Genetic Myopathies in Ruminants and Pigs
Myotonia congenita in goats is due to an autosomal dominant mutation that has incomplete penetrance in the skeletal muscle chloride channel. Goats with this mutation have been selected for as a breed and are commonly referred to as “fainting goats.” Clinical signs ranging from stiffness after rest to marked general rigidity after visual, tactile, or auditory stimulation usually develop by 6 wk of age. These signs remain throughout the animal’s life but are not progressive. A diagnosis of myotonia is made by identifying the characteristic “dive bomber” discharges in electromyography and/or by genetic testing.
A mutation in the myophosphorylase gene in Charolais cattle produces signs of exercise intolerance and muscle necrosis very similar to nutritional myodegeneration. The disease has been recognized in many countries, including the USA and New Zealand. Animals become exercise intolerant, may collapse when forced to exercise, and develop muscle necrosis characterized by increased serum CK and prolonged recumbency. Supportive care during episodes has allowed many young animals to survive and, unknowingly to the owner, enter the breeding herd.
Malignant hyperthermia in swine (see Malignant Hyperthermia) is due to an autosomal recessive genetic mutation in the skeletal muscle ryanodine receptor 1 gene (RYR1) that causes abnormal meat quality in swine. Pietrain, Poland China, and certain strains of Landrace pigs are affected. During transportation or anesthesia, pigs develop increased body temperature, extreme rigidity of the skeletal muscles, and lactic acidosis. At slaughter, affected muscles become pale, soft, and exudative, which diminishes meat quality. A genetic test is available to diagnose affected and carrier animals.
The RN(−) (rendement Napole) phenotype is common in Hampshire pigs. It is due to an autosomal dominant mutation in the protein kinase AMP-activated gamma 3 subunit gene (PRKAG3), which encodes the gamma 3 isoform of AMP-activated protein kinase (AMPK). Clinically, pigs appear healthy; however, the 70% increase in glycogen content in skeletal muscle causes poor meat quality at slaughter.
Bovine congenital pseudomyotonia is an impairment of muscle relaxation induced by exercise that prevents animals from performing rapid movements. Different mutations in the ATP2A1 gene that encodes the Ca2+-ATPase in skeletal muscle sarcoplasmic reticulum have been identified in the Chianina, Belgian Blue, and Romagnola breeds.