Originally published to Purina Pro Club, September 2012.
When a scaling skin disorder first showed up in Golden Retrievers in the 1990s, it was frequently misdiagnosed as seborrhea, a condition that also causes scaling and dandruff. More than a decade later, veterinary specialists began studying the disorder and realized it is specific to Golden Retrievers. More recently, geneticists identified the causative mutation and developed a direct DNA test to identify affected and carrier dogs.
Fortunately, Golden Retriever ichthyosis is seldom severe. The disorder is named for the Greek word ichthys, meaning fish, because it looks like fish scales. Breeders sometimes refer to the condition as “puppy dandruff” since puppies usually outgrow signs of flaky skin as they mature, although ichthyosis also occurs in adult dogs. Research in Goldens led to the discovery that the disorder is similar to one of the human autosomal recessive congenital ichthyoses (ARCI).
An ongoing survey conducted by the Orthopedic Foundation for Animals in conjunction with the Golden Retriever Club of America (GRCA) reports that ichthyosis accounts for 0.2 percent of skin disorders in the breed. “Overall, this is a very low percentage,” says Rhonda Hovan, the GRCA research facilitator. “The percentage of affected Goldens in this survey probably does not reflect how many dogs have the disease, especially if the diagnosis is based on physical signs alone. It is likely that mild cases go undetected or ignored. It also is possible that ichthyosis is inadvertently diagnosed as seborrhea, which accounts for 0.9 percent of skin disorders.”
An Inherited Congenital Disorder
Cindy Williamson of Harford County, Md., who breeds Golden Retrievers under the Lycinan prefix, describes unknowingly breeding litters with ichthyosis since 1992. “It was not diagnosed then as ichthyosis,” she says. “The veterinarian suggested it was ‘walking dandruff,’ which is caused by mites, but the puppies outgrew the condition and quit showing signs by 8 weeks of age. We called the puppies ‘peelers’ because of their flaky skin. We didn’t think much about it because it went away and never persisted into adulthood.”
Ichthyoses are a diverse group of hereditary, usually congenital, diseases characterized by faulty formation of the outer layer of the epidermis, the stratum corneum, with resultant scaling.2,3,4 Skin biopsies have revealed two types of ichthyoses: epidermolytic ichthyosis (EI) and nonepidermolytic ichthyosis (NI).
Golden Retrievers develop the NI type, which also occurs in American Bulldogs and Jack Russell Terriers though in a more serious, yet not common, form. Among the breeds that develop EI are Norfolk Terriers, Rhodesian Ridgebacks and Labrador Retrievers.
A University of Pennsylvania study of 46 Golden Retrievers diagnosed with ichthyosis from January 2004 to January 2007 found that all the dogs had mild to moderate dry scaling with variable hyperpigmentation on their abdomens. Large, loose scales ranging from soft white to gray were described as looking like snowflakes.
“The dogs looked like walking snow globes,” says Elizabeth Mauldin, D.V.M., associate professor of pathology and dermatology at the University of Pennsylvania School of Veterinary Medicine. “We documented that the dogs suffered from a primary disorder of cornification, the process of making scale in natural skin turnover. We determined that a skin biopsy can be used to diagnose the disorder.”
The 46 dogs in the study consisted of 25 females and 21 male Golden Retrievers. Twenty-two dogs had skin lesions when they were younger than 1 year of age; three dogs developed the disorder between 1 and 2 years of age; and 13 dogs were older than 2 years of age. The age of onset was unknown for eight dogs.
The wide age distribution reflects the subtle nature of the phenotype.2 The challenge of determining an exact age of onset is difficult as mild scaling could have been overlooked. Hereditary ichthyosis is often present at birth, but it is not uncommon for signs to develop later in adulthood.
“All the dogs had strikingly similar histopathologic changes consisting of mild to moderate hyperkeratosis, or thickening of the outer layer of skin, and an absence of epidermal hyperplasia, which is the proliferation of normal cells, and dermal inflammation,” Mauldin explains. “Environmental factors including nutrition, supplements, humidity, estrous cycle, and other skin conditions likely impacted the degree of scale formation.”
