Transcriptional diversity of human mediodorsal thalamus and its role in psychiatric disease - Schizophrenia (SZ) and bipolar disorder (BP) are devastating, highly heritable brain disorders that affect millions of people worldwide. Current treatment options often fail to adequately control symptoms, and relapses are common. Numerous genetic variants associated with risk have been identified, but progress is hindered by limited understanding of the mechanisms by which genetic variants act within the brain to confer disease risk. One consistent finding in patients with SZ and BP is reduced connectivity between the thalamus, particularly the mediodorsal thalamus, and prefrontal cortex. While the molecular basis of this reduced connectivity remains unclear, recent findings link brain connectivity to regional variation in gene expression, itself a function of cell type and inherited genetic variation. In this K22 application, I propose an integrated training and research program aimed at developing my knowledge and skills in human genetics and bioinformatics, while building upon my strong background in molecular neuroscience and single-cell genomics. I will investigate the overarching hypothesis that known differences in thalamocortical connectivity in SZ and BP are accompanied by transcriptional differences in the mediodorsal thalamus. These transcriptional differences can now be precisely defined by new technologies that illuminate genomics and transcriptomics at single-cell resolution in post-mortem human brain. I will also investigate how inherited genetic variants affect cell type-specific gene expression in thalamus. Such variants will then be compared to common variants known to confer risk for BP or SZ, potentially elucidating one mechanism whereby genetic variants confer risk for major mental illness. In Aim 1 I will use single-nucleus RNA-sequencing to define transcriptional signatures of diverse cell types in human mediodorsal thalamus and then use these signatures to explore differences in cell type-specific gene expression in post- mortem brain obtained from people diagnosed with SZ or BP and non-psychiatric controls. I will also explore the effects of known risk alleles on cell type-specific gene expression in mediodorsal thalamus and compare the thalamic transcriptomes with those of interconnected cortical regions in the same patients. Since gene transcription is often driven by dynamic changes in chromatin, in Aim 2 I will use single-cell ATAC-seq to identify regions of accessible chromatin in post-mortem human mediodorsal thalamus. By integrating these data with common genetic variants, I will identify candidate cis-regulatory elements that influence chromatin accessibility in thalamus. These variants will be good candidates for future work aimed at linking genetic risk for SZ and BP to cell type-specific gene expression changes within the mediodorsal thalamus and associated brain regions. If successful, this novel integration of genetic, transcriptomic, and chromatin-accessibility data in human thalamus will deliver new insights into the mechanisms by which common disease-associated genetic variants affect cellular physiology in the brain. This project will provide me with a foundation of training and research experience that is essential for my future plans to study the thalamus and its role in the pathogenesis of SZ and BP.
