The dorsal root ganglion in Friedreich's ataxia

Atrophy of dorsal root ganglia (DRG) and thinning of dorsal roots (DR) are hallmarks of Friedreich's ataxia (FRDA). Many previous authors also emphasized the selective vulnerability of larger neurons in DRG and thicker myelinated DR axons. This report is based on a systematic reexamination of DRG, DR and ventral roots (VR) in 19 genetically confirmed cases of FRDA by immunocytochemistry and single- and double-label immunofluorescence with antibodies to specific proteins of myelin, neurons and axons; S-100alpha as a marker of satellite and Schwann cells; laminin; and the iron-responsive proteins ferritin, mitochondrial ferritin, and ferroportin. Confocal images of axons and myelin allowed the quantitative analysis of fiber density and size, and the extent of DR and VR myelination. A novel technology, high-definition X-ray fluorescence (HDXRF) of polyethylene glycol-embedded fixed tissue, was used to "map" iron in DRG. Unfixed frozen tissue of DRG in three cases was available for the chemical assay of total iron. Proliferation of S-100alpha-positive satellite cells accompanied neuronal destruction in DRG of all FRDA cases. Double-label visualization of peripheral nerve myelin protein 22 and phosphorylated neurofilament protein confirmed the known loss of large myelinated DR fibers, but quantitative fiber counts per unit area did not change. The ratio of myelinated to neurofilament-positive fibers in DR rose significantly from 0.55 to 0.66. In VR of FRDA patients, fiber counts and degree of myelination did not differ from normal. Pooled histograms of axonal perimeters disclosed a shift to thinner fibers in DR, but also a modest excess of smaller axons in VR. Schwann cell cytoplasm in DR of FRDA was depleted while laminin reaction product remained prominent. Numerous small axons clustered around fewer Schwann cells. Ferritin in normal DRG localized to satellite cells, and proliferation of these cells in FRDA caused wide rims of reaction product about degenerating nerve cells. Mitochondrial ferritin was not detectable. Ferroportin was present in the cytoplasm of normal satellite cells and neurons, and in large axons of DR and VR. In FRDA, some DRG neurons lost their cytoplasmic ferroportin immunoreactivity, whereas the cytoplasm of satellite cells remained ferroportin positive. Ferroportin in DR axons disappeared in parallel with atrophy of large fibers. HDXRF of DRG detected regional and diffuse increases in iron fluorescence that matched ferritin expression in satellite cells. The observations support the conclusions that satellite cells and DRG neurons are affected by iron dysmetabolism; and that regeneration and inappropriate myelination of small axons in DR are characteristic of the disease.