Associations between methylenetetrahydrofolate reductase polymorphisms, serum homocysteine levels, and incident cortical cataract

摘要

Importance: Methylenetetrahydrofolate reductase (MTHFR) polymorphisms have been shown to influence homocysteine levels; homocysteine has been implicated as a cataractogenic stressor. Objective: To investigate the associations of MTHFR polymorphisms and serum homocysteine levels with incident cortical cataract in an older population. Design, Setting, and Participants: From 1992 to 1994, a population-based cohort study, the Blue Mountains Eye Study, was conducted with 3654 residents (82.4% of eligible participants) of the Blue Mountains region aged 49 years and older. At the second (1997-1999, 5-year follow-up) and third (2002-2004, 10-year follow-up) surveys, 2334 (75.8% of survivors) and 1952 (76.7% of survivors) were examined, respectively. For this report, the second survey serves as baseline when homocysteine levels were assessed, and 5-year incidence of cataract refers to incidence estimated from the second to the third survey. After excluding participants with no follow-up data or DNA or who had previous cortical cataract or cataract surgery, 757 participants were included in gene and environment analyses. This current project on associations with cataract was designed initially March 19, 2013, and completed April 14, 2014. Cataract was assessed using the Wisconsin Cataract Grading system. Two MTHFR polymorphisms, C677T (rs1801133) and A1298C (rs1801131), were included. Serum homocysteine levels were assessed following standard methods. Main Outcomes and Measures: Logistic regression models were used to estimate odds ratios (ORs) and 95% confidence intervals for incident cortical cataract, after adjusting for age, sex, smoking status, hypertension, diabetes, education, and myopia. Path analysis was performed to explore a possible pathway of MTHFR polymorphisms via homocysteine levels to cortical cataract. Results: The mean (SD) age of the 1726 participants in the Blue Mountains Eye Study 2 cohort with normal homocysteine levels was 68.3 (8.1) years and 73.2 (8.5) years for those with elevated homocysteine levels. Both the C677T polymorphism (CT/TT vs CC: OR = 1.50; 95% CI = 1.01-2.23) and elevated homocysteine levels (>15 µmol/L: OR = 2.24; 95% CI = 1.38-3.63) were independently associated with increased risk of cortical cataract. Path analysis showed that the genetic effect on cortical cataract was partially mediated via homocysteine levels. Combined CT/TT genotypes and elevated homocysteine levels were associated with a 3-fold risk of cortical cataract (OR = 3.74; 95% CI = 1.79-7.80). The synergy index of both exposures was 1.34 (95% CI = 0.44-4.01). Conclusions and Relevance: MTHFR polymorphism and elevated homocysteine levels contributed separately and jointly to increased risk of cortical cataract. If these findings are confirmed, homocysteine levels may be a therapeutic target to reduce risk of cortical cataract in persons carrying genetic risk.