In Canadian Oji-Cree subjects with type 2 diabetes, we found that: 1) MTHFR 677T carriers had an increased risk of PAD [OR 3.54 (1.01, 12.4), P = 0.049], adjusting for age, sex, duration of diabetes, hypertension, current smoking habits, and diabetes treatment; and 2) there was no significant association between MTHFR genotype and intermittent claudication, a much more advanced stage of PAD.
Our finding of a significant association between MTHFR genotype and PAD for subjects with type 2 diabetes is novel, to our knowledge. Only one other study to date has examined the relationship between PAD and MTHFR genotype in subjects with type 2 diabetes. In 135 patients with PAD and 219 controls, Ciccarone et al. found that though elevated homocysteine levels associated with the severity of PAD, as assessed by colour-duplex ultrasound, there was no significant association between MTHFR genotype and PAD . Our observed association is of borderline statistical significance, yet given that under multivariate analysis MTHFR genotype was the most significant risk factor in comparison to other traditional risk factors, including smoking, hypertension, and duration of diabetes, investigation into its potential importance is warranted.
Though not significant, a 3-fold increased risk of PAD was observed for subjects who were current smokers. This finding stresses the importance of smoking cessation programmes in aboriginal communities, especially given the high prevalence of smoking in this population (~50% current smokers for both males and females). Taking preventative measures now is also critical considering the relatively young age of the population. Though the prevalence of PAD was ~14%, and the prevalence of intermittent claudication was only ~6%, the mean age of the study participants was under 50, younger than the majority of studies involving PAD [9, 10, 13]. A full expression of disease phenotypes associated with diabetes for the Oji-Cree is yet to be seen.
A limitation of our study was the small sample size. With less than 200 subjects included in each analysis, the power to detect significant associations was low. Most notably, the observation of no significant association for any of the well-known risk factors in the logistic regression analysis for PAD was likely due to the small sample size. In spite of the small sample size, the observation of an OR of greater than 3.5 for the MTHFR genotype and PAD suggests an association of notably strong magnitude which would be of interest to confirm in future studies.
Our results are also limited by the absence of both dietary information and plasma folate and homocysteine concentrations for our study subjects. Previous studies have found the prevalence of inadequate dietary folate intake in the Oji-Cree to be 37%, which is more than twice the average for the general Canadian population, and may contribute to compromised MTHFR enzyme activity . These studies, however, took place in an era prior to the supplementation of the food source with folate, and thus may be an overestimation of the present situation. Further detailed clinical measurements would have helped to provide insight on the relationships between MTHFR genotype, plasma homocysteine concentration, and PAD. Other studies, however, have found significant associations for homocysteine levels with increased atherosclerosis, yet no significant association for MTHFR genotype [9, 35], raising the question of possible alternative causes for hyperhomocysteinemia and questioning the real importance of the MTHFR genotype. Nonetheless, our data are consistent with the concept that MTHFR genotype may have a mechanistic role in the development and progression of atherosclerosis in the lower extremities.