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Canine Spinocerebellar Ataxia (SCA)
| Acronym: | SCA |
| Gene: | KCNJ10 |
| Mutation: | c.627C>G |
| Inheritance: | Autosomal recessive |
| Sample type: | CHS (Cheek Swab), WBE (Whole Blood EDTA) |
Genetics and characteristics
Canine spinocerebellar ataxia (SCA) is an inherited disorder occurring in Jack Russell Terriers, Smooth-haired Fox Terriers, and Toy Fox Terriers. The first time form of hereditary ataxia was reported in the Smooth-Haired Fox Terrier in 1957, which was followed by a report of a disease with similar symptoms in the Jack Russell Terrier in 1973. Different forms of ataxia differ from each other in age at onset, clinical signs, and histopathologic changes. Jack Russell Terriers, Parson Russell Terriers, and Russell Terriers are recognized as separate breeds and it is believed they have common descent, originating from the stock of Parson Jack Russell, a 19th-century dog breeder from Great Britain. The age of onset of canine spinocerebellar ataxia is between 2 months to 9 months of age. In beginning, the dog’s owner may notice a lack of coordination, pelvic limb swaying when walking, and difficulties in climbing upstairs or jumping. SCA is a progressive disorder, and with its progress, the ”prancing” type of gaint is noticed, especially in the pelvic limbs. Affected dogs may have difficulties while standing, and may fall frequently, with troubles in attempts of returning to a standing position. Neurological examinations have revealed symmetric spinocerebellar ataxia, characterized by hyperthermia and spasticity, especially in the pelvic limbs. Postural reactions appear to be delayed and hyperthermic, while spinal reflexes are normal to increase. Brain stem auditory-evoked potentials (BAEPs) identified abnormalities in some but not all SCA-affected dogs. These abnormalities include a lack of waves III, IV, and V.
Post-mortem histopathological examination showed bilateral symmetrical myelopathy, myelin loss, swelling of axons, moderate diffuse gliosis, and marked loss of myelinated nerve fibers. All of the SCA-affected Jack Russell Terriers exhibit myokymia. Myokymia is an involuntary tremor of muscles, where muscles become hyperactive and heat is created. Excessive heat causes hyperthermia and can result in a seizure. In SCA-affected dogs, it usually appears between 2 months to 3 years of age. Common symptoms of myokymia are stiffness of hind limbs and forelimbs, curled paws, facial rubbing, and extremely high core body temperature. Myokymia is exhibited by human patients with mutations in voltage-gated potassium channel genes. Since SCA is an inherited disorder with a progressive nature, with no cure developed, most of the SCA-affected dogs are euthanized by 2 years of age due to poor quality of life. Canine spinocerebellar ataxia (SCA) is caused by a point mutation in the KCNJ10, a potassium channel gene known to cause myokymia in human patients. SCA is inherited as an autosomal recessive disorder. Healthy parents of an affected dog are obligate heterozygotes and therefore carry one mutant allele. Heterozygotes have no symptoms. At conception, each cub 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.
Results Reported As
Test Result |
Interpretation of test result |
CLEAR |
Tested mutation was not detected in animal with „clear“ result. Animal tested as clear has wild-type allele in homozygous state (i.e. two pairs of healthy alleles). It will not develop disease caused by tested mutation.* It will pass only wild-type allele to its offspring. |
CARRIER |
Tested mutation was detected in animal with „carrier“ result. Animal tested as carrier has one wild-type and one mutation allele, it is in heterozygous state. It will not develop disease caused by tested mutation.* It can pass wild-type or mutation allele to its offspring. |
AFFECTED |
Tested mutation was detected in animal with „affected“ result. Animal tested as affected has two copies of mutation alleles affecting the gene. It is likely the animal will experience a genetic disorder due to this mutation.** It will pass only mutation allele to its offspring. |
*Test excludes only tested mutation but not possible unknown mutations or factors that can lead to similar condition/symptoms.
** Potential unknown mutations or multiple other factors can possibly affect the likelihood of experiencing a genetic disorder.
References:
Forman OP, De Risio L, Mellersh CS (2013) Missense Mutation in CAPN1 Is Associated with Spinocerebellar Ataxia in the Parson Russell Terrier Dog Breed. PLoS ONE 8(5): e64627.
Gilliam D, O’Brien DP, Coates JR, Johnson GS, Johnson GC, Mhlanga-Mutangadura T, Hansen L, Taylor JF, Schnabel RD. A Homozygous KCNJ10 Mutation in Jack Russell Terriers and Related Breeds with Spinocerebellar Ataxia with Myokymia, Seizures, or Both. J Vet Intern Med.
Rohdin C, Gilliam D, O’Leary CA, O’Brien DP, Coates JR, Johnson GS, Jaderlund KH. A KCNJ10 mutation previously identified in the Russell group of terriers also occurs in smooth-haired fox terriers with hereditary ataxia and in related breeds. Acta Vet Scand. 2015 May 23;57:26.