Variabilities in retinal function and structure in a canine model of cone-rod dystrophy associated with RPGRIP1 support multigenic etiology

Defects in the cilia gene RPGRIP1 cause Leber congenital amaurosis and cone-rod dystrophy in humans. A form of canine cone-rod dystrophy (cord1) was originally associated with a homozygous insertion in RPGRIP1 (RPGRIP1 (ins/ins)) as the primary disease locus while a homozygous deletion in MAP9 (MAP9 (del/del)) was later identified as a modifier associated with the early onset form. However, we find further variability in cone electroretinograms (ERGs) ranging from normal to absent in an extended RPGRIP1 (ins/ins) canine colony, irrespective of the MAP9 genotype. Ophthalmoscopically, cone ERG(absent) RPGRIP1 (ins/ins) eyes show discolouration of the tapetal fundus with varying onset and disease progression, while sd-OCT reveals atrophic changes. Despite marked changes in cone ERG and retinal morphology, photopic vision-guided behaviour is comparable between normal and cone ERG(absent) RPGRIP1 (ins/ins) littermates. Cone morphology of the dogs lacking cone ERG are truncated with shortened outer and inner segments. Immunohistochemically, cone ERG(absent) RPGRIP1 (ins/ins) retinas have extensive L/M-opsin mislocalization, lack CNGB3 labelling in the L/M-cones, and lack GC1 in all cones. Our results indicate that cord1 is a multigenic disease in which mutations in neither RPGRIP1 nor MAP9 alone lead to visual deficits, and additional gene(s) contribute to cone-specific functional and morphologic defects.