Population-based study of l-transposition of the great arteries: possible associations with environmental factors

Familial Recurrence Patterns in Congenitally Corrected Transposition of the Great Arteries: An International Study

BACKGROUND

Congenitally corrected transposition of the great arteries (ccTGA) is a rare disease of unknown cause. We aimed to better understand familial recurrence patterns.

METHODS

An international, multicentre, retrospective cohort study was conducted in 29 tertiary hospitals in 6 countries between 1990 and 2018, entailing investigation of 1043 unrelated ccTGA probands.

RESULTS

Laterality defects and atrioventricular block at diagnosis were observed in 29.9% and 9.3%, respectively. ccTGA was associated with primary ciliary dyskinesia in 11 patients. Parental consanguinity was noted in 3.4% cases. A congenital heart defect was diagnosed in 81 relatives from 69 families, 58% of them being first-degree relatives, including 28 siblings. The most prevalent defects in relatives were dextro-transposition of the great arteries (28.4%), laterality defects (13.6%), and ccTGA (11.1%); 36 new familial clusters were described, including 8 pedigrees with concordant familial aggregation of ccTGA, 19 pedigrees with familial co-segregation of ccTGA and dextro-transposition of the great arteries, and 9 familial co-segregation of ccTGA and laterality defects. In one family co-segregation of ccTGA, dextro-transposition of the great arteries and heterotaxy syndrome in 3 distinct relatives was found. In another family, twins both displayed ccTGA and primary ciliary dyskinesia.

CONCLUSIONS

ccTGA is not always a sporadic congenital heart defect. Familial clusters as well as evidence of an association between ccTGA, dextro-transposition of the great arteries, laterality defects and in some cases primary ciliary dyskinesia, strongly suggest a common pathogenetic pathway involving laterality genes in the pathophysiology of ccTGA.

Novel MYO1D Missense Variant Identified Through Whole Exome Sequencing and Computational Biology Analysis Expands the Spectrum of Causal Genes of Laterality Defects

Laterality defects (LDs) or asymmetrically positioned organs are a group of rare developmental disorders caused by environmental and/or genetic factors. However, the exact molecular pathophysiology of LD is not yet fully characterised. In this context, studying Arab population presents an ideal opportunity to discover the novel molecular basis of diseases owing to the high rate of consanguinity and genetic disorders. Therefore, in the present study, we studied the molecular basis of LD in Arab patients, using next-generation sequencing method. We discovered an extremely rare novel missense variant in MYO1D gene (Pro765Ser) presenting with visceral heterotaxy and left isomerism with polysplenia syndrome. The proband in this index family has inherited this homozygous variant from her heterozygous parents following the autosomal recessive pattern. This is the first report to show MYO1D genetic variant causing left-right axis defects in humans, besides previous known evidence from zebrafish, frog and Drosophila models. Moreover, our multilevel bioinformatics-based structural (protein variant structural modelling, divergence, and stability) analysis has suggested that Ser765 causes minor structural drifts and stability changes, potentially affecting the biophysical and functional properties of MYO1D protein like calmodulin binding and microfilament motor activities. Functional bioinformatics analysis has shown that MYO1D is ubiquitously expressed across several human tissues and is reported to induce severe phenotypes in knockout mouse models. In conclusion, our findings show the expanded genetic spectrum of LD, which could potentially pave way for the novel drug target identification and development of personalised medicine for high-risk families.

The assessment of patients with situs inversus and corrected transposition of the great arteries

INTRODUCTION

Congenitally corrected transposition of the great arteries (ccTGA) is a rare congenital heart defect. There are different subgroups according to the location of the heart in the thorax, apical position and situs.

OBJECTIVES

The purpose of this study was to assess pediatric patients with situs inversus (SI) ccTGA (SI-ccTGA), a rare subgroup of this condition, in detail.

METHODS

The records of patients with SI-ccTGA followed between January 1, 2010 and January 1, 2019 in our clinic were analyzed retrospectively. Demographic features, associated cardiac defects, arrhythmias and follow-up data were recorded.

