What Is dHMN-9?
dHMN-9 is a type of CMT caused by autosomal dominant mutations in the WARS1 gene. This gene provides instructions for making tryptophanyl-tRNA synthetase, an enzyme that attaches the amino acid tryptophan to its transfer RNA during protein synthesis. Mutations in the WARS1 gene disrupt this function, leading to impaired nerve signal transmission.
dHMN-9 is autosomal dominant, meaning that just one of the gene’s two copies needs a mutation to cause this subtype.
Clinical Features
The age of symptom onset in dHMN-9 is variable, ranging from childhood to adulthood. As a distal hereditary motor neuropathy, dHMN-9 predominantly affects the motor nerves, producing weakness and atrophy of the distal limbs while sensation is typically preserved. Nerve conduction studies usually show somewhat slowed conduction velocities and reduced amplitudes, consistent with an axonal form of CMT.
dHMN-9 symptoms may include:
- Weakness in the feet and lower legs
- Muscle atrophy
- Foot drop
- Reduced or absent reflexes
- A steppage-style walking pattern
- Foot deformities, including high arches, and hammertoes (clawed toes)
- Progressive involvement of the hands
- Difficulty with fine motor skills and manual dexterity
- Additional symptoms not listed here
Disease Course
dHMN-9 shows wide variability in severity and progression. Some individuals are mildly affected, while others develop a more severe disease. Disease progression is generally slow, and life expectancy is not reduced.
