Mechanisms Underlying the Pathogenesis of SEL1L-HRD1 ERAD Disease Variants - Mechanisms Underlying the Pathogenesis of SEL1L-HRD1 ERAD Disease Variants ABSTRACT Our laboratories are interested in the physiological role of endoplasmic reticulum (ER)-associated protein degradation (ERAD), a principal mechanism for the recruitment and retro-translocation of misfolded ER proteins for cytosolic proteasomal degradation. The SEL1L-HRD1 protein complex represents the most conserved branch of ERAD, where single-span transmembrane protein SEL1L functions as an obligatory cofactor for the multi-span transmembrane protein E3 ligase HRD1. Using mouse models, recent evidence has demonstrated pivotal roles of SEL1L-HRD1 ERAD in many physiological processes such as energy metabolism, water balance and innate immunity; however, no disease mutant of SEL1L or SYVN1 (encoding HRD1 protein) has been identified in humans. In collaboration with three clinical geneticists in Saudi Arabia, France and Italy, we have identified three bi-allelic missense point mutations in SEL1L (G585D, M528R) and SYVN1 (P398L) in three families, all of whom exhibit similar developmental and neurological defects, including global developmental delay and severe intellectual disability. However, the causality of these novel SEL1L/SYVN1 variants and their underlying mechanisms remain largely unclear. In the preliminary data of this application, we found that a mutation of SEL1L at Serine 658 to Proline (S658P) identified in Finnish hound with early onset cerebellar ataxia caused early-onset ataxia in mice due to the disruption of the interaction between SEL1L and HRD1. Furthermore, human SEL1L G585D variant attenuates the interaction between SEL1L and substrates. We hypothesize that these variants are hypomorphic disease-causing mutations in humans by attenuating SEL1L-HRD1 ERAD function. Using knock-in mouse and cell models powered by state-of-art imaging and high throughput techniques, we will accomplish the following two Aims: (1) Determine the causality and pathophysiological significance of human SEL1L and SYVN1 variants in vivo; and (2) Delineate the molecular mechanism underlying the pathogenicity associated with human SEL1L and SYVN1 variants. This study will not only transform our understanding of the importance of SEL1L-HRD1 ERAD in humans, but provide future framework for targeting SEL1-HRD1 ERAD therapeutically in humans. RELEVANCE TO HUMAN HEALTH: This study represents the first attempt to identify and establish disease- causing mutations of SEL1L-HRD1 ERAD in humans. It will significantly advance our understanding of the importance of SEL1L-HRD1 ERAD in health and disease.
