Spatial and temporal expression of the cystathionine beta-synthase gene during early human development

Molecular Engines, Therapeutic Targets, and Challenges in Pediatric Brain Tumors: A Special Emphasis on Hydrogen Sulfide and RNA-Based Nano-Delivery

Pediatric primary brain tumors represent a real challenge in the oncology arena. Besides the psychosocial burden, brain tumors are considered one of the most difficult-to-treat malignancies due to their sophisticated cellular and molecular pathophysiology. Notwithstanding the advances in research and the substantial efforts to develop a suitable therapy, a full understanding of the molecular pathways involved in primary brain tumors is still demanded. On the other hand, the physiological nature of the blood-brain barrier (BBB) limits the efficiency of many available treatments, including molecular therapeutic approaches. Hydrogen Sulfide (H2S), as a member of the gasotransmitters family, and its synthesizing machinery have represented promising molecular targets for plentiful cancer types. However, its role in primary brain tumors, generally, and pediatric types, particularly, is barely investigated. In this review, the authors shed the light on the novel role of hydrogen sulfide (H2S) as a prominent player in pediatric brain tumor pathophysiology and its potential as a therapeutic avenue for brain tumors. In addition, the review also focuses on the challenges and opportunities of several molecular targeting approaches and proposes promising brain-delivery strategies for the sake of achieving better therapeutic results for brain tumor patients.

Molecular characterization and immune protection by cystathionine β-synthase from Eimeria tenella

Eimeria tenella is an obligate intracellular apicomplexan parasite that causes avian coccidiosis and leads to severe economic losses in the global poultry industry. Cystathionine β-synthase (CBS) and cystathionine γ-lyase (CGL) act together to generate H2S in the reverse transsulfuration pathway. In this study, E. tenella Cystathionine β-synthase (EtCBS) was cloned using rapid amplification of cDNA 5'-ends (5'RACE) and characterized, and its immunoprotective effects were evaluated. The recombinant EtCBS protein (rEtCBS) was expressed and successfully recognized by anti-sporozoites (Spz) protein rabbit serum. EtCBS mRNA levels were highest in Spz by qPCR, and the protein expression levels were higher in unsporulated oocysts (UO) than in other stages by Western blot. Indirect immunofluorescence showed that EtCBS protein was found on the surface of Spz and second-generation merozoites (Mrz). The invasion inhibition assays showed that rabbit anti-rEtCBS polyclonal antibodies effectively inhibited parasite invasion host cells. Chickens immunized with rEtCBS protein showed prominently increased weight gains and decreased oocyst output compared to nonimmunized and infected control group. The results suggest that EtCBS could be a potential vaccine candidate against E. tenella.

A narrative review on the role of folate-mediated one-carbon metabolism and its associated gene polymorphisms in posing risk to preeclampsia

Preeclampsia (PE) presents a major obstetrical problem for mother and fetus which is characterized by the onset of hypertension and proteinuria in formerly normotensive women. Altered folate-mediated one-carbon metabolism is one of the factors for PE development either due to nutritional insufficiencies such as folate deficiency or polymorphisms in genes that code for the key enzymes of the cycle. Commonly, there are four genes in the cycle whose polymorphisms have been described in relation to PE. These factors could cause elevation of homocysteine; the toxic metabolite, which subsequently leads to the development of PE. Sufficient levels of folate have been considered important during pregnancy and may reduce the risk of development of PE. This review aims at discussing genetic polymorphisms and nutritional deficiencies as probable predisposing factors and suggests considering fetal genotypes, varied ethnicities, and interaction of various other factors involved to render better conclusiveness to the present studies.

Homocysteine in Neurology: A Possible Contributing Factor to Small Vessel Disease

Homocysteine (Hcy) is a sulfur-containing amino acid generated during methionine metabolism, accumulation of which may be caused by genetic defects or the deficit of vitamin B12 and folate. A serum level greater than 15 micro-mols/L is defined as hyperhomocysteinemia (HHcy). Hcy has many roles, the most important being the active participation in the transmethylation reactions, fundamental for the brain. Many studies focused on the role of homocysteine accumulation in vascular or degenerative neurological diseases, but the results are still undefined. More is known in cardiovascular disease. HHcy is a determinant for the development and progression of inflammation, atherosclerotic plaque formation, endothelium, arteriolar damage, smooth muscle cell proliferation, and altered-oxidative stress response. Conversely, few studies focused on the relationship between HHcy and small vessel disease (SVD), despite the evidence that mice with HHcy showed a significant end-feet disruption of astrocytes with a diffuse SVD. A severe reduction of vascular aquaporin-4-water channels, lower levels of high-functioning potassium channels, and higher metalloproteinases are also observed. HHcy modulates the N-homocysteinylation process, promoting a pro-coagulative state and damage of the cellular protein integrity. This altered process could be directly involved in the altered endothelium activation, typical of SVD and protein quality, inhibiting the ubiquitin-proteasome system control. HHcy also promotes a constant enhancement of microglia activation, inducing the sustained pro-inflammatory status observed in SVD. This review article addresses the possible role of HHcy in small-vessel disease and understands its pathogenic impact.

Chemistry and Biochemistry of Sulfur Natural Compounds: Key Intermediates of Metabolism and Redox Biology

Sulfur contributes significantly to nature chemical diversity and thanks to its particular features allows fundamental biological reactions that no other element allows. Sulfur natural compounds are utilized by all living beings and depending on the function are distributed in the different kingdoms. It is no coincidence that marine organisms are one of the most important sources of sulfur natural products since most of the inorganic sulfur is metabolized in ocean environments where this element is abundant. Terrestrial organisms such as plants and microorganisms are also able to incorporate sulfur in organic molecules to produce primary metabolites (e.g., methionine, cysteine) and more complex unique chemical structures with diverse biological roles. Animals are not able to fix inorganic sulfur into biomolecules and are completely dependent on preformed organic sulfurous compounds to satisfy their sulfur needs. However, some higher species such as humans are able to build new sulfur-containing chemical entities starting especially from plants' organosulfur precursors. Sulfur metabolism in humans is very complicated and plays a central role in redox biochemistry. The chemical properties, the large number of oxidation states, and the versatile reactivity of the oxygen family chalcogens make sulfur ideal for redox biological reactions and electron transfer processes. This review will explore sulfur metabolism related to redox biochemistry and will describe the various classes of sulfur-containing compounds spread all over the natural kingdoms. We will describe the chemistry and the biochemistry of well-known metabolites and also of the unknown and poorly studied sulfur natural products which are still in search for a biological role.

