Telomere shortening in mTR-/- embryos is associated with failure to close the neural tube.
Mice genetically deficient for the telomerase RNA (mTR) can be propagated for only a limited number of generations. In particular, mTR-/- mice of a mixed C57BL6/129Sv genetic background are infertile at the sixth generation and show serious hematopoietic defects. Here, we show that a percentage of mTR-/- embryos do not develop normally and fail to close the neural tube, preferentially at the forebrain and midbrain. The penetrance of this defect increases with the generation number, with 30% of the mTR-/- embryos from the fifth generation showing the phenotype. Moreover, mTR-/- kindreds in a pure C57BL6 background are only viable up to the fourth generation and also show defects in the closing of the neural tube. Cells derived from mTR-/- embryos that fail to close the neural tube have significantly shorter telomeres and decreased viability than their mTR-/- littermates with a closed neural tube, suggesting that the neural tube defect is a consequence of the loss of telomere function. The fact that the main defect detected in mTR-/- embryos is in the closing of the neural tube, suggests that this developmental process is among the most sensitive to telomere loss and chromosomal instability. (+info)
The "thermolabile" variant of methylenetetrahydrofolate reductase and neural tube defects: An evaluation of genetic risk and the relative importance of the genotypes of the embryo and the mother.
Recent reports have implicated the "thermolabile" (T) variant of methylenetetrahydrofolate reductase (MTHFR) in the causation of folate-dependent neural tube defects (NTDs). We report herein the largest genetic study of NTD cases (n=271) and families (n=218) to date, establishing that, in Ireland, the "TT" genotype is found in 18.8% of cases versus 8.3% of controls (odds ratio 2.57; confidence interval [CI] 1.48-4.45; P=.0005). The maternal and paternal TT genotypes have intermediate frequencies of 13.8% and 11.9%, respectively, indicating that the predominant MTHFR-related genetic effect acts via the TT genotype of the developing embryo. Analysis of the 218 family triads of mother, father, and affected child with log-linear models supports this interpretation, providing significant evidence that the case TT genotype is associated with NTDs (P=.02) but no evidence of a maternal TT genotypic effect (P=. 83). The log-linear model predicted that the risk of NTDs conferred by the case TT genotype is 1.61 (CI 1.06-2.46), consistent with the paramount importance of the case TT genotype in determining risk. There is no compelling evidence for more than a modest additional risk conferred by a maternal TT genotype. These results favor a biological model of MTHFR-related NTD pathogenesis in which suboptimal maternal folate status imposes biochemical stress on the developing embryo, a stress it is ill-equipped to tolerate if it has a TT genotype. (+info)
Polymorphisms of methylenetetrahydrofolate reductase and other enzymes: metabolic significance, risks and impact on folate requirement.
A common genetic polymorphism results from a C-->T substitution in the gene encoding methylenetetrahydrofolate reductase (MTHFR), the enzyme that produces 5-methyltetrahydrofolate (5-methyl-THF) required for the conversion of homocysteine to methionine. In individuals with the T/T genotype (T/T), functional metabolic effects include changes in one-carbon folate derivatives, elevations in plasma homocysteine and differences in response to folic acid supplementation compared with normal (C/C) or heterozygous (C/T) genotypes. The metabolic changes associated with the T/T genotype are postulated to modify risk for chronic disease (e.g., vascular disease and cancer) and neural tube defects (NTD) when accompanied by folate deficiency. The modulation of these metabolic abnormalities by increasing folate intake suggests that folate requirements may be different in affected individuals (T/T) relative to normal (C/C) or heterozygous (C/T) individuals. The complex interaction between this common genetic polymorphism of MTHFR and folate intake is the focus of intense investigation. (+info)
Molecular genetic analysis of human folate receptors in neural tube defects.
Neural tube defects (NTDs) are the most common congenital malformations and are considered to have a multifactorial origin, having both genetic and environmental components. Periconceptional folate administration reduces the recurrence and occurrence risk by 70-100%. Recently we discovered the first genetic risk factors for NTDs: the 677 C-->T and the 1298 A-->C mutations in the methylenetetrahydrofolate reductase gene explaining at the most 35-50% of the protective effect of folate. In this study we further explored the genetic component of NTDs by analysing the coding region, including the intron-exon boundaries and signal sequences of the folate receptor genes by SSCP analysis. Among 39 patients with spina bifida (SB), 47 mothers with a child with SB, and 10 controls, no polymorphism was present in the folate receptor alpha (FR-alpha) gene or in the folate receptor beta (FR-beta) gene. (+info)
Homocysteine--a pathophysiological cornerstone in obstetrical and gynaecological disorders?
