PROJECT SUMMARY Autism spectrum disorder (ASD) is one of the most common neurodevelopmental disorders, diagnosed in one in 44 children in the United States. Sex is among the most significant risk factors for ASD, with males being three to five times more likely than females to manifest the disorder. While epidemiological data documenting these sex differences in autism are abundant, there is currently little understanding of the fundamental differences between male and female biology that contribute to the sex biases in autism prevalence, severity, and clinical presentation. Based on mounting evidence implicating i) important microglial contributions to neurodevelopment, ii) sex-biased microglial activation in individuals with ASD, and iii) our preliminary findings regarding microglia- specific sex-linked gene expression, this proposal is built around the central hypothesis that genetic sex differences in microglia contribute the male bias in ASD. Sex chromosomes may be especially important drivers of sexual dimorphism in human brain development. For one, ASD manifests at an early age, before the puberty-associated increase in sex hormone production. Further, progressive chromosome Y aneuploidy (e.g. XY, XYY, XYYY, XYYY) leads to incrementally increased incidence of ASD. These observations suggest a significant role for chromosome Y for presenting ASD-related behaviors. Yet, there is a gap in knowledge about Y-linked genes and their contribution to sex dimorphism outside the reproductive tract and to ASD pathophysiology. Therefore, the project goal is to test the hypothesis that increasing chromosome Y dose in human microglia in vitro (Aim 1) and in a xenotransplantation model (Aim 3) results in microglial pathology, neuropathological defects, and ASD-associated behavioral deficits. Delineating genome wide transcriptional disruption and genome wide reorganization (Aim 2) will further uncover how chromosome Y-linked genes contribute to microglia-specific gene networks and expand our knowledge on Y- linked genes associated with ASD. The long-term goal is to generate a framework for studying genetic sex differences in microglia and their contributions to sex-biased brain development. A deeper understanding of the contribution and molecular regulation of microglia in ASD will lay the groundwork to ultimately identify novel potential sex-specific interventions to improve the outcomes for individuals with ASD. The proposed research will take place in the Coufal and Glass laboratories at UC San Diego. The Coufal lab expertise is in human induced pluripotent stem cell models of neuroimmunology, and the Glass lab has expertise in tissue-resident macrophage gene regulation and epigenetics. Through graduate coursework, mentorship, and hands-on learning, Celina will gain experience in stem cell modeling systems for human brain cell types and for approaching large datasets from a quantitative perspective. These skills will be valuable for the completion of th...