MOLECULAR MECHANISMS IN MITOCHONDRIAL-DNA DEPLETION SYNDROME

Depletion of mitochondrial DNA (mtDNA) appears to be an important caus e of mitochondrial dysfunction in neonates and infants, We have identi fied another child in whom depletion of mtDNA was demonstrated in live r and serial skeletal muscle biopsies. A primary myoblast culture from the patient initially showed normal levels of mtDNA, but there was a progressive loss of mtDNA in later cell passages and clonal myoblast c ell cultures, similar to that observed in the skeletal muscle tissue o f the patient, Thus, these clonal myoblast cultures provide an in vitr o model of the in vivo mtDNA dynamics, The levels of mitochondrial mRN As for subunits I and II of cytochrome c oxidase declined with declini ng mtDNA levels, but the fall in mitochondrial transcript levels lagge d behind that of the mtDNA levels, Levels of cytochrome c oxidase subu nit I and II polypeptides, however, declined ahead of declining mtDNA levels, Immunocytochemistry showed that between individual cells of th e clonal myoblast cultures, the expression of the mitochondrially enco ded subunit I of cytochrome c oxidase was heterogeneous, suggesting va riable levels of mtDNA, Transfer of patient mitochondria with residual mtDNA levels to control cells devoid of mtDNA (po cells) led to resto ration of mtDNA levels and, hence, suggests a nuclear involvement in t he depletion.