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We have shown previously that mice deficient in the activin type IIB receptor (ActRIIB) exhibit right isomerism, which is characterized by mirror-image symmetrical right lungs, complex cardiac malformations, and hypoplasia of the spleen. These observations led us to hypothesize that the signaling of a TGF-beta family member by means of ActRIIB is necessary for the determination of the left-sidedness of the visceral organs. To test this hypothesis, we examined laterality defects in mice carrying mutations in both ActRIIB and inversus viscerum (iv) genes, because iv(-/-) mice display a spectrum of laterality defects, including situs inversus, right isomerism, and left isomerism. We found that all mice homozygous for both iv and ActRIIB mutations displayed the right isomerism. The phenotype of right isomerism in the double mutants was also more severe than that in ActRIIB(-/-) mice as shown by persistent left inferior vena cava, right atrial isomerism, and hypoplasia of spleen. Interestingly, the incidence of right isomerism also increased significantly in iv(-/-);ActRIIB(+/-) and iv(+/-);ActRIIB(-/-) mice compared with homozygous mice carrying either of single gene mutations. A mechanism of the genetic modulation between ActRIIB and iv genes may be that iv modulates the asymmetric expression of a TGF-beta family member that signals through activin type II receptors, ActRIIA and ActRIIB, to specify the "left-sidedness." Nodal is the most likely candidate. We show here that the penetrance and severity of the right isomerism is significantly elevated in nodal(+/-); ActRIIB(-/-) mice, compared with ActRIIB(-/-) mice. Furthermore, the chimeric mice derived from nodal(-/-) ES cells displayed right isomerism, indistinguishable from that in (iv(-/-);ActRIIB(-/-)) mice. We propose that iv functions to establish asymmetric expression of nodal in a gene-dosage-sensitive manner and that nodal signals through the activin type II receptors to specify the left-sidedness by means of a threshold mechanism., (Copyright 2002 Wiley-Liss, Inc.)

A growing number of DNA polymerases have been identified, although their physiological function and relation to human disease remain mostly unknown. DNA polymerase lambda (Pol lambda; also known as Pol beta2) has recently been identified as a member of the X family of DNA polymerases and shares 32% amino acid sequence identity with DNA Pol beta within the polymerase domain. With the use of homologous recombination, we generated Pol lambda(-/-) mice. Pol lambda(-/-) mice develop hydrocephalus with marked dilation of the lateral ventricles and exhibit a high rate of mortality after birth, although embryonic development appears normal. Pol lambda(-/-) mice also show situs inversus totalis and chronic suppurative sinusitis. The surviving male, but not female, Pol lambda(-/-) mice are sterile as a result of spermatozoal immobility. Microinjection of sperm from male Pol lambda(-/-) mice into oocytes gives rise to normal offspring, suggesting that the meiotic process is not impaired. Ultrastructural analysis reveals that inner dynein arms of cilia from both the ependymal cell layer and respiratory epithelium are defective, which may underlie the pathogenesis of hydrocephalus, situs inversus totalis, chronic sinusitis, and male infertility. Sensitivity of Pol lambda(-/-) cells to various kinds of DNA damage is indistinguishable from that of Pol lambda(+/+) cells. Collectively, Pol lambda(-/-) mice may provide a useful model for clarifying the pathogenesis of immotile cilia syndrome.

Primary ciliary dyskinesia (PCD, MIM 242650) is characterized by recurrent infections of the respiratory tract due to reduced mucociliary clearance and by sperm immobility. Half of the affected offspring have situs inversus (reversed organs), which results from randomization of left-right (LR) asymmetry. We previously localized to chromosome 5p a PCD locus containing DNAH5, which encodes a protein highly similar to the Chlamydomonas gamma-dynein heavy chain. Here we characterize the full-length 14-kb transcript of DNAH5. Sequence analysis in individuals with PCD with randomization of LR asymmetry identified mutations resulting in non-functional DNAH5 proteins.