RESULTS

Twenty-one out of 120 ccTGA patients had SI. The median age was 30 months (4 days-18 years). There were hemodynamically significant associated lesions in 85.7% (n=18) of the patients. A large ventricular septal defect was found in 16 patients (76.2%), severe pulmonary stenosis in 11 (52.4%), pulmonary atresia in six (28.5%), and severe tricuspid regurgitation in two (9.5%). Eleven out of 21 patients had biventricular physiology and eight had single-ventricle physiology. Bidirectional cavopulmonary anastomosis followed by a hemi-Mustard-Rastelli operation were planned for the remaining two patients. Twelve out of 18 patients with associated defects (66.6%) were operated and surgery was planned for three more patients (16.6%). The remaining three patients were scheduled for clinical follow-up. Arrhythmias developed in two (9.5%) patients on follow-up; ablation was performed in one of them and pacemaker implantation followed by cardiac resynchronization therapy was performed in the other. Two patients died during follow-up, one after a central shunt operation and the other preoperatively due to pneumonia and sepsis.

CONCLUSION

SI-ccTGA is not a mirror image of situs solitus ccTGA (SS-ccTGA) due to important anatomic and physiologic differences between them. SI-ccTGA patients have a lower risk of tricuspid valve regurgitation than SS-ccTGA patients. The timing of clinical presentation of these patients mainly depends on the type and severity of the associated lesions, as in all subtypes of ccTGA.

Environmental Contaminants and Congenital Heart Defects: A Re-Evaluation of the Evidence

Congenital heart defects (CHDs) are a common birth defect of largely unknown etiology, with high fetal and neonatal mortality. A review of CHDs and environmental contaminant exposure found that meta-analyses showed only modest associations for smoking, vehicle exhaust components, disinfectant by-products and proximity to incinerators, with stronger results from the newer, larger and better quality studies masked by the typical absence of effect in older studies. Recent studies of exposure to agricultural pesticides, solvents, metals and landfill sites also showed associations. Certain contaminants have been associated with certain CHDs, with septal defects being the most common. Frequent methodological problems include failure to account for potential confounders or maternal/paternal preconception exposure, differences in diagnosing, defining and classifying CHDs, grouping of defects to increase power, grouping of contaminants with dissimilar mechanisms, exclusion of pregnancies that result in death or later life diagnosis, and the assumption that maternal residence at birth is the same as at conception. Furthermore, most studies use measurement estimates of one exposure, ignoring the many additional contaminant exposures in daily life. All these problems can distort and underestimate the true associations. Impaired methylation is a common mechanism, suggesting that supplementary folate may be protective for any birth defect.

Rhythm disturbances and treatment strategies in children with congenitally corrected transposition of the great arteries

BACKGROUND

We aimed to evaluate rhythm abnormalities in cases of congenitally corrected transposition of the great arteries (ccTGA) and associated treatment strategies.

PATIENTS AND METHODS

This retrospective cohort study included 65 pediatric patients with ccTGA who were admitted to the clinic between 2009 and 2017. The patients were divided into two groups, and surgical data, Holter electrocardiographic (ECG) recordings, ECG recordings, electrophysiological data, and device implantation data on the two groups were compared.

RESULTS

Group I (n = 53, 82%) consisted of patients with significant associated lesions, and Group II (n = 12, 18%) consisted of those with minor or no associated lesions (isolated ccTGA). Rhythm abnormalities were diagnosed in 22 (34%) of the patients based on initial ECG findings and Holter ECG recordings. Eleven (17%) of these patients had atrioventricular (AV) block of different degrees, and the other 11 (17%) had supraventricular arrhythmia (SVA). The median follow-up was 49 months (range, 9-89 months), and the rhythm remained normal in 26 (42%) of the patients. Three patients died on follow-up. Of 40 patients with normal initial findings, nine required pacemaker implantation due to complete heart block, and SVA developed in seven patients on follow-up. No ventricular tachycardia was seen initially or on follow-up. Ablation was performed in four patients. During the follow-up period, pacemakers were implanted in 12 (23%) of patients in Group I and 4 (33%) of patients in Group II due to complete heart block. Cardiac resynchronization therapy (CRT) was performed in four patients due to systemic ventricular dysfunction. Notably, all four of these patients had a pacemaker implanted postoperatively.

