TRAPPC6B biallelic variants cause a neurodevelopmental disorder with TRAPP II and trafficking disruptions.

Highly conserved transport protein particle (TRAPP) complexes regulate subcellular trafficking pathways. Accurate protein trafficking has been increasingly recognized to be critically important for normal development, particularly in the nervous system. Variants in most TRAPP complex subunits have been found to lead to neurodevelopmental disorders with diverse but overlapping phenotypes. We expand on limited prior reports on TRAPPC6B with detailed clinical and neuroradiologic assessments, and studies on mechanisms of disease, and new types of variants. We describe 29 additional patients from 18 independent families with biallelic variants in TRAPPC6B. We identified seven homozygous nonsense (n = 12 patients) and eight canonical splice-site variants (n = 17 patients). In addition, we identified one patient with compound heterozygous splice-site/missense variants with a milder phenotype and one patient with homozygous missense variants. Patients displayed non-progressive microcephaly, global developmental delay/intellectual disability, epilepsy and absent expressive language. Movement disorders including stereotypies, spasticity and dystonia were also observed. Brain imaging revealed reductions in cortex, cerebellum and corpus callosum size with frequent white matter hyperintensity. Volumetric measurements indicated globally diminished volume rather than specific regional losses. We identified a reduced rate of trafficking into the Golgi apparatus and Golgi fragmentation in patient-derived fibroblasts that was rescued by wild-type TRAPPC6B. Molecular studies revealed a weakened interaction between mutant TRAPPC6B (c.454C>T, p.Q152*) and its TRAPP binding partner TRAPPC3. Patient-derived fibroblasts from the TRAPPC6B (c.454C>T, p.Q152*) variant displayed reduced levels of TRAPPC6B as well as other TRAPP II complex-specific members (TRAPPC9 and TRAPPC10). Interestingly, the levels of the TRAPPC6B homologue TRAPPC6A were found to be elevated. Moreover, co-immunoprecipitation experiments showed that TRAPPC6A co-precipitates equally with TRAPP II and TRAPP III, while TRAPPC6B co-precipitates significantly more with TRAPP II, suggesting enrichment of the protein in the TRAPP II complex. This implies that variants in TRAPPC6B may preferentially affect TRAPP II functions compared to TRAPP III functions. Finally, we assessed phenotypes in a Drosophila TRAPPC6B-deficiency model. Neuronal TRAPPC6B knockdown impaired locomotion and led to wing posture defects, supporting a role for TRAPPC6B in neuromotor function. Our findings confirm the association of damaging biallelic TRAPPC6B variants with microcephaly, intellectual disability, language impairments, and epilepsy. A subset of patients also exhibited dystonia and/or spasticity with impaired ambulation. These features overlap with disorders arising from pathogenic variants in other TRAPP subunits, particularly components of the TRAPP II complex. These findings suggest that TRAPPC6B is essential for brain development and function, and TRAPP II complex activity may be particularly relevant for mediating this function.

Further Delineation of the TRAPPC6B Disorder: Report on a New Family and Review.

Pathogenic variants in the TRAPPC6B gene were recently found to be associated in three consanguineous families, with microcephaly, epilepsy, and brain malformations. Here, we report on a 3.5-year-old boy, born to consanguineous Lebanese parents, who presented with developmental delay, lactic acidosis, postnatal microcephaly, and abnormal brain magnetic resonance imaging. By whole exome sequencing, a novel homozygous likely pathogenic variant in exon 1 of the TRAPPC6B gene (c.23T > A; [p.Leu8*]) was identified. A review of the clinical description and literature is discussed, pointing out the phenotypic heterogeneity associated with mutations in this gene.

A homozygous founder mutation in TRAPPC6B associates with a neurodevelopmental disorder characterised by microcephaly, epilepsy and autistic features.

BACKGROUND: Transport protein particle (TRAPP) is a multisubunit complex that regulates membrane trafficking through the Golgi apparatus. The clinical phenotype associated with mutations in various TRAPP subunits has allowed elucidation of their functions in specific tissues. The role of some subunits in human disease, however, has not been fully established, and their functions remain uncertain. OBJECTIVE: We aimed to expand the range of neurodevelopmental disorders associated with mutations in TRAPP subunits by exome sequencing of consanguineous families. METHODS: Linkage and homozygosity mapping and candidate gene analysis were used to identify homozygous mutations in families. Patient fibroblasts were used to study splicing defect and zebrafish to model the disease. RESULTS: We identified six individuals from three unrelated families with a founder homozygous splice mutation in TRAPPC6B, encoding a core subunit of the complex TRAPP I. Patients manifested a neurodevelopmental disorder characterised by microcephaly, epilepsy and autistic features, and showed splicing defect. Zebrafish trappc6b morphants replicated the human phenotype, displaying decreased head size and neuronal hyperexcitability, leading to a lower seizure threshold. CONCLUSION: This study provides clinical and functional evidence of the role of TRAPPC6B in brain development and function.

Trs33-Containing TRAPP IV: A Novel Autophagy-Specific Ypt1 GEF.

Ypt/Rab GTPases, key regulators of intracellular trafficking pathways, are activated by guanine-nucleotide exchange factors (GEFs). Here, we identify a novel GEF complex, TRAPP IV, which regulates Ypt1-mediated autophagy. In the yeast Saccharomyces cerevisiae, Ypt1 GTPase is required for the initiation of secretion and autophagy, suggesting that it regulates these two distinct pathways. However, whether these pathways are coordinated by Ypt1 and by what mechanism is still unknown. TRAPP is a conserved modular complex that acts as a Ypt/Rab GEF. Two different TRAPP complexes, TRAPP I and the Trs85-containing TRAPP III, activate Ypt1 in the secretory and autophagic pathways, respectively. Importantly, whereas TRAPP I depletion copies Ypt1 deficiency in secretion, depletion of TRAPP III does not fully copy the autophagy phenotypes of autophagy-specific ypt1 mutations. If GEFs are required for Ypt/Rab function, this discrepancy implies the existence of an additional GEF that activates Ypt1 in autophagy. Trs33, a nonessential TRAPP subunit, was assigned to TRAPP I without functional evidence. We show that in the absence of Trs85, Trs33 is required for Ypt1-mediated autophagy and for the recruitment of core-TRAPP and Ypt1 to the preautophagosomal structure, which marks the onset of autophagy. In addition, Trs33 and Trs85 assemble into distinct TRAPP complexes, and we term the Trs33-containing autophagy-specific complex TRAPP IV. Because TRAPP I is required for Ypt1-mediated secretion, and either TRAPP III or TRAPP IV is required for Ypt1-mediated autophagy, we propose that pathway-specific GEFs activate Ypt1 in secretion and autophagy.