Ezh2 Is An Essential Regulator Of T-Cell Development and Oncogenic Transformation

Enhancer of zeste homolog 2 (EZH2) catalyzes di- and trimethylation of lysine 27 on histone H3 (H3K27me2/3) and establishes chromatin marks associated with gene silencing. We and others have recently shown that Ezh2 and its partners act as tumour suppressor genes in mouse and likely human lymphoblastic leukemia. Moreover some studies also suggest that Ezh2 is strongly required during B and T cell differentiation. However, the function of EZH2 during these processes remains unclear. For functional study we exploited an Ezh2 conditional knockout mouse model. The Cre-mediated deletion generates a mutated Ezh2^Δ allele and abrogates production of EZH2 protein. Upon gene inactivation we monitored T-cell maturation and cancer development. We found that Ezh2 inactivation induces a block at the DN3-DN4 transition of TCRab⁺T-cells while TCRγδ T-cells were increased by 5 fold compared to wild type animals. Cell cycle analysis revealed increase in the proportions of TCRγδ⁺T-cells in the G2 phase compared to TCRβ⁺T-cells and wild type controls. This observation suggested a possibility of G2/M checkpoint activation resulting from either improper DNA replication, or a non-repaired DNA damage. Moreover we found that the Ezh2 deficient TCRγδ⁺ leukemia were prone to genomic instability. A majority of leukemias analyzed were aneuploid, and ∼50% were near-tetraploid. These observations were confirmed by Spectral Karyotyping (SKY), which also enabled detection of several chromosomal rearrangements. Consistent with these observations, analysis of global gene expression data from various RNA-Seq-derived datasets revealed that the genes having the highest correlation factor with Ezh2 are involved in cell division, DNA replication and DNA damage repair. Together, these studies show that Ezh2 is an essential regulator of the TCRγδ T-cell state, and prevents T-cell transformation, likely through regulation of DNA replication, cell division or DNA damage repair.