Research Of Cleft Palate Defects And DMVD Expected To Benefit Affected Breeds
As researchers learn more about cleft palate defects and degenerative mitral valve disease (DMVD), two inherited conditions that affect some toy breeds, breeders may one day be able to reduce disease incidence. Here is a review of the research.
Cleft palate defects occur in dogs with an incidence rate of up to 25 percent. Though it occurs in nearly all breeds, brachycephalic breeds, have a 30 percent increased risk for developing the disease. The birth defects occur when the roof of the mouth and/or lip fail to close during gestation. Often, the first sign that a puppy has a cleft defect is seen when it is nursing and milk bubbles out of the nose.
The recent discovery by scientists at the University of California-Davis of the causative mutation for cleft palate in Nova Scotia Duck Tolling Retrievers has led to funding of $2 million from the National Institutes of Health (NIH) to study the disorder in other breeds of dog. The collaborative study, involving scientists at the University of Sydney in Australia, aims to identify mutations responsible for clefts that will help breeders to produce healthy litters and help advance research of orofacial clefting in humans.
Lead investigator Danika Bannasch, D.V.M., Ph.D., professor of genetics at the University of California-Davis School of Veterinary Medicine, says, "Many of the epidemiological characteristics of cleft palate in dogs resemble cleft palate in humans. We think that dogs may be a suitable model for learning more about the developmental biology of cleft formation."
The palate is the partition that divides the mouth and the nose cavities. It consists of the hard palate formed by bony plates and the soft palate shaped by muscle fibers used in swallowing. The incomplete closure of the palate may affect the hard and/or soft components.
Oftentimes, breeders opt to have cleft palate puppies euthanized. Attempts to raise puppies require bottle or tube feeding because they cannot suckle as a normal puppy would. Initially, they must be fed every two hours, gradually decreasing to every three hours at night if they are gaining weight. The goal is a slow, steady weight gain.
Coughing and gagging are common, especially if the soft palate is affected, and some puppies sneeze and snort because food and saliva pass through the cleft into the nose. Complications include risk of malnourishment and aspiration pneumonia from the regurgitation of food through the nose. Around 4 weeks of age, puppies can be introduced to dry food and water.
Corrective surgery is an option for puppies that are 3 months of age or older, though it is expensive and has a low success rate. Surgery ranges from simple closure of the cleft to sliding grafts or prosthetic implants, depending on the severity and location of the defect. Severely affected dogs may require multiple surgeries.
Because surgery is performed on young, often underweight dogs with breathing problems, anesthesia and surgery are considered high risk. Post-surgical complications include puppies licking the surgical site and picking up objects with their mouths that put pressure on the repair. Growth of the upper jaw may cause parts of the cleft repair to become thin and open up. Additional surgeries may be needed to close openings.
Environmental factors also may cause cleft defects in dogs. Maternal nutrition, drug or chemical exposure, and some viral infections during pregnancy are known to increase the risk. Anti-inflammatory drugs, including prednisone and prednisolone, the anti-fungal drug griseofulvin, and insecticides and pesticides given to pregnant bitches are considered culprits. 1
Discovery of CP1 Mutation in Tollers
The University of California-Davis researchers who discovered the gene mutation for the CP1 form of cleft palate in Nova Scotia Duck Tolling Retrievers are preparing the data for publication in a scientific journal. The AKC Canine Health Foundation helped to fund the research that showed that the CP1 mutation, one of two that cause cleft palates in Tollers, is the most common that contributes to the autosomal recessive form of cleft palate disease. The CP1 mutation will be known by a scientific name when the journal article is published.
"Tollers with the CP1 form of cleft palate have a large insertion in a gene known to affect development of the palate," says Bannasch, who made the discovery after six years of study. "The mutation is not present in any other breed based on our genetic testing of more than 300 dogs of 80 breeds."
DNA tests are available for cleft palate in Tollers through the Orthopedic Foundation for Animals. The tests identify carriers as well as normal and affected dogs. Meanwhile, Bannasch encourages breeders to continue submitting DNA samples, as her laboratory is looking for additional cleft palate genes.
The four-year, $2 million NIH grant is allowing the cleft palate research to expand to include all breeds of dog. Collaborator Claire Wade, Ph.D., professor and chair of computational biology and animal genomics at the University of Sydney in Australia, is busy collecting DNA and classifying dogs' phenotypes and environmental data.
"We will perform a genomewide association study using affected and unaffected dogs that are related and unrelated," Bannasch says. "Our goal is to identify a possible genomic region for the mutation and then perform candidate gene sequencing or whole genome sequencing to identify mutations. If we find the mutation in a particular breed, we will look for the causative mutation in other breeds."
"Fortunately, we are able to use new sequencing technologies and computational approaches to study the data," Wade says. "The sequencing is a second step in the discovery process. It is expected to yield high-quality information if we first have genetic mapping information."
The research requires a minimum of 10 DNA samples from affected dogs to analyze the genetics involved in cleft palate disease. Comparative data from normal relatives is needed as well. "Blood samples can be obtained from pups, and we can receive donated tissue samples too," Wade says. "It is important that people contact us before sending samples, so we can ensure that the samples are handled safely to preserve the DNA."
As more DNA samples are contributed, the research may lead to the identification of genes and pathways that are critical to development of cleft palate in various breeds. Breeders may one day be equipped with a DNA test to make prudent breeding decisions that will ensure no puppies are born with this heartbreaking defect.
