Much of the information on mtDNA copy number, replication and the bottleneck has been obtained from studying mouse and other animal embryos. Further research on mitochondrial biology in early human embryos, and in the germline in particular, is encouraged and should be supported by research funding agencies.
Review of preimplantation genetic diagnosis to avoid mitochondrial disease
Preimplantation genetic diagnosis (PGD) involves the removal of one or more cells from an early embryo as a biopsy for genetic diagnosis to identify those embryos that are suitable for transferring to the uterus in a treatment cycle. PGD is currently carried out to test for both nuclear DNA mutations and mtDNA mutations. However, nuclear mitochondrial disease and mtDNA disease have different modes of inheritance and therefore the benefits of PGD differ.
Effectiveness and safety of PGD for nuclear mitochondrial disease
PGD for nuclear genetic diseases examines well-understood patterns of chromosomal inheritance. The presence or absence of a nuclear mutation will help to determine disease susceptibility. PGD is possible for mitochondrial diseases resulting from mutations in nuclear DNA, providing that the affected gene is known. Such nuclear mutations are responsible for about 80% of cases of mitochondrial diseases in childhood, but as they often lead to early death, their prevalence is only about 50% in adult cases. However, given the number of nuclear genes involved in mitochondrial function, accurate diagnosis of the nuclear mutation(s) causing the disease is still difficult and PGD is only possible if the causative mutations are known. Without knowing the nuclear gene, the possibility remains that the disease is due to a mutation in mtDNA; this may be the wrong diagnosis. Sequencing the whole mtDNA genome from the affected child or the mother could in principle exclude mtDNA mutations, though
Page 10 of 45