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.