The Paw Print Genetics Blog

One thing a veterinarian does when either assisting a dog during whelping, or in the immediate follow up to a cesarean section is examine the puppies.  Included in this examination is an oral exam to check both suckle reflex and for any congenital defect which may compromise the puppy’s quality of life.  In effect, the vet is looking for any sign of a cleft lip/palate (CL/P).  A CL/P is a relatively common congenital defect of the craniofacial region.  The development of the palate includes the soft palate, the rostral (frontal) hard palate, the premaxilla section of the skull and the lips¹.  This defect creates an opening between the oral and nasal cavities.  This opening is concerning because the puppy will have difficulties nursing leading to malnutrition and may inhale milk into the respiratory system which can lead to a sinus infection or pneumonia¹¹.  Some dog breeds more commonly present with cleft defects than others suggesting a genetic component to this condition¹.  Genetics do play a role in the formation of this defect, but they are not the only cause of a cleft lip/palate.  Genetic testing the appropriate dog breed may help reduce the chance of producing puppies with this defect, but a breeder must understand the parameters for this testing.  When a new litter has one or more affected puppies, what are the treatment options and how successful are they?  Hopefully, the more we understand about this congenital abnormality, the more it can be prevented. 

The oral and nasal cavities become distinct separate openings by fusion of certain tissues during the 23^rd to the 28^th day in embryonic development^1,2,3.    To achieve this separation of cavities, five different embryonic structures need to develop appropriately with coordination of a complex series of events including cell growth, cell migration, differentiation, and apoptosis (cellular destruction)^1,2,3.  These five structures are the frontonasal prominence, which will become the primary palate and delineate into the incisors and medial portion of the upper lip.  The right and left maxillary prominences, which will develop into the secondary hard palate and lateral portions of the upper lip. Successful fusion of the secondary palate results in complete separation of the nasal and oral cavities.  Lastly, the right and left mandibular prominence will develop into the two halves of the lower jaw^1,2.  Given the complexity of these structures, it is easy to understand that a breakdown in any one of these results in a CL/P.

Genetics play a prominent role in determining this tissue development.  However, the underlying genetic basis for cleft lip and palate is extremely complicated⁴.  There can be many genes involved and only a few have been identified through research.  The mode of inheritance for cleft palate depends on the breed of dog affected¹.  Not all mutations that cause CL/P, will manifest the condition. This is known as penetrance of a genetic variant or mutation.  Likewise, the expression of a mutation that does present in a dog can be variable and inconsistent¹. 

CL/P may be an isolated abnormality with variable presentation, or it may be part of a larger syndrome with other medical problems or features. It may involve just the lip, the lip and palate or only the palate where the cleft is found.  When a cleft is the only defect the puppy presents, this is classified as a nonsyndromic¹.  When a cleft is one of several abnormalities presenting in an affected puppy, these are considered syndromic clefts¹.  Nonsyndromic clefts are anecdotally more common than syndromic issues, but this may be skewed by the fact that many puppies with syndromic presentations are immediately euthanized and records are not always accurately kept¹.  

Despite being presumptively more common, genetic testing for nonsyndromic clefts is lacking.  This is in part due to a multifactorial inheritance pattern which limits the ability to study these types of clefts¹.  Multifactorial inheritance means many sections of DNA along with external environmental factors are involved.  Skipping generations and presenting in atypical patterns, nonsyndromic clefts limit the predictability of inheritance¹. Yet, recurrent instances of CL/P in a pedigree are strongly suggestive of a genetic condition. 

The alternative to nonsyndromic clefts is, of course, syndromic clefts.  A “syndrome” is a set of abnormalities that occur jointly.  In 2015 researchers examine 13 cases of Nova Scotia duck tolling retrievers (NSDTR) presenting with a variety of cleft defects¹⁰.  10 of the affected dogs also had syndactyly (webbed or conjoined digits) in their paws.  A mutation in the ADAMTS20 gene was associated with the presentation of this condition¹⁰.  The inheritance of this mutation was determined to be autosomal recessive with variable expression due to the variety of clefts demonstrated¹⁰.  Genetic testing of the ADAMTS20 gene in the NSDTR will reliably determine whether a dog is a genetic carrier of cleft palate and syndactyly. In general, carrier dogs do not have features of the disease but when bred with another carrier of the same mutation, there is a risk of having affected pups. This mutation has not been found in other breeds so, at this time genetic testing is only appropriate for the NSDTR.  This is not the only cause of CL/P in this breed, as a mutation in the DLX6 gene has also been found to cause cleft defects in NSDTR’s⁹.

Although genetics are clearly responsible for some presentations of CL/P, changes in the uterine environment during gestation can also result in cleft defects⁵.  In general, illness of the mother during pregnancy such as bacterial or viral infections can cause CL/P in the puppies.  Certain nutritional changes such as excessive amounts of Vitamin A, much higher than recommended therapeutic levels, or conversely Folic Acid deficiency has been associated with cleft defects^5,6,7.  Medications such as aspirin, the anti-fungal drug, Griseofulvin, antibiotics like metronidazole and corticosteroids can cause CL/P⁸.  When a chemical or drug causes a birth defect, this is called a teratogen. The effects of teratogens on the developing fetus or puppies are based on research across many species. 