Understanding DMVD
At the University of Pennsylvania School of Veterinary Medicine, researchers aim to learn more about diagnosing and treating dogs with degenerative mitral valve disease. The slow, progressive condition is believed to affect 30 percent of small-breed dogs over the age of 10.
"In the early stages of the disease, no clinical signs are apparent and a dog appears healthy," says lead investigator Mark Oyama, D.V.M., DACVIM-Cardiology, professor of clinical studies and chief of cardiology. "We are studying the dogs that have what we believe is a very early 'silent' form of the disease, when early degeneration of the mitral valve is present but the telltale sign of a heart murmur cannot yet be heard."
A previous University of Pennsylvania study of DMVD in Norfolk Terriers found that 12 of 48 dogs considered clinically normal actually showed evidence of the disease on an echocardiogram test, despite not having a detectable heart murmur from an auscultation examination by a veterinary cardiologist. An echocardiogram is a cardiac ultrasound used to detect heart structure and function, whereas an auscultation involves a veterinarian using a stethoscope to listen to a dog's heart and lungs for abnormalities.
"This study raised the question whether an echocardiogram rather than auscultation should be used to identify dogs for DMVD," Oyama says. "It provided the foundation for our second study where we followed dogs with what we suspected was early disease over time to see if they eventually progressed to developing a heart murmur."
This study, supported by the AKC Canine Health Foundation, will be completed this year. Although it is too early to report on the findings, Oyama notes that so far some dogs have subtle, mild changes in the shape and function of the mitral valve — the early signs of DMVD — despite the absence of a heart murmur.
"We plan to continue monitoring these dogs to determine whether they develop other classic signs of mitral valve disease," he says.
Among the findings reported in the first study, the research team discovered that dogs with severe DMVD had changes in six serum amino acids and a hormone called NT-proBNP that is produced by the heart when stressed or diseased. These indicators could be used to predict severity of disease.
DMVD is believed to be an inherited condition. Many small-breed dogs, including the Cavalier King Charles Spaniel and Toy Poodle, develop the condition. Cavaliers, which have the greatest risk, develop an early-onset form with rapid progression.
"No one knows why these small breeds are prone to DMVD," says Oyama. "However, affected breeds have other diseases associated with connective tissue problems, such as luxating patella and collapsing trachea. The mitral valve has a lot of connective tissue, so it's possible these various conditions are somehow related."
The majority of dogs diagnosed with DMVD are recognized by their veterinarians during an auscultation examination. An audible heart murmur usually is followed by an echocardiogram that provides a visual tool to see early changes in the mitral valve and/or thoracic, or chest, radiographs.
Owners are not likely to notice signs of the disease unless it reaches the advanced stage. Excessive panting when exercising, shortness of breath, coughing, a distended abdomen, loss of appetite, and weight loss are the signs owners commonly report.
Degeneration of the mitral valve happens slowly. Located between the left atrium and left ventricle of the heart, the mitral valve helps regulate the flow of blood in and out of the heart and prevents a back flow from going into the atrium. The mitral valve is made of thin flaps of tissue, or valve leaflets, attached by long, tendon-like structures, the chordae tendineae, to the muscles of the left ventricle. These valve leaflets open and close to regulate the flow of blood, but as the disease progresses, they begin to thicken, contract and lose flexibility.
When the mitral valve functions correctly, blood in the left ventricle is pumped to the body as the heart contracts. As the mitral valve degrades, it cannot close properly and small amounts of blood leak back into the left atrium. Over time, the valve will continue to degrade until the heart can no longer compensate. Stress from the leak causes the heart to enlarge, eventually resulting in congestive heart failure. In severe cases, the chordae tendineae may rupture, damaging or causing complete collapse of the mitral valve.
A disease that is managed rather than cured, DMVD has limited treatment options for dogs with advanced disease and congestive heart failure. Although surgical repair currently is not available in the United States, a research team in Japan at Nihon University in Tokyo has reported good results with valve repairs using an open heart surgical procedure.
"Many dogs will never develop the severe form of the disease that causes clinical signs requiring medications," Oyama says. "A relatively small percentage will have progressive heart enlargement and be at risk for heart failure."
Breeders often debate whether they should breed a dog diagnosed with DMVD since the disease is believed to be an inherited condition. On the other hand, not breeding dogs diagnosed with DMVD could potentially reduce genetic diversity and proliferate other diseases.
"I recommend that breeders monitor dogs showing mild changes in mitral valve structure from an echocardiogram test," Oyama says. "I don't think these dogs should automatically be removed from breeding programs. Doing so would further limit the already small gene pool, and until we know more about what echocardiographic changes constitute a definitive diagnosis of very early 'silent' mitral valve disease, auscultation for a heart murmur remains the recommended gold standard for screening for DMVD."
[1] Mulvihill JJ, Mulvihill CG, Priester WA. Cleft Palate in Domestic Animals: Epidemiological Features. Teratology. 1980;21:109-112.
Owners May Contribute to Cleft Palate Research
Breeders and owners of dogs affected by cleft palate and/or cleft lip defects are encouraged to contribute DNA samples to help advance research of the disorder. The researchers also need DNA from their normal littermates and relatives. The collaborative study involves researchers at the University of California-Davis and the University of Sydney in Australia, who are using sophisticated sequencing and computational data analytics to help advance information about the inherited birth defect.
Before sending DNA samples, please contact Zena Wolf, a graduate student researcher at the University of California-Davis, for information. She can be reached by email at ztwolf@ucdavis.edu.