Mitochondrial disease genetics is uniquely tricky: mitochondrial DNA (mtDNA) is maternally inherited, heteroplasmic (variable mutant load per tissue), and frequently missed on standard NGS panels that only sequence nuclear DNA. Nuclear-encoded mitochondrial genes (POLG, SURF1, TWNK, others) follow Mendelian inheritance. The right test depends on the suspected phenotype, the inheritance pattern, and which tissue you can sample.

Why Mitochondrial Genetics Is Different

• Two genomes: nuclear DNA (Mendelian inheritance) AND mitochondrial DNA (maternal inheritance, ~16.5 kb circular, 37 genes).
• Heteroplasmy: cells contain hundreds-to-thousands of mtDNA copies. Mutant mtDNA + wild-type mtDNA coexist. Phenotype depends on mutant load + tissue.
• Threshold effect: clinical disease typically requires mutant load >60–90% in affected tissue.
• Maternal inheritance: all children of an affected mother are at risk; no father-to-child transmission.
• Tissue selection for testing: blood is often insensitive for mtDNA point mutations (white cells selectively clear mutant mtDNA over time) and especially insensitive for large mtDNA deletions in adult KSS/CPEO. Urine sediment or buccal swab for point mutations; skeletal muscle when blood/urine is non-diagnostic and a deletion syndrome remains strongly suspected. A negative blood test does NOT exclude mtDNA disease — escalate tissue when suspicion is high.

MELAS (Mitochondrial Encephalopathy, Lactic Acidosis, Stroke-like episodes)

• First-line test: targeted mtDNA testing for m.3243A>G in MT-TL1 (tRNA leucine).
• Yield: ~80% of MELAS patients have m.3243A>G.
• If negative: full mtDNA sequencing (m.3271T>C, m.13513G>A, others).
• Tissue: urine sediment > buccal swab >> blood. Muscle if non-invasive samples negative + high suspicion.
• Clinical clues: stroke-like episodes not respecting vascular territories (typically posterior, gyriform cortical/subcortical T2 hyperintensity); seizures; lactic acidosis (elevated lactate in serum AND CSF); short stature; sensorineural hearing loss; diabetes.
• Imaging: parieto-occipital cortical lesions that cross vascular boundaries; “ragged red fibers” on muscle biopsy.
• Treatment: supportive; L-arginine for acute stroke-like episodes; CoQ10, riboflavin, taurine. Drug cautions (situational, not blanket): valproate is contraindicated in POLG-related disease (fatal hepatotoxicity) and used cautiously in other mitochondrial disease; metformin can precipitate lactic acidosis in MELAS or otherwise lactic-prone patients; statins occasionally exacerbate mitochondrial myopathy and should be used with judgment, not universally avoided; aminoglycosides cause ototoxicity in m.1555A>G carriers.

MERRF (Myoclonic Epilepsy with Ragged Red Fibers)

• First-line test: targeted mtDNA testing for m.8344A>G in MT-TK (tRNA lysine).
• Yield: ~80%.
• If negative: full mtDNA sequencing.
• Clinical clues: myoclonic epilepsy + cerebellar ataxia + sensorineural hearing loss + lipomas (cervical/dorsal); ragged red fibers on muscle biopsy.

Leber Hereditary Optic Neuropathy (LHON)

• First-line test: targeted mtDNA testing for the three primary LHON mutations:
  • m.11778G>A in MT-ND4 (~70%).
  • m.14484T>C in MT-ND6 (~15%).
  • m.3460G>A in MT-ND1 (~13%).
• Yield: ~90% of LHON.
• If negative + high suspicion: full mtDNA sequencing.
• Tissue: blood (often homoplasmic, so blood is OK).
• Clinical clues: subacute painless bilateral central vision loss in young men (M:F ~4:1, incomplete penetrance); centrocecal scotoma; optic nerve pseudo-edema → atrophy.
• Treatment: idebenone (variable benefit); gene therapy trials (lumevoq/lenadogene nolparvovec).
• Counseling: penetrance only ~50% in men, ~10% in women; environmental triggers (smoking, alcohol) increase risk.

NARP / Leigh Syndrome

• First-line test: targeted mtDNA testing for m.8993T>G or m.8993T>C in MT-ATP6.
• Phenotype depends on mutant load:
  • Low (~70%) heteroplasmy: NARP (Neuropathy, Ataxia, Retinitis Pigmentosa).
  • High (>90%) heteroplasmy: maternally-inherited Leigh syndrome (subacute necrotizing encephalomyelopathy) — fatal in infancy.
• Leigh syndrome NGS panel: if m.8993 negative, panel covers >75 nuclear genes (SURF1, NDUFV1, PDHA1, others) + mtDNA.
• Imaging: symmetric T2 hyperintensities in basal ganglia + brainstem.

Kearns-Sayre Syndrome (KSS) and CPEO

• First-line test: mtDNA large-scale deletion analysis by Southern blot or long-range PCR on muscle.
• Why muscle? mtDNA deletions are typically NOT detectable in blood (cleared by mitotic tissues). Skeletal muscle retains them.
• KSS classic triad: PEO + pigmentary retinopathy + onset <20 years; plus cardiac conduction defects + CSF protein elevation + cerebellar ataxia + short stature.
• Single deletion vs. multiple deletions:
  • Single deletion → typically sporadic mtDNA disease (KSS, CPEO, Pearson syndrome).
  • Multiple deletions on muscle Southern blot → likely nuclear gene defect causing mtDNA instability → reflex to nuclear panel: POLG, POLG2, TWNK, RRM2B, OPA1, SLC25A4.
• Cardiac surveillance: KSS patients need annual ECG + early pacemaker for conduction block.

