Juvenile Myoclonic Epilepsy Rhodesian Ridgeback Type (JME)

Juvenile Myoclonic Epilepsy Rhodesian Ridgeback Type (JME) is an inherited disorder characterized by myoclonic jerks and photosensitivity. JME in Rhodesian Ridgeback dogs is a canine equivalent to the human form of JME, with which it shares the same symptoms and age of onset. These similarities made the canine equivalent of this disorder important study model and contributed to the identification of causative mutation.

Characteristics and Symptoms

Studied affected dogs displayed frequent myoclonic jerks or twitches, which start to occur around 6 months of age. Owners of the affected dogs described the myoclonic jerks as severe startling or even resembling an electric shock. The myoclonic seizures can be triggered by visual stimuli, such as light flashes, a sudden incidence of light or flashing light on the sea waves. Photosensitivity was recognized in 35% of dogs. The twiches mostly occur when the dog is in the relaxed state, drowsy or when falling asleep or napping. Twitches can occur also during sitting, standing or walking. There were no noticed changes in the behavior. The myoclonic jerks were mostly limited to the trunk, proximal limb musculature, cervical musculature producing nodding movements of the head, and the face. The intensity can vary between events and affected dogs. After the seizure, the dogs seem confused and scared. Due to the seizures, sleep among those dogs was impaired. The frequency of the twitches can go up to 150 twitches per day. Affected dogs are usually euthanized upon owner’s request.

Genetics

Juvenile Myoclonic Epilepsy in Rhodesian Ridgeback Dogs (JME) is caused by a 4-bp deletion in the exon 2 of the DIRAS1 gene, causing a frameshift and a loss of the stop signal. Research among other breeds affected with epilepsy revealed that this mutation is specific only for Rhodesian Ridgeback breed. A carrier rate of 15% was determined.

The expression of the DIRAS1 is limited to the brain and heart and it is suggested that DIRAS1 protein is needed for acetylcholine transmission at neuromuscular junctions and neuronal development. Nicotinergic cholinergic activity influences brain excitability and cognition, regulates the excitatory/inhibitory switch during neuronal development, stimulates glutamate release from thalamocortical terminals, and maintains nonrapid eye movement sleep by low levels of acetylcholine. Abnormal DIRAS1 function could alter cholinergic neurotransmission or formation of neuronal circuits and network assembly in the developing brain resulting in myoclonic epilepsy and photosensitivity.

Juvenile Myoclonic Epilepsy Rhodesian Ridgeback Type (JME)is inherited in an autosomal recessive pattern. Healthy parents of affected puppy are obligate heterozygotes and therefore carry one mutant allele. Heterozygotes have no symptoms. Dogs homozygous for the mutation will display the symptoms of the JME. At conception, when mating two carrier dogs, 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.

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Test for ridge disposition in Rhodesian ridgeback

The Rhodesian ridgeback’s distinguishing feature is the ridge of hair running along its back in the opposite direction from the rest of its coat. This characteristic is genetically determined and governed by the dominant inheritance. To understand dominant inheritance, it is necessary first to realize that every dog has inherited from its parents, two copies of each gene (one from its mother and one from its father).

Dominant inheritance means that if the puppy inherited the ridge gen just from one parent, there ridge is formed on the puppy´s back. In this case, the dog with only one ridge gene is called heterozygote (Rr). If the dog inherited ridge gene from both parents, the dog has two ridge genes, will form a ridge on the back and is called dominant homozygote (RR).

However, in the litter there may ridgeless puppies. Ridgeless puppies are born to two heterozygotes (male and female dog with only one ridge gene). Statistically, in a litter of two heterozygotes there will be born 25% ridgeless puppies .

However, according to the test results in practice, it appears that the ridge genetics in Rhodesian ridgebacks is not so simple and is characterized by incomplete manifestation of ridge gene. This means that all dominant homozygotes have ridge. Also, approximately 90% of the heterozygotes have ridge, but 10% of the heterozygotes are ridgeless. Very likely, the ridge gene is suppressed and ridge is not formed on the back. All individuals who lack ridge gene (rr) are ridgeless.

In collaboration with the Veterinary Research Institute in 2014, we have for the first time developed the genetic test, which distinguishes dominant homozygotes (two ridge genes) from heterozygotes (one ridge gene). This is the most accurate way how to predict birth of ridgeless puppies, if both parents are tested (see. Table below). Our test determines whether and to what extent there will ridgeless puppies in a litter!

Further, it was also observed the association between genetic disposition for the ridge and the occurrence of dermoid sinus. According to the research, dermoid sinus occurs approximately 5 times more often in dominant homozygotes. Our test may reduce the incidence of dermoids in Rhodesian Ridgebacks by a controlled selection of tested individuals for mating.

We have  developed the genetic test, which distinguishes dominant homozygotes (R/R – two ridge genes) from heterozygotes (R/r – one ridge gene). Using our genetic test, a breeder may accuratelly predict birth of ridgeless puppies (see Table below).

Test for ridge disposition in Rhodesian ridgeback