Pathogenic <i>TNNI1</i> variants disrupt sarcomere contractility resulting in hypo- and hypercontractile muscle disease

Abstract

Troponin I (TnI) regulates thin filament activation and muscle contraction. Two isoforms, TnI-fast (<i>TNNI2</i>) and TnI-slow (<i>TNNI1</i>), are predominantly expressed in fast- and slow-twitch myofibers, respectively. <i>TNNI2</i> variants are a rare cause of arthrogryposis, whereas <i>TNNI1</i> variants have not been conclusively established to cause skeletal myopathy. We identified recessive loss-of-function <i>TNNI1</i> variants as well as dominant gain-of-function <i>TNNI1</i> variants as a cause of muscle disease, each with distinct physiological consequences and disease mechanisms. We identified three families with biallelic <i>TNNI1</i> variants (F1: p.R14H/c.190-9G>A, F2 and F3: homozygous p.R14C), resulting in loss of function, manifesting with early-onset progressive muscle weakness and rod formation on histology. We also identified two families with a dominantly acting heterozygous <i>TNNI1</i> variant (F4: p.R174Q and F5: p.K176del), resulting in gain of function, manifesting with muscle cramping, myalgias, and rod formation in F5. In zebrafish, TnI proteins with either of the missense variants (p.R14H; p.R174Q) incorporated into thin filaments. Molecular dynamics simulations suggested that the loss-of-function p.R14H variant decouples TnI from TnC, which was supported by functional studies showing a reduced force response of sarcomeres to submaximal [Ca²⁺] in patient myofibers. This contractile deficit could be reversed by a slow skeletal muscle troponin activator. In contrast, patient myofibers with the gain-of-function p.R174Q variant showed an increased force to submaximal [Ca²⁺], which was reversed by the small-molecule drug mavacamten. Our findings demonstrated that <i>TNNI1</i> variants can cause muscle disease with variant-specific pathomechanisms, manifesting as either a hypo- or a hypercontractile phenotype, suggesting rational therapeutic strategies for each mechanism.