Decoding the Genetic and Phenotypic Spectrum of MT-TI Mitochondrial Diseases.

BACKGROUND: Variants in the MT-TI gene, which encodes mitochondrial transfer RNA for isoleucine, have been associated with neuromuscular, cardiac, auditory, renal, and metabolic disorders, but their clinical interpretation remains difficult. OBJECTIVE: To integrate clinical, familial, heteroplasmy, and functional evidence across the reported MT-TI variant spectrum and clarify its implications for variant interpretation and diagnosis. METHODS: We conducted a narrative review of reported MT-TI variants, with detailed comparison of seven representative variants and synthesis of phenotypic, familial, tissue-specific heteroplasmy, and functional findings. RESULTS: Evidence was derived mainly from case reports and small pedigrees. Heteroplasmy differed markedly among blood, skeletal muscle, and myocardium, indicating that blood may not represent variant loads in energy-demanding tissues. Reported values generally reflected the lowest observed levels in affected individuals or family-specific boundaries rather than validated pathogenic cutoffs. Functional findings support a staged mechanism involving disturbed transfer RNA processing, structure, stability, or aminoacylation, followed by impaired mitochondrial protein synthesis and respiratory-chain dysfunction. Integrated mechanistic support was limited to a few variants, including m.4295A>G; evidence for most variants remained incomplete or indirect. CONCLUSION: Diagnosis requires tissue-informed heteroplasmy assessment integrated with phenotype, maternal family history, and functional evidence. Current treatment is supportive, and proposed reproductive and molecular strategies lack MT-TI-specific clinical-trial evidence.