Matern, Dietrich

Abstract Background: Tyrosinemia type I (TYR 1) is a disorder causing early death if left untreated. Newborn screening (NBS) for this condition is problematic because determination of the diagnostic marker, succinylacetone (SUAC), requires a separate first-tier or only partially effective second-tier analysis based on tyrosine concentration. To overcome these problems, we developed a new assay that simultaneously determines acylcarnitines (AC), amino acids (AA), and SUAC in dried blood spots (DBS) by flow injection tandem mass spectrometry (MS/MS). Methods: We extracted 3/16-inch DBS punches with 300 μL methanol containing AA and AC stable isotope-labeled internal standards. This extract was derivatized with butanol-HCl. In parallel, we extracted SUAC from the residual filter paper with 100 μL of a 15 mmol/L hydrazine solution containing the internal standard 13C5-SUAC. We combined the derivatized aliquots in acetonitrile for MS/MS analysis of AC and AA with additional SRM experiments for SUAC (m/z 155–137) and 13C5-SUAC (m/z 160–142). Analysis time was 1.2 min. Results: SUAC was increased in retrospectively analyzed NBS samples of 11 TYR 1 patients (length of storage, 52 months to 1 week; SUAC range, 13–81 μmol/L), with Tyr concentrations ranging from 65 to 293 μmol/L in the original NBS analysis. The mean concentration of SUAC in 13 521 control DBS was 1.25 μmol/L. Conclusion: The inclusion of SUAC analysis into routine analysis of AC and AA allows for rapid and cost-effective screening for TYR 1 with no tangible risk of false-negative results.

AbstractBackground: Newborn screening for maple syrup urine disease (MSUD) relies on finding increased concentrations of the branched-chain amino acids (BCAAs) leucine, isoleucine, and valine by tandem mass spectrometry (MS/MS). d-Alloisoleucine (allo-Ile) is the only pathognomonic marker of MSUD, but it cannot be identified by existing screening methods because it is not differentiated from isobaric amino acids. Furthermore, newborns receiving total parenteral nutrition often have increased concentrations of BCAAs. To improve the specificity of newborn screening for MSUD and to reduce the number of diet-related false-positive results, we developed a LC-MS/MS method for quantifying allo-Ile.Methods: Allo-Ile and other BCAAs were extracted from a 3/16-inch dried blood spot punch with methanol/H2O, dried under nitrogen, and reconstituted into mobile phase. Quantitative LC-MS/MS analysis of allo-Ile, its isomers, and isotopically labeled internal standards was achieved within 15 min. To determine a reference interval for BCAAs including allo-Ile, we analyzed 541 dried blood spots. We also measured allo-Ile in blinded samples from 16 MSUD patients and 21 controls and compared results to an HPLC method.Results: Intra- and interassay imprecision (mean CVs) for allo-Ile, leucine, isoleucine, and valine ranged from 1.8% to 7.4%, and recovery ranged from 91% to 129%. All 16 MSUD patients were correctly identified.Conclusions: The LC-MS/MS method can reliably measure allo-Ile in dried blood spots for the diagnosis of MSUD. Applied to newborn screening as a second-tier test, it will reduce false-positive results, which produce family anxiety and increase follow-up costs. The assay also appears suitable for use in monitoring treatment of MSUD patients.

We report 2 patients with isolated 3-methylcrotonyl-coenzyme A carboxylase deficiency whose urine was devoid of, or contained only trace, 3-methylcrotonylglycine, the pathognomonic marker for this disorder. The first patient, a girl with trisomy 21, was detected through newborn screening with an elevated 5 carbon hydroxycarnitine species level, and the second patient came to clinical attention at the age of 5 months because of failure to thrive and developmental delay. Investigation of urinary organic acids revealed an elevated 3-hydroxyisovaleric acid level but no demonstrable 3-methylcrotonylglycine in both patients. Enzyme studies in cultured fibroblasts confirmed isolated 3-methylcrotonyl-coenzyme A carboxylase deficiency with residual activities of 5% to 7% and 12% of the median control value, respectively. Incorporation of 14C-isovaleric acid into intact fibroblasts was essentially normal, showing that the overall pathway was at least partially functional and potentially explaining the absence of 3-methylcrotonylglycine in urine. Mutation analysis of the MCCA and MCCB genes revealed that both patients were compound heterozygous for a missense mutation, MCCB-c.1015G→A (p.V339M), and a second mutation that leads to undetectable MCCB messenger (poly A+) RNA. Absent or trace 3-methylcrotonylglycine levels in urine raises the potential for misdiagnosis in the clinical biochemical genetics laboratory based solely on urine organic acid analysis using combined gas chromatography-mass spectrometry.

