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Volume 17, Number 20,
Issue of October 15, 1997
pp. 7746-7753
Copyright ©1997 Society for Neuroscience
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
Received Feb. 27, 1997; revised July 15, 1997; accepted July 30, 1997.
Li Zhou1,
Anne Chomyn2,
Giuseppe Attardi2, and
Carol A. Miller1
1 Departments of Pathology and Neurology, University of
Southern California School of Medicine, Los Angeles, California 90033, and 2 Division of Biology, California Institute of
Technology, Pasadena, California 91125
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.
Key words:
MERRF syndrome;
CNS microdissection;
PCR;
neurodegeneration;
tRNALys;
mtDNA
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