Vertebrate internal organs of the body develop with a distinctive left-right asymmetry. The mouse inversion of embryonic turning (inv) mutation which was created by random insertional mutagenesis causes a constant reversal of the left-right polarity (situs inversus) as well as cyst formation in the kidney in homozygotes. Since this phenotype is tightly linked to the transgenic integration site, the disrupted gene is likely to be located near this site. To analyse the transgenic integration site, ICRF yeast artificial chromosome (YAC) clones were screened by hybridization with a probe containing this site. Three positive YACs were identified and cosmid libraries were constructed from these. The insertional mutagenesis created a 60 kb genomic deletion on mouse chromosome 4. A cosmid contig spanning the whole deleted region was generated by genomic walking. In an effort to identify transcript(s), we used cosmid insert DNA to screen mouse embryo cDNA libraries directly after pre-competition with Cot1 DNA to suppress the non-specific signal. Only one cDNA clone (3 kb, clone 28) was identified, and we then obtained full-length cDNA. Genomic Southern hybridization indicated that this gene spans the whole deleted region, implying that the homozygous inv mice have an intragenic deletion of this gene. Northern hybridization showed that the gene is expressed as early as embryonic day 7, and its size was approximately 5.6 kb. As the highest expression is seen in kidney and liver in the adult, the expression patterns are consistent with our postulate that the gene is responsible not only for reversal of left-right asymmetry but also for cyst formation in the kidney in the inv mutant. The deduced amino acid sequence shows that the gene has 15 successive repeats of an Ank/Swi6 motif in its N-terminal domain. These repeats are considered to provide a mechanism for recognition of proteins, such as to fix target proteins at certain cellular locations or alter intracellular location of target binding proteins. In conclusion, we have cloned an inv candidate gene that is responsible for both the situs inversus and cyst-forming kidney. The gene product involves ankyrin motif repeats, indicating that a cytoskeleton disruption induces the situs inversus and the cystic kidney.

Congenital anatomic anomalies often present technical obstacles during liver transplantation. Biliary atresia (BA) is the most common indication for liver transplantation in children, and up to 28% of children with situs inversus are complicated by BA. A boy aged 2 years 11 months with BA, situs inversus, and dextrocardia received a liver transplant from his father. The donor also had situs inversus and dextrocardia without other anomalies. Graft function was excellent postoperatively, and no significant complications were encountered. This is only the second report of the successful use of a living related donor graft for a patient with BA and situs inversus. This case was particularly rare because the donor also had situs inversus, which made the present procedure more feasible.

The inv/inv mouse carries an insertional mutation in the inversin gene, (inv, for inversion of embryonic turning). Previously it had been reported that almost 100% of the homozygous offspring (inv/inv) were characterized by situs inversus totalis. In this report we identify the spectrum of cardiopulmonary anatomical abnormalities in inv/inv mice surviving to birth to determine whether the abnormalities seen are of the categories classically associated with human situs abnormalities. Stillborn mice, offspring that died unexpectedly (within 48 hr after birth), and neonates with phenotypic characteristics of situs inversus (right-sided stomachs, growth failure or jaundice) were processed for standard histological examination. Of 173 offspring, 34 (20%) neonates (11 stillborn, 9 unexpected deaths, and 14 mice with situs inversus phenotype) were examined, 27 of which were genotyped to be inv/inv. Interestingly, three inv/inv mice (11%) were found to have situs solitus. Twenty-four had situs inversus with normal, mirror-image cardiac anatomy (dextrocardia with atrioventricular concordance, ventriculoarterial concordance and a right aortic arch). The overall incidence of cardiovascular anomalies observed was 10 out of 27 (37%). The most frequent severe malformation, identified in 3 out of 27 animals, was a complex consisting of pulmonary infundibular stenosis/atresia with absence of pulmonary valve tissue and a ventricular septal defect. The pulmonary phenotype in inv/inv mice was situs inversus with occasional minor lobar abnormalities. We conclude that 1) cardiopulmonary malformations in inv/inv mice are not rare (37%), 2) the cardiopulmonary malformations observed in inv/inv specimens are not of the spectrum typically associated with human heterotaxia. In particular, inv/inv mice have a propensity for defects in the development of the right ventricular outflow tract and the interventricular septum, and 3) approximately one out of ten inv/inv mice is born with situs solitus and shows cardiac anomalies that correspond to those observed in inv/inv specimens with situs inversus. Our data therefore suggest that inversin, the product of the inv locus, may have specific roles in cardiac morphogenesis independent of its role in situs determination.