The effects of gasotransmitters on bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD), which remains a major clinical problem for preterm infants, is caused mainly by hyperoxia, mechanical ventilation and inflammation. Many approaches have been developed with the aim of decreasing the incidence of or alleviating BPD, but effective methods are still lacking. Gasotransmitters, a type of small gas molecule that can be generated endogenously, exert a protective effect against BPD-associated lung injury; nitric oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H₂S) are three such gasotransmitters. The protective effects of NO have been extensively studied in animal models of BPD, but the results of these studies are inconsistent with those of clinical trials. NO inhalation seems to have no effect on BPD, although side effects have been reported. NO inhalation is not recommended for BPD treatment in preterm infants, except those with severe pulmonary hypertension. Both CO and H₂S decreased lung injury in BPD rodent models in preclinical studies. Another small gas molecule, hydrogen, exerts a protective effect against BPD. The nuclear factor erythroid-derived 2 (Nrf2)/heme oxygenase-1 (HO-1) axis seems to play a central role in the protective effect of these gasotransmitters on BPD. Gasotransmitters play important roles in mammals, but further clinical trials are needed to explore their effects on BPD.

Feasibility of domino liver transplantation from hyperhomocsyteinemia

Hyperhomocysteinemia, resulting from a cystathionine beta synthase (CBS) deficiency, is an autosomal recessive disease associated with high levels of homocysteine. Such patients can present with severe mental retardation, ectopia lentis and osteoporosis and thromboembolic disease. To the best of our knowledge, only two cases of liver transplantation for CBS deficiency have been published. Here, we report a case of an 8-year-old male with a CBS deficiency that underwent living donor liver transplantation. The postoperative course was uneventful and homocysteine levels remained normal. The liver of this CBS deficiency patient was then successfully used in domino transplantation. The domino liver transplantation recipient was a 41-year-old male diagnosed with acute liver failure following hemi-liver resection due to cholangiocarcinoma. The domino recipient developed acquired hyperhomocysteinemia, which was controlled with a special regimen of medications. No complications relative to CBS deficiency were observed up to 11 months post-transplant. At this time, the patient expired as a result of cholangiocarcinoma recurrence. In conclusion, our data suggest that liver transplantation for CBS deficiency can be effective, safe and beneficial. It is possible to be both safe and beneficial to use a CBS deficiency patient as a domino donor for salvage liver transplantation in a selective category of recipients.

The Controversial Role of Homocysteine in Neurology: From Labs to Clinical Practice

Homocysteine (Hcy) is a sulfur-containing amino acid that is generated during methionine metabolism. Physiologic Hcy levels are determined primarily by dietary intake and vitamin status. Elevated plasma levels of Hcy can be caused by deficiency of either vitamin B12 or folate. Hyperhomocysteinemia (HHcy) can be responsible of different systemic and neurological disease. Actually, HHcy has been considered as a risk factor for systemic atherosclerosis and cardiovascular disease (CVD) and HHcy has been reported in many neurologic disorders including cognitive impairment and stroke, independent of long-recognized factors such as hyperlipidemia, hypertension, diabetes mellitus, and smoking. HHcy is typically defined as levels >15 micromol/L. Treatment of hyperhomocysteinemia with folic acid and B vitamins seems to be effective in the prevention of the development of atherosclerosis, CVD, and strokes. However, data from literature show controversial results regarding the significance of homocysteine as a risk factor for CVD and stroke and whether patients should be routinely screened for homocysteine. HHcy-induced oxidative stress, endothelial dysfunction, inflammation, smooth muscle cell proliferation, and endoplasmic reticulum (ER) stress have been considered to play an important role in the pathogenesis of several diseases including atherosclerosis and stroke. The aim of our research is to review the possible role of HHcy in neurodegenerative disease and stroke and to understand its pathogenesis.

Association of Cystathionine β-Synthase Gene Polymorphisms With Preeclampsia

Preeclampsia (PE) is a pregnancy disorder that increases maternal and fetal morbidity and mortality worldwide. High plasma levels of homocysteine (Hcy) are a risk factor for several cardiovascular diseases. Cystathionine β-synthase (CBS) plays an important role in Hcy homeostasis catalyzing the irreversible degradation of Hcy to cystathionine, protecting the endothelium from injury caused by hypoxia. Several mutations and polymorphisms may alter the expression of the CBS gene, resulting in variable levels of Hcy. The purpose of this study was to investigate the association of CBS gene polymorphisms with PE in Mexican women. A case–control study consisting of 129 pregnant women with PE (37 severe and 92 mild) and 173 women with uncomplicated pregnancies was performed. Polymorphisms, such as G797A, C785T, T833C, G919A, T959C, C1105T, and 844ins68 base pair, in the CBS gene were genotyped. The polymorphism G797A was monomorphic in cases with the presence of only G797A-G allele. Allele C785T-T and genotype C785T-C/T were associated with susceptibility in severe and mild PE. Alleles G797A-G and T959C-T were associated with susceptibility only in severe PE. Haplotype TGTWGTC was of susceptibility for severe PE and of protection for mild PE. Haplotypes CGTWGCC and CATWGTC seem to be protective for severe PE, but the latter is related to susceptibility in mild PE. The results suggest that C785T, G797A, and T959C mutations are contributing in different ways in severe and mild PE in our population and could be count as another related factor for this disease.

Effect of homocysteine on pregnancy: A systematic review

Research purpose was to put together the available pieces of present scientific data and to close the gap in the knowledge of Hcy levels in pregnancy and its association with some pregnancy complications. Scientific data were taken from research papers published between January 1990 and December 2017, and found on the Internet (PubMed, ClinicalKey and Embase databases) by the following tags entered in English, Russian, French and German languages: pregnancy, homocysteine, pregnancy complications, pregnancy loss, preeclampsia, intrauterine growth restriction, and placental abruption. The review showed that Hcy levels range in uncomplicated pregnancy. Upon that, Hcy level tends to decrease during the second and third trimesters. Some studies have revealed a link between polymorphism and abortion. Sufficient data were obtained indicating the relationship between HHcy and PE. Placental abruption was also associated with high Hcy levels increasing the risk 5.3-fold, but still there are data not supporting the hypothesis that Hcy levels correlate with placental abruption.

Causes of hyperhomocysteinemia and its pathological significance

In the last 10 years, homocysteine has been regarded as a marker of cardiovascular disease and a definite risk factor for many other diseases. Homocysteine is biosynthesized from methionine through multiple steps and then goes through one of two major metabolic pathways: remethylation and transsulfuration. Hyperhomocysteinemia is a state in which too much homocysteine is present in the body. The main cause of hyperhomocysteinemia is a dysfunction of enzymes and cofactors associated with the process of homocysteine biosynthesis. Other causes include excessive methionine intake, certain diseases and side effects of some drugs. Hyperhomocysteinemia is a trigger for many diseases, such as atherosclerosis, congestive heart failure, age-related macular degeneration, Alzheimer's disease and hearing loss. There are many studies showing a positive relationship between homocysteine level and various symptoms. We speculate that a high level of homocysteine can be the sole reason or an aggravating factor in numerous diseases for which causal links are not fully understood.

