CNS atrophy with cerebellar ataxia (CACA) in Belgian Shepherds

CNS (central nervous system) atrophy with cerebellar ataxia (CACA) is a disease found in Belgian Shepherds. The affected puppies show ataxia, which is characterized by uncoordinated movements and intention tremors. Clinical signs are relatively non-specific, and other variants in other genes may lead to similar forms of ataxia in Belgian Shepherds (SDCA1, SDCA2).

CACA is caused by a mutation in the SELENOP gene. It is a homozygous ~17 kb deletion involving the entire protein coding sequence of the SELENOP gene in the critical interval of 52 Mb. The exact genomic designation of the deletion is Chr4:66,946,539_66,963,863del17,325 (CanFam 3.1). The SELENOP gene encodes selenoprotein P, which is involved in the deposition and transport of selenium to the brain and other organs. Incorporation of selenium into selenoprotein P prevents the toxic effects of free selenium. A deletion in the SELENOP gene results in the complete absence of encoded selenoprotein P and causes a defect in selenium transport to the CNS.

Clinical signs, such as uncoordinated movements, intention tremors, general elevated muscle tone and reduced swallowing reflex can be observed in puppies at 2 weeks of age. Gradual progression leads to such a degree of severity of clinical signs that the animals must be euthanized. Affected animals show atrophy of the CNS, especially of the cerebellum. Atrophy affects all layers of the cerebellar cortex with depletion of Purkinje and granular cells. Neuroaxonal degeneration affects the midbrain, brain stem and spinal cord. There is also a marked reduction in the amount of myelin in the white matter of the brain and spinal cord. Blood selenium concentration is reduced by 70% in affected homozygotes and 30% in heterozygotes compared to healthy dogs.

CACA is a recessively inherited disease. The disease develops in dogs which inherit the mutated gene from each parent.  These dogs are designated as P/P (positive/positive). The carriers of the mutated gene are designated as N/P (negative/positive). The carriers inherited the mutated gene from one parent only and are without clinical signs. However, they pass the disease on to their offspring.  When mating two heterozygotes (N/P), there will be theoretically 25% of the offspring healthy, 50% of the offspring will be carriers and 25% of the offspring will inherit the mutated gene from both parents and will be affected by CACA.  Mating one healthy dog (N/N) with a carrier of this mutation (N/P) will theoretically produce 50% carriers and 50% healthy offspring.  If a carrier (N/P) is mated with an affected dog (P/P), there will be theoretically 50% affected dogs and 50% carriers.

The genetic test can reveal the genotype of the animal and is a useful tool for breeders to prevent unintentional breeding of affected puppies.

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Reference:

Christen, M., Högler, S., Kleiter, M., Leschnik, M., Weber, C., Thaller, D., Jagannathan, V., Leeb, T. : Deletion of the SELENOP gene leads to CNS atrophy with cerebellar ataxia in dogs. PLoS Genet 17:e1009716, 2021. Pubmed reference: 34339417. DOI: 10.1371/journal.pgen.1009716.