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Occipitoatlantoaxial Malformation (OAAM)
| Acronym: | OAAM |
| Gene: | HOXD3 |
| Mutation: | 2.7 kb deletion |
| Inheritance: | Autosomal Recessive |
| Sample type: | WBE (Whole Blood EDTA), HA (Hairs - with roots) |
Genetics and characteristics
Occipitoatlantoaxial Malformation (OAAM) is an inherited developmental disease that affects horses. Horses with OAAM have an abnormal head position, lack of muscle control and coordination, extended neck posture, and visible irregularities along the cervical spine. The disease affects the mobility of the neck, with loss of extension and flexibility. OAAM is caused by a genetic mutation in the homeobox D3 (HOXD3) gene responsible for normal development of the back of the head, atlas (first cervical vertebra), and axis (second cervical vertebra). The deletion mutation in the HOXD3 gene causes a partial fusion of the atlas with the back of the head so that the axis resembles the atlas, leading to characteristic clinical signs.
OAAM is inherited as an autosomal recessive trait, meaning that two copies of the mutation are needed for a horse to be affected. Although OAAM is typically found in horses of the Arabian breed, it has also been reported infrequently in other breeds, such as Quarter Horse, an Appaloosa, and a Miniature Horse. The disease is incurable, but early detection by genetic testing can help find the right diagnosis and prevent further breeding of the affected horse by careful selection of mating pairs.
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:
Bell, S., Detweiler, D., Benak, J., & Pusterla, N. (2007). What Is Your Diagnosis? Journal of the American Veterinary Medical Association, 231(7), 1033–1034. doi: 10.2460/javma.231.7.1033
Bordbari, M. H., Penedo, M. C. T., Aleman, M., Valberg, S. J., Mickelson, J., & Finno, C. J. (2017). Deletion of 2.7 kb near HOXD3 in an Arabian horse with occipitoatlantoaxial malformation. Animal Genetics, 48(3), 287–294. doi:10.1111/age.12531