Phenytoin-induced teratogenesis: a molecular basis for the observed developmental delay during neurulation

Trim69 regulates zebrafish brain development by ap-1 pathway

Proteins belonging to the TRIM family have been implicated in a variety of cellular processes such as apoptosis, differentiation, neurogenesis, muscular physiology and innate immune responses. Trim69, previously identified as a novel gene cloned from a human testis cDNA library, has a homologous gene in zebrafish and this study focused on investigating the function of trim69 in zebrafish neurogenesis. Trim69 was found to be expressed in zebrafish embryo brain at the early stages. Knockdown of trim69 led to deformed brain development, obvious signs of apoptosis present in the head, and decreased expression of neuronal differentiation and stem cell markers. This phenotype was rescued upon co-injection of human mRNA together along with the trim69 knockdown. Results of this study also showed an interaction between TRIM69 and c-Jun in human cells, and upon TRIM69 knock down c-Jun expression subsequently increased, whereas the over-expression of TRIM69 led to the down-regulation of c-Jun. Additionally, knockdown both c-Jun and trim69 can rescue the deformed brain, evident cellular apoptosis in the head and decreased expression of neuronal differentiation and stem cell markers. Overall, our results support a role for trim69 in the development of the zebrafish brain through ap-1 pathway.

DACT1 is involved in human placenta development by promoting Wnt signaling

OBJECTIVE

The aim of this study was to evaluate the expression of DACT1 in human placenta tissue and the relationship between DACT1 and target genes of the Wnt signaling pathway.

METHOD

Real-time PCR and western blotting were used to detect the expression of DACT1 and the target genes of Wnt signaling pathway in human placenta tissue. And the relationship between them was analyzed using SPSS 19.

RESULTS

Real-time PCR results showed that DACT1 expression was significantly higher in 49- to 71-day placenta tissues (mean value = 0.020) than that in 39- to 48-day (the mean value = 0.009). The mRNA expressions of the Wnt signaling pathway target genes, CCND1, CCND2, FOSL1, DAB2 and JUN, were also increased expressed in human placenta tissues. Significant positive associations between DACT1 and CCND1, CCND2, FOSL1, DAB2 and JUN were observed. Western blotting analysis showed that the protein expression of DACT1, CCND1, CCND2, FOSL1, DAB2 and JUN displayed the increasing trend in 43-, 49- and 71-day placenta samples.

CONCLUSION

DACT1 might play an important role in human placenta development via promoting Wnt signaling.

AP-1/c-Jun transcription factors: regulation and function in malignant melanoma

Malignant melanoma is an aggressive form of skin cancer with an increasing incidence worldwide. One way to address the pathology of the disease is through molecular research. In addition to the analysis of melanoma-relevant signaling pathways, the investigation of important transcription factors is a fundamental objective. The AP-1 transcription factor family is known to play an important role in melanoma progression and development. The AP-1 family member c-Jun is highly expressed and active in melanoma cells, and the mechanisms and signaling pathways regulating c-Jun protein are diverse. In addition to the common regulation and activation of c-Jun by mitogen-activated protein kinases (MAPKs), there are several other signaling pathways and interactions leading to c-Jun protein expression and thus AP-1 activation. In malignant melanoma, and many other cancer types, c-Jun has mainly oncogenic functions; however, other AP-1 proteins also have anti-oncogenic roles. Interestingly, several studies have revealed that a strong AP-1 activity in melanoma mainly depends on c-Jun. Recently, it has also been shown that the c-Jun protein is regulated and activated by several other mechanisms, including miRNAs and the cytoskeleton. In summary, there are a variety of mechanisms underlying the induction of c-Jun protein expression and activity leading to tumor progression and development, and this diverse regulatory machinery is due to the heterogeneity of different tumor types, particularly in malignant melanoma.

