Variations in mitochondrial DNA copy numbers in MS brains

The aim of this study is to determine if there is a pathology-related variation in mitochondrial (mt)DNA copy numbers in brains of patients with multiple sclerosis (MS). Our recent study demonstrated an age-dependent but excluded a MS pathology-related increase in the proportion of cytochrome oxidase (COX)-negative cells and deleted mtDNA molecules in postmortem brain tissue specimens of patients and controls (Blokhin et al., Neuromolecular Medicine, in press, 2008). This corollary study further extends our efforts defining mitochondrial contributions to tissue degeneration associated with inflammatory demyelination. Copy number variations of mtDNA molecules were defined by quantifying the mtDNA ND1 gene copies relative to the invariable nuclear ribosomal 18S gene copies (ND1/r18S) using real-time polymerase chain reaction analyses in laser dissected, COX-positive and COX-negative single neurons and glial cells from frozen postmortem normal-appearing gray (NAGM) and white matter (NAWM) regions and chronic active plaques of MS patients, and gray matter (GM) and white matter (WM) regions of age matched non-neurological disease (NND) controls. ND1/r18S values were correlated with tissue regions, pathology, and age. While the ND1/r18S values were similar in NAWM and plaque-containing specimens of MS patients as well as in NAWM of patients and WM of age-matched NND controls, we found significantly higher mtDNA copy number values in neurons of NAGM than in cells of other MS brain regions. The ND1/r18S values were even higher in NAGM than in GM of age-matched NND controls. An age-related decline in ND1/r18S values was also noted in neurons of both MS patients and NND controls. These observations exclude a change in mtDNA copy numbers in plaques, however, suggest a compensatory replication of mtDNA or mitochondria in the cortex with neuroaxonal loss in MS. The age-related decline in mtDNA copy numbers may explain some features of late-onset MS.