TET Proteins and Histone Modifications in Hematopoiesis

DNA methylation has been implicated in numerous biological processes including long-term gene silencing, and the development of various diseases, including cancer. A recent advance key in the epigenetics field was the discovery that the TET family of proteins can hydroxylate methylated DNA converting 5-methylcytosine (mC) to 5-hydroxymethylcytosine (hmC). While TET-mediated hydroxylation may serve as part of a DNA demethylation process, its impact on methylation patterns is only beginning to be unraveled. Previously, our team and others have presented the genome-wide mapping of hmC and Tet1 in mouse embryonic stem cells (mESCs). Tet1 binds throughout the genome being specifically enriched at CpG-rich sequences. These are generally depleted of DNA methylation and often cover cis-regulatory elements, such as enhancers and promoters. hmC is mostly enriched in the regions with lower CpG-density, where it overlaps with Tet1. Our recent work has focused on understanding the importance of the TET proteins in regulating DNA methylation patterns and transcription in biologically relevant systems. To this extent we have used mouse embryonic stem cells and an acute myeloid leukemia models based on the inactivation of Tet2, developed in our lab. Mouse embryonic stem cells depleted of either Tet protein display a partial reduction in hmC levels accompanied by minor changes in DNA methylation patterns. In contrast, co-depletion of Tet1 and Tet2 leads to a dramatic, global loss of hmC and importantly, causes distinct alterations in DNA methylation levels also affecting gene-regulatory elements. Nevertheless, mESCs depleted of both Tet1 and Tet2 proliferate, express pluripotency markers and are capable of differentiating into the three germ layers. These results demonstrate that both Tet1 and Tet2 function to prevent aberrant DNA methylation in mESCs and further suggest that they serve to ensure transcriptional plasticity during development. The frequent mutation of TET2 in acute myeloid leukemia (AML) patients suggests that inactivation of TET2 plays a central role in disease development. However, deletion of Tet2 in transgenic mouse models does not lead to development of leukemia indicating that additional oncogenic events are necessary for leukemic transformation. To understand the potential role of TET2 mutations in the development of AML and in the regulation of DNA methylation patterns, we tested the ability of TET2 to collaborate with oncogenic fusion proteins in AML. The recurrent translocation t(8;21) results in expression of AML1-ETO fusion proteins and is present in approximately 10 percent of AML patients. We show that disruption of TET2 synergizes with AML1-ETO fusion proteins to induce and accelerate an aggressive and transplantable AML-like condition mimicking the human disease. Preliminary studies suggest that TET2 deletion in pre-leukemic cells leads to progressive hypermethylation of gene-regulatory elements that results in altered expression of several genes implicated in tumorigenesis. Taken together, these results illustrate how aberrant DNA methylation patterns can contribute to disease and confirm the role of TET2 as a tumor suppressor and as a regulator of DNA methylation.