Background: Genetic background of the fetus contributes to the pathogenesis of congenital malformation after teratogen exposure. Such contribution is illustrated in left-right axis malformations observed in the F1 offspring of nonobese diabetic (NOD) mouse dams and sires from different strains. When sires of the NOD, ICR, or C57BL/6J were mated with NOD dams, incidence varied depending on the fetal genotype, with 65% in NOD x NOD, 24% in NOD x ICR, and 7% in NOD x C57BL/6J., Methods: As a first step in elucidating the molecular basis of the interstrain differences in susceptibility to situs defects, we compared genomic sequences of six genes HNF3beta, Acvr2b, Nodal, ZIC3, Lefty1, and Smad2, which are involved in the normal development of left-right axis among NOD, ICR, and C57BL/6J strains., Results: The outbred strain ICR had 1) a 0.2-kb insertion in the putative promoter region of the isoform E of HNF3beta together with a G to A change that could create a potential splice acceptor in the exon 3 of HNF3beta (gene frequency P = 0.36), 2) five single base substitutions within the 5' controlling element and a proline to serine substitution (P2S) of Lefty1 (P = 0.77), and 3) a tyrosine to histidine substitution within the prodomain of Nodal (P = 0.48). The inbred strain NOD had the same G to A change as ICR and a three-base deletion in the putative promoter of isoform E of HNF3beta., Conclusions: We suggest that sequence variations in HNF3beta, Lefty1, and Nodal might account, in part, for the interstrain differences in susceptibility to situs abnormalities among the offspring of diabetic dams., (Copyright 2001 Wiley-Liss, Inc.)

Heterotaxy syndromes, otherwise laterality defects, are variations from anatomic left-right asymmetry. Situs inversus is the complete reversal of the normal situs, still situs ambiguus is the randomisation of the normal organ position. Situs ambiguus may be manifested as asplenia or polysplenia syndrome. Normal situs and both types of the heterotaxy syndromes may appear among some affected families, whereas the different situs are rarely expressed in the same family. We describe an autosomal-recessive inherited familial heterotaxy syndrome with two affected siblings - one of whom has situs inversus, and the other with polysplenia syndrome. The polysplenia syndrome was diagnosed by fetal echocardiography. Since the chromosomal or molecular diagnosis of laterality defects are accessible only in X-linked heterotaxy syndromes, the fetal echocardiography is the earliest available diagnostic method in this field. Therefore, fetal echocardiography has great importance for affected families., (Copyright 2001 S. Karger AG, Basel)

Proper morphogenesis and positioning of internal organs requires delivery and interpretation of precise signals along the anterior-posterior, dorsal-ventral, and left-right axes. An elegant signaling cascade determines left- versus right-sided identity in visceral organs in a concordant fashion, resulting in a predictable left-right (LR) organ asymmetry in all vertebrates. The complex morphogenesis of the heart and its connections to the vasculature are particularly dependent upon coordinated LR signaling pathways. Disorganization of LR signals can result in myriad congenital heart defects that are a consequence of abnormal looping and remodeling of the primitive heart tube into a multi-chambered organ. A framework for understanding how LR asymmetric signals contribute to normal organogenesis has emerged and begins to explain the basis of many human diseases of LR asymmetry. Here we review the impact of LR signaling pathways on cardiac development and congenital heart disease.

All vertebrates display a characteristic asymmetry of internal organs with the cardiac apex, stomach and spleen towards the left, and the liver and gall bladder on the right. Left-right (L-R) axis abnormalities or laterality defects are common in humans (1 in 8,500 live births). Several genes (such as Nodal, Ebaf and Pitx2) have been implicated in L-R organ positioning in model organisms. In humans, relatively few genes have been associated with a small percentage of human situs defects. These include ZIC3 (ref. 5), LEFTB (formerly LEFTY2; ref. 6) and ACVR2B (encoding activin receptor IIB; ref. 7). The EGF-CFC genes, mouse Cfc1 (encoding the Cryptic protein; ref. 9) and zebrafish one-eyed pinhead (oep; refs 10, 11) are essential for the establishment of the L-R axis. EGF-CFC proteins act as co-factors for Nodal-related signals, which have also been implicated in L-R axis development. Here we identify loss-of-function mutations in human CFC1 (encoding the CRYPTIC protein) in patients with heterotaxic phenotypes (randomized organ positioning). The mutant proteins have aberrant cellular localization in transfected cells and are functionally defective in a zebrafish oep-mutant rescue assay. Our findings indicate that the essential role of EGF-CFC genes and Nodal signalling in left-right axis formation is conserved from fish to humans. Moreover, our results support a role for environmental and/or genetic modifiers in determining the ultimate phenotype in humans.

