Concomitant loss of TET2 and TET3 results in T cell expansion and genomic instability [WGS]
TET proteins are tumor suppressors that through their catalytic activity oxidize 5-methylcytosine to 5-hydroxymethylcytosine, to promote DNA demethylation and to regulate gene expression. Notably, TET2 is one of the most frequently mutated genes in hematological malignancies, including T cell lymphomas. However, murine models with deletion of TET2 do not exhibit T cell expansion, presumably due to redundancy with other members of the TET family of proteins. In order to gain insight on the TET mediated molecular events that safeguard T cells from aberrant proliferation we performed serial adoptive transfers of murine CD4 T cells that lack concomitantly TET2 and TET3 to fully immunocompetent congenic mice. Our data reveal a progressive acquisition of malignant traits upon loss of TET2 and TET3 that is characterized by loss of genomic integrity, acquisition of aneuploidy and upregulation of the protooncogene Myc. Overall design: To test whether loss of TET2 and TET3 could endow Tet2/3 DKO CD4 cells with hyperproliferative capacity, we performed serial transplantations of sorted, highly pure Tet2/3 DKO CD4 cells in congenic CD45.1+ recipients, that have not received any irradiation. After 16 weeks, we witnessed that the transferred CD45.2+ Tet2/3 DKO cells have evaded the immune surveillance of the recipients and have expanded. At this point, we isolated CD45.2+ cells by FACS sorting  and re-transplanted them in new CD45.1+ immunocompetent recipients. The second transplantation resulted in significant acceleration of the expansion of the transplanted cells which became apparent within 2-4 weeks. To assess chromosomal copy number we performed whole genome sequencing. Genomic DNA was isolated from sorted wild type CD4 cells, or sorted expanded CD45.2+ Tet2/3 DKO cells after one or two serial transplantations in congenic CD45.1+ recipient mice.