ClinVar Genomic variation as it relates to human health

This recessive variant was identified in two brothers diagnosed with profound bilateral hearing loss. Both patients harbour also a second variant (see below) in this gene in compound heterozygosity

The p.R214H variant (also known as c.641G>A), located in coding exon 1 of the TBC1D24 gene, results from a G to A substitution at nucleotide position 641. The arginine at codon 214 is replaced by histidine, an amino acid with highly similar properties. This variant was identified with another TBC1D24 variant in three families with non-syndromic hearing loss; the variant was suggested to act as a hypomorph (Bakhchane A et al. PLoS ONE, 2015 Sep;10:e0138072; Rehman AU et al. Oral Dis, 2017 Jul;23:551-558). This variant was also detected in a patient with neonatal seizures and pervasive developmental disorder; however, the patient also had a de novo KCNQ2 mutation (Della Mina E et al. Eur. J. Hum. Genet., 2015 Mar;23:354-62). This amino acid position is highly conserved in available vertebrate species. In addition, this alteration is predicted to be tolerated by in silico analysis. Since supporting evidence is limited at this time, the clinical significance of this variant remains unclear.

Identified with a second TBC1D24 variant on the opposite allele (in trans) in siblings with nonsyndromic hearing loss in published literature; authors propose that the p.(R214H) variant may be hypomorphic, resulting in a milder phenotype (PMID: 26371875); In silico analysis indicates that this missense variant does not alter protein structure/function; This variant is associated with the following publications: (PMID: 27259978, 22277662, 24848745, 29741288, 32987832, 28428906, 34426522, 32860223, 29358611, 28301460, 27281533, 34440452, 35580552, 26371875, 36374051, 33986365, 36515421, 36964972, 37811145)

This variant was observed as part of a predisposition screen in an ostensibly healthy population. A literature search was performed for the gene, cDNA change, and amino acid change (where applicable). Publications were found based on this search. The evidence from the literature, in combination with allele frequency data from public databases where available, was sufficient to rule this variant out of causing disease. Therefore, this variant is classified as benign.

The p.Arg214His variant in TBC1D24 is classified as likely benign due to its high allele frequency. Although it has been reported in trans with a likely pathogenic variant in two Morroccan probands with hearing loss (2/272 alleles), it was also identified at a similar frequency in a race matched control population (1%, 4/400 chromosomes; Bakhchane 2015). Furthermore, this variant has been reported in several populations by gnomAD with a total allele frequency of 0.1% (271/280420) and an allele frequency of 0.19% (58/30602) of South Asian chromosomes including 1 homozygote. ACMG/AMP criteria: BS1.

TBC1D24: BP4, BS1

Based on the classification scheme VCGS_Germline_v1.1.1, this variant is classified as likely pathogenic. Following criteria are met: 0102 - Loss-of-function is a known mechanism of disease for this gene (PMID: 27281533). (N) 0108 - This gene is known to be associated with both recessive and dominant disease. Only a single missense has been reported to cause autosomal dominant disease (OMIM). (N) 0200 - Variant is predicted to result in a missense amino acid change from arginine to histidine (exon 2). (N) 0251 - Variant is heterozygous. (N) 0304 - Variant is present in gnomAD <0.01 for a recessive condition (269 heterozygotes, 1 homozygote). (P) 0309 - An alternative amino acid change at the same position has been observed in gnomAD (3 heterozygotes, 0 homozygotes). (N) 0502 - Missense variant with conflicting in silico predictions and/or uninformative conservation. (N) 0600 - Variant is located in an annotated domain or motif, (Rab-GTPase-TBC domain; PDB, NCBI). (N) 0705 - A comparable variant (p.Arg214Cys) has been previous reported as a VUS (ClinVar). (N) 0808 - Previous reports of pathogenicity are conflicting. This variant has been reported as a VUS, likely benign and likely pathogenic (ClinVar, LOVD). All reports of this variant being benign are in relation to epilepsy studies (PMID: 24848745, PMID: 29358611), however, this variant has been called a hypomorphic allele, and disease causing within deafness patients (PMID: 26371875, PMID: 28951997). (N) 0901 - Strong evidence for segregation with disease. This variant has been shown to segregate in two families with hearing loss (PMID: 26371875). (P) 1007 - No published functional evidence has been identified for this variant. (N) 1201 - Heterozygous variant detected in trans with a second (at least likely) pathogenic heterozygous variant in a recessive disease (IGV). (P) 1208 - Inheritance information for this variant is not currently available. (N) Legend: (P) - Pathogenic, (N) - Neutral, (B) - Benign