Type B1 progressive retinal atrophy of the Miniature Schnauzer (photoreceptor dysplasia, pd): progressive degeneration of photoreceptors leading to blindness, with a normal ocular fundus until 10 months and advanced disease by around 3 years. The commercial test detects an intronic SNV in HIVEP3 strongly associated with the phenotype: it is a useful and robust linked marker, although it is probably not the causal variant. The most likely causal variant is a complex structural rearrangement of PPT1 (PPT1dci), very difficult to genotype routinely.

Affected breed: Miniature Schnauzer (the only documented breed, OMIA:001311). Population carrier frequencies: limited data.

- Nyctalopia as the initial sign
- Hyperreflective tapetal changes
- Retinal vascular attenuation
- Progressive blindness

Photoreceptor dysplasia (pd) of the Miniature Schnauzer was described in 1991 with autosomal recessive inheritance demonstrated by pedigree and crosses (Parshall et al.). The phosducin (PDC) variant proposed in 1998 was later ruled out as not associated. Murgiano et al. (2019) identified as the probable cause a complex structural rearrangement of PPT1 (PPT1dci), confirmed in homozygosity in a cohort of 22 cases. Kaukonen et al. (2020) proposed an intronic SNV of HIVEP3 and recognised two clinical subtypes (types 1 and 2). The researchers of both studies published a consensus (Aguirre et al. 2020): the PPT1 variant is the most probable causal one, but the HIVEP3 SNV, strongly associated, serves as a marker because it is genotyped robustly.

- Test breeding animals with the HIVEP3 marker test before mating.
- Do not mate two carriers/homozygotes with each other: risk of affected offspring (recessive).
- A carrier can be mated with a free animal; test the offspring intended for breeding.
- Interpret the result as a linked marker (not as direct causality) and confirm with the laboratory the variant covered by the assay.
- Periodic ophthalmological examination of breeding animals, because other PRAs exist in the breed.

The Miniature Schnauzer may present several forms of PRA with similar ophthalmoscopic pictures. Confirmation requires a specific genetic test for the HIVEP3 mutation. Differentiate from other PRAs (prcd-PRA, other Type A forms) by age of onset and molecular test.

1. Kaukonen M et al. 2020, variante silenciadora putativa en un modelo canino de retinosis pigmentaria; PRA tipos 1 y 2 del Schnauzer miniatura (PMID 32150541)
2. Murgiano L et al. 2019, variante estructural compleja de PPT1 asociada a degeneración retiniana canina no sindrómica (PMID 30541930)
3. Aguirre GD et al. 2020, comentario de consenso sobre HIVEP3 vs PPT1 (PMID 33151924)
4. Parshall C et al. 1991, displasia de fotorreceptores del Schnauzer miniatura (herencia AR demostrada)
5. OMIA:001311 Displasia de fotorreceptores (PRA tipo 1/pd) del Schnauzer miniatura