Human exposure to environmental contaminants and congenital anomalies: a critical review

Congenital anomalies are an important cause of infant mortality and disability. Developmental exposure to environmental contaminants is thought to increase the risk for congenital anomalies. Herein, we describe a critical review of the literature conducted between February and March 2014 yielding 3057 references from which 97 unique relevant articles published from 2003 through 2014 were evaluated. Common congenital anomalies including hypospadias, cryptorchidism, anogenital distance (AGD), congenital heart defects and oral clefts were well represented in the literature whereas other outcomes such as neural tube defects, limb deficiency defects and gastroschisis were rarely described. While definitions used for congenital anomalies and methods of ascertainment were usually consistent across studies, inconsistencies were frequently found in grouping of different congenital heart defects. Despite strong links between some congenital anomalies and parental occupation, these studies are unable to provide clear insight into the specific chemicals responsible owing to lack of direct measures of exposure. In comparison, data are mixed for contaminant exposures at concentrations representative of results from contemporary biomonitoring studies. Of the environmental contaminants studied, the association between phthalate exposures and developmental abnormalities of the male reproductive tract received the greatest attention. Important limitations of the literature studied relate to adequacy of sample size, absence of or weaknesses in exposure assessment methodologies, failure to account for biological plausibility and grouping of congenital anomalies with divergent mechanisms. We conclude that the literature is inadequate at this time to support a conclusion that exposure to environmental contaminants are or are not associated with increased risks for congenital anomalies in the general population.

Genetic basis of human left-right asymmetry disorders

Humans and other vertebrates exhibit left-right (LR) asymmetric arrangement of the internal organs, and failure to establish normal LR asymmetry leads to internal laterality disorders, including situs inversus and heterotaxy. Situs inversus is complete mirror-imaged arrangement of the internal organs along LR axis, whereas heterotaxy is abnormal arrangement of the internal thoraco-abdominal organs across LR axis of the body, most of which are associated with complex cardiovascular malformations. Both disorders are genetically heterogeneous with reduced penetrance, presumably because of monogenic, polygenic or multifactorial causes. Research in genetics of LR asymmetry disorders has been extremely prolific over the past 17 years, and a series of loci and disease genes involved in situs inversus and heterotaxy have been described. The review highlights the classification, chromosomal abnormalities, pathogenic genes and the possible mechanism of human LR asymmetry disorders.

Congenitally corrected transposition

Congenitally corrected transposition is a rare cardiac malformation characterized by the combination of discordant atrioventricular and ventriculo-arterial connections, usually accompanied by other cardiovascular malformations. Incidence has been reported to be around 1/33,000 live births, accounting for approximately 0.05% of congenital heart malformations. Associated malformations may include interventricular communications, obstructions of the outlet from the morphologically left ventricle, and anomalies of the tricuspid valve. The clinical picture and age of onset depend on the associated malformations, with bradycardia, a single loud second heart sound and a heart murmur being the most common manifestations. In the rare cases where there are no associated malformations, congenitally corrected transposition can lead to progressive atrioventricular valvar regurgitation and failure of the systemic ventricle. The diagnosis can also be made late in life when the patient presents with complete heart block or cardiac failure. The etiology of congenitally corrected transposition is currently unknown, and with an increase in incidence among families with previous cases of congenitally corrected transposition reported. Diagnosis can be made by fetal echocardiography, but is more commonly made postnatally with a combination of clinical signs and echocardiography. The anatomical delineation can be further assessed by magnetic resonance imaging and catheterization. The differential diagnosis is centred on the assessing if the patient is presenting with isolated malformations, or as part of a spectrum. Surgical management consists of repair of the associated malformations, or redirection of the systemic and pulmonary venous return associated with an arterial switch procedure, the so-called double switch approach. Prognosis is defined by the associated malformations, and on the timing and approach to palliative surgical care.