Electron microscopic analysis of the skin of five affected dogs compared with two normal control dogs showed crystalline structures in the outer layer of skin, the stratum corneum. Pedigree analysis of 14 dogs indicated that Golden Retriever ichthyosis is an autosomal recessive condition in which a dog inherits a copy of the mutated gene from both its sire and dam. Not curable, ichthyosis is treated with moisturizers and emollients to provide palliative care. “Corticosteroids, such as prednisone, do not improve skin condition and actually worsen the skin barrier,” explains Mauldin. “This further impairs the body’s natural defenses and puts a dog at risk for secondary bacterial and possibly yeast infections.” Two other studies were simultaneously performed on ichthyosis in Golden Retrievers. The studies, which took place in England3 and France,4 concluded similar clinical and histopathological results. Thus, the three studies led to the hypothesis of an autosomal recessive transmission mode.
DNA Discovery Aids Understanding
The discovery of the causative mutation for Golden Retriever ichthyosis was made in 2010 by a group of French geneticists led by Catherine André, Ph.D., head of the Canine Genetics team at CNRS/University of Rennes. The project was funded by CNRS and the European Commission under the LUPA initiative that brought together genomic experts at university and private laboratories to study the genetics of dogs and learn more about human diseases. A genomewide association study of 40 affected dogs and 40 unrelated healthy ones allowed them to identify a genomic region on canine chromosome 12.
The researchers used candidate gene sequencing in 12 affected and 12 healthy Goldens to identify the mutation in the PNPLA1 (patatin-like phospholipase) protein. They found a homozygous insertion-deletion, or indel, mutation in PNPLA1 that leads to a premature stop codon and then an altered protein in the affected dogs.
“We considered PNPLA1 to be a promising candidate gene,” André says. “This premature stop codon causes a loss of 74 amino acids in the C-terminal region, affecting enzyme activity and potentially lipid droplet binding, of the PNPLA1 protein.”
The research was confirmed by sequencing 320 Golden Retrievers, consisting of 120 affected and 200 unaffected healthy dogs. All the affected dogs were homozygous for the mutation, about 30 percent, and the healthy ones were either homozygous for the normal allele, about 30 percent, or heterozygous, about 40 percent, which is consistent with an autosomal recessive mode of inheritance.
The mutation was not found in 180 healthy dogs representing the other four retriever types: Labrador Retriever, Flat-Coated Retriever, Curly Coated Retriever, and Chesapeake Bay Retriever. Nor was the mutation found in 300 healthy dogs of 25 other breeds.
“The data support the causative nature of the mutation in Goldens, as well as the specific founder effect in the breed,” André explains. “The indel introduced a frameshift and thus a premature stop codon in the open reading frame of the gene, which led to a loss of 74 amino acids in the highly conserved C-terminal region of the PMPLA1 protein.”
The geneticists applied information from the Golden genetic model to human research. “We screened the PNPLA1 gene in humans affected with ARCI,” says André. “The research translated well from Goldens to humans through collaborations with Dr. Judith Fisher, a specialist of human ichthyoses from the Institute of Human Genetics at Freiburg University in Germany. Mutations in the PNPLA1 gene were identified in two human families affected by ARCI. This was the first evidence for the involvement of PNPLA1 in ichthyosis in dogs and humans.”
Random sampling of 500 Golden Retrievers in France further supports the autosomal recessive inheritance and the mutation frequency as follows:
- Forty percent of dogs are carriers that inherit one copy of the mutated allele. Though they will not be affected, they transmit the mutated allele to 50 percent of their offspring.
- Thirty percent are affected, inheriting two copies of the mutated allele and passing one mutated allele on to 100 percent of their offspring. Many affected dogs do not show signs of disease, while others develop mild to moderate disease.
- Thirty percent are normal. They inherit two copies of the normal allele, and their offspring cannot get the disease even if the other parent is affected or a carrier.
