Genetics

      The study entitled "Duplication of FGF3, FGF4, FGF 19 and ORAOV1 causes hair ridge and predisposition to dermoid sinus in Ridgeback dogs" by Salmon Hillbertz et al. (2007) finds that the ridges* on the backs of the Thai and Rhodesian Ridgebacks are caused by an autosomal dominant mutation which also predisposes to the congenital disorder named dermoid sinus**. 

The dermoid sinus condition in dogs resembles the "dermal sinus" condition found in humans. In dogs, the dermoid sinus is located at the beginning and the end of the ridge; only ridged dogs have the dermoid sinus condition. No ridgeless dogs with dermoid sinus condition have ever been reported.
In Sweden, the Rhodesian Ridgeback population has 5-6% of ridgeless dogs, 8-10% have dermoid sinus. Ridgeless and dermoid sinus affected Rhodesian Ridgebacks are excluded from breeding.

Results
Samples from 9 ridgeless and 12 ridged dogs, 11 with dermoid sinus enabled the authors to place the "Ridge" locus on a 750-kb region of chromosome 18.
10 out of 11 dermoid sinus affected dogs were homozygous for a haplotype not found in the ridgeless dogs. In addition, all Rhodesians were hererozygous for a SNP (SNP_51,399,353) within the 750-kb region. Further, 43 out of 45 additional Rhodesian Ridgebacks were also heterozygous for this SNP, suggesting that the SNP is part of a duplicated region.

The authors used the Multiple Ligation Dependent Genome Amplification (MLGA) technique to assess whether the allele for the ridge is associated with a duplication. The results show that ridged Rhodesian and Thai Ridgebacks can be homozygous or heterozygous for a large duplication.

The precise location of the duplication breakpoints was determined by using interspersed probes in the critical intervals. The duplication breakpoint between the tandem copies was amplified and sequenced using PCR. 
1) In the CanFam2.0 genome assembly, the 133.4-kb duplication was found to extend from nucleotide position 51,398,518 to 51,531,941with a single-nucleotide insertion at the breakpoint. 
2) Identical breakpoints were found in Rhodesian and Thai Ridgebacks, although Mitochondrial DNA analysis revealed no close relationship between these two breeds.
These results indicate that this duplication is what causes the ridge, and that the ridge has either introgressed from one population into another, or that the mutation is old enough to have been present in the very early phases of the evolution of the domestic dog.
3) There is a complete association between the duplication and the presence of a ridge in more than 50 Rhodesian and Thai Ridgebacks, confirming a dominant inheritance of the ridge.
4) 13 of 15 Ridgebacks with dermoid sinus were found to be homozygous for the duplication. This indicates that the mutation for a ridge predisposes for dermoid sinus, with a low penetrance in dogs heterozygous and high penetrance in those homozygous for the mutation.
5) The ridge and dermoid sinus result probably from a dysregulated gene expression during embryogensis.
Of the genes included in the duplication: FGF3, FGF4, FGF19, ORAOV1 and the 3' end of CCND1 encoding cyclin D1, the last two are probably of less importance. Tight regulation of FGFs expression is however known to be critical during the development of the embryo and the morphogenesis of hair follicles. Several of the 22 FGF genes known for mammals are implicated in the regulation of hair growth and skin development.
6) It seems that dogs with ridges have a defect of the planar cell polarity system required for the normal orientation of hair follicles and the closure of the neural tube.

Conclusions:

The authors propose that it is a dysregulation of the FGF genes expression along the dorsal midline which leads to the ridge and a higher risk of dermoid sinus.
They also state that their results have implications for the way ridged dogs could be bred, in that the dermoid sinus condition could be eliminated by allowing ridgeless dogs to breed with ridged dogs, while avoiding mating ridged dogs to one another.

Acknowledgements

I thank L. Andersson and G. Andersson for fine tuning this text and their positive comments.

Author's notes

*Ridges are hairs along the back which are oriented in the opposite direction of those on the rest of the body ending in various crowns resembling cow -licks. They can be found in 3 breeds, the Thai Ridgeback (Thailand) , the Rhodesian Ridegeback (South Africa) and the Phu Quoc dog (Vietnam). Although all 3 breeds have ridges, the ridges themselves look different between the dogs and between the breeds. The AfriCanis of South Africa occasionaly show ridges on their backs as well.

**Dermoid sinus is a genetic skin disorder in dogs. It is a neural tube defect which can appear as single or multiple lumps on the dorsal midline. During normal development, the neural tube from which the spine develops separates from the skin. In the case of a dermoid sinus, this separation is not complete, with the dermoid sinus remaining connected to the animal's spinal cord in some cases, whereas the sinus terminates in a "blind sac" in others. Although the dermoid sinus is more commonly found in ridged breeds, it also occurs occasionally in other breeds such as Chow Chow, Shih Tzu, Yorkshire Terrier, Boxer, Siberian Husky, Golde Retriever and English Springer Spaniel. See this reference

References

Salmon Hillbertz N H C, Isaksson M, Karlsson E K, Hellmen E, Rosengren Pielberg G, Savolainen P, Wad C M, von Euler E, Gustafson U, Hedhammar A, Nilsson M, Lindblad-Toh K, Andersson L, Andersson G (2007): "Duplication of FGF3, FGF4, FGF 19 and ORAOV1 causes hair ridge and predisposition to dermoid sinus in, Ridgeback dogs" in Nature Genetics, Vol. 39, Number 11, November 2007.
 

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