Primary midline developmental field. II. Clinical/epidemiological analysis of alteration of laterality (normal body symmetry and asymmetry)

Selective Serotonin Reuptake Inhibitor Use During Pregnancy and Major Malformations: The Importance of Serotonin for Embryonic Development and the Effect of Serotonin Inhibition on the Occurrence of Malformations

Bioelectric signaling is transduced by neurotransmitter pathways in many cell types. One of the key mediators of bioelectric control mechanisms is serotonin, and its transporter SERT, which is targeted by a broad class of blocker drugs (selective serotonin reuptake inhibitors [SSRIs]). Studies showing an increased risk of multiple malformations associated with gestational use of SSRI have been accumulating but debate remains on whether SSRI as a class has the potential to generate these malformations. This review highlights the importance of serotonin for embryonic development; the effect of serotonin inhibition during early pregnancy on the occurrence of multiple diverse malformations that have been shown to occur in human pregnancies; that the risks outweigh the benefits of SSRI use during gestation in populations of mild to moderately depressed pregnant women, which encompass the majority of pregnant depressed women; and that the malformations seen in human pregnancies constitute a pattern of malformations consistent with the known mechanisms of action of SSRIs. We present at least three mechanisms by which SSRI can affect development. These studies highlight the relevance of basic bioelectric and neurotransmitter mechanism for biomedicine.

From cytoskeletal dynamics to organ asymmetry: a nonlinear, regulative pathway underlies left-right patterning

Consistent left-right (LR) asymmetry is a fundamental aspect of the bodyplan across phyla, and errors of laterality form an important class of human birth defects. Its molecular underpinning was first discovered as a sequential pathway of left- and right-sided gene expression that controlled positioning of the heart and visceral organs. Recent data have revised this picture in two important ways. First, the physical origin of chirality has been identified; cytoskeletal dynamics underlie the asymmetry of single-cell behaviour and patterning of the LR axis. Second, the pathway is not linear: early disruptions that alter the normal sidedness of upstream asymmetric genes do not necessarily induce defects in the laterality of the downstream genes or in organ situs Thus, the LR pathway is a unique example of two fascinating aspects of biology: the interplay of physics and genetics in establishing large-scale anatomy, and regulative (shape-homeostatic) pathways that correct molecular and anatomical errors over time. Here, we review aspects of asymmetry from its intracellular, cytoplasmic origins to the recently uncovered ability of the LR control circuitry to achieve correct gene expression and morphology despite reversals of key 'determinant' genes. We provide novel functional data, in Xenopus laevis, on conserved elements of the cytoskeleton that drive asymmetry, and comparatively analyse it together with previously published results in the field. Our new observations and meta-analysis demonstrate that despite aberrant expression of upstream regulatory genes, embryos can progressively normalize transcriptional cascades and anatomical outcomes. LR patterning can thus serve as a paradigm of how subcellular physics and gene expression cooperate to achieve developmental robustness of a body axis.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.

Establishing the Embryonic Axes: Prime Time for Teratogenic Insults

A long standing axiom in the field of teratology states that the teratogenic period, when most birth defects are produced, occurs during the third to eighth weeks of development post-fertilization. Any insults prior to this time are thought to result in a slowing of embryonic growth from which the conceptus recovers or death of the embryo followed by spontaneous abortion. However, new insights into embryonic development during the first two weeks, including formation of the anterior-posterior, dorsal-ventral, and left-right axes, suggests that signaling pathways regulating these processes are prime targets for genetic and toxic insults. Establishment of the left-right (laterality) axis is particularly sensitive to disruption at very early stages of development and these perturbations result in a wide variety of congenital malformations, especially heart defects. Thus, the time for teratogenic insults resulting in birth defects should be reset to include the first two weeks of development.

Overlap syndrome: a diagnostic dilemma

We report an interesting case of a young patient who came with a concern for missing teeth and lack of hair on scalp and body. Examination revealed complete absence of teeth, absence of eyebrows, eyelashes and hair over scalp. He was short-statured, had hyperextensible joints and hyperelastic skin, protuberant lips and many other anomalies such that the overall pattern of defects was not recognisable. A wide array of investigations involving the dental and medical faculties were done; however, the final diagnosis could not be reached, since it appeared to involve features of more than one syndrome, thus the name 'overlap syndrome'.

Segmentation anomalies of vertebrae and ribs with other abnormalities of blastogenesis: syndromes or associations?

