Laura E. Mitchell

Frequency of 22q11 deletions in patients with conotruncal defects

OBJECTIVES This study was designed to determine the frequency of 22q11 deletions in a large, prospectively ascertained sample of patients with conotruncal defects and to evaluate the deletion frequency when additional cardiac findings are also considered. BACKGROUND Chromosome 22q11 deletions are present in the majority of patients with DiGeorge, velocardiofacial and conotruncal anomaly face syndromes in which conotruncal defects are a cardinal feature. Previous studies suggest that a substantial number of patients with congenital heart disease have a 22q11 deletion. METHODS Two hundred fifty-one patients with conotruncal defects were prospectively enrolled into the study and screened for the presence of a 22q11 deletion. RESULTS Deletions were found in 50.0% with interrupted aortic arch (IAA), 34.5% of patients with truncus arteriosus (TA), and 15.9% with tetralogy of Fallot (TOF). Two of 6 patients with a posterior malalignment type ventricular septal defect (PMVSD) and only 1 of 20 patients with double outlet right ventricle were found to have a 22q11 deletion. None of the 45 patients with transposition of the great arteries had a deletion. The frequency of 22q11 deletions was higher in patients with anomalies of the pulmonary arteries, aortic arch or its major branches as compared to patients with a normal left aortic arch regardless of intracardiac anatomy. CONCLUSIONS A substantial proportion of patients with IAA, TA, TOF and PMVSD have a deletion of chromosome 22q11. Deletions are more common in patients with aortic arch or vessel anomalies. These results begin to define guidelines for deletion screening of patients with conotruncal defects.

De novo mutations in histone-modifying genes in congenital heart disease.

Congenital heart disease (CHD) is the most frequent birth defect, affecting 0.8% of live births. Many cases occur sporadically and impair reproductive fitness, suggesting a role for de novo mutations. Here we compare the incidence of de novo mutations in 362 severe CHD cases and 264 controls by analysing exome sequencing of parent–offspring trios. CHD cases show a significant excess of protein-altering de novo mutations in genes expressed in the developing heart, with an odds ratio of 7.5 for damaging (premature termination, frameshift, splice site) mutations. Similar odds ratios are seen across the main classes of severe CHD. We find a marked excess of de novo mutations in genes involved in the production, removal or reading of histone 3 lysine 4 (H3K4) methylation, or ubiquitination of H2BK120, which is required for H3K4 methylation. There are also two de novo mutations in SMAD2, which regulates H3K27 methylation in the embryonic left–right organizer. The combination of both activating (H3K4 methylation) and inactivating (H3K27 methylation) chromatin marks characterizes ‘poised’ promoters and enhancers, which regulate expression of key developmental genes. These findings implicate de novo point mutations in several hundreds of genes that collectively contribute to approximately 10% of severe CHD.

Epidemiology of neural tube defects

The epidemiological investigation of the common open neural tube defects (NTDs), anencephaly, and spina bifida, has a long history. The most significant finding from these past studies of NTDs was the identification of the protective effect of maternal, periconceptional supplementation with folic acid. Fortuitously, the association between folic acid and NTDs became widely accepted in the early 1990s, at a time when genetic association studies of complex traits were becoming increasingly feasible. The confluence of these events has had a major impact on the direction of epidemiological, NTD research. Association studies to evaluate genes that may influence the risk of NTDs through their role in folate‐related processes, or through other metabolic or developmental pathways are now commonplace. Moreover, the study of genetic as well as non‐genetic, factors that may influence NTD risk through effects on the nutrient status of the mother or embryo has emerged as a major research focus. Research efforts over the past decade indicate that gene–gene, gene–environment, and higher‐order interactions, as well as maternal genetic effects influence NTD risk, highlighting the complexity of the factors that underlie these conditions. The challenge for the future is to design studies that address these complexities, and are adequately powered to detect the factors or combination of factors that influence the development of NTDs.