Most CL/P defects will require surgical correction.  Unfortunately, surgery should be delayed until the puppy is of an age when anesthesia is better tolerated and will not place the puppy at undo risk.  This means until that time, the puppy must be carefully managed in terms of feeding and cleaning the oral cavity.  Sometimes a stomach tube must be placed to ensure the puppy received adequate nutrition Complications during this time are always possible so owners must be aware of signs of distress or issues with the affected puppy.  Non-resolving sinus infections or pneumonia regularly require lengthy and aggressive veterinary treatment. Clefts that only affect the lip may not be more than a cosmetic problem, but any defect in the palate will require considerable effort from the owner.  Often, owners will elect to euthanize these dogs soon after birth. 

The best surgical time is at the discretion and experience of the veterinarian, but 4 to 6 months of age is a common time when this issue is addressed^11,12.  Surgery prior to the eruption of the permanent teeth may cause further dental issues later^11,12.  By 6 months of age the tissue around the cleft defect has mostly stabilized and the success of the corrective procedure is improved.  The surgery itself depends on the planning of the veterinary surgeon.  This can be done in a single procedure, but often requires multiple efforts to fully correct the defect.  Post-surgical complications are common, and owners must be prepared for arduous postoperative care^11,12. 

Cleft Lip/Palates are the most common congenital defect of the craniofacial region.  These defects create an opening between the nasal and oral cavities.  This is concerning for the puppy as it prevents them from nursing properly.  This leads to severe nutritional deficiencies or respiratory disease.   Multiple locations during embryonic development produce the normal anatomy that separate the nasal and oral cavities.  Any disruption in this development may produce a range of cleft defects.   Genetics are partially responsible given that some breeds are pre-disposed to CL/P, but cleft defects can also develop because of toxins or trauma during gestation.  Although several breeds are suspected of having genetic mutations that cause cleft defects, the Nova Scotia duck tolling retriever has had two mutations identified.  Reliable genetic testing is important for determining breeding practices. To eliminate this mutation from breeding lines and to avoid the potential of producing affected pups, breeding of known carriers to each other is not recommended.  If you have any questions about the ADAMTS20 gene please contact us for more information at AskUs@pawprintgenetics.com or give us a call 509-483-5950 (Mon. to Fri., 8 am to 5 pm Pacific time) to receive a personal consultation with one of our veterinarians or geneticists.

References

1. Enio Moura and Cláudia Turra Pimpão (March 22nd 2017). Cleft Lip and Palate in the Dog: Medical and Genetic Aspects, Designing Strategies for Cleft Lip and Palate Care, Mazen Ahmad Almasri, IntechOpen, DOI: 10.5772/67049. Available from: https://www.intechopen.com/books/designing-strategies-for-cleft-lip-and-palate-care/cleft-lip-and-palate-in-the-dog-medical-and-genetic-aspects
2. Bush JO, Jiang R. Palatogenesis: morphogenetic and molecular mechanisms of secondary palate development. Development. 2012;139(2):231–243. doi: 10.1242/dev.067082
3. Evans HE. Reproduction and prenatal development. In: Evans HE, Christensen GC, editors. Miller’s Anatomy of The Dog. 2nd edition. Philadelphia: W. B. Saunders Company; 1993. pp. 13–77.
4. Padgett GA. Control of Canine Genetic Diseases. New York: Howell Book House; 1998. 264 p.
5. Freytag T L, Liu S M, Rogers Q R, Morris J G: Teratogenic effects of chronic ingestion of high levels of vitamin A in cats. J Anim Physiol Anim Nutr (Berl) 2003 Vol 87 (1-2) pp. 42-51.
6. Davies M: Excess vitamin A intake during pregnancy as a possible cause of congenital cleft palate in puppies and kittens. Vet Rec 2011 Vol 169 (4) pp. 107.
7. Domoslawska A, Jurczak A, Janowski T: Oral folic acid supplementation decreases palate and/or lip cleft occurrence in Pug and Chihuahua puppies and elevates folic acid blood levels in pregnant bitches. Pol J Vet Sci 2013 Vol 16 (1) pp. 33-7.
8. Gillick A, Bulmer W S: Griseofulvin, a possible teratogen. Can Vet J 1972 Vol 13 (10) pp. 244.
9. Wolf ZT, Leslie EJ, Arzi B, Jayashankar K, Karmi N, Jia Z, Rowland DJ, Young A, Safra N, Sliskovic S, Murray JC, Wade CM, Bannasch DL. A LINE-1 insertion in DLX6 is responsible for cleft palate and mandibular abnormalities in a canine model of Pierre Robin sequence. PLoS Genet. 2014;10(4):e1004257. doi: 10.1371/journal.pgen.1004257.
10. Wolf ZT, Brand HA, Shaffer JR, Leslie EJ, Boaz A, Willet CE, Cox TC, McHenry T, Narayan N, Feingold E, Wang X, Sliskovic S, Karmi N, Safra N, Sanchez C, Deleyiannis FWB, Murray JC, Wade CM, Marazita ML, Bannasch DL. Genome-Wide Association Studies in Dogs and Humans Identify ADAMTS20 as a Risk Variant for Cleft Lip and Palate. PLoS Genet. 2015 Mar 23;11(3):e1005059. doi: 10.1371/journal.pgen.1005059.
11. DavidsonA P. Gregory C, Dedrick P: Successful management permitting delayed operative revision of cleft palate in a labrador retriever. Vet Clin North Am Small Anim Pract 2014 Vol 44 (2) pp. 325-9.
12. Fiani N, Verstraete FJ M , Arzi B: Reconstruction of Congenital Nose, Cleft Primary Palate, and Lip Disorders. Vet Clin North Am Small Anim Pract 2016 Vol 46 (4) pp. 663-75.