Mitochondrial cytochrome b gene deletion in Kearns-Sayre syndrome associated with a subclinical type of peripheral neuropathy.

Kearns-Sayre syndrome (KSS) is a progressive neuromuscular disease characterized by ophthalmoplegia, cardiac conduction block, and pigmentary retinopathy associated with abnormal mitochondrial structure and function. Usually mitochondrial DNA (mtDNA) deletions have been associated with Kearns-Sayre syndrome and chronic progressive external ophthalmoplegia. Size and position of the deletions differ markedly among these patients. The present study confirms this observation for a patient with KSS by a muscle and nerve biopsy in which we detected a 1.2 kb mtDNA deletion. The location of the deletion, however, is unusual in this case: its position comprises nucleotides 14952 to 15739. The defect is heteroplasmic and concerns the cytochrome b and tRNA genes. Complex I and IV of the respiratory chain were intact in this case, indicating that below a threshold of tRNA formation, the impaired biosynthesis and membrane integration of one respiratory complex may cause the phenotypical appearance of the KSS syndrome associated with a subclinical neuropathy.