Alternative titles; symbols
HGNC Approved Gene Symbol: BCKDHA
Cytogenetic location: 19q13.2 Genomic coordinates (GRCh38) : 19:41,397,818-41,425,002 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 19q13.2 | Maple syrup urine disease, type Ia | 248600 | Autosomal recessive | 3 |
The BCKDHA gene encodes the E1-alpha subunit of the branched-chain alpha-keto acid (BCAA) dehydrogenase complex (BCKD; EC 1.2.4.4), an inner-mitochondrial enzyme complex that catalyzes the oxidative decarboxylation of the branched-chain alpha-ketoacids derived from isoleucine, leucine, and valine. This reaction is the second major step in the catabolism of the branched-chain amino acids (Wynn et al., 1998).
The BCKD complex consists of 3 catalytic components: a heterotetrameric (alpha2-beta2) branched-chain alpha-keto acid decarboxylase (E1), a homo-24-meric dihydrolipoyl transacylase (E2; 248610), and a homodimeric dihydrolipoamide dehydrogenase (E3; 238331). E1 is a thiamine pyrophosphate (TPP)-dependent enzyme. The reaction is irreversible and constitutes the first committed step in BCAA oxidation. The BCKDHB gene (248611) encodes the beta subunit of E1. The complex also contains 2 regulatory enzymes, a kinase and a phosphorylase.
Tiu et al. (1988) cloned the gene for the E1-alpha subunit of branched-chain keto acid dehydrogenase, which they symbolized BCKAD. They found 91% amino acid sequence homology to the rat protein and noted that the phosphorylation site described in other species was conserved in human. Northern blot analysis of human liver showed a single 1.6-kb mRNA transcript. Zhang et al. (1988) isolated a cDNA corresponding to the BCKDHA gene from a human liver cDNA library. The deduced 378-amino acid protein shows 96% identity to the rat enzyme subunit, and the amino acid residues surrounding the phosphorylation site are completely conserved between human and rat. Northern blot analysis demonstrated a single 1.8-kb mRNA transcript, with a higher level of transcript in liver than in normal fibroblasts.
Fisher et al. (1989) found that the E1-alpha subunit comprises 400 amino acids with a calculated molecular mass of approximately 46 kD. Dariush et al. (1991) reported the complete sequence of the 3-prime untranslated region of the BCKDHA gene and found that the gene has multiple transcription initiation sites at bases +1, +18, and +22.
The BCKDHA protein has a molecular mass of 45.5 kD (Wynn et al., 1998).
Dariush et al. (1991) determined that the BCKDHA gene contains 9 exons.
Using somatic cell hybrids, Tiu et al. (1988) mapped the E1-alpha gene to human chromosome 19 and mouse chromosome 7. Crabb et al. (1989) also mapped the human BCKDHA gene to chromosome 19. By in situ hybridization, Fekete et al. (1989) assigned the BCKDHA gene to 19q13.1-q13.2.
Wynn et al. (1998) performed functional studies of the wildtype E1-alpha protein in an E. coli expression system augmented by the chaperonins GroEL (HSPD1; 118190) and GroES (HSPE1; 600141), which are essential for efficient folding and assembly of the E1 tetramer in E. coli. Studies of the wildtype protein showed that E1-alpha and E1-beta assembled in 10 to 20 minutes. Alpha-beta heterodimers (80 kD) were formed at 1 hour, and a minor species of alpha2-beta2 tetramers (165 kD) appeared at 2 hours. At 3 hours and 20 hours, the major species were alpha2-beta2 tetramers, and the minor species were alpha-beta dimers. The data indicated that the subunits initially formed dimers, which later slowly dimerize to produce heterotetramers. Alpha-beta dimers had no enzyme activity. Further studies of mutant E1-alpha proteins demonstrated that the E1-alpha C-terminal aromatic residues, phe364, tyr368, and tyr393, are crucial for complete and proper assembly with E1-beta to form tetrameric E1.
In a patient with the classic type of maple syrup urine disease (MSUD1A; 248600), Zhang et al. (1989, 1991) identified a mutation in the BCKDHA gene (608348.0001). Chuang et al. (1994) later identified a second mutation in the BCKDHA gene in this patient (608348.0002). Each parent was heterozygous for 1 of the mutations.
In 3 of 4 unrelated Hispanic Mexican patients with intermediate MSUD, Chuang et al. (1995) identified a homozygous mutation in the BCKDHA gene (608348.0003). The fourth patient was homozygous for a different mutation in the same gene (608348.0004).