SummaryThe continued expansion of newborn screening programmes to include additional conditions increases the responsibility of newborn screening laboratories to provide testing with the highest sensitivity and specificity to allow for identification of affected patients while minimizing the false‐positive rate. Some assays and analytes are particularly problematic. Over recent years, our laboratory tried to improve this situation by developing second‐tier tests to reduce false‐positive results in the screening for congenital adrenal hyperplasia (CAH), tyrosinaemia type I, methylmalonic acidaemias, homocystinuria, and maple syrup urine disease (MSUD). Beginning in 2004, this approach was applied to Mayo's newborn screening programme and resulted in a false‐positive rate of 0.09%, a positive predictive value of 41%, and a positive detection rate of 1 affected case in 1672 babies screened.

Abstract Background: Mitochondrial respiratory chain complex (RCC) disorders may occur as commonly as 1 in 8500 individuals. Because of the great variability of phenotypic presentations, measurement of individual RCC enzyme activities is a crucial diagnostic process. Current assay methods are time-consuming and labor-intensive and thus constitute a major impediment to clinical practice. A method with a faster turnaround time would therefore be beneficial. Method: We developed an automated spectrophotometric method for measuring the respiratory chain enzyme activities of complex I, complex II + III, and complex IV with the Hitachi 912, an automated spectrophotometer. Mitochondrial citrate synthase was also determined for normalization of the RCC activities. Results: A blinded method comparison with samples from an external testing center yielded a 91% concordance of interpretations. Mean intraassay imprecision (as CV; n = 20) in a single batch analysis of each RCC was 5.9%. Interassay imprecision, evaluated on 2 samples harvested and analyzed 3 times each, gave mean CVs of 10%–18%. Conclusions: With this automated method, a panel of RCC enzyme activities can be determined in <2 h. In addition, an immunoblot assay using monoclonal antibodies against specific subunits of RCC enzyme complexes can be informative in cases of borderline enzyme activity. Our results suggest that in vitro diagnosis of RCC enzyme deficiencies in skin fibroblasts is an effective alternative to invasive muscle biopsy.

Objective. 2-Methylbutyryl-CoA dehydrogenase deficiency, also known as short/branched-chain acyl-CoA dehydrogenase (SBCAD) deficiency, is a recently described autosomal recessive disorder of l-isoleucine metabolism. Only 4 affected individuals in 2 families have been described. One patient developed athetoid cerebral palsy, and another had severe motor developmental delay with muscle atrophy. A sibling of the first patient is asymptomatic after prenatal diagnosis and early treatment. Family investigations in the second family revealed that the patient’s mother was also affected but asymptomatic. Methods. We report 8 additional patients identified by prospective newborn screening using tandem mass spectrometry. Results. Molecular genetic analysis performed for 3 of these patients revealed that all are homozygous for an 1165A>G mutation that causes skipping of exon 10 of the SBCAD gene. Although there was no obvious consanguinity, all patients belong to the Hmong, an ancient ethnic group that originated in China and constitutes only 0.8% and 0.6% of the Minnesota and Wisconsin population, respectively. Dietary treatment was initiated in the neonatal period. Except for 1 patient who developed mild muscle hypotonia, all patients remain asymptomatic at ages ranging from 3 to 14 months of age. Conclusions. These cases suggest that SBCAD deficiency is another inborn error of metabolism detectable by newborn screening using tandem mass spectrometry. The continued efficacy of long-term dietary therapy instituted presymptomatically remains to be established.

Abstract Background: Newborn screening for congenital adrenal hyperplasia (CAH) involves measurement of 17α-hydroxyprogesterone (17-OHP), usually by immunoassay. Because this testing has been characterized by high false-positive rates, we developed a steroid profiling method that uses liquid chromatography–tandem mass spectrometry (LC-MS/MS) to measure 17-OHP, androstenedione, and cortisol simultaneously in blood spots. Methods: Whole blood was eluted from a 4.8-mm (3/16-inch) dried-blood spot by an aqueous solution containing the deuterium-labeled internal standard d8-17-OHP. 17-OHP, androstenedione, and cortisol were extracted into diethyl ether, which was subsequently evaporated and the residue dissolved in LC mobile phase. This extract was injected into a LC-MS/MS equipped with pneumatically assisted electrospray. The steroids were quantified in the selected-reaction monitoring mode by use of peak areas in reference to the stable-isotope-labeled internal standard. We analyzed 857 newborn blood spots, including 14 blood spots of confirmed CAH cases and 101 of false-positive cases by conventional screening. Results: Intra- and interassay CVs for 17-OHP were 7.2–20% and 3.9–18%, respectively, at concentrations of 2, 30, and 50 μg/L. At a cutoff for 17-OHP of 12.5 μg/L and a cutoff of 3.75 for the sum of peak areas for 17-OHP and androstenedione divided by the peak area for cortisol, 86 of the 101 false-positive samples were within reference values by LC-MS/MS, whereas the 742 normal and 14 true-positive results obtained by conventional screening were correctly classified. Conclusion: Steroid profiling in blood spots can identify false-positive results obtained by conventional newborn screening for CAH.