Valproic acid downregulates RBP4 and elicits hypervitaminosis A-teratogenesis--a kinetic analysis on retinol/retinoic acid homeostatic system

Background

Valproic acid (VPA) is an antiepileptic and anti-migraine prophylactic drug. VPA exhibits two severe side effects, namely acute liver toxicity and teratogenicity. These side effects are usually seen at the genetic and somatic levels. The cited action mechanisms involve inhibition of histone deacetylase, hypofolatenemia, hyperhomocysteinemia, and reactive oxidative stress. The proteomic information associated with VPA teratogenicity is still unavailable. We hypothesized that proteomic analysis might help us identify functional proteins that could be relevantly affected by VPA, and this phenomenon could be very sensitive in early embryonic stage, resulting in VPA teratogenicity.

Methodology/Principal Findings

Proteomic analysis on the chicken embryos at Hamburger and Hamilton (HH) stage 28 showed that there were significant downregulations of ovotransferrins, carbonic anhydrase-2, retinol binding protein-4 (RBP4), NADH cytochrome b5 reductase 2 (CYB5R2), apolipoprotein A1, and protein SET, together with upregulation of 60S ribosomal protein L22. Among these, RBP4 was the most significantly downregulated (−32%). Kinetic analysis suggested that this situation could trigger hypervitaminosis A (+39.3%), a condition that has been well known to induce teratogenesis..

Conclusions/Significance

This is the first report showing that VPA dowregulates RBP4. Our finding not only has led to a possible mechanism of VPA teratogenesis, but also has initiated new preventive strategies for avoiding VPA teratogeneis.

Multiple transcription factor families regulate axon growth and regeneration

Understanding axon regenerative failure remains a major goal in neuroscience, and reversing this failure remains a major goal for clinical neurology. Although an inhibitory central nervous system environment clearly plays a role, focus on molecular pathways within neurons has begun to yield fruitful insights. Initial steps forward investigated the receptors and signaling pathways immediately downstream of environmental cues, but recent work has also shed light on transcriptional control mechanisms that regulate intrinsic axon growth ability, presumably through whole cassettes of gene target regulation. Here we will discuss transcription factors that regulate neurite growth in vitro and in vivo, including p53, SnoN, E47, cAMP-responsive element binding protein (CREB), signal transducer and activator of transcription 3 (STAT3), nuclear factor of activated T cell (NFAT), c-Jun activating transcription factor 3 (ATF3), sex determining region Ybox containing gene 11 (Sox11), nuclear factor κ-light chain enhancer of activated B cells (NFκB), and Krüppel-like factors (KLFs). Revealing the similarities and differences among the functions of these transcription factors may further our understanding of the mechanisms of transcriptional regulation in axon growth and regeneration.

Effect of sodium phenytoin concentration on neural tube development in the early stages of chicken embryo development

Animal and human research has shown that anticonvulsants are teratogens and pose a risk of causing fetal malformations. In various studies, the teratogenic effects of sodium phenytoin (PTH) in several systems have been investigated. Toe and finger, renal, and even facial malformations have been described in the literature. However, there is debate about whether the true risk of teratogenesis is lower or higher than previously reported for PTH. There is also little published information on the effect of this agent on neural tube closure in an embryological model. In this study, 0.1 mL of three different concentrations of PTH solution (mg/mL: 1, 3, 5) or vehicle was applied under the embryonic disc of specific pathogen-free Leghorn chicken embryos after 24 hours' incubation. Incubation was continued until 72 hours of maturation. At 72 hours, all embryos were evaluated macroscopically and microscopically. There were serious neural tube closure defects in the embryos administered large amounts (0.5 mg) of PTH, but doses of 0.1 mg (subtherapeutic concentration for humans) and 0.3mg (therapeutic concentration for humans) produced no statistically significant defects (p=0.05). The difference between the defects in the high concentration group and the other three groups was statistically significant. In our study PTH administered in a strict concentration regimen produced a lower level of neural tube closure-related defects than previously reported.

Cognitive/behavioral teratogenetic effects of antiepileptic drugs

The majority of children of mothers with epilepsy are normal, but they are at increased risk for developmental delay. Antiepileptic drugs (AEDs) appear to play a role. Our current knowledge is reviewed, including research design issues and recommendations for future research. In animals, exposure of the immature brain to some AEDs can produce widespread neuronal apoptosis and behavioral deficits. The risks of AEDs in humans are less clear, but recent studies raise concerns, especially for valproate. There is a critical need for well-designed systematic research to improve our understanding of AED effects on the fetal brain.

