Analogous to the use of DNA demethylating agents, therapeutic interventions that enhance TET2 enzymatic activity, decrease 2-HG inhibition, or restore TET2 transcription may be clinically beneficial in hematological malignancies. In humans, TET2 mutations are frequently associated with age-related clonal hematopoiesis ( 15), and children carrying autosomal recessive mutations in TET2 exhibit skewed T-cell development, altered B-cell maturation, and a pronounced susceptibility for development of lymphomas ( 16). In support of a tumor suppressive role for TET2 in hematopoiesis, mice lacking Tet2 display an array of hematopoietic abnormalities including enhanced stem cell renewal, myeloproliferation, decreased common lymphoid progenitors, and a predisposition to development of hematological malignancies ( 12 ⇓– 14). Loss of 5hmC in ATLL was shown to be independent of mutations in TET2 but was instead caused by reduced TET2 expression ( 11), indicating that TET2 function can be inhibited by mutation, 2-HG production or altered transcription. In adult T-cell leukemia/lymphoma (ATLL), reduced genomic 5hmC is associated with a more acute, aggressive form of the malignancy and worse prognosis. In addition, loss of TET2 function can be phenocopied by gain-of-function mutations in the isocitrate dehydrogenase genes ( IDH1 and IDH2), resulting in production of 2-hydroxygluterate (2-HG), a potent inhibitor of TET2 activity ( 9, 10). TET2 mutations are particularly common in hematopoietic malignancies including acute myeloid leukemia (23%), myelodysplastic syndromes (25%), chronic myelomonocytic leukemia (50%), and peripheral T-cell lymphomas (60%) ( 1). The TET methylcytosine dioxygenases (TET1-3) initiate DNA demethylation by conversion of 5-methylcystosine to 5-hydroxymethylcytosine (5hmC) ( 6 ⇓– 8). Loss-of-function mutations in enzymes involved in the homeostasis of DNA methylation are frequent across a range of malignancies ( 1), and DNA-demethylating drugs have been successfully used in treatment of myeloid malignancies ( 2 ⇓ ⇓– 5), supporting a role for DNA methylation in tumorigenesis and its clinical relevance as a target for treatment.
Together, our results clearly identify 5-aza as a potential targeted therapy for TET2-silenced T-ALL.Īberrant DNA methylation is a common feature of cancer, typified by locus-specific gains of methylation at gene promoters and genome-wide loss of DNA methylation in intergenic regions. Additionally, 5-aza led to up-regulation of methylated genes and human endogenous retroviruses (HERVs), which was further enhanced by the addition of physiological levels of vitamin C, a potent enhancer of TET activity. Importantly, treatment with the DNA demethylating agent, 5-azacytidine (5-aza), was significantly more toxic to TET2-silenced T-ALL cells and resulted in stable re-expression of the TET2 gene. Furthermore, we show that TET2 silencing is often associated with hypermethylation of the TET2 promoter in primary T-ALL.
Here, we analyzed RNA-sequencing data of 321 primary T-ALLs, 20 T-ALL cell lines, and 25 normal human tissues, revealing that TET2 is transcriptionally repressed or silenced in 71% and 17% of T-ALL, respectively. Whereas mutations in the DNA demethylating enzyme TET2 are frequent in adult T-cell malignancies, TET2 mutations in T-ALL are rare. As survival rates for relapsed T-ALL remain dismal (10 to 25%), development of targeted therapies to prevent relapse is key to improving prognosis. Pediatric T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive malignancy resulting from overproduction of immature T-cells in the thymus and is typified by widespread alterations in DNA methylation.