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Burmese Head Defect (BHD)
| Acronym: | BHD |
| Gene: | ALX1 |
| Mutation: | c.497_508del |
| Inheritance: | Autosomal co-dominant |
| Sample type: | CHS (Cheek Swab), WBE (Whole Blood EDTA) |
Genetics and characteristics
Burmese head defect (BHD) is a craniofacial deformity that occurs in the Burmese cat breed. The occurence of a head defect in Burmese is connected to selected breed phenotype. The Burmese are a cat breed with an extreme brachycephalic phenotype. In the late 1970s, a male Burmese cat in the USA with a more brachycephalic head type became a highly popular sire and his lineage became known as the “Contemporary” Burmese. More brachycephalic individuals were positively selected in the breed, and matings between two „Contemporary“ individuals produced 25% offspring with craniofacial defects.
Burmese head defects genetic mutation causes disruption of the normal developmental processes in skull formation and facial features. In affected cats, a duplication of the tissue of the upper jaw is present, together with the development of two hard palates and two sets of whisker pads. In addition, the region of the skull above the upper jaw does not form properly. Eyes and ears are absent, or malformed, and there is incomplete closure of the fontanelles of the skull. The brain appears to protrude from the skull, although it is generally covered by skin. These craniofacial malformations are severe and immediately apparent at birth. Affected kittens are generally born live and require euthanasia as the condition is incompatible with life.
Burmese head defect (BHD) is associated with a deletion in the ALX1 gene (Aristaless-Like Homeobox protein 1). The defect is inherited in an autosomal recessive manner, however, carriers of the mutation are more brachycephalic individuals than wildtype, thus the trait has also been described as co-dominant. When mating two carriers (heterozygotes) at conception each kitten has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Although the frequency of the mutation is higher in „Contemporary“ lines, it is detected in other Burmese breed lines also. A DNA test can be used to identify cats that are carriers (heterozygotes) of the mutation that causes Burmese head defect.
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. |
AFFECTED HETEROZYGOTE |
Tested mutation was detected in animal with „affected heterozygote“ result. Animal tested as affected heterozygote has one wild-type and one mutation allele, it is in heterozygous state. It is likely to 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.
**Penetrance of tested mutation, and potential unknown mutations or multiple other factors can possibly affect the likelihood of experiencing a genetic disorder.
References:
Lyons, L.A., Erdman, C.A., Grahn, R.A., Hamilton, M.J., Carter, M.J., Helps, C.R., Alhaddad, H., and Gandolfi, B. (2016). Aristaless-Like Homeobox protein 1 (ALX1) variant associated with craniofacial structure and frontonasal dysplasia in Burmese cats. Developmental Biology 409, 451–458.