The 9-bp deletion in region V of mtDNA: a risk factor of hearing loss and encephalomyopathy in Caucasian populations?

A deletion of one of the two copies of the 9-bp tandem repeat sequence (CCCCCTCTA), in the small non-coding/untranslated segment located between the cytochrome oxidase II and lysine tRNA genes of mitochondrial DNA (mtDNA), has previously been used as a polymorphic anthropological marker (MIC9D) for people of Africa and Asia, but it has been rarely reported in Europe. 32 Sicilian patients with syndromic hearing loss, negative for mutations in GJB2 and GJB6 genes, were tested for mtDNA known point mutations associated with syndromic or non-syndromic hearing loss by RFLP and/or direct sequencing. We identified the presence of the MIC9D in homoplasmy in lymphocytes and muscle of three subjects with sensorineural hearing loss and encephalomyopathy, two of these also presented moderate mental retardation. This deletion was absent in 300 Caucasian controls. Although further studies are warranted, our results suggest that the MIC9D polymorphism could have a susceptibility role in Caucasus, such as Sicily population.