Exploring the impact of altitude on congenital heart defects in Saudi Arabia

Congenital heart defects (CHD) have a complex etiology in which environmental factors play a key role. High altitude has already been established as a risk factor. Data from the CHD Registry in Riyadh were used to explore the impact of altitude throughout the Kingdom, including the mountain ranges of the south-west; 2,787 Saudi Arabian CHD cases were studied from 104 different cities and villages. Altitude was estimated using a geographical information system. The CHD “burden” was estimated by normalizing the number of CHD cases according to the city population. Cities at both the highest and the lowest altitudes displayed an increased CHD burden, indicating that CHD burden in Saudi Arabia is impacted by at least two different environmental factors — high altitude and coastal proximity. There is also an indication of a separate south-western influence which may reflect a unique cultural or genetic factor.

Maternal residential proximity to waste sites and industrial facilities and conotruncal heart defects in offspring

Most studies of the relationship between maternal residential proximity to sources of environmental pollution and congenital cardiovascular malformations have combined heart defects into one group or broad subgroups. The current case-control study examined whether risk of conotruncal heart defects, including subsets of specific defects, was associated with maternal residential proximity to hazardous waste sites and industrial facilities with recorded air emissions. Texas Birth Defects Registry cases were linked to their birth or fetal death certificate. Controls without birth defects were randomly selected from birth certificates. Distances from maternal addresses at delivery to National Priority List (NPL) waste sites, state superfund waste sites, and Toxic Release Inventory (TRI) facilities were determined for 1244 cases (89.5% of those eligible) and 4368 controls (88.0%). Living within 1 mile of a hazardous waste site was not associated with risk of conotruncal heart defects [adjusted odds ratio (aOR) = 0.83, 95% confidence interval (CI) = 0.54, 1.27]. This was true whether looking at most types of defects or waste sites. Only truncus arteriosus showed statistically elevated ORs with any waste site (crude OR: 2.80, 95% CI 1.19, 6.54) and with NPL sites (crude OR: 4.63, 95% CI 1.18, 13.15; aOR 4.99, 95% CI 1.26, 14.51), but the latter was based on only four exposed cases. There was minimal association between conotruncal heart defects and proximity to TRI facilities (aOR = 1.10, 95% CI = 0.91, 1.33). Stratification by maternal age or race/ethnic group made little difference in effect estimates for waste sites or industrial facilities. In this study population, maternal residential proximity to waste sites or industries with reported air emissions was not associated with conotruncal heart defects or its subtypes in offspring, with the exception of truncus arteriosus.

Paroxetine and fluoxetine in pregnancy: a prospective, multicentre, controlled, observational study

AIMS

Recent studies have suggested a possible association between maternal use of selective serotonin reuptake inhibitors (SSRIs) in early pregnancy and cardiovascular anomalies. The aim of the present study was to evaluate the teratogenic risk of paroxetine and fluoxetine.

METHODS

This multicentre, prospective, controlled study evaluated the rate of major congenital anomalies after first-trimester gestational exposure to paroxetine, fluoxetine or nonteratogens.

RESULTS

We followed up 410 paroxetine, 314 fluoxetine first-trimester exposed pregnancies and 1467 controls. After exclusion of genetic and cytogenetic anomalies, there was a higher rate of major anomalies in the SSRI groups compared with the controls [paroxetine 18/348 (5.2%), fluoxetine 12/253 (4.7%) and controls 34/1359 (2.5%)]. The main risk applied to cardiovascular anomalies [paroxetine 7/348 (2.0%), crude odds ratio (OR) 3.47, 95% confidence interval (CI) 1.13, 10.58; fluoxetine 7/253 (2.8%), crude OR, 4.81 95% CI 1.56, 14.71; and controls 8/1359 (0.6%)]. On logistic regression analysis only cigarette smoking of >or=10 cigarettes day(-1) and fluoxetine exposure were significant variables for cardiovascular anomalies. The adjusted ORs for paroxetine and fluoxetine were 2.66 (95% CI 0.80, 8.90) and 4.47 (95% CI 1.31, 15.27), respectively.

