Two FGF4 retrogenes on chromosomes 12 (12-FGF4RG) and 18 (18-FGF4RG) contribute to short-limbed phenotypes in dogs. 12-FGF4RG has also been associated with intervertebral disc disease (IVDD). Both of these retrogenes were found to be widespread among dog breeds with allele frequencies ranging from 0.02 to 1; however, their additive contribution to disease is unknown. Surgical cases of IVDD (n = 569) were evaluated for age of onset, disc calcification, and genotypes for the FGF4 retrogenes. Multivariable linear regression analysis identified the presence of one or two copies of 12-FGF4RG associated with significantly younger age at first surgery in a dominant manner. 18-FGF4RG had only a minor effect in dogs with one copy. Multivariable logistic regression showed that 12-FGF4RG had an additive effect on radiographic disc calcification, while 18-FGF4RG had no effect. Multivariable logistic regression using mixed breed cases and controls identified only 12-FGF4RG as highly associated with disc herniation in a dominant manner (Odds Ratio, OR, 18.42, 95% Confidence Interval (CI) 7.44 to 50.26; P < 0.001). The relative risk for disc surgery associated with 12-FGF4RG varied from 5.5 to 15.1 within segregating breeds and mixed breeds. The FGF4 retrogene on CFA12 acts in a dominant manner to decrease the age of onset and increase the overall risk of disc disease in dogs. Other modifiers of risk may be present within certain breeds, including the FGF4 retrogene on CFA18.

Two transcribed retrocopies of the fibroblast growth factor 4 (FGF4) gene have previously been described in the domestic dog. An FGF4 retrocopy on chr18 is associated with disproportionate dwarfism, while an FGF4 retrocopy on chr12 is associated with both disproportionate dwarfism and intervertebral disc disease (IVDD). In this study, whole-genome sequencing data were queried to identify other FGF4 retrocopies that could be contributing to phenotypic diversity in canids. Additionally, dogs with surgically confirmed IVDD were assayed for novel FGF4 retrocopies. Five additional and distinct FGF4 retrocopies were identified in canids including a copy unique to red wolves (Canis rufus). The FGF4 retrocopies identified in domestic dogs were identical to domestic dog FGF4 haplotypes, which are distinct from modern wolf FGF4 haplotypes, indicating that these retrotransposition events likely occurred after domestication. The identification of multiple, full length FGF4 retrocopies with open reading frames in canids indicates that gene retrotransposition events occur much more frequently than previously thought and provide a mechanism for continued genetic and phenotypic diversity in canids.