ACCUMULATION OF MITOCHONDRIAL-DNA DELETIONS IN MYOTUBES CULTURED FROMMUSCLES OF PATIENTS WITH MITOCHONDRIAL MYOPATHIES

Myoblast cultures were established from muscle biopsies of two patient s harboring heteroplasmic mitochondrial (mt) DNA deletions. The accumu lation kinetics of the deleted mtDNA was followed during myoblast to m yotube differentiation. The percentage of deleted mtDNA was determined by quantitative PCR in myoblasts, myotubes, and muscle biopsies. The deleted form accounted for 65% of the mtDNA present in a muscle biopsy from a patient harboring a 5.6-kb deletion. The percentage of deleted mtDNA was 1.2% in myoblasts and increased progressively after differe ntiation, up to 12% at 21 days after the commitment time. In a second patient harboring a 2.8-kb deletion, the percentage of deleted mtDNA i ncreased much more slowly: from 0.07% in myoblasts to 0.21% after 22 d ays of differentiation, as compared with 45% in the muscle biopsy. Thu s, a three- and ten-fold increase, respectively, in the fraction of de leted mtDNA occurred during the differentiation of myoblasts to myotub es from the two patients. The faster accumulation of deleted mtDNA in the first patient's cells was linked to an earlier myoblast to myotube differentiation, suggesting that the level of deleted mtDNA is invers ely related to the rate of cell proliferation.