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Genetic Testing in Infants Carol L. Beck, MSN, NNP-BC Lori Baas Rubarth, PhD, NNP-BC Continuing Nursing Education (CNE) Credit A total of 2 contact hours may be earned as CNE credit for reading the articles in this issue identified as CNE and for completing an online posttest and evaluation. To be successful the learner must obtain a grade of at least 80% on the test. Test expires three (3) years from publication date. Disclosure: The author/planning committee has no relevant financial interest or affiliations with any commercial interes ts related to the subjects discussed within this article. No commercial support or sponsorship was provided for this educational activity. ANN/ANCC does not endorse any commercial products discussed/displayed in conjunction with this educational activity. The Academy of Neonatal Nursing is accredited as a provider of continuing nursing education by the American Nurses Credentialing Center ’s Commission on Accreditation. Provider, Academy of Neonatal Nursing, approved by the California B oard of R egis tered Nursing, Provider #CEP 6261; and Florida Board of Nursing, Provider #FBN 3218, content code 2505. The purpose of this article is to review the types of genetic testing available for the neonatal population and to discuss the utility of these tests.

Abstract Genetic testing has made diagnosis and treatment possible for many infants. With the addition of many new tests over the past few years, it is important to understand the clinical usefulness of each of these tests. Selecting the correct method of genetic testing assists in obtaining an accurate diagnosis and development of a plan of care for the infant. Keywords: genetics; genetic disorders; testing; microarray; FISH; chromosomes

G

en etic testi ng is a n i m porta n t

t ool in the diagnosis of chromosomal abnormalities. It is important to familiarize yourself with the most common phenotypes and be aware of which tests to perform to obtain an accurate diagnosis of each genetic disorder. Chromosomes are microscopic structures found in each cell of the body (except red blood cells) carrying genetic information. Human cells have 46 chromosomes grouped into 23 pairs. One chromosome from each pair is inherited from the biological mother and father. The chromosome pairs are numbered one through 23. The first through 22nd chromosomes are called autosomes, and the 23rd pair is called the sex chromosomes (also called the “X” [female] and “Y” [male] chromosomes). Females have two X chromosomes, and males have one X and one Y chromosome as their sex chromosomes. Mothers give an X chromosome to their children, and fathers give either an X or Y chromosome to their children, which then determine the genetic sex of the child.

CHROMOSOME ANALYSIS

Accepted for publication January 2014.

Chromosome analysis is the examination of chromosomes for number and structure. Chromosomes are first stained and magnified under a microscope. Chromosomes have distinct patterns of light and dark banding, producing horizontal stripes when stained.

These bands are then numbered from low to high outward from the centromere, the center point where the long arm (q) and short arm (p) meet (Figure 1). The bands form when the chromosome is stained, and the cytogenetic location of specific genes can be identified based on their DNA sequencing. The sample is compared with a control for the addition, deletion, or rearrangement of genetic material. For example, DiGeorge syndrome is also called 22q11.2 deletion syndrome because the piece of genetic material is missing from the 22nd chromosome, on the “q” arm (lower segment), at the 11.2 position from the center or centromere of the chromosome. There are two types of chromosomal abnormalities: numeric and structural. The numeric type is often called aneuploidy. Examples of numeric abnormalities would include monosomy with 45 chromosomes as seen in Turner’s syndrome and trisomy with 47 chromosomes (trisomy 13, 18, and 21). Types of structural abnormalities include deletion, duplication, inversion, translocation (balanced or unbalanced), ring, insertion, and isochromosome. Deletions are a loss of chromosomal material, and duplications are a gain of genetic material. Inversions have a segment of chromosomal material in reverse order. Translocation is the exchange of material between two chromosomes that can result in a balanced or unbalanced amount of genetic material. A ring chromosome is

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FIGURE 1  n  Diagram of a chromosome.

formed when one or both ends of a chromosome are missing and it forms a ring. Insertion is where a segment of a chromosome is removed and inserted into another chromosome. An isochromosome occurs when an arm of a chromosome is missing and is replaced by an exact copy of the opposite arm, making a mirrored image (two p-arms or two q-arms).1 When rearrangement of genetic material occurs, it may not result in obvious health concerns, congenital anomalies, or dysmorphic features. When the rearrangement results in a balanced amount of genetic material, the results of the arrangement may not be obvious until there is pregnancy loss or an infant is born with genetic abnormalities. This occurs because there is an imbalance in the genetic material divided between the sex chromosomes, resulting in too much or too little genetic material.

