Methionine synthase polymorphism is a risk factor for Alzheimer disease:

Methionine synthase 2756AA polymorphism is associated with the risk of cognitive impairment in patients with late-life depression

BACKGROUNDS

Apolipoprotein E epsilon-4 (APOE ε4) allele, methylenetetrahydrofolate reductase (MTHFR C677T), and methionine synthase (MTR A2756G) were tested their associations with cognitive impairment in people with late-life depression (LLD).

METHODS

People with LLD were assessed by mini-mental state examination and were examined the distribution of APOE ε4 allele, MTHFR, and MTR polymorphisms.

RESULTS

Odds ratio of MTR 2756 AA to MTR 2756 AG and GG genotypes for the risk of cognitive impairment was 5.80 (95% confidence interval = 1.18-28.50; P = 0.03).

CONCLUSION

People with LLD carrying MTR2756 AA genotype have higher risk of cognitive impairment than those carrying G allele.

Potential Links between Impaired One-Carbon Metabolism Due to Polymorphisms, Inadequate B-Vitamin Status, and the Development of Alzheimer's Disease

Alzheimer's disease (AD) is the major cause of dementia and no preventive or effective treatment has been established to date. The etiology of AD is poorly understood, but genetic and environmental factors seem to play a role in its onset and progression. In particular, factors affecting the one-carbon metabolism (OCM) are thought to be important and elevated homocysteine (Hcy) levels, indicating impaired OCM, have been associated with AD. We aimed at evaluating the role of polymorphisms of key OCM enzymes in the etiology of AD, particularly when intakes of relevant B-vitamins are inadequate. Our review indicates that a range of compensatory mechanisms exist to maintain a metabolic balance. However, these become overwhelmed if the activity of more than one enzyme is reduced due to genetic factors or insufficient folate, riboflavin, vitamin B6 and/or vitamin B12 levels. Consequences include increased Hcy levels and reduced capacity to synthetize, methylate and repair DNA, and/or modulated neurotransmission. This seems to favor the development of hallmarks of AD particularly when combined with increased oxidative stress e.g., in apolipoprotein E (ApoE) ε4 carriers. However, as these effects can be compensated at least partially by adequate intakes of B-vitamins, achieving optimal B-vitamin status for the general population should be a public health priority.

Methionine synthase A2756G transition might be a risk factor for male infertility: Evidences from seven case-control studies

Methionine synthase (MTR) has a crucial role in DNA synthesis and methylation reactions. The aim of this study was to investigate the association of the MTR-A2756G polymorphism with idiopathic male infertility. Blood samples were collected from 217 idiopathic infertile- and 233 healthy-men, and MTR-A2756G genotyping was performed by PCR-RFLP. Meta-analysis was conducted by pooling our data with the data obtained from 6 previous studies. Also, the effects of this substitution on protein structure were evaluated by bioinformatics tools. Our study revealed the association of AG-genotype, GG-genotype, and G-allele with male infertility. Meta-analysis showed a significant association between A2756G transition and male infertility. In addition, structural analysis of the transition effect on protein revealed a significant influence on MTR function (with score: 38; expected accuracy: 66%). These findings suggest that the A2756G substitution might be a genetic risk factor and a potential biomarker for idiopathic male infertility.

Developmental lead exposure and lifespan alterations in epigenetic regulators and their correspondence to biomarkers of Alzheimer's disease

INTRODUCTION

Early life lead (Pb) exposure results in a latent increase in Alzheimer's disease (AD)-related proteins, and cognitive deficits late in life in both rodents and primates. This study was conducted to investigate if these late life changes were accompanied by epigenetic alterations.

METHODS

Western blot analysis and RT-PCR were used to measure Deoxyribonucleic acid methylation regulators (DNMT1, DNMT3a, MeCP2, MAT2A) and histone proteins (H3K9Ac, H3K4me2, H3K27me3).

RESULTS

Cerebral levels of DNMT1 and MeCP2 were significantly reduced in mice exposed to Pb early in life, whereas the expression of DNMT3a was not altered. Levels of MAT2a were increased in the Pb-exposed mice across the lifespan. H3K9Ac and H3K4me2, involved in gene activation, were decreased, whereas the repressive mark H3K27me3 was elevated.

DISCUSSION

Epigenetic modifiers are affected by the developmental exposure to Pb and may play a role in mediating the latent increases in AD-related proteins in the brain.