On the basis of two recently studied human fetuses and the historical records and remnant 19th century skeletons in the Museum Vrolik in Amsterdam, we have begun an analysis of an unusual form of somite dysgenesis. This disorder includes vertebral and costal segmentation defects with or without (distal) limb malformation and deformities, anogenital anomalies, unusual colonic atresia, abdominal wall and diaphragmatic defect, Central nervous system abnormality with large head, and severe neurohypotrophic lower limb deformities. This study suggests the existence of an axial vertebral/costal dysgenesis complex with apparently or nearly normal number of cervical vertebrae. There also is some overlap with lumbosacral agenesis but different from the autosomal recessive entities Jarcho-Levin syndrome or spondylocostal dysostosis. To date, no associated heart defects have been noted.

XX-agonadism in a fetus with multiple congenital anomalies

We report on an 18-week gestation fetus with 46,XX karyotype, gonadal agenesis, meningo-encephalocele, spina bifida, omphalocele, webbing of right upper limb, deformed right clavicle and right sided ribs, absent interventricular septum, hypoplastic aorta, hypoplastic spleen, and single umbilical artery. This case is similar to the one previously described by Kennerknecht et al. in 1997 and may represent a unique syndrome.

Defects of blastogenesis

Ever more frequent and closer involvement by clinical geneticists and counselors in the prenatal assessment of development mandates a better understanding of all stages of human ontogeny, but especially those of earliest development during which most of the lethal and all of the gross, multiple and complex defects of morphogenesis arise. Because of the phenomenon of universality, i.e., identical molecular inductive mechanisms involved in the process of embryonic pattern formation in all vertebrates, experimental animals indeed are a most valuable approach to an understanding of the causal and formal aspects of development and are beginning to forge essential, strong bonds between molecular biologists and clinicians in a mutually supportive discipline of developmental biology. However, to grieving parents of a stillborn fetus with, say, Pentalogy of Cantrell, sirenomelia or otocephaly, mouse data offer little comfort or reassurance about recurrence; thus, it is imperative to make ever more effective a science of human teratology (sensu lato) with participating reproductive geneticists, obstetricians, neonatologists, ultrasonographers, pediatric/fetal pathologists, cytogeneticists and pediatric geneticists to generate the diagnostic, pathogenetic and causal data necessary to counsel and to comfort the parents. Few molecular data exist on causes of blastogenetic defects in humans; however, the phenomenon of parsimony, whereby the same "morphogenetic" molecule, say, sonic hedgehog (SHH), is "deployed" simultaneously or sequentially during the morphogenesis (and even the histogenesis) of several/many embryonic primordia, makes it likely that a genetic/epigenetic disturbance of such an inductive system will have multiple effects on blastogenetic, organogenetic and perhaps also histogenetic events in the embryo. If causally defined, such a pattern of anomalies constitutes pleiotropy, and the embryo/fetus can be said to have a syndrome. If cause is unknown, the presumption of pleiotropy is less certain, and the fetus/infant may be said to have an "association" with low empiric recurrence risk.

Heterotaxia as an outcome of maternal diabetes: an epidemiological study

There are very few publications on the possible relationship between maternal diabetes and infants presenting heterotaxia-asymmetry defects. In mice, there is a relationship between maternal diabetes and heterotaxia, although this is influenced by the fetal genotype. An epidemiological analysis of heterotaxia-asymmetry in children born to diabetic mothers (diabetes mellitus or gestational diabetes) is presented here. The analysis is based in the case-control study of the ECEMC database. However, due to the very small sample size for each type of study of heterotaxia-asymmetry alterations, up to 10 control infants for each case were selected. Although the sample size is small, the results suggest that only maternal diabetes mellitus significantly increases the risk for transposition of great vessels (OR=61.87; CI:7.36-519.82), and transposition of viscera (OR=24.82; CI;1.84-335.44).

On the symmetry of limb deficiencies among children with multiple congenital anomalies