Homocysteine, a product of the methionine cycle, is known to play an important role in cardiovascular diseases, neurological disorders and embryology, and in very important, fast growing fields concerning obstetrics and gynaecology. Therefore, we attempted an actual overview on possible obstetrical and gynaecological disorders as a consequence of an impaired methionine cycle. We tried to evaluate all mechanisms concerning homocysteine metabolism in order to look for hypothetical possibilities of therapeutic interventions. Using MEDLINE starting in January 1966, a search was conducted for articles published in which homocysteine was included as a subject heading or a text word. This search was also specified in combination with other key words such as obstetrics, pregnancy, gynaecology and cancer. Additional sources were identified through cross-referencing. All sources found were examined with regard to providing substantial information on our topic. The information obtained was divided into articles dealing with homocysteine and the methionine cycle itself, homocysteine and pregnancy, and homocysteine and hormones, including menopause, hormone replacement therapy and oral contraceptives. Another group was concerned with other special gynaecological aspects of the methionine cycle. We suggest that elevated concentrations of homocysteine could be a marker and perhaps a cause of, or contributive to, a wide range of obstetrical and gynaecological disorders. (+info)
Pax2/5 and Pax6 subdivide the early neural tube into three domains.
The nested expression patterns of the paired-box containing transcription factors Pax2/5 and Pax6 demarcate the midbrain and forebrain primordium at the neural plate stage. We demonstrate that, in Pax2/5 deficient mice, the mesencephalon/metencephalon primordium is completely missing, resulting in a fusion of the forebrain to the hindbrain. Morphologically, in the alar plate the deletion is characterized by the substitution of the tectum (dorsal midbrain) and cerebellum (dorsal metencephalon) by the caudal diencephalon and in the basal plate by the replacement of the midbrain tegmentum by the ventral metencephalon (pons). Molecularly, the loss of the tectum is demonstrated by an expanded expression of Pax6, (the molecular determinant of posterior commissure), and a rostral shift of the territory of expression of Gbx2 and Otp (markers for the pons), towards the caudal diencephalon. Our results suggest that an intact territory of expression of Pax2/5 in the neural plate, nested between the rostral and caudal territories of expression of Pax6, is necessary for defining the midbrain vesicle. (+info)
Accentuated apoptosis in normally developing p53 knockout mouse embryos following genotoxic stress.
In order to identify the alternative pathways which may substitute for the p53 function during embryogenesis, we have focused our studies on p53 -/- normally developing mouse embryos that survived a genotoxic stress. We assumed that under these conditions p53-independent pathways, which physiologically control genomic stability, are enhanced. We found that while p53 +/+ mouse embryos elicited, as expected, a p53-dependent apoptosis, p53-/- normally developing mice exhibited an accentuated p53-independent apoptotic response. The p53-dependent apoptosis detected in p53+/+ embryos, was an immediate reaction mostly detected in the brain, whereas the p53-independent apoptosis was a delayed reaction with a prominent pattern observed in epithelial cells of most organs in the p53-deficient mice only. These results suggest that in the absence of p53-dependent apoptosis, which is a fast response to damaged DNA, p53-independent apoptotic pathways, with slower kinetics, are turned on to secure genome stability. (+info)
Knowledge and use of folic acid by women of childbearing age--United States, 1995 and 1998.
In the United States, approximately 4000 pregnancies are affected by neural tube defects each year; 50%-70% of these developmental defects could be prevented with daily intake of 400 microg of the B vitamin folic acid throughout the periconceptional period. In 1992, the Public Health Service recommended that all women capable of becoming pregnant consume 400 microg of folic acid daily throughout their childbearing years to reduce their risk for having a pregnancy affected by neural tube defects. In 1998, the Institute of Medicine recommended that all women of childbearing potential consume 400 microg of synthetic folic acid per day from fortified foods and/or a supplement in addition to food folate from a varied diet. This report summarizes the findings of a survey conducted during July-August 1998 to assess folic acid knowledge and practices among women of childbearing age in the United States and compares these results with those from a similar survey conducted in 1995. The findings indicate that 7% of women know folic acid should be taken before pregnancy to reduce the risk for neural tube defects. (+info)