Decreased Brain Levels of Vitamin B12 in Aging, Autism and Schizophrenia

Many studies indicate a crucial role for the vitamin B₁₂ and folate-dependent enzyme methionine synthase (MS) in brain development and function, but vitamin B₁₂ status in the brain across the lifespan has not been previously investigated. Vitamin B₁₂ (cobalamin, Cbl) exists in multiple forms, including methylcobalamin (MeCbl) and adenosylcobalamin (AdoCbl), serving as cofactors for MS and methylmalonylCoA mutase, respectively. We measured levels of five Cbl species in postmortem human frontal cortex of 43 control subjects, from 19 weeks of fetal development through 80 years of age, and 12 autistic and 9 schizophrenic subjects. Total Cbl was significantly lower in older control subjects (> 60 yrs of age), primarily reflecting a >10-fold age-dependent decline in the level of MeCbl. Levels of inactive cyanocobalamin (CNCbl) were remarkably higher in fetal brain samples. In both autistic and schizophrenic subjects MeCbl and AdoCbl levels were more than 3-fold lower than age-matched controls. In autistic subjects lower MeCbl was associated with decreased MS activity and elevated levels of its substrate homocysteine (HCY). Low levels of the antioxidant glutathione (GSH) have been linked to both autism and schizophrenia, and both total Cbl and MeCbl levels were decreased in glutamate-cysteine ligase modulatory subunit knockout (GCLM-KO) mice, which exhibit low GSH levels. Thus our findings reveal a previously unrecognized decrease in brain vitamin B₁₂ status across the lifespan that may reflect an adaptation to increasing antioxidant demand, while accelerated deficits due to GSH deficiency may contribute to neurodevelopmental and neuropsychiatric disorders.

Correlation of homocysteine metabolic enzymes gene polymorphism and mild cognitive impairment in the Xinjiang Uygur population

Background

The aim of this study was to investigate the genetic polymorphisms in the homocysteine (HCY) metabolic enzymes in the Xinjiang Uygur population who have mild cognitive impairment (MCI).

Material/Methods

Based on the epidemiological investigation, 129 cases of diagnosed Uygur MCI patients and a matched control group with 131 cases were enrolled for analyzing the association between the polymorphisms in the HCY metabolism related genes (C677T, A1298C, and G1968A polymorphisms in MTHFR, as well as the A2756G polymorphism in MS) and MCI by using the SNaPshot method. We then determined the homocysteine level in patients.

Results

In Xinjiang Uygur subjects, the A1298C polymorphisms in MTHFR and the A2756G polymorphisms in the MS gene in the MCI group were different from those in the control group. However, the C677T and G1968A polymorphisms in the MTHFR gene in MCI patients were not different from those in the control group. Multivariate logistic regression showed that, in addition to the well-known risk factors, such as low education level, high cholesterol level, high level of low-density lipoprotein, and high homocysteine levels, the A>G mutation in the MS gene at the rs1805087 locus was another independent risk factor for MCI in the Uyghur MCI population. The risk of MCI in G allele carriers was 2.265 times higher than that in matched control individuals (95% CI: 1.205~4.256, P<0.05).

Conclusions

The genetic polymorphism of HCY metabolizing enzymes is correlated to the occurrence of MCI in the Xinjiang Uygur population. The A2756G polymorphism in the MS gene could be an independent risk factor for MCI in the Xinjiang Uygur population.

B vitamin polymorphisms and behavior: evidence of associations with neurodevelopment, depression, schizophrenia, bipolar disorder and cognitive decline

The B vitamins folic acid, vitamin B12 and B6 are essential for neuronal function, and severe deficiencies have been linked to increased risk of neurodevelopmental disorders, psychiatric disease and dementia. Polymorphisms of genes involved in B vitamin absorption, metabolism and function, such as methylene tetrahydrofolate reductase (MTHFR), cystathionine β synthase (CβS), transcobalamin 2 receptor (TCN2) and methionine synthase reductase (MTRR), have also been linked to increased incidence of psychiatric and cognitive disorders. However, the effects of these polymorphisms are often quite small and many studies failed to show any meaningful or consistent associations. This review discusses previous findings from clinical studies and highlights gaps in knowledge. Future studies assessing B vitamin-associated polymorphisms must take into account not just traditional demographics, but subjects' overall diet, relevant biomarkers of nutritional status and also analyze related genetic factors that may exacerbate behavioral effects or nutritional status.

Association Study between Folate Pathway Gene Single Nucleotide Polymorphisms and Gastric Cancer in Koreans

