We show that both the wild type and mutant allele of the FGF4-retrogene segregate in the Norwegian population of Havanese dogs and that it is associated with shoulder height. Our results support that the short and bowed forelegs seen in some Havanese could potentially be a result of chondrodystrophy, rather than a breed specific syndrome as previously suggested.
It should be noted, that the prevalence of the risk allele is high, but the number of severely affected individuals (e.g. those requiring surgery) is low, which means that modifying genes probably affect the degree of foreleg bowing and elbow incongruity in the chondrodystrophic dogs. Our result does not uncover other associated genes, but highlight the increasing population frequency of an unnecessary, underlying risk factor.
For most dogs in the study, we genotyped a very closely linked variant rather than the causative insert itself. The studied SNP is, however, located only 1272 base pairs away from the insert site (~ 0.001 cM), which means the likelihood of a recombination is very low. We have verified a complete LD between the variant and the retrogene in a selection of 44 Havanese with genotypes A/G (n = 22) and A/A (n = 22). The strong association between the marker and phenotype also point towards true variation in the presence/absence of the retrogene.
Shoulder height was selected as a phenotypic marker for foreleg shortening, because it could be easily and reliably measured by the owner. A more standardized measure, e.g. using radiographs to evaluate the degree of foreleg bowing or having one person measure all the dogs, could have improved precision of the measurements, but would significantly reduce the number of dogs we were able to include in the study. We believe the degree of error in owner measurements is similar in the two genotype-groups and should therefore not affect the result of our association analysis.
The primary aim of the study was to investigate the frequency of the FGF4 retrogene and discuss potential effects on the population risk of foreleg pathology. A thorough clinical evaluation of the dogs, which would be necessary to accurately classify the degree of foreleg bowing and give a conclusive description of prevalence, was beyond the scope of this study. All owner-reported cases of severe foreleg bowing have been from dogs that are risk allele homozygotes.
We did not identify any wild type homozygotes. This is not surprising, given the low population frequency of the wild type allele. The absence of G/G-individuals prevent us from investigating possible phenotypic differences between G/G-dogs and heterozygotes. Traditionally, chondrodystrophy has been considered a dominant trait in dog, but the significant height difference we found between A/G- and A/A-individuals show that at least in this breed, the dominance is incomplete. Some forms of chondrodysplasia in human, also show incomplete dominance .
The Fédération Cynologique Internationale (FCI) breed standard for Havanese , states that the height at the withers should be between 23 cm and 27 cm (tolerance 21 cm to 29 cm), which means the average height of the A/A-dogs is correct. Increasing the number of A/A- x A/G-matings, would reduce the prevalence of dogs with disproportionally short legs, with a risk that some offspring might be too tall according to standard. We believe that preserving the wild type allele before it is lost should be of high priority. We therefore suggest allowing a limited increase in height for the first generations that may be corrected in succeeding generations through traditional selection.
A slight increase in the height acceptance in the breed standard could also be considered. This would allow a faster change in allele frequency and still leave room to focus on other traits, because the need to select for height would decrease. Increasing the height acceptance to 30 cm, which equals the median height of the A/G-dogs, would be enough to ensure most A/G-dogs are still within standard. This is also in accordance with what some consider to be the original, Cuban standard .
Lastly, it should be noted that the standard lists a “French front” (pasterns to close and feet turned outwards) as an important fault .
We show a decline in the population frequency of the wild type allele during the past two decades, with A/G-dogs being on average 4.7 older than A/A-dogs. This finding is supported by statements from breeders, who indicate that there has been a “trend” of selection for longer backs and shorter legs in recent years. It is possible that a selection for certain conformational traits have unintentionally turned a primarily non-chondrodystrophic breed, chondrodystrophic.
Chondrodystrophy is associated with increased risk of angular limb deformity and elbow disease . If the shortened and bowed forelegs seen in Havanese are directly associated with chondrodystrophy, increasing the prevalence of the non-chondrodystrophic wild type in the population could reduce the number of dogs with increased risk of foreleg pathology, subsequently reducing the number of clinically affected individuals. This would benefit the health and welfare of the breed.
Marker-assisted selection should be implemented to gradually increase the population frequency of the beneficial allele and ensure that the non-chondrodystrophic type is not lost. We believe any increase in the frequency of the wild type allele has the potential to reduce risk of foreleg pathology and that ideally, the wild type should eventually become be the predominant variant. However, it is challenging to obtain a fast change in allele frequency without negatively influencing genetic variation and/or other traits. The initial goal should therefore be to recover a sustainable population of non-chondrodystrophic individuals and avoid that the risk allele becomes fixed.
DNA-testing as many Havanese as possible for the FGF4-retrogene on CFA18, would be valuable to identify the rare, wild type carriers for breeding purposes. Litters from wild type carriers should be tested prior to adoption, to ensure continuation of the breeding program.
To avoid loss of genetic variation through selection for the low frequency wild type, it may also be worth considering a limited outcross to wild type carriers in closely related breeds like the bichon frisé. If done right, such an outcross could increase the prevalence of the wild type allele and speed up the reversal process, without much negative effect on other traits because the breeds are so similar.
Parallel to breeding for a gradual increase in the population frequency of the non-chondrodystrophic genotype, efforts should be made to reduce the degree of foreleg deformities and elbow incongruity among the chondrodystrophic Havanese. Selection response in other chondrodystrophic breeds have shown that it is possible to reduce the degree of foreleg bowing by selection based simply on visual inspection. A suggested protocol for classification of elbow incongruity in chondrodystrophic breeds , could potentially be used to screen chondrodystrophic Havanese prior to breeding.