Background: Normal development of the left/right (L/R) body axis leads to the characteristic sidedness of asymmetric body structures, e.g., the left-sided heart. Several genes are now known to be expressed with L/R asymmetry during embryogenesis, including nodal, a member of the transforming growth factor-beta (TGF-beta) family. Mutations or experimental treatments that affect L/R development, such as those that cause situs inversus (reversal of the sidedness of asymmetric body structures), have been shown to alter or abolish nodal's asymmetric expression., Methods: In the present study, we examined the effects on nodal expression of alpha(1)-adrenergic stimulation, known to cause a 50% incidence of situs inversus in rat embryos grown in culture, using reverse transcription-polymerase chain reaction assay and whole-mount in situ hybridization assay., Results: In embryos cultured with phenylephrine, an alpha(1)-adrenergic agonist, nodal's normal asymmetric expression only in the left lateral plate mesoderm was altered. In some treated embryos, nodal expression was detected in either the left or right lateral plate mesoderm. However, most treated embryos lacked lateral plate mesoderm expression. In addition, the embryos that did show expression were at a later stage than when nodal expression is normally found., Conclusions: Our results demonstrate that alpha(1)-adrenergic stimulation delays the onset and perturbs the normal asymmetric pattern of nodal expression. Either of these effects might contribute to situs inversus., (Copyright 2000 Wiley-Liss, Inc.)

Recent advances have given us new insights into the molecular basis of organ position. A gene cascade that determines left-right positioning of organ primordia has emerged. In here we present the current knowledge of the molecular determinants of organ positioning during vertebrate embryogenesis.

Cilia have been classified as sensory or motile types on the basis of functional and structural characteristics; however, factors important for regulation of assembly of different cilia types are not well understood. Hepatocyte nuclear factor-3/forkhead homologue 4 (HFH-4) is a winged helix/forkhead transcription factor expressed in ciliated cells of the respiratory tract, oviduct, and ependyma in late development through adulthood. Targeted deletion of the Hfh4 gene resulted in defective ciliogenesis in airway epithelial cells and randomized left-right asymmetry so that half the mice had situs inversus. In HFH-4-null mice, classic motile type cilia with a 9 + 2 microtubule ultrastructure were absent in epithelial cells, including those in the airways. In other organs, sensory cilia with a 9 + 0 microtubule pattern, such as those on olfactory neuroepithelial cells, were present. Ultrastructural analysis of mutant cells with absent 9 + 2 cilia demonstrated that defective ciliogenesis was due to abnormal centriole migration and/or apical membrane docking, suggesting that HFH-4 functions to direct basal body positioning or anchoring. Evaluation of wild-type embryos at gestational days 7.0 to 7.5 revealed Hfh4 expression in embryonic node cells that have monocilium, consistent with a function for this factor at the node in early determination of left- right axis. Analysis of the node of HFH-4 mutant embryos revealed that, in contrast to absent airway cilia, node cilia were present. These observations indicate that there are independent regulatory pathways for node ciliogenesis compared with 9 + 2 type ciliogenesis in airways, and support a central role for HFH-4 in ciliogenesis and left-right axis formation.

The study of left-right axis malformations in man and mouse has greatly advanced understanding of the mechanisms regulating vertebrate left-right axis formation. Recently, the roles of the TGF-beta family, Sonic hedgehog and fibroblast growth factor signaling, homeobox genes, and cilia in left-right axis determination have been more clearly defined. The identification of genes and environmental factors affecting left-right axis formation has important implications for understanding human laterality defects.

