The Invitae Maple Syrup Urine Disease panel analyzes the three genes (BCKDHA, BCKDHB, and DBT) that are associated with maple syrup urine disease (MSUD). This test is indicated for any individual with a positive newborn screen for MSUD, elevated branched-chain amino acids on plasma amino acid analysis (especially elevated leucine), the presence of alloisoleucine on plasma amino acid analysis, or a suspected diagnosis of MSUD based on clinical presentation. Newborn screening may miss intermittent MSUD, so any individual with a clinical or biochemical phenotype suggestive of MSUD should be tested, even if a prior newborn screen was negative. Age of diagnosis and subsequent metabolic control are the greatest determinants of long-term outcome.
BCKDHA BCKDHB DBT
BCKDHA BCKDHB DBT
The Invitae Organic Acidemias Panel has been designed to provide a broad genetic analysis of this class of disorders. Depending on the individual’s clinical and family history, this broader panel may be appropriate. It can be ordered at no additional cost.
MSUD is an inborn error of metabolism that results from an inability to completely catabolize the branched-chain amino acids (BCAAs) leucine, isoleucine, and valine. The second step of BCAA catabolism involves oxidative decarboxylation by the branched-chain alpha keto-acid dehydrogenase (BCKDH) complex. Impairment in any of the BCKDH subunits results in a metabolic block that leads to elevated plasma and tissue concentrations of BCAAs. The accumulation of BCAAs and their metabolic by-products causes the characteristic odor that is often detectable in the urine and cerumen of affected individuals.
MSUD is caused by decreased activity of the branched-chain alpha keto-acid dehydrogenase complex (BCKHD), which is involved in the second step of catabolism of the BCAAs leucine, isoleucine, and valine. BCKHD is a mitochondrial multienzyme complex that is composed of several subunits (E1α, E1β, E2, E3, and regulatory regions). A deficiency in the E2, E1α, or E1β subunits causes isolated MSUD. Impairment of the E3 subunit produces a phenotypically and biochemically distinct condition called dihydrolipoamide dehydrogenase deficiency (congenital lactic acidosis, branched-chain ketoaciduria, 2-ketoglutaric aciduria) that is not tested for in this MSUD panel.
MSUD is genetically heterogeneous (see the table below). An affected individual is homozygous or compound heterozygous for pathogenic variants in the same gene. No cases have been identified with pathogenic variants in different genes. Biallelic variants in any of the three genes can cause any of the four clinical phenotypes. Pathogenic variants are most common in BCKDHA and BCKDHB. Thiamin responsive patients have been found to have a greater incidence of missense DBT variants that result in a full-length mutant protein.
|BCKDHA||E1α||Branched-chain keto acid dehydrogenase E1, alpha subunit|
|BCKDHB||E1β||Branched-chain keto acid dehydrogenase E1, beta subunit|
|DBT||E2||Dihydrolipoamide branched-chain transacylase E2|
MSUD has a broad phenotypic spectrum: classic, intermediate, intermittent, and thiamin-responsive. Classic cases generally have a residual enzyme activity of less than 3% while intermediate, intermittent, and thiamin-responsive MSUD have variable residual enzyme activity ranging from 3%–40%.
Classic MSUD represents most known cases and presents in the neonatal period following protein catabolism. Affected neonates present within the first 48 hours of life with irritability, poor feeding, and rapid onset of ketoacidosis. If untreated, the patients rapidly deteriorate and may become lethargic with intermittent apnea, neurologic abnormalities, and even coma and respiratory failure. Intermediate MSUD has greater enzymatic activity and may present in infancy with feeding problems, growth delays, and developmental delay, or in later childhood with an insidious developmental delay. Most individuals with intermediate MSUD are diagnosed between five and seven years of age. Cases of intermittent MSUD are clinically well during infancy and childhood but present with clinical and biochemical abnormalities during times of illness or physiologic stress. Patients with thiamin-responsive MSUD also present later in life; their enzymatic activity can be increased with thiamin supplementation because thiamin is a cofactor for the BCKDH complex. All forms of MSUD are susceptible to periods of acute metabolic decompensation due to BCAA build-up. Such episodes are frequently precipitated by infection, injury, or physiologic stress.
MSUD is treatable by lifetime dietary restriction of branched-chain amino acids and aggressive intervention during metabolic crisis. Dietary therapy must be managed by a nutritionist to prevent malnutrition. Some cases are responsive to pharmacologic doses of thiamin. Thiamin treatment does not eliminate the need for dietary intervention, but it reduces the degree of restriction. Some affected individuals have been successfully treated by liver transplant.
For patients with a clinical and biochemical diagnosis of MSUD, approximately 100% will have two pathogenic variants in one of these three genes.
|Gene||Proportion of MSUD cases|
MSUD is inherited in an autosomal recessive manner.
MSUD is fully penetrant, with a broad phenotypic spectrum.
The general population incidence of MSUD has been reported as 1 in 120,000–500,000. This may be underestimated due to intermittent biochemical presentations and a mild disease that results in a subclinical phenotype.
MSUD has a much higher incidence in the Old Order Mennonite population of southeastern Pennsylvania due to a BCKDHA founder mutation (p.Y393N). Incidence has been reported as high as 1 in 176 live births.
There is an increased carrier frequency of 1 in 113 among the Ashkenazi Jewish population for the p.R133P mutation in BCKDHB.
Any individual with a positive newborn screen for MSUD, elevated branched-chain amino acids upon plasma amino-acid analysis (especially elevated leucine), the presence of alloisoleucine upon plasma amino-acid analysis, or a suspected diagnosis of MSUD based on clinical presentation, should be tested for MSUD. Newborn screening may miss intermittent MSUD, so any individual with a clinical or biochemical phenotype suggestive of MSUD should be tested, even if a prior newborn screen had a negative result. Age of diagnosis and subsequent metabolic control are the greatest determinants of long-term outcome.
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Invitae is a College of American Pathologists (CAP)-accredited and Clinical Laboratory Improvement Amendments (CLIA)-certified clinical diagnostic laboratory performing full-gene sequencing and deletion/duplication analysis using next-generation sequencing technology (NGS).
Our sequence analysis covers clinically important regions of each gene, including coding exons, +/- 10 base pairs of adjacent intronic sequence, and select noncoding variants. Our assay provides a Q30 quality-adjusted mean coverage depth of 350x (50x minimum, or supplemented with additional analysis). Variants classified as pathogenic or likely pathogenic are confirmed with orthogonal methods, except individual variants that have high quality scores and previously validated in at least ten unrelated samples.
Our analysis detects most intragenic deletions and duplications at single exon resolution. However, in rare situations, single-exon copy number events may not be analyzed due to inherent sequence properties or isolated reduction in data quality. If you are requesting the detection of a specific single-exon copy number variation, please contact Client Services before placing your order.
|Gene||Transcript reference||Sequencing analysis||Deletion/Duplication analysis|