Mild form of disproportionate dwarfism in the Labrador Retriever with short limbs (mainly the forelimbs) and a body of normal length and width, with no ocular or auditory involvement. Height at the withers is reduced by around 6 cm compared with the standard, and the disease is not associated with significant secondary joint problems. It is recessively inherited with incomplete penetrance, so not all homozygotes show an evident phenotype. It predominates in working lines.

Specific to the Labrador Retriever, with a predominance in working lines. Carrier frequency of 12% in the European population at the time the mutation was discovered. Height at the withers reduced by ~6 cm in phenotypically expressive homozygotes.

- Short limbs with a body of normal length and width\n- Forearms somewhat more affected than the hindlimbs\n- Long bones short and sometimes slightly curved\n- Wide, rounded epiphyses\n- Reduction in height at the withers of ~6 cm compared with the standard\n- No relevant ocular, auditory or secondary joint involvement\n- Overlap in height between mildly affected dogs and small unaffected dogs

Frischknecht and colleagues (2013) described a mild disproportionate dwarfism in the Labrador Retriever and designated it skeletal dysplasia 2 (SD2) to distinguish it from other skeletal dysplasias of the breed. Using GWAS and homozygosity mapping they located a critical interval on chromosome 12, and whole-genome sequencing revealed two missense variants perfectly associated with the trait: LTA c.140C>T and COL11A2 c.143G>C (p.Arg48Pro). Given the known role of COL11A2 in bone development, the authors proposed the latter as the candidate causal variant. The defect is concentrated in working lines, with a carrier frequency of 12% in the European population at the time of discovery.

- Test breeding animals before mating, especially in working lines where the allele is more prevalent\n- Do not cross two carriers: 25% risk of homozygotes (with variable expressivity)\n- A carrier can be mated to a clear animal; offspring intended for breeding must be tested\n- Remember the incomplete penetrance: a homozygote may have a height close to small unaffected dogs, which makes the molecular test essential\n- Distinguish from OSD (COL9A3) and other skeletal dysplasias of the Labrador: the SD2 test is specific

The differential diagnosis includes other skeletal dysplasias of the Labrador (OSD due to COL9A3 with ocular involvement, other chondrodysplasias) and normal height variation. Molecular confirmation is key because the phenotype is subtle and overlaps with normal variation. Height is a complex trait: the SD2 test is not an absolute predictor of adult height. COL11A2 in humans causes Stickler syndrome type 3, OSMED and Weissenbacher-Zweymüller, which are more severe than canine SD2.

1. Frischknecht M, et al. A COL11A2 mutation in Labrador retrievers with mild disproportionate dwarfism. PLoS One. 2013;8(3):e60149. PMID: 23527306
2. OMIA:001772-9615. Skeletal dysplasia 2, COL11A2-related in Canis lupus familiaris. https://omia.org/OMIA001772/9615/