Study on Environmental Causes and SNPs of MTHFR, MS and CBS Genes Related to Congenital Heart Disease

PURPOSE

Congenital heart diseases (CHD) are among the most common birth defects in China. Environmental causes and folate metabolism changes may alter susceptibility to CHD. The aim of this study is to evaluate the relevant risk-factors of children with CHD and their mothers.

METHODS

138 children with CHD and 207 normal children for controls were recruited. Their mothers were also enlisted in this study and interviewed following a questionnaire about their pregnant history and early pregnancy situation. Five single nucleotide polymorphisms (SNPs) in methylenetetrahydrofolate reductase (MTHFR), methionine synthase (MS) and cystathionine β-synthase (CBS) of mothers and children were genotyped.

RESULTS

There were significant differences in the gender of children, occupation of mothers, family history with CHD, history of abortion, history of adverse pregnancy, early pregnancy health, fetus during pregnancy, pesticide exposure and drug exposure in CHD group and control group ( P < 0.05). Logistic regression analyses showed that after adjustment for above factors, MTHFR rs1801131 were significantly associated with their offspring CHD risk in mothers. Compared with the mothers whose MTHFR were rs1801131 AA and AC genotypes, the mothers who got a mutation of MTHFR rs1801131 CC genotypes had a 267% increase in risk of given birth of a CHD children (OR = 3.67,95%CI = 1.12-12.05). Meanwhile, MTHFR rs1801131 were significantly associated with CHD susceptibility in children (OR = 1.42, 95% CI = 1.00-2.44 in additive model).

CONCLUSIONS

Besides mothers' social and fertility characteristics, our results suggested that the genetic variants in folate metabolism pathway might be one of the most related risk-factors of CHD. MTHFR rs1801131 were identified as loci in Chinese population that were involved in CHD.

L-cystathionine inhibits oxidized low density lipoprotein-induced THP-1-derived macrophage inflammatory cytokine monocyte chemoattractant protein-1 generation via the NF-κB pathway

This study aimed to explore whether and how L-cystathionine had any regulatory effect on the inflammatory response in THP-1-derived macrophages cultured in vitro under oxidized low-density lipoprotein (ox-LDL) stimulation. The human monocyte line THP-1 cell was cultured in vitro and differentiated into macrophages after 24 hours of PMA induction. Macrophages were pretreated with L-cystathionine and then treated with ox-LDL. The results showed that compared with the controls, ox-LDL stimulation significantly upregulated the expression of THP-1-derived macrophage MCP-1 by enhancing NF-κB p65 phosphorylation, nuclear translocation and DNA binding with the MCP-1 promoter. Compared with the ox-LDL group, 0.3 mmol/L and 1.0 mmol/L L-cystathionine significantly inhibited the expression of THP-1-derived macrophage MCP-1. Mechanistically, 0.3 mmol/L and 1.0 mmol/L L-cystathionine suppressed phosphorylation and nuclear translocation of the NF-κB p65 protein, as well as the DNA binding activity and DNA binding level of NF-κB with the MCP-1 promoter, which resulted in a reduced THP-1-derived macrophage MCP-1 generation. This study suggests that L-cystathionine could inhibit the expression of MCP-1 in THP-1-derived macrophages induced by ox-LDL via inhibition of NF-κB p65 phosphorylation, nuclear translocation, and binding of the MCP-1 promoter sequence after entry into the nucleus.

Hydrogen Sulfide as a Potential Therapeutic Target in Fibrosis

Hydrogen sulfide (H₂S), produced endogenously by the activation of two major H₂S-generating enzymes (cystathionine β-synthase and cystathionine γ-lyase), plays important regulatory roles in different physiologic and pathologic conditions. The abnormal metabolism of H₂S is associated with fibrosis pathogenesis, causing damage in structure and function of different organs. A number of in vivo and in vitro studies have shown that both endogenous H₂S level and the expressions of H₂S-generating enzymes in plasma and tissues are significantly downregulated during fibrosis. Supplement with exogenous H₂S mitigates the severity of fibrosis in various experimental animal models. The protective role of H₂S in the development of fibrosis is primarily attributed to its antioxidation, antiapoptosis, anti-inflammation, proangiogenesis, and inhibition of fibroblasts activities. Future studies might focus on the potential to intervene fibrosis by targeting the pathway of endogenous H₂S-producing enzymes and H₂S itself.

L-cystathionine inhibits the mitochondria-mediated macrophage apoptosis induced by oxidized low density lipoprotein

This study was designed to investigate the regulatory role of l-cystathionine in human macrophage apoptosis induced by oxidized low density lipoprotein (ox-LDL) and its possible mechanisms. THP-1 cells were induced with phorbol 12-myristate 13-acetate (PMA) and differentiated into macrophages. Macrophages were incubated with ox-LDL after pretreatment with l-cystathionine. Superoxide anion, apoptosis, mitochondrial membrane potential, and mitochondrial permeability transition pore (MPTP) opening were examined. Caspase-9 activities and expression of cleaved caspase-3 were measured. The results showed that compared with control group, ox-LDL treatment significantly promoted superoxide anion generation, release of cytochrome c (cytc) from mitochondrion into cytoplasm, caspase-9 activities, cleavage of caspase-3, and cell apoptosis, in addition to reduced mitochondrial membrane potential as well as increased MPTP opening. However, 0.3 and 1.0 mmol/L l-cystathionine significantly reduced superoxide anion generation, increased mitochondrial membrane potential, and markedly decreased MPTP opening in ox-LDL + l-cystathionine macrophages. Moreover, compared to ox-LDL treated-cells, release of cytc from mitochondrion into cytoplasm, caspase-9 activities, cleavage of caspase-3, and apoptosis levels in l-cystathionine pretreated cells were profoundly attenuated. Taken together, our results suggested that l-cystathionine could antagonize mitochondria-mediated human macrophage apoptosis induced by ox-LDL via inhibition of cytc release and caspase activation.

