Feline Myotonia Congenita (MC)
Feline Myotonia Congenita (MC) is a hereditary neuromuscular disorder affecting skeletal muscles in the domestic shorthair cats. Myotonia is defined as delayed relaxation of voluntarily or reflexively contracted muscle and the term derives from Greek word ”Myo-” for muscle, and Latin word ”Tonus” for tension. The condition is classified as channelopathy, a disease caused by disturbed function of ion channel subunits of the proteins that regulate them. Myotonia congenita was first described in 1876 in a human patient, while report about feline form of the disorder did not appear until 1998. Except in humans and cats, myotonia congenita was identified in dogs, goats, horses, water buffalo and mice, and in all those species MC is associated to mutation in same gene, the CLCN1 gene. Patterns of the disorders vary from dominant to recessive, as well as the degree of its severity.
Voltage-dependent chloride (Cl–) channels are transmembrane proteins. They are critical for the normal function of skeletal muscle cells and participate in many physiological processes, such as maintenance of normal cellular excitability, the control of neurotransmitter release and the transport of ions across epithelial cells. In case of lack of sufficient functional chloride channels, the muscle fiber membrane becomes hyper-excitable and continues to be electrically active when stimulated for longer period of time than a normal muscle fiber. In this case, muscle relaxation is delayed and contraction is prolonged.
Characteristics and Symptoms
General physical examination revels in affected cats restricted jaw opening, halitosis, varying degrees of gingivitis, pseudoptyalism (the dribbling of saliva caused by a difficulty in swallowing or paralysis of the lips), marked dental calculus accumulation with palpable loose teeth and evidence of poor grooming habits. Severe muscle hypertrophy is present along the cervical spine. In one affected cat a respiratory distress, such as stridor and open mouth breathing, has been reported. Mental activity in all affected cats appears as normal. The gait shows as stiff, choppy and short-strided in all limbs, especially in the pelvic limbs, followed by a decreased ability to adduct all limbs while walking. Affected cats show a protruding tongue with excessive drooling, prominent neck and proximal limb musculature, decreased range of jaw motion, oral disease, and a progressive decrease through successive attempts of visual placing in the thracic limbs. No impact has been noticed on smooth or cardiac muscles, which is consistent with previous reports of myotonia congenita.
Feline Myotonia Congenita (MC) is caused by a mutation within CLCN1 gene, which causes expression of truncated protein which is malfunctional.
MC in cats is inherited in an autosomal recessive mode of inheritance. Cat carrying one copy of the mutated gene is heterozygous and will not show the myotonia congenita symptoms. When mating two carriers (heterozygotes) at conception each kitten has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.
Gandolfi B, Daniel RJ, O’Brien DP, Guo LT, Youngs MD, et al. (2014) A Novel Mutation in CLCN1 associated with Feline Myotonia Congenita. PloS ONE 9(10): e109926. doi:10.1371/journal.pone.0109926.