Spectrum of mutations in MMACHC, allelic expression, and evidence for genotype-phenotype correlations.
Methylmalonic aciduria and homocystinuria, cblC type, is a rare disorder of intracellular vitamin B(12) (cobalamin [Cbl]) metabolism caused by mutations in the MMACHC gene. MMACHC was sequenced from the gDNA of 118 cblC individuals. Eleven novel mutations were identified, as well as 23 mutations that were observed previously. Six sequence variants capture haplotype diversity in individuals across the MMACHC interval. Genotype-phenotype correlations of common mutations were apparent; individuals with c.394C>T tend to present with late-onset disease whereas patients with c.331C>T and c.271dupA tend to present in infancy. Other missense variants were also associated with late- or early-onset disease. Allelic expression analysis was carried out on human cblC fibroblasts compound heterozygous for different combinations of mutations including c.271dupA, c.331C>T, c.394C>T, and c.482G>A. The early-onset c.271dupA mutation was consistently underexpressed when compared to control alleles and the late-onset c.394C>T and c.482G>A mutations. The early-onset c.331C>T mutation was also underexpressed when compared to control alleles and the c.394C>T mutation. Levels of MMACHC mRNA transcript in cell lines homozygous for c.271dupA, c.331C>T, and c.394C>T were assessed using quantitative real-time RT-PCR. Cell lines homozygous for the late onset c.394C>T mutation had significantly higher levels of transcript when compared to cell lines homozygous for the early-onset mutations. Differential or preferential MMACHC transcript levels may provide a clue as to why individuals carrying c.394C>T generally present later in life.
Department of Medical Genetics, McGill University Health Centre, Montreal, Quebec, Canada. lerner-ellis@molbio.mgh.harvard.edu
Jordan P
JP
Lerner-Ellis
Natascia
N
Anastasio
Junhui
J
Liu
David
D
Coelho
Terttu
T
Suormala
Martin
M
Stucki
Amanda D
AD
Loewy
Scott
S
Gurd
Elin
E
Grundberg
Chantal F
CF
Morel
David
D
Watkins
Matthias R
MR
Baumgartner
Tomi
T
Pastinen
David S
DS
Rosenblatt
Brian
B
Fowler
D017668:Age of Onset; D000483:Alleles; D002170:Canada; D002352:Carrier Proteins / Q000235:genetics*; D004252:DNA Mutational Analysis; D005260:Female; D005786:Gene Expression Regulation; D005838:Genotype; D006712:Homocystinuria / Q000235:genetics; D006801:Humans; D007558:Italy; D008297:Male; D009154:Mutation; D010088:Oxidoreductases; D010641:Phenotype; D012333:RNA, Messenger / Q000032:analysis; D014806:Vitamin B 12 Deficiency / Q000235:genetics
D002352:Carrier Proteins; D012333:RNA, Messenger; C508359:MMACHC protein, human; D010088:Oxidoreductases