The immunomodulation of inducible hydrogen sulfide in Pacific oyster Crassostrea gigas

Hydrogen sulfide (H2S) is an important gasotransmitter, which plays indispensable roles in cardiovascular, nervous and immune systems of vertebrates. However, the information about the immunomodulation of H2S in invertebrates is still very limited. In the present study, the temporal expression profile of cystathionine γ lyase in oyster Crassostrea gigas (CgCSE) was investigated after the oysters were stimulated by lipopolysaccharide. The expression levels of CgCSE mRNA transcripts in hemocytes increased significantly at 12h (1.31-fold of the PBS group, P<0.05) after LPS stimulation. The immunomodulation of inducible H2S in oyster was examined by monitoring the alterations of both cellular and humoral immune parameters in response to the stimulations of LPS, LPS+Na2S and LPS+propargylglycine (PAG). The total hemocyte counts (THC) and hemolymph PO activity increased significantly after LPS stimulation, and the increase could be further enhanced by adding PAG, while inhibited by appending Na2S. The phagocytosis activity of hemocytes was also increased firstly after LPS treatment, and the increase was enhanced by adding Na2S but inhibited after appending PAG. The anti-bacterial activity in hemolymph increased at 3h post LPS treatment, and then decreased after adding PAG. The total SOD activity of hemolymph was also elevated at 6h post LPS treatment, and the elevated activity was depressed by adding Na2S. These results collectively indicated that H2S might play crucial roles in the immune response of oyster via modulating the turnover and phagocytosis of hemocytes, and regulating the anti-bacterial activity and proPO activation in the hemolymph.

A genetic association study detects haplotypes associated with obstructive heart defects

The development of congenital heart defects (CHDs) involves a complex interplay between genetic variants, epigenetic variants, and environmental exposures. Previous studies have suggested that susceptibility to CHDs is associated with maternal genotypes, fetal genotypes, and maternal-fetal genotype (MFG) interactions. We conducted a haplotype-based genetic association study of obstructive heart defects (OHDs), aiming to detect the genetic effects of 877 SNPs involved in the homocysteine, folate, and transsulfuration pathways. Genotypes were available for 285 mother-offspring pairs with OHD-affected pregnancies and 868 mother-offspring pairs with unaffected pregnancies. A penalized logistic regression model was applied with an adaptive least absolute shrinkage and selection operator (lasso), which dissects the maternal effect, fetal effect, and MFG interaction effects associated with OHDs. By examining the association between 140 haplotype blocks, we identified 9 blocks that are potentially associated with OHD occurrence. Four haplotype blocks, located in genes MGMT, MTHFS, CBS, and DNMT3L, were statistically significant using a Bayesian false-discovery probability threshold of 0.8. Two blocks in MGMT and MTHFS appear to have significant fetal effects, while the CBS and DNMT3L genes may have significant MFG interaction effects.

Hydrogen sulfide regulates Ca(2+) homeostasis mediated by concomitantly produced nitric oxide via a novel synergistic pathway in exocrine pancreas

AIM

The present study was designed to explore the effects of hydrogen sulfide (H2S) on Ca(2+) homeostasis in rat pancreatic acini.

RESULTS

Sodium hydrosulfide (NaHS; an H2S donor) induced a biphasic increase in the intracellular Ca(2+) concentration ([Ca(2+)]i) in a dose-dependent manner. The NaHS-induced [Ca(2+)]i elevation persisted with an EC50 of 73.3 μM in the absence of extracellular Ca(2+) but was abolished by thapsigargin, indicating that both Ca(2+) entry and Ca(2+) release contributed to the increase. The [Ca(2+)]i increase was markedly inhibited in the presence of NG-monomethyl L-arginine or 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), and diaminofluorescein-2/diaminofluorescein-2 triazole (DAF-2/DAF-2T) fluorometry demonstrated that nitric oxide (NO) was also produced by H2S in a dose-dependent manner with an EC50 of 64.8 μM, indicating that NO was involved in the H2S effect. The H2S-induced [Ca(2+)]i increase was inhibited by pretreatment with U73122, xestospongin C, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, KT5823, and GP2A, indicating that phospholipase C (PLC), the inositol 1,4,5-trisphosphate (IP3) receptor, soluble guanylate cyclase (sGC), protein kinase G (PKG), and Gq-protein play roles as intermediate components in the H2S-triggered intracellular signaling.

INNOVATION

To our knowledge, our study is the first one highlighting the effect of H2S on intracellular Ca(2+) dynamics in pancreatic acinar cells. Moreover, a novel cascade was presumed to function via the synergistic interaction between H2S and NO.

CONCLUSION

We conclude that H2S affects [Ca(2+)]i homeostasis that is mediated by H2S-evoked NO production via an endothelial nitric oxide synthase (eNOS)-NO-sGC-cyclic guanosine monophosphate-PKG-Gq-protein-PLC-IP3 pathway to induce Ca(2+) release, and this pathway is identical to the one we recently proposed for a sole effect of NO and the two gaseous molecules synergistically function to regulate Ca(2+) homeostasis.

L-Cysteine promotes the proliferation and differentiation of neural stem cells via the CBS/H₂S pathway

Growing evidence has suggested that hydrogen sulfide (H₂S) acts as a novel neuro-modulator and neuroprotective agent; however, it remains to be investigated whether H2S has a direct effect on neural stem cells (NSCs). We report here that NSCs expressed cystathionine β synthase (CBS) and addition of exogenous H2S donor, L-cysteine, stimulated proliferation and increased the differentiation potential of NSCs to neurons and astroglia. Moreover, pre-treatment with aminooxyacetic acid, the inhibitor of CBS or knockdown of CBS in using siRNA, significantly attenuated the effects of L-cysteine on elevated H₂S levels and the cell proliferation; it also effectively suppressed L-cysteine-induced neurogenesis and astrocytogenesis. Further analysis revealed that L-cysteine-induced proliferation was associated with phosphorylation of extracellular signal-regulated kinases 1/2 and differentiation with altered expression of differentiation-related genes. Taken together, the present data suggest that L-cysteine can enhance proliferation and differentiation of NSCs via the CBS/H2S pathway, which may serve as a useful inference for elucidating its role in regulating the fate of NSCs in physiological and pathological settings.

