Detection of 677CT/1298AC "double variant" chromosomes: implications for interpretation of MTHFR genotyping results.
OBJECTIVE
Genotyping 37,026 individuals as part of a thrombophilia evaluation, we determined and analyzed the genotypic frequencies of the 677CT and 1298AC mutations in the methylenetetrahydrofolate reductase (MTHFR) gene.


METHODS
The 677CT and 1298AC mutations in the MTHFR gene were determined by either a laboratory-developed test involving PCR amplification and restriction digestion utilizing the ABI 3100 capillary electrophoresis apparatus (Applied Biosystems Inc) or by using an Analyte Specific Reagent (ASR) supplied by Third Wave Technologies. The genotype for three specimens with triple variant MTHFR mutations were confirmed by DNA sequencing on the ABI 3100 capillary electrophoresis apparatus.


RESULTS
The MTHFR frequencies of the 677CT/1298AA, 677CC/1298AC, 677CT/1298AC, 677CC/1298AA, 677TT/1298AA, 677CC/1298CC, 677TT/1298AC, and 677CT/1298CC genotypes were 0.228, 0.208, 0.198, 0.153, 0.122, 0.088, 0.0005, and 0.0003, respectively.


CONCLUSIONS
Individuals containing double variant MTHFR mutations on one allele (cis) cannot be distinguished between compound heterozygotes (trans) for 677CT and 1298AC mutations in routine clinical testing, a genotype associated with thrombophilia. Such patients could be inappropriately counseled for being at high risk for thrombotic episodes. Until information regarding prevalence and the clinical consequences of this double variant (cis) allele becomes available, caution should be used in interpreting the genotyping results of compound heterozygosity for 677CT and 1298AC.
Brown
Nicholas M
NM
Department of Molecular Genetics, Quest Diagnostics, Nichols Institute, Chantilly, Virginia 20151, USA.
Pratt
Victoria M
VM
Buller
Arlene
A
Pike-Buchanan
Lisa
L
Redman
Joy B
JB
Sun
Weimin
W
Chen
Rebecca
R
Crossley
Beryl
B
McGinniss
Matthew J
MJ
Quan
Franklin
F
Strom
Charles M
CM
D001483:Base Sequence; D017931:DNA Primers; D019075:Electrophoresis, Capillary; D006580:Genetic Carrier Screening; D005820:Genetic Testing / Q000379:methods*; D005838:Genotype; D006801:Humans; D020138:Hyperhomocysteinemia / Q000235:genetics*; D042965:Methylenetetrahydrofolate Reductase (NADPH2) / Q000235:genetics*; D020125:Mutation, Missense / Q000235:genetics*; D016133:Polymerase Chain Reaction; D017422:Sequence Analysis, DNA; D019851:Thrombophilia / Q000235:genetics*
D017931:DNA Primers; D042965:Methylenetetrahydrofolate Reductase (NADPH2)