Spongy cerebellar degeneration with cerebellar ataxia (SDCA2) in Belgian shepherds

The SDCA2 is a subtype of a neurodegenerative disease known as spongy degeneration with cerebellar ataxia that affects Malinois dogs and other varieties of the Belgian shepherd breed.

SDCA2 is relatively variable as to disease onset, severity and histopathological lesions. The cerebellar dysfunction occurs at the age of 4 to 6 weeks. The main symptoms are ataxic gait, balance loss and insufficient movement coordination (stumbling, staggering, falling and intention tremor). All affected puppies show wide-based ataxic gait, which is more obvious in the hind limbs. In this way the puppies try to keep stability and improve the movement coordination. The prognosis is poor and usually ends by euthanizing the dog.

The disease is caused by the mutation c.130_131ins227 in ATP1B2 gene that is encoding the beta 2 subunit isoform Na+/K+ ATPase.  The beta 2 is expressed predominantly in brain, particularly in the cerebellum. The Na+/K+ ATPase is a protein in the plasma membrane. Its main task is to regulate the intracellular homeostasis. A failure of K+ homeostasis in the central nervous system is often associated with the neurological disorders such as cerebellar dysfunction and epilepsy. The mutation was found in the Belgian Shepherds Malinois and Tervueren. In other varieties of Belgian Shepherd, this mutation has not been found so far. However detection of such mutation in future is not excluded. The occurrence of this mutation in breeds that were cross-bred with Belgian shepherds in the past, for example in some lineages of Dutch shepherds, cannot be excluded.

SDCA2 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 SDCA2.  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.

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References:

Nico Mauri, Miriam Kleiter, Elisabeth Dietschi, Michael Leschnik, Sandra Högler, Michaela Wiedmer, Joëlle Dietrich, Diana Henke, Frank Steffen, Simone Schuller, Corinne Gurtner, Nadine Stokar-Regenscheit, Donal O'Toole, Thomas Bilzer, Christiane Herden, Anna Oevermann, Vidhya Jagannathan and Tosso Leeb: A SINE Insertion in ATP1B2 in Belgian Shepherd Dogs Affected by Spongy Degeneration with Cerebellar Ataxia (SDCA2); G3: GENES, GENOMES, GENETICS Early online June 15, 2017; https://doi.org/10.1534/g3.117.043018