Teratogenicity of valproate conjugates with anticonvulsant activity in mice

Valproic acid (VPA) is an effective antiepileptic medication, the use of which in females of childbearing age is complicated by its ability to induce birth defects, including neural tube defects (NTDs), in exposed embryos. In experimental settings, VPA reproducibly induces NTDs in laboratory animals such as the highly inbred SWV/Fnn mice. In search of new, efficacious derivatives of VPA that lack toxicity, the conjugates of VPA with amantadine (VPA-AMA) and N-3-aminopropyl-2-pyrrolidinone (VPA-PYR) have been synthesized and evaluated for their anticonvulsant activity. In the present study, the authors evaluated the teratogenicity potential of VPA-AMA and VPA-PYR using a well-established mouse model for antiepileptic drug teratogenicity. All tested compounds were injected intraperitoneally to pregnant dams on gestational day 8.5, and the fetuses examined on day 18.5. At the highest dose tested (3.61 mmol/kg), VPA-PYR was maternally lethal, whereas VPA-AMA induced excessive embryonic lethality. At a dose of 2.20 mmol/kg, VPA-PYR was not teratogenic to the exposed embryos; VPA-AMA induced NTDs in 8.2% of embryos, VPA caused 5.5% NTDs. 0.80 mmol/kg amantadine induced NTDs in 2.2% of the exposed fetuses. In conclusion, VPA-AMA has a comparable teratogenicity as does VPA, and it is proposed that the teratogenicity of VPA-AMA is due to the parent compound. Additional studies are needed to fully define and understand the structure-teratogenicity relationships of VPA analogues.

Developmental outcome of levetiracetam, its major metabolite in humans, 2-pyrrolidinone N-butyric acid, and its enantiomer (R)-alpha-ethyl-oxo-pyrrolidine acetamide in a mouse model of teratogenicity

PURPOSE

The purpose of this study was to test the teratogenic potential of the antiepileptic drug (AED) levetiracetam (LEV), its major metabolite in humans, 2-pyrrolidone-N-butyric acid (PBA), and enantiomer, (R)-alpha-ethyl-oxo-pyrrolidine acetamide (REV), in a well-established mouse model.

METHODS

All compounds were administered by intraperitoneal injections once daily to SWV/Fnn mice on gestational days 8-1/2 to 12-1/2. LEV was administered at doses of 600, 1,200, and 2,000 mg/kg/day, piracetam (PIR) and PBA, at 600 and 1,200 mg/kg/day, and REV, at 600 mg/kg/day. On gestational day 18(1/2), fetuses were examined for gross external malformations and prepared for skeletal analysis by using Alizarin Red S staining.

RESULTS

No significant gross external malformations were observed in any of the study groups. Fetal weights were significantly reduced in most study groups. Resorption rates were significantly increased only in the 2,000-mg/kg/day LEV group. The overall incidence of skeletal abnormalities and specifically of hypoplastic phalanges was significantly increased in both PBA treatments and in the intermediate 1,200-mg/kg/day LEV group. In contrast to that in humans, 24-h urinary excretion analysis in mice showed that 65-100% of the LEV doses were excreted unchanged, whereas only 4% was excreted as the metabolite PBA.

CONCLUSIONS

Results of this study demonstrate that both LEV and its major metabolite in humans, PBA, do not induce major structural malformations in developing SWV/Fnn embryos and suggest that they provide a margin of reproductive safety for the pregnant epileptic population when compared with other AEDs tested in this mouse model.

