TGM6 L517W is not a pathogenic variant for spinocerebellar ataxia type 35

The Evolution of Transglutaminases Underlies the Origin and Loss of Cornified Skin Appendages in Vertebrates

Transglutaminases (TGMs) cross-link proteins by introducing covalent bonds between glutamine and lysine residues. These cross-links are essential for epithelial cornification which enables tetrapods to live on land. Here, we investigated which evolutionary adaptations of vertebrates were associated with specific changes in the family of TGM genes. We determined the catalog of TGMs in the main clades of vertebrates, performed a comprehensive phylogenetic analysis of TGMs, and localized the distribution of selected TGMs in tissues. Our data suggest that TGM1 is the phylogenetically oldest epithelial TGM, with orthologs being expressed in the cornified teeth of the lamprey, a basal vertebrate. Gene duplications led to the origin of TGM10 in stem vertebrates, the origin of TGM2 in jawed vertebrates, and an increasing number of epithelium-associated TGM genes in the lineage leading to terrestrial vertebrates. TGM9 is expressed in the epithelial egg tooth, and its evolutionary origin in stem amniotes coincided with the evolution of embryonic development in eggs that are surrounded by a protective shell. Conversely, viviparous mammals have lost both the epithelial egg tooth and TGM9. TGM3 and TGM6 evolved as regulators of cornification in hair follicles and underwent pseudogenization upon the evolutionary loss of hair in cetaceans. Taken together, this study reveals the gain and loss of vertebrate TGM genes in association with the evolution of cornified skin appendages and suggests an important role of TGM9 in the evolution of amniotes.

Clinical Spectrum of TGM6-Related Movement Disorders: A New Report with a Pooled Analysis of 48 Patients

Background  Spinocerebellar ataxias (SCAs) are a diverse group of progressive neurodegenerative disorders. Until now, more than 20 genes have been implicated to be associated with this phenotype and TGM6 is one of these genes, associated with spinocerebellar ataxia-35 (SCA-35). The majority of disease-causing variants in the TGM6 gene predominantly have been reported from China and Taiwan and the association with Parkinson's disease (PD) have also been reported recently.

Methods  We report the first Indian case with SCA-35 in a 16-year-old-boy with atypical age of onset at 9 years, prominent extrapyramidal features, intellectual disability, and a novel missense mutation in the TGM6 gene. We also reviewed and collated all previously published cases with pathogenic TGM6 variants.

Results  Including the index case, 54 cases were identified from 10 relevant articles in literature and 48 cases had adequate clinical details to be included in the pooled analysis. Around two-thirds of reported cases had SCA-35 phenotype, with cerebellar atrophy. Onset in the majority of cases was the fourth decade of life onwards. A proportion of SCA-35 cases also had spasmodic torticollis, impaired proprioception, extrapyramidal features, and myoclonic jerks. The patients with PD had often early-onset milder symptoms, slower progression, and favorable response to levodopa/carbidopa. One patient each presented with episodic ataxia and dystonic tremor of the upper limb. Most of the cases had missense mutations, without any definite hotspot or genotype–phenotype correlation.

Conclusions  TGM6 mutation should be suspected in patients with SCA like presentation, especially when it is accompanied by extrapyramidal features, spasmodic torticollis, impaired proprioception, or myoclonus.

TGM6 might not be a specific causative gene for spinocerebellar ataxia resulting from genetic analysis and functional study

OBJECTIVE

To investigate whether TGM6 is a specific causative gene for spinocerebellar ataxia type 35 (SCA35).

MATERIALS AND METHODS

The next-generation sequencing (NGS) data consisted of 47 SCA, 762 non-SCA patients and 2827 normal controls were analyzed. The allele frequencies of low frequent and deleterious TGM6 variants were compared. Functional studies were performed in five widely distributed variants (V314M, R342Q, P347L, V391M, L517W).

RESULTS

Two TGM6 detrimental variants were identified in one SCA patient, 14 in non-SCA patients and 43 in normal controls, the allele frequencies of TGM6 variants did not differ among the SCA and other controls. Seven reported pathogenic variants (c.7 + 1G > T, c.331C > T, c.1171G > A, c.1478C > T, c.1528G > C, c.1550 T > G and c.1722_1724delAGA) were identified in patients with various neurologic diseases or normal controls. All the 5 widely distributed variants led to destabilization and significantly reduction of enzymatic activity of TG6 as the reported pathogenic mutations.

CONCLUSIONS

TGM6 might not be a specific causative gene for SCA35, the relevant clinical consult or diagnostic should be pay more attention.