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Multiplex Genetic Testing

Resolution 8 (A-02), introduced by the American College of Medical Genetics at the 2002 Annual Meeting and referred to the Board of Trustees, asked:

Policy H-480.966 on multiplex genetic testing states that physicians should not routinely order DNA-based tests for multiple genetic conditions and that tests for more than one genetic condition should be ordered only when clinically relevant and after the patient has had counseling and has given informed consent . A recent Council on Scientific Affairs (CSA) report (I-01) reviewed issues pertinent to newborn screening programs in the United States.¹ Screening of newborns for metabolic disorders using biochemical tests is commonplace but DNA-based screening is not, and when DNA analysis is used it functions primarily as a confirmatory test (see below). Accordingly, this report provides an update on the status of newborn screening and genetic testing in order to evaluate the recommendation in Resolution 8 (A-02).

Methods

Literature searches were conducted in the MEDLINE database for English-language articles published between January 1998 and February 2003 using the search terms newborn screening and genetic testing. The current U.S. National Screening Status Report (updated December 2002) was consulted as were sites on the World Wide Web relating to newborn screening.

Background

Approximately 4 million infants are born in the United States each year. It has been estimated that as many as 5% of these children can be expected to develop a disease with a genetic component and this rate increases to 8% if congenital abnormalities are included.² Many of the genetic disorders that cause life-threatening symptoms in newborns involve enzyme deficiencies that lead to severe defects in metabolism.³ The most common metabolic errors result in accumulations of amino acids, fatty acids, or other metabolites that cause serious health and developmental problems and in some instances are fatal. As discussed in CSA Report 4 (I-01), laboratory tests that detect the accumulation of many of these metabolites form the basis of newborn screening programs in this country. The primary goal of these screening programs is to identify infants affected by conditions for which prompt application of confirmed interventions can prevent or reduce disease, disability, and death. Unfortunately, no national standards govern newborn screening and so each state determines its own list of diseases and methods for screening.⁴ In addition, a multitude of supplemental laboratory tests are available for infants through nonprofit and commercial laboratories that specialize in newborn screening programs.⁵

Newborn Screening Tests

The National Newborn Screening & Genetics Resource Center (NNSGRC) publishes a report that lists the current newborn screening tests administered by each state (U.S. National Screening Status Report).⁶ Although tests are available for identifying more than 50 different genetic and metabolic disorders prevalent in newborns (see Table), there is a wide variation in tests conducted in each state. For example, certain states require screening for 26 specific disorders while others mandate testing for only 3 of these diseases. Currently, only 2 disorders (phenylketonuria and congenital hypothyroidism) are screened for in all 50 states and the District of Columbia. A majority of states also have mandatory screening for other disorders in newborns including galactosemia, sickle cell disease, congenital adrenal hyperplasia, maple syrup urine disease, homocystinuria, and biotinidase deficiency.

The vast majority of state-mandated newborn screening involves the use of biochemical or bio-analytical testing. These tests range in complexity from a simple biochemical evaluation (eg, radioimmunoassay) that identifies a single metabolite to the use of sophisticated analytical techniques (eg, high performance liquid chromatography and mass spectroscopy) that can detect multiple metabolites in a single blood sample. In fact, 23 states now utilize a technique called tandem mass spectroscopy (MS/MS) that enables detection of at least 28 different metabolic disorders from a few drops of dried blood. Techniques such as MS/MS offer advantages of high throughput, extreme sensitivity, and proven reliability.⁷

Recent advances in medical genetics have introduced DNA-based analyses into newborn screening programs. However, DNA tests are not normally used for the initial diagnostic screening but rather as confirmatory tests (two-tiered testing) to identify specific allelic variants or to determine carrier status. For example, certain states use DNA analyses to confirm diagnosis of sickle cell disease following initial detection by hemoglobin electrophoresis. The limited use of DNA-based tests in newborn screening is due to a number of factors including technical issues associated with extracting and ensuring DNA integrity in dried blood samples. In addition, many disorders detectable in newborns have either no known associated genetic defect (eg, congenital hypothyroidism) or are linked to a large number of possible mutations (eg, cystic fibrosis) that makes identification of specific DNA lesions problematic. As an example, more than 900 mutations associated with cystic fibrosis have been described in the literature. Recent findings that a subset of these mutations are more prevalent in certain ethnic and cultural groups (eg, Northern Europeans and Ashkenazi Jews) have resulted in the formation of a panel of 25 mutations that is recommended for carrier testing in these vulnerable populations.

Other issues limiting the use of DNA analyses in newborn screening include questions about the history of the disease, disease prevalence within a population carrying a specific mutation (penetrance), and questions involving the provision of care for asymptomatic newborns with a confirmed mutation. Because of these and other uncertainties surrounding the genetic aspects of many newborn disorders, states do not require that such testing be performed. Thus, DNA testing is often used in a confirmatory capacity and to supplement the needs of individual patients, physicians, and hospitals.

