MYT1L deficiency impairs excitatory neuron trajectory during cortical development [E14]

MYT1L is a neuron-specific transcription factor routinely used in fibroblast-to-neuron transdifferentiation. Furthermore, mutations that reduce MYT1L function are associated with autism. Likewise, MYT1L has been hypothesized to play a role in the trajectory of neuronal specification and subtype specific maturation, but this hypothesis has not been directly tested, nor is it clear which neuron types are most impacted by MYT1L loss, and the cumulative impact of haploinsufficiency on chromatin has been unclear. In this study, we profiled 277,698 nuclei from the forebrains of wild-type and MYT1L-deficient mice at two developmental stages: E14 at the peak of neurogenesis and P21, when neurogenesis is complete. We found that MYT1L deficiency had the strongest impact on deep layer excitatory neurons, specifically disrupting their trajectory of development and preventing arrival at a mature state. We also demonstrated that MYT1L acts primarily, but not exclusively, as a transcriptional repressor in these cell types, and most effects on gene expression are cell-autonomous. Furthermore, we developed and applied single-nucleus combinatorial indexing Calling Cards (sci-CC), an exponentially scalable method to simultaneously record gene expression and longitudinal enhancer activity in single nuclei over time. We find that the disruptions persist throughout neurodevelopment as excitatory neurons were disproportionately affected. These findings illuminate the intricate role of MYT1L in orchestrating gene expression dynamics during neuronal development, providing insights into the molecular underpinnings of MYT1L syndrome. For E14 samples, forebrains from mice were dissected and nuclei were harvested from 3 replicates each of MYT1L WT, Het, and KO mice. Males and females were collected and mixed for each genotype. Libraries were made using the Scale Biosciences Single Cell RNA Kit.