The transforming growth factor-beta (TGF-beta) superfamily encompasses a large group of structurally related polypeptides that are capable of regulating cell growth and differentiation in a wide range of embryonic and adult tissues. Growth/differentiation factor-1 (Gdf-1, encoded by Gdf1) is a TGF-beta family member of unknown function that was originally isolated from an early mouse embryo cDNA library and is expressed specifically in the nervous systemin late-stage embryos and adult mice. Here we show that at early stages of mouse development, Gdfl is expressed initially throughout the embryo proper and then most prominently in the primitive node, ventral neural tube, and intermediate and lateral plate mesoderm. To examine its biological function, we generated a mouse line carrying a targeted mutation in Gdf1. Gdf1-/- mice exhibited a spectrum of defects related to left-right axis formation, including visceral situs inversus, right pulmonary isomerism and a range of cardiac anomalies. In most Gdf1-/- embryos, the expression of Ebaf (formerly lefty-1) in the left side of the floor plate and Leftb (formerly lefty-2), nodal and Pitx2 in the left lateral plate mesoderm was absent, suggesting that Gdf1 acts upstream of these genes either directly or indirectly to activate their expression. Our findings suggest that Gdf1 acts early in the pathway of gene activation that leads to the establishment of left-right asymmetry.

We examined the nodal flow of well-characterized mouse mutants, inversus viscerum (iv) and inversion of embryonic turning (inv), and found that their laterality defects are always accompanied by an abnormality in nodal flow. In a randomized laterality mutant, iv, the nodal cilia were immotile and the nodal flow was absent. In a situs inversus mutant, inv, the nodal cilia was motile but could only produce very weak leftward nodal flow. These results consistently support our hypothesis that the nodal flow produces the gradient of putative morphogen and triggers the first L-R determination event.

Extrahepatic biliary atresia (BA) is a devastating disease of the neonate in which the hepatic and/or common bile duct is obliterated or interrupted. Infants and children with this diagnosis constitute 50% to 60% of the pediatric population that undergoes orthotopic liver transplantation. However, there is still very little known about the etiology and pathogenesis of BA. Several recent studies have demonstrated that anomalies of situs determination are more commonly associated with BA than previously recognized. In this study, we examined the pathogenesis of jaundice in the inv mouse, a transgenic mouse in which a recessive deletion of the inversin gene results in situs inversus and jaundice. The results show that these mice have cholestasis with conjugated hyperbilirubinemia, failure to excrete technetium-labeled mebrofenin from the liver into the small intestine, lack of continuity between the extrahepatic biliary tree and the small intestine as demonstrated by Trypan blue cholangiography, and a liver histological picture indicative of extrahepatic biliary obstruction with negligible inflammation/necrosis within the hepatic parenchyma. Lectin histochemical staining of biliary epithelial cells in serial sections suggests the presence of several different anomalies in the architecture of the extrahepatic biliary system. These results suggest that the inversin gene plays an essential role in the morphogenesis of the hepatobiliary system and raise the possibility that alterations in the human orthologue of inversin account for some of the cases of BA in which there are also anomalies of situs determination.

We report on two cases of microdeletion of chromosome sub-band 2q37.3 with abnormal situs viscerum. The first patient had dextrocardia, duodenal and jejunal atresia, and an abdominal hernia. The liver was in the left upper quadrant, stomach in the right upper quadrant. In contrast anema the ascending colon was in the left, and descending colon on the right, with an area of atresia in the mid-jejunum. The second patient had malrotation and malposition of large and small bowel, with most of the bowels positioned above the liver and spleen. There was incomplete rotation of the cecum. The right kidney was malrotated and mal-positioned. The finding of 2q37.3 deletion in both patients implies that a locus or loci involved in the development of normal body situs lies within this chromosome region. Molecular cytogenetic evaluation for a possible 2q37.3 deletion should be considered in patients with abnormal situs viscerum.

Targeted disruption of the mouse activin receptor type IIB gene (Acvr2b) results in abnormal left-right (LR) axis development among Acvr2b-/- homozygotes [Oh and Li, 1997: Genes Dev 11:1812-1826]. The resulting malformations include atrial and ventricular septal defects, right-sided morphology of the left atrium and left lung, and spleen hypoplasia. Based on these results, we hypothesized that mutations in the type IIB activin receptor gene are associated with some cases of LR axis malformations in humans. We report here characterization of the ACVR2B genomic structure, analysis of ACVR2B splice variants, and screening for ACVR2B mutations among 112 sporadic and 14 familial cases of LR axis malformations. Two missense substitutions have been identified, one of which appears in two unrelated individuals. Neither of these nucleotide changes has been found in 200 control chromosomes. We conclude that ACVR2B mutations are present only rarely among human LR axis malformation cases.

