Laboratory Studies
Routine newborn metabolic screening for maple syrup urine disease (MSUD) has been available since 1964. This screening is performed in all 50 United States and in various parts of the world. The test is performed within 24-48 hours following birth. Newborn screening for maple syrup urine disease is performed with tandem mass spectrometry (MS/MS) using concentrations of leucine and isoleucine and the Fisher ratio (branch-chain amino acids/phenylalanine and tyrosine) as diagnostic measures. Immediate treatment should follow the identification of affected newborn infants.
Plasma amino acids testing
Plasma amino acids (PAA) testing should be performed to assess for elevated levels of branched-chain amino acids (BCAAs) and to detect l-alloisoleucine (derived from l-isoleucine).
The detection of l-alloisoleucine (also termed alloisoleucine) is diagnostic for maple syrup urine disease. Alloisoleucine reference values in plasma were established in healthy adults (1.9 ± 0.6 μmol/L [mean ± SD]; n = 35), children aged 3-11 years (1.6 ± 0.4 μmol/L; n = 17), and infants younger than 3 years (1.3 ± 0.5 μmol/L; n = 37). The effect of dietary isoleucine was assessed in oral loading tests. A plasma l-alloisoleucine level greater than 5 μmol/L is the most specific and most sensitive diagnostic marker for all forms of maple syrup urine disease. [17]
Plasma alloisoleucine may not appear until the sixth day of life, even when leucine levels are elevated. Transient elevations of BCAAs (without the presence of alloisoleucine) may develop in patients with ketotic hypoglycemia. Infants in the neonatal intensive care unit receiving total parental nutrition (TPN) may have artificially elevated plasma amino acid levels.
Urine organic acids
Urine organic acids (UOA) should be analyzed using gas chromatography-mass spectrometry (GC-MS) to detect alpha-hydroxyisovalerate, lactate, pyruvate, and alpha-ketoglutarate. A random urine specimen is usually sufficient for this study.
BCKD enzyme activity
BCKD enzyme activity can be measured in lymphocytes, cultured fibroblasts, or both, although this test is not required for diagnosis.
Molecular testing
Molecular testing confirmation should be pursued in all patients diagnosed with maple syrup urine disease to confirm the diagnosis, to provide additional information about prognosis, to provide the knowledge for in-depth genetic counseling for the family, and to provide criteria for prenatal testing. Molecular testing is available for the three genes that have been reported in patients with maple syrup urine disease, as follows:
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BCKDHA gene located at 19q13.2, which encodes BCKA decarboxylase (E1) alpha subunit gene (maple syrup urine disease type 1A)
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BCKDHB gene located at 6q14.1, which encodes BCKA decarboxylase (E1) beta subunit gene (maple syrup urine disease type 1B)
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DBT gene located at 1p21.2, which encodes dihydrolipoyl transacylase (E2) subunit gene (maple syrup urine disease type 2) (see also OMIM #248600, GeneTests)
Prenatal diagnosis
Prenatal diagnosis can be performed by measuring BCKD enzyme activity in cultured amniocytes or chorionic villus cells, by performing mutation analysis, or by measuring branched-chain amino acid concentrations in amniotic fluid. However, molecular analysis is the most reliable and therefore preferred method for prenatal diagnosis. Prenatal diagnostic studies require identification of two pathogenic mutations in the index patient.
