Identifying the Genetic Basis of Hypermobile Ehlers-Danlos Syndrome (hEDS)

Abstract

Ehlers-Danlos Syndrome (EDS) describes a group of 14 heritable connective tissue disorders that have in common joint hypermobility. The most prevalent of the subtypes is the recently reclassified hypermobile EDS (hEDS) characterized by generalized joint hypermobility, skin features such as velvety texture and/or skin hyperextensibility, and musculoskeletal pain. Secondary features include autonomic dysfunction, gastrointestinal dysfunction, and fatigue. While the most prevalent EDS subtype, hEDS is the least understood without any consensus on associated causal genes or variants. Due to its complex and heterogeneous symptomatology, we hypothesize that hEDS is a genetically heterogenous disorder but with primary causal variants and/or genes. The focus of this dissertation, as part of the hEDS GENE study led by Dr. Christina Laukaitis, is to define the updated hEDS phenotype and identify both structural and rare single nucleotide variants that contribute to the hEDS phenotype. To explore the 2017 hEDS reclassification, 148 people were enrolled in the hEDS GENE study with 98 meeting the 2017 hEDS criteria, 27 having a hypermobility spectrum disorder (HSD) and 23 being asymptomatic family members. More than 80% of the participants were female. Each participant completed seven clinal questionnaires to thoroughly describe and quantify hEDS symptomatology including the 36-Item Short Form Health Survey, a subscale of the full Patient Health Questionnaire, the Tampa Scale for Kinesiophobia, the Fatigue Severity Scale, the Epworth Sleepiness Scale, the Gastro-Questionnaire, and a short form version of the Composite Autonomic Symptom Score. We concluded that symptom frequency and severity were generally indistinguishable between participants with hEDS and HSDs with both experiencing significant decrease in health-related quality of life, however, we also observed subtle differences in autonomic symptoms between the two hypermobile groups. In participants with a clearly-defined hEDS phenotype, I sought to identify previously reported structural variants and novel single nucleotide variants in the fibrinogen (FBG)-like domain of TNXB. This gene encodes the protein tenascin X, which is suggested to play a role in regulating the extracellular matrix. The tenascin X FBG-like domain is hypothesized to activate the transforming growth factor beta (TGF-β) signaling pathway. Using copy number data from individuals undergoing whole-genome sequencing (WGS), we identified 31 out of 68 hEDSaffected individuals with deletions spanning the FBG-like domain of TNXB. I also identified three previously reported variants and two novel TNXB missense variants that may potentially contribute to an hEDS phenotype. Using a multivariant group differences analysis, I determined that individuals with and without TNXB deletions experienced subtle differences in GI symptomology. Finally, in order to identify potentially causal hEDS variants, I performed a linkage-like analysis using the Rare Variant Sharing (RVS) statistical framework on our multiplex families. I identified three novel variants, occurring in intronic regions and previously undescribed, in three genes that replicated in multiple families and/or replicated in individuals. These variants occurred in the Complement Factor H Related 4 (CFHR4) gene, the Endoplasmic Reticulum Aminopeptidase 1 (ERAP1) gene, and the RasGEF Domain Family Member 1B (RASGEF1B) gene. Further genetic and functional studies will need to be done to validate these findings.