Early onset or syndromic epilepsy

Gene: TBC1D2B

Green List (high evidence)

TBC1D2B variants have been associated with Neurodevelopmental disorder with seizures and gingival overgrowth (OMIM:619323) and as definitive Gen2Phen gene for TBC1D2B-related neurodevelopmental disorder. So far, 11 TBC1D2B variants have been reported in 8 unrelated families. Global developmental delay (HP:0001263) was reported in 5/8 families, mental deterioration (HP:0001268) was seen in 5/8 families and seizures (HP:0001250) were reported in 8/8 families (four of these were controlled with medication)(PMID: 38374468).

Created: 11 Apr 2024, 11:22 a.m. | Last Modified: 11 Apr 2024, 11:22 a.m.

Panel Version: 4.191

The rating of this gene has been updated following NHS Genomic Medicine Service approval.

Created: 3 Mar 2022, 5:34 p.m. | Last Modified: 3 Mar 2022, 5:34 p.m.

Panel Version: 2.491

Comment on list classification: New gene added by Konstantinos Varvagiannis. Updating rating from Grey to Amber based on one publication (PMID:32623794).

There are sufficient cases to rate this gene Green at the next GMS panel update (added 'for-review' tag) - all 4 individuals (3 families) present seizures, although the age of onset is variable (19-years, 18-months, 3-months).

Created: 16 Nov 2020, 2:43 p.m. | Last Modified: 16 Nov 2020, 2:43 p.m.

Panel Version: 2.214

Green List (high evidence)

Harms et al (2020 - PMID: 32623794) report on 3 unrelated individuals with biallelic pLoF TBC1D2B variants.

Features included cognitive impairment (mild ID in one case, regression at the age of 12y in another, hypotonia and delayed milestones in a third aged 8m), seizures (3/3 - variable age of onset) and/or gingival overgrowth (2/3 - prior to initiation of AEDs). Other findings included behavioral abnormalities, mandibular anomalies, abnormal brain imaging and ophthalmologic or (rarely) audiometric evaluations.

All were born to non-consanguineous couples and additional investigations were performed in some.

Variants were identified by WES or trio WGS, with Sanger confirmation/compatible segregation analyses.

In line with the pLoF variants, mRNA studies in fibroblasts from 2 unrelated affected individuals demonstrated significantly reduced (~80-90%) TBC1C2D mRNA levels compared to controls, restored following cycloheximide treatment. Protein was absent in patient fibroblasts.

TBC-domain containing GTPase activating proteins are known as key regulators of RAB GTPase activity. TBC1D2B was shown to colocalize with RAB5-positive endocytic vesicles. CRISPR/Cas9-mediated ko of TBC1D2B in HeLa cells suggested a role in EGF receptor endocytosis and decreased cell viability of TBC1D2B-deficient HeLa cells upon serum deprivation.

Genes encoding other TBC domain-containg GTPase-activating proteins, e.g. TBC1D7 and TBC1D20, TBC1D24 are associated with recessive neurodevelopmental disorders (with ID and/or seizures) and the pathophysiological defect in TBC1D2B-related disorder (deficit in vesicle trafficking and/or cell survival) is proposed to be similar to that of TBC1D24.

Overall this gene can be considered for inclusion with amber/green rating in the ID panel and green in epilepsy panel.
Sources: Literature

Created: 13 Jul 2020, 7:04 p.m.

Mode of inheritance
BIALLELIC, autosomal or pseudoautosomal

Phenotypes
Global developmental delay; Intellectual disability; Seizures; Gingival overgrowth; Behavioral abnormality; Abnormality of the mandible; Abnormality of brain morphology; Abnormality of the eye; Hearing abnormality

Publications

• 32623794