Anticonvulsant activity, teratogenicity and pharmacokinetics of novel valproyltaurinamide derivatives in mice

1 The purpose of this study was to synthesize novel valproyltaurine (VTA) derivatives including valproyltaurinamide (VTD), N-methyl-valproyltaurinamide (M-VTD), N,N-dimethyl-valproyltaurinamide (DM-VTD) and N-isopropyl-valproyltaurinamide (I-VTD) and evaluate their structure-pharmacokinetic-pharmacodynamic relationships with respect to anticonvulsant activity and teratogenic potential. However, their hepatotoxic potential could not be evaluated. The metabolism and pharmacokinetics of these derivatives in mice were also studied. 2 VTA lacked anticonvulsant activity, but VTD, DM-VTD and I-VTD possessed anticonvulsant activity in the Frings audiogenic seizure susceptible mice (ED(50) values of 52, 134 and 126 mg kg(-1), respectively). 3 VTA did not have any adverse effect on the reproductive outcome in the Swiss Vancouver/Fnn mice following a single i.p. injection of 600 mg kg(-1) on gestational day (GD) 8.5. VTD (600 mg kg(-1) at GD 8.5) produced an increase in embryolethality, but unlike valproic acid, it did not induce congenital malformations. DM-VTD and I-VTD (600 mg kg(-1) at GD 8.5) produced a significant increase in the incidence of gross malformations. The incidence of birth defects increased when the length of the alkyl substituent or the degree of N-alkylation increased. 4 In mice, N-alkylated VTDs underwent metabolic N-dealkylation to VTD. DM-VTD was first biotransformed to M-VTD and subsequently to VTD. I-VTD's fraction metabolized to VTD was 29%. The observed metabolic pathways suggest that active metabolites may contribute to the anticonvulsant activity of the N-alkylated VTDs and reactive intermediates may be formed during their metabolism. In mice, VTD had five to 10 times lower clearance (CL), and three times longer half-life than I-VTD and DM-VTD, making it a more attractive compound than DM-VTD and I-VTD for further development. VTD's extent of brain penetration was only half that observed for the N-alkylated taurinamides suggesting that it has a higher intrinsic activity that DM-VTD and I-VTD. 5 In conclusion, from this series of compounds, although VTD caused embryolethality, this compound emerged as the most promising new antiepileptic drug, having a preclinical spectrum characterized by the highest anticonvulsant potential, lowest potential for teratogenicity and favorable pharmacokinetics.

The effects of folic acid in the prevention of neural tube development defects caused by phenytoin in early chick embryos

STUDY DESIGN

The effects of phenytoin and folic acid on the development of neural tube defects in early chick embryos were studies.

OBJECTIVE

To investigate the effects of folic acid in the prevention of neural tube development defects.

SUMMARY OF BACKGROUND DATA

Several studies have shown that phenytoin selectively inhibits neural tube closure. Folic acid supplementation has been reported to decrease the occurrence of neural tube defects.

METHODS

This study shows the effects of folic acid in preventing neural tube development defects caused by phenytoin in chicks based on light microscopy, transmission electron microscopy, and histopathological examination. Forty-five fertile Hubbard Broil eggs, all at Stage 8 (four somite) of development, were divided into three equal groups: Group 1 embryos (n = 15), the control group, were explanted and grown for 18 hours in a nutrient medium (thin albumin). Group 2 embryos (n = 15) were explanted and grown for 18 hours in a nutrient medium containing 500 microg/mL of phenytoin. Group 3 embryos (n = 15) were explanted and grown for 18 hours in a nutrient medium containing 500 microg/mL of phenytoin and 0.4 microg/mL of folic acid.

RESULTS

After the incubation period, 86.6% of the control embryos (Group 1) had intact neural tubes; 80% of Group 2 and 46.6% of Group 3 embryos showed neural tube defects.

CONCLUSIONS

The results of this study suggest that phenytoin causes neural tube defects, whereas folic acid decreases the incidence of neural tube development defects caused by phenytoin in early chick embryos.

Neurodevelopmental effects of antiepileptic drugs

Although the vast majority of children born to women with epilepsy are normal, these children are at increased risk for both anatomic and cognitive impairments. Current evidence suggests that the defects are the result of in utero antiepileptic drug (AED) exposure combined with a genetic predispositon. However, the exact mechanisms underlying these effects remain to be delineated. AED polytherapy increases the risk, but it remains uncertain if specific AEDs pose an overall greater threat. Most women with epilepsy cannot avoid AEDs during pregnancy because of the greater risks posed by seizures to the mother and fetus. Therefore, current recommendations emphasize definitive diagnosis and the use of AED monotherapy at the lowest effective dose if treatment is indicated. Prenatal folate and multivitamins should also be given routinely to women of childbearing age who require AED therapy.