A functional variant in the cystathionine β-synthase gene promoter significantly reduces congenital heart disease susceptibility in a Han Chinese population

Homocysteine is an independent risk factor for various cardiovascular diseases. There are two ways to remove homocysteine from embryonic cardiac cells: remethylation to form methionine or transsulfuration to form cysteine. Cystathionine β-synthase (CBS) catalyzes the first step of homocysteine transsulfuration as a rate-limiting enzyme. In this study, we identified a functional variant -4673C>G (rs2850144) in the CBS gene promoter region that significantly reduces the susceptibility to congenital heart disease (CHD) in a Han Chinese population consisting of 2 340 CHD patients and 2 270 controls. Individuals carrying the heterozygous CG and homozygous GG genotypes had a 15% (odds ratio (OR) = 0.85, 95% confidence interval (CI) = 0.75-0.96, P = 0.011) and 40% (OR = 0.60, 95% CI = 0.49-0.73, P = 1.78 × 10(-7)) reduced risk to develop CHD than the wild-type CC genotype carriers in the combined samples, respectively. Additional stratified analyses demonstrated that CBS -4673C>G is significantly related to septation defects and conotruncal defects. In vivo detection of CBS mRNA levels in human cardiac tissues and in vitro luciferase assays consistently showed that the minor G allele significantly increased CBS transcription. A functional analysis revealed that both the attenuated transcription suppressor SP1 binding affinity and the CBS promoter hypomethylation specifically linked with the minor G allele contributed to the remarkably upregulated CBS expression. Consequently, the carriers with genetically increased CBS expression would benefit from the protection due to the low homocysteine levels maintained by CBS in certain cells during the critical heart development stages. These results shed light on unexpected role of CBS and highlight the importance of homocysteine removal in cardiac development.Cell Research advance online publication 18 September 2012; doi:10.1038/cr.2012.135.

Cystathionine protects against endoplasmic reticulum stress-induced lipid accumulation, tissue injury, and apoptotic cell death

Cystathionine (R-S-(2-amino-2-carboxyethyl)-l-homocysteine) is a non-proteinogenic thioether containing amino acid. In mammals, cystathionine is formed as an intermediate of the transsulfuration pathway by the condensation of serine and homocysteine (Hcy) in a reaction catalyzed by cystathionine β-synthase (CBS). Cystathionine is subsequently converted to cysteine plus ammonia and α-ketobutyrate by the action of cystathionine γ-lyase (CGL). Pathogenic mutations in CBS result in CBS-deficient homocystinuria (HCU) which, if untreated, results in mental retardation, thromboembolic complications and connective tissue disorders. Currently there is no known function for cystathionine other than serving as an intermediate in transsulfuration and to date, the possible contribution of the abolition of cystathionine synthesis to pathogenesis in HCU has not been investigated. Using both mouse and cell-culture models, we have found that cystathionine is capable of blocking the induction of hepatic steatosis and kidney injury, acute tubular necrosis, and apoptotic cell death by the endoplasmic reticulum stress inducing agent tunicamycin. Northern and Western blotting analysis indicate that the protective effects of cystathionine occur without any obvious alteration of the induction of the unfolded protein response. Our data constitute the first experimental evidence that the abolition of cystathionine synthesis may contribute to the pathology of HCU and that this compound has therapeutic potential for disease states where ER stress is implicated as a primary initiating pathogenic factor.

Overview of homocysteine and folate metabolism. With special references to cardiovascular disease and neural tube defects

This overview addresses homocysteine and folate metabolism. Its functions and complexity are described, leading to explanations why disturbed homocysteine and folate metabolism is implicated in many different diseases, including congenital birth defects like congenital heart disease, cleft lip and palate, late pregnancy complications, different kinds of neurodegenerative and psychiatric diseases, osteoporosis and cancer. In addition, the inborn errors leading to hyperhomocysteinemia and homocystinuria are described. These extreme human hyperhomocysteinemia models provide knowledge about which part of the homocysteine and folate pathways are linked to which disease. For example, the very high risk for arterial and venous occlusive disease in patients with severe hyperhomocysteinemia irrespective of the location of the defect in remethylation or transsulphuration indicates that homocysteine itself or one of its "direct" derivatives is considered toxic for the cardiovascular system. Finally, common diseases associated with elevated homocysteine are discussed with the focus on cardiovascular disease and neural tube defects.

Folic acid, methylation and neural tube closure in humans

This review provides a brief description of folate use and folic acid metabolism in relation to neural tube defect (NTD) risk. First, a meta-analysis of reduction in NTD recurrence and occurrence risk with periconceptional folic acid supplementation is presented. Second, an overview of the complex folate metabolism is given. Third, SNPs for genes involved in folate and homocysteine metabolism that have been studied in relation to NTD riskare discussed. Fourth, the questions whether folate receptor autoantibodies or hampered methylation are mechanisms underlying NTDs are briefly discussed.

Comparative localization of cystathionine beta-synthase and cystathionine gamma-lyase in retina: differences between amphibians and mammals

Hydrogen sulfide (H(2)S) is a gaseous neuromodulator that can be synthesized by the transsulfuration enzymes cystathionine beta-synthase (CBS) and cystathionine gamma-lyase (CGL). In this study we examined H(2)S as a potential neuromodulator in vertebrate retina. CBS-like immunoreactivity (LI) was found in somas in the inner nuclear layer and as punctate staining in the inner and outer plexiform layers in the salamander retina. CGL-LI was most clearly characterized in salamander, where it was localized in Müller cells. Western blots indicated proteins with the correct molecular weights for both enzymes in both species for liver and cerebellum. Correct molecular weight proteins were identified for both CGL and CBS in salamander retina. The CBS antiserum did not recognize the correct molecular weight protein in mouse retina but the CGL antiserum recognized the correct molecular weight protein for mouse retina. Enzyme assays indicated both CGL and CBS enzyme activity in all three tissues in the salamander. There was good CBS activity in the liver and cerebellum of the mouse but no activity in the retina. CGL activity was clearly present only in the mouse liver, with only trace activity in the cerebellum and retina. In conclusion, both CBS and CGL are present in the amphibian retina, which suggests either a potential role for H(2)S as a gaseous neuromodulator in both neurons and glia in the retina or a requirement for cysteine and glutathione synthesis via the transsulfuration pathway as a defense against oxidative stress.

Folate-mediated one-carbon metabolism and neural tube defects: balancing genome synthesis and gene expression

Neural tube defects (NTDs) refer to a cluster of neurodevelopmental conditions associated with failure of neural tube closure during embryonic development. Worldwide prevalence of NTDs ranges from approximately 0.5 to 60 per 10,000 births, with regional and population-specific variation in prevalence. Numerous environmental and genetic influences contribute to NTD etiology; accumulating evidence from population-based studies has demonstrated that folate status is a significant determinant of NTD risk. Folate-mediated one-carbon metabolism (OCM) is essential for de novo nucleotide biosynthesis, methionine biosynthesis, and cellular methylation reactions. Periconceptional maternal supplementation with folic acid can prevent occurrence of NTDs in the general population by up to 70%; currently several countries fortify their food supply with folic acid for the prevention of NTDs. Despite the unambiguous impact of folate status on NTD risk, the mechanism by which folic acid protects against NTDs remains unknown. Identification of the mechanism by which folate status affects neural tube closure will assist in developing more efficacious and better targeted preventative measures. In this review, we summarize current research on the relationship between folate status and NTDs, with an emphasis on linking genetic variation, folate nutriture, and specific metabolic and/or genomic pathways that intersect to determine NTD outcomes.

