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X-Linked Progressive Retinal Atrophy 2 (RPGR gene, Mongrel)

X-Linked Progressive Retinal Atrophy 2 (RPGR gene, Mongrel)

X-linked progressive retinal atrophy 2 is an eye disease caused by early onset degeneration of the retinal rods and cones, the photoreceptor cells that transmit light signals in nerve impulses.

Symptoms

Symptoms appear earlier than in progressive X-linked retinal atrophy type 1. Visual deficits are evident around 6 weeks of age in the affected puppy. As the disease progresses, the photoreceptor cells, cones and rods, gradually degenerate until the animal loses vision completely by about two years of age.

Disease Management

There is no treatment for progressive vision loss. Thanks to their acute senses of smell and hearing, dogs can compensate for this loss, especially in familiar environments. You can help your dog by establishing regular exercise routes and gradually introducing the necessary changes.

Genetic basis

This disease follows an X-linked recessive mode of inheritance. Recessive X-linked inheritance means that female dogs must receive two copies of the mutation or pathogenic variant (one from each parent) to develop the disease, whereas males need only one copy of the mutated gene or variant from the dam to develop the disease. Male dogs usually show symptoms of the disease. Each male puppy born to a mother carrying the mutation has a 50% chance of inheriting the mutation and thus the risk of developing the disease. Bitches that do not carry the mutation are not at increased risk of having affected puppies. Breeding between dogs carrying genetic variants that can cause disease, even if they do not show symptoms, is not recommended.

Technical report

The RPGR gene codes for a protein called "regulatory retinitis pigmentosa GTPase". Mutations in this gene are primarily responsible for X-linked retinitis pigmentosa in humans and also in dogs, a disease that causes retinal degeneration and vision loss. The RPGR gene is expressed in the retina and is thought to produce a protein that interacts with other molecules to form complexes and is essential for the formation of the cilia of photoreceptor cells (cones and rods) as well as their maintenance and function. In Mongrel dogs the variant causing this type of X-linked hereditary retinal atrophy is a two nucleotide deletion known as c.3472_3473del (Zhang et al., 2002). This variant affects normal retinal development, damages both the function and morphology of photoreceptor cells, and causes a more severe phenotype of earlier onset than that seen in type 1 atrophy.

Most affected breeds

Mongrel

Bibliography

Appelbaum T, Santana E, Aguirre GD. Critical Decrease in the Level of Axon Guidance Receptor ROBO1 in Rod Synaptic Terminals Is Followed by Axon Retraction. Invest Ophthalmol Vis Sci. 2020 Mar 9;61(3):11.

Zhang Q, Acland GM, Wu WX,et al. Different RPGR exon ORF15 mutations in Canids provide insights into photoreceptor cell degeneration. Hum Mol Genet. 2002 May 1;11(9):993-1003.