Myopathic changes associated with psychomotor delay and seizures caused by a novel homozygous mutation in TBCK

Heterozygous variants in TBCK cause a mild neurologic syndrome in humans and mice

TBCK-related encephalopathy is a rare pediatric neurodegenerative disorder caused by biallelic loss-of-function variants in the TBCK gene. After receiving anecdotal reports of neurologic phenotypes in both human and mouse TBCK heterozygotes, we quantified if TBCK haploinsufficiency causes a phenotype in mice and humans. Using the tbck+/- mouse model, we performed a battery of behavioral assays and mTOR pathway analysis to investigate potential alterations in neurophysiology. We conducted as well a phenome-wide association study (PheWAS) analysis in a large adult biobank to determine the presence of potential phenotypes associated to this variant. The tbck+/- mouse model demonstrates a reduction of exploratory behavior in animals with significant sex and genotype interactions. The concurrent PheWAS analysis of 10,900 unrelated individuals showed that patients with one copy of a TBCK loss-of-function allele had a significantly higher rate of acquired toe and foot deformities, likely indicative of a mild peripheral neuropathy phenotype. This study presents an example of what may be the underappreciated occurrence of mild neurogenic symptoms in heterozygote individuals of recessive neurogenetic syndromes.

Expanding the phenotype of children presenting with hypoventilation with biallelic TBCK pathogenic variants and literature review

Individuals with biallelic TBCK pathogenic variants present in infancy with distinctive facial features, profound hypotonia, severe intellectual impairment and epilepsy. Although rare, it may mimic other neurogenetic disorders leading to extensive investigations. Improved understanding of the clinical phenotype can support early monitoring of complications due to respiratory insufficiency. We present six individuals who were found to have pathogenic biallelic TBCK variants. The clinico-radiological and diagnostic records were reviewed. Five individuals were diagnosed with hypoventilation, requiring respiratory support, highlighting the need for early respiratory surveillance. Characteristic brain imaging in our cohort included periventricular leukomalacia-like changes. We recommend screening for TBCK in hypotonic children with periventricular leukomalacia-like changes, particularly in the absence of prematurity.

Multiple functions of TBCK protein in neurodevelopment disorders and tumors

TBC1 domain containing kinase (TBCK) protein is composed of three conserved domains, including N-terminal Serine/Threonine kinase domain, central TBC domain and C-terminal rhodanese homology domain (RHOD). A total of 9 different transcripts (classified as long and short TBCK) generated by alternative splicing have been reported in different cell lines. Exogenous expression of long TBCK has been identified to function as a suppressor of cell growth in certain cell types. On the contrary, TBCK has also been reported to serve a tumor-promoting role in other cell lines, indicating that TBCK might function differentially, depending on the context in different cellular environments. Furthermore, deleterious homozygous or compound heterozygous mutations identified by whole-exome sequencing in the TBCK gene could ablate the function of TBCK, further impacting the mTOR signaling pathway and leading to neurogenetic disorders, such as hypotonia, global developmental delay, facial dysmorphic features and brain abnormalities. However, as a poorly explored protein, there are a lot of studies associated with the functions of TBCK that need to be performed in the future. The present review summarizes data regarding the structural features and potential roles of TBCK in developmental and neurological diseases and tumorigenesis. Future prospects of TBCK research lie in revealing numerous biological functions of TBCK.