A novel missense mutation of TNNI2 in a Chinese family cause distal arthrogryposis type 1

Distal arthrogryposis in a girl arising from a novel TNNI2 variant inherited from paternal somatic mosaicism

TNNI2 at 11p15.5 encodes troponin I2, fast skeletal type, which is a member of the troponin I gene family and a component of the troponin complex. Distal arthrogryposis (DA) is characterized by congenital limb contractures without primary neurological or muscular effects. DA is inherited in an autosomal dominant fashion and is clinically and genetically heterogeneous. Exome sequencing identified a causative variant in TNNI2 [NM_003282.4:c.532T>C p.(Phe178Leu)] in a Japanese girl with typical DA2b. Interestingly, the familial study using Sanger sequencing suggested a mosaic variant in her healthy father. Subsequent targeted amplicon-based deep sequencing detected the TNNI2 variant with variant allele frequencies of 9.4-17.7% in genomic DNA derived from peripheral blood leukocytes, saliva, hair, and nails in the father. We confirmed a disease-causing variant in TNNI2 in the proband inherited from her asymptomatic father with its somatic variant. Our case demonstrates that careful clinical and genetic evaluation is required in DA.

A TNNI2 variant c.525G>T causes distal arthrogryposis in a Chinese family

BACKGROUND

Distal arthrogryposis (DA) is a group of congenital autosomal-dominant disorders secondary to defects in joint and muscle function, characterized by multiple joint contractures of the hands and feet. DA can be divided into 10 types according to clinical features. DA has been confirmed to be caused by mutations in genes encoding components of the contractile apparatus of skeletal muscle fibers, such as troponin I2 (TNNI2).

METHODS

In this study, we report a three-generation DA family belonging to the DA2B type. The clinical characteristics of affected members are genetically stable and consistent, with severe deformities in hands and feet, and two affected adults had short stature. None exhibited facial abnormalities. Blood from three affected and three healthy members were collected for whole-exome sequencing and Sanger sequencing.

RESULTS

A missense variant in TNNI2 (NM_003282.4: c.525G>T: p.K175N) was successfully identified, which resulted in the substitution of amino acid at position 175 of TNNI2 from lysine to asparagines.

CONCLUSION

The variant c.525G>T in TNNI2 explains the cause of DA in the family. This variant was identified in Chinese people for the first time, and the same variant had been reported in another study but no description of clinical symptoms. Our study comprehensively characterized the c.525G>T variant in TNNI2.

The distal arthrogryposis-linked p.R63C variant promotes the stability and nuclear accumulation of TNNT3

Background

Distal arthrogryposis (DA) is comprised of a group of rare developmental disorders in muscle, characterized by multiple congenital contractures of the distal limbs. Fast skeletal muscle troponin‐T (TNNT3) protein is abundantly expressed in skeletal muscle and plays an important role in DA. Missense variants in TNNT3 are associated with DA, but few studies have fully clarified its pathogenic role.

Methods

Sanger sequencing was performed in three generation of a Chinese family with DA. To determine how the p.R63C variant contributed to DA, we identified a variant in TNNT3 (NM_006757.4): c.187C>T (p.R63C). And then we investigated the effects of the arginine to cysteine substitution on the distribution pattern and the half‐life of TNNT3 protein.

Results

The protein levels of TNNT3 in affected family members were 0.8‐fold higher than that without the disorder. TNNT3 protein could be degraded by the ubiquitin‐proteasome complex, and the p.R63C variant did not change TNNT3 nuclear localization, but significantly prolonged its half‐life from 2.5 to 7 h, to promote its accumulation in the nucleus.

Conclusion

The p.R63C variant increased the stability of TNNT3 and promoted nuclear accumulation, which suggested its role in DA.

