Understanding the function of mitochondria and inheritance of mitochondrial DNA are crucial to comprehending the severity of mitochondrial disease, and the potential impact of Proposition 44.
Mitochondrial disease was very poorly understood until very recently with more efficient gene sequencing, which as allowed scientists to identify “mtDNA disorders as a frequent cause of genetic disease” (Taylor, 2005). The prevalence of mtDNA disease is difficult to accurately estimate because of its varying symptoms across patients. Data from Finland estimates around 1 in 6,000 people affected, while studies in Britain estimate about 1 in 3,500 people to be affected (Taylor, 2005).
Each cell can have thousands of mitochondria each, meaning each cell also has thousands of copies of the mitochondrial genome. Homoplasmy occurs when all the copies of the genome are the same, and heteroplasmy means there is a mixture of genotypes (Taylor, 2005). This distinction determines the severity of disease. If all or no copies of the mtDNA are affected by a mutation, there is homoplasmy. If some are affected ands some are not due to a genetic bottleneck, there is heteroplasmy. In the case of heteroplasmy, there is a “threshold level of mutation”, that can affect the severity of symptoms (Taylor, 2005).
Since mitochondria are so crucial to cell survival, “mtDNA diseases affect many tissues” and have highly variable clinical features.
Even in homoplasmic mutations, only some offspring who inherit the mutation develop disease, depending on their environments. In heteroplasmic mutations, clinical outcomes depend on the “proportions of mutated versus wild-type mtDNA” transmitted, making predictions for each pregnancy very difficult (Taylor, 2005). >>Types and Symptoms of Disease