In a classic case of maple syrup urine disease (MSUD1A; 248600), Zhang et al. (1989, 1991) identified a T-A transversion in the E1-alpha subunit gene, resulting in a tyr394-to-asn (TYR394ASN) substitution. The patient's unaffected father was heterozygous for this mutant allele, while the patient's unaffected mother did not carry the mutation, but still expressed only about half of the normal mRNA and protein. The patient was heterozygous for the Y394N mutation, although only the abnormal allele was expressed as mRNA. Zhang et al. (1989) concluded that the patient was a compound heterozygote, inheriting from the father the mutant Y394N allele, and from the mother a different allele perhaps containing a defect in regulation which abolished the expression of one of the E1-alpha alleles. On further study of the same family, however, Chuang et al. (1994) demonstrated that the affected child and his mother were heterozygous for an 8-bp deletion in exon 7 of the E1-alpha subunit gene (887-894del; 608348.0002), resulting in a nonsense codon at 255. Chuang et al. (1994) noted that the second allele had previously been postulated to be a regulatory mutation, and that the patient and his mother were heterozygous for an A-T transversion in the promoter regulatory region of the gene, but this was found to be a normal polymorphism.
In cell lines from 2 Mennonite cases of MSUD1A, Matsuda et al. (1990) demonstrated the same Y394N mutation. They noted that although the E1-beta subunit gene is normal, no crossreacting material associated with this gene could be demonstrated, suggesting that the mutation in the E1-alpha gene results in instability of the E1-beta subunit.
On reporting additional information about BCKDHA gene structure and sequence, Dariush et al. (1991) concluded that exon 9 contains the Mennonite mutation, referred to by them as tyr393-to-asn (Y393N), reflecting a difference in numbering system. They reported that the homozygous mutation appears to cause skipping of exon 6 in the mutant E1-alpha transcript. Fisher et al. (1991, 1991) also identified the Y393N mutation in Pennsylvania Mennonites with MSUD1A, and presented evidence that the Y393N mutation impedes assembly of the E1 component.
Mitsubuchi et al. (1992) described a technique involving primer-specific restriction map modification for the rapid and easy detection of the mutant BCKDHA gene in the Pennsylvania Mennonite population.
Wynn et al. (1998) performed detailed functional expression studies of the Y393N mutant protein in E. coli. The mutant protein showed slow assembly kinetics with the E1-beta protein compared to wildtype protein (over 2 hours versus 10 to 20 minutes) and only formed dimers with a molecular mass of 83 kD, not complete tetramers. There was no residual enzyme activity, and thermal stability of the mutant protein was decreased compared to wildtype.
Puffenberger (2003) reported that the Y393N mutation has a high frequency in the Old Order Mennonites of southeastern Pennsylvania. From 1985 to 1994, 19 of 6,810 Old Order Mennonite children were born with MSUD1A (0.279%, or approximately 1 in 358 births). Puffenberger (2003) estimated the carrier frequency as 7.96% and the mutant allele frequency as 4.15%. The average inbreeding coefficient for all Old Order Mennonite MSUD1A sibships was 2.19%.
For discussion of the 8-bp deletion (887_894del) in the BCKDHA gene that was found in compound heterozygous state in patients with classic maple syrup urine disease (MSUD1A; 248600) by Chuang et al. (1994), see 608348.0001.
In 3 of 4 unrelated Hispanic Mexican patients with intermediate maple syrup urine disease (MSUD1A; 248600), Chuang et al. (1995) found homozygosity for an 895G-A transition in exon 7 of the BCKDHA gene, resulting in a gly245-to-arg (G245R) substitution.
Wynn et al. (1998) performed detailed functional expression studies of the G245R mutant protein in E. coli. The mutant protein showed moderately slow assembly kinetics with the E1-beta protein compared to wildtype protein (30 to 40 versus 10 to 20 minutes), but the G245R protein formed complete tetramers. There was some residual enzyme activity at 2.66% of normal controls. Thermal stability of the mutant protein was decreased compared to wildtype.
In a Hispanic Mexican patient with intermediate maple syrup urine disease (MSUD1A; 248600), Chuang et al. (1995) identified a homozygous 1253T-G transversion in the BCKDHA gene, resulting in a phe364-to-cys (F364C) substitution.
