ALTERED MITOCHONDRIAL-FUNCTION IN FIBROBLASTS CONTAINING MELAS OR MERRF MITOCHONDRIAL-DNA MUTATIONS

A number of human diseases are caused by inherited mitochondrial DNA m utations. Two of these diseases, MELAS (mitochondrial myopathy, enceph alopathy, lactic acidosis and stroke-like episodes) and MERRF (myoclon ic epilepsy and ragged-red fibres), are commonly caused by point mutat ions to tRNA genes encoded by mitochondrial DNA. Here we report on how these mutations affect mitochondrial function in primary fibroblast c ultures established from a MELAS patient containing an A to G mutation at nucleotide 3243 in the tRNA(Leu(UUR)) gene and a MERRF patient con taining an A to G mutation at nucleotide 8344 in the tRNA(Lys) gene. B oth mitochondrial membrane potential and respiration rate were signifi cantly decreased in digitonin-permeabilized MELAS and MERRF fibroblast s respiring on glutamate/malate. A similar decrease in mitochondrial m embrane potential was found in intact MELAS and MERRF fibroblasts. The mitochondrial content of these cells, estimated by stereological anal ysis of electron micrographs and from measurement of mitochondrial mar ker enzymes, was similar in control, MELAS and MERRF cells. Therefore, in cultured fibroblasts, mutation of mitochondrial tRNA genes leads t o assembly of bioenergetically incompetent mitochondria, not to an alt eration in their amount. However, the cell volume occupied by secondar y lysosomes and residual bodies in the MELAS and MERRF cells was great er than in control cells, suggesting increased mitochondrial degradati on in these cells. In addition, fibroblasts containing mitochondrial D NA mutations were 3-4-fold larger than control fibroblasts. The implic ations of these findings for the pathology of mitochondrial diseases a re discussed.