Gastric cancer is ranked as the most common cancer in Koreans. A recent molecular biological study about the folate pathway gene revealed the correlation with a couple of cancer types. In the folate pathway, several genes are involved, including methylenetetrahydrofolate reductase (MTHFR), methyltetrahydrofolate-homocysteine methyltransferase reductase (MTRR), and methyltetrahydrofolate-homocysteine methyltransferase (MTR). The MTHFR gene has been reported several times for the correlation with gastric cancer risk. However, the association of the MTRR or MTR gene has not been reported to date. In this study, we investigated the association between the single nucleotide polymorphisms (SNPs) of the MTHFR, MTRR, and MTR genes and the risk of gastric cancer in Koreans. To identify the genetic association with gastric cancer, we selected 17 SNPs sites in folate pathway-associated genes of MTHFR, MTR, and MTRR and tested in 1,261 gastric cancer patients and 375 healthy controls. By genotype analysis, estimating odds ratios and 95% confidence intervals (CI), rs1801394 in the MTRR gene showed increased risk for gastric cacner, with statistical significance both in the codominant model (odds ratio [OR], 1.39; 95% CI, 1.04 to 1.85) and dominant model (OR, 1.34; 95% CI, 1.02 to 1.75). Especially, in the obese group (body mass index ≥ 25 kg/m²), the codominant (OR, 9.08; 95% CI, 1.01 to 94.59) and recessive model (OR, 3.72; 95% CI, 0.92 to 16.59) showed dramatically increased risk (p < 0.05). In conclusion, rs1801394 in the MTRR gene is associated with gastric cancer risk, and its functional significance need to be validated.

Effect of common polymorphisms in folate uptake and metabolism genes on frequency of micronucleated lymphocytes in a South Australian cohort

Single nucleotide polymorphisms (SNPs) in genes that control folate uptake and metabolism may have an important effect on chromosomal stability. The present study investigated the effect of common SNPs in some of these critical genes on frequency of lymphocytes with micronuclei, a biomarker of chromosome breakage or loss. 164 individuals (94 males and 70 females) of different age ranging from 18 to 73 years participated in this study. Polymorphisms in GCPII (C1561T), RFC (G80A), MTR (A2756G), MTRR (A66G and C524T), TS (tandem repeats, 6bp deletion in 3'-UTR region) and MTHFR (C677T and A1298C) were detected using PCR-based methods. Frequency of binucleated (BN) lymphocytes containing one or more micronuclei (BN-MN) was determined using the cytokinesis-block micronucleus (CBMN) assay and adjusted for the effects of age and gender. We did not find any significant association between BN-MN frequency and the common SNPs in GCPII, MTRR, TS and MTHFR genes. BN-MN frequency in individuals who carried at least one copy of the rarer G allele for MTR (A2756G) or were homozygotes for the more common G allele for RFC (G80A) had a 14% or 19% lower BN-MN frequency compared to the alternative genotypes for that SNP respectively. It was evident from genotype combination analyses that BN-MN frequency per 1000 BN cells was highest in those with the combined MTR (2756) AA and RFC (80) GA or AA genotype (13.6 per thousand) and lowest in those with the combined MTR (2756) AG or GG and RFC (80) GG genotypes (9.5 per thousand) (P trend=0.015). The RFC G80A and MTR A2756G polymorphisms and their combinations may be important variables that substantially affect lymphocyte BN-MN frequency in this South Australian cohort.

Methotrexate decreases PP2A methylation and increases tau phosphorylation in neuron

Folate deficiency is associated with Alzheimer's disease (AD) and has been suggested to contribute to the pathogenesis of AD. Hyperphosphorylation of tau and deposition of beta-amyloid derived from amyloid precursor protein (APP) are characteristic features of the neurodegenerative pathology of AD. To investigate the molecular mechanisms underlying the association between folate deficiency and AD pathogenesis, we treated rat primary neuron cultures with methotrexate (MTX), a folate antagonist. After MTX treatment, levels of phosphorylated tau, APP, and beta-secretase were increased, as shown by Western-blot and immunocytochemistry analyses, and the neuronal viability was reduced, as assessed by the MTS assay, indicating that folate deficiency increases characteristic AD pathologies. Interestingly, levels of methylated protein phosphatase-2A (PP2A), which is the active form of the putative tau phosphatase PP2A, were reduced. These novel findings indicate that folate deficiency increases the characteristic AD pathology including tau phosphorylation presumably by PP2A inactivation.

Genetic and Environmental Influences on Plasma Homocysteine: Results from a Danish Twin Study

Background: Increased plasma homocysteine has been linked to many clinical conditions including atherosclerosis and ischemic stroke. We assessed the genetic and environmental influences on homocysteine in adult twins and tested the influence of 3 candidate polymorphisms.

Methods: Homocysteine was analyzed in 1206 healthy twins, who were genotyped for 3 polymorphisms: MTHFR 677C&gt;T, MTR 2756A&gt;G, and NNMT (dbSNP: rs694539). To perform quantitative trait linkage analysis of the MTHFR locus, the genotyping was supplemented with 2 genetic markers localized on each site of the MTHFR locus. The twin data were analyzed using biometric structural equation models as well as a combined association and linkage analysis in 2 age cohorts.