CONCLUSION

This study suggests a possible association between cardiovascular anomalies and first-trimester exposure to fluoxetine.

Getting to the heart of the matter: epidemiology of cyanotic heart defects

Congenital heart defects (CHDs) are the most common type of birth defect, making significant contributions to infant morbidity and mortality, but not all CHDs contribute equally to such outcomes. Although cyanotic CHDs constitute some of the most serious CHDs, its epidemiology is poorly understood. We present a comprehensive systematic review of the literature on the epidemiology of cyanotic CHD, with emphasis on the most current knowledge on identified risk/etiologic factors. Literature for this review was identified by searching the PubMed database from the National Center for Biotechnology Information at the US National Library of Medicine as well as bibliographies of identified papers. The 100 reports that contributed to this review describe risk factors such as infant sex, race, and ethnicity, environmental exposures, and maternal and paternal age. Several studies reported differences in prevalence rates by race and ethnicity and elevated sex ratios, and they identified some risk factors, including advanced maternal age. Investigators have made significant progress in the effort to describe the etiology of cyanotic CHDs, but discrepancies, such as the variation in prevalence rates by race and ethnicity and the impact of environmental exposures, still need to be addressed.

Noninherited risk factors and congenital cardiovascular defects: current knowledge: a scientific statement from the American Heart Association Council on Cardiovascular Disease in the Young: endorsed by the American Academy of Pediatrics

Prevention of congenital cardiovascular defects has been hampered by a lack of information about modifiable risk factors for abnormalities in cardiac development. Over the past decade, there have been major breakthroughs in the understanding of inherited causes of congenital heart disease, including the identification of specific genetic abnormalities for some types of malformations. Although relatively less information has been available on noninherited modifiable factors that may have an adverse effect on the fetal heart, there is a growing body of epidemiological literature on this topic. This statement summarizes the currently available literature on potential fetal exposures that might alter risk for cardiovascular defects. Information is summarized for periconceptional multivitamin or folic acid intake, which may reduce the risk of cardiac disease in the fetus, and for additional types of potential exposures that may increase the risk, including maternal illnesses, maternal therapeutic and nontherapeutic drug exposures, environmental exposures, and paternal exposures. Information is highlighted regarding definitive risk factors such as maternal rubella; phenylketonuria; pregestational diabetes; exposure to thalidomide, vitamin A cogeners, or retinoids; and indomethacin tocolysis. Caveats regarding interpretation of possible exposure-outcome relationships from case-control studies are given because this type of study has provided most of the available information. Guidelines for prospective parents that could reduce the likelihood that their child will have a major cardiac malformation are given. Issues related to pregnancy monitoring are discussed. Knowledge gaps and future sources of new information on risk factors are described.

