Muscular Dystrophy (Golden Retriever Type)

Description

Golden Retriever Muscular Dystrophy (GRMD or just MD) is a sex-linked recessive disorder. A sex-linked recessive disorder means that the mutation occurs on the chromosome that determines sex (the X or Y chromosomes – XX codes for a female, while XY codes for a male). Dystrophin protein is required in order to connect muscles to bone, acting as an anchor for muscular cells.The mutation of the dystrophin gene is what causes a deficiency of dystrophin proteins in Golden Retrievers.

If the dystrophin gene has a mutation, this production is interrupted, leading to a reduced amount of dystrophin protein. The lack of dystrophin protein leads to the progressive degeneration of skeletal and cardiac muscles. The disease is similar to the human disease, muscular dystrophy.

Symptoms appear relatively quickly, at about six weeks to two months of age. A dog with muscular dystrophy will exhibit muscle weakness, difficulty standing or walking normally, and difficulty swallowing. Symptoms can range from relatively mild to severe, but GRMD is generally fatal by about 6 months of age.

The GRMD mutation is located on the X chromosome. Many sex-linked recessive diseases are located on the X-chromosome. Sex-linked recessive diseases typically affect males. This is because males inherit only one copy of the X chromosome (XY), while females inherit two copies (XX). This means that females will only ever inherit a single recessive copy of an affected gene (one X from mom and one X from dad). Because they have another X chromosome, they have a “second chance” in not displaying symptoms of the disease. The other X chromosome would have to be affected as well in order for them to display symptoms. Most females are carriers of the disease due to the low likelihood of an affected male and a carrier female breeding and producing a litter because of the fatality of the disease at a young age.

In contrast, a male that inherits only one copy of X does not have that “second chance.” A male receives one X from mom and one Y from dad. If the male inherits an affected X chromosome from mom, there is not another normal X chromosome that would replace the mutated gene. This means the male could inherit the affected X chromosome and will develop symptoms. So while both male and female dogs can be affected, GRMD is mostly a disease related to male Golden Retrievers. Females can be carriers of the mutation and will not exhibit any symptoms. A helpful website regarding sex-linked recessive disease inheritance can be found here.

It is important to genetically test female dogs to see if they are carriers of the disease. If a female carrier is bred, even to a male dog that does not have GRMD, there is a 50% chance per male puppy born that he will develop GRMD, which is fatal. There is a 50% chance that each female puppy born will also be a carrier and will not develop symptoms of GRMD. However, they now possess the ability to pass on GRMD to her offspring. It is a good idea to spay any female golden retrievers with GRMD (especially if they’re a common household pet) in order to prevent any accidents that could occur outside the home.