Maple syrup urine disease (MSUD) is an inborn error of metabolism caused by branched-chain α-ketoacid dehydrogenase (BCKD)deficiency. In the classical form, where there is less than 3% residual enzyme activity, symptoms occur soon after birth. In the untreated neonate, on a normal intake of protein, the odor of maple syrup may be detected in the cerumen as early as 12-24 hours, and in the urine by 48-72 hours, after birth. Elevated plasma concentrations of the branched-chain amino acids (BCAA), leucine (LEU), isoleucine (ILE), valine (VAL) and alloisoleucine (allo-ILE), as well as a generalized disturbance of plasma amino acid concentration ratios are present by 12-24 hours of age; elevated branched-chain α-ketoacids (BCKA) and generalized ketonuria, irritability, and poor feeding by 24-72 hours; deepening encephalopathy manifesting as lethargy, intermittent apnea, opisthotonus, and stereotyped movements such as "fencing" and "bicycling" by 4-5 days; and coma and central respiratory failure that may occur by 7-10 days. Other phenotypes with various degrees of partial enzyme activity include the intermediate, thiamin-responsive, and intermittent forms of MSUD that can lead to severe metabolic intoxication and encephalopathy with sufficient catabolic stress. Characteristics of the different phenotypes are in TABLE #1, MSUD Phenotypes. Further information can be found at: http://www.ncbi.nlm.nih.gov/books/NBK1319/

TABLE 1 - MSUD Phenotypes

Phenotype	Clinical Symptoms (prior to treatment)	Biochemical	% BCKD activity
Classical	Neonatal onset, poor feeding, lethargy, altered tone, ketoacidosis, seizures. Symptoms often present prior to learning NBS results. Prenatal testing in at-risk siblings can allow dietary intervention at birth. Nearly all due to mutations in the E1 BCKD subunits	↑↑ allo-ILE, BCAA, BCKA	0 -2
Intermediate	Failure to thrive, ketoacidosis and developmental delay; classical symptoms can occur during catabolic illness/stress	↑ allo-ILE, BCAA, BCKA	3 - 30
Intermittent	Normal early development, episodic ataxia/ketoacidosis, severe symptoms may be precipitated by catabolic illness/stress. May be missed by MS/MS NBS	Normal BCAA, BCKA when asymptomatic	5 - 20
Thiamin (B1) responsive	Similar to intermediate. Often due to mutations in E2 BCKD subunit	↑ allo-ILE, BCAA, BCKA ↓BCKA and/or BCAA with thiamin therapy	2 - 40
Lipamide dehydrogenase deficiency	Normal neonatal period, failure to thrive, hypotonia, lactic acidosis, developmental delay, movement disorder. Due to mutations in the E3 BCKD subunit - a component of both pyruvate dehydrogenase and α-ketoglutarate dehydrogenase	Moderate BCAA and BCKA,  ↑α-ketoglutarate, pyruvate	0 - 25

The analytes used as markers for mass spectrometry (MS/MS) newborn screening for MSUD are LEU (LEU and its isomers ILE, allo-ILE) and VAL. The ratio of LEU to phenylalanine helps distinguish possible MSUD from an infant on parenteral/supplemental amino acid or generalized aminoacidemia.  Another analyte, hydroxyproline (elevated in hydroxyprolinemia), has the same mass-to-charge ratio as the LEU isomers and may (rarely) lead to false-positive results (L.1).

See: http://www.newbornscreening.info/Pro/aminoaciddisorders/MSUD.html.

Newborn screening alone is not diagnostic. Those with a positive newborn screen require follow-up biochemical testing and clinical evaluation for confirmation.

• Plasma amino acid analysis: BCAA and presence of allo-ILE.
• Urine organic acid analysis: presence of BCKA. The urine DNPH reaction: presence of α-ketoacids and ketones

http://www.acmg.net/StaticContent/ACT/Leucine.pdf

• Mutation analysis and enzymatic testing: although not necessary for diagnosis, may help predict severity of the disorder or thiamin responsiveness. Individuals with MSUD are always homozygous or compound heterozygous for mutations in the same BCKD subunit gene. See: http://www.ncbi.nlm.nih.gov/sites/GeneTests/lab/clinical_disease_id/22186?db=genetests

MSUD is inherited as an autosomal recessive disorder. Carriers of MSUD have not shown any signs/symptoms of impaired BCAA catabolism. The genes encoding the various catalytic subunits/components (E1a, E1b, E2, E3) have been mapped to chromosome loci: 19q13.1-13.2; 6q14; 1p31; and 7q31-32, respectively. In a total of 78 cell lines from MSUD subjects, the large majority had mutations in the E1 subunits. A common mutation among the Old Order Mennonites is Y393N, a point mutation in the E1a subunit. See: http://omim.org/entry/248600