DOT1L Drives Endothelial‐to‐Mesenchymal Transition and Fibrotic Vascular Remodeling via H3K79 Methylation

ABSTRACT The lung is a highly vascularized organ in which endothelial cells (ECs) play a pivotal role in maintaining tissue homeostasis and regulating gas‐blood exchange. Increasing evidence suggests that endothelial‐to‐mesenchymal transition (EndoMT) contributes to fibrosis; however, the underlying epigenetic mechanisms remain incompletely understood. Here, we identify disruptor of telomeric silencing 1‐like (DOT1L), a histone H3 lysine 79 (H3K79) methyltransferase, as a key epigenetic regulator of EndoMT and fibrotic progression. In human umbilical vein ECs, TGFβ stimulation upregulated DOT1L expression and increased H3K79me2 levels during EndoMT. DOT1L knockdown abrogated H3K79 methylation and suppressed the expression of fibrosis‐associated genes. Chromatin immunoprecipitation analysis revealed that direct binding of SMAD2 to the DOT1L promoter increased its transcription and promoted H3K79me2 deposition at fibrosis‐related gene loci following TGFβ2 stimulation. In vivo, endothelial lineage‐tracing in mice demonstrated H3K79me2 accumulation in ECs undergoing EndoMT during bleomycin‐induced pulmonary fibrosis. Importantly, endothelial‐specific deletion of Dot1L significantly attenuated fibrotic remodeling, collagen deposition, and mesenchymal marker expression. Collectively, these findings establish DOT1L as a critical epigenetic driver of EndoMT and pulmonary fibrosis through H3K79me2‐mediated transcriptional activation, highlighting it as a potential therapeutic target in fibrotic lung disease.