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Class III Malocclusions in Miniature Donkeys

Miniature donkeys are a breed of novelty animals that are rapidly gaining popularity. Miniature donkeys were first imported into the United States in 1929, and a registry was established for them in 1958. They serve as both pets and as show animals.  They are members of the equine family and the U. S. population derives from a limited gene pool imported from Sicily and Sardinia. Class III malocclusions are a congenital defect that occurs in this breed more frequently than in other equids. Affected individuals display a protrusion of the lower incisors in front of the upper incisors, sometimes referred to as "monkey mouth." Severity of the underbite varies and the degree of malocclusion may change over time. It is a conformational flaw in mild cases but  leads to prehensile difficulties in severe cases.  Depending on severity it can prevent free side to side chewing and result in improper and excessive molar wear.  Dental problems can occur as the donkey is forced to chew incorrectly. Breeders report wide-spread occurrence of this condition. The condition runs in families but phenotypically unaffected animals can produce affected offspring.  The occurrence does not appear to be random, as a high proportion of offspring from select sires are affected whereas a high proportion of offspring from other sires are not affected. 

Coronal Craniosynostosis

Premature fusion of the coronal cranial suture, a membrane-like joint in the skull, results in a shortened maxilla and brachycephaly.   This premature fusion occurs in humans and can be familial or sporatic.  Many craniosynostosis syndromes are accompanied by limb abnormalities.   Mutations of FGFR, TWIST, and MSX2 genes can cause craniosynostosis syndromes with dominant inheritance.  Fibroblast growth factor receptor genes (FGFR2, FGFR3, FGFR4) are important for normal development of the cranial suture. We hypothesize that coronal craniosynostosis may cause malocclusion in miniature donkeys.  The problem is known to occur in dog and cat breeds in addition to humans.

Our Research Goals

Studies utilizing diagnosis, pedigrees, and DNA from miniature donkeys are ongoing to uncover the mode of inheritance and test candidate genes.  We have constructed a pedigree containing over 250 individuals to help us determine if we can trace and predict inheritance.  Additionally, we hope to ascertain if the condition is controlled by a single gene or is affected by non-genetic influences.  Candidate genes include FGFR, TWIST, and MSX2.  Using DNAs extracted from blood samples, 2,299 nucleotides of FGFR2 have been sequenced.  No polymorphisms have been detected so far.  These methods include obtaining blood samples and do not involve sacrifice or significant discomfort to the subjects.

Principle Investigator: Susan Ewart, DVM, PhD

Co-Investigators: Judy Marteniuk, DVM; Diana Rosenstein, DVM, MS

Related Links

National Miniature Donkey Association- Background information on the breed

International Miniature Donkey Registry- Official registry for miniature donkeys

Related publications

Wilkie, AO. Craniosynostosis: genes and mechanisms. Human Molecular Genetics, 1997;6:1647-56.