- Globular head
- Shortened torso/neck
- Enlarged joints
- Bowed forelegs
- Under bite
- Cleft palate (resulting in difficulties breathing)
- Four causal variants for ACAN dwarfism have been identified and validated to date: D1, D2, D3* and D4.
- All variants show similar phenotypes and lead to dwarfism in Miniature Horses and Shetland ponies.
- An affected pony may have a two of the same, or two different variants. Any combination with D1 results in early spontaneous abortion of the affected foal (D1/D1, D1/D2, D1/D3, D1/D4).
- Other combinations show the phenotype described above. Symptoms may vary from mild to severe. Affected ponies are often euthanized because of poor quality of life.
- Some combinations are very rare (D3/D3, D4/D4). The effects of these genotypes on the appearance of the disease are yet unknown.
- Originally five different variants were reported to cause the ACAN dwarfism. The original D3 variant has been proven incorrect, and the fifth variant reported (Metzger et al. 2017) been named D3*. CAG tests for this corrected D3* variant.
- Another form of dwarfism also results in health issues in Shetland ponies and Miniature Horses: Skeletal Atavism (SA, Del-dwarfism). It is possible for ponies to be carriers of both ACAN and SA.
→ Only animals with two copies of the variant (d/d) are affected. Animals with only one copy (N/d) are clinically normal carriers.
|Genotype:||The horse is:
|N/N||normal.||The horse/pony has no copies of the mutations responsible for ACAN dwarfism and therefore cannot pass it on to its offspring.|
|N/d||a carrier.||The horse/pony is a clinically normal carrier. The variant will be passed on to its offspring with a probability of 50%.|
|d/d||affected.||The variant will be passed on to all of its offspring. All offspring will be carriers (N/d). Any combination with the D1 variant will lead to early abortion of the affected foal.|
- Carriers may be bred to normal animals (N/d x N/N) without any risk of producing affected offspring. The offspring should also be tested before breeding to determine if they are carriers or normal.
- Breeding two carriers (N/d x N/d) is not recommended due to the possibility of 25% of the offspring being affected.
- Affected animals (d/d) should not be used for breeding.
Testinformation: This test detects all four variants of the ACAN gene.
Eberth, J.E. : Chondrodysplasia-Like Dwarfism in the Miniature Horse. Theses and Dissertations–Veterinary Science. Paper 11. Entire thesis is available at http://uknowledge.uky.edu/gluck_etds/11:, 2013.
Metzger, J., Gast, A.C., Schrimpf, R., Rau, J., Eikelberg, D., Beineke, A., Hellige, M., Distl, O. : Whole-genome sequencing reveals a potential causal mutation for dwarfism in the Miniature Shetland pony. Mamm Genome 28:143-151, 2017. Pubmed reference: 27942904. DOI: 10.1007/s00335-016-9673-4.
Further information is available at Online Mendelian Inheritance in Animals.