PROJECT SUMMARY This proposal aims to identify the role of the histone lysine methyltransferase Dot1L in neuronal function and its contribution to neurodevelopmental disorders (NDDs). NDDs include a spectrum of highly prevalent conditions that manifest during development that can cause intellectual disability, developmental delays, and autism spectrum disorder. Recent work demonstrated that many chromatin regulators are mutated in NDDs, including the histone methyltransferase Dot1L. Dot1L methylates histone 3 of lysine 79 (H3K79me) which is associated with active transcription. We found that H3K79me is highly abundant and dynamically regulated in postmitotic neurons. Our preliminary data also indicate that H3K79me is critical for neuronal function. We found that patient mutations result in a loss of Dot1L methyltransferase activity indicating that depletion of H3K79me can cause NDDs. Further, we found that Dot1L depletion alters transcription of synaptic genes and bidirectionally regulates GluA2, an AMPA receptor subunit. Finally, we found long-term memory deficits in Dot1L conditional knockout (cKO) mice. However, the role of Dot1L in neuronal function and cognition remain unclear. I hypothesize that Dot1L regulates synaptic gene expression and that partial Dot1L loss disrupts this regulation leading to NDDs. In Aim 1, I will define chromatin and transcriptional disruptions caused by partial Dot1L loss using a heterozygous Dot1L cKO mouse model coupled with H3K79me2 cleavage under targets and tagmentation (CUT&Tag) and RNA-sequencing. In Aim 2, I will examine the impact of partial Dot1L loss on neuronal function and cognition by using the heterozygous Dot1L cKO mouse model and controls to perform electrophysiology and behavioral experiments. Cumulatively, this work will establish a role for Dot1L in neuronal function and NDDs and more broadly will contribute to understanding of the role of chromatin regulators in brain function.