Methylenetetrahydrofolate reductase genetic polymorphisms in patients with cataract

Association of OGG1 and MTHFR polymorphisms with age-related cataract: A systematic review and meta-analysis

PURPOSE

To discern and confirm genetic biomarkers that help identify populations at high risk for age-related cataract (ARC).

METHODS

A literature search was performed in the PubMed, Web of Science and China National Knowledge Internet databases for genetic association studies published before June 26, 2016 regarding ARC susceptibility. All genetic polymorphisms reported were systematically reviewed, followed by extraction of candidate genes/loci with sufficient genotype data in ≥3 studies for the meta-analysis. A random/fixed-effects model was used to calculate the pooled odds ratios and 95% confidence intervals to evaluate the associations considering multiple genetic models. Sensitivity analysis was also performed.

RESULTS

A total of 144 polymorphisms in 36 genes were reported in the 61 previous genetic association studies. Thereby, three polymorphisms of two genes (8-oxoguanine DNA glycosylase-1 [OGG1]; methylenetetrahydrofolate reductase NADPH [MTHFR]) in eight studies were included in the meta-analysis. Regarding the OGG1-rs1052133, the GG (OR = 1.925; 95%CI, 1.181-3.136; p = 0.009) and CG (OR = 1.384; 95%CI, 1.171-1.636; p<0.001) genotypes indicated higher risk of ARC. For the MTHFR gene, the CC+TT genotype of rs1801133 might be protective (OR, 0.838; 95%CI, 0.710-0.989; p = 0.036), whereas the AA+CC genotype of rs1801131 indicated increased risk for the mixed subtype (OR = 1.517; 95%CI, 1.113-2.067; p = 0.008).

CONCLUSIONS

Polymorphisms of OGG1 and MTHFR genes are associated with ARC susceptibility and may help identify populations at high risk for ARC.

Associations of Polymorphisms in MTHFR Gene with the Risk of Age-Related Cataract in Chinese Han Population: A Genotype-Phenotype Analysis

Homocysteine (Hcy) is a potential risk factor for age-related cataract (ARC). Methylenetetrahydrofolate reductase (MTHFR) is the key enzyme for Hcy metabolism, and variants of MTHFR may affect MTHFR enzyme activity. This study mainly evaluated the associations between variants in MTHFR gene, plasma MTHFR enzyme activity, total Hcy (tHcy) levels and ARC risk in Chinese population. Four single nucleotide polymorphisms (SNPs) in MTHFR gene were genotyped using the high-resolution melting (HRM) method in 502 ARC patients (mean age, 70.2 [SD, 9.0], 46.0% male) and 890 healthy controls (mean age, 67.1 [SD, 11.1], 47.6% male). The plasma MTHFR activity, folic acid (FA), vitamins B12 and B6 levels were detected by enzyme-linked immunosorbent assays (ELISA). The plasma tHcy levels were measured by an automated enzymatic assay. After the Bonferroni correction, the minor allele T of SNP rs1801133 showed a significant association with an increased risk of overall ARC (OR = 1.26, P = 0.003). Consistent association was also found between SNP rs1801133 and cortical ARC risk (OR = 1.44, P = 0.003). Haplotype analyses revealed an adverse effect of the haplotype "C-A-T-C" (alleles in order of SNPs rs3737967, rs1801131, rs1801133 and rs9651118) on ARC risk (OR = 1.55, P = 0.003). Moreover, in a joint analysis of SNPs rs9651118 and rs1801133, subjects with two unfavorable genotypes had a 1.76-fold increased risk of ARC compared with the reference group, and a statistically significant dose-response trend (P_trend = 0.001) was also observed. Further, in healthy controls and patients with cortical ARC, the allele T of SNP rs1801133 and the increasing number of unfavorable genotypes were significantly correlated with decreased MTHFR activity as well as increased tHcy levels. However, there was no significant association between FA, vitamins B12, B6 levels and MTHFR variants. Our data indicated that variants in MTHFR gene might individually and jointly influence susceptibility to ARC by affecting MTHFR enzyme activity and tHcy levels.

Molecular Genetics of Cataract

Lens opacities or cataract(s) represent a universally important cause of visual impairment and blindness. Typically, cataract is acquired with aging as a complex disorder involving environmental and genetic risk factors. Cataract may also be inherited with an early onset either in association with other ocular and/or systemic abnormalities or as an isolated lens phenotype. Here we briefly review recent advances in gene discovery for inherited and age-related forms of cataract that are providing new insights into lens development and aging.

Comparative quantification of plasma TDRD7 mRNA in cataract patients by real-time polymerase chain reaction

Purpose

To investigate the relationship between plasma TDRD7 mRNA and lens transparency, and to evaluate plasma TDRD7 mRNA as a potential marker for cataracts and its sub-type by quantitatively analyzing human peripheral blood.

