A mutation in the fast skeletal muscle troponin I gene causes myopathy and distal arthrogryposis

Objective: To describe a three-generation family with distal arthrogryposis associated with myopathy and caused by a mutation in the gene encoding for sarcomeric thin filament protein troponin I, TNNI2. Methods: The authors performed clinical investigations and reviewed medical records. Muscle biopsy specimens were obtained for morphologic analysis. Genomic DNA was extracted from blood and analyzed for mutations in TNNI2. Results: The five affected individuals had predominantly distal congenital joint contractures, mild facial involvement (mild micrognathia, narrow palpebral fissures), and no detectable muscle weakness. The four affected adults had slightly increased levels of creatine kinase in blood, and muscle biopsy specimens showed findings of myopathy with changes restricted to type 2 fibers. These included variability of muscle fiber size, internalized nuclei, and increased interstitial connective tissue. Analysis of TNNI2 encoding the troponin I isoform expressed in type 2 muscle fibers disclosed a heterozygous three-base in-frame deletion, 2,918-2,920del, skipping the highly conserved lysine at position 176. The mutation was present in all 5 affected individuals but was not identified in any of the 11 unaffected family members. Conclusion: Distal arthrogryposis type 1 is genetically heterogeneous, and myopathy due to sarcomeric protein dysfunction may be one underlying cause of the disease.