Primary open-angle glaucoma (POAG) in the Fawn Brittany Basset

Glaucomas represent a diverse group of diseases that gradually lead to blindness due to the loss of retinal ganglion cells and damage to the optic nerve. It is one of the most common causes of irreversible blindness and visual impairment in humans and animals worldwide (Bouhenni et al., 2012). The disease is characterized by a gradual and chronic increase in intraocular pressure and lens subluxation. In the late stages of the disease, there is enlargement of the eyeball, degeneration of the retina, and deformation of the optic nerve papilla.

Glaucoma is generally classified as primary (usually hereditary) and secondary (resulting from other damage to the eye, e.g., injury, tumor etc.) (Kotman et al., 2003).

Primary glaucoma is further divided into three main groups according to the iridocorneal angle (the angle between the iris, cornea, and sclera:

• Primary open-angle glaucoma (POAG)
• Primary angle-closure glaucoma (PACG)
• Primary congenital glaucoma (PCG)

POAG in a Fawn Brittany Basset is caused by a missense mutation c.1552G>A in the ADAMTS17 gene. The ADAMTS17 gene is a member of the ADAMTS family of extracellular proteases. The mutation likely disrupts the enzymatic function of the protein.

The mutation is inherited in an autosomal recessive manner. This means that the disease only develops in individuals who inherit the mutated gene from both parents. Carriers of the mutated gene are clinically healthy, but they pass the mutation on to their offspring. In the case of mating two heterozygous individuals, theoretically 25% of offspring will be completely healthy, 50% will be carriers, and 25% will inherit the mutated gene from both parents and will therefore be affected by the disease.

Genetic testing can clearly reveal an animal's genotype and is a useful tool for breeders to prevent the unintentional breeding of affected puppies.

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References:

Oliver, J.A., Forman, O.P., Pettitt, L., Mellersh, C.S. : Two independent mutations in ADAMTS17 are associated with primary open angle glaucoma in the Basset Hound and Basset Fauve de Bretagne breeds of dog. PLoS One 10:e0140436, 2015. Pubmed reference: 26474315