mtDNA heteroplasmy

mtDNA heteroplasmy is when a single cell, tissue, or person carries two or more different mitochondrial DNA sequences. It is a mix of normal (wild-type) and mutant copies. The heteroplasmy level (the percent of mutant copies) is measured by next-generation sequencing or droplet digital PCR. There is a threshold effect for disease. Mitochondrial disorders usually need a mutant load above 60 to 90% before energy production fails enough to cause symptoms. But even sub-clinical levels matter. A 2018 analysis of 789 Health ABC participants is telling. It looked at a common variant (m.3243A>G) at just 0 to 19% heteroplasmy. Even at those low levels, it was tied to lower grip strength, worse cognition, and stiffer arteries. And it carried a 96% higher dementia-mortality risk in the top versus bottom third (HR = 1.96; Tranah et al.). Somatic mutations build up in you with age. A 2026 Nature analysis of about 750,000 whole genomes (Gupta et al.) found a sharp rise after 60. The pattern pointed to copying errors, not oxidative damage, revising a long-held assumption. The buildup is tissue-specific. Non-dividing tissues keep clonal expansions; blood cells dilute them more easily (Sanchez-Contreras et al. 2023, eLife). There is also a 'germline bottleneck' during egg formation (about 30 to 35 segregating units). It makes mutant fractions jump between generations. And older maternal age independently raises heteroplasmy in offspring (Rebolledo-Jaramillo et al. 2014). Whether the somatic buildup actually causes aging, or is mainly a biomarker of copying stress, is still under investigation.

Last reviewed: June 22, 2026

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