Wynn et al. (1998) performed detailed functional expression studies of the F364C mutant protein in E. coli. The mutant protein showed slow assembly kinetics with the E1-beta protein compared to wildtype protein (over 2 hours versus 10 to 20 minutes) and only formed dimers, not complete tetramers. There was no residual enzyme activity, and thermal stability of the mutant protein was decreased compared to wildtype. Wynn et al. (1998) stated that the associated phenotype was classic MSUD.
In fibroblasts derived a patient with classic maple syrup urine disease (MSUD1A; 248600), Wynn et al. (1998) identified compound heterozygosity for 2 mutations in the BCKDHA gene: a C-to-T transition resulting in an arg220-to-trp (R220W) substitution and Y393N (608348.0001). Detailed functional expression studies in E. coli found that the R220W protein showed normal assembly kinetics with the E1-beta subunit and formed a complete tetramer. However, there was no detectable enzyme activity and mildly decreased thermal stability.
In fibroblasts derived from a patient with classic maple syrup urine disease (MSUD1A; 248600), Wynn et al. (1998) identified compound heterozygosity for 2 mutations in the BCKDHA gene: a G-to-A transition resulting in a gly204-to-ser (G204S) substitution and Y393N (608348.0001). Detailed functional expression studies in E. coli found that the G204S protein showed slow assembly kinetics with the E1-beta subunit compared to wildtype (over 2 hours versus 10 to 20 minutes), although tetramers were formed. Enzyme activity was undetectable and the G204S protein showed decreased thermal stability compared to wildtype.
In fibroblasts derived from a patient with classic maple syrup urine disease (MSUD1A; 248600), Wynn et al. (1998) identified compound heterozygosity for 2 mutations in the BCKDHA gene: a C-to-G transversion resulting in a thr265-to-arg (T265R) substitution and a truncation mutation. Detailed functional expression studies in E. coli found that the T265R protein was insoluble and failed to coassemble with the E1-beta subunit at 37 degrees C. However, the T265R protein did assemble with E1-beta at 28 degrees C and formed small amounts of complete tetramers with no enzyme activity. The T265R protein also showed significantly decreased thermal stability compared to wildtype. Wynn et al. (1998) suggested that the thr265 residue is important for proper folding.
In an Israeli boy with classic maple syrup urine disease (MSUD1A; 248600), Chuang et al. (2004) identified a homozygous C-to-G transversion in the BCKDHA gene, resulting in a cys219-to-trp (C219W) substitution. In vitro functional expression studies with the C219W mutant subunit showed no catalytic activity with defective BCKD subunit assembly and reduced binding affinity for the cofactor thiamine.
In 11 children with classic maple syrup urine disease (MSUD1A; 248600) from a Gypsy community in southern Portugal, Quental et al. (2008) identified a homozygous 1-bp deletion (117delC) in the BCKDHA gene, resulting in a frameshift and a truncated protein with only 61 residues. The mutation likely results in nonsense-mediated mRNA decay. By haplotype analysis, Quental et al. (2009) showed that the 117delC mutation is a founder mutation in Portuguese Gypsies with a carrier frequency estimated to be 1.4%. An unrelated Spanish patient with the deletion who was not of Gypsy origin did not share the haplotype, indicating that it occurred independently. The deletion occurs within a poly-C tract and may represent a mutation hotspot in the BCKDHA gene.
