Recall rate and positive predictive value of MSUD screening is not influenced by hydroxyproline

Abstract

Maple syrup urine disease (MSUD) is an autosomal recessive inherited disorder caused by the deficiency of the branched-chain 2-oxo acid dehydrogenase (BCOA-DH) complex. The worldwide incidence is approximately 1 in 185,000. MSUD is integrated in many "expanded" newborn screening (NBS) programs that use electrospray ionization tandem mass spectrometry (ESI-MS/MS). Elevated leucine, isoleucine, and alloisoleucine in the dried blood samples (DBS) of newborns are diagnostic parameters. However, with the applied method, it is not possible to distinguish the amino acids from each other, and also not from the other isobaric amino acid, hydroxyproline. While the branched chain amino acids (BCAA) leucine, isoleucine, and alloisoleucine are no diagnostic problem, because they are all elevated in MSUD patients, and, rather, increase the diagnostic sensitivity and specificity, hydroxyproline may cause false-positive screening results. Hydroxyproline is elevated in the benign familial condition hyperhydroxyprolinemia, which needs no medical intervention. The detection of cases with hyperhydroxyprolinemia have formerly been reported from screening programs that used thin-layer chromatography for phenylketonuria (PKU) screening, and, recently, two more cases have been reported, detected by ESI-MS/MS-based NBS. However, the detection of non-diseases is a heavy burden for screening programs and should be avoided if possible. With optimal settings for the quantitation of BCAAs and interpretation rules, elevated hydroxyproline should not cause false-positive screening results.

A Rapid and Sensitive UPLC-MS/MS-Method for the Separation and Quantification of Branched-Chain Amino Acids from Dried Blood Samples of Patients with Maple Syrup Urine Disease (MSUD)

“…Data from 1.6 million newborns in Bavaria/Germany revealed an incidence of one in 135,000 (95% C.I. ; 1:76,000-1:260,000) [4]. For Switzerland we would estimate the same incidence, which is supported from data collected between 1965 to 1989, with an incidence of one in 143,000 (95% C.I.…”

“…The merely combined concentrations of leucine, isoleucine, alloisoleucine, and hydoxyproline (xle) are not helpful, as demonstrated in Table 2. Since the concentration of alanine is frequently decreased or in the low normal range, the inclusion of alanine (ala) and/or the ratios xle/ala and val/ala have already improved the interpretation of MSUD-NBS [2,4]. The biochemical and metabolic basis for this phenomenon has been described already in 1978 [14].…”

“…The positive effect of early detection, especially of the severe cases, is well documented [7]; however, NBS results cannot discriminate between severe and milder forms [8]. There are also rare cases of hyperhydroxyprolinemia resulting in a false-positive MSUD screening result [9,10], but this seems to be a minor problem in general [4]. Several methods have been published for second-tier measurement of alloisoleucine [11][12][13], which all use a second extraction from the initial NBS blood spot and chromatographic separation of the isobaric amino acids leucine, isoleucine, and alloisoleucine.…”

Screening newborns for metabolic disorders based on targeted metabolomics using tandem mass spectrometry

“…An elevated leucine, isoleucine, and valine appear in maple syrup urine disease (MSUD) 35) . If it is concomitant with C2 increase, odds are high that it is due to the ketosis-related expedited catabolism.…”

Exploratory Untargeted Metabolomics of Dried Blood Spot Samples from Newborns with Maple Syrup Urine Disease

“…If the NBS results are positive, then follow-up genetic tests are performed using high-throughput genomic sequencing combined with Sanger sequencing for their confirmation and validation [ 12 ]. Notably, NBS and genetic testing have advanced the identification of MSUD cases [ 13 , 14 ], but these approaches have certain limitations that must be resolved. Additionally, genetic testing can be expensive, is occasionally accompanied by delayed results, and detects variants of uncertain significance (VUSs) [ 15 , 16 ].…”