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Glycogenosis (GSD) Type IV - Glycogen Branching Enzyme Deficiency

Glycogen storage disease type IV (GSD IV) of the Norwegian forest cat is an inherited abnormality of glucose metabolism. Normally, excess glucose obtained in the diet or produced by conversion of proteins and fats is stored in many tissues as a very highly branched polymer (chain) of glucose residues called glycogen. A special biological catalyst called glycogen branching enzyme (GBE for short) is necessary during glycogen synthesis to produce the branching structure. When needed for energy, glucose molecules are removed from glycogen and digested within the tissue or released into the blood circulation for use by other tissues. The ability to add and remove glucose molecules from glycogen efficiently is dependent on its highly branched structure. Our studies have determined that GSD IV in Norwegian forest cats is due to an inherited deficiency of GBE. Affected kittens store a very abnormal glycogen, a glucose polymer devoid of branch points, in most tissues.

GSD IV is inherited as a simple autosomal recessive trait. Practically, this means that, though clinically-normal, both parents of an affected kitten are obligate carriers of the trait. The parents will pass their carrier status along to 50% of all their offspring, both male and female, when mated to a non-carrier cat. When two carriers are mated, 25% of the offspring will be affected and two-thirds of the clinically-normal littermates will be carriers. Because they are clinically-normal, carriers of GSD IV may be active breeders in a cattery, passing their carrier status along to the next generation, and never suspected until an affected kitten is born. Furthermore, because stillbirth or early death is not so uncommon for any of many reasons, a GSD IV affected kitten may be discarded without diagnosis and its carrier parents continue breeding unsuspected. One condition that can masquerade as neonatal death from GSD IV is neonatal isoerythrolysis (NI), a blood type-incompatibility problem that can cause death of newborn type A or type AB kittens born to type B queens a few hours to days after first nursing.

To overcome these problems we have developed a definitive, DNA-based test that is not subject to the ambiguities we experienced previously when attempting to determine GBE activity in blood samples or muscle biopsies. This test will allow conscientious Norwegian forest cat breeders to eliminate GSD IV carrier cats from their breeding programs while preserving desirable traits. The new DNA test detects directly whether the mutation that causes GSD IV is present in a cat's DNA in two copies, as in affected kittens, one copy, as in carriers, or not at all, as in genetically normal cats. In a few catteries, we have found several carriers and these catteries have used results of this test in their breeding programs.

As neonatal isoerythrolysis (NI) is also a recognized blood incompatibility problem that causes fading kittens and death during the first week of life, in collaboration with Dr. Fyfe's laboratory at the Michigan State University College of Veterinary Medicine, we provided GSD IV testing and blood typing to be performed on the same samples, a real convenience to cat breeders. Dr. Fyfe has worked closely with the Section of Medical Genetics in the past and will remain a consultant to the testing laboratory regarding the GSD IV test. A GSD IV test will be performed for $75.00, blood typing will continue to cost $20.00, but the combined testing will be performed for $85.00. The required sample for both tests is 1 - 2 mL of whole blood collected in EDTA anticoagulant (a purple top tube). The samples should be transported to the laboratory in a padded mailer within 2 days, preferably early in the week, but cooling is not necessary. The $3.00 express mail service available in all U.S. Post Offices is adequate and reliable. Each sample must be properly labeled with the cat's identification, and a written test request and payment must accompany each package of samples. Questions regarding testing and proper collection and delivery of samples may be answered by calling 215-898-3375; penngen@vet.upenn.edu.

Clinical Signs:
The disease caused by GBE deficiency in Norwegian forest cats is of two types. By far, the most common form is stillbirth or death within a few hours of birth. We believe this to be due to insufficient glucose available to produce energy during the birth process and the first hours of life. On rare occasions, an affected kitten will survive the neonatal period and appear normal until 5 months of age. Though less common, this is the more devastating form of the disease because by this age new owners are very attached to their kitten, and witness it go through months of neuromuscular degeneration. By eight months of age, GSD IV results in severe muscular weakness, atrophy and contractures, and inability to use the limbs. The cat may die suddenly from heart failure.

Affected Breeds:
Norwegian Forest Cat

Required Samples:
EDTA Blood, 1-2 mL or
2 buccal swabs

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