The regulatory region polymorphisms of the MTHFR gene are not associated with Alzheimer's disease

Homocysteine Level and Mechanisms of Injury in Parkinson's Disease as Related to MTHFR, MTR, and MTHFD1 Genes Polymorphisms and L-Dopa Treatment

An elevated concentration of total homocysteine (tHcy) in plasma and cerebrospinal fluid is considered to be a risk factor for Alzheimer's disease (AD) and Parkinson's disease (PD). Homocysteine (Hcy) levels are influenced by folate concentrations and numerous genetic factors through the folate cycle, however, their role in the pathogenesis of PD remains controversial. Hcy exerts a neurotoxic action and may participate in the mechanisms of neurodegeneration, such as excitotoxicity, oxidative stress, calcium accumulation, and apoptosis. Elevated Hcy levels can lead to prooxidative activity, most probably through direct interaction with N-methyl-D-aspartate (NMDA) receptors and sensitization of dopaminergic neurons to age-related dysfunction and death. Several studies have shown that higher concentration of Hcy in PD is related to long-term administration of levodopa (L-dopa). An elevation of plasma tHcy levels can also reflect deficiencies of cofactors in remethylation of Hcy to methionine (Met) (folates and vitamin B12) and in its transsulfuration to cysteine (Cys) (vitamin B6). It is believed that the increase in the concentration of Hcy in PD can affect genetic polymorphisms of the folate metabolic pathway genes, such as MTHFR (C677T, A1298C and G1793A), MTR (A2756G), and MTHFD1 (G1958A), whose frequencies tend to increase in PD patients, as well as the reduced concentration of B vitamins. In PD, increased levels of Hcy may lead to dementia, depression and progression of the disease.

One-carbon metabolism and Alzheimer's disease: focus on epigenetics

Alzheimer’s disease (AD) represents the most common form of dementia in the elderly, characterized by progressive loss of memory and cognitive capacity severe enough to interfere with daily functioning and the quality of life. Rare, fully penetrant mutations in three genes (APP, PSEN1 and PSEN2) are responsible for familial forms of the disease. However, more than 90% of AD is sporadic, likely resulting from complex interactions between genetic and environmental factors. Increasing evidence supports a role for epigenetic modifications in AD pathogenesis. Folate metabolism, also known as one-carbon metabolism, is required for the production of S-adenosylmethionine (SAM), which is the major DNA methylating agent. AD individuals are characterized by decreased plasma folate values, as well as increased plasma homocysteine (Hcy) levels, and there is indication of impaired SAM levels in AD brains. Polymorphisms of genes participating in one-carbon metabolism have been associated with AD risk and/or with increased Hcy levels in AD individuals. Studies in rodents suggest that early life exposure to neurotoxicants or dietary restriction of folate and other B vitamins result in epigenetic modifications of AD related genes in the animal brains. Similarly, studies performed on human neuronal cell cultures revealed that folate and other B vitamins deprivation from the media resulted in epigenetic modification of the PSEN1 gene. There is also evidence of epigenetic modifications in the DNA extracted from blood and brains of AD subjects. Here I review one-carbon metabolism in AD, with emphasis on possible epigenetic consequences.

Non-apolipoprotein E and apolipoprotein E genetics of sporadic Alzheimer's disease

The genetic epidemiology of sporadic Alzheimer's disease (SAD) remains a very active area of research,making it one of the most prolifically published areas in medicine and biology. Numerous putative candidate genes have been proposed. However, with the exception of apolipoprotein E (APOE), the only confirmed genetic risk factor for SAD, all the other data appear to be not consistent. Nevertheless, the genetic risk for SAD attributable to the APOE gene in the general population is 20-0%, providing a strong evidence for the existence of additional genetic risk factors. The first part of the present article was dedicated to non-APOE genetics of SAD, reviewing chromosomes-by-chromosomes the available data concerning the major candidate genes. The second part of this article focused on some recently discovered aspects of the APOE polymorphism and their implications for SAD. An attempt to identify the future directions for non-APOE genetic research in SAD was also discussed.

