Maple syrup urine disease: identification and carrier-frequency determination of a novel founder mutation in the Ashkenazi Jewish population

Abstract

Maple syrup urine disease (MSUD) is a rare, autosomal recessive disorder of branched-chain amino acid metabolism. We noted that a large proportion (10 of 34) of families with MSUD that were followed in our clinic were of Ashkenazi Jewish (AJ) descent, leading us to search for a common mutation within this group. On the basis of genotyping data suggestive of a conserved haplotype at tightly linked markers on chromosome 6q14, the BCKDHB gene encoding the E1beta subunit was sequenced. Three novel mutations were identified in seven unrelated AJ patients with MSUD. The locations of the affected residues in the crystal structure of the E1beta subunit suggested possible mechanisms for the deleterious effects of these mutations. Large-scale population screening of AJ individuals for R183P, the mutation present in six of seven patients, revealed that the carrier frequency of the mutant allele was approximately 1/113; the patient not carrying R183P had a previously described homozygous mutation in the gene encoding the E2 subunit. These findings suggested that a limited number of mutations might underlie MSUD in the AJ population, potentially facilitating prenatal diagnosis and carrier detection of MSUD in this group.

Figure 1

Genomic organization of polymorphic markers tightly linked to the genes encoding E1α, E1β, and E2 and genotype data for markers flanking E1β. A, Organization of STR markers used in analysis of the BCKDHB locus, and their relative spacing, in both physical and genetic distance. BCKDHB spans ∼240 kb, and the 3′ end lies ∼1 Mb upstream of D6S251. B, Results of genotyping analysis of patients with intermediate (boxed) or classic (underlined) MSUD. The parents of individual 5 are denoted by circles. Individual 9 is a patient with a known mutation (i.e., E2ΔAT).

Figure 2

Results of mutation analysis of AJ patients with MSUD. A, C, and D, Changes (arrows) from expected wild-type sequence, which is shown below the traces. The affected codon is boxed, and the predicted amino acid change is indicated below the codon. The different sequence traces correspond to exon 5 in individual 4 (A), to exon 10 in individual 2 (C), and to exon 7 in individual 1 (D). B, Representative digestion of exon 5 amplimers with NlaIV (New England Biolabs). The sizes of 100-bp-ladder marker fragments (lane M) are indicated to the left of the gel. The samples shown were amplified from the genomic DNA of individuals 1 (lane 1), 5 (lane 2), 6 (lane 3), 7 (lane 4), and 9 (lane 5). Undigested amplimers are 237 bp in length, whereas digested products are predicted to be 99 bp and 128 bp in length.

Figure 3

Ribbon representations of portions of E1β crystal structure, showing positions of mutated residues. The E1 crystal-structure data were obtained from the NCBI Structure (Molecular Modeling Database). Swissprot-Pdb Viewer was used to generate models and image files. A, Ribbon diagram of β-sheets d, e, and g, with the side chains of E146, R183, and E239, respectively, which are shown in white. The mutated residue, R183, is indicated by the arrow, side-chain hydrogen-bond interactions are indicated by dashed lines, and the coordinated K⁺ ion is labeled directly. For clarity of presentation, main-chain hydrogen interactions have been omitted. B, Overview of entire molecule, except for structures overlying the sheet containing R183. The residues altered by missense mutations, R183 and G228, are indicated by bent and dashed arrows, respectively, and their side chains are shown in white. The α-carbon backbone of residues truncated by the nonsense mutation E372X are shown in white.