Incorporation of Second-Tier Biomarker Testing Improves the Specificity of Newborn Screening for Mucopolysaccharidosis Type I

Abstract

Enzyme-based newborn screening for Mucopolysaccharidosis type I (MPS I) has a high false-positive rate due to the prevalence of pseudodeficiency alleles, often resulting in unnecessary and costly follow up. The glycosaminoglycans (GAGs), dermatan sulfate (DS) and heparan sulfate (HS) are both substrates for α-l-iduronidase (IDUA). These GAGs are elevated in patients with MPS I and have been shown to be promising biomarkers for both primary and second-tier testing. Since February 2016, we have measured DS and HS in 1213 specimens submitted on infants at risk for MPS I based on newborn screening. Molecular correlation was available for 157 of the tested cases. Samples from infants with MPS I confirmed by IDUA molecular analysis all had significantly elevated levels of DS and HS compared to those with confirmed pseudodeficiency and/or heterozygosity. Analysis of our testing population and correlation with molecular results identified few discrepant outcomes and uncovered no evidence of false-negative cases. We have demonstrated that blood spot GAGs analysis accurately discriminates between patients with confirmed MPS I and false-positive cases due to pseudodeficiency or heterozygosity and increases the specificity of newborn screening for MPS I.

Current Practices for U.S. Newborn Screening of Pompe Disease and MPSI

“…For MPSI, eighteen states report current or planned exclusive molecular testing if the initial NBS is positive. Similar to Pompe disease, a smaller fraction of states who are currently or planning to start screening for MPSI use additional biochemical testing (glycosaminoglycans on dried blood spot) [ 21 ] with or without molecular testing in addition to repeating the NBS with or without subsequent molecular testing. The majority of reflex molecular testing was performed at commercial labs (eleven states for Pompe disease, ten states for MPSI) compared to state NBS or academic laboratories.…”

“…Extensive work has been conducted to decrease NBS false positive rates with implementation of reflex testing [ 20 , 21 , 22 , 23 , 24 , 25 ]. There are benefits and disadvantages associated with both molecular and biochemical reflex testing.…”

Evaluation of Multiple Methods for Quantification of Glycosaminoglycan Biomarkers in Newborn Dried Blood Spots from Patients with Severe and Attenuated Mucopolysaccharidosis-I

“…Furthermore, only mass spectrometry assays can be applied to DBS and thus be incorporated into NBS. This is an important consideration given that the majority of at-risk newborns identified only by low IDUA activity in DBS are false positives, due in part to common pseudodeficiencies [ 8 ], and there is the obvious desire to reduce anxiety to families by minimizing the NBS follow-up referrals that may take weeks to collect and analyze diagnostic data.…”

“…However, there is a place for GAG measurement in DBS as part of NBS. Recent data shows that this measurement is a powerful tool for reducing false positives when carried out as a second stage NBS test after measurement of IDUA activity [ 8 ].…”

The Combined Impact of CLIR Post-Analytical Tools and Second Tier Testing on the Performance of Newborn Screening for Disorders of Propionate, Methionine, and Cobalamin Metabolism

“…A normal 2TT result overrides the initial observation, and a normal report is released as mentioned above. A practical example of the clinical utility of DSP tools has been reported recently [24].…”