Chuang, J. L., Davie, J. R., Chinsky, J. M., Wynn, R. M., Cox, R. P., Chuang, D. T. Molecular and biochemical basis of intermediate maple syrup urine disease: occurrence of homozygous G245R and F364C mutations at the E1-alpha locus of Hispanic-Mexican patients. J. Clin. Invest. 95: 954-963, 1995. [PubMed: 7883996] [Full Text: https://doi.org/10.1172/JCI117804]
Chuang, J. L., Fisher, C. R., Cox, R. P., Chuang, D. T. Molecular basis of maple syrup urine disease: novel mutations at the E1-alpha locus that impair E1(alpha-2-beta-2) assembly or decrease steady-state E1-alpha mRNA levels of branched-chain alpha-keto acid dehydrogenase complex. Am. J. Hum. Genet. 55: 297-304, 1994. [PubMed: 8037208]
Chuang, J. L., Wynn, R. M., Moss, C. C., Song, J., Li, J., Awad, N., Mandel, H., Chuang, D. T. Structural and biochemical basis for novel mutations in homozygous Israeli maple syrup urine disease patients. J. Biol. Chem. 279: 17792-17800, 2004. [PubMed: 14742428] [Full Text: https://doi.org/10.1074/jbc.M313879200]
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Dariush, N., Fisher, C. W., Cox, R. P., Chuang, D. T. Structure of the gene encoding the entire mature E1-alpha subunit of human branched-chain alpha-keto acid dehydrogenase complex. FEBS Lett. 284: 34-38, 1991. Note: Erratum: FEBS Lett. 291: 376 only, 1991. [PubMed: 2060625] [Full Text: https://doi.org/10.1016/0014-5793(91)80755-r]
Fekete, G., Plattner, R., Crabb, D. W., Zhang, B., Harris, R. A., Heerema, N., Palmer, C. G. Localization of the human gene for the El-alpha subunit of branched chain keto acid dehydrogenase (BCKDHA) to chromosome 19q13.1-q13.2. Cytogenet. Cell Genet. 50: 236-237, 1989. [PubMed: 2805821] [Full Text: https://doi.org/10.1159/000132768]
Fisher, C. R., Chuang, J. L., Cox, R. P., Fisher, C. W., Star, R. A., Chuang, D. T. Maple syrup urine disease in Mennonites: evidence that the Y393N mutation in E1-alpha impedes assembly of the E1 component of branched-chain alpha-keto acid dehydrogenase complex. J. Clin. Invest. 88: 1034-1037, 1991. [PubMed: 1885764] [Full Text: https://doi.org/10.1172/JCI115363]
Fisher, C. R., Fisher, C. W., Chuang, D. T., Cox, R. P. Occurrence of a tyr393-to-asn (Y393N) mutation in the E1-alpha gene of the branched-chain alpha-keto acid dehydrogenase complex in maple syrup urine disease patients from a Mennonite population. Am. J. Hum. Genet. 49: 429-434, 1991. [PubMed: 1867199]
Fisher, C. W., Chuang, J. L., Griffin, T. A., Lau, K. S., Cox, R. P., Chuang, D. T. Molecular phenotypes in cultured maple syrup urine disease cells: complete E(1)-alpha cDNA sequence and mRNA and subunit contents of the human branched chain alpha-keto acid dehydrogenase complex. J. Biol. Chem. 264: 3448-3453, 1989. [PubMed: 2914958]
Matsuda, I., Nobukuni, Y., Mitsubuchi, H., Indo, Y., Endo, F., Asaka, J., Harada, A. A T-to-A substitution in the E1-alpha subunit gene of the branched-chain alpha-keto acid dehydrogenase complex in two cell lines derived from Menonite (sic) maple syrup urine disease patients. Biochem. Biophys. Res. Commun. 172: 646-651, 1990. [PubMed: 2241958] [Full Text: https://doi.org/10.1016/0006-291x(90)90723-z]
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Puffenberger, E. G. Genetic heritage of the Old Order Mennonites of southeastern Pennsylvania. Am. J. Med. Genet. 121C: 18-31, 2003. [PubMed: 12888983] [Full Text: https://doi.org/10.1002/ajmg.c.20003]
Quental, S., Gusmao, A., Rodriguez-Pombo, P., Ugarte, M., Vilarinho, L., Amorim, A., Prata, M. J. Revisiting MSUD in Portuguese Gypsies: evidence for a founder mutation and for a mutational hotspot within the BCKDHA gene. Ann. Hum. Genet. 73: 298-303, 2009. [PubMed: 19456321] [Full Text: https://doi.org/10.1111/j.1469-1809.2009.00518.x]
Quental, S., Macedo-Ribeiro, S., Matos, R., Vilarinho, L., Martins, E., Teles, E. L., Rodrigues, E., Diogo, L., Garcia, P., Eusebio, F., Gaspar, A., Sequeira, S., Furtado, F., Lanca, I., Amorim, A., Prata, M. J. Molecular and structural analyses of maple syrup urine disease and identification of a founder mutation in a Portuguese Gypsy community. Molec. Genet. Metab. 94: 148-156, 2008. [PubMed: 18378174] [Full Text: https://doi.org/10.1016/j.ymgme.2008.02.008]
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Wynn, R. M., Davie, J. R., Chuang, J. L., Cote, C. D., Chuang, D. T. Impaired assembly of E1 decarboxylase of the branched-chain alpha-ketoacid dehydrogenase complex in type IA maple syrup urine disease. J. Biol. Chem. 273: 13110-13118, 1998. [PubMed: 9582350] [Full Text: https://doi.org/10.1074/jbc.273.21.13110]
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