Pitx2, a member of the bicoid-related family of homeobox-containing genes, is asymmetrically expressed in the left lateral plate mesoderm and derived tissues during chick and mouse development. Modifications of Pitx2 pattern of expression in the iv mouse mutation correlate with the situs alterations characteristic of the mutation. Misexpression experiments demonstrate that Shh and nodal positively regulate Pitx2 expression. Our results are compatible with a Pitx2 function in the late phase of the gene cascade controlling laterality.

The inv mouse was reported as a novel strain with situs inversus Yokoyama et al., '93), and a few cases with heterotaxy were found in homozygotes. The original report by Yokoyama et al. described the location of the heart and the stomach using the index of arrangement of body structure. We newly examined 40 homozygous offspring for phenotypes of visceroatrial situs and the incidence of cardiovascular anomalies making use of morphological details defined in each organ structure. According to the arrangement of each organ, which ranged from the almost complete form of situs inversus to left isomerism, visceroatrial situs was classified into four categories: Situs inversus (4 cases), "variation type" of situs (17 cases), "abdominal heterotaxy" (15 cases), and visceroatrial heterotaxy (4 cases). In offspring with situs inversus, only one had aortic stenosis (25%). Seven with the "variation type" of situs had cardiovascular anomalies, such as aortic stenosis, endocardial cushion defect, and posterior vena cava interruption (41%). All 15 offspring with "abdominal heterotaxy" had anomalies of the posterior vena cava, and three of them also had tetralogy of Fallot. The remaining four with visceroatrial heterotaxy had endocardial cushion defect, which was associated with outflow tract anomaly in two cases (i.e. tetralogy of Fallot in one case and transportation of the great arteries in the other). These results revealed that visceroatrial heterotaxy frequently occurred in the inv homozygotes, especially in the abdomen, and often showed a propensity to left isomerism with posterior vena cava interruption.

We present the clinical and postmortem findings of the first photographically documented case of asplenia and interrupted inferior vena cava and the anatomic findings of 5 previously reported cases. A brief review of the various hereditary patterns of visceral situs abnormalities suggests that, at least in some cases, the asplenia and polysplenia syndromes are etiologically and pathogenetically interrelated.

A novel extracellular matrix protein flectin (250 kD M(r)) shows specific left-right asymmetric expression before and throughout the looping process during heart development in avian embryos [Tsuda et al., 1996]. Flectin is a candidate molecule to provide directionality to the looping process in the avian model. In this study on mouse embryonic heart development, flectin is shown to be developmentally regulated and to be expressed in a specific asymmetric fashion, but in a different pattern from that observed in avian hearts. The molecules involved in development tend to be the same, but timing of expression, modulation, and asymmetry are different. In the mouse embryo, flectin is expressed symmetrically when the cardiogenic plate is formed. As looping progresses, flectin expression becomes asymmetric. There is right side predominance at the outflow tract and left side predominance at the ventricular portion of the tubular heart. The left side predominance of flectin develops in an anteroposterior direction, while right side predominance of the outflow tract remains relatively unchanged. These differential expression patterns of flectin decrease once the looping process is completed. After looping, flectin becomes restricted to the epicardium and subepicardial extracellular regions. In inv/inv mice, a known mouse model for human situs inversus, in which the directionality of heart looping is inverted, flectin expression pattern is mirror image of that of normal mouse embryos during looping stages. Our study indicates that, in the mouse, flectin shows a specific asymmetric expression pattern after initiation of heart looping and that this asymmetric expression pattern is related to the directionality of looping. The remodeling of the extracellular matrix (ECM) including specific flectin expression begins with the looping process. This morphogenetic change of the ECM coincides with the differentiation of each region of the tubular heart.

Like all vertebrates, humans establish anatomical left-right asymmetry during embryogenesis. Variation from this normal arrangement (situs solitus) results in heterotaxy, expressed either as randomization (situs ambiguus) or complete reversal (situs inversus) of normal organ position. Familial heterotaxy occurs with autosomal dominant, recessive and X-linked inheritance. All possible situs variants, solitus, ambiguus and inversus, can appear among some heterotaxy families. Positional cloning has led to the identification of a gene on the X chromosome responsible for some cases of human heterotaxy. Additional candidate genes have emerged from recent studies of left-right axis development in chick, frog and mouse, which have begun to elucidate a tightly regulated genetic cascade that differentiates the left and right sides prior to the appearance of morphological asymmetry.

