Article on Mitochondrial Disorders

This is a great article that discusses some statistics on the prevalences and other aspects of mitochondrial disorders. The author (Thorburn, 2004) discusses the way mtDNA disorders are not usually "maternally inherited," even though mtDNA is maternally-donated (almost always but not always--there can be some paternal mtDNA that survives in the fertilized egg, rarely). The author notes that there's been no evidence of mtDNA disorders having occurred in past generations, in most cases, and it's more or less a random event (given the present knowledge) that produces a mutation in mtDNA. The mutation then becomes "amplified" by the replication of mutant copies, and the amplification may not occur until a person is 2 or 3 years old. The author also talks about the way there's not usually any way of predicting the risk of a mtDNA mutation between siblings, and there can be anywhere from 0 to 100 percent of siblings who have mtDNA mutations that produce mitochondrial disorders (but some nuclear gene mutations cause mitochondrial disorders, and those can be inherited paternally or maternally). The author also talks about the way mtDNA mutations of the respiratory chain enzyme subunits can, in fact, persist in proliferating cells such as fibroblasts and bone marrow stem cells. Although mtDNA mutations tend to cause symptoms by affecting postmitotic tissues such as the brain, mtDNA mutations in bone marrow cells can cause myelodysplastic syndromes. The degree of heteroplasmy [in this case, the presence of both mutant and normal (wild-type) mtDNA copies in a single cell or mitochondrion] in proliferative cells can gradually increase (or decrease) throughout life, also.

http://www.ncbi.nlm.nih.gov/pubmed/15190193