Amyloidosis is the deposition of an abnormal substance called amyloid in the tissues of the body. These amyloid deposits are composed of protein fibrils formed by the polymerization of protein subunits forming a specific pattern called the beta-pleated sheet. The specific biophysical arrangement of this sheet gives the amyloid deposits their unique staining and optical properties. Due to this structure amyloid is insoluble and can be thought of as “wax”. It is also important to realize that amyloidosis is not a single disease, but can be the end point of many diseases. The structure of amyloid also is responsible for the characteristic green color after staining with Congo red.
In Shar-Pei amyloidosis is a reactive amyloidosis. This form of systemic amyloidosis usually occurs with chronic inflammatory diseases and is characterized by the presence of amyloid protein AA. Amyloid protein AA is derived from an acute phase protein called serum amyloid A protein (SAA) produced by the liver. There are many other acute phase proteins produced by the liver which have important roles in the inflammatory process and in tissue repair after injury. It is important to understand that amyloid protein AA is a normal protein and that it’s production is a normal response to tissue injury and inflammation. It is also important to realize that many diseases, traumatic injuries, cancer disorders, stresses, etc. can stimulate the production of the acute phase proteins. There appears to be a balance between the production of SAA and the degradation and excretion of SAA from the body. It is not known whether the development of amyloidosis in the Shar-Pei is due to prolonged excessive SAA production by the liver which overwhelms the degradation mechanisms or a defect in the degradation process itself, or a combination of both. We do know that Familial Shar-Pei Fever is an inflammatory process which does stimulate the synthesis and release of acute phase proteins from the liver. That this occurs can be surmised from the changes seen on the hemogram and biochemical profiles of Shar-Pei during, or shortly after, an FSF episode. It certainly appears that the cause of amyloidosis in Shar-Pei has a genetic basis.
Reactive amyloidosis results in extracellular deposition of amyloid protein in tissues. This means the “waxy” amyloid is surrounding the cells and slowly crushes them as well as interfering with nutrition of the cells. These cells die and the structures they make up are replaced by fibrous, nonfunctional scar tissue. There are species differences as to which tissues amyloid will accumulate in. In dogs, the kidney is the primary organ involved with the spleen and liver affected less often. The kidney is especially vulnerable due to its decreased ability to replace damaged cells and ultimately, when a certain number of cells have been irreparably damaged, kidney failure with its accompanying clinical signs develops. Once amyloid is deposited in the tissues it appears that nothing can remove it.
Why does amyloidosis have so many different clinical presentations? Why does it occur in some Shar-Pei at 2 years of age and in others at 10 years of age? Why do some Shar-Pei develop amyloidosis and others don’t? Why is it a genetic disease in Shar-Pei? There are many questions which have no answers at this time. I think several theories are plausible to explain the variations we see:
- The underlying basis of amyloidosis in Shar-Pei is Familial Shar-Pei Fever (FSF). It is quite possible that FSF has variable age of onset and variable degrees of severity in terms of the inflammatory disease it causes. This may result in a variable rate of progression in the development of amyloidosis in different individuals. For example, the response of the liver to FSF and the synthesis and release of the acute phase proteins, especially SAA, may be more acute in some dogs resulting in a more rapid deposition of amyloid. In other individuals, the response to FSF may be more chromic and result in slower deposition of amyloid. In effect, there may be milder forms and more severe forms of the same disease.
- The exact mechanism of amyloid deposition may be different in different individuals.
- There may be other effects of FSF on the body which are additive with the amyloidosis. As an example, we know Shar-Pei are more susceptible to disseminated intravascular coagulation (internal blood clotting) during an episode of FSF and blood clots in the kidneys may cause more kidney damage than just amyloid deposition itself.
- Some dogs may have other disease processes going on which can be additive with the effects of amyloidosis.
These are just some ideas on why we see different presentations of the same disease. One fact remains – any amyloid deposits found in a Shar-Pei have to be regarded as related to FSF and genetic until proven otherwise. It really doesn’t matter whether a little or a large amount of amyloid is found. Another point to keep in mind is that the mechanisms initiating amyloid deposition are normal protective responses seen in any breed of dog. It appears in our breed that the mechanisms, which regulate the inflammatory response, don’t work properly allowing this normal response to go out of control and cause disease.