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A form of progressive retinal atrophy designated PRA4, described in the Lhasa Apso and caused by an insertion of a LINE-1 element in the promoter of the IMPG2 gene. It produces progressive retinal degeneration with initial nyctalopia and blindness; most diagnosed cases occur between 5 and 8 years of age. Its molecular identification made it possible to develop a breed-specific genetic test.

It affects the Lhasa Apso, in which the variant is breed-private. The estimated mutant allele frequency in the United Kingdom population is 0.07-0.1, which corresponds to a carrier frequency of approximately 18 %.

- Genotype breeding animals before mating\n- Do not cross carrier×carrier (25 % risk of affected homozygotes); carrier×clear produces 0 % affected and 50 % carriers\n- An affected animal must not be bred; a carrier can be crossed with a clear dog without producing affected offspring\n- Annual ophthalmological examination (ECVO) complementary

The differential diagnosis includes other forms of PRA described in small breeds (more than 10 genes involved). The IMPG2 LINE-1 insertion is private to the Lhasa Apso, so a negative result does not rule out other forms of PRA in the breed; complement with serial ophthalmological examination (ECVO).

1. Hitti-Malin RJ et al. (2020) A LINE-1 insertion situated in the promoter of IMPG2 is associated with autosomal recessive progressive retinal atrophy in Lhasa Apso dogs. BMC Genet 21:100. PMID: 32894063
2. OMIA:002289-9615. Retinal atrophy, progressive, IMPG2-related in Canis lupus familiaris. https://omia.org/OMIA002289/9615/