Myoclonic epilepsy in Rhodesian ridgebacks

Juvenile myoclonic epilepsy is a genetic disease characterized by sudden short and uncontrolled muscle jerks or twitches (electric shock-like seizures). The epileptic seizures affect mainly the muscles of proximal limbs and trunk, cervical muscles, head muscles and facial muscles.

The disease occurs in young dogs and the onset of the first seizures is around 6 months of age. The seizures occur very often (daily or almost daily) and the owners of the affected dogs reported up to 150 twitches per day. The frequency and intensity of the seizures can differ among the individual dogs or among the seizures of an individual dog respectively.  The seizures occur most commonly when the animals are relaxed, drowsy or in the first stages of sleep. Some affected dogs show visually induced seizures (described as photosensitivity) e.g. by light flashes (sudden light incidence when opening the windows or sunshine flashing through the trees during a walk in the forest).

The mutation c.564_567delAGAC in DIRAS1-gene has been identified as the cause of this disease.  Genetic analyses revealed 4-bp deletion in this gene. DIRAS1 belongs to the Ras family GTPases that are in the cells connected through many signalling pathways involved in cell growth and cell differentiation and synaptic plasticity necessary for  learning and memory. DIRAS1 is widely expressed in the brain and some studies pointed out its possible connection with acetylcholine transmission and regulation of acetylcholine release at myoneural junctions and its probable role in cell migration, axonal growth and development of neuronal dendrites.

Epilepsy is one of the most common chronic neurological diseases in dogs. However, it seems that particularly this mutation is specific only to the Rhodesian ridgeback breed.  The occurrence of the mutant allele is approximately in 15% of dogs. Genetic tests enable to diagnose this specific epilepsy and identify the carrier.

Myoclonic epilepsy is inherited autosomally recessively which means that the disease develops only in those dogs who inherit mutated allele from both parents. Carriers of mutated allele (heterozygotes) are clinically healthy but transmit the mutation on their descendants. In case of mating two heterozygous dogs there is a theoretical chance that 25% of descendants will be absolutely healthy, 50% will be carriers a 25% will inherit from both parents mutated allele and therefore will be affected by myoslonic epilepsy.

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References:

Franziska Wielaendera et al.: Generalized myoclonic epilepsy with photosensitivity in juvenile dogs caused by a defective DIRAS family GTPase 1; PNAS | March 7, 2017 | vol. 114 | no. 10 | 2669–2674