Atherosclerosis 2001 Jun;156(2):409-15
The 1298A-->C polymorphism in methylenetetrahydrofolate reductase
(MTHFR): in vitro expression and association with homocysteine.
Weisberg IS, Jacques PF, Selhub J, Bostom AG, Chen Z, Curtis Ellison R, Eckfeldt JH,
Rozen R.
Departments of Human Genetics and Pediatrics, McGill University Health Centre, Montreal,
Canada
A common mutation in methylenetetrahydrofolate reductase (MTHFR), 677C-->T, is associated
with reduced enzyme activity, a thermolabile enzyme and mild hyperhomocysteinemia, a risk
factor for vascular disease. Recently, a second common mutation (1298A-->C; glutamate to
alanine) was reported, but this mutation was suggested to increase homocysteine only in
individuals who carried the bp677 variant. To evaluate the functional consequences of this
mutation, we performed site-directed mutagenesis and in vitro expression. For in vivo assessment
of clinical impact, we examined the 1298A-->C genotypes and plasma homocysteine in 198
individuals from the NHLBI Family Heart Study that had previously been assessed for the 677
substitution. Site-directed mutagenesis of the human cDNA was performed to generate enzymes
containing each of the two mutations, as well as an enzyme containing both substitutions. Enzyme
activity and thermolability were assessed in bacterial extracts. The activity of the wild-type cDNA
was designated as 100%; mutant enzymes containing the 1298 and 677 mutations separately had
68% (+/-5.0) and 45% (+/-10.8), respectively, of control activity while the enzyme containing
both mutations had 41% (+/-12.8) of control activity. The 1298 mutation was not associated
with a thermolabile enzyme. In the Family Heart Study, fasting homocysteine was significantly
higher (P<0.05) in individuals heterozygous for both substitutions, compared to individuals who
carried only the 677C-->T variant. This study suggests that two variants in MTHFR should be
assessed as genetic risk factors for hyperhomocysteinemia.
Hum Mol Genet 2001 Mar 1;10(5):433-43
Mice deficient in methylenetetrahydrofolate reductase exhibit
hyperhomocysteinemia and decreased methylation capacity, with
neuropathology and aortic lipid deposition.
Chen Z, Karaplis AC, Ackerman SL, Pogribny IP, Melnyk S, Lussier-Cacan S, Chen
MF, Pai A, John SW, Smith RS, Bottiglieri T, Bagley P, Selhub J, Rudnicki MA, James
SJ, Rozen R.
Departments of Biology, Human Genetics and Pediatrics, Department of Medicine, Department
of Pathology, McGill University, Montreal, Quebec H3A 1B1, Canada.
Hyperhomocysteinemia, a risk factor for cardiovascular disease, is caused by nutritional and/or
genetic disruptions in homocysteine metabolism. The most common genetic cause of
hyperhomocysteinemia is the 677C-->T mutation in the methylenetetrahydrofolate reductase
(MTHFR) gene. This variant, with mild enzymatic deficiency, is associated with an increased risk
for neural tube defects and pregnancy complications and with a decreased risk for colon cancer
and leukemia. Although many studies have reported that this variant is also a risk factor for
vascular disease, this area of investigation is still controversial. Severe MTHFR deficiency results
in homocystinuria, an inborn error of metabolism with neurological and vascular complications.
To investigate the in vivo pathogenetic mechanisms of MTHFR deficiency, we generated mice
with a knockout of MTHFR: Plasma total homocysteine levels in heterozygous and homozygous
knockout mice are 1.6- and 10-fold higher than those in wild-type littermates, respectively. Both
heterozygous and homozygous knockouts have either significantly decreased
S-adenosylmethionine levels or significantly increased S-adenosylhomocysteine levels, or both,
with global DNA hypomethylation. The heterozygous knockout mice appear normal, whereas the
homozygotes are smaller and show developmental retardation with cerebellar pathology.
Abnormal lipid deposition in the proximal portion of the aorta was observed in older
heterozygotes and homozygotes, alluding to an atherogenic effect of hyperhomocysteinemia in
these mice. |
