Certain variations of mitochondrial DNA are protective against strokes, according to a recent study in The Lancet Neurology.

Scientists at the University of Newcastle examined the mitochondrial DNA of over 4000 volunteers. 950 volunteers had previously had a stroke, 240 had heart disease symptoms and 2939 were healthy controls.

In those volunteers with a stroke history, fewer people had a subset of mutations known as 'haplotype K'. The scientists suggested that 'haplotype K' - present in nine per cent of Europeans - is protective against stroke, decreasing someone's risk of stroke by an estimated 50 per cent. These statistics mean that haplotype K, along with aggressive blood pressure control, is now among the best single predictors of stroke risk. The beneficiary effects of haplotype K did not extend to heart disease.

Patrick Chinnery, lead author on the study, said: 'This could explain why genome-wide association studies [which only look at nuclear DNA] have failed to identify new stroke targets'. He adds that the findings highlight the 'importance of mitochondrial DNA as a genetic risk factor'.

The team hope to identify the mechanism behind the protective effect of haplotype K. Chinnery thinks it might have something to do with reactive oxygen species - the damaging by-products of energy generation in cells.

Mitochondria are the energy-producing powerhouses of cells. Separated from the rest of the cell by their own membrane, they contain their own DNA. Until recently, only a few rare conditions linked to mutations in mitochondrial DNA, for example, Leber's hereditary optic neuropathy (LHON). Now more common mitochondrial mutations - producing more subtle effects - are being linked to complex diseases such as Parkinson's disease.