A Neurodevelopmental Disorder Caused by Mutations in the VPS51 Subunit of the GARP and EARP Complexes

Abstract

GARP and EARP are related heterotetrameric protein complexes that associate with the cytosolic face of the trans-Golgi network and recycling endosomes, respectively. At these locations, GARP and EARP function to promote the fusion of endosome-derived transport carriers with their corresponding compartments. GARP and EARP share three subunits, VPS51, VPS52 and VPS53, and each has an additional complex-specific subunit, VPS54 or VPS50, respectively. The role of these complexes in human physiology, however, remains poorly understood. By exome sequencing, we have identified compound heterozygous mutations in the gene encoding the shared GARP/EARP subunit VPS51 in a six-year-old patient with severe global developmental delay, microcephaly, hypotonia, epilepsy, cortical vision impairment, pontocerebellar abnormalities, failure to thrive, liver dysfunction, lower extremity edema and dysmorphic features. The mutation in one allele causes a frameshift that produces a longer but highly unstable protein that is degraded by the proteasome. In contrast, the other mutant allele produces a protein with a single amino-acid substitution that is stable but assembles less efficiently with the other GARP/EARP subunits. Consequently, skin fibroblasts from the patient have reduced levels of fully-assembled GARP and EARP complexes. Likely because of this deficiency, the patient's fibroblasts display altered distribution of the cation-independent mannose 6-phosphate receptor, which normally sorts acid hydrolases to lysosomes. Furthermore, a fraction of the patient's fibroblasts exhibit swelling of lysosomes. These findings thus identify a novel genetic locus for a neurodevelopmental disorder and highlight the critical importance of GARP/EARP function in cellular and organismal physiology.

The Parkinson’s Disease Protein LRRK2 Interacts with the GARP Complex to Promote Retrograde Transport to the trans-Golgi Network

“…It is worth noting that we observed enlarged lysosomes in the kidneys of our KO mice, which could be the result of nondegraded cargo due to impaired trafficking. This observation is in line with the known effect of GARP disruption in lysosomal morphology (Gershlick et al, 2019;Pé rez-Victoria et al, 2008). Dysregulation of lysosomal proteins in LRRK2 PD cases is not just limited to cathepsin D, as significant decreases in GBA levels have also been reported in postmortem brains of LRRK2 mutant cases (Zhao et al, 2018).…”

A systematic approach to identify recycling endocytic cargo depending on the GARP complex

“…Better protocols for the purification and lipidomic analysis of vacuoles will be crucial to answer these questions in the future. Understanding the lipid related phenotypes will be important to understand the effects of mutations in GARP subunits in human disease (Feinstein et al, 2014; Gershlick et al, 2018).…”

The Golgi-associated retrograde protein (GARP) complex plays an essential role in the maintenance of the Golgi glycosylation machinery

“…GARP depletion also results in alterations of autophagy (Pérez-Victoria et al, 2010b) (Dotiwala et al, 2013), anterograde transport of GPI-anchored and transmembrane proteins (Hirata et al, 2015), and sphingolipid homeostasis (Fröhlich et al, 2015). Furthermore, mutations in GARP subunits have been found to cause neurodevelopmental disorders in humans (Feinstein et al, 2014) (Hady-Cohen et al, 2018 (Gershlick et al, 2019) (Uwineza et al, 2019), and a mutation in VPS54 is the cause of progressive motor neuron death in the wobbler mouse, an animal model for Amyotrophic Lateral Sclerosis (ALS) (Schmitt-John et al, 2005). Inhibition of sphingolipid synthesis showed improvement in neuropathology and survival in mutant wobbler mice, suggesting that altered sphingolipid homeostasis contributes to the pathogenesis of GARP-deficiency disorders (Petit et al, 2020).…”