Characterization of a novel mitochondrial DNA deletion in a patient with avariant of the Pearson marrow-pancreas syndrome

We have recently diagnosed a patient with anaemia, severe tubulopathy, and diabetes mellitus. As the clinical characteristics resembled Pearson marrow -pancreas syndrome, despite the absence of malfunctioning of the exocrine p ancreas in this patient, we have performed DNA analysis to seek for deletio ns in mtDNA. DNA analysis showed a novel heteroplasmic deletion in mtDNA of 8034bp in length, with high proportions of deleted mtDNA in leukocytes, li ver, kidney, and muscle. No deletion could be detected in mtDNA of leukocyt es from her mother and young brother, indicating the sporadic occurrence of this deletion. During culture, skin fibroblasts exhibited a rapid decrease of heteroplasmy indicating a selection against the deletion in proliferati ng cells. We estimate that per cell division heteroplasmy levels decrease b y 0.8%. By techniques of fluorescent in situ hybridisation (FISH) and mitoc hondria-mediated transformation of rho(0) cells we could show inter- as wel l as intracellular variation in the distribution of deleted mtDNA in a cell population of cultured skin fibroblasts. Furthermore, we studied the mitoc hondrial translation capacity in cybrid cells containing various proportion s of deleted mtDNA. This result revealed a sharp threshold, around 80%, in the proportion of deleted mtDNA, above which there was strong depression of overall mitochondrial translation, and below which there was complementati on of the deleted mtDNA by the wild-type DNA. Moreover, catastrophic loss o f mtDNA occurred in cybrid cells containing 80% deleted mtDNA.