Hypomorphic variants of SEL1L-HRD1 ER-associated degradation are associated with neurodevelopmental disorders

Recent studies using cell type-specific knockout mouse models have improved our understanding of the pathophysiological relevance of SEL1L-HRD1 endoplasmic reticulum (ER)-associated degradation (ERAD); however, its importance in humans remains unclear as no disease variant has been identified. Here we report the identification of three bi-allelic missense variants of SEL1L and HRD1 (or SYVN1) in six children from three independent families presenting with developmental delay, intellectual disability, microcephaly, facial dysmorphisms, hypotonia and/or ataxia. These SEL1L (p.Gly585Asp, p.Met528Arg) and HRD1 (p.Pro398Leu) variants were hypomorphic and impaired ERAD function at distinct steps of ERAD including substrate recruitment (SEL1L p.Gly585Asp), SEL1L-HRD1 complex formation (SEL1L p.Met528Arg), and HRD1 activity (HRD1 p.Pro398Leu). Our study not only provide new insights into the structure-function relationship of SEL1L-HRD1 ERAD, but also establish the importance of SEL1L-HRD1 ERAD in humans.