Second-Tier Assay for MPS I Newborn Screening Shows Remarkable Reduction of False Positives
Mucopolysaccharidosis type I (MPS I) is a rare genetic disorder that typically presents with progressive multisystem involvement in early childhood. This condition results from the deficiency of the enzyme, alpha-L-iduronidase (IDUA), which is responsible for breaking down complex sugars called glycosaminoglycans (GAGs). When GAGs cannot be properly metabolized, they accumulate in the body, resulting in symptoms such as enlargement of the liver and spleen, coarse facial features, skeletal and cardiac problems, and delays in development. Early diagnosis is crucial, as it has been shown that early initiation of treatment drastically improves outcomes. Because of this, MPS I was added to the Recommended Uniform Screening Panel (RUSP) in 2016. RUSP is a list of 35 core conditions that the U.S. secretary of health and human services recommends all U.S. states include in their newborn screening panel.
In a new collaborative study of 1,213 dried blood spot specimens submitted on infants at risk for MPS I, Mayo Clinic’s Biochemical Genetics Laboratory has developed a second-tier approach to newborn screening for this disease. The method can accurately discriminate newborns with severe and attenuated (later onset of the disease with less severity of symptoms) forms of MPS I from unaffected newborns.
The high specificity of this approach is good news, given that current newborn screening for this disease, which measures IDUA enzyme activity, has poor specificity, resulting in false positives and costly (often unnecessary) follow-up testing.
“In terms of poor specificity, this means that if we base our interpretation only on IDUA enzyme levels, we are either going to miss several cases (setting a low cutoff) or have a fairly large number of false-positive cases (setting a high cutoff),” says Silvia Tortorelli, M.D., Ph.D., Co-Director of the Biochemical Genetics Laboratory. The Mayo Clinic method, published in The International Journal of Neonatal Screening, measures GAGs, which are complex sugar molecules found in the body’s connective tissue.
“Patients with MPS I can’t properly break down two of these GAGs—dermatan and heparan sulfate—because of an inherited deficiency of the IDUA enzyme,” says Dawn Peck, a genetic counselor at the Biochemical Genetics Laboratory. “Affected patients will have elevated levels of dermatan and heparan sulfate, while unaffected patients do not. So we can increase the specificity of newborn screening by performing second-tier GAG testing following a reduced enzyme value.”
Most published reports from newborn screening programs highlight a low positive predictive value for MPS I, when using enzyme activity alone. “The majority of babies with positive screens end up being unaffected, false-positive cases,” explains Peck, who co-authored the study paper. “This not only causes anxiety for new parents, but also overwhelms the health care providers who have to sort out these cases. The addition of molecular testing certainly increases the positive predictive value, but most second-tier molecular results take several days, if not weeks, to return. Here at Mayo, our turnaround is one or two days.”
An Early Diagnosis Equals a Better Outcome
Severe manifestations of MPS I can also cause significant vision loss, a large head, and skeletal and joint abnormalities, among a host of other symptoms. As yet, there is no cure for this disease, but early intervention with appropriate treatment(s) can mitigate progression and severity.
“Patients with severe disease are referred for hematopoietic stem cell transplantation (HSCT) assessment early on, and likely started on enzyme replacement therapy (ERT) until the HSCT is performed,” says Dr. Tortorelli, senior author on the study. “We know that early diagnosis and early treatment are highly correlated with better cognitive, behavioral, and musculoskeletal outcomes. For patients with the attenuated form, ERT is recommended and believed to improve clinical outcomes.”
So far, most states that are screening for MPS I only use the IDUA enzyme test method for detection.
“Published experience of several newborn screening programs demonstrates that the majority of cases that are flagged by newborn screening turn out to be false positives due to pseudodeficiency (when IDUA activity appears low with the enzyme test but doesn’t result in disease),” says Peck. “So you end up recalling a large number of babies for additional testing or evaluation who turn out to be unaffected. Enzyme screening alone cannot differentiate between affected and unaffected patients.”
False positives from pseudodeficiency represent the greatest challenge to conventional newborn screening for MPS I. According to the study paper, the incidence of pseudodeficiency is 16 times higher than true disease. Hence, newborn screening programs and clinicians that only use enzymatic screening will continue to be overwhelmed with false-positive results.
Making Second-Tier GAG Testing the Norm, Not the Exception
The GAG assay can also differentiate between affected patients and non-affected carriers, who can sometimes “fool” the enzyme test. Dr. Tortorelli explains: “In the case of an unaffected carrier, the individual is carrying a DNA variant that is causing disease, but without a second one on the other allele (mutated gene), which means the person will not develop any disease. These carriers can also show a diminished enzyme level, although usually to a lesser degree than affected cases.”
In other words, this second-tier GAG approach avoids the identification of patients with ambiguous findings who face the potential of a lifetime of unnecessary monitoring.
Analysis of the study’s testing population and correlation with molecular results identified few discrepant outcomes and uncovered no evidence of false-negative cases. According to the study, "The incorporation of post-analytical interpretation and second-tier biomarker testing in the newborn screening process limits detection to high-risk patients."
Fortunately, some states are now using Mayo Clinic’s second-tier approach as part of their newborn screening programs. GAG testing doesn’t require a new blood sample; it can be done with the original blood spot from the infant (taken as part of newborn screening protocol on the second day of life via a heel prick), sparing the child from further invasive procedures.
“We already receive samples from three state laboratories that perform the first-tier test (enzyme analysis), and more state laboratories have expressed interest,” says Dr. Tortorelli. “We’d love to have more states and follow-up facilities send us positive (low enzymes) cases to help reduce the number of these cases, and to reduce the cost of other unnecessary follow-up testing.”
Peck echoes her colleague in advocating for this study approach: “We believe that we can avoid costly molecular testing by incorporating GAGs into the newborn screening process,” she says. “This would limit unnecessary testing and the potential for ambiguous results. So, yes, we would love to receive more samples from newborn screening programs or clinicians.”