Myoclonic Epilepsy and Ragged Red Fibers (MERRF) Syndrome: Selective Vulnerability of CNS Neurons Does Not Correlate with the Level of Mitochondrial TRNAlys Mutation in Individual Neuronal Isolates

Selective vulnerability of subpopulations of neurons is a striking feature of neurodegeneration. Mitochondrially transmitted diseases are no exception. In this study CNS tissues from a patient with myoclonus epilepsy and ragged red fibers (MERRF) syndrome, which results from an A to G transition of nucleotide (nt) 8344 in the mitochondrial tRNALys gene, were examined for the proportion of mutant mtDNA. Either individual neuronal somas or the adjacent neuropil and glia were microdissected from cryostat tissue sections of histologically severely affected brain regions, including dentate nuclei, Purkinje cells, and inferior olivary nuclei, and from a presumably less affected neuronal subpopulation, the anterior horn cells of the spinal cord. Mutant and normal mtDNA were quantified after PCR amplification with a mismatched primer and restriction enzyme digestion. Neurons and the surrounding neuropil and glia from all CNS regions that were analyzed exhibited high proportions of mutant mtDNA, ranging from 97.6 +/- 0.7% in Purkinje cells to 80.6 +/- 2.8% in the anterior horn cells. Within each neuronal group that was analyzed, neuronal soma values were similar to those in the surrounding neuropil and glia or in the regional tissue homogenate. Surprisingly, as compared with controls, neuronal loss ranged from 7% of the Purkinje cells to 46% of the neurons of the dentate nucleus in MERRF cerebellum. Thus, factors other than the high proportion of mutant mtDNA, in particular nuclear-controlled neuronal differences among various regions of the CNS, seem to contribute to the mitochondrial dysfunction and ultimate cell death.