Alterations in neurofilament mRNA in hereditary canine spinal muscular atrophy

Hereditary canine spinal muscular atrophy (HCSMA) is a dominantly inherited motor neuron disease in which distal axonal caliber is reduced in lower motor neurons. Because several animal models show that neurofilament protein gene expression is a major determinant of axonal caliber, we began an examination of neurofilament gene expression in HCSMA early in the clinical disease to determine whether this family of proteins was selectively affected and could thus possibly contribute to the morphologic and functional alterations characteristic of the disease.

We used quantitative in situ hybridization to compare levels of mRNA encoding neurofilament protein subunits in lateral ventral horn neurons from the cervical spinal cord enlargement (C7-C8) in 10-week-old homozygous HCSMA and control dogs. Each slide contained a spinal cord section from a control and an HCSMA dog in order to make within-slide comparisons. The mean number of grains/neuron and the mean neuronal grain density for the HCSMA section were divided by that value for the control section on each slide. The means of these ratios for each mRNA species (i.e., neurofilament subunits and total polyadenylated mRNA (poly-A+) were then compared statistically.

The levels of mRNA encoding the low molecular weight neurofilament protein subunit were significantly different from levels of mRNA encoding the high molecular weight neurofilament protein subunit and poly-A+ mRNA in dogs with HCSMA compared with control dogs. The neuronal levels of poly-A+ mRNA were comparable in dogs with HCSMA and controls.

If neurofilament protein subunit levels are found to follow the mRNA levels in this animal model, our results would suggest that decreased expression of the low molecular weight neurofilament gene is sufficient to inhibit neurofilament function, i.e., maintenance of axonal caliber, probably by disrupting normal neurofilament assembly.