“Bartter-like” phenotype in Kearns–Sayre syndrome

Kearns–Sayre syndrome (KSS) is a mitochondrial disease caused by large deletions in mitochondrial DNA (mtDNA). In most patients the disease is characterized by mtDNA heteroplasmy, where a mixture of wild-type and mutated mtDNA co-exist within cells in variable proportion, modulating the severity of the phenotype in different tissues. We report on the case of a 14-year-old child with classical symptoms of KSS and a renal phenotype characterized by hypokalaemic alkalosis, hypomagnesaemia, hyperreninaemia, hyperaldosteronism and nephrocalcinosis, resembling Bartter syndrome. Analysis of mtDNA demonstrated an 8,661 bp deletion involving eight mitochondrial genes. Uneven degrees of mtDNA heteroplasmy were demonstrated in several tissues, ranging from 24% to 60% of deleted/total mtDNA. Variable degrees of expression of mitochondrial enzymes were also found in biopsy specimens of renal and skeletal muscle by histocytochemistry. In particular, preserved cytochrome c oxidase was observed in tubular structures within medullary rays. It is proposed that a “Bartter-like” phenotype can arise in some patients with KSS as a result of heteroplasmy. In these cases aldosterone-responsive tubular structures have been spared during renal embryogenesis, allowing for the development of hypokalaemic alkalosis in response to salt and water losses from the more damaged tubular segments.