Pathogenic TNNI1 variants disrupt sarcomere contractility resulting in hypo- and hypercontractile muscle disease

Donkervoort, Sandra; van de Locht, Martijn; Ronchi, Dario; Reunert, Janine; McLean, Catriona A.; Zaki, Maha; Orbach, Rotem; de Winter, Josine M.; Conijn, Stefan; Hoomoedt, Daan; Neto, Osorio Lopes Abath; Magri, Francesca; Viaene, Angela N.; Foley, A. Reghan; Gorokhova, Svetlana; Bolduc, Veronique; Hu, Ying; Acquaye, Nicole; Napoli, Laura; Park, Julien H.; Immadisetty, Kalyan; Miles, Lee B.; Essawi, Mona; McModie, Salar; Ferreira, Leonardo F.; Zanotti, Simona; Neuhaus, Sarah B.; Medne, Livija; ElBagoury, Nagham; Johnson, Kory R.; Zhang, Yong; Laing, Nigel G.; Davis, Mark R.; Bryson-Richardson, Robert J.; Hwee, Darren T.; Hartman, James J.; Malik, Fady I.; Kekenes-Huskey, Peter M.; Comi, Giacomo Pietro; Sharaf-Eldin, Wessam; Marquardt, Thorsten; Ravenscroft, Gianina; Bonnemann, Carsten G.; Ottenheijm, Coen A. C.

Zusammenfassung

Troponin I (TnI) regulates thin filament activation and muscle contraction. Two isoforms, TnI-fast (TNNI2) and TnI-slow (TNNI1), are predominantly expressed in fast- and slow-twitch myofibers, respectively. TNNI2 variants are a rare cause of arthrogryposis, whereas TNNI1 variants have not been conclusively established to cause skeletal myopathy. We identified recessive loss-of-function TNNI1 variants as well as dominant gain-of-function TNNI1 variants as a cause of muscle disease, each with distinct physiological consequences and disease mechanisms. We identified three families with biallelic TNNI1 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 TNNI1 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 TNNI1 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.