Methods

Plasma RNA was extracted from 40 patients with cataracts, and 30 normal controls of matched age and gender. Blood cholesterol and fasting glucose were measured, and the RNA extracted from the sample was synthesized into cDNA. After polymerase chain reaction, the results were compared after quantifying the TDRD7 mRNA using ABL1 mRNA for normalization. We analyzed the relative gene expression data via the ΔΔCt method.

Results

The normalized 2^-ΔΔCt of plasma TDRD7 mRNA based on ABL1 mRNA was 1.52 ± 0.63 in the case of the control group and 1.05 ± 0.34 in the case of the cataract patients, and the TDRD7 expression level of the cataract patients was lower than that of the control group (p = 0.048). The comparison of the genetic values of different types of cataracts demonstrated that the TDRD7 expression level of the cortical type and mixed type were lower than those of the nuclear type and posterior subcapsular opacity type (p = 0.017).

Conclusions

Human cataracts and the TDRD7 gene loss-of-function mutations are strongly causally related, as the expression level of plasma TDRD7 mRNA in patients with cataracts was statistically significantly lower than in the normal control group.

Age-related cataracts: homocysteine coupled endoplasmic reticulum stress and suppression of Nrf2-dependent antioxidant protection

To determine whether high levels of homocysteine (Hcy) induce endoplasmic reticulum (ER) stress with suppression of the nuclear factor-erythroid-2-related factor 2 (Nrf2)-dependent antioxidant protection in lens epithelial cells (LECs). ER stress was acutely induced by exposure of LECs to 100 μM Hcy without FCS and also by exposure to 5 mM Hcy with 10% FCS. After exposure to Hcy, significant changes were found in P-PERK, P-eIF2α, XBP1, Nrf2, and Keap1 within 24 h. The production of reactive oxygen species (ROS) was increased after Hcy exposure. The downstream enzymes of Nrf2 like, catalase, and glutathione reductase, were significantly decreased. These results suggested that the Hcy-induced ER stress suppressed the Nrf2-dependent antioxidant protection and simultaneously generated ROS which resulted in further oxidation and death of LECs. The loss of Nrf2 is mainly due to proteosomal degradation and m-calpain activation by the increased levels of cytoplasmic Ca(++). The caspases also play a role in the degradation of Nrf2. Our findings demonstrated that high levels of Hcy induce ER stress, chronic UPR, alter the levels of UPR specific proteins, increase the production of ROS, degrade Nrf2 and block the Nrf2-dependent antioxidant defense protection in LECs. Thus, the upregulation of ROS might exceed the Nrf2 dependent antioxidant defense protection in the LECs and result in the highly oxidized lenses and resulted in ARCs.

Methylenetetrahydrofolate Reductase Genetic Polymorphisms in Patients with Primary Open-Angle Glaucoma

PURPOSE

Hyperhomocysteinemia has been found in patients with primary open-angle glaucoma. The purpose of the present study was to determine if hyperhomocysteinemia-associated polymorphisms of the methylenetetrahydrofolate reductase gene (MTHFR) are overrepresented in primary open-angle glaucoma.

METHODS

Patients with primary open-angle glaucoma (n = 243) and controls (n = 187) were analyzed for the MTHFR 677 C > T and 1298 A > C polymorphisms using minisequencing technique.

RESULTS

No significant differences were observed in allele and genotype frequencies of the MTHFR 677C > T and 1298A > C polymorphisms between controls and the primary open-angle glaucoma group.

CONCLUSIONS

If hyperhomocysteinemia is important in the pathogenesis of glaucoma, this study does not support a role for MTHFR polymorphisms in this context.

Ocular defects associated with a null mutation in the mouse arylamine N-acetyltransferase 2 gene

The xenobiotic metabolizing enzyme, mouse arylamine N-acetyltransferase type 2 (Nat2), is expressed during embryogenesis from the blastocyst stage and in the developing neural tube and eye. Mouse Nat2 is widely believed to have an endogenous role distinct from xenobiotic metabolism, and polymorphisms in the human ortholog have been implicated in susceptibility to spina bifida and orofacial clefting. The developmental role of Nat2 was investigated using transgenic Nat2 knockout/lacZ knockin (Nat2 (tm1Esim)) mice. The transgene was bred onto an A/J background and offspring were scored for developmental defects at weaning. After backcross generation eight, an ocular defect, ranging from cataract to microphthalmia and anophthalmia, was recorded among offspring of backcross and intercross pairs. Histologic analysis of cataract cases revealed a failure of the lens to separate from the cornea and plaques within the lens tissue. While Nat2 ( -/- ) mice have been described as overtly aphenotypic, the presence of a Nat2 null allele in one or both parents can result in ocular defects. These ocular phenotypes and their association with Nat2 genotype indicate that the Nat2 locus may be responsible for the previously described microphthalmic Cat4 phenotype and implicate the orthologous human NAT as a phenotypic modifier of microphthalmia and anophthalmia.