LAUSR

Mitochondria aremultifaceted: the ‘‘powerhouses of the cell’’ and integrally involved in many other cellular functions, such as autophagy and apoptosis. As a consequence, inherited or spontaneous mutations in mitochondrial genes give rise to a heterogeneous group of genetic diseases impacting multiple organs, with high-energy-demand tissues such as skeletal muscle and brain the most commonly affected. Patients with mitochondrial disease have mitochondrial DNA (mtDNA) copies with and without the harmful mutation, and it is the percentage of mutated mtDNA copies, also referred to as the ‘‘heteroplasmic ratio,’’ which determines disease incidence and severity. Clinical symptoms typically appear when more than 60% of mtDNA is mutated and the higher this percentage, themore severe the disease. Newwork suggests how this ratio might be tweaked for therapeutic benefit using gene editing technology. These reports come in the context of existingmitochondrial replacement techniques that have been developed to prevent the transmission of mitochondrial disease, in particular pronuclear transfer in which nuclear material from thewoman whose eggs carry mutated mitochondria is placed in an enucleated egg from an unaffected donor. If successful, a one-cell embryo containing the nuclear DNA from themother and father is created and can be transplanted back into the uterus. The genetic contribution from the mtDNA donor is small, constituting just 0.1% of the total DNA but this does not negate the argument that such children have a genetic and potentially legal connection to three parents (Mitalipov and Wolf 2014). Mitochondrial replacement was approved in the UK in 2015 but has seen resistance from regulatory bodies in other countries, notably the US Food and Drug Administration (FDA; Castro 2016). Although babies born using such replacement therapies suggest these treatments can successfully prevent transmis-