Mapping SNPs BCKDHA

General

Maple syrup urine disease is an autosomal recessive disorder that affects the amino acid metabolism. The disease is caused by a defect in the branched-chain alpha-keto acid dehydrogenase complex which blocks oxidative decarboxylation. The result is a rising concentration of branched-chain amino acids. MSUD is caused by mutations in the gene coding for the alpha subunit of the branched-chain keto acid dehydrogenase(BCKDHA).

HGMD

Searching for "BCKDHA" a total of 39 mutations are reported, comprised of the following mutation types:

• missense/nonsense: 33 mutations
• small deletions: 3 mutations
• small insertions: 1 mutation
• gross deletions: 1 mutation
• complex rearrangements: 1 mutation

For us the missense/nonsense mutations are the most interesting ones, as a single nucleotide change can lead to the phenotype of Maple Syrup Urine Disease.

Codon change	Amino Acid change	Codon number
gCAG-GAG	Gln-Glu	80
ACG-ATG	THr-Met	106
cCGG-TGG	Arg-Trp	114
gTAT-AAAT	Tyr-Asn	121
CGG-CAG	Arg-Gln	122
cCAG-AAG	Gln-Lys	145
ATC-ACC	Ile-Thr	168
GCG-GTG	Ala-Val	171
GCG-GTG	Ala-Val	175
cGGC-AGC	Gly-Ser	204
cGCT-ACT	Ala-Thr	208
TGC-TAC	Cys-Thr	213
cCGG-TGG	Arg-Trp	220
AAT-AGT	Asn-Ser	222
GGC-GAC	Gly-Asp	238
tGCA-CCA	Ala-Pro	240
aCGA-TGA	Arg-Term	242
cGGG-AGG	Gly-Arg	245
cCGC-TGC	Arg-Cys	252
CGC-CAC	Arg-His	252
tGGT-AGT	Gly-Ser	255
GAT-GCT	Asp-Ala	257
ACA-AGA	Thr-Arg	265
cCGA-TGA	Arg-Term	269
ATC-ACC	Ile-Thr	281
cGAG-AAG	Glu-Lys	282
gGCC-ACC	Ala-Thr	283
CGC-CAC	Arg-His	301
cCGG-TGG	Arg-Trp	318
TTC-TGC	Phe-Cys	364
cGTG-ATG	Val-Met	367
TAT-TGT	Tyr-Cys	368
cTAC-AAC	Tyr-Asn	393

dbSNP

results for the SNPs search with BCKDHA:

-all: 742

-human: 371

SNPs in human

ID	mutation in sequence	amino acid	position
rs137852876	CTTGAGTGCCCCATCATCTTCTTCTG[C/G]CGGAACAATGGCTACGCCATCTCCA	Ser	pos=251
rs137852875	TGATGTGTTTGCCGTATACAACGCCA[C/G]AAAGGAGGCCCGACGGCGGGCTGTG	Ser	pos=251
rs137852874	CAGCCAGTGAGGGGGACGCCCATGCC[A/G]GCTTCAACTTCGCTGCCACACTTGA	Arg	pos=251
rs137852873	TTGAGTGCCCCATCATCTTCTTCTGC[C/T]GGAACAATGGCTACGCCATCTCCAC	Tyr	pos=251
rs137852872	GCCCAAACCCAACCCCAACCTACTCT[G/T]CTCAGACGTGTATCAGGAGATGCCC	Lys	pos=251
rs137852871	GTGTCCCCACAGCAGCACGAGGCCCC[A/G]GGTATGGCATCATGTCAATCCGCGT	Arg	pos=251

results for the gene search with BCKDHA

mRNA pos	aa position	function	mutation	aa change
54	5	synonymous	C/T	-
72	12	synonymous	C/A	-
125	29	missense	G/A	Gly/Glu
153	83	synonymous	C/G, C/T	-
155	39	missense	C/A	Pro/His
156	39	synonymous	C/A	-
238	82	missense	A/C	Met/Leu
330	97	synonymous	G/C	-
491	151	missense	C/T	Thr/Met
547	170	missense	C/T	Pro/Ser
678	213	synonymous	C/T	-
687	216	synonymous	G/T	-
769	244	missense	G/A	Gly/Arg
879	280	synonymous	C/T	-
1011	324	synonymous	C/T	-
1012	325	frame shift	C/	-
1014	325	synonymous	C/T	-
1120	361	missense	A/G	Ile/Val
1260	407	synonymous	A/G	-
1299	420	synonymous	C/T	-

Point mutations can have an influence on the amino acids or not depending on kind of point mutation. There are two different types: synonymous and non-synonymous mutations.

If a point mutation is synonymous it means that the change occurs only in the nucleotide sequence but not in the amino acid sequence. This is possible because of the fact that amino acids are encoded by three nucleotides (codons) and some of the amino acids are encoded by more than one possible arrangement of nucleotides. So it can happen that when there is a mutation in the nucleotide sequence there is also a change in the codon but both codons encode the same amino acid.

The other possibility is that a mutation is non-synonymous which means that the mutation has an influence on the amino acid sequence and the amino acid changes. This change can have more or less severe because amino acids have several properties. When an amino acid is replaced by an amino acid which has the same properties the change is not so grave as the change to an amino acid with completely different properties.