Mitochondrial leukoencephalopathy and complex II deficiency associated with a recessive SDHB mutation with reduced penetrance

Mitochondrial disease involving complex II is rare among respiratory chain deficiencies and its genetic cause remains often unknown. Two main clinical presentations are associated with this biochemical defect: mitochondrial encephalomyopathy and susceptibility to tumors. Only one homozygous SDHBmutation has been described in a patient with mitochondrial disorder. We report here two sisters, who presented highly different phenotypes (neurological impairmentwith leukoencephalopathy vs. asymptomatic status) and harbored the same homozygous SDHB mutation, suggesting reduced penetrance.