Screening newborn babies for SCID

Severe combined immunodeficiency (SCID, or “bubble-boy disease”) is an inherited genetic condition resulting from a severe defect in the immune system that makes it difficult or impossible for the affected individual to fight off infections. It impacts an estimated one in 58,000 newborns each year, according to the Journal of the American Medical Association.

Laboratories globally can screen newborns for SCID with the appropriate tests. Though SCID can be fatal if not detected at birth before symptoms have begun, it is treatable, and in most cases curable, if detected and addressed early in newborns. Treatments include stem cell transplants that use cells obtained from a family member or donor whose blood has been banked.

SCID screening at the start of life

SCID is a group of disorders characterized by a severe defect in T cell production and function. Typically, infants with SCID will die due to infection by one year of age unless the infant’s immune system is restored through treatment.¹ The defining characteristic for SCID is always a severe deficiency in T-cell production and function, with defects in B-lymphocytes as a primary or secondary problem, and in some genetic types, production of natural killer (NK) cells as well.^2-4 SCID is also known as “bubble-boy disease” due to affected children having to live in an isolated germ-free environment.

The preferred treatment for SCID is bone marrow/stem-cell transplantation. Evidence from large case series indicates that children receiving early stem-cell transplant for SCID have improved outcomes compared with children who are treated later.⁵ Enzyme replacement therapy is available for adenosine deaminase deficiency-SCID as well as gene therapy, in Europe.^6,7

The T-cell receptor excision circle (TREC) assay is a dried blood spot (DBS) assay employing polymerase chain reaction (PCR)-based nucleic acid amplification and time-resolved fluorescence resonance energy transfer (TR-FRET) technology. The assay provides an effective semi-quantitative determination of TREC DNA in blood specimens dried on a filter paper as an aid in screening newborns for SCID.^8,9

TRECs are stable circular DNA fragments generated during T-cell receptor rearrangement.¹⁰ In healthy newborns, TRECs are made in large numbers, while in newborns with SCID TRECs are barely detectable. The distinguishing measurement can be made on the dried blood spots routinely collected from newborns. Following DNA amplification, TREC copy number in blood can be used to distinguish T-cell lymphopenic SCID newborns from healthy newborns. However, low TREC copy numbers can also be the result of other immunodeficient disorders, such as DiGeorge Syndrome, or be a result of the use of immunosuppression drugs and an underdeveloped immune system, as in the case of premature babies. Confirmatory testing is required for the diagnosis of SCID and for the determination of the form of SCID.^11,12 Compiled information on SCID screening can be found in the SCID-specific guidelines.¹³ The TREC assay takes a simplified approach to screening, with accurate results that identify newborns for confirmatory testing which leads to diagnosis. The assay clearly identifies SCID positive samples with a reasonable presumed false positive rate.¹⁴

A practical approach to SCID screening

The TREC assay is a combination of PCR-based nucleic acid amplification and time-resolved fluorescence resonance energy transfer (TR-FRET)-based detection.^8,9 The assay detects two targets: TREC, the marker of SCID, and beta-actin, which is used as an internal control in each individual test.^15,16 Beta-actin is used as a control for monitoring specimen amplification. Determination of TREC and beta-actin is performed simultaneously for each specimen. Result interpretation is based on two separate calibration curves, and the assay quality control is based on three kit control result interpretations. Each assay generates results for TREC and beta-actin.

The TREC assay itself has four steps: punching, elution, amplification, and measurement. There is no DNA extraction and there are no transfers. The assay is performed through the elution and amplification steps, and is measured all on the same microplate. The absence of transfers minimizes the risk of sample contamination. The assay’s workflow yields a benefit versus current laboratory tests by reducing steps and minimizing manual work, which helps enhance screening efficiency. The kit contains all reagents ready to use, and control and calibrator materials supplied in DBS format to make them as closely representative as possible. The TREC assay does not require laboratory staff to have any extensive previous molecular biology experience. The kit also contains the necessary quality controls and assay calibrators.¹⁴

In the United States, newborn screening has been implemented on a state-by-state basis.^17,18 However, there is no consensus for a SCID screening method. Laboratories use different, non-standardized, assays for detection of TRECs in the newborn dried blood spots (DBS).^12,19-22

As for other parts of the world, Israel and Taiwan are currently screening for SCID, and other countries, such as France, Germany, Sweden, Italy, Spain, Japan, Australia, and the Netherlands are conducting pilot studies for SCID screening.

REFERENCES

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Linh Hoang, MD, PhD, serves as Vice President of Neonatal Screening for PerkinElmer, which offers comprehensive newborn screening solutions to laboratories globally for a wide range of conditions and disorders.