Testing Options

Genetic testing became an option in the late 1950s. 2 There has been an increase in the precision and resolution in the testing methods over the past four decades. There are now multiple testing options available for chromosome analysis of infants with a possible genetic disorder. There are several testing options that will be discussed, including karyotype, high-resolution chromosome analysis, fluorescence in situ hybridization (FISH), and microarray. Karyotype. Karyotyping is an analysis performed on cells undergoing mitosis to detect abnormal chromosome numbers and major structural rearrangements. This test has the ability to identify gains or losses of genetic material or any rearrangements. It can identify trisomies, monosomies, and larger duplications or deletions but may miss small or subtle gains or losses of genetic material.1 Indications for ordering a karyotype would be ambiguous genitalia, confirmation of Down syndrome, or a possible balanced translocation. High-Resolution Chromosome A nalysis. Highresolution testing is sometimes referred to as extended

banding chromosome analysis. This test examines the chromosome at a higher resolution than low-resolution chromosome analysis, allowing for increased examination of the bands. High-resolution testing can detect subtle balanced rearrangements, partial trisomies or monosomies, translocations, inversions, microdeletions, and microduplications that would not otherwise be detected on low-resolution chromosome analysis.2 This test would assist parents with a known balanced rearrangement prior to conception, parents who wish to evaluate a fetus prenatally, and physicians attempting to diagnose any genetic cause for an infant with multiple congenital anomalies after birth. With the improved technology available in the twenty-first century, low-­resolution chromosome analysis is no longer used. Observation of the bands on chromosomes with staining has gone from the ability to visualize 300–450 bands with low-resolution analysis to .800 bands with high-­resolution analysis. Fluorescence in Situ Hybridization. Fluorescence in situ hybridization, better known as FISH, is the analysis of a single strand of DNA to detect partial monosomy, trisomy, X-Y determination, and specific microdeletion/duplication of small areas within the chromosome. Analysis for trisomy is usually done in conjunction with standard chromosome analysis. FISH for microduplication or microdeletion can be ordered when clinical features associated with specific syndromes are identified involving the addition or deletion of chromosomal material. These are specific to certain areas of the chromosome as seen in Prader-Willi syndrome on chromosome 15 and Cri-du-chat (“Cry of the cat”) syndrome with partial deletion of the short arm of the fifth chromosome. FISH is used in gender determination of infants with ambiguous genitalia to obtain sex chromosome identification. The single-stranded DNA (probe) is coated with a fluorescent dye to identify the abnormality. Location of the gene of interest is vital to FISH because one must know the general location to use this test.2 FISH provides better resolution than standard chromosomal analysis, and results for this particular test can be obtained in 24–48 hours. Another advantage is that multiple probes can be done simultaneously.2 FISH tests are routinely followed by high-resolution chromosome analysis for confirmation of results. Microarray. Array comparative genomic hybridization, often called microarray or aCGH, is an advanced technology that identifies chromosomal abnormalities on all chromosomes simultaneously. The resolution is much higher than with standard chromosome analysis. Microarray compares the DNA against a control sample to identify any gains or losses of genetic material. Some of the major advantages of microarray are the identification of chromosomal imbalances when there is no known diagnosis, when there is no anomaly identified, and when the disorder would not be identified by other testing methods such as FISH. The microarray also provides additional information

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TABLE 1  n  Common Genetic Tests for Newborns Name of Test (amount of blood required)5

Usefulness and Limitations of Major Genetic Tests in Infants

Cost*

Low-resolution chromosome analysis (1 mL blood in a sodium heparin tube)

Low band count (350–400 bands) Use when you think you know the diagnosis (e.g., trisomy) Least expensive Results 5 7–14 d

Most labs no longer use this.

High-resolution chromosome analysis (1 mL blood in a sodium heparin tube)

High band count (.550 bands) Used when you do not know the diagnosis or are looking for balanced translocations (e.g., Robertsonian translocation with trisomy 21) Takes longer and is more expensive than low-resolution chromosomes Preliminary results 5 48–72 h Final results 5 7–10 d

$1,000

Fluorescence in situ hybridization (FISH) (1 mL blood in a sodium heparin tube)

Much faster than other tests Used for rapid screen of aneuploidy if suspected anomaly or gene is known; follow with high-resolution chromosome analysis Results can be available in 24 h.

$250–$500

Array comparative genomic hybridization (aCGH) (1 mL blood in ethylenediaminetetraacetic acid [EDTA] tube)

Used for unknown congenital anomalies or microdeletions/ microduplications Highest detection rate for significant anomalies Results 5 3–5 d May soon be preferred first-line test

$1,200–$4,000

*Approximate costs obtained from Warren Sanger, PhD, FFACMG at University of Nebraska Medical Center’s Human Genetics Laboratory.

concerning minor deletions or duplications in known genetic disorders. The microarray can detect imbalances and some cases of mosaicism such as Beckwith-Wiedemann syndrome (chromosome 11). It is also three times more likely to detect an abnormality than standard analysis.3 Limitations of this test include not being able to detect balanced translocations, inversion, or some polyploidy or triploidy. The test can only detect if there is a gain or loss of genetic material. The test cannot detect a translocation of trisomy 21 since the genetic material is still present but in a different location. A FISH would be required to identify the translocation defect. A disadvantage of the microarray is that it may identify copy number variants.3 These are common or benign variants in the general population and may have no clinical significance, or they can be the cause of minor dysmorphic features, congenital anomalies, or mental retardation and autism.3 Therefore, the microarray can be used not only to identify these small chromosomal changes that are irrelevant to the individual but also to identify the cause of some significant disabilities in other individuals. The identification of these small deletions or copies may or may not be significant to the individual or the family. This is where the geneticist or physician must understand the types of variations that can be identified with this method of genetic testing.