Distal Arthrogryposis and Lethal Congenital Contracture Syndrome - An Overview

Distal arthrogryposis (DA) is a skeletal muscle disorder which can be classified under a broader term as Arthrogryposis multiplex contractures. DA is characterized by the presence of joint contractures at various parts of the body, particularly in distal extremities. It is identified as an autosomal dominant and a rare X-linked recessive disorder associated with increased connective tissue formation around joints in such way that immobilizes muscle movement causing deformities. DA is again classified into various types since it manifests as a range of conditions representing different etiologies. Myopathy is one of the most commonly listed etiologies of DA. The mutations in sarcomeric protein-encoding genes lead to decreased sarcomere integrity, which is often associated with this disorder. Also, skeletal disorders are often associated with cardiac disorders. Some studies mention the presence of cardiomyopathy in patients with skeletal dysfunction. Therefore, it is hypothesized that the congenitally mutated protein that causes DA can also lead to cardiomyopathy. In this review, we will summarize the different forms of DA and their clinical features, along with gene mutations responsible for causing DA in its different forms. We will also examine reports that list mutations also known to cause heart disorders in the presence of DA.

Exome sequencing reveals a novel MFN2 missense mutation in a Chinese family with Charcot-Marie-Tooth type 2A

Charcot-Marie-Tooth (CMT) is a group of inherited peripheral neuropathies. To date, mutations in >80 genes are reportedly associated with CMT. Protein mitofusin 2 encoded by MFN2 serves an essential role in mitochondrial fusion and regulation of apoptosis, which has previously been reported to be highly associated with an axonal form of neuropathy (CMT2A). In the present study, a large Chinese family with severe CMT was reported and a genetic analysis of the disease was performed. A detailed physical examination for CMT was performed in 13 family members and electrophysiological examinations were performed in 3 affected family members. Whole-exome sequencing was performed on the proband, and the suspected variants were identified by Sanger sequencing. The pathogenicity of mutation was verified by restriction fragment length polymorphism analysis in the family followed by a bioinformatics analysis. A novel c.1190G>C; p.(R397P) mutation in the MFN2 gene was identified in the proband, and co-segregated between genotype and phenotype in the family. The substituted amino acid changed the hydrophobicity and charge characteristics of the mitofusin 2 coiled-coiled domain; thus it may affect its biological function. In summary, a novel pathogenic mutation was identified in a Chinese family with CMT, which expands the phenotypic and mutational spectrum of CMT2A, and provides evidence for prenatal interventions and more precise pharmacological treatments to this family.

A MYH3 mutation identified for the first time in a Chinese family with Sheldon-Hall syndrome (DA2B)

Sheldon-Hall syndrome is the most common type of distal arthrogryposis syndromes, also known as distal arthrogryposis 2B (DA2B). Sheldon-Hall syndrome is caused by mutations in the TPM2, TNNI2, TNNT3 or MYH3 gene and characterized by ulnar deviation, camptodactyly, overlapping fingers and scoliosis from birth. We investigated a Chinese family with multiple members who clinically presented with distal arthrogryposis of the hands. In total, 261 subjects including one proband and ten family members from the non-consanguineous Chinese family and 250 healthy volunteers were included and had their genomic DNA extracted. A novel missense mutation in exon 13 of the MYH3 gene, c.1160A > G (p.Tyr387Cys), was identified in the proband and his father through whole-exome sequencing. The proband and six affected family members were confirmed to carry this mutation by Sanger sequencing, although the mutation was not detected in the four unaffected individuals or 250 volunteers. This is the first report of a novel MYH3 mutation being identified as the cause of DA2B in a Chinese family. Our findings confirm that MYH3 gene mutations can be a pathogenic cause of DA2B in Asian patients. This study increases the mutational spectrum in MYH3 and aids genetic counseling and prenatal diagnosis.

Distal arthrogryposis with variable clinical expression caused by TNNI2 mutation

Distal arthrogryposis (DA) is a clinically and genetically heterogeneous disorder with multiple joint contractures. We describe a female DA patient with hand and foot deformities, and right-sided torticollis. Using exome sequencing, we identified a novel TNNI2 mutation (c.485>A, p.Arg162Lys) in the patient and her father. The father has no typical DA but hip dysplasia. This may explain the clinical features of DA2B in this family, but with variable clinical expression.