Heteroplasmy

Heteroplasmy 16169 C/T of Nicholas II of Russia. From Coble et al., 2009.

Heteroplasmy is the presence of a mixture of more than one type of an organellar genome (mitochondrial DNA (mtDNA) or plastid DNA) within a cell or individual. It is a factor for the severity of mitochondrial diseases, since most eukaryotic cells contain many hundreds of mitochondria with hundreds of copies of mtDNA, it is possible and indeed very frequent for mutations to affect only some of the copies, while the remaining ones are unaffected.

Symptoms of severe heteroplasmic mitochondrial disorders frequently do not appear until adulthood because many cell divisions and a lot of time is required for a cell to receive enough mitochondria containing the mutant alleles to cause symptoms. An example of this phenomenon is Leber optic atrophy. Individuals with this condition often do not experience vision difficulties until they have reached adulthood. Another example is MERRF syndrome (or Myoclonic Epilepsy with Ragged Red Fibers). In MELAS, heteroplasmy explains the variation in severity of the disease among siblings.

Notable cases

One notable example of an otherwise healthy individual whose heteroplasmy was discovered incidentally is Nicholas II of Russia, whose heteroplasmy (and that of his brother) served to convince Russian authorities of the authenticity of his remains.

See also

• homoplasmy
• microheteroplasmy
• Paternal mtDNA transmission