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The systemic amyloidoses are a group of diseases that result from the abnormal deposition of amyloid in various tissues of the body. They have been classified into 3 major types: primary, secondary, and hereditary. The most common form of amyloidosis (AL) is a disease of the bone marrow called primary systemic AL (immunoglobulin light chain). Secondary AL usually occurs in tandem with chronic infectious or inflammatory diseases, such as rheumatoid arthritis, tuberculosis, or osteomyelitis. Familial or hereditary AL is the least common form. Determining the specific type of AL is imperative in order to provide both an accurate prognosis and appropriate therapies.
Familial or hereditary transthyretin AL is an autosomal dominant disorder caused by mutations in the transthyretin gene (TTR). The resulting amino acid substitutions lead to a relatively unstable, amyloidogenic transthyretin (TTR) protein. Most individuals begin to exhibit clinical symptoms between the third and seventh decades of life. TTR-associated AL is progressive over a course of 5 to 15 years and usually ends in death from cardiac or renal failure or malnutrition. Affected individuals may present with a variety of symptoms including peripheral neuropathy, blindness, cardiomyopathy, nephropathy, autonomic nervous dysfunction, and bowel dysfunction.
More than 90 mutations that cause TTR-associated familial AL have now been identified within the TTR gene. Most of the mutations described to date are single base pair changes that result in an amino acid substitution. Some of these mutations correlate with the clinical presentation of AL.
The Division of Laboratory Genetics recommends a testing strategy that includes both protein analysis by mass spectrometry (MS) and TTR gene analysis by DNA sequencing (ATTRZ / TTR Gene, Full Gene Analysis) for patients in whom TTR-associated familial AL is suspected. The structure of TTR protein in plasma is first determined by MS. Only the transthyretin (also known as prealbumin) is analyzed for amino acid substitutions. Other proteins known to be involved in other less common forms of familial amyloidosis are not examined. If no alterations are detected, gene analysis will not be performed unless requested by the provider (ie, when the diagnosis is still strongly suspected; to rule-out the possibility of a false-negative by MS). In all cases demonstrating a structural change by MS, the entire TTR gene will be analyzed by DNA sequence analysis to identify and characterize the observed alteration (gene mutation or benign polymorphism).
For predictive testing in cases where a familial mutation is known, testing for the specific mutation by DNA sequence analysis (ATTRX / TTR Gene, Known Mutation) is recommended. These assays do not detect mutations associated with non-TTR forms of familial AL. Therefore, it is important to first test an affected family member to determine if TTR is involved and to document a specific mutation in the family before testing at risk individuals.
Diagnosis of adult individuals suspected of having transthyretin-associated familial amyloidosis
The presence of a structural change in transthyretin (TTR) is suggestive of a gene mutation that requires confirmation by DNA sequence analysis. A negative result by mass spectrometry does not rule-out a TTR mutation. Mass spectrometric (MS) results are falsely negative if the amino acid substitution does not produce a measurable mass shift for the mutation transthyretin. Approximately 90% of the TTR mutations are positive by MS (see Cautions).
After identification of the mutation at the DNA level, predictive testing for at-risk family members can be performed by molecular analysis (ATTRX / TTR Gene, Known Mutation).
There are 3 circumstances where testing by mass spectrometry will not identify amyloid-causing mutations:
-If the amino acid change results in a protein different by <10 atomic mass units (amu), the mutation will not be reliably detected.
-If an amino acid change results from a frequent nondisease-causing mutation (+30 amu). Since over 12% of the population has this innocuous polymorphism, it is an instance in which molecular testing must be done.
-Coinheritance of the polymorphism with a -30 amu mutation would also result in a transthyretin mass indistinguishable from normal.
An interpretive report will be provided.
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4. Boston University School of Medicine, BUSM. Mass Spectrometry Resource. Available from URL: bumc.bu.edu/Dept/Home.aspx?DepartmentID=354
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