I-6. Apoptosis in mitochondrial encephalomyopathies; how frequent, how extensive, how relevant

Apoptosis has recently been invoked as a possible pathogenic mechanism in mitochondrial encephalomyopathies (ME) because the respiratory chain is a major source of reactive oxygen species (ROS) and because the latter can activate apoptosis. However there are reports disputing both its occurrence and its extend of participation in ME, raising doubts as to its relevance. Most of the studies documenting the presence of apoptosis in ME are based on a small number of well characterized patients exhibiting a recognized syndrome, such as MELAS, MERRF and mitochondrial DNA mutations. It is now well recognized that the majority of ME are caused by nuclear mutations reflecting the fact that the mtDNA codes for only 37 genes while most of the approximately 900 gene products in the organelle are encoded by nuclear DNA. We therefore decided to investigate the frequency and extend of apoptosis in a heterogeneous group of patients fulfilling Bernier’s criteria of definite or probable ME. Methods and patients; Sixty patients were included in the study, 41 adults and 19 children (less than 18 years of age). The patients were identified from the muscle tissue bank of the CING and Aiginition Hospital in Athens. Sixteen patients had well recognized ME syndromes while 24 had a myopathic presentation and 20 multisystemic disorders (mainly children). Twenty six (43%) had abnormal mtDNA; 15 had either single or multiple deletions, 7 had depletion, 3 had duplication and 1 had the A3243G mutation. The rest had presumably nuclear mutations. Immunocytochemistry of serial muscle sections was carried out to visualize oxidative stress (8-OH-dG, 4-HNE and Mn SOD), pro apoptotic proteins (bax, cytochrome c, apaf 1), anti apoptotic proteins (bcl2, XIAP) and evidence for completed apoptosis (activated caspase 3 and TUNEL). Results;The proportion of patients exhibiting cytochrome IV deficient fibers (COX) was 77%, expression of ROS 55%, expression of pro- and/or anti- apoptotic proteins 50% and expression of Tunnel pos fibers 25%. The number of fibers exhibiting positivity varied from 0 to more than 75%. There was a statistical correlation between the number of COX def fibers and the number of fibers expressing oxidative stress, pro- or anti- apoptotic proteins and Tunnel pos nuclei. Conclusion; Apoptosis was widely expressed in muscle tissue of a heterogeneous group of ME patients probably driven by ROS. Its pathogenic significance however needs to be explored in animal models of the disease.