Linkage mapping of a mouse gene, iv, that controls left-right asymmetry of the heart and viscera

Extrahepatic biliary atresia and associated anomalies: etiologic heterogeneity suggested by distinctive patterns of associations

Fifty-one cases of extrahepatic biliary atresia (EHBA) with associated anomalies were found in a study of EHBA (251 cases). Analysis of segregation patterns of these anomalies in individual patients suggested the existence of 2 major groups: (1) 15 cases (29.4%) with various combinations of anomalies within the laterality sequence, and (2) 30 cases (58.8%) with one or 2 anomalies mostly involving the cardiac, gastrointestinal, and urinary systems. These latter anomalies did not follow any recognizable pattern. The third group of 6 cases all had intestinal malrotation, some with preduodenal portal vein; these cases show some similarity to the laterality sequence group and may represent a more confined phenotypic result of faulty situs determination. This previously unattempted classification of patients with EHBA and associated anomalies might enable a more targeted approach towards identification of causes in this heterogeneous disorder. EHBA within the laterality sequence might prove a suitable candidate for a major gene mutation. Teratogenic, infectious and polygenic multifactorial causes might play a more significant role in EHBA associated with "nonsyndromic" organ system anomalies.

A detailed linkage map of subtelomeric murine Chromosome 12 region including the situs inversus mutation locus IV

A mouse model is an invaluable tool to tackle genesis of human congenital diseases that have so far eluded human studies. Homozygote for the i.v. mutation, the murine Si/Col strain presents a left-right lateralization defect of thoracic and abdominal organs and heart defects very similar to human ones. This i.v. mutation has been mapped to the region between the Aat and Igh-C loci, suggesting the presence of an equivalent human gene in the human syntenic 14q3 region. A precise linkage map of the region is, therefore, of great interest since it will contribute to the genetic approach of the i.v. gene. Analysis of 242 back-cross progeny from Mus musculus (MAI) or spretus strains of mice and SI/Col mice has allowed mapping of the i.v. gene to a linkage group of eight markers. It includes four genes: Aat (alpha 1-antitrypsin), Ckb (creatine kinase, brain form), Crip (cysteine-rich intestinal protein), and Igh-C (immunoglobulin heavy chain constant region complex); three murine microsatellites: D12Mit6, D12Mit7, and D12Mit8; and one new marker, D12Mtp1, defined by a minisatellite human probe, pYNZ2. After analysis of the data by the LINKAGE program, the following multilocus map has been constructed: centromere-D12Mit6-6.9 cM-D12Mit7-1.7 cM-D12Mtp1-2.6 cM-Aat-5.0 cM-(Ckb, Igh-C)-0.4 cM-D12Mit8-0.4 cM-Crip-11.2 cM-i.v.-telomere. This map differs from the previous map in placing i.v. locus telomeric to Igh-C. D12Mit6 and D12Mit7 are now precisely mapped centromeric to the locus Aat. In addition, a new locus D12Mtp1 is located between Aat and D12Mit7.

Abnormal laterality and congenital cardiac anomalies. Relations of visceral and cardiac morphologies in the iv/iv mouse

BACKGROUND

In the management of hearts with deranged laterality, it is essential that the left and right atrial chambers are correctly identified. There are two major approaches, which are based on venous connections or on the morphology of the atrial appendages, and there is no consensus as to which is the most useful. We used the iv/iv mouse mutant, which is known to be pertinent to this problem, to evaluate the relations of cardiac defects with atrial, venous, and other visceral morphologies.

METHODS AND RESULTS

The morphology of the heart and other organs was examined in 275 iv/iv mice using criteria based on abnormal laterality in humans. The arrangement of the atrial appendages was determined by morphological examination of the junction between the appendage and the venous component of the atrium. On this basis, 45.1% of cases were shown to have usual atrial arrangement, 50.2% had mirror imagery, 1.5% had right isomerism, and 3.3% had left isomerism. Every case of atrial isomerism had a cardiac lesion; the morphological types were similar to those seen in human cases. Of cases with either usual or mirror-image arrangement of the appendages, 33.2% had abnormal spleens, but only 3.1% had cardiac defects. Similarly, venous abnormalities were much more common (30.1%) than cardiac defects.

CONCLUSIONS

Study results endorse the importance of the morphology of atrial appendages in predicting cardiac abnormalities and point to the marked inconsistency of the arrangement of other organs, including the spleen and the connections of the systemic veins.

Directional control of neurite outgrowth from cultured hippocampal neurons is modulated by the lectin concanavalin A

Cell surface carbohydrates play an important role in the regulation of neurite outgrowth during neuronal development. We have investigated the actions of the plant lectin concanavalin A (Con A), a carbohydrate-binding protein, on neurite outgrowth from hippocampal pyramidal neurons in primary cell culture. Neurons plated in culture medium containing nanomolar concentrations of Con A have a larger number of primary neurites arising directly from the cell soma than do neurons plated in culture medium alone. Furthermore, Con A causes counterclockwise turning of neurites in over 70% of the cultured neurons. Both of these effects of Con A are blocked by the hapten sugar alpha-methyl-D-mannopyranoside, suggesting that they result from the interaction of Con A with a cell surface carbohydrate. Another lectin with a different sugar specificity, wheat germ agglutinin, does not modulate neurite outgrowth. Analysis of neurite outgrowth using video-enhanced microscopy reveals that the counterclockwise turning is accompanied by directionally biased extension of filopodia from the growth cones of growing neurites. Treatment of the neurons with cytochalasin, which disrupts actin polymerization, eliminates the neurite turning induced by Con A, suggesting that actin microfilaments are involved in directional control of neurite outgrowth.

