Two genes, one culprit - a functional candidate validation of a SPATA7 deletion in dogs with day blindness/retinal degeneration

Inherited retinal diseases (IRDs) are a diverse group of disorders that share common vision deficits ranging from early onset blindness to severe and progressive later-onset disease. We report a form of early-onset day-vision loss, cone-rod dystrophy, in the Standard poodle. Through GWAS and homozygosity mapping, a large deletion on CFA8:NC_049229.1:g.60,022,583_60,040,453del was found which removes 3' portions of two different genes, PTPN21 and SPATA7, presenting a challenge for assessing the actual causative gene in a multi-gene large deletion. All affected dogs were homozygous for the mutant allele, which segregated perfectly with the phenotype within the breed. The variant was absent in 1879 dogs from the Dog10K database. While the role of SPATA7 for retinal disease has been established in human patients and genetically engineered mice, the role of PTPN21 in the retina is unclear even though it is expressed in rod and cone photoreceptors. Expression of whole and truncated transcripts for both genes was detected in skin fibroblasts from controls and cases. Retinal RNA analysis of PTPN21 splicing suggests that at least one unmodified transcript is still present in mutants. Ptpn21-/- knockout mice did not have an ocular phenotype, and IHC for rod- and cone-specific opsins detected no cone or rod abnormalities suggesting that PTPN21 loss has minimal to no contributory role towards the retinal phenotype in mutants. The variant leads to a deletion of the 3'-end of the SPATA7 transcript: XM_038545497.1:r.1,314_1,629delins[g.60,018,954-60,018,990], p.(XP_038401425.1: Asp361GlufsTer2), reducing the predicted protein from 595 to 361 AA. Ultrastructure expansion microscopy (U-ExM) enabled the detection of a distinct SPATA7 signal around the transition zone of the primary cilium in photoreceptors and fibroblasts of WT dogs, which was absent in affected dog. We posit that SPATA7 deficiency is the main cause of the condition, and propose this disease as a model for the SPATA7-related form of cone-rod dystrophy in humans. Our work shows an example of functional refinement of a multi-gene deletion variant using a multi-technique approach.