Discovery and Functional Interpretation of Genetic Risk in Autoimmune Diseases

Abstract

Autoimmune diseases are chronic and debilitating conditions arising from abnormal immune responses directed against normal body tissues; they collectively affect the lives of 5-10% of the world population. These diseases often show familial clustering, suggesting strong genetic heritability. For many of autoimmune diseases, variation in the human leukocyte antigen (HLA) genes is the primary modulator of genetic risk. Recently, genome-wide association studies (GWAS) identified hundreds of genomic regions outside the HLA that harbor additional risk-conferring variants. The ultimate goal is to identify the precise causal variants and understand the mechanisms by which they lead to autoimmunity, which is challenged by complexities of the genome and the immune system. In this work, my colleagues and I developed and applied experimental and computational tools to reveal critical clues from multiple genetic and biological data types. First, we devised a statistical algorithm to identify the critical cell types involved in different autoimmune diseases. Two strongly heritable and common diseases, rheumatoid arthritis (RA) and type 1 diabetes (T1D), both involve the adaptive immune system, specifically the CD4+ T cells. We then conducted focused studies in CD4+ T cells using high-throughput genomic and proteomic technologies, and showed that immunological phenotypes and functions varied with genetic differences across individuals. To facilitate this study, we developed an automated computational tool to efficiently and reliably analyze the large-scale data. Finally, the HLA genes, which encode a family of highly variable antigen-recognition proteins, are the longest-known and strongest modulators of genetic risk in T1D. However, the extraordinary level of polymorphism and complex structure in the HLA region largely hindered precise localization and functional investigation of the causal mutations. We used dense-genotyping and robust statistical analyses to pinpoint the amino acid residue changes at a few key amino acid positions that explained the majority of disease risk within the HLA. The work presented in this dissertation revealed the specific immune cell populations, genetic variants, and cellular functions that affect RA, T1D, and other autoimmune diseases. Furthermore, it offers a rational framework, as well as powerful open-source computational tools, that can be applied in future functional genomic studies.Medical Science