Mitochondrial DNA Mutation Load: Chance or Destiny?

It is well known that mitochondrial DNA (mtDNA)–related disorders are clinically heterogeneous, even within the same family.1 This is because in most patients, pathogenic mtDNA mutations are heteroplasmic, that is, a mixture of normal and mutated mtDNAs. Moreover, applying Mendelian terminology to mitochondrial genetics, heteroplasmic mtDNA mutations are recessive in the sense that very high proportions of a mutation are needed before a clinical phenotype becomes manifest. This threshold effect varies among mutation types and is generally higher for transfer RNA (tRNA) mutations than for protein-coding gene mutations or large-scale single deletions. Another common feature of mtDNA-related disorders is that the mutation load (and the clinical phenotype) may vary widely between a mother and each of her children owing to a mitochondrial genetic bottleneck. The mechanism(s) through which the mitochondrial genetic bottleneck operates in humans is still hotly debated and the subject of a vast literature (for review, see article by Folmes et al2). A major factor dictating levels of maternally transmitted mtDNA variants is germline segregation (ie, parceling of mtDNA during the development of primordial germ cells into oocytes). Germline segregation coupled with reduced numbers of mtDNA molecules per cell produces an undefined germline bottleneck mechanism initially proposed to explain rapid shifts of heteroplasmy within one generation in Holstein cows.3–5 Experiments in mice heteroplasmic for neutral mtDNA variants showed that the physical bottleneck associated with the decrease of mtDNA from the zygote to primordial germ cells and immature oocytes is not accompanied by variations in heteroplasmy, whereas the resumption of mtDNA replication in postnatal oocytes during folliculogenesis rapidly segregates sequence variants and often in a tissue-specific manner.6,7 Although these data were obtained in animal models and did not deal with deleterious mtDNA mutations, they clearly showed the importance of nuclear factors in determining the mitochondrial genetic bottleneck. Nevertheless, human studies also comparing heteroplasmy concordance for a neutral mtDNA trait in monozygotic and dizygotic twins failed to show any nuclear genetic contribution to heteroplasmy.8 It is generally accepted that mature oocytes from normal women contain about 150 000 mtDNAs, and a steep decline in the number of mitochondrial genomes from the zygote to the inner cell mass of the blastocysts allows only a minority of the maternal mtDNA to populate the fetus producing a second embryonic bottleneck. If the oocyte is heteroplasmic for a pathogenic mtDNA mutation, random segregation of mutant and wild-type mitochondrial genomes in the germ layers can explain both the rapid change of mutation load from one generation to the next and the wide variation of tissue involvement within members of the same family. Let us consider bottleneck studies in humans harboring known pathogenic mtDNA mutations. There is a general consensus that the bottleneck is due to the dramatic reduction of mtDNA number during early oogenesis (germline bottleneck) and is based mainly on random genetic drift.2,9–11 However, the tightness of the bottleneck varies depending on the type of the mutation, the tissue, and the nuclear background. In general terms, a tight bottleneck is our best protection against the accumulation of dangerously deleterious mtDNA mutations,10 which may explain why bottleneck tightness seems to be directly related to mutation severity; for example, the bottleneck for mtDNA deletions is tighter than that for mutations in protein-coding genes and this is tighter than mutations in tRNA genes.9–11 In this issue of JAMA Neurology, Spyropoulos et al12 describe a previously documented pathogenic mtDNA mutation in monozygotic twins that is interesting both clinically and because it contributes to a better understanding of the mtDNA bottleneck. The mutation was in MTND6 and caused an unusual association of Leber hereditary optic neuropathy–like optic atrophy with extraocular muscle involvement and fluctuating encephalopathy. The almost identical clinical features and the similar tempo of their appearance were a fitting prelude to the molecular findings: almost identical mutation loads in blood, urinary epithelium, and hair follicles (muscle was unfortunately evaluated only in 1 brother). Although both patients originated from a single oocyte, random postfertilization genetic drift per se does not explain the similarity of the mutation loads in 3 tissues and a casual event appears unlikely. Thus, this experiment of nature demonstrates the importance of nuclear factors in determining homogeneous mtDNA segregation in different tissues. Indirectly, this article also showed the importance of the mutation type in modulating mtDNA segregation because a previous report by the same group showed diametrically opposite results in monozygotic twins harboring a single large-scale mtDNA deletion.13 One brother had ptosis, progressive external ophthalmoplegia, and proximal weakness; he harbored 66% deleted mtDNA in muscle. The other brother was asymptomatic and had barely detectable deleted mtDNA molecules. In this case, uneven random distribution of the deleted mtDNAs at zygote separation obviously trumped nuclear control. A third set of twins, this time dizygotic, harbored a tRNA mutation, the m.8344A>G in MTTK: one had typical MERRF and a high mutation load, the other was asymptomatic and had a very low level of heteroplasmy, which is one more example of uneven random distribution of yet another mutation type during oogenesis.14 In conclusion, the mtDNA mutation load that defines the presence and severity of a disease appears to be more often a matter of stochastic chance than of destiny, but we still have much to learn about the mitochondrial genetic bottleneck.