Long term health and neurodevelopment in children exposed to antiepileptic drugs before birth

OBJECTIVE

To investigate the frequency of neonatal and later childhood morbidity in children exposed to antiepileptic drugs in utero.

DESIGN

Retrospective population based study.

SETTING

Population of the Grampian region of Scotland.

PARTICIPANTS

Mothers taking antiepileptic drugs in pregnancy between 1976 and 2000 were ascertained from hospital obstetric records and 149 (58% of those eligible) took part. They had 293 children whose health and neurodevelopment were assessed.

MAIN OUTCOME MEASURES

Frequencies of neonatal withdrawal, congenital malformations, childhood onset medical problems, developmental delay, and behaviour disorders.

RESULTS

Neonatal withdrawal was seen in 20% of those exposed to antiepileptic drugs. Congenital malformations occurred in 14% of exposed pregnancies, compared with 5% of non-exposed sibs, and developmental delay in 24% of exposed children, compared with 11% of non-exposed sibs. After excluding cases with a family history of developmental delay, 19% of exposed children and 3% of non-exposed sibs had developmental delay, 31% of exposed children had either major malformations or developmental delay, 52% of exposed children had facial dysmorphism compared with 25% of those not exposed, 31% of exposed children had childhood medical problems (13% of non-exposed sibs), and 20% had behaviour disorders (5% of non-exposed).

CONCLUSION

Prenatal antiepileptic drug exposure in the setting of maternal epilepsy is associated with developmental delay and later childhood morbidity in addition to congenital malformation.

Anticonvulsant profile and teratogenicity of N-methyl-tetramethylcyclopropyl carboxamide: a new antiepileptic drug

PURPOSE

The studies presented here represent our efforts to investigate the anticonvulsant activity of N-methyl-tetramethylcyclopropyl carboxamide (M-TMCD) and its metabolite tetramethylcyclopropyl carboxamide (TMCD) in various animal (rodent) models of human epilepsy, and to evaluate their ability to induce neural tube defects (NTDs) and neurotoxicity.

METHODS

The anticonvulsant activity of M-TMCD and TMCD was determined after intraperitoneal (i.p.) administration to CF#1 mice, and either oral or i.p. administration to Sprague-Dawley rats. The ability of M-TMCD and TMCD to block electrical-, chemical-, or sensory-induced seizures was examined in eight animal models of epilepsy. The plasma and brain concentrations of M-TMCD and TMCD were determined in the CF#1 mice after i.p. administration. The induction of NTDs by M-TMCD and TMCD was evaluated after a single i.p. administration at day 8.5 of gestation in a highly inbred mouse strain (SWV) that is susceptible to valproic acid-induced neural tube defects.

RESULTS

In mice, M-TMCD afforded protection against maximal electroshock (MES)-induced, pentylenetetrazol (Metrazol)-induced, and bicuculline-induced seizures, as well as against 6-Hz "psychomotor" seizures and sound-induced seizures with ED50 values of 99, 39, 81, 51, and 10 mg/kg, respectively. In rats, M-TMCD effectively prevented MES- and Metrazol-induced seizures and secondarily generalized seizures in hippocampal kindled rats (ED50 values of 82, 45, and 39 mg/kg, respectively). Unlike M-TMCD, TMCD was active only against Metrazol-induced seizures in mice and rats (ED50 values of 57 and 52 mg/kg, respectively). Neither M-TMCD nor TMCD was found to induce NTDs in SWV mice.

CONCLUSIONS

The results obtained in this study show that M-TMCD is a broad-spectrum anticonvulsant drug that does not induce NTDs and support additional studies to evaluate its full therapeutic potential.

A clinical study of 57 children with fetal anticonvulsant syndromes

BACKGROUND

Anticonvulsants taken in pregnancy are associated with an increased risk of malformations and developmental delay in the children. To evaluate the pattern of abnormalities associated with prenatal anticonvulsant exposure further, we undertook a clinical study of 57 children with fetal anticonvulsant syndromes.