Hydrogen sulfide: neurochemistry and neurobiology

Current evidence suggests that hydrogen sulfide (H2S) plays an important role in brain functions, probably acting as a neuromodulator as well as an intracellular messenger. In the mammalian CNS, H2S is formed from the amino acid cysteine by the action of cystathionine beta-synthase (CBS) with serine (Ser) as the by-product. As CBS is a calcium and calmodulin dependent enzyme, the biosynthesis of H2S should be acutely controlled by the intracellular concentration of calcium. In addition, it is also regulated by S-adenosylmethionine which acts as an allosteric activator of CBS. H2S, as a sulfhydryl compound, has similar reducing properties as glutathione. In neurons, H2S stimulates the production of cAMP probably by direct activation of adenylyl cyclase and thus activate cAMP-dependent processes. In astrocytes, H2S increases intracellular calcium to an extent capable of inducing and propagating a "calcium wave", which is a form of calcium signaling among these cells. Possible physiological functions of H2S include potentiating long-term potentials through activation of the NMDA receptors, regulating the redox status, maintaining the excitatory/inhibitory balance in neurotransmission, and inhibiting oxidative damage through scavenging free radicals and reactive species. H2S is also involved in CNS pathologies such as stroke and Alzheimer's disease. In stroke, H2S appears to act as a mediator of ischemic injuries and thus inhibition of its production has been suggested to be a potential treatment approach in stroke therapy.

Genetic variation in genes of folate metabolism and neural-tube defect risk

Neural-tube defects (NTD) are common congenital malformations that can lead to severe disability or even death. Periconceptional supplementation with the B-vitamin folic acid has been demonstrated to prevent 50-70% of NTD cases. Since the identification of the first genetic risk factor of NTD, the C677T single-nucleotide polymorphism (SNP) in the methylenetetrahydrofolate reductase (MTHFR) gene, and the observation that elevated plasma homocysteine levels are associated with NTD, research has focused on genetic variation in genes encoding for enzymes of folate metabolism and the closely-related homocysteine metabolism. In the present review relevant SNP in genes that code for enzymes involved in folate transport and uptake, the folate cycles and homocysteine metabolism are summarised and the importance of these SNP discussed in relation to NTD risk.

Hydrogen sulphide regulates calcium homeostasis in microglial cells

Hydrogen sulphide (H2S), which is produced endogenously from L-cysteine in mammalian tissues, has been suggested to function as a neuromodulator in the brain. However, the role of H2S in microglial cells is unclear. In this study, the effect of exogenous and endogenous H2S on intracellular calcium homeostasis was investigated in primary cultured microglial cells. Sodium hydrosulphide (NaHS), a H2S donor, caused a concentration-dependent (0.1-0.5 mM) increase in intracellular calcium concentration ([Ca2+]i). This effect was significantly attenuated in the presence of a calcium-free extracellular solution, Gd3+ (100 microM), a nonselective Ca2+ channel blocker, or thapsigargin (2 microM), an inhibitor of the sarcoplasmic/endoplasmic reticulum Ca2+ -ATPase. These observations suggest that the increase in [Ca2+]i in response to H2S involves both calcium influx across the plasma membrane and calcium release from intracellular stores. The H2S-induced calcium elevation is partly attenuated by H-89, a selective cAMP-dependent protein kinase (PKA) inhibitor, but not by U73122, a phospholipase C (PLC) inhibitor, and chelerythrine, a selective protein kinase C (PKC) inhibitor, suggesting the involvement of cAMP/PKA, but not PLC/PKC/phosphoinositol-3,4,5-inositol (IP3) pathway. Using RT-PCR, only cystathionine gamma-lyase (CSE), a H2S producing enzyme, was detected in primary cultures of microglia. Lowering endogenous H2S level with, D,L-propargylglycine and beta-cyano-L-alanine, two CSE inhibitors, significantly decreased [Ca2+]i, suggesting that endogenous H2S may have a positive tonic influence on [Ca2+]i homeostasis. These findings support the possibility that H2S may serve as a neuromodulator to facilitate signaling between neurons and microglial cells.

The distribution of cystathionine beta-synthase (CBS) in the eye: implication of the presence of a trans-sulfuration pathway for oxidative stress defense

Clinical abnormalities in cystathionine-beta-synthase (CBS) deficiency, a key enzyme in the trans-sulfuration pathway, associate with many eye disorders. However, little is known about this enzyme in the eye. The goal of this study is to examine the distribution of CBS in the various regions of the eye, including conjunctiva, cornea, iris, lens, vitreous, retina and optic nerve using fresh eyes from both pigs (6 months) and humans (4-82 years). We have found that pig eye showed the highest CBS protein presence in cornea, conjunctiva and iris, followed by retina and optic nerve. The whole lens had a relatively lower amount and vitreous body had none. CBS protein distribution in the human eyes showed a similar pattern, with high level in the anterior segments but much lower amount in retina and optic nerve. CBS in anterior segments remained high throughout the lifespan, but retinal CBS showed a trend of age-dependent increase. The presence of CBS in human and pig eye tissues was further confirmed by RT-PCR, CBS activity assay, both showed similar distribution profiles as the Western blot analysis. This is the first evidence of the presence of CBS enzyme in the eye outside of the lens, which indicates that a functional trans-sulfuration pathway may be present in various eye tissues.

The production of transgenic mice expressing human cystathionine beta-synthase to study Down syndrome

Down syndrome (DS) is the most common genetic cause of significant cognitive disability. We hypothesize that by identifying metabolic alterations associated with cognitive impairment, it may be possible to develop medical or dietary interventions to ameliorate cognitive disabilities in persons with DS. Evidence suggests that one-carbon/transsulfuration (1C-TS) metabolism is abnormal in persons with DS. Cystathionine beta-synthase (CBS) plays a critical role in this metabolic system. The gene for CBS is on human chromosome 21, and there is evidence of elevated CBS enzyme activity in tissues and cells from individuals with DS. To analyze the possible role of CBS in Down syndrome, we have produced several lines of transgenic mice expressing the human CBS gene. We describe the use of Florescence Situ Hybridization (FISH) analysis to characterize the transgene insertion site for each line. Our initial expression analysis of each transgenic line by RT-PCR shows that the tissue specificity of human CBS mRNA levels in these mice may differ from the tissue specificity of mouse CBS mRNA levels in the same animals. These mice will be invaluable for assessing the regulation of the CBS gene and the role of CBS in cognition. They can also be used to develop therapies that target abnormalities in 1C-TS metabolism to improve cognition in persons with DS.

