Molecular basis of intermittent maple syrup urine disease: novel mutations in the E2 gene of the branched-chain α-keto acid dehydrogenase complex

Abstract

The E2 gene of the branched-chain α-keto acid dehydrogenase (BCKDH) complex was studied at the molecular level in three patients with intermittent maple syrup urine disease (MSUD). All three patients had higher BCKDH activity than did those with the classical phenotype. In the first patient, a single base substitution from A to G in intron 8 created a new 5Ј splice site and caused an insertion of 126 nucleotides between exons 8 and 9 by activating an upstream cryptic 3Ј splice site in the same intron. The predicted mRNA encoded a truncated protein with 282 amino acids including 4 novel ones at the carboxyl terminus, compared with the normal protein with 421 amino acids. In vitro, the region from the patient but not from a normal control was recognized and was recovered as a novel exon, indicating that the single substitution was responsible for incorporation of the region into mRNA. This mutation probably supports an exon definition model in which the spliceosome recognizes a 3Ј splice site and then scans downstream for an acceptable 5Ј splice site, thereby defining an exon. The second patient was homozygous for a G to T transversion at nucleotide 1463 in exon 11, which predicted a substitution of the termination codon by a leucine residue and the addition of 7 extra amino acids at the carboxyl terminus. For each mutation, these two patients were homozygous and their parents were heterozygous. The third patient was a compound heterozygote for a C to G transversion at nucleotide 309 in exon 4 and a G to A transition at nucleotide 1165 in exon 9, causing an Ile-toMet substitution at amino acid 37 and a Gly-to-Ser substitution at amino acid 323, respectively. Taken together, these results indicate that the molecular basis of intermittent phenotype MSUD in some patients can be due to mutations in the E2 gene, giving rise to a low but significant residual activity of the BCKDH complex.

Mutational spectrum of maple syrup urine disease in Spain

“…In line with previous reports (Chuang, et al, 1997;Tsuruta, et al, 1998), most of the EII MSUD Spanish patients show a milder phenotype of the disease (Table 4) and have been considered as variants. It is interesting to remark that the mutation pattern identified in this group correspond to missense mutations, affecting residues in a conserved core of the E2 protein.…”

Maple syrup urine disease in the Austronesian aboriginal tribe Paiwan of Taiwan: a novel DBT (E2) gene 4.7 kb founder deletion caused by a nonhomologous recombination between LINE-1 and Alu and the carrier-frequency determination

“…The changed or missing amino acids after Lys366 may also destroy the structure of the E2 catalytic domain and disrupt the assembly of intermediated active trimers that interlock through carboxyl-terminal hydrophobic knobs to produce the native 24-meric core, 14 thus affecting the assembly of the correct 24-mers cubic structure.…”

Applying Genomic Analysis to Newborn Screening

“…Individual 2 had 3 potentially relevant variants, including 2 that have been previously reported (in ACADS , associated with short chain acyl-CoA dehydrogenase deficiency, MIM 201470; and HPD , associated with hawkinsinuria, allelic with tyrosinemia type III, MIM 140350), and 1 novel variant (in OTOA , associated with autosomal recessive deafness, MIM 607039) [Tomoeda et al, 2000;Corydon et al, 2001;Zwaenepoel et al, 2002]. Individual 3 had two such variants, neither of which were novel (in DBT , associated with maple syrup urine disease, MIM 248600; and HPD , associated with hawkinsinuria, allelic with tyrosinemia type III, MIM 140350) [Tsuruta et al, 1998;Tomoeda et al, 2000].…”