Variations in the mitochondrial helicase Twinkle (PEO1) gene are usually associated with autosomal dominant chronic progressive external ophthalmoplegia (PEO). We describe five patients from two unrelated Alsatian families with the new R374W variation in the Twinkle linker region who progressively developed an autosomal dominant multisystem disorder with PEO, hearing loss, myopathy, dysphagia, dysphonia, sensory neuropathy, and late-onset dementia resembling Alzheimer's disease. These observations demonstrate that Twinkle variations in the linker domain alter cerebral function and further implicate disrupted mitochondrial DNA integrity in the pathogenesis of dementia.