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Skeletal Dysplasia 2 – Dwarfism (SD2)
| Acronym: | SD2 |
| Gene: | COL11A2 |
| Mutation: | c.143G>C |
| Inheritance: | Autosomal recessive |
| Sample type: | CHS (Cheek Swab), WBE (Whole Blood EDTA) |
Genetics and characteristics
Skeletal dysplasia 2 (SD2) is a form of dwarfism known as mild disproportionate dwarfism. It is characterized by disproportion in body parts; one or more body parts are smaller in comparison to the overall size. Short legs, which are one of the typical characteristics of dwarfism, are an expected standard in some breeds, but in many other breeds are an undesirable trait, especially among working breeds, such as Labrador retrievers. Skeletal dysplasia 2 (SD2) is caused by a mutation in the COL11A2 gene and is inherited in an autosomal recessive manner. Dogs with an inherited SD2 develop short legs and front legs are commonly more severely affected than the back legs. While the legs develop shorter than the standards for a particular breed, the body’s development occurs normally.
Skeletal dysplasia 2 can be easily mistaken with retinal dysplasia/oculoskeletal dysplasia (RD/OSD). Oculoskeletal dysplasia (OSD) is an inherited disease, whose phenotype displays as short legs and severe ocular defects. It is known OSD is caused by a mutation in the COL9A3 gene encoding the α3-chain of collagen type IX. In contrast to retinal dysplasia/oculoskeletal dysplasia (RD/OSD), skeletal dysplasia 2 (SD2) in Labrador Retrievers has a very subtle phenotype without any obvious ocular or auditory involvement. Also in research made, the owners of affected dogs did not report signs of degenerative joint disease in their affected dogs, which was in accordance with the radiological findings. This leads to the conclusion that SD2 does not cause joint problems, in contrast to OSD.
While working with affected dogs, for purposes of researching, it was established that all affected dogs were related to each other and had the same ancestor, a male Labrador Retriever born in 1966. This male was a working dog that became a popular sire and contributed to the spreading of the SD2 gene in the population. It is suspected that this exact male is the actual founder of the mutation. If this is true, the mutation would have occurred less than 10 generations ago. Knowing all this, it is easy to conclude that SD2 is a relatively rare and young trait.
Results Reported As
Test Result |
Interpretation of test result |
CLEAR |
Tested mutation was not detected in animal with „clear“ result. Animal tested as clear has wild-type allele in homozygous state (i.e. two pairs of healthy alleles). It will not develop disease caused by tested mutation.* It will pass only wild-type allele to its offspring. |
CARRIER |
Tested mutation was detected in animal with „carrier“ result. Animal tested as carrier has one wild-type and one mutation allele, it is in heterozygous state. It will not develop disease caused by tested mutation.* It can pass wild-type or mutation allele to its offspring. |
AFFECTED |
Tested mutation was detected in animal with „affected“ result. Animal tested as affected has two copies of mutation alleles affecting the gene. It is likely the animal will experience a genetic disorder due to this mutation.** It will pass only mutation allele to its offspring. |
*Test excludes only tested mutation but not possible unknown mutations or factors that can lead to similar condition/symptoms.
** Potential unknown mutations or multiple other factors can possibly affect the likelihood of experiencing a genetic disorder.
References:
Frischknecht (2013): A COL11A2 Mutation in Labrador Retrievers with Mild Disproportionate Dwarfism. PLoS One. 2013;8(3):e60149. doi: 10.1371/journal.pone.0060149.