André notes that thus far all affected Golden Retrievers have the same mutation. She believes the disease is less prevalent in Golden Retrievers in the U.S. compared to those in France and other European countries, where the frequency of the mutation now reaches about 50 percent.
She is hopeful that ongoing research will help identify and clarify the role of the PNPLA1 protein in normal and affected dogs. “This is a big concern for breeders,” André says. “Some dogs are so mildly affected they are considered clinically unaffected, but if they are bred, they will spread the disease.”
Realizing Breeding Implications
A direct DNA test now is available for determining if a Golden Retriever carries the PNPLA1 mutation or is affected by the scaling disorder, although the DNA test cannot predict which affected dogs will actually show clinical signs. Margret Casal, Ph.D., D.V.M., associate professor of medical genetics at the University of Pennsylvania School of Veterinary Medicine, cautions that a positive ichthyosis test should not discourage breeders. “Breeders should not remove affected or carrier dogs from the gene pool,” she says. “This would reduce genetic diversity and create a super bottleneck.”
The best approach is to gradually reduce the mutation over six or seven generations, Casal advises. “You should consider the entire dog — all his or her qualities and characteristics. An affected or carrier dog that has much to contribute should be bred, although you should avoid breeding two affected dogs. Instead, breed outstanding affected or carrier dogs to clear dogs. This provides a choice of dogs to progressively decrease the frequency of the PNPLA1 gene mutation.”
The GRCA Health & Genetics Committee endorses this approach. It also echoes Casal’s caution regarding unnecessarily reducing the genetic diversity that is so vital for long-term breed health, Hovan says.
Golden Retriever breeder Gayle Watkins of Cold Spring, N.Y., who breeds under the Galyan prefix, concedes that she has not been too concerned about ichthyosis since it appears so mildly in most Golden Retrievers. A recent litter of seven affected puppies made her think twice.
“We had DNA testing performed on the dam when she was pregnant,” Watkins says. “Both she and the sire tested positive. After this happened, I realized how upset prospective buyers can become about buying puppies with a known flaw.”
Two of the puppies had moderate to severe flakes, or dandruff. Scales could be seen when the hairs on their coat were parted. “When the puppies were 19 weeks old, the signs had disappeared, and eventually, there were no clinical signs,” she says.
Once the genetic test was offered, Williamson tested the sire and dam of a recent litter that produced a puppy with lightly, flaking dandruff. Both the sire and dam came back as carriers. “The dam was bred three times before the DNA test was available,” she says. “What’s more, all three sires were carriers. I’ve always placed puppies with dandruff into pet homes, so while I don’t have any affected dogs at my kennel, I have plenty of carriers.”
Watkins worries that as DNA tests become available for milder conditions, breeders will be forced to avoid producing dogs with any diseases. “We are going to be pushed into making poor decisions for the breed, such as removing dogs from the gene pool for minor conditions,” she says. “Fortunately, ichthyosis does not affect a dog’s ability to hunt, retrieve, swim or participate in all kinds of activities. It is so important to not lose dogs with great qualities. A DNA test can be helpful as long as we use smart breeding.”
Purina appreciates the support of the Golden Retriever Club of America and particularly Rhonda Hovan, the GRCA research facilitator, in helping to identify topics for the Purina Pro Club Golden Retriever Update newsletter.
1 Grall A, et al. PNPLA1 mutations cause autosomal recessive congenital ichthyosis in golden retriever dogs and humans. Nature Genetics. 2012(Jan15);44(2):140-147.
2 Mauldin EA, et al. The clinical and morphologic features of nonepidermolytic ichthyosis in the golden retriever. Veterinary Pathology. 2008(Mar); 45(2):174-180.
3 Cadiergues MC, et al. Cornification defect in the golden retriever: clinical, histopathological, ultrastructural and genetic characterisation. Veterinary Dermatology. 2008:19;120-129.
4 Guague`re E, et al. Clinical, histopathological and genetic data of ichthyosis in the golden retriever: a prospective study. Journal of Small Animal Practice. 2009:50(5);227-235.
Category : Health