Clinical Utility of Genotyping the 677C>T Variant of Methylenetetrahydrofolate Reductase in Humans Is Decreased in the Post-Folic Acid Fortification Era

Abstract

Moderate hyperhomocysteinemia is associated with many diseases. Major factors affecting plasma total homocysteine (tHcy) concentrations include folate concentrations and polymorphisms in the methylenetetrahydrofolate reductase (MTHFR) gene. Because U.S.-mandated fortification of grain products with folic acid has improved folate and tHcy status in Americans, we investigated the effect of the MTHFR 677C>T variant before and after fortification. We determined tHcy and folate concentrations in sera from 844 Caucasian and 587 African American participants in the Coronary Artery Risk Development in Young Adults study before and after fortification and we genotyped the MTHFR 677C>T variant. MTHFR 677TT homozygotes had higher (P < 0.01) tHcy concentrations both before and after fortification compared with MTHFR 677CC homozygotes. However, the difference between these 2 genotypes decreased from 2.5 micromol/L before fortification to <0.7 micromol/L postfortification (P < 0.01). In addition, the prevalence of moderate hyperhomocysteinemia (tHcy > 13 micromol/L) in 677TT homozygotes decreased from 33% before fortification to 12% postfortification (P < 0.01). Using a cutoff value of 13 micromol/L to define moderate hyperhomocysteinemia, the sensitivity of the MTHFR 677TT genotype to predict elevations in homocysteine was low (approximately 30%) both before and after folic acid fortification. Increasing the cutoff from 13 to 19 micromol/L increased the sensitivity of the assay before fortification to 62% but decreased the sensitivity to 17% postfortification. We conclude that after folic acid fortification in the US, measurement of tHcy rather than genotyping of MTHFR 677TT should be used as the primary assay for the diagnosis and monitoring of moderate hyperhomocysteinemia.

Polygenic association with total homocysteine in the post-folic acid fortification era: The CARDIA study

“…In the current study we are able to compare the association of polymorphisms in the CBS, MS, MTRR and MTHFR genes with tHcy concentrations before and after folic acid fortification. We have previously shown in a smaller subset of CARDIA that folic acid fortification was associated with increased concentrations of folate and decreased prevalence of moderate hyperhomocysteinemia [31], similar to results from the Framingham and the National Health and Nutrition Examination Survey (NHANES) studies [29, 30]. In this study, we also examined how folic acid fortification influenced the association of the MTHFR 677C>T polymorphism with tHcy concentrations.…”

“…As a result, studies have shown that folate status has significantly improved with a concurrent decrease in prevalence of moderate hyperhomocysteinemia [29, 30]. Our group, in a previous study of a subset of 844 Caucasian and 587 African American participants of the Coronary Artery Risk Development in Young Adults (CARDIA) study showed that homozygosity of the MTHFR 677T allele plays a less significant role in predisposing the United States population to moderate hyperhomocysteinemia in the post folic acid fortification era [31]. Although polygenic regulation of tHcy has been examined in populations with premature CAD or at high risk for CAD [2,32], there are no studies investigating the effect of multiple homocysteine metabolizing genes on the level of tHcy after folic acid fortification in the U.S.…”

ACMG Practice Guideline: lack of evidence for MTHFR polymorphism testing

“…59 This public health initiative may be incidentally reducing some of the perceived risk associated with MTHFR polymorphisms. 60,61 This is hypothesized to be one reason that an association between the "thermolabile" variant and venous thromboembolism is no longer observed in the North-American population. 21 The American Congress of Obstetricians and Gynecologists does not recommend the measurement of homocysteine or MTHFR polymorphisms in the evaluation of the etiology of venous thromboembolism.…”

Prospective study of methylenetetrahydrofolate reductase (MTHFR) variant C677T and risk of all-cause and cardiovascular disease mortality among 6000 US adults

“…The geographic differences might indicate that the influence of the TT genotype is dependent on folate status, because the folate intake increased substantially in North American countries after January 1998, when folic acid fortification was introduced (43). After that time, the average serum folate concentration increased by 2-fold, the average homocysteine concentration fell significantly among the general population (44), and the difference in the homocysteine concentration by the MTHFR C677T genotype was significantly attenuated (45). However, we cannot rule out the possibility that the temporal association between when the inverse association of CVD mortality and MTHFR TT genotype began to become apparent and when mandatory folate fortification was implemented in the United States was merely coincidental.…”