Our a priori hypothesis was that established functional polymorphisms of enzymes in the folate/homocysteine pathway would be associated with increased homocysteine concentrations in patients with SLE, and hence with CAC scores, a clinical finding for which elevated homocysteine concentrations are predictive19
. The polymorphisms selected for this study (MTHFR
677C>T and 1298A>C, MTR
1494del6, and DHFR
c.86+60_78) were those for which there is evidence for significant effects on homocysteine concentrations7-16
. The differences in the distributions of genotype frequencies between African American and Caucasian controls mandated that associations of homocysteine with genotype be stratified by race. More specifically, it is known that MTHFR
677C>T and 1298A>C28
, CBS 844ins6829,30
, and TYMS
have different genotype frequencies in African Americans and Caucasians. Our study also found that frequencies of DHFR
c.86+60_78 differed between races.
None of the genotypes for the polymorphisms under test differed significantly in distribution between SLE patients and controls, even after stratification by race, indicating that none are genetic risk factors for SLE per se. A study by Fijnheer, et al31
found that MTHFR
677C>T did not explain elevated homocysteine levels in SLE patients, which agrees with our findings. Our results contrast with a smaller Italian study reporting that SLE patients had a higher prevalence of the MTHFR
. A Polish study found that frequencies of MTHFR
677C>T were not different between SLE patients and controls, but the authors found that the MTR
2756G allele was overrepresented in SLE patients32
, which contrasts with our finding of lack of an association.
Although folate, B12
, and B6
levels were not available for our study, use of folic acid, B12
, and B6
supplements was studied as a surrogate, but was found to be not associated with homocysteine concentrations. GFR was correlated negatively with homocysteine, and other studies have found a similar relationship33
. In SLE patients none of the 7 polymorphisms were associated with homocysteine levels. In controls there were 2 polymorphisms associated with homocysteine levels, but only in Caucasians. The MTHFR
677CT and TT genotypes were associated with an increase in homocysteine levels compared to CC, as expected from the literature7,8
. Carriers of the MTHFR
1298C allele had lower homocysteine levels compared to AA. This finding is concordant with a study by Parle-McDermott, et al
that found that MTHFR
1298AC and CC genotypes were associated with increased red-cell folate and a nonsignificant decrease in homocysteine within the MTHFR
677CC genotype in pregnant women34
. A large study by Ulvik, et al
used an approach similar to the Parle-McDermott study and stratified their analysis of biochemical variables based on both MTHFR
677 and 1298 genotypes. Ulvik, et al
found that MTHFR
1298AC and CC genotypes were associated with higher homocysteine levels and lower serum folate levels9
. The differences in findings among the studies on MTHFR
1298A>C may be due to the number of subjects studied or to the genetic variability of the populations studied.
Although our study has some limitations, the magnitude of the difference in homocysteine concentrations between SLE patients and controls was large enough to suggest that genetic factors might be responsible, at least in part, for the elevation of homocysteine in the patients. Such a genetic effect (i.e., the MTHFR 677C>T polymorphism) was observed in the controls, but not in patients with SLE. While we acknowledge that the size of our study population precludes a conclusion that there are no contributing genetic factors, our study suggests that if such factors are involved they are likely to have no more than a relatively small effect. Thus, the increase in homocysteine among SLE patients is probably due primarily to other variables that are components of the SLE disease process itself.
African American patients with SLE had elevated homo-cysteine levels compared to Caucasian patients and African American controls. None of the tested functional polymorphisms of enzymes in the folate/homocysteine pathway were associated with SLE. In addition, in SLE patients, none of the polymorphisms were associated with homocysteine concentrations even when adjusted for covariates, including GFR, and there were no associations with median CAC scores. It is unlikely that polymorphisms in folate/homocysteine-metabolizing enzymes contribute substantially to the elevated homocysteine levels observed in patients with SLE. Therefore the mechanism whereby SLE patients achieve elevated homocysteine concentrations and high CAC scores relative to controls is most likely due to inflammatory aspects of the disease process that dominate any genetic effects intrinsic to enzymes of the folate/homocysteine pathway.