MITOCHONDRIAL MYOPATHY - CORRELATION BETWEEN OXIDATIVE DEFECT AND MITOCHONDRIAL-DNA DELETIONS AT SINGLE-FIBER LEVEL

In situ hybridization combined with immunohistochemical techniques has been applied to study patients affected by mitochondrial myopathies w ith large mitochondrial (mt)DNA deletions. All patients' muscle biopsi es showed ragged red fibers (RRFs) and cytochrome oxidase (COX) defici ency. Two digoxygenin-labeled, polymerase chain reaction (PCR)-amplife d DNAs were used as probes. One probe was designed to hybridize only w ith wild-type mtDNAs, while the other recognized both wild-type and de leted mtDNAs. Concomitant immunocytochemical analysis using antibodies against subunits II, III, (encoded by mtDNA) and IV (encoded by nucle ar DNA) of COX was carried out. In our patients deleted mtDNAs are ove rexpressed in COX-negative RRFs, while wild-type mtDNAs are decreased in the same fibers. Immunohistochemistry studies show that COX IV is o verexpressed in RRFs and that COX II and COX III subunits are still pr esent. Deleted mtDNAs are spatially segregated in muscle fibers, where they interfere with the local population of normal mitochondrial geno mes, causing a regional deficiency of the mitochondrial respiratory ac tivity.