2024.02.11 | Questions 73-74
Newborn screening (2/3) - Pompe disease
Hello,
This is the 2nd in a series of 3 posts related to newborn screening. This post focuses on the diagnostic approach to Pompe disease in the context of newborn screening (NBS). Knowing about Pompe is important clinically and for exams. I also uploaded a video to YouTube on Pompe disease and included an audio voiceover of this post.
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Please feel free to email with any comments or questions about this post (daniel@studyrare.com). If you have suggestions for future posts, also feel free to reach out.
Have a great week!
-Daniel
Questions
Question 73
A 5-day-old girl presents to her pediatrician for a routine visit. Her pediatrician received a call that her newborn screen was positive for Pompe disease. Which of the following techniques was most likely used by the state newborn screening lab to identify the initial positive result?
Question 74
Later that day, the pediatrician receives a faxed report from the state newborn screening lab showing that the infant was found to have reduced acid alpha-glucosidase (GAA) enzyme activity. No other abnormalities are noted on the newborn screen. The newborn is feeding well and is otherwise asymptomatic. What is the most appropriate next step in the management of this infant?
Explanation
Q73: Enzyme activity assay
Q74: Send confirmatory molecular testing
Pompe disease is an autosomal recessive lysosomal and glycogen storage disorder caused by mutations in the gene encoding the acid alpha-glucosidase (GAA) enzyme. The lack of the GAA enzyme results in the accumulation of lysosomal glycogen in skeletal and cardiac muscle. Clinically, there are 2 forms of Pompe disease: early-onset and late-onset. Early-onset Pompe presents with hypotonia and cardiac hypertrophy, often within the first several months of life. Late-onset Pompe disease, in contrast, does not typically present with cardiac involvement, though can present with skeletal muscle weakness (including the diaphragm, which can result in respiratory insufficiency). Newborn screening for Pompe disease aims to identify affected infants so that treatment with enzyme replacement therapy can begin as soon as possible. Without treatment, most newborns affected with early-onset Pompe will die within the first year of life.
💡Positive newborn screening (NBS) results for Pompe disease should be acted upon promptly, as confirmation of the diagnosis and early initiation of enzyme replacement therapy can improve clinical outcomes. Pompe is one of approximately ~15 conditions (indicated in the ACMG ACT sheets as “Time Critical”) where rapid intervention with medical or dietary therapy can significantly improve the newborn’s outcome.
Question 73 addresses the technique used by state newborn screening labs to screen for Pompe disease. Enzyme activity assays are commonly used as part of the initial screening process for several lysosomal and glycogen storage diseases, including Pompe disease. These assays measure the activity of the enzyme (e.g. GAA) in blood samples using an artificial substrate that releases a fluorophore ☀️, which can be quantified. Infants who screen positive for Pompe based on low acid alpha-glucosidase enzyme activity must undergo confirmatory molecular testing (Question 74) via sequencing of the GAA gene. This is because while enzyme activity assays are an effective screening tool, they cannot differentiate between Pompe disease (a true enzyme deficiency), a pseudodeficiency allele, and a carrier.
Pseudodeficiency alleles are genetic variants that reduces enzyme activity below the threshold of detection for the assay, but not so much as to cause disease. A pseudodeficiency allele in one or both copies of GAA can reduce enzyme activity enough to cause a positive initial newborn screen. Carriers of Pompe disease have a single heterozygous variant in GAA and can also flag positive on NBS due to reduced total enzyme expression. Carriers and pseudodeficiency alleles are responsible for false positive results on a variety of NBS enzyme activity assays (e.g. Duarte variant causing a false positive result for galactosemia), and molecular testing can distinguish these from true enzymatic deficiencies seen in affected patients.
💡When addressing a false positive NBS screening result with a patient, appropriate counseling and reassurance is important to prevent misunderstanding and reinforce that the newborn is not affected with a disease.
Incorrect answers (Question 73)
Capillary electrophoresis (Choice A) is used for the separation of hemoglobin variants and can screen for the presence of hemoglobinopathies. It is not used in the initial screening for Pompe disease. Polymerase chain reaction (Choice B), or PCR, is used to amplify DNA segments in preparation for DNA sequencing. PCR is therefore part of the workflow used for confirmatory testing (but not in screening) for Pompe, among other conditions. The enzyme-linked immunosorbent assay (Choice D), or ELISA, is an immunoassay used for quantifying levels of immunoreactive trypsinogen (IRT), which serves as a first-line screening test for cystic fibrosis on NBS. ELISA are not used to screen for Pompe.
Incorrect answers (Question 74)
Initiating enzyme replacement therapy (Choice A) would be the next best step once the diagnosis of Pompe is confirmed. At this point, however, additional testing with genetic sequencing of the GAA gene is needed to confirm the diagnosis of Pompe. Continue routine newborn care (Choice B) would be the next best step if the confirmatory GAA sequencing was negative or showed the baby was a carrier or had a pseudodeficiency allele. Referral to a pediatric cardiologist (Choice C), along with an EKG and echocardiogram, would be appropriate once the diagnosis of Pompe disease was confirmed.
💡 Additional biomarkers for Pompe are available and can help expedite a diagnostic workup. These include a urine biomarker called HEX4, a glucose tetrasaccharide, which is elevated in patients with Pompe disease and other glycogen storage disorders. A CK level (which would be elevated in Pompe) and EKG (which could show signs of hypertrophic cardiomyopathy) may also be indicated as part of an expedited evaluation.
Learning objective
Pompe disease is an autosomal recessive lysosomal and glycogen storage disorder that affects skeletal and cardiac muscle. Patients with Pompe have reduced acid alpha-glucosidase (GAA) enzyme activity. The newborn screen for Pompe, as for other lysosomal and glycogen storage disorders, relies on an enzyme-based assay. Confirmatory molecular testing is required after a positive NBS result to rule out a false positive, which can be due to a pseudodeficiency allele or to the patient being a carrier of a pathogenic allele. Early identification of Pompe disease allows for early initiation of enzyme replacement therapy, which improves patient outcomes.
2023 ABMGG General Exam Blueprint | IX. Population screening → a. Newborn screening → i) Blood spot screening | V. Single gene inheritance → d. Single gene disorders → ix. Metabolic disease → 6) Lysosomal storage disorders
2023 ABGC Exam Content Outline | Domain 1C. Genetic Conditions → 3. Screening, surveillance, and risk reduction | Domain 3A. Testing and Interpretation → 1. Test methodologies/technologies and applicable limitations
References
Pompe disease testing (from Rare Disease Advisor)