METHODS

Fifty two children were ascertained through the Fetal Anticonvulsant Syndrome Association and five were referred to the Aberdeen Medical Genetics Service. Pregnancy and medical history were obtained through a standardised questionnaire and interview and the children were examined.

RESULTS

Thirty four (60%) were exposed in utero to valproate alone, four (7%) to carbamazepine alone, four (7%) to phenytoin alone, and 15 (26%) to more than one anticonvulsant. Forty six (81%) reported behavioural problems, 22 (39%) with hyperactivity or poor concentration of whom four (7%) had a diagnosis of attention deficit and hyperactivity disorder. Thirty four (60%) reported two or more autistic features, of whom four had a diagnosis of autism and two of Asperger's syndrome. Forty four (77%) had learning difficulties, 46 (81%) had speech delay, 34 (60%) had gross motor delay, and 24 (42%) had fine motor delay. Nineteen (33%) had glue ear and 40 (70%) had joint laxity involving all sizes of joints. Of 46 who had formal ophthalmic evaluation, 16 (34%) had myopia.

CONCLUSIONS

Speech delay, joint laxity, glue ear, and myopia are common in the fetal anticonvulsant syndromes and autistic features and hyperactivity form part of the behavioural phenotype.

Developmental lead exposure disturbs expression of synaptic neural cell adhesion molecules in herring gull brains

Neurobehavioral testing of herring gull chicks (Larus argentatus) in both laboratory and field studies indicates that lead exposure during critical periods of development causes neurological deficits that may compromise survival in the wild. Accumulating evidence suggests that lead impairs neurodevelopment, in part, by altering the expression of cell adhesion molecules (CAMs) responsible for the proper formation and maintenance of neural structure and synaptic function. We examined the adhesion molecules NCAM, L1, and N-cadherin in gull brains to determine whether these CAMs are altered by lead exposure and might serve as markers of developmental neurotoxicity. One-day-old chicks were collected from nesting colonies and were laboratory housed. On post-hatching day (PHD) 2, chicks were given 100 mg/kg lead acetate or saline (intraperitoneally). Birds were killed on PHD 34, 44, or 55 (blood-lead levels averaged 27.4, 20.8, and 19.5 microg/dl, respectively). Brains were removed and stored at -70 degrees C until analysis. Expression of CAMs was determined in synaptosomal preparations by Western blotting and the activity of NCAM-associated sialyltransferase (ST) was determined in purified whole brain golgi apparatus. Elevation in synaptosomal polysialylated NCAM expression and a significant increase in golgi ST activity was observed in lead-treated animals at PHD 34. Reductions in synaptosomal N-cadherin were observed at PHD 34 and 44, while L1 expression appeared unaffected by lead at any time-point. By 55 days post-hatching, no differences in N-cadherin expression, polysialylated NCAM expression or NCAM-associated ST activity were seen in lead-treated animals as compared with age-matched control animals. Lead-induced disruption of CAM expression during early neurodevelopment may contribute to behavioral deficits observed in herring gulls in both the laboratory and the field, and may serve as a marker for heavy metal exposure during postnatal development.

Pharmacogenetic screening for susceptibility to fetal malformations in women

OBJECTIVE

To present a review of the literature and research on the pharmacogenetics of congenital defects, with a focus on the need for predictive maternal genotype assays.

DATA SOURCE

MEDLINE searches (January 1985-January 1999), past reference reviews, and unpublished research.

STUDY SELECTION

Review of relevant human, animal, and basic science studies.

DATA EXTRACTION

Data on research on polymorphisms, genotyping, cytochrome P450 enzyme systems, epoxide hydrolase, folate metabolism, metabolism of anticonvulsant medications, molecular genetics of neural tube defects, variations in drug metabolism, and environmental exposures were evaluated.

DATA SYNTHESIS

Data synthesis includes not only a review of the literature but suggests ways such data might be used to facilitate the development of maternal genotype assays, with the goal of preventing birth defects.