Environmental hazards: evidence for effects on child health

The human fetus, child, and adult may experience adverse health outcomes from parental or childhood exposures to environmental toxicants. The fetus and infant are especially vulnerable to toxicants that disrupt developmental processes during relatively narrow time windows. This review summarizes knowledge of associations between child health and development outcomes and environmental exposures, including lead, methylmercury, polychlorinated biphenyls (PCBs), dioxins and related polyhalogenated aromatic hydrocarbons (PHAHs), certain pesticides, environmental tobacco smoke (ETS), aeroallergens, ambient air toxicants (especially particulate matter [PM] and ozone), chlorination disinfection by-products (DBPs), sunlight, power-frequency magnetic fields, radiofrequency (RF) radiation, residential proximity to hazardous waste disposal sites, and solvents. The adverse health effects linked to such exposures include fetal death, birth defects, being small for gestational age (SGA), preterm birth, clinically overt cognitive, neurologic, and behavioral abnormalities, subtle neuropsychologic deficits, childhood cancer, asthma, other respiratory diseases, and acute poisoning. Some environmental toxicants, notably lead, ionizing radiation, ETS, and certain ambient air toxicants, produce adverse health effects at relatively low exposure levels during fetal or child developmental time windows. For the many associations supported by limited or inadequate epidemiologic evidence, major sources of uncertainty include the limited number of studies conducted on specific exposure-outcome relationships and methodologic limitations. The latter include (1) crude exposure indices, (2) limited range of exposure levels, (3) small sample sizes, and (4) limited knowledge and control of potential confounders. Important knowledge gaps include the role of preconceptual paternal exposures, a topic much less studied than maternal or childhood exposures. Large longitudinal studies beginning before or during early pregnancy are urgently needed to accurately measure and assess the relative importance of parental and childhood exposures and evaluate relatively subtle health outcomes such as neuropsychologic and other functional deficits. Large case-control studies are also needed to assess the role of environmental exposures and their interactions with genetic factors in relatively uncommon outcomes such as specific types of birth defects and childhood cancers. There is also an urgent need to accelerate development and use of biomarkers of exposure and genetic susceptibility in epidemiologic studies. This review supports the priority assigned by international agencies to relationships between child health and air quality (indoor and outdoor), lead, pesticides, water contaminants, and ETS. To adequately address such priorities, governments and agencies must strengthen environmental health research capacities and adopt policies to reduce parental and childhood exposures to proven and emerging environmental threats.

Genetic and environmental influences on malformations of the cardiac outflow tract

Congenital cardiovascular malformations are the most common form of birth defect recorded. Specific malformations of the outflow portions of the heart are termed conotruncal malformations and arise from the septation of the common conotruncus of the heart. There are multiple lines of evidence that point towards genetic-environmental interactions in the genesis of conotruncal congenital cardiovascular malformations. In particular, environmental exposures that involve vitamin A, retinol, folic acid or retinol receptors are identified as cardiac teratogens. Other environmental agents for which there is evidence of cardiac teratogenicity for outflow tract malformations include nitrofen, ambient air pollution, chlorinated hydrocarbons and pesticides. Genetic polymorphisms of xenobiotic metabolism are clearly differentiating in the effect of potential teratogens. Work in this field is at a new cusp, with the ability to measure xenobiotic exposure, document xenobiotic metabolizing genetic polymorphisms and integrate these data into models of cardiac teratogenesis.

Major congenital malformations and residential proximity to a regional industrial park including a national toxic waste site: an ecological study

BACKGROUND

Public concern about exposure to emissions from the regional industrial park (IP), including 17 chemical plants and the national industrial toxic waste site, initiated this study of the possible association between major congenital malformations (MCM) and residence near the IP in Israel's Southern District.

METHODS

The study was conducted during the period 1995-2000 and included 63,850 deliveries. Data on deliveries and MCM detected at births were obtained from the regional medical center, and stratified by ethnicity and type of locality. As exposure indicator we used distance categories (proximal and distant) and predominant wind direction from the IP. Distance stratification was based on the geographical distribution of the localities and complaints about the odor related to IP emissions. Based on these complaints, localities up to 20 km from the IP were considered proximal to the IP.

RESULTS

Average rates of MCM were 5.0% and 4.1% for Bedouin and Jewish newborns, respectively. The rate of MCM for Bedouin from proximal localities was significantly greater compared with distant localities (5.6% vs. 4.8%; RR = 1.17 with 95% CI: 1.04-1.29). In the proximal Bedouin permanent localities, the MCM rate reached 8.2 %, which was significantly higher than in distant areas (RR = 1.63, 95% CI: 1.39-1.80). Significant risk increase of central nervous system MCM was found in these localities, compared to distant ones (RR = 2.27, 95% CI: 1.44-3.60). Among newborns from the traditional tribal settlements, proximity to the IP was associated with increased rates of the following MCM: 1) all combined, 2) those associated with chromosomal abnormalities, and 3) those defined as "others unclassified MCM." Comparison of autosomal recessive disease rates by proximity to the IP in Bedouin newborns indicates that the observed increased risk of MCM is not explained by consanguineous marriages. The rates of MCM in the Jewish population were similar among "exposed" and "unexposed" inhabitants.

