Women at risk of passing on mitochondrial disease to their children could use PGD to give birth to an unaffected child. The scientists at Maastricht University Medical Centre, Netherlands, claim their work has the potential to prevent the transmission of mitochondrial diseases.
Mitochondria are known as the 'power houses' of the cell as they convert the energy contained in food to energy for the cell. They are membrane-enclosed structures inside cells that use their own DNA (mitochondrial DNA - mtDNA) as well as DNA in the cell's nucleus to direct their function. Mutations in mtDNA are varied, and have serious implications. For example, if the mutations do not cause an early death, children may be affected by visual and hearing problems, loss of muscle co-ordination, poor growth, mental retardation, heart, liver and kidney disease, neurological problems, respiratory disorders and dementia.
However, there are two key difficulties in predicting the inheritance of mitochondrial diseases. Firstly, there is a threshold of mutant mtDNA which must be exceeded before clinical symptoms occur. Secondly, the mtDNA mutation level inherited by the children of a female carrier can vary greatly between siblings and even twins.
Prenatal diagnosis cannot be used to predict mitochondrial disease since the mutation load in mtDNA detected in chorionic villus samples does not correlate to future severity of disease. The multinational team from Holland, Australia and the UK therefore decided to investigate whether PGD would be a better alternative. Leader of the study, Dr Debby Hellebrekers said: 'If we could find a minimal level of mtDNA mutation load below which the chance for an embryo of being affected was acceptably low, we could offer PGD to women who otherwise had little chance of giving birth to a healthy child'.
The team studied 327 unrelated patients or families who between them had 159 different disease-causing mtDNA mutations. They measured the amount of mutated mtDNA in muscle tissue — as this correlates best with prenatal tissues - of affected individuals and compared this to their unaffected relatives from their maternal family (because mitochondria are maternally inherited). They found that if less than 18 percent of mtDNA was mutated, that individual had a 95 percent or greater chance of being unaffected.
'Being able to find the minimal level of mutant mtDNA below which the chances of passing on a disorder is low was therefore very important', said Dr Hellebrekers. 'Currently, there are no effective treatments for mtDNA disorders. Although we cannot guarantee that a mutant mtDNA level of 18 percent or lower will result in the birth of an unaffected child, we think that the chances of having a healthy child are high enough to make using PGD in this instance morally acceptable'.
'Our research enables us to give genetic counselling to women at risk with respect to their reproductive choices and to provide them, for the first time, with the opportunity to give birth to a healthy baby. The prevalence of mtDNA disorders is 1 in 5,000, which means that the families of about 146,000 patients in Europe can now have the option of having a healthy child. This is a choice that they do not currently have', she said.
The findings were reported at the European Human Genetics Conference in Amsterdam, May 2011.