Familial amyloid polyneuropathy (FAP) is a rare genetic condition that results in symptoms often found in neurological disorders. Mutations in a gene that encodes for a protein called transthyretin (TTR) leads to the production of abnormal protein deposits called amyloids. Amyloids can build up in tissues over time and cause problems, including neuropathy (nerve cell death).
To aid in the diagnosis of FAP, biopsies of affected tissues can be surgically removed and analyzed for the presence of amyloid deposits.
What is a biopsy?
A biopsy is a surgical procedure in which a small piece of tissue is removed and retained for testing. Amyloid deposits can occur in multiple tissues in FAP and a biopsy can be taken from these different areas.
If a patient has symptoms of neuropathy, then a biopsy of an affected nerve might be taken. Conventional areas from which a biopsy is taken include the sural nerve (at the ankle), the superficial radial nerve (at the wrist), or the ulnar nerve (also at the wrist). To extract the sample, a small part of the skin is numbed and a cut is made to expose the nerve. Then, a small piece of the nerve is removed before the cut is stitched. The piece of nerve is then sent to a laboratory for analysis.
While nerve biopsy used to be the most prevalent form of biopsy to diagnose FAP, less invasive biopsies are used more often now. One area often biopsied is abdominal (belly) fat. To extract the tissue sample, the area is numbed and a needle is inserted into the abdomen in order to take out some of the fat located just under the skin.
Other, less-invasive areas for biopsy include the inner lining of the rectum (rectal mucosa), skin or salivary glands in the mouth. In all of these cases, a small area of the tissue is surgically removed after a local anesthetic is applied.
Tissue analysis
The samples taken during the biopsy procedure are then tested to look for the presence of amyloid deposits. The tissue can be analyzed using different methods. The tissue sample is usually sliced very thin and mounted on microscope slides and then processed with different dyes or treatments before being examined under a microscope.
The most common method is using a dye called Congo red, which stains amyloid deposits apple green in the tissue slice. Other dyes, including BSB (1-Bromo-2,5-bis(3-carboxy-4-hydroxyethyl)benzene) and FSB (1-Fluoro-2,5-bis(3-carboxy-4hydroxystyryl)benzene), also can be used to look for amyloid.
Another analysis technique that may be used is immunohistochemistry, in which antibodies are used to detect specific proteins. In this case, antibodies specific for TTR are used to stain deposits of TTR in the tissue.
Biopsy results
Results of the biopsy can lend evidence for a diagnosis of FAP. If the results indicate a potential case of FAP, genetic testing might be recommended to confirm the type of mutation present in the TTR gene and which treatment options are available.
Risks
Biopsies are minor procedures and the risks are small. Since a tissue sample is being surgically removed, there is always the potential of bleeding and infection-related complications.
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