CONCLUSIONS

Individuals vary in how they metabolize drugs and handle toxic environmental exposures. In an ideal pregnancy, there is no or limited exposure to medications and environmental agents. However, in women with chronic medical conditions such as heart disease and seizures, this is often not possible. Unfortunately, no techniques have been available to identify those at risk in this population. Gene polymorphisms for a specific enzyme may result in an absence or reduction in the level of enzyme activity or in no change at all, with little effect on the structure/function of the gene product(s); they are not associated with clinical phenotypes in either the mother or the fetus. Other polymorphisms may be only markers. Thus, developing genotyping assays for women that are predictive of phenotype expression in the fetus is the key to screening for polymorphisms. As more mutations are identified and clinical, pharmacologic, biologic, and pharmacokinetic relationships are established, using these polymorphisms to develop a genotyping assay for women may become a clinical reality, possibly leading to preventive prepregnancy or prenatal treatment that may play an increasingly effective role in maternal care.

Apparent lability of neural tube closure in laboratory animals and humans

Neural tube defects (NTDs), a set of structural abnormalities affecting the brain, spinal cord, and the skeletal and connective tissues that protect them, are common malformations among humans and laboratory animals. The embryogenesis of the neural tube is presented to convey the complexity of the phenomenon, the multiplicity of requisite cellular and subcellular processes, and the precise timing of events that must occur for successful neural tube development. Interruption, even transitory, of any of these intricate processes or disruption of an embryo's developmental schedule can lead to an NTD. The population distribution of human NTDs demonstrates that genetic predisposition functions in susceptibility to NTDs. Data from animal studies support these concepts. NTDs are common outcomes in developmental toxicity safety assessments, occurring among control and treated groups. Numerous agents have caused increased levels of NTDs in laboratory animals, and species with shorter gestational periods appear more prone to toxicant-induced NTDs than those with longer gestations. Data from post-implantation whole embryo culture, although not predictive of human risk, are useful in studying neurulation mechanisms and in demonstrating the importance of maintaining embryonic schedules of development. We conclude that the concept that NTDs are produced by only a few toxicants that selectively target the developing nervous system is untenable. Rather, the combination of the time in gestation that an agent is applied, its dose, and its ability to disrupt critical processes in neurulation leads to NTDs. We further conclude that, because of both the relatively high prevalence and the multifactorial nature of NTDs, the mere occurrence of an NTD is insufficient for inferring that the defect was caused by an exogenous agent.

Phenytoin-induced alterations in craniofacial gene expression

In utero exposure to the anticonvulsant drug phenytoin has been shown to alter normal embryonic development, leading to a pattern of dysmorphogenesis known as the Fetal Hydantoin Syndrome. This embryopathy is characterized by growth retardation, microcephaly, mental deficiency, and craniofacial malformations, although the precise mechanism(s) by which phenytoin alters normal developmental pathways remains unknown. To better understand the molecular events involved in the pathogenesis of phenytoin-induced congenital defects, alterations in gene expression were examined during critical periods of craniofacial development. Pregnant SWV mice were administered phenytoin (60 mg/kg/day) from gestational day 6.5 until they were sacrificed at selected developmental time points. Tissue from the craniofacial region of control and exposed embryos was isolated, and samples were subjected to in situ transcription, antisense RNA amplification, and hybridization on reverse Northern blots to quantitatively assess expression of 36 candidate genes. Chronic phenytoin exposure significantly altered expression of several genes at distinct times during morphogenesis. Results of these studies show that expression of the retinoic acid receptors (RAR) alpha, beta, and gamma were significantly increased by phenytoin exposure. Elevations in gene expression of laminin beta 1, and the growth factors IGF-2, TGF alpha, and TGF beta 1, were also demonstrated in the craniofacial region of phenytoin-exposed embryos. As several of these genes are transcriptionally regulated by retinoic-acid-responsive elements in their promoter regions, phenytoin-induced alterations in expression of the RAR isoforms may have severe downstream consequences in the regulation of events necessary for normal craniofacial development. Such alterations occurring coordinately at critical times during craniofacial development may account for the dysmorphogenesis often associated with phenytoin exposure.

