Multiple Endocrine Neoplasia Type 2 (MEN 2)
There is considerable diversity in the techniques used and the approach to RET mutation testing among the various laboratories that perform this procedure. Methods used to detect mutations in RET include polymerase chain reaction (PCR) followed by restriction enzyme digestion of PCR products, heteroduplex analysis, single-stranded conformation polymorphism (SSCP) analysis, denaturing high-performance liquid chromatography (DHPLC), and DNA sequencing.[88,89,90,91] Most testing laboratories, at a minimum, offer testing using a targeted exon approach; that is, the laboratories look for mutations in the exons that are most commonly found to carry mutations (exons 10, 11, 13, 14, 15 and 16). Other laboratories offer testing for all exons. If targeted exon testing in a family with a high clinical suspicion for MEN 2 is normal, sequencing of the remaining exons can then be performed.
These differences in mutation detection method and targeted versus full gene testing represent important considerations for selecting a laboratory to perform a test and in interpreting the test result. (Refer to the PDQ summary Cancer Genetics Risk Assessment and Counseling for more information on clinical validity.)
Genotype-Phenotype Correlations and Risk Stratification
Genotype-phenotype correlations in MEN 2 are well-established and have long been used to guide clinicians in making medical management recommendations. Several groups have developed mutation-stratification tables based on clinical phenotype, age of onset, and aggressiveness of MTC.[22,24,63] This classification strategy was first put forth after the Seventh International Workshop on MEN in 2001, which provided guidelines for the age of genetic testing and prophylactic thyroidectomy. This stratification was revised by the American Thyroid Association (ATA). The original classification scheme provided three levels of risk based on the genetic mutation of an individual. The new guidelines by the ATA added a fourth category for codon 634 mutations, in recognition of their aggressive clinical course. The specific mutations and their ATA classification are summarized in Table 2 below. It should be noted, however, that this approach has not been prospectively validated as a basis for clinical decision-making.
Level D mutations are the most aggressive and carry the highest risk of developing MTC. These mutations, which are typically seen in MEN 2B, are associated with the youngest age at disease onset and the highest risk of mortality. ATA level C mutations (codon 634) are associated with a slightly lower risk, yet the MTC in patients with these mutations is still quite aggressive and may present at an early age. ATA level A and B mutations are associated with a lower risk of aggressive MTC relative to the risk seen in level C and D mutation carriers. However, the risk of MTC is still substantially elevated over the general population risk and consideration of risk-reducing thyroidectomy is warranted.