Possibly relevant human trait(s) and/or gene(s) (MIM number): 300029

Mendelian trait/disorder: yes

Mode of inheritance: X-linked

Considered a defect: yes

Key variant known: yes

Year key variant first reported: 2002

Cross-species summary: This disorder has been renamed in OMIA on the basis of the review by Miyadera et al. (2012)

Species-specific symbol: XLPRA2

Species-specific description: The difference between XLPRA1 (OMIA 000831-9615) and XLPRA2 (this entry) is summarised by Appelbaum et al. (2020) as "XLPRA1-affected dogs have normal PR morphogenesis, after which progressive rod–cone degeneration develops in the peripheral retina, gradually advancing toward the optic disc. . . . The phenotype associated with XLPRA2 is very severe and manifests during early retinal development."

Molecular basis: Based on a comparative positional cloning approach (the canine disorder maps to a location on the canine X chromosome that is homologous with the location of the same disorder (RP3) in humans, which is due to mutations in the RPGR gene), Zhang et al. (2002) identified a "a two-nucleotide deletion (delGA) in 1084–1085" in the canine RPGR gene as a causal mutation for a form of X-linked PRA they call XPRA2. The authors noted that this deletion "results in a frameshift that significantly changes the deduced peptide sequence, causing an increased isoelectric point (4.30 versus 4.01), and leads to the inclusion of 34 additional basic residues before prematurely terminating translation 71 amino acids downstream".

Clinical features: Beltran et al. (2006): "Abnormal development of photoreceptors was recognizable as early as 3.9 weeks of age. Outer segment (OS) misalignment was followed by their disorganization and fragmentation. Reduction in length and broadening of rod and cone inner segments (IS) was next observed, followed by the focal loss of rod and cone IS at later time points. The proportion of dying photoreceptors peaked at approximately 6 to 7 weeks of age and was significantly reduced after 12 weeks. In addition to rod and cone opsin mislocalization, there was early rod neurite sprouting, retraction of rod bipolar cell dendrites, and increased Müller cell reactivity. Later in the course of the disease, changes were also noted in horizontal cells and amacrine cells." As summarised by these authors: "XLPRA2 is an early-onset model of XLRP that is morphologically characterized by abnormal photoreceptor maturation followed by progressive rod-cone degeneration and early inner retina remodeling. "

Breed: Mongrel.

Associated gene: