Mutations in fast skeletal troponin I, troponin T, and β‐tropomyosin that cause distal arthrogryposis all increase contractile function

Abstract

Distal arthrogryposes (DAs) are a group of disorders characterized by congenital contractures of distal limbs without overt neurological or muscle disease. Unexpectedly, mutations in genes encoding the fast skeletal muscle regulatory proteins troponin T (TnT), troponin I (TnI), and beta-tropomyosin (beta-TM) have been shown to cause autosomal dominant DA. We tested how these mutations affect contractile function by comparing wild-type (WT) and mutant proteins in actomyosin ATPase assays and in troponin-replaced rabbit psoas fibers. We have analyzed all four reported mutants: Arg63His TnT, Arg91Gly beta-TM, Arg174Gln TnI, and a TnI truncation mutant (Arg156ter). Thin filaments, reconstituted using actin and WT troponin and beta-TM, activated myosin subfragment-1 ATPase in a calcium-dependent, cooperative manner. Thin filaments containing either a troponin or beta-TM DA mutant produced significantly enhanced ATPase rates at all calcium concentrations without alternating calcium-sensitivity or cooperativity. In troponin-exchanged skinned fibers, each mutant caused a significant increase in Ca2+ sensitivity, and Arg156ter TnI generated significantly higher maximum force. Arg91Gly beta-TM was found to have a lower actin affinity than WT and form a less stable coiled coil. We propose the mutations cause increased contractility of developing fast-twitch skeletal muscles, thus causing muscle contractures and the development of the observed limb deformities.

Gain-of-function mutations in the mechanically activated ion channel PIEZO2 cause a subtype of Distal Arthrogryposis

“…This could suggest that these specific DA mutations cause congenital contractures on the basis of hypercontractility and increased tension in developing fast twitch skeletal muscles, resulting in diminished muscle movement in utero. The possibility that PIEZO2 senses forces associated with muscle contraction, combined with the finding that mutant PIEZO2 proteins have increased activity, is likewise consistent with the view that the integrated development of muscles and joints during embryogenesis is an active rather than a passive process (13). For example, increased ion flux could result in increased neural firing during fetal muscle movement, and this in turn could inhibit overall muscle movement via a negative feedback loop that integrates muscle contraction and movement.…”

“…Interestingly, previously identified DA mutations reside in MYH3 and MYH8 that encode sarcomeric proteins, or in TNNI2, TNNT3 and TPM2, respectively (2,(12)(13)(14). These proteins are expressed in fast twitch fibers, and functional studies have shown that DA (DA1 and DA2) mutations in TNNI2, TNNT3, and TPM2 produce increased contractile function in troponinreplaced rabbit psoas fibers (13).…”

“…Recently, loss-of-function mutations in endothelin converting enzyme-like 1 (ECEL1) have been shown to be responsible for a specific, recessively inherited subtype of DA (10,11). Meanwhile, other types of DAs have been shown to involve mutations in the myosin heavy chain genes MYH3 and MYH8, in the myosin-binding protein C gene, MYBPC1, and in the genes TNNI2, TNNT3 and TPM2 that encode the muscle regulatory proteins troponin I, troponin T and beta-tropomyosin, respectively (2,(12)(13)(14). Here, we show that a subtype of DA5 that includes restrictive pulmonary disease is caused by gain-of-function mutations in the mechanically activated (MA) cation channel PIEZO2 (15).…”

Skeletal Muscle Contractile Gene (TNNT3, MYH3, TPM2) Mutations Not Found in Vertical Talus or Clubfoot

“…This temporal expression pattern supports a theory that congenital contractures may result from a period of fetal akinesia [14]. However, recent in vitro evidence suggests human mutations in TNNT3, TNNI2, and TPM2 increase the contractility of fast-twitch muscle fibers [19], with the result being that the contracture forms because of excess contractility. Muscle abnormalities limited to the fetal period are consistent with the natural history of most cases of successfully treated idiopathic clubfoot, in which the contracture does not recur and the foot demonstrates minimal weakness [6].…”

Arthrogryposis: A Review and Update

“…Similarly, the results from in vitro contractility studies in which recombinant mutant TnI and b-tropomyosin 34 molecules were used suggest that distal arthrogryposis syndromes are, in some cases, caused by increased myofiber contractility. However, the mechanism by which contractility is altered is not yet clear 35 . In contrast, direct measurement of the contractile properties of chemically skinned single muscle fibers sampled from affected muscles in individuals with MYH3 mutations suggests that maximal force normalized to fiber cross-sectional area is less than that observed in myofibers from unaffected individuals (Bamshad and Beck, unpublished data).…”