Maternal Genetic Effects, Exerted by Genes Involved in Homocysteine Remethylation, Influence the Risk of Spina Bifida

There is currently considerable interest in the relationship between variation in genes that are involved in the folate-homocysteine metabolic axis and the risk of spina bifida. The evaluation of this relationship is, however, complicated by the potential involvement of both the maternal and the embryonic genotype in determination of disease risk. The present study was designed to address questions regarding both maternal and embryonic genetic risk factors for spina bifida by use of the two-step transmission/disequilibrium test. Analysis of data on variants of two genes involved in homocysteine remethylation/methionine biosynthesis--methionine synthase (MTR) A2756G and methionine synthase reductase (MTRR) A66G--provided evidence that both variants influence the risk of spina bifida via the maternal rather than the embryonic genotype. For both variants, the risk of having a child with spina bifida appears to increase with the number of high-risk alleles in the maternal genotype: MTR (R1=2.16, 95% CI 0.92-5.06; R2=6.58, 95% CI 0.87-49.67) and MTRR (R1=2.05, 95% CI 1.05-3.99; R2=3.15, 95% CI 0.92-10.85). These findings highlight the importance of considering both the maternal and embryonic genotype when evaluating putative spina bifida susceptibility loci.

Evidence for an association between dehydroepiandrosterone sulfate and nonfatal, premature myocardial infarction in males.

BACKGROUND Several studies indicate that endogenous hormones play a role in the etiology of coronary artery disease, either as independent risk factors or indirectly, via an effect on lipids, lipoproteins, or other heart disease risk factors. METHODS AND RESULTS The relation between endogenous hormone levels and premature (< 56-year-old patients) myocardial infarction was assessed in a retrospective study involving 49 male survivors of premature myocardial infarction and 49 age-matched, volunteer male controls. Serum samples were obtained for each subject the morning after a > or = 12-hour fast and frozen at -70 degrees C for subsequent hormonal analysis. Among the male patients, the average duration between the most recent myocardial infarction and blood sampling was 3.4 years (range, 0.7 to 19.2 years). Individuals reporting the use of any medications with the potential to alter lipid, lipoprotein, or hormone levels were excluded from these analyses. Dehydroepiandrosterone sulfate levels were significantly lower in the patients than in the control subjects. This association remained statistically significant even after accounting for the effects of total cholesterol, triglycerides, the ratio of total to high-density lipoprotein (HDL) cholesterol, HDL, apolipoprotein A-I, apolipoprotein A-II, apolipoprotein B, and body mass index. There were no significant differences in the levels of estradiol, testosterone, or free testosterone or the ratio of estradiol to testosterone between patients and control subjects. CONCLUSIONS Our conclusions are limited by the retrospective nature of this study. However, these data indicate that serum dehydroepiandrosterone sulfate levels are inversely related to premature myocardial infarction in males and that this association is independent of the effects of several known risk factors for premature myocardial infarction.

Embryonic development of folate binding protein-1 (Folbp1) knockout mice: Effects of the chemical form, dose, and timing of maternal folate supplementation

Inactivation of folate binding protein‐1 (Folbp1) adversely impacts murine embryonic development, as nullizygous embryos (Folbp1‐/‐) die in utero. Administration of folinic acid (N5‐formyl‐tetrahydrofolate) to Folbp1‐deficient dams before and throughout gestation rescues the majority of embryos from premature death; however, a portion of surviving embryos develop structural malformations, including neural tube defects. We examined whether maternal supplementation with L‐N5‐methyl‐tetrahydrofolate (L‐5M‐THF) has superior protective effects on embryonic development of Folbp1‐/‐ fetuses compared with L‐N5‐formyl‐tetrahydrofolate (L‐5F‐THF). We also examined the critical period during gestation when folate supplementation is most beneficial to the developing Folbp1‐/‐ embryos. Folbp1‐/‐ pups presented with a range of malformations involving the neural tube, craniofacies, eyes, and abdominal wall. The frequencies of these malformations decreased with increasing folate dose, regardless of the form used. There was no additional benefit provided by L‐5M‐THF compared with L‐5F‐THF. Despite rescuing the phenotype in Folbp1‐/‐ embryos, no significant elevation of Folbp1‐/‐ maternal folate levels was observed with supplementation. Developmental Dynamics 231:221–231, 2004.

Guidelines for the design and analysis of studies on nonsyndromic cleft lip and cleft palate in humans: Summary report from a workshop of the International Consortium for Oral Clefts genetics

OBJECTIVE The members of the International Consortium for Oral Clefts Genetics recognize the need for collaboration between researchers involved in etiologic studies of nonsyndromic cleft lip and palate and cleft palate. To address this need, the consortium established four working subcommittees: diagnostic and phenotypic assessment, molecular genetic studies, epidemiologic data collection and analysis, and genetic data collection and analysis. These subcommittees were charged with the development of guidelines for data collection and analysis that would facilitate both a priori and a posteriori comparisons and pooling of data from multiple centers. This report presents summary statements of the four subcommittees.

