TNNT1

Three homologous genes have evolved in vertebrates, encoding three muscle type specific isoforms of TnT.[9] Each of the TnT isoform genes is linked to one of the three troponin I[11] isoform genes encoding the inhibitory subunit of the troponin complex, in chromosomal DNA to form three gene pairs: The fast skeletal muscle TnI (fsTnI)-fsTnT, ssTnI-cardiac (cTnT) and cTnI-ssTnT gene pairs. Sequence and epitope conservation studies suggested that genes encoding the muscle type specific TnT and TnI isoforms may have evolved from duplications of a fsTnI-like-fsTnT-like gene pair.[12] Evolutionary lineage of the three TnI-TnT gene pairs shows that cTnI-ssTnT is the newest[12] and most closely linked.[13]

Protein sequence alignment demonstrated that TNNT1 genes are highly conserved among vertebrate species (Fig. 2), especially in the middle and C-terminal regions, while the three muscle type isoforms are significantly diverged among vertebrate species.[8][9]

In mammalian and avian species, TNNT1 gene has a total of 14 exons, among which exon 5 encoding an 11-amino acid in the N-terminal region is alternatively spliced, generating a high molecular weight and a low molecular weight slow TnT splice forms (Jin, Chen et al. 1998).[14][15] Biochemical studies showed that TnT splice forms have detectable different molecular conformation in the middle and C-terminal regions, producing different binding affinities for TnI and tropomyosin.[8][9] The alternative splice forms of ssTnT play a role in skeletal muscle adaptation in physiologic and pathological conditions.[16] Alternative splicing at alternative acceptor sites of intron 5 generates a single amino acid difference in the N-terminal region of ssTnT,[15] of which functional significance has not been established.

A nonsense mutation E180X in the exon 11 of TNNT1 gene causes Amish Nemaline Myopathy (ANM), which is a severe form of recessive nemaline myopathy originally found in the Old Order Amish population in Pennsylvania, USA.[17][18] Truncation of the ssTnT polypeptide chain by the E180X mutation deletes the tropomyosin-binding site 2[19] as well as the binding sites for TnI and troponin C (TnC) in the C-terminal region (Fig. 3). Consistent with the recessive phenotype, the truncated ssTnT is incapable of incorporation into the myofilaments and completely degraded in muscle cells.[20]

Tnnt1 gene targeted mouse studies reproduced the myopathic phenotypes of ANM.[21][22] ssTnT null mice showed significantly decreased type I slow fibers in diaphragm and soleus muscles with hypertrophy of type II fast fibers, increased fatigability, and active regeneration of slow fibers (Wei, Lu et al. 2014).[21]

Recent case reports identified three more mutations in TNNT1 gene to cause nemaline myopathies outside the Amish population. A nonsense mutation S108X in exon 9 was identified in a Hispanic male patient with severe recessive nemaline myopathy phenotype.[23] A Dutch patient with compound heterozygous TNNT1 gene mutations that cause exon 8 and exon 14 deletions also presents nemaline myopathy phenotypes.[24] A rearrangement in TNNT1 gene (c.574_577 delins TAGTGCTGT) leading to aberrant splicing that causes C-terminal truncation of the protein (L203 truncation) was reported in 9 Palestinian patients from 7 unrelated families with recessively inherited nemaline Myopathy.[25]

Illustrated in Fig. 3, the S108X mutation truncates ssTnT protein to cause a loss of functional structures equivalent to that of E180X. The exon 8 deletion destructs the middle region tropomyosin-binding site 1.[19][26] The L203 truncation deletes the binding sites for TnI and TnC but preserves both tropomyosin-binding sites 1 and 2.[19] It remains to be investigated whether this novel mutation is able to bind the actin-tropomyosin thin filament in vivo and how it causes recessive nemaline myopathy.

Alternative splicing of exon 5 produces high and low molecular weight splice forms of ssTnT. The low molecular ssTnT was significantly upregulated in type 1 (demyelination) but not type 2 (axon loss) Charcot-Marie-Tooth disease,[16] suggesting that structural modification of TnT in the myofilaments may contribute to adaptation to abnormalities in neuronal activation.

TNNT1 has been shown to interact with PRKG1.[27]

[28] [29] [30]

• GeneReviews/NCBI/NIH/UW entry on Nemaline Myopathy