Genetic epilepsy syndromesGene: TRAPPC4
Van Bergen et al. (2019 - PMID: 31794024) report on 7 affected individuals from 3 famillies (only 1 of which consanguineous), all homozygous for a TRAPPC4 splicing variant.
Overlapping features included feeding difficulties, progressive microcephaly, severe to profound developmental disability (7/7 - DD also prior to the onset of seizures / regression also reported in 3), epilepsy (7/7 - onset in the first year), spastic quadriparesis. Other findings in some/few incl. scoliosis, cortical visual and hearing impairment. Some facial features were shared (eg. bitemporal narrowing, long philtrum, open mouth with thin tented upper lip, pointed chin, etc). Brain imaging demonstrated abnormalities in those performed (among others cerebral with/without cerebellar atrophy).
Work-up prior to exome sequencing was normal (highly variable incl. metabolic testing, CMA, MECP2, CDKL5, mitochondrial depletion studies, etc).
Exome of affected individuals (and parents +/- affected sibs in some families) revealed a homozygous TRAPPC4 splicing variant [NM_016146.5:c.454+3A>G / chr11:g.118890966A>G (hg19)]. Sanger sequencing confirmed variant in affecteds, heterozygosity in parents and compatible genotypes with disease status in sibs/other members.
Families were of Caucasian/Turkish and French-Canadian ethnicities. SNP array to compare haplotypes between affecteds in 2 families did not reveal a shared haplotype (/founder effect) and the variant is present in gnomAD (68/281054 - no hmz) in many populations (European/Asian/African/Latino) [https://gnomad.broadinstitute.org/variant/11-118890966-A-G].
mRNA studies in fibroblasts from an affected individual confirmed the splicing defect (2 RT-PCR products corresponding to wt and a shorter due to skipping of exon 3, the latter further confirmed by Sanger sequencing. The shorter transcript is not present in controls). qPCR revealed that the normal transript in patient fibroblasts was present at 6% of the level observed in control fibroblasts (or 54% in the case of a heterozygote parent compared to controls).
Western blot in patient fibroblasts, revealed presence of full-length protein in significantly reduced levels compared to fibroblasts from carrier parents or controls. There was no band using an antibody targeting the N-terminal region of the protein prior to exon 3, suggesting that NMD applies (skipping of ex3 is also predicted to lead to frameshift).
TRAPPC4 encodes one of the core proteins of the TRAPP complex. Use of different accessory proteins leads to formation of 2 distinct complexes (TRAPPII / III). The complex has an important role in intracellular trafficking. Both TRAPPII & TRAPPIII have a function in the secretory pathway, while complex III has a role also in autophagy. Core proteins are important for the complex stability. The TRAPP complex serves as a GEF for Ypt/Rab GTPases [several refs in article].
Mutations in genes for other proteins of the complex lead to neurodevelopmental disorders with associated ID ('TRAPPopathies' used by the authors / TRAPPC12, C6B, C9 green in the current panel).
Western blot suggested that levels of other TRAPP subunits (TRAPPC2 or C12) under denaturing conditions, although PAGE/size exclusion chromatography suggested that the levels of fully-assembled TRAPP complexes were lower in affected individuals.
Studies in patient fibroblasts showed a secretory defect (between ER, Golgi and the plasma membrane) which was restored upon lentiviral transduction with wt TRAPPC4 construct. Basal and starvation-induced autophagy were also impaired in patient fibroblasts (increased LC3 marker and LC3-positive structures / impaired co-localization with lysosomes) partly due to defective autophagosome formation (/sealing).
TRAPPC4 is the human orthologue of the yeast Trs23. In a yeast model of reduced Trs23 (due to temperature instability) the authors demonstrated impaired assembly of the TRAPP core. The yeast model recapitulated the autophagy as well as well as the secretory defect observed in patient fibroblasts.
Created: 9 Dec 2019, 5:37 a.m.
Mode of inheritance
BIALLELIC, autosomal or pseudoautosomal
Feeding difficulties; Progressive microcephaly; Intellectual disability; Seizures; Spastic tetraparesis; Abnormality of the face; Scoliosis; Cortical visual impairment; Hearing impairment
gene: TRAPPC4 was added gene: TRAPPC4 was added to Genetic epilepsy syndromes. Sources: Literature Mode of inheritance for gene: TRAPPC4 was set to BIALLELIC, autosomal or pseudoautosomal Publications for gene: TRAPPC4 were set to 31794024 Phenotypes for gene: TRAPPC4 were set to Feeding difficulties; Progressive microcephaly; Intellectual disability; Seizures; Spastic tetraparesis; Abnormality of the face; Scoliosis; Cortical visual impairment; Hearing impairment Penetrance for gene: TRAPPC4 were set to Complete Review for gene: TRAPPC4 was set to GREEN