In humans, unpaired organs are placed in a highly ordered pattern along the left-right axis. As indicated by animal studies, a cascade of signaling molecules establish left-right asymmetry in the developing embryo. Some of the same genes are involved also in limb patterning. To provide a better insight into the connection between these processes in humans, we analysed the symmetry of limb deficiencies among infants with multiple congenital anomalies. The study was based on data collected by the International Clearinghouse for Birth Defects Monitoring Systems (ICBDMS). Registries of the ICBDMS provided information on infants who, in addition to a limb deficiency, also had at least one major congenital anomaly in other organ systems. We reviewed 815 such cases of which 149 cases (18.3 %) were syndromic and 666 (81.7 %) were nonsyndromic. The comparisons were made within the associated limb deficiencies, considering the information on symmetry, using a comparison group with malformations associated not involved in the index association. Among the non-syndromic cases, the left-right distribution of limb deficiencies did not differ appreciably between limb deficiency subtypes (e.g., preaxial, transverse, longitudinal). The left-right distribution of limb anomalies did not differ among most types of non-limb anomalies, though a predominance of left-sided limb deficiencies was observed in the presence of severe genital defects - odds ratio [OR], 2.6; 95 % CI, 1.1-6.4). Limb deficiencies (LDs) were more often unilateral than bilateral when accompanied by gastroschisis (OR, 0.1) or axial skeletal defects (OR, 0.5). On the contrary, LDs were more often bilateral than unilateral when associated with cleft lip with or without cleft palate (OR, 3.9) or micrognathia (OR, 2.6). Specifically, we found an association between bilateral preaxial deficiencies and cleft lip, bilateral amelia with gastroschisis and urinary tract anomalies, and bilateral transverse deficiencies and gastroschisis and axial skeleton defects. Of 149 syndromic cases, 62 (41.6 %) were diagnosed as trisomy 18. Out of the 30 cases of trisomy 18 with known laterality, 20 cases were bilateral. In the remainder the right and left sides were equally affected. Also, in most cases (74.4 %) only the upper limbs were involved. In conclusion the left-right distribution of limb deficiencies among some non-limb anomalies may suggest a relationship between the development of the limb and the left-right axis of the embryo.

Patterning the heart's left-right axis: from zebrafish to man

Normal left-right asymmetry is highly conserved among vertebrates. Errors in the proper patterning of this axis are believed to lead to congenital anomalies of the heart and abdominal viscera, often with profound clinical consequences. We review briefly the nature of potential signals and signaling sources that lead to the break in left-right symmetry. The evidence suggests that left-right reversal, or homogenization, of these signals may lead to different consequences, and we explain some malpositions and malalignments of the atria, ventricles, and/or outflow tract that are seen in a variety of congenital cardiac diseases. We speculate that there are units of organ assembly responsive to laterality signals, and these units may be driven independently. One crucial source of signals appears to be the notochord and floorplate. In order to examine the clinical relationship of these midline structures to putative disorders of laterality, we review all patients with disturbances of normal laterality seen at the Massachusetts General Hospital over the past 20 years. We find a significant association between laterality defects and anomalies of the spine and other midline structures.

Heterotaxy-neural tube defect and holoprosencephaly occuring independently in two sib fetuses

We report on two sib fetuses, products of a consanguineous union, who had multiple and apparently unrelated malformations. The first fetus, a female, had trilobed lungs, a single cardiac ventricle, asplenia, situs ambiguus of the liver, and a lumbosacral meningomyelocele. The brain of this fetus was normal. The second fetus, a male, had bilobed lungs, a single cardiac ventricle, situs solitus of the abdominal organs and spleen, and a semilobar holoprosencephaly. The occurrence of these malformations in sibs of different sexes and the parental consanguinity suggest a recessive mutation in a gene responsible for both heterotaxy and midline defects, including holoprosencephaly.

Autosomal recessive lateralization and midline defects: blastogenesis recessive 1

In this report, we present 2 sibships in which midline and lateralization anomalies are demonstrated. Because midline and lateralization processes are early embryological events, we suggest calling this sequence Blastogenesis Recessive 1 (BGR1). Since connexin 43 gene mutations were demonstrated in some polyasplenia patients and according to connexin 43 temporospatial tissue expression, we hypothesize that this gene could bear mutations responsible for the anomalies reported in these two sibships.

Blastogenesis dominant 1: a sequence with midline anomalies and heterotaxy

Lateralization defect is a heterogeneous condition with different modes of transmission (autosomal recessive, dominant or X-linked). Here, we report on 3 additional families that contribute to the description of phenotypic anomalies of the autosomal dominant type. Phenotypic anomalies include: lateralization defects, cardiac malformations, diaphragmatic hernia, urologic and neurologic anomalies. We suggest calling this sequence BGD1 for blastogenesis dominant 1 because the deleterious effect probably occurs during blastogenesis and involves not only lateralization but other defects as well.

Defects in the determination of left-right asymmetry

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Clinical/epidemiological analysis of congenital anomalies associated with diaphragmatic hernia

Several studies have been published on congenital diaphragmatic hernia (CDH), either as an isolated defect or as part of a multiple congenital anomaly (MCA) pattern. Here we present an epidemiological study designed to measure the association between CDH and a group of 17 selected congenital anomalies in an attempt to identify groups of specific defect patterns. This analysis was done using the data from the Spanish Collaborative Study of Congenital Malformations (ECEMC).