Test Results

Genetic testing is performed to diagnose those infants who are born with signs of a genetic disorder or to predict those infants who may be at risk for inheriting a genetic

abnormality from the parents. When ordering genetic testing, there is a need to assess the clinical usefulness and limitations of each test (Table 1). The tests are only accurate if they effectively identify the disorder. The usefulness of the test can be limited if it takes too much time to obtain confirmation of the results, causing increased anxiety in the parents. Many genetic diagnoses can be ascertained prenatally. There is a new prenatal genomic screening test that can identify and test the fetal cells within the maternal circulation. This test of maternal blood would be ideal for DNA testing of a fetus prenatally to preclude the possible complications associated with chorionic villi sampling or amniocentesis.4 Certain clinical situations are commonly found in the newborn nursery or in the NICU. If there was no diagnosis prenatally and an infant has signs of Down syndrome (trisomy 21), the provider requires confirmation of the diagnosis. A routine karyotype would be an economic choice. If a quick evaluation and confirmation is needed, choose a FISH test, which can usually be completed within 24 hours. If a child has some minor defects or developmental delays, a microarray will reveal minor and/or major chromosomal abnormalities and can assist with a diagnosis.

CONCLUSION

Major advances in genetic testing have occurred, and those advances have produced new testing modalities. Karyotype is helpful to identify gains/losses of genetic material. FISH

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probes can identify monosomies, trisomies, microdeletions, and microduplications and assist with gender determination. Results can be obtained within 24–48 hours. Microarrays can identify chromosomal abnormalities on all of the chromosomes simultaneously and are helpful in identifying unexplained multiple anomalies. High-resolution testing can assist in identifying known balanced translocations and multiple congenital anomalies. Careful consideration should be given when ordering genetic testing to identify the correct type of testing required to ensure an accurate diagnosis.

REFERENCES

1. ten Bosch JR, Manning MA, Cherry AM. Further advances in neonatal genetic testing. NeoReviews. 2012;13:e20-e29. http://dx.doi.org10 .1542/neo.13-1-220. 2. Goodin K, Chen M, Lose E, Mikhail FM, Korf BR. Advances in genetic testing and applications in newborn medicine. NeoReviews. 2008;9:e282-e290. http://dx.doi.org/10.1542/neo.9-7-e282. 3. Lu XY, Phung MT, Shaw CA, et al. Genomic imbalances in neonates with birth defects: high detection rates by using chromosomal microarray analysis. Pediatrics. 2008;122:1310-1318. http://dx.doi.org/10.1542/ peds.2008-0297. 4. Caplan AL. Will new genetic tests lead to more and earlier abortions? http://www.medscape.com/viewarticle/813186. Published October 29, 2013. Accessed January 3, 2014.

5. Sanger W, University of Nebraska Medical Center. Specimen requirements and turn around times. http://www.unmc.edu/mmi/geneticslab/ docs/SpecReqsTATs_POSTNATAL.pdf. Updated January 25, 2013. Accessed January 3, 2014.

About the Author

Carol L. Beck, MSN, NNP-BC, is a neonatal nurse practitioner (NNP) at The Medical Center of Aurora in Aurora, Colorado. Carol received her master’s degree from Creighton University, Omaha, Nebraska and her bachelor’s degree from The Ohio State University, Columbus, Ohio. She has worked as a certified NNP for over 20 years in a Level III NICU and has worked in Ohio, Texas, and Colorado. Lori Baas Rubarth, PhD, NNP-BC, has spent more than 30 years as an NNP and is an associate professor and coordinator of the NNP program at Creighton University, Omaha, Nebraska. She received her doctorate degree from the University of Arizona; her master’s degree from Wayne State University, Detroit, Michigan; and her bachelor’s degree in nursing from Grand Valley State University, Grand Rapids, Michigan. She also works as an NNP at both Level II and Level III NICUs in the Alegent-Creighton Health System and at Methodist Women’s Hospital’s Level III NICU. For further information, please contact: Lori Baas Rubarth, PhD, NNP-BC Creighton University College of Nursing 2500 California Plaza Omaha, NE 68178 E-mail: [email protected]

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Genetic testing in infants.

Genetic testing has made diagnosis and treatment possible for many infants. With the addition of many new tests over the past few years, it is importa...
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