Vertebrates position unpaired organs of the chest and abdomen asymmetrically along the left-right (LR) body axis. Each structure comes to lie non-randomly with respect to the midline in an overall position designated situs solitus, exemplified in humans by placement of the heart, stomach and spleen consistently to the left. Aberrant LR axis development can lead to randomization of individual organ position (situs ambiguus) or to mirror-image reversal of all lateralized structures (situs inversus). Previously we mapped a locus for situs abnormalities in humans, HTX1, to Xq26.2 by linkage analysis in a single family (LR1) and by detection of a deletion in an unrelated situs ambiguus male (Family LR2; refs 2,3). From this chromosomal region we have positionally cloned ZIC3, a gene encoding a putative zinc-finger transcription factor. One frameshift, two missense and two nonsense mutations have been identified in familial and sporadic situs ambiguus. The frameshift allele is also associated with situs inversus among some heterozygous females, suggesting that ZIC3 functions in the earliest stages of LR-axis formation. ZIC3, which has not been previously implicated in vertebrate LR-axis development, is the first gene unequivocally associated with human situs abnormalities.

The development of characteristic visceral asymmetries along the left-right (LR) axis in an initially bilaterally symmetrical embryo is an essential feature of vertebrate patterning. The allelic mouse mutations inversus viscerum (iv) and legless (lgl) produce LR inversion, or situs inversus, in half of live-born homozygotes. This suggests that the iv gene product drives correct LR determination, and in its absence this process is randomized. These mutations provide tools for studying the development of LR-handed asymmetry and provide mouse models of human lateralization defects. At the molecular level, the normally LR asymmetric expression patterns of nodal and lefty are randomized in iv/iv embryos, suggesting that iv functions early in the genetic hierarchy of LR specification. Here we report the positional cloning of an axonemal dynein heavy-chain gene, left/right-dynein (lrd), that is mutated in both lgl and iv. lrd is expressed in the node of the embryo at embryonic day 7.5, consistent with its having a role in LR development. Our findings indicate that dynein, a microtubule-based motor, is involved in the determination of LR-handed asymmetry and provide insight into the early molecular mechanisms of this process.

Abnormal left-right-axis formation results in heterotaxy, a multiple-malformation syndrome often characterized by severe heart defects, splenic abnormalities, and gastrointestinal malrotation. Previously we had studied a large family in which a gene for heterotaxy, HTX1, was mapped to a 19-cM region in Xq24-q27.1. Further analysis of this family has revealed two recombinations that place HTX1 between DXS300 and DXS1062, an interval spanning approximately 1.3 Mb in Xq26.2. In order to provide independent confirmation of HTX1 localization, a PCR-based search for submicroscopic deletions in this region was performed in unrelated males with sporadic or familial heterotaxy. A cluster of sequence-tagged sites failed to amplify in an individual who also had a deceased, affected brother. FISH identified the mother as a carrier of the deletion, which arose as a new mutation from the maternal grandfather. The deletion interval spans 600-1,100 kb and lies wholly within the 1.3-Mb region identified by recombination. Discovery of this deletion supports localization of HTX1 to Xq26.2 and reveals the first molecular-genetic abnormality associated with human left-right-asymmetry defects.

Vertebrates have characteristic and conserved left-right (L-R) visceral asymmetries, for example the left-sided heart. In humans, alterations of L-R development can have serious clinical implications, including cardiac defects. Although little is known about how the embryonic L-R axis is established, a recent study in the chick embryo revealed L-R asymmetric expression of several previously cloned genes, including Cnr-1 (for chicken nodal-related-1), and indicated how this L-R molecular asymmetry might be important for subsequent visceral morphogenesis. Here we show that nodal is asymmetrically expressed in mice at similar stages, as is Xnr-1 (for Xenopus nodal related-1) in frogs. We also examine nodal expression in two mouse mutations that perturb L-R development, namely situs inversus viscerum (iv), in which assignment of L-R asymmetry is apparently random and individuals develop either normally or are mirror-image-reversed (situs inversus), and inversion of embryonic turning (inv), in which all individuals develop with situs inversus. In both, nodal expression is strikingly affected, being reversed or converted to symmetry. These results further support a key role for nodal and nodal-related genes in interpreting and relaying L-R patterning information in vertebrates. To our knowledge, our results provide the first direct evidence that iv and inv normally function well before the appearance of morphological L-R asymmetry.

