ALTERATIONS IN NEUROFILAMENT MESSENGER-RNA IN HEREDITARY CANINE SPINAL MUSCULAR-ATROPHY

BACKGROUND: Hereditary canine spinal muscular atrophy (HCSMA) is a dom inantly 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 a xonal caliber, we began an examination of neurofilament gene expressio n in HCSMA early in the clinical disease to determine whether this fam ily of proteins was selectively affected and could thus possibly contr ibute to the morphologic and functional alterations characteristic of the disease. EXPERIMENTAL DESIGN: We used quantitative in situ hybridi zation to compare levels of mRNA encoding neurofilament protein subuni ts in lateral ventral horn neurons from the cervical spinal cord enlar gement (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/n euron and the mean neuronal grain density for the HCSMA section were d ivided by that value for the control section on each slide. The means of these ratios for each mRNA species (i.e., neurofilament subunits an d total polyadenylated mRNA (poly-A+) were then compared statistically . RESULTS: The levels of mRNA encoding the low molecular weight neurof ilament protein subunit were significantly different from levels of mR NA encoding the high molecular weight neurofilament protein subunit an d poly-A+ mRNA in dogs with HCSMA compared with control dogs. The neur onal levels of poly-A+ mRNA were comparable in dogs with HCSMA and con trols. CONCLUSIONS: If neurofilament protein subunit levels are found to follow the mRNA levels in this animal model, our results would sugg est that decreased expression of the low molecular weight neurofilamen t gene is sufficient to inhibit neurofilament function, i.e., maintena nce of axonal caliber, probably by disrupting normal neurofilament ass embly.