CONCLUSION

Residential proximity to the IP is associated with increased rates of MCM among Arab-Beduin but not in Jewish populations. These observations indicate the need for public health protection of a vulnerable society in transition, although the relative importance of chemical exposure and health care utilization requires further study.

Familial recurrence of heart defects in subjects with congenitally corrected transposition of the great arteries

Familial recurrence of congenitally corrected transposition of the great arteries (CCTGA) is considered uncommon. Most of the previous familial studies involved a small number of patients and referred to all situs and looping anomalies including single ventricle, heterotaxia, and other cardiac defects different from CCTGA. We performed a large, consecutive clinical case series study in order to detect the recurrence of congenital heart defects in families of children with the classic form of CCTGA. From January 1997 through December 2004, 102 consecutive patients with CCTGA were evaluated in four institutions. There were 59 male (57.8%) and 43 female (42.2%). Mean age was 8.6 +/- 7.8 years. Eighty-eight patients (86.3%) had situs solitus of the atria, 14 (13.7%) situs inversus. The cardiac and extracardiac anomalies among relatives and the patterns of familial recurrence were investigated. Relatives with congenital heart defects were found in 16/102 families (15.7%). Transposition of the great arteries (TGA) was the most common recurrent defect (6/102 families). Consanguinity was identified in the parents of three probands. Six probands had an unaffected twin-sib. Recurrence risks for congenital heart defects were calculated at 5.2% (6/116) for siblings. In conclusion, CCTGA is not always sporadic in families. The pattern of inheritance, the presence of consanguinity among parents and the recurrence of situs inversus could suggest, in some families, an autosomal recessive mechanism with similarities with that occurring in some pedigrees with heterotaxia. The recurrence of TGA and CCTGA in the same family suggests a pathogenetic link between these two anatomically different malformations.

Effect of proximity to hazardous waste sites on the development of congenital heart disease

The authors sought to determine whether the risk of congenital heart disease (CHD) was greater for the children of mothers who lived close to a hazardous waste site (HWS) than for those who lived farther away. All cases (n = 1283) of confirmed CHD, and a random sample of 2,292 controls, born in Dallas County, Texas, from 1979-1984 were linked with 276 HWSs present during the study. The authors ascertained locations of households and determined the distance to the nearest HWS. They obtained odds ratios (ORs) for CHD. A 20% increased risk for CHD was found in association with maternal residence within 1 mile of an HWS (OR = 1.2, 95% confidence interval = 1.1-1.4). The results of this study suggests that a statistically significant additional risk for CHD is associated with maternal proximity to an HWS.

Molecular genetics of heterotaxy syndromes

PURPOSE OF REVIEW

Heterotaxy is a complex set of birth defects in which the normal concordance of asymmetric thoracic and abdominal organs is disturbed. In this review the authors summarize recent research on the etiology of heterotaxy syndromes. Improved understanding of the genetic control of left-right patterning in the early embryo is leading to the identification of candidate genes that may be mutated in heterotaxy patients, and epidemiologic studies are helping to define nongenetic mechanisms of embryopathy.

RECENT FINDINGS

Several genes have now been implicated in heterotaxy and related isolated congenital heart malformations. These studies indicate that heterotaxy can be caused by single gene mutations. They also demonstrate that there is probably extensive locus heterogeneity. Heterotaxy may be caused by teratogenic exposures, especially maternal diabetes. Isolated congenital heart defects resulting from isomerisms and disturbed looping may be caused by mutations in genes that control early left-right patterning and the earliest steps in cardiogenesis. Genes currently implicated in human heterotaxy include ZIC3, LEFTYA, CRYPTIC, and ACVR2B. Roles for NKX2.5 and CRELDA are suggested by recent case reports.

SUMMARY

Active research on the etiology of heterotaxy is leading to a reformulation of the likely etiologies. Its complex inheritance likely results from a mix of teratogenic and single gene disorders with variable expression and incomplete penetrance.