The mouse mutation muscle deficient (mdf) is characterized by a progressive motoneuron disease.

Muscle deficient (mdf) is an autosomal-recessive mutation mapped to mouse chromosome 19. The clinical phenotype and the muscle histopathology, briefly described in 1980, and the nervous system histopathology are detailed in the present study. Homozygotes develop a posterior waddle at 4 to 8 weeks of age. Soon thereafter, the hindlimbs become paralyzed and weakness appears in forelimbs, leading to a serious disability. The disease progresses slowly and the mean lifespan is reduced to 8 months. Skeletal muscles exhibit a neurogenic atrophy with signs of reinnervation. Peripheral nerves display axonal degeneration. Neurons within the spinal cord ventral horn, and some motor nuclei of the brain stem, are affected by a cytoplasmic vacuolar degeneration. Ascending and descending spinal cord tracts appear normal. An astrogliosis, restricted to the ventral horn of the spinal cord, occurs in mdf/mdf mice of 10 weeks of age. These clinical and histological features are indicative of a progressive motor neuronopathy. Among the murine spinal muscular atrophies, the programmed cell death of the mdf motoneurons is morphologically similar to wobbler. Because of the long time course, the mdf mutation may represent a valuable tool for understanding juvenile motoneuron diseases with chronic evolution, even though the murine locus is not syntenic with the human ones.