Table 1. Clinical Utility of Genetic/Genomic Testsa continued...
In some genes, the same mutation has been found in multiple, apparently unrelated families. This observation is consistent with a founder effect, wherein a mutation identified in a contemporary population can be traced back to a small group of founders isolated by geographic, cultural, or other factors. For example, two specific BRCA1 mutations (185delAG and 5382insC) and one BRCA2 mutation (6174delT) have been reported to be common in Ashkenazi Jews. Other genes also have reported founder mutations. The presence of founder mutations has practical implications for genetic testing. Many laboratories offer directed testing specifically for ethnic-specific alleles. This greatly simplifies the technical aspects of the test but is not without limitations. For example, approximately 15% of BRCA1 and BRCA2 mutations that occur among Ashkenazim are nonfounder mutations. Also, for genes in which large genome rearrangements are founder mutations, ordering additional testing using different techniques may be needed.
Allelic heterogeneity (i.e., different mutations within the same gene) can confer different risks or be associated with a different phenotype. For example, though the general rule is that adenomatous polyposis coli (APC) gene mutations are associated with hundreds or thousands of colonic polyps and colon cancer of the classical FAP syndrome, some APC mutations cause a milder clinical picture, with fewer polyps and lower colorectal cancer risk.[8,9] In addition, other disorders may be part of the FAP spectrum. Mutations in a certain portion of the APC gene also predispose to retinal changes, for example, when mutations in a different region of APC predispose to desmoid tumors. Thus, selection of the appropriate genetic test for a given individual requires considerable knowledge of genetic diagnostic methods, correlation between clinical and molecular findings, and access to information about rapidly changing testing options. These issues are addressed in detail in PDQ summaries on the genetics of specific cancers. (Refer to the PDQ summaries on Genetics of Breast and Ovarian Cancer; Genetics of Colorectal Cancer; and Genetics of Endocrine and Neuroendocrine Neoplasias for more information.)
Regulation of genetic tests
Government regulation of genetic tests to date remains extremely limited in terms of both analytic and clinical validity with little interagency coordination. The Centers for Medicare & Medicaid Services, using the Clinical Laboratory Improvement Act (CLIA), regulates all clinical human laboratory testing performed in the United States for the purposes of generating diagnostic or other health information. CLIA regulations address personnel qualifications, laboratory quality assurance standards, and documentation and validation of tests and procedures. For laboratory tests themselves, CLIA categorizes tests based on the level of complexity into waived tests, moderate complexity, or high complexity. Genetic tests are considered high complexity, which indicates that a high degree of knowledge and skill is required to perform or interpret the test. Laboratories conducting high complexity tests must undergo proficiency testing at specified intervals, which consists of an external review of the laboratory's ability to accurately perform and interpret the test.[10,12] However, a specialty area specific for molecular and biologic genetic tests has yet to be established; therefore, specific proficiency testing of genetic testing laboratories is not required by CLIA.