Same mitochondrial gene, many diseases: a 34-patient map shows why the tissue you test matters

Complex III deficiency is one of the rarer oxidative phosphorylation disorders, and MT-CYB variants are a genetically awkward cause: because the gene sits in mitochondrial DNA, every cell carries a mix of normal and variant copies, and the mix differs from tissue to tissue. Researchers at Beijing Children's Hospital set out to map that variability — one deeply worked-up child plus every published case they could systematically gather.

What did the index case show?

The child carrying the m.15045G>A variant presented with exercise intolerance, borderline intellectual functioning and elevated medium- and long-chain acylcarnitines. Muscle biopsy showed mitochondrial myopathic changes, and respiratory chain analysis demonstrated isolated complex III deficiency. Heteroplasmy — the variant's share of mitochondrial DNA — measured 22.5% in blood, 70.5% in urine and 95.5% in muscle, further evidence for the variant's pathogenicity and a textbook demonstration that blood can badly understate what muscle carries.

What did the 34-patient map find?

Across 34 patients with disease-associated MT-CYB variants drawn from 29 articles, mitochondrial myopathy was the most common phenotype (50%). Patients with myopathy had later onset, and muscle-restricted variants were markedly enriched in that group (p < 0.001). By contrast, muscle heteroplasmy level, variant type and isolated complex III deficiency were not significantly associated with phenotype. Unsupervised clustering of genetic and functional features matched the clinical classification in 88.2% of cases.

What should patients and clinicians take from this?

Two practical points. First, tissue choice is diagnostic strategy: when mitochondrial disease is suspected and blood testing is unrevealing, urine and especially muscle can carry the answer — as the validated workup covered on this site has long emphasized. Second, prediction stays humble: tissue distribution helped explain phenotype, but no single number — not even heteroplasmy — foretold how a given patient would present.

Where the evidence stands

• Established: MT-CYB encodes the only mitochondrially encoded complex III subunit; its variants cause isolated complex III deficiency with wide clinical heterogeneity, and heteroplasmy varies by tissue.
• Preliminary: In a pediatric case plus a PRISMA-guided review of 34 patients (Frontiers in Neuroscience, 2026), m.15045G>A heteroplasmy was 22.5% (blood), 70.5% (urine) and 95.5% (muscle); mitochondrial myopathy was the commonest phenotype (50%), muscle-restricted variants were enriched in myopathy (p<0.001), and heteroplasmy level itself did not predict phenotype.
• Absent: Prospective validation of tissue-based prediction rules, and any therapy specific to MT-CYB disease.