Response to Letter Regarding Article, “Ten-Eleven Translocation-2 (TET2) Is a Master Regulator of Smooth Muscle Cell Plasticity”

Abstract

We thank Drs Li and Yu for their enthusiastic comments on our recent work 1 identifying Ten-Eleven-Translocation 2 (TET2) as a novel epigenetic master regulator of smooth muscle cell (SMC) phenotype. We also appreciate their highlighting the implications of our study regarding a role for TET2 in cellular reprogramming. We concur that this is an exciting example of a single epigenetic agent promoting the SMC phenotype in fibroblasts, but we hypothesize that TET2 in combination with other regulatory factors may be even more potent in conversion of non-SMC to the SMC lineage. We are currently working to identify these cofactors.Extensive work from many labs over the past 30 years has revealed the importance of epigenetic modifications, including changes in DNA methylation and histone acetylation, in cellular reprogramming and transdifferentiation. In one of the earliest demonstrations, the DNA methylation inhibitor 5-azacytidine could revert 3T3 and C3H10T1/2 cells to a more pluripotent state that then permitted cellular differentiation into the bone, muscle, and adipogenic lineages. Similarly, inhibition of histone deacetylation and DNA methylation has been shown to improve reprogramming efficiency of somatic cell nuclear transfer, 3 as well as in the production of induced pluripotent stem cells after ectopic expression of defined factors in fibroblasts. 4 The actions of the TET enzymes indeed contribute to DNA demethylation and thus may also contribute to the reprogramming process. Notably, we concur with Drs Li and Yu that generation of 5hmC as a stable epigenetic mark could contribute to reprogramming in addition to acting as an intermediate leading to demethylation. Indeed, our work unexpectedly revealed the stable presence of 5hmC in differentiated SMC in vivo. 1 We reported that ectopic expression of TET2 is able to convert MRC5 fibroblasts into the smooth muscle lineage as demonstrated by the robust upregulation of SMC markers including SMA and MYH11. Interestingly, TET2 overexpression was only able to partially convert endothelial cells to the smooth muscle lineage.1 The mechanisms underlying the cell type-specific differences in TET2-mediated reprogramming efficiency remain to be explored. We speculate that the embryonic origin of MRC5, a cell line commonly used for reprogramming studies, may confer a greater plasticity that can therefore be more readily differentiated into multiple lineages compared with the more fully differentiated adult endothelial cells. Our results are reminiscent of those of Cordes and colleagues, 5 who reported that ectopic miR145 is inefficient to induce SMC phenotype in C3H10T1/2 embryonic fibroblasts, but is highly potent to induce differentiation in the more pluripotent Joma1.3 neural crest stem cell line.We are pursuing new studies to further address the roles of TET2 in vascular disease. The fact that TET2 overexpression does not convert endothelial cells to the SMC lineage suggests potential for TET2-based therapies for vascular diseases. Further exploration of the role for...