DIFFERENT ALLELE-DISTRIBUTION OF MTHFR 677 C → T AND MTHFR −393 C → A IN PATIENTS CLASSIFIED ACCORDING TO SUBTYPES OF LESCH'S TYPOLOGY

AIMS

The typology by Lesch distinguishes between four subtypes: type 1 (model of allergy), type 2 (model of anxiety or conflict), type 3 (alcohol as an antidepressant), and type 4 (alcohol as adaptation). Taking into account that alcohol dependence is associated with elevated homocysteine levels, this study was undertaken to investigate different MTHFR (methylenetetrahydrofolate reductase) genotypes related to homocysteine metabolism in patients with alcohol dependence who were classified according to Lesch's typology (LT).

SUBJECTS AND METHODS

134 non-abstinent chronic alcoholics (112 males, 22 females; mean age 44.2 (SD 8.9) years) were classified according to LT and divided into four groups: LT 1 (n = 26), LT 2 (n = 65), LT 3 (n = 58), and LT 4 (n = 18). Total plasma homocysteine levels and MTHFR genotypes -393, 677, and 1,793 were determined.

RESULTS

We observed a significantly higher frequency of the thermolabile MTHFR 677 C-->T variant (TT) in patients classified as subtype LT4 when compared with subtypes LT2 and LT3 (P = 0.005). Furthermore, for the MTHFR -393 C --> A-polymorphism, significantly more AC/AA variants were found in subtype LT4 (P = 0.034). No differences in allele-distribution were detected for MTHFR 1793.

CONCLUSION

To our knowledge, this is the first study evaluating MTHFR genotypes in patients who were classified according to LT. Significantly different distributions of MTHFR 677 and -393 variants within Lesch Type 4 as compared with Types 2 and 3 hint at genetic determination of Lesch subtypes.

Genomic characterization of Alzheimer’s disease and genotype-related phenotypic analysis of biological markers in dementia

More than 180 genes distributed across the human genome are potentially involved in the pathogenesis of Alzheimer’s disease (AD). The AD population shows a higher genetic variation rate than the control population. Significant differences in allelic distribution and frequency exist when AD-related polygenic clusters are compared with other forms of dementia, indicating that the genetic component in neurodegenerative dementia differs from that of other CNS disorders. The characterization of AD genotype-related phenotypic profiles reveals substantial differences in biological markers among AD clusters associated with different genes and/or allelic combinations. AD and dementia with vascular component (DVC) are the most prevalent forms of dementia. Both clinical entities share many similarities, but they differ in their major phenotypic and genotypic profiles, as revealed by structural and functional genomics studies. Comparative phenotypic studies have identified significant differences in 25% of more than 100 parametric variables, including anthropometric values, cardiovascular function, blood pressure, lipid metabolism, uric acid metabolism, peripheral calcium homeostasis, liver function, alkaline phosphatase, lactate dehydrogenase, red and white blood cells, regional brain atrophy, and brain blood flow velocity. Functional genomic studies incorporating apolipoprotein E (APOE)-related changes in biological markers extended the difference between AD and DVC by up to 57%. Structural genomic studies with AD-related genes, including APP, MAPT, APOE, PS1, PS2, A2M, ACE, AGT, cFOS, and PRNP, demonstrate different genetic profiles in AD and DVC, with an absolute genetic variation rate in the range of 30–80%, depending upon genes and genetic clusters. The relative polymorphic variation in genetic clusters integrated by two, three or four genes associated with AD ranges from 1 to 3%. The main phenotypic differences in AD are genotype dependent, indicating a powerful influence of polygenic factors on the AD phenotypic profile. All these genotypic and phenotypic variations bring about important consequences for the pharmacogenomics of AD.