<i>EIPR1</i> variants cause a neurodevelopmental disorder with endolysosomal and dense core vesicle defects

EIPR1 (EARP-interacting protein 1, formerly known as TSSC1) is a WD40-domain protein that interacts with the EARP (endosome-associated recycling protein) and GARP (Golgi-associated retrograde protein) complexes in the process of delivering endosome-derived transmembrane cargos to the plasma membrane and the trans-Golgi network (TGN), respectively. Additionally, EIPR1 cooperates with EARP in the biogenesis of dense core vesicles. While these properties of EIPR1 were established in cultured cells and model organisms, the physiological and pathological importance of EIPR1 in humans remains to be determined. Here we report the identification of five EIPR1 homozygous missense variants [NM_003310.5:c.835C>G p.(Arg279Gly), NM_003310.5:c.813C>G p.(His271Gln), NM_003310.5:c.694C>T p.(Arg232Trp), NM_003310.5:c.47G>A p.(Arg16His) and NM_003310.5:c.419T>A p.(Val140Asp)] in eight individuals from six unrelated families with a neurological disorder featuring a spectrum of global neurodevelopmental delay, microcephaly, ataxia, spasticity, delayed myelination, callosal hypoplasia, cerebellar atrophy, walking and speech impairments, dysmorphic facies, and neutropenia. Cellular studies using a heterologous transfection system demonstrate that these variants reduce EIPR1 protein levels and its physical interaction with EARP and GARP complexes. Furthermore, we show that the Arg279Gly and His271Gln variants reduce the ability of EIPR1 to promote EARP association with endosomes in non-neuronal cells and dense core vesicle biogenesis in iPSC-derived neurons. Additionally, skin fibroblasts from one of the Arg279Gly affected individuals shows reduced recycling of internalized transferrin to the plasma membrane (an EARP-deficiency phenotype) and impaired retrograde transport of internalized Shiga toxin B-subunit to the TGN (a GARP-deficiency phenotype) compared to fibroblasts from an unaffected parent. Moreover, these patient fibroblasts exhibit enlarged lysosomes, increased levels of the lysosomal membrane protein LAMP1, and increased levels of the autophagic markers LC3B-II and SQSTM1, all phenotypes previously associated with GARP deficiency. Knockout of the orthologous eipr1 in zebrafish results in neurodevelopmental and locomotor defects consistent with the clinical phenotype of the human patients. Injection of WT human EIPR1 mRNA into eipr1 KO zebrafish rescues these defects, whereas mRNAs encoding the human EIPR1 variants Arg279Gly or His271Gln fail to do so, confirming the impaired activity of these variants. These findings identify EIPR1 as a novel genetic locus associated with a neurodevelopmental disorder and underscore its critical role in endosomal recycling and dense core vesicle biogenesis, processes essential for the development and function of the nervous system.