Heterotaxy results from failure to establish normal left-right asymmetry during embryonic development. Most familial cases are thought to be autosomal recessive. We have identified a family in which 4 individuals from 3 generations manifest laterality defects. Twenty-five family members have been examined. Two have complete reversal of normal laterality (situs inversus) while 2 others have asplenia, midline liver, and complex cardiac malformations (situs ambiguus). Two additional obligate gene carriers are anatomically normal (situs solitus). Male-to-male transmission confirms autosomal inheritance. Identification of this family establishes an autosomal dominant form of laterality defect, suggesting that a portion of sporadic cases may be new-mutation dominant or unrecognized familial cases. The finding of all forms of laterality (solitus, ambiguus, and inversus) among obligate disease gene carriers within a single family may be relevant to genetic evaluation and counseling in apparently isolated patients with laterality disturbance.

Previous familial cases of recurrent heterotaxia have suggested an autosomal recessive or exceptionally X-linked or dominant inheritance. Here, we report six families including 18 affected members, consistent with autosomal dominant inheritance. Among these, four families have more than one case of heterotaxia. The other two families have one member with heterotaxia and at least one other affected member with an "isolated" heart malformation, which could be considered as a mild form of heterotaxia. In five families, the disorder is transmitted through two or three generations. In one family, the patients are of the same generation but are linked to each other by obligate carriers. We suggest a rule to classify these families with heart malformations, according to the etiologic factor involved (rule of precocity). This rule might be useful to other disruptions of morphogenetic processes.

We report on a family with asplenia syndrome in one and total anomalous pulmonary venous connection (TAPVC) in the other sib. Both conditions are rare, may have a genetic cause and belong to a spectrum of laterality disorders. This suggests that both asplenia syndrome and TAPVC in this family are the clinical expression of a single genetic disorder.

A report is given of a newborn girl with situs inversus and Turner syndrome that presented respiratory distress. The patient had a mosaic karyotype 45,X/46,X + mar (80%/20%). Ciliary motion analysis demonstrated a total absence of ciliary motion whereas, ultrastructural studies revealed typical features of primary ciliary dyskinesia (PCD) (absence or short outer/inner dynein arms in 90% of the cilia). We regard this rare combination (PCD, situs inversus and Turner syndrome) as a coincidental occurrence.

X-linked laterality sequence (XLLS) consists of situs inversus, complex cardiac defects, and alterations in the development of the spleen. We describe a family in which two male cousins had XLLS with caudal manifestations. In our family, the obligate carrier females had uterine septum and hypertelorism, which may be gene carrier manifestations.

We report on a newborn infant with clinical and radiological manifestations of some type of short rib-polydactyly syndrome who died soon after birth. Chromosomal studies on peripheral blood lymphocytes and chondrocytes demonstrated an apparently balanced pericentric inversion of chromosome 4 (present in the mother also). This association may have occurred by chance but, if not, the chromosomal breakpoints could interrupt the gene responsible for short rib-polydactyly syndromes, or else be related to the mechanism of short rib-polydactyly syndromes.

Ambiguous abdominal situs, asplenia/polysplenia and severe cardiac malformations characterize heterotaxy in humans. These anomalies result from the inability of the developing embryo to establish normal left-right asymmetry. We have studied an interesting family in which the heterotaxy phenotype segregates as an X-linked recessive trait. In order to map the heterotaxy locus (HTX), we have analysed 39 family members using highly-polymorphic microsatellite markers from the X chromosome. One of these markers, DXS994, shows no recombination with the disease locus in 20 informative meioses. Linkage analysis results in a maximum lod score of 6.37. Current genetic and physical mapping data assign the order of loci in Xq24-q27.1 as cen-DXS1001-(DXS994, HTX)-DXS984-tel. These results establish the first mapping assignment of situs abnormalities in humans.