Pharmacogenomics - it's not just pharmacogenetics

New technologies in both combinatorial chemistry and combinatorial biology promise to unlock new opportunities for drug discovery and lead optimisation. Using such genome-based technologies to measure the dynamic properties of pharmacological systems, pharmacogenomics can now provide an objective measure of a drug's biological efficacy, including its potential adverse effects.

Corticosteroid regulation of ion channel conductances and mRNA levels in individual hippocampal CA1 neurons

Overexposure to corticosteroid hormones is harmful to hippocampal neuronal integrity, likely by perturbation of calcium homeostasis. To identify molecular mechanisms at the single-cell level, we characterized mRNA expression corresponding to voltage- and ligand-gated Ca channels in individual dissociated CA1 neurons in response to long-term corticosterone (CORT) exposure. Predominant mineralocorticoid receptor occupation (ADC-LO group) resulted in low levels of P/Q- and L-type Ca channel mRNAs, high levels of GluR-2 versus GluR-1, and a high ratio of NMDAR-2A to NMDAR-2B mRNA. Corresponding alterations in protein expression were consistent with the restriction of Ca influx. In contrast, additional glucocorticoid receptor occupation (ADC-HI group) altered the expression of these mRNAs in a manner consistent with enhanced Ca influx; interestingly, qualitatively similar alterations were seen in control ADX neurons. Electrophysiological data from the same neurons indicate that Ca current amplitudes also are modulated by CORT, although on a shorter time scale. Finally, principal components analysis (PCA) suggests that neuronal AMPA and NMDA receptor composition may be regulated by MR and GR activation in a complex manner. Therefore, our data implicate molecular events by which CORT may regulate Ca influx into CA1 hippocampal neurons.

Genetic toxicities of human teratogens

Birth defects cause a myriad of societal problems and place tremendous anguish on the affected individual and his or her family. Current estimates categorize about 3% of all newborn infants as having some form of birth defect or congenital anomaly. As more precise means of detecting subtle anomalies become available this estimate, no doubt, will increase. Even though birth defects have been observed in newborns throughout history, our knowledge about the causes and mechanisms through which these defects are manifested is limited. For example, it has been estimated that around 20% of all birth defects are due to gene mutations, 5-10% to chromosomal abnormalities, and another 5-10% to exposure to a known teratogenic agent or maternal factor [D.A. Beckman, R.L. Brent, Mechanisms of teratogenesis. Ann. Rev. Pharmacol. Toxicol. 24 (1984) 483-500; K. Nelson, L.B. Holmes Malformations due to presumed spontaneous mutations in newborn infants, N. Engl. J. Med. 320 (1989) 19-23.]. Together, these percentages account for only 30-40%, leaving the etiology of more than half of all human birth defects unexplained. It has been speculated that environmental factors account for no more than one-tenth of all congenital anomalies [D.A. Beckman, R.L. Brent, Mechanisms of teratogenesis, Ann. Rev. Pharmacol. Toxicol. 24 (1984) 483-500]. Furthermore, since there is no evidence in humans that the exposure of an individual to any mutagen measurably increases the risk of congenital anomalies in his or her offspring' [J.F. Crow, C. Denniston, Mutation in human populations, Adv. Human Genet. 14 (1985) 59-121; J.M. Friedman, J.E. Polifka, Teratogenic Effects of Drugs: A Resource for Clinicians (TERIS). The John Hopkins University Press, Baltimore, 1994], the mutagenic activity of environmental agents and drugs as a factor in teratogenesis has been given very little attention. Epigenetic activity has also been given only limited consideration as a mechanism for teratogenesis. As new molecular methods are developed for assessing processes associated with teratogenesis, especially those with a genetic or an epigenetic basis, additional environmental factors may be identified. These are especially important because they are potentially preventable. This paper examines the relationships between chemicals identified as human teratogens (agents that cause birth defects) and their mutagenic activity as evaluated in one or more of the established short-term bioassays currently used to measure such damage. Those agents lacking mutagenic activity but with published evidence that they may otherwise alter the expressions or regulate interactions of the genetic material, i.e. exhibit epigenetic activity, have likewise been identified. The information used in making these comparisons comes from the published literature as well as from unpublished data of the U.S. National Toxicology Program (NTP).