Newborn Screening: How It Relates to Primary Immunodeficiency

Modified from Artemio Jongco, MD, PhD, MPH
Northwell Health

This article originally appeared in the IDF ADVOCATE Fall 2016.

What is newborn screening?
All babies born in the U.S. undergo newborn screening (NBS) in the first days of life. NBS consists of a panel of laboratory tests for a variety of genetic or metabolic disorders, and more recently some immunologic disorders, which without intervention may permanently impact newborns and their families. For example, there is a test for phenylketonuria (PKU) as well as tests for hemoglobin disorders (sickle cell disease), hormonal insufficiency (hypothyroidism) and Severe Combined Immunodeficiency (SCID) or other causes of a low lymphocyte count. The most common way NBS is performed is by heel stick to collect drops of blood onto a filter paper called a Guthrie card, which is then sent to the state Department of Health for processing and interpretation. Although NBS is mandatory, each state develops and manages its own program, and determines how many disorders are tested. Most states screen for a standard number of conditions, but some states may screen for more or less. Remember that any abnormal NBS result should be followed up with an examination and additional confirmatory testing by an experienced healthcare provider such as an immunologist in the case of a positive SCID screen.

Why is early diagnosis of SCID important?
SCID, a group of genetic diseases that are fatal in infancy if left untreated, has been characterized in the medical community as a pediatric emergency. However, if a baby with SCID receives a bone marrow transplant in the first 3.5 months of life, the survival rate can be as high as 94%. Early recognition and treatment of SCID, as well as the other disorders identified through NBS, can lead to a better outcome for the baby.

What has newborn screening taught us about SCID?
In 2010, the U.S. Department of Health and Human Services recommended that all states include SCID screening in their NBS programs. SCID is a group of disorders caused by mutations in a number of different genes, all of which have very low or absent T cells, resulting in death unless immune reconstitution can be achieved by a bone marrow transplant or possibly gene therapy very early in life. Prior to newborn screening for SCID, the estimated annual incidence of SCID in the U.S. was approximately 1 in 100,000 live births, or a total of approximately 40 new cases annually. As more states screen for SCID, it is becoming clear that SCID incidence is higher than previously thought. In fact, in New York State, the incidence is 1 in 48,500 lives births. In 11 states routinely screening, the incidence was 1 in 58,000 in 3,030,083 infants. Screening for SCID is feasible, cost-effective, facilitates timely referral for confirmatory immunologic evaluation and therapeutic intervention if indicated, which greatly improves clinical outcomes. SCID screening is now being conducted or planned to begin in all but three states.

What other types of PI can be identified by SCID newborn screening?
The SCID NBS test measures circular pieces of DNA called TRECS within newly developed T cells that have recently entered the bloodstream from the thymus gland. Therefore, any disorder characterized by low T cell numbers can have low TRECs. For example, a PI such as DiGeorge Syndrome has been detected frequently in SCID NBS programs, whereas PI’s that do not result in low T cells, such as X-linked agammaglobulinemia or Chronic Granulomatous Disease, are not detected. There are other diseases such as Ataxia-Telangiectasia and Down’s syndrome or Trisomy 21 that have also been detected by this test. Therefore, all infants with positive TREC screens should be evaluated by an immunologist before any treatment considerations are made, so as to make certain of the correct diagnosis.

Currently, no U.S. state screens for a PI other than SCID or other causes of T cell lymphopenia in routine NBS. However, researchers have shown that the technique used for SCID screening can also be adapted to PI characterized by low B cells. Similar to T cells, B cells also undergo maturational steps during their development resulting in the formation of B-cell kappa chain excision circles, or KRECs that can be measured during NBS. Some European countries have started pilot programs to screen for PIs with low B cells at birth, and the results of such pilot programs have been promising. A current shortcoming is that PI’s associated with decreased or absent antibody production but not low B cells, such as Common Variable Immunodeficiency, would be missed by such strategies. Being able to expand NBS to include other types of PIs without low T or B cells is an active area of research.

What does the future hold for newborn screening of other types of PI?
All primary immunodeficiency diseases, not just SCID, stand to benefit from early diagnosis. Continued advances in molecular technology may soon allow screening for other types of PI. Moreover, it is possible that future newborns will have extensive testing for known genetic mutations that cause PI or even eventually sequencing of their entire genome. Even predisposition to the more common multifactorial immune disorders with later onset may become possible through deep sequence analysis of DNA from newborns. However, since the mere presence of a mutation does not fully predict phenotype for these conditions, much more needs to be learned about the true predictive value of each proposed type of screening.

What is the underlying biology for SCID newborn screening?
Babies with SCID lack T cells, a type of white blood cells that mature in the thymus, which is an organ of the immune system located in the upper chest. T cells are different from other cells of the immune system because they have a T cell receptor (TCR) on the cell surface. Each TCR is unique and has a specialty — they are specific for one antigen, which is a foreign substance. Our immune system’s ability to fight various infections partly depends on the presence of thousands of T cells with these different specialties.

The first step in generating a functional TCR involves the successful recombination of variable (V), diversity (D) and joining (J) gene segments which are encoded on different parts of the human genome. During normal T cell maturation, the developing T cell must cut one V, D and J segment from the genome, and stitch them back together in a process called V(D)J recombination to form a mature TCR. As a consequence of these rearrangements, a circular piece of DNA, called a T cell receptor excision circle (TREC), is formed as a byproduct, consisting of the extra DNA not used in the mature TCR. Thus, TRECs are a surrogate marker for normal T cell development because TRECs are only formed when V(D)J recombination occurs successfully, and the resulting T cell with a functional TCR exits the thymus.

TRECs are markedly reduced or absent in infants with primary immunodeficiency diseases characterized by T cell defects, including SCID. TREC detection has emerged as the preferred NBS test because genetic sequencing is too expensive, and there are many known, and yet to be identified, genes that can cause SCID.

How does the laboratory measure TRECs during newborn screening?
SCID NBS uses quantitative polymerase chain reaction (qPCR) to quantify TREC copy number in DNA extracted from Guthrie cards. PCR is a molecular laboratory technique commonly used to amplify DNA of interest. In qPCR, the DNA of interest is fluorescently labeled and the amount of fluorescence released during amplification is directly proportional to the amount of amplified DNA. Although various laboratories may use different qPCR machines and reagents for SCID NBS, all these assays rely on the same basic principle. Remember that each state Department of Health decides what cut-off value designates an abnormal result. Unsurprisingly, cut-off values differ from state to state. Nonetheless, an infant with SCID will most likely have an abnormal TREC copy number regardless of which laboratory performs the qPCR assay. Remember that any abnormal NBS result should be followed up with an examination and additional confirmatory testing by an experienced immunologist.

References:

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