POLG Spectrum (Alpers, Ataxia-Neuropathy, PEO+)

• First-line test: POLG sequencing.
• Phenotypes:
  • Alpers syndrome: infantile/childhood progressive encephalopathy + intractable epilepsy + liver failure.
  • POLG-related sensory ataxic neuropathy (SANDO / MEMSA / MIRAS): adult onset.
  • Autosomal dominant PEO with multiple mtDNA deletions.
• CRITICAL contraindication: NEVER give valproate to a POLG-mutation patient — fatal hepatotoxicity. Always test POLG in unexplained pediatric refractory epilepsy BEFORE using valproate.

Mitochondrial mtDNA Depletion Syndromes

• Nuclear-encoded; depletion = reduced mtDNA copy number per cell.
• Genes: POLG, TWNK, DGUOK, MPV17, SUCLA2, SUCLG1, TK2, RRM2B, FBXL4.
• Phenotypes: hepatocerebral, myopathic, encephalomyopathic.
• TK2 deficiency: doxecitine/doxribtimine emerging as disease-modifying nucleoside therapy.

Other Important Mitochondrial Syndromes

• MNGIE (mitochondrial neurogastrointestinal encephalomyopathy): TYMP sequencing + thymidine/deoxyuridine assay. Treatable with allogeneic HSCT or carrier-erythrocyte enzyme replacement.
• Pearson syndrome: infantile sideroblastic anemia + pancreatic exocrine dysfunction; single mtDNA deletion (overlap with KSS).
• Wolfram syndrome (DIDMOAD): WFS1 sequencing. Diabetes Insipidus + Diabetes Mellitus + Optic Atrophy + Deafness. Note: most cases nuclear, not mtDNA.
• Optic atrophy type 1 (Kjer / OPA1): OPA1 sequencing — the commonest hereditary optic atrophy; AD with incomplete penetrance.

Mitochondrial Disease Panels (Practical)

• Tier 1 “is this mitochondrial?” workup: combined nuclear NGS + full mtDNA sequencing + mtDNA copy-number/deletion analysis from blood or buccal swab.
• Tier 2 (if negative + high suspicion): muscle biopsy for mtDNA deletion analysis, respiratory chain enzymology, histochemistry (ragged red fibers, COX-negative fibers).
• Tier 3: trio WES or WGS — increasingly the first-line in pediatric cases.

Disease → Test Quick Reference Table

Pitfalls and Pearls

• Standard NGS panels miss mtDNA: ordering “WES” alone will miss MELAS, MERRF, LHON, KSS. Always confirm the lab’s mitochondrial coverage.
• Heteroplasmy quantification is part of the result — report the percentage in the tested tissue.
• Mother-to-all transmission: in mtDNA disease, fathers do not transmit; mothers transmit to all children but at variable loads.
• Lactate is suggestive, not diagnostic: ~30% of mitochondrial patients have normal serum/CSF lactate.
• Ragged red fibers / COX-negative fibers on muscle biopsy support mitochondrial myopathy but are not specific or sensitive.
• Valproate is contraindicated in POLG; in other mitochondrial disease, drug choice is condition-specific — metformin (lactic acidosis in MELAS), aminoglycosides (m.1555A>G ototoxicity), statins (selected mitochondrial myopathy) are weighed against indication, not universally avoided.
• Cardiac surveillance in KSS / CPEO / m.3243A>G carriers.
• PGD / mitochondrial replacement (3-parent IVF) available in select jurisdictions for maternally-transmitted mtDNA disease.
• Trio WES / WGS increasingly the first-line in pediatric encephalopathy of unknown cause + suspected mitochondrial.

References

1. Gorman GS, Chinnery PF, DiMauro S, et al. Mitochondrial diseases. Nat Rev Dis Primers. 2016;2:16080.
2. El-Hattab AW, Adesina AM, Jones J, Scaglia F. MELAS syndrome: clinical manifestations, pathogenesis, and treatment options. Mol Genet Metab. 2015;116(1-2):4-12.
3. Yu-Wai-Man P, Newman NJ, Carelli V, et al. Bilateral visual improvement with unilateral gene therapy injection for Leber hereditary optic neuropathy. Sci Transl Med. 2020;12(573):eaaz7423.
4. Pfeffer G, Majamaa K, Turnbull DM, et al. Treatment for mitochondrial disorders. Cochrane Database Syst Rev. 2012;(4):CD004426.
5. Schaefer AM, McFarland R, Blakely EL, et al. Prevalence of mitochondrial DNA disease in adults. Ann Neurol. 2008;63(1):35-39.
6. Saneto RP, Cohen BH, Copeland WC, Naviaux RK. Alpers-Huttenlocher syndrome. Pediatr Neurol. 2013;48(3):167-178.
7. Parikh S, Goldstein A, Karaa A, et al. Patient care standards for primary mitochondrial disease: a consensus statement from the Mitochondrial Medicine Society. Genet Med. 2017;19(12).