor p300. Tet2-DMCs showed significantly lower 5-hydroxymethylcytosine in TET2-MT cases. We conclude that leukemia-associated TET2 mutations affect DNA methylation at NCGI regions containing hematopoietic-specific enhancers and transcription factor-binding sites.

Effects of TET2 mutations on DNA methylation in chronic myelomonocytic leukemia.

TET2 enzymatically converts 5-methyl-cytosine to 5-hydroxymethyl-cytosine, possibly leading to loss of DNA methylation. TET2 mutations are common in myeloid leukemia and were proposed to contribute to leukemogenesis through DNA methylation. To expand on this concept, we studied chronic myelomonocytic leukemia (CMML) samples. TET2 missense or nonsense mutations were detected in 53% (16/30) of patients. In contrast, only 1/30 patient had a mutation in IDH1 or IDH2, and none of them had a mutation in DNMT3A in the sites most frequently mutated in leukemia. Using bisulfite pyrosequencing, global methylation measured by the LINE-1 assay and DNA methylation levels of 10 promoter CpG islands frequently abnormal in myeloid leukemia were not different between TET2 mutants and wild-type CMML cases. This was also true for 9 out of 11 gene promoters reported by others as differentially methylated by TET2 mutations. We found that two non-CpG island promoters, AIM2 and SP140, were hypermethylated in patients with mutant TET2. These were the only two gene promoters (out of 14,475 genes) previously found to be hypermethylated in TET2 mutant cases. However, total 5-methyl-cytosine levels in TET2 mutant cases were significantly higher than TET2 wild-type cases (median = 14.0% and 9.8%, respectively) (p = 0.016). Thus, TET2 mutations affect global methylation in CMML but most of the changes are likely to be outside gene promoters.

DNA methylation does not stably lock gene expression but instead serves as a molecular mark for gene silencing memory.

DNA methylation is commonly thought of as a "molecular lock" that leads to permanent gene silencing. To investigate this notion, we tested 24 different histone deacetylase inhibitors (HDACi) on colon cancer cells that harbor a GFP locus stably integrated and silenced by a hypermethylated cytomegalovirus (CMV) promoter. We found that HDACi efficiently reactivated expression of GFP and many other endogenous genes silenced by DNA hypermethylation. After treatment, all promoters were marked with active chromatin, yet DNA hypermethylation did not change. Thus, DNA methylation could not prevent gene reactivation by drug-induced resetting of the chromatin state. In evaluating the relative contribution of DNA methylation and histone modifications to stable gene silencing, we followed expression levels of GFP and other genes silenced by DNA hypermethylation over time after treatment with HDACi or DNA-demethylating drugs. Reactivation of methylated loci by HDACi was detectable for only 2 weeks, whereas DNA-demethylating drugs induced permanent epigenetic reprogramming. Therefore, DNA methylation cannot be considered as a lock for gene expression but rather as a memory signal for long-term maintenance of gene silencing. These findings define chromatin as an important druggable target for cancer epigenetic therapy and suggest that removal of DNA methylation signals is required to achieve long-term gene reactivation.

Cancer epigenetics.

Epigenetics refers to stable alterations in gene expression with no underlying modifications in the genetic sequence and is best exemplified by differentiation, in which multiple cell types diverge physiologically despite a common genetic code. Interest in this area of science has grown over the past decades, especially since it was found to play a major role in physiologic phenomena such as embryogenesis, imprinting, and X chromosome inactivation, and in disease states such as cancer. The latter had been previously thought of as a disease with an exclusive genetic etiology. However, recent data have demonstrated that the complexity of human carcinogenesis cannot be accounted for by genetic alterations alone, but also involves epigenetic changes in processes such as DNA methylation, histone modifications, and microRNA expression. In turn, these molecular alterations lead to permanent changes in the expression of genes that regulate the neoplastic phenotype, such as cellular growth and invasiveness. Targeting epigenetic modifiers has been referred to as epigenetic therapy. The success of this approach in hematopoietic malignancies validates the importance of epigenetic alterations in cancer, not only at the therapeutic level but also with regard to prevention, diagnosis, risk stratification, and prognosis.