The role of cystathionine beta-synthase in homocysteine metabolism

Cystathionine beta-synthase (CBS) is the first enzyme in the transsulfuration pathway, catalyzing the conversion of serine and homocysteine to cystathionine and water. The enzyme contains three functional domains. The middle domain contains the catalytic core, which is responsible for the pyridoxal phosphate-catalyzed reaction. The C-terminal domain contains a negative regulatory region that is responsible for allosteric activation of the enzyme by S-adenosylmethionine. The N-terminal domain contains heme, and this domain regulates the enzyme in response to redox conditions. Besides its canonical reaction, CBS can catalyze alternative reactions that produce hydrogen sulfide, a novel neuromodulator in the brain. Mutations in human CBS result in homocystinuria, an autosomal recessive disorder characterized by defects in a variety of different organ systems. The most common CBS allele is 833T>C (I278T), which is associated with pyridoxine-responsive homocystinuria. A complementation system in S. cerevisiae has been developed for analysis of human CBS mutations. Using this system, it has been discovered that deletion of the C-terminal domain of CBS can suppress the functional defects of many patient-derived mutations. This finding suggests it may be possible to develop drugs that interact with the C-terminal domain of CBS to treat elevated homocysteine in humans.

Human embryonic stem cell methyl cycle enzyme expression: modelling epigenetic programming in assisted reproduction?

To investigate a possible mechanism for inducing epigenetic defects in the preimplantation embryo, a human embryonic stem cell model was developed, and gene expression of the key methyl cycle enzymes, MAT2A, MAT2B, GNMT, SAHH, CBS, CGL, MTR, MTRR, BHMT, BHMT2, mSHMT, cSHMT and MTHFR was demonstrated, while MAT1 was barely detectable. Several potential acceptors of cycle-generated methyl groups, the DNA methyltransferases (DNMT1, DNMT3A, DNMT3B and DNMT3L), glycine methyltransferase and the polyamine biosynthetic enzymes, SAM decarboxylase and ornithine decarboxylase, were also expressed. Expression of folate receptor alpha suggests a propensity for folate metabolism. Methotrexate-induced depletion of folate resulted in elevated intracellular homocysteine concentration after 7 days in culture and a concomitant increase in cysteine and glutathione, indicating clearance of homocysteine through the transulphuration pathway. These studies indicate that altered methyl group metabolism provides a potential mechanism for inducing epigenetic changes in the preimplantation embryo.

Sulfur amino acid metabolism: pathways for production and removal of homocysteine and cysteine

Tissue concentrations of both homocysteine (Hcy) and cysteine (Cys) are maintained at low levels by regulated production and efficient removal of these thiols. The regulation of the metabolism of methionine and Cys is discussed from the standpoint of maintaining low levels of Hcy and Cys while, at the same time, ensuring an adequate supply of these thiols for their essential functions. S-Adenosylmethionine coordinately regulates the flux through remethylation and transsulfuration, and glycine N-methyltransferase regulates flux through transmethylation and hence the S-adenosylmethionine/S-adenosylhomocysteine ratio. Cystathionine beta-synthase activity is also regulated in response to the redox environment, and transcription of the gene is hormonally regulated in response to fuel supply (insulin, glucagon, and glucocorticoids). The H2S-producing capacity of cystathionine gamma-lyase may be regulated in response to nitric oxide. Cys is substrate for a variety of anabolic and catabolic enzymes. Its concentration is regulated primarily by hepatic Cys dioxygenase; the level of Cys dioxygenase is upregulated in a Cys-responsive manner via a decrease in the rate of polyubiquitination and, hence, degradation by the 26S proteasome.

The dominant role of Sp1 in regulating the cystathionine beta-synthase -1a and -1b promoters facilitates potential tissue-specific regulation by Kruppel-like factors

Cystathionine beta-synthase (CBS) catalyzes the condensation of serine with homocysteine to form cystathionine and occupies a crucial regulatory position between the methionine cycle and transsulfuration. The human cystathionine beta-synthase gene promoters -1a and -1b are expressed in a limited number of tissues and are coordinately regulated with proliferation through a redox-sensitive mechanism. Site-directed mutagenesis, DNase I footprinting and deletion analysis of 5276 bp of 5' proximal -1b flanking sequence revealed that this region does not confer tissue-specific expression and that 210 bp of proximal sequence is sufficient for maximal promoter activity. As little as 32 bp of the -1b proximal promoter region is capable of driving transcription in HepG2 cells, and this activity is entirely dependent upon the presence of a single overlapping Sp1/Egr1 binding site. Co-transfection studies in Drosophila SL2 cells indicated that both promoters are transactivated by Sp1 and Sp3 but only the -1b promoter is subject to a site-specific synergistic regulatory interaction between Sp1 and Sp3. Sp1-deficient fibroblasts expressing both Sp3 and NF-Y were negative for CBS activity. Transfection of these cells with a mammalian Sp1 expression construct induced high levels of CBS activity indicating that Sp1 has a critical and indispensable role in the regulation of cystathionine beta-synthase. Sp1 binding to both CBS promoters is sensitive to proliferation status and is negatively regulated by Kruppel-like factors in co-transfection experiments suggesting a possible mechanism for the tissue specific regulation of cystathionine beta-synthase.

Expression of the cystathionine beta synthase (CBS) gene during mouse development and immunolocalization in adult brain

Hyperhomocysteinemia, caused by a lack of cystathionine beta synthase (CBS), leads to elevated plasma concentrations of homocysteine. This is a common risk factor for atherosclerosis, stroke, and possibly neurodegenerative diseases. However, the mechanisms that link hyperhomocysteinemia due to CBS deficiency to these diseases are still unknown. Early biochemical studies describe developmental and adult patterns of transsulfuration and CBS expression in a variety of species. However, there is incomplete knowledge about the regional and cellular expression pattern of CBS, notably in the brain. To complete the previous data, we used in situ hybridization and Northern blotting to characterize the spatial and temporal patterns of Cbs gene expression during mouse development. In the early stages of development, the Cbs gene was expressed only in the liver and in the skeletal, cardiac, and nervous systems. The expression declined in the nervous system in the late embryonic stages, whereas it increased in the brain after birth, peaking during cerebellar development. In the adult brain, expression was strongest in the Purkinje cell layer and in the hippocampus. Immunohistochemical analyses showed that the CBS protein was localized in most areas of the brain but predominantly in the cell bodies and neuronal processes of Purkinje cells and Ammon's horn neurons.

