Temporal and Spatial Multi-omics Characterization of Smith-Lemli-Opitz Syndrome

Abstract

Smith-Lemli-Opitz syndrome (SLOS) is a metabolic disorder caused by genetic mutations in the DHCR7 gene, leading to defective 3β-hydroxysterol-Δ7-reductase (DHCR7), the enzyme that catalyzes the last step of cholesterol synthesis. The resulting deficiency in cholesterol and accumulation of its precursor, 7-dehydrocholesterol (7-DHC), have a profound impact on brain development, manifesting as developmental delay, cognitive impairment, moderate-to-severe intellectual disability, and behavioral deficits. Additionally, disorder severity and phenotype vary widely between individuals. Anomalies of the CNS are frequently observed in patients, with microcephaly, ventricular dilatation, and malformations of the corpus callosum and cerebellum being some of the most common. However, therapeutic interventions for the neurological aspects of SLOS are lacking. Further neurophysiological studies on SLOS mouse models are needed to understand how the loss of Dhcr7 and disruption of cholesterol biosynthesis affect the course of neurodevelopment. The objective of this dissertation is to establish the spatial and temporal lipidomic and transcriptomic changes that occur during development in a Dhcr7-KO mouse model of SLOS. Chapter 1 provides background on the history, genetic cause, and clinical phenotype of the disorder. Chapter 2 presents the results from an untargeted lipidomics analysis of developing mouse brains in wild-type (WT) and Dhcr7-KO mice. We compared relative lipid levels throughout development, from embryonic day 12.5 to postnatal day 0. The study identified differentially expressed brain lipids between WT and Dhcr7-KO mice at specific developmental time points, revealing lipid metabolic pathways that are affected in SLOS beyond the cholesterol biosynthesis pathway, such as glycerolipid, glycerophospholipid, and sphingolipid metabolism. In Chapter 3, we mapped the spatial distribution of sterols and lipids in neonatal Dhcr7-KO mouse brains using mass spectrometry imaging. We determined the sites of accumulation of sterols and major oxysterol metabolites of 7-DHC and identified differentially expressed lipids between WT and Dhcr7-KO brains. Chapter 4 presents a complementary transcriptomics analysis to Chapter 2, where we carried out RNA sequencing analysis of whole brains from WT and Dhcr7-KO mice at the same four time points spanning development. We discuss the impact of these gene expression changes and affected signaling pathways as they relate to the neurological phenotype of SLOS. In Chapter 5, we summarize the overall project findings and directions for future research.