Transforming growth factor α locus and nonsyndromic cleft lip with or without cleft palate: A reappraisal

An association between nonsyndromic cleft lip with or without cleft palate (CL±P) and genetic variation at the transforming growth factor α (TGFA) locus was originally reported in 1989. Subsequent population‐based studies of this association have provided conflicting results. The present analyses were undertaken to determine if the cumulative weight of the available data convincingly supports or refutes this association. The published data were analyzed for differences in allele frequencies between Caucasian CL±P patients (i.e., cases) and controls, and for heterogeneity between Caucasian samples. When all data except the original report were considered, there was a statistically significant association between TGFA and CL±P (M.H.O.R. = 1.43; 95% C.I. 1.12–1.80). However, there was evidence of significant heterogeneity in the TGFA allele frequencies between cases, but not controls, from different studies. The data suggest that the observed heterogeneity is unlikely to be attributable to differences in the ethnic composition of the cases among the various studies but may reflect differences in the proportion of cases with bilateral lip defects and/or with positive family histories of CL±P. Definitive conclusions regarding the source(s) of the observed heterogeneity could not, however, be drawn on the basis of the available data. Hence, at present, the evidence regarding an association between genetic variation at the TGFA locus and CL±P remains inconclusive. Genet. Epidemiol. 14:231–240,1997.

Differences in exposure assignment between conception and delivery: the impact of maternal mobility.

In studies of reproductive outcomes, maternal residence at delivery is often the only information available to characterise environmental exposures during pregnancy. The goal of this investigation was to describe residential mobility during pregnancy and to assess the extent to which change of residence may result in exposure misclassification when exposure is based on the address at delivery. Maternal residential mobility was compared between neural tube defect cases and unaffected controls from Texas participants in the National Birth Defects Prevention Study (NBDPS). Maternal residential information was obtained from the NBDPS interview. Data from the U.S. EPA National Air Toxics Assessment [Assessment System for Population Exposure Nationwide (ASPEN)], modelled at the census tract level, were used to estimate benzene exposure based on address at conception and address at delivery. Quartiles of exposure were assigned based on these estimates and the quartile assignments based on address at conception and address at delivery were compared using traditional methods (kappa statistics) and a novel application of mixed-effects ordinal logistic regression. Overall, 30% of case mothers and 24% of control mothers moved during pregnancy. Differences in maternal residential mobility were not significant between cases and controls, other than case mothers who moved did so earlier during pregnancy than control mothers (P = 0.01). There was good agreement between quartiles of estimated benzene exposure at both addresses (kappa = 0.78, P < 0.0001). Based on the mixed-effects regression model, address at delivery was not significantly different from using address at conception when assigning quartile of benzene exposure based on estimates from ASPEN (odds ratio 1.03, 95% confidence interval 0.85, 1.25). Our results indicate that, in this Texas population, maternal residential movement is generally within short distances, is typically not different between cases and controls, and does not significantly influence benzene exposure assessment.

Maternal Exposure to Ambient Levels of Benzene and Neural Tube Defects among Offspring: Texas, 1999–2004

Background Previous studies have reported positive associations between maternal exposure to air pollutants and several adverse birth outcomes. However, there have been no studies assessing the association between environmental levels of hazardous air pollutants, such as benzene, and neural tube defects (NTDs), a common and serious group of congenital malformations. Objective Our goal was to conduct a case–control study assessing the association between ambient air levels of benzene, toluene, ethylbenzene, and xylene (BTEX) and the prevalence of NTDs among offspring. Methods The Texas Birth Defects Registry provided data on NTD cases (spina bifida and anencephaly) delivered between 1999 and 2004. The control group was a random sample of unaffected live births, frequency matched to cases on year of birth. Census tract–level estimates of annual BTEX levels were obtained from the U.S. Environmental Protection Agency 1999 Assessment System for Population Exposure Nationwide. Restricted cubic splines were used in mixed-effects logistic regression models to determine associations between each pollutant and NTD phenotype. Results Mothers living in census tracts with the highest benzene levels were more likely to have offspring with spina bifida than were women living in census tracts with the lowest levels (odds ratio = 2.30; 95% confidence interval, 1.22–4.33). No significant associations were observed between anencephaly and benzene or between any of the NTD phenotypes and toluene, ethylbenzene, or xylene. Conclusion In the first study to assess the relationship between environmental levels of BTEX and NTDs, we found an association between benzene and spina bifida. Our results contribute to the growing body of evidence regarding air pollutant exposure and adverse birth outcomes.