An integrative approach to linking genes, brain, and behavior in 22q11.2 copy number variations

Efforts to bridge the gap between genes, brain, and behavior are facilitated by the study of highly-penetrant, disease-associated copy number variations (CNV). CNVs at the 22q11.2 locus present as deletions and duplications, each associated with clinical phenotypes that implicate perturbed neurodevelopmental processes. In fact, 22q11.2 CNVs confer some of the greatest known genetic risks for psychiatric disorders, and thus represent a compelling model to yield biological insights into how gene dosage may influence downstream brain and behavior. In this body of work, I present three studies that offer a comprehensive approach tethering cortical morphometric changes, cognitive and behavioral impairments, and transcriptomic dysregulation to reveal novel insights about the molecular effects of 22q11.2 CNVs. The first two studies are published in The Journal of Neuroscience and Biological Psychiatry, respectively, while the third study will soon be submitted to Brain, Behavior, and Immunity. In Chapter 1, I briefly introduce the challenge of studying neurodevelopmental psychiatric disorders, and how the use of highly penetrant, disease-associated CNVs can help to mitigate some of these biological complexities. Then, I explain how reciprocal 22q11.2 CNVs have great potential to offer key insights into the underlying biology of neuropsychiatric disorders. In Chapter 2, I show for the first time that 22q11.2 deletions (22qDel) and duplications (22qDup) confer global opposing effects on brain morphometry by comparing 66 molecularly-confirmed 22qDel carriers, 21 22qDup carriers, and 56 demographically-matched controls. 22qDel was associated with widespread reductions in cortical surface area, with corresponding enlargement in 22qDup. Cortical thickness showed the opposite pattern, but with more localized effects. Regional patterns of thickness difference between 22qDel and 22qDup carriers were also observed in subcortical regions which, in concert with the diffuse surface area changes, imply global and early impacts of 22q11.2 CNVs on brain development. Lastly, these findings were not accounted for by a subset of the sample, but rather the entire distribution was shifted, which suggests a highly penetrant effect of gene dosage. In Chapter 3, I systematically examine neurodevelopmental phenotypes relevant to intellectual functioning, Autism Spectrum Disorder (ASD), and psychosis in 106 22qDel carriers, 38 22qDup carriers, and 82 demographically-matched controls. This study aimed to leverage dimensional phenotyping, based on the hypothesis that variation in specific quantitative traits may better reflect underlying biological variation than categorical diagnoses. I found that reciprocal 22q11.2 deletions and duplications were associated with distinct impacts on intellectual functioning and psychosis-related symptomatology, but shared global deficits in the domain of ASD-related symptomatology. However, when more fine-grained subdomain measures of ASD-related traits were assayed, subtle differences in ASD profiles were found to distinguish deletion and duplication carriers. Lastly, while controls showed an inverse relationship with cortical thickness and processing speed, this association was absent in both CNV groups. This altered relationship between normative cortical thinning and cognitive processing speed implies disrupted development of the cortical mantle may underlie impaired processing efficiency in 22q11.2 CNV carriers. In Chapter 4, I leverage high-throughput transcriptomic data as an additional lens by which to gain mechanistic insights. This study represents the first genome-wide gene expression study to characterize transcriptomic dysregulation in peripheral blood in reciprocal 22q11.2 CNV carriers. Specifically, analyzing samples from 82 22qDel carriers, 29 22qDup carriers, and 68 demographically-matched controls, I showed that the 22q11.2 deletion significantly alters gene expression across the genome, and that these differences are substantially attenuated after adjustment for cell type heterogeneity. Moreover, expression of two cytoskeletal- and ASD-relevant genes differed between 22q11.2 CNV carriers with and without a diagnosis of ASD. I also extend findings regarding cell type differences between 22q11.2 deletion carriers and controls beyond known T cell differences to include mast cell and macrophage subtypes, two cell types that have multifunctional, immune-related roles throughout the body. These findings demonstrate the challenges inherent in using peripheral blood, a common tissue of analysis in psychiatric genetics, to study brain-related diseases. Nevertheless, this work shows how blood tissue, which contains a wealth of information regarding the immune system, can still offer valuable insights regarding immune mechanisms in psychiatric disorders, provided confounds such as cell type heterogeneity are properly accounted for. In sum, the work described herein seeks to reveal fundamental biology, generate hypotheses for future targeted experiments to validate and test in animal or in vitro models, and ultimately inform novel therapeutic targets