Results: Age, sex, and MTHFR genotype have a significant impact on homocysteine concentrations, whereas the other genotypes were not associated with homocysteine concentrations. The variance in homocysteine could be solely ascribed to additive genetic and nonshared environmental factors, with an estimated additive genetic proportion of total variation at age 18–39 years of 0.63 (95% CI, 0.53–0.71) and at age 40–65 years of 0.27 (95% CI, 0.10–0.41). The impact of the MTHFR locus is estimated to explain 53% (95% CI, 0.07–0.67) of the total phenotypic variation in persons 18–39 years old and 24% (95% CI, 0.00–0.39) in persons 40–65 years old, i.e., almost all additive genetic variance.

Conclusions: Homocysteine concentrations have a high heritability that decreases with age. The MTHFR gene locus is responsible for almost all the variation attributable to genetic factors, leaving very little influence of other genetic variations.

Methylenetetrahydrofolate Reductase 677C>T and Methionine Synthase 2756A>G Mutations: No Impact on Survival, Cognitive Functioning, or Cognitive Decline in Nonagenarians

BACKGROUND

Several reports have shown an association between homocysteine, cognitive functioning, and survival among the oldest-old. Two common polymorphisms in the genes coding for methylenetetrahydrofolate reductase (MTHFR 677C>T) and methionine synthase (MTR 2756A>G) have an impact on plasma homocysteine level.

METHODS

We examined the effect of the MTHFR 677C>T and MTR 2756A>G genotypes on baseline cognitive functioning, cognitive decline over 5 years measured in three assessments, and survival in a population-based cohort of 1581 nonagenarians. Cognitive functioning was assessed by using the Mini-Mental State Examination (MMSE) and five brief cognitive tests (cognitive composite).

RESULTS

There are no differences in MMSE score (p =.83) or in cognitive composite (p =.56) at intake as a function of genotype tested by analysis of variance, whereas sex and social group have a impact on MMSE (p < or =.03), and social group on the cognitive composite (p <.01). The mean MMSE was lower for women than for men. However, considering the group participating in all three assessments, there were no sex-related differences in MMSE (p =.34). The cognitive decline in the group participating in all three assessments was investigated using regression models for the relationship between cognitive performance and genotype, age, sex, and social group and revealed no significant difference. Furthermore, the MTHFR 677T and MTR 2756A heterozygous and homozygous genotype had no significant impact on survival, with hazard ratios of 1.05 (95% confidence interval [CI], 0.93-1.17), 0.93 (95% CI, 0.77-1.14), 1.05 (95% CI, 0.94-1.18), and 0.97 (95% CI, 0.74-1.28).

CONCLUSIONS

MTHFR and MTR genotypes are not associated with cognitive functioning, cognitive decline, or survival among nonagenarians.

S-adenosylmethionine/homocysteine cycle alterations modify DNA methylation status with consequent deregulation of PS1 and BACE and beta-amyloid production

Few diseases are characterized by high homocysteine (HCY) and low folate and vitamin B12 blood levels. Alzheimer disease (AD) is among these. It has already been shown that DNA methylation is involved in amyloid precursor protein (APP) processing and beta-amyloid (A beta) production through the regulation of Presenilin1 (PS1) expression and that exogenous S-adenosylmethionine (SAM) can silence the gene reducing A beta production. Here we demonstrate that BACE (beta-secretase), as well as PS1, is regulated by methylation and that the reduction of folate and vitamin B12 in culture medium can cause a reduction of SAM levels with consequent increase in presenilin1 and BACE levels and with increase in A beta production. The simultaneous administration of SAM to the deficient medium can restore the normal gene expression, thus reducing the A beta levels. The use of deprived medium was intended to mimic a mild nutritional deficit involved in the onset of AD.

Downregulation of protein phosphatase 2A carboxyl methylation and methyltransferase may contribute to Alzheimer disease pathogenesis

ABalphaC, a major protein phosphatase 2A (PP2A) heterotrimeric enzyme, binds to and regulates the microtubule cytoskeleton and tau. We have shown that ABalphaC protein expression levels are selectively reduced in Alzheimer disease (AD). Notably, the carboxyl methylation of PP2A catalytic subunit (PP2A(C)) is critically required for ABalphaC holoenzyme assembly, and catalyzed by a specific methyltransferase (PPMT). Here, we provide the first analysis of human PPMT and methylated PP2A(C) in brain regions from AD, non-AD demented, and aged control autopsy cases. Immunoblotting analyses revealed that PPMT protein expression and PP2A(C) methylation levels were quantitatively decreased in AD-affected brain regions. Immunohistochemical studies showed that PPMT was abundant in neurons throughout the cortex in normal control and non-AD demented cases. However, in AD, there was a regional loss of PPMT immunoreactivity that closely paralleled the severity of tau pathology, but not amyloid plaque burden. We propose that the deregulation of PPMT and PP2A methylation/demethylation cycles contributes to AD pathogenesis, by inducing changes in PP2A heteromultimeric composition and substrate specificity. In turn, PP2A dysfunction compromises the mechanisms that control tau, neuronal plasticity, and survival.