Tau tubulin kinase 2 (TTBK2) is a ubiquitous serine-threonine protein kinase implicated in diverse cellular processes, including microtubule regulation, ciliogenesis, synaptic signaling, and the phosphorylation of key proteins like TDP-43. Despite its relevance, many aspects of TTBK2 function in both physiological and pathological conditions remain poorly understood. Truncating variants in TTBK2 gene cause spinocerebellar ataxia type 11 (SCA11), a rare form of autosomal dominant cerebellar ataxia. However, the functional consequences and pathogenic potential of missense variants have yet to be elucidated. In this study, we developed a CRISPR/Cas9 knock-in cell model harboring a missense variant in TTBK2 kinase domain (NM_173500.4:c.625 C > T; p.Leu209Phe) to evaluate its impact on TTBK2 expression, associated protein levels, and phosphoproteomic profiles. TTBK2 missense variant (TTBK2-L209F) was associated with reduced TTBK2 protein levels, altered levels of cytoskeleton-related proteins, and impaired kinase activity, namely toward TDP-43. Phosphoproteomic analyses identified dysregulation in pathways linked to gene regulation, protein degradation, cytoskeletal organization, and TGF-β signaling. These findings provide valuable insights into the biological roles of TTBK2 in cellular signaling. Moreover, this study underscores the importance of functional studies to better understand the consequences of TTBK2 missense variants, particularly those affecting the kinase domain, and their potential contribution to disease.

Spinocerebellar ataxia type 11 (SCA11) is a rare disease and the tau tubulin kinase 2 (TTBK2) gene was the causative gene. To date, only six SCA11 families have been reported. Here, we reported a Chinese SCA11 pedigree with cerebellar ataxia. Both patients in the family demonstrated typical clinical features of cerebellar ataxia and cerebellar atrophy on brain MRI. A novel heterozygous duplication mutation (c.1211_1217dupAGGAGAA) of the TTBK2 gene was identified in the proband using whole-exome sequencing (WES), which resulted in a frameshift mutation and formed a premature stop codon (p. N406Kfs*47). The mutation was detected in the proband's affected brother, and his unaffected mother, who with a lower percentage of the mutation and considered as an asymptomatic mutation carrier. Our study delineated the genotypic spectrum of SCA11.

Spinocerebellar ataxia type 11 (SCA11) is a rare type of autosomal dominant cerebellar ataxia, mainly characterized by progressive cerebellar ataxia, abnormal eye signs and dysarthria. SCA11 is caused by variants in TTBK2, which encodes tau tubulin kinase 2 (TTBK2) protein. Only a few families with SCA11 were described to date, all harbouring small deletions or insertions that result in frameshifts and truncated TTBK2 proteins. In addition, TTBK2 missense variants were also reported but they were either benign or still needed functional validation to ascertain their pathogenic potential in SCA11. The mechanisms behind cerebellar neurodegeneration mediated by TTBK2 pathogenic alleles are not clearly established. There is only one neuropathological report and a few functional studies in cell or animal models published to date. Moreover, it is still unclear whether the disease is caused by TTBK2 haploinsufficiency of by a dominant negative effect of TTBK2 truncated forms on the normal allele. Some studies point to a lack of kinase activity and mislocalization of mutated TTBK2, while others reported a disruption of normal TTBK2 function caused by SCA11 alleles, particularly during ciliogenesis. Although TTBK2 has a proven function in cilia formation, the phenotype caused by heterozygous TTBK2 truncating variants are not clearly typical of ciliopathies. Thus, other cellular mechanisms may explain the phenotype seen in SCA11. Neurotoxicity caused by impaired TTBK2 kinase activity against known neuronal targets, such as tau, TDP-43, neurotransmitter receptors or transporters, may contribute to neurodegeneration in SCA11.

Spinocerebellar ataxias, also called autosomal dominant cerebellar ataxias, are a group of neurological genetic diseases characterised by chronic, progressive cerebellar ataxia. The clinical hallmark of spinocerebellar ataxia is the loss of balance and coordination, accompanied by slurred speech. Spinocerebellar ataxia type 11 is a rare subtype of spinocerebellar ataxia caused by mutations in the tau tubulin kinase 2 gene. Patients with spinocerebellar ataxia are clinically characterised by slowly progressive cerebellar ataxia, trunk and limb ataxia, and eye movement abnormalities with occasional pyramidal features. Peripheral neuropathy and dystonia are rare. According to the literature, only nine families affected with spinocerebellar ataxia have been reported worldwide. Herein, a series of spinocerebellar ataxia cases are discussed in detail to determine the potential research direction of this dysfunction, including its epidemiology, clinical features, genetic characteristics, diagnosis and differential diagnosis, pathogenic mechanisms, treatment, prognosis, follow-up, genetic counselling and future perspectives, and to improve the overall understanding of spinocerebellar ataxia among clinicians, researchers and patients.

Frameshift mutations in Tau Tubulin Kinase 2 (TTBK2) cause spinocerebellar ataxia type 11 (SCA11), which is characterized by the progressive loss of Purkinje cells and cerebellar atrophy. Previous work showed that these TTBK2 variants generate truncated proteins that interfere with primary ciliary trafficking and with Sonic Hedgehog (SHH) signaling in mice. Nevertheless, the molecular mechanisms underlying the dominant interference of mutations remain unknown. Herein, we discover that SCA11-associated variants contain a bona fide peroxisomal targeting signal type 1. We find that their expression in RPE1 cells reduces peroxisome numbers within the cell and at the base of the cilia, disrupts peroxisome fission pathways, and impairs trafficking of ciliary SMO upon SHH signaling activation. This work uncovers a neomorphic function of SCA11-causing mutations and identifies requirements for both peroxisomes and cholesterol in trafficking of cilia-localized SHH signaling proteins. In addition, we postulate that molecular mechanisms underlying cellular dysfunction in SCA11 converge on the SHH signaling pathway.