Although, DNA analysis in newborn screening programs has been generally relegated to a confirmatory role, there is growing potential for its use as a diagnostic tool in healthy infants (presymptomatic) known to be at increased risk for certain disorders. An example of such a disorder is retinoblastoma, a rare condition (1/15,000 infants) resulting in malignant tumor formation in the retina that is one of the most common cancers affecting children.⁸ Approximately 40% of retinoblastoma cases are familial and can be traced to mutations in the tumor suppressor gene RB1 located on chromosome 13. Hereditary retinoblastoma is nearly 100% penetrant, meaning virtually all individuals with genetic mutation will present symptoms, and can be fatal if left untreated. Early diagnosis and better treatment have contributed to increasing survival rates (>90%) for children with this condition. Currently, mutational analysis is available in many clinical laboratories and is estimated to identify genetic defects in about 80% of individuals with a hereditary predisposition. Because affected newborns usually exhibit symptoms within the first year of life, early identification facilitates clinical diagnosis of at-risk children through frequent ophthalmologic examinations and prevents unnecessary medical surveillance of low-risk children in high-risk families.⁹

Retinoblastoma represents just one of many hereditary disorders that can be identified in newborns through DNA testing. The use of these tests is warranted for such disorders because of the life-threatening symptoms that develop early in childhood. However, controversies arise when DNA analysis is used to screen newborns for congenital disorders for which symptoms do not appear until much later in life. Deciphering of the human genome and the knowledge it provides of possible links between gene variants and disease susceptibility will undoubtedly place increasing pressure on newborn screening programs to expand and include more DNA-based tests. Development of multiplex testing formats (eg, microarrays) in which multiple genetic conditions are analyzed in parallel will further encourage the use of DNA genotyping in newborn screening. Although microarrays specific for newborn screening are not commercially available at the present time, a company called Neo Gen Screening has recently announced plans to develop different multiplex screening assays utilizing BioChips for such disorders as sickle cell disease, hereditary hemochromatosis, alpha-1-antityrpsin deficiency, hearing impairment, and insulin dependent diabetes.¹¹ While the use of DNA microarrays in clinical genetic testing appears inevitable, the incorporation of any new genetic test into existing newborn screening programs must be carefully scrutinized to ensure that it meets the following criteria recommended by the Institute of Medicine in 1994: (1) there is clear indication of benefit to the newborn; (2) a system is in place to confirm the diagnosis; and (3) treatment and follow-up are available for affected newborns.¹⁰

Summary and Conclusion

The vast majority of all mandatory tests associated with newborn screening programs involve tests based on biochemical and not DNA analyses. One of the primary reasons for this discrepancy is that most of the disorders these tests are used to detect involve metabolic defects resulting from excessive buildup of metabolites that can be readily identified by simple biochemical assays or more sophisticated analytical instruments now available. Although the use of genetic tests in newborn screening is currently limited, their potential to confirm clinical diagnosis and identify disorders in presymptomatic children portends more widespread use in the near future. However, many questions still exist regarding the suitability of such genetic testing in newborns. Thus, concerns for routine ordering of tests for multiple genetic conditions in newborns are warranted and their use should be closely monitored to ensure they are used appropriately.

RECOMMENDATIONS

The following statements, recommended by the Council on Scientific Affairs, were adopted by the AMA House of Delegates as AMA directives and policy at the 2003 AMA Annual Meeting:

1. Resolution 8 (A-02) is not adopted. (Directive)
2. The AMA will continue to monitor developments in newborn screening and revisit this issue should the use of DNA-based newborn screening tests be more widely adopted by states. (Directive)
3. That Policy H-480.966 is amended and with a change in title as follows:

References

1. Council on Scientific Affairs Report 4. Newborn screening: Challenges for the coming decade. American Medical Association, Interim Meeting, New Orleans, LA; December 2001. Available at: /ama/pub/category/8026.html. Accessed: 2-26-2003.
2. Ross LF, Moon MR. Ethical issues in genetic testing of children. Arch Pediatr Adolesc Med. 2000;154:873-879.
3. Khoury MJ, McCabe LL, McCabe ERB. Population screening in the age of genomic medicine. N Engl J Med. 2003;348:50-58.
4. Newborn Screening Task Force. Serving the family from birth to the medical home: newborn screening: a blueprint for the future-a call for a national agenda on state newborn screening programs. Pediatrics. 2000;106:389-422.
5. National Newborn Screening & Genetics Resource Center. List of commercial and non-profit laboratories involved in newborn screening. Available at: http://genes-r-us.uthscsa.edu/resources/newborn/commercial.htm. Accessed: 2-25-2003.
6. National Newborn Screening & Genetics Resource Center. U.S. National Screening Status Report. Available at: http://genes-r-us.uthscsa.edu/resources/newborn/screenstatus.htm. Accessed: 2-13-2003.
7. Chace DH, Kalas TA, Naylor EW. The application of tandem mass spectroscopy to neonatal screening for inherited disorders of intermediary metabolism. Annu Rev Genomics Hum Genet. 2002;3:17-45.
8. Birch JM. Genes and cancer. Arch Dis Child. 1999;80:1-3.
9. Cohen JG, Dryja TP, Davis KB, Diller LR, Li FP. RB1 genetic testing as a clinical service: a follow-up study. Med Pediatr Oncol. 2001;37:372-8.
10. Andrews LB, Fullarton JE, Holtzman NA, Motulsky AG, eds. Assessing Genetic Risks. Implications for Health and Social Policy. Washington DC: National Academy of Sciences; 1994.
11. Neo Gen Screening research and development programs. Available at http://www.neogenscreening.com/rd.cfm. Accessed: 4-14-2003.