Cystathionine beta-synthase is coordinately regulated with proliferation through a redox-sensitive mechanism in cultured human cells and Saccharomyces cerevisiae

Cystathionine beta-synthase (CBS) catalyzes the condensation of serine with homocysteine to form cystathionine and occupies a crucial regulatory position between the methionine cycle and the biosynthesis of cysteine by transsulfuration. Analysis of CBS activity under a variety of growth conditions indicated that CBS is coordinately regulated with proliferation in both yeast and human cells. In batch cultures of Saccharomyces cerevisiae, maximal CBS activities were observed in the exponential phase of cells grown on glucose, while growth-arrested cultures or those growing non-fermentatively on ethanol or glycerol had approximately 3-fold less activity. CBS activity assays and Western blotting indicated that growth-specific regulation of CBS is evolutionarily conserved in a range of human cell lines. CBS activity was found to be maximal during proliferation and was reduced two- to five-fold when cells became quiescent at confluence. In cultured HepG2 cells, the human CBS gene is induced by serum and basic fibroblast growth factor and is downregulated, but not abolished, by contact inhibition, serum-starvation, nutrient depletion, or the induction of differentiation. Consequently, for certain cell types, CBS may represent a novel marker of both differentiation and proliferation. The intracellular level of the CBS regulator compound, S-adenosylmethionine, was found to reflect the proliferation status of both yeast and human cells, and as such, constitutes an additional mechanism for proliferation-specific regulation of human CBS. Our data indicates that screening compounds for the ability to affect transsulfuration in cultured cell models must take proliferation status into account to avoid masking regulatory interactions that may be of significance in vivo.

High homocysteine and thrombosis without connective tissue disorders are associated with a novel class of cystathionine beta-synthase (CBS) mutations

Cystathionine beta-synthase (CBS) is a crucial regulator of plasma levels of the thrombogenic amino acid homocysteine (Hcy). Homocystinuria due to CBS deficiency confers a dramatically increased risk of thrombosis. Early diagnosis usually occurs after the observation of ectopia lentis, mental retardation, or characteristic skeletal abnormalities. Homocystinurics with this phenotype typically carry mutations in the catalytic region of the protein that abolish CBS activity. We describe a novel class of missense mutations consisting of I435T, P422L, and S466L that are located in the non-catalytic C-terminal region of CBS that yield enzymes that are catalytically active but deficient in their response to S-adenosylmethionine (AdoMet). The P422L and S466L mutations were found in patients suffering premature thrombosis and homocystinuric levels of Hcy but lacking any of the connective tissue disorders typical of homocystinuria due to CBS deficiency. The P422L and S466L mutants demonstrated a level of CBS activity comparable to that of the AdoMet stimulated wild-type CBS but could not be further induced by the addition of AdoMet. In terms of temperature stability, oligomeric organization, and heme saturation the I435T, P422L, and S466L mutants are indistinguishable from wild-type CBS. Our findings illustrate the importance of AdoMet for the regulation of Hcy metabolism and are consistent with the possibility that the characteristic connective tissue disturbances observed in homocystinuria due to CBS deficiency may not be due to elevated Hcy.

Spina bifida and folate-related genes: a study of gene-gene interactions

PURPOSE

To assess whether interactions of common alleles of two folate genes contribute to spina bifida risk.

METHODS

Case-control study, comparing 203 children with spina bifida to 583 controls.

RESULTS

Homozygosity for the 677C-T allele of 5,10-methylenetetrahydrofolate reductase (MTHFR) alone was associated with an odds ratio for spina bifida of 1.57 (95% confidence interval [CI], 1.02-2.38). For the 844ins68 allele of cystathionine-beta-synthase alone, the odds ratio was 0.83 (95% CI, 0.39-1.64). For the joint genotype, the odds ratio was 3.69 (95% CI, 1.04-13.50).

CONCLUSIONS

Interactions between common alleles of folate genes might contribute to the risk for spina bifida.

Folate, homocysteine and neural tube defects: an overview

Folate administration substantially reduces the risk on neural tube detects (NTD). The interest for studying a disturbed homocysteine (Hcy) metabolism in relation to NTD was raised by the observation of elevated blood Hcy levels in mothers of a NTD child. This observation resulted in the examination of enzymes involved in the folate-dependent Hcy metabolism. Thus far, this has led to the identification of the first and likely a second genetic risk factor for NTD. The C677T and A1298C mutations in the methylenetetrahydrofolate reductase (MTHFR) gene are associated with an increased risk of NTD and cause elevated Hcy concentrations. These levels can be normalized by additional folate intake. Thus, a dysfunctional MTHFR partly explains the observed elevated Hcy levels in women with NTD pregnancies and also, in part, the protective effect of folate on NTD. Although the MTHFR polymorphisms are only moderate risk factors, population-wide they may account for an important part of the observed NTD prevalence.

Lack of embryotoxicity of homocysteine thiolactone in mouse embryos in vitro

Recent work from humans and chick embryos has suggested that homocysteine may play a role in producing neural tube defects (NTDs). In an effort to determine if homocysteine is able to produce NTDs in mammalian embryos, mouse embryos were explanted on GD 8 and cultured for 44 h. When either homocysteine or homocysteine thiolactone was added to the culture medium, treated embryos developed as well as controls and had closed neural tubes. Homocysteine thiolactone was also microinjected into the amniotic sac of mouse embryos. Again, development proceeded normally with no significant increase in the number of embryos with open neural tubes at the end of the culture period. HPLC analysis of embryonic thiols 24 h after microinjection revealed a significant increase in embryonic cystathionine levels. These data suggest that homocysteine does not produce NTDs in mouse embryos cultured in vitro and that early organogenesis-stage embryos are able to metabolize homocysteine.

Antisense modulation of 5,10-methylenetetrahydrofolate reductase expression produces neural tube defects in mouse embryos

The role of folate metabolism in producing neural tube defects (NTDs) in humans is unknown. In the current study, antisense oligodeoxyribonucleotide technology was utilized to disrupt normal expression of the gene for 5,10-methylenetetrahydrofolate reductase (MTHFR) in organogenesis-stage mouse embryos. Two different antisense probes were microinjected into the amniotic sac of gestation day (GD) 8 mouse embryos with PBS or scrambled sense oligodeoxyribonucleotides injected into control embryos. Concentration-dependent increases in the frequencies of embryos with NTDs were observed for both antisense sequences. The level of mRNA for MTHFR was decreased in embryos treated with the higher concentration of one antisense sequence, indicating that the sequence is able to decrease gene expression. 5-methyltetrahydrofolate, the product of the MTHFR reaction, was able to decrease the incidence of antisense-induced NTDs, but co-injection with L-methionine did not. These results suggest that reduced expression of MTHFR may play a role in producing NTDs.