Endocardial cushion defect: further studies of "isolated" versus "syndromic" occurrence

The isolated occurrence of endocardial cushion defect (ECD) has been suggested to differ from its occurrence within the context of a syndrome, with regard to the nature (complete or partial) of the defect and the associated cardiovascular malformations. Analysis of data derived from the Baltimore-Washington Infant Study of congenital cardiovascular malformations supports the observation that "syndromic" ECD tends to be of the complete atrioventricular canal type and is less frequently associated with left cardiac anomalies than the isolated form. However, each syndrome has a unique impact on the overall cardiovascular "phenotype", including the ECD. This is especially true for Down and Ivemark syndromes, which are most frequently associated with ECD, but also for other syndromes as well. It is also suggested that isolated ECD is specifically associated with gastrointestinal and urinary tract anomalies. However, in Down syndrome ECD appears to be a specific cardiovascular expression of the trisomic state that is unrelated to other noncardiac malformations. Additional information on the association of ECD with other less common genetic syndromes is needed in order to further investigate the possible genetic basis of this cardiac defect.

Regulation of vertebrate left-right asymmetries by extracellular matrix

The vertebrate body is organized along three geometric axes: anterior-posterior, dorsal-ventral and left-right. Left-right axis formation, displayed in heart and gut development, is the least understood, even though it has been studied for many years. In Xenopus laevis gastrulae, a fibronectin-rich extracellular matrix is deposited on the basal surface of ectoderm cells over which cardiac and visceral primordia move during development. Here I report experiments in which localized perturbation of a small patch of extracellular matrix by microsurgery was correlated with localized randomization of left-right asymmetries. Global perturbation of the extracellular matrix by microinjection of Arg-Gly-Asp peptides or heparinase into the blastocoel resulted in global randomization of left-right asymmetries. From these observations, I suggest that left-right axial information is contained in the extracellular matrix early in development and is independently transmitted to cardiac and visceral primordia.

Angular homeostasis. VIII. Pursuit of a slowly moving target in a plane: relevance to lateralization in cardiovascular ontogeny

We explore the pursuit in a plane of a target moving at constant slow speed in a straight line. Two models of the pursuit are given. In the continuous case, the pursuer is moving at constant speed and is subject to proportionate angular homeostasis with correction constant b. In the discrete version movement occurs at a constant speed in a sequence of straight line segments of constant length (called the step size, s) the end of the segments being called the vertices. The pattern considered is not the absolute position of the pursuer, but its distance and orientation relative to the target. Both the transients and the asymptotic orbit are addressed. A key quantity is r, the speed of the target expressed as a fraction of that of the pursuer. If the speed of the pursuer is defined as unity, r is also the ratio of the speeds. There exists a critical speed fraction, R(b,s), a function of b and s, that defines what the term slow designates. R(b,s), which has to be found numerically, has the following property. For r less than R(b,s), the asymptotic path is a simple closed curve. In the discrete case the vertices converge to a simple closed curve. The larger r, the more the path (or in the discrete analogue its set of vertices) departs from a circle, and the more eccentric the target is with respect to it. Interest centers on two issues. First we address the transient patterns of the path, notably whether or not the sense of any particular path (clockwise or counterclockwise) is the same throughout, or changes at some stage.(ABSTRACT TRUNCATED AT 250 WORDS)

Spectrum of heart malformations in mice with situs solitus, situs inversus, and associated visceral heterotaxy

BACKGROUND

We present a study of the heart malformations found in a collection of mouse fetuses of the iv/iv strain between days 16.5 and 18.5 of gestation.

METHODS AND RESULTS

One hundred hearts were serially sectioned and studied by segmental analysis with a light microscope. Forty additional hearts were analyzed with a scanning microscope. Forty percent of the hearts were found to be malformed. The most frequently occurring heart malformations were persistence of the sinus venosus (9%), common atrium (17%), common atrioventricular canal (24%), double-outlet right ventricle (12%), Fallot's tetralogy (8%), and transposition of the great arteries (5%). These malformations do not usually occur in isolation but rather appear in the formation of complex cardiopathies. The most severe and frequent is the combination of persistence of sinus venosus, common atrium, common atrioventricular canal, and double-outlet right ventricle; this is the "bulboventricular heart." The morphology of each lesion, as well as the degree of association, is similar to that found in human hearts with complex cardiopathies. Some of these cardiopathies appear to be directly related to formation of the cardiac loop. The iv/iv mouse appears to constitute an excellent model with which to study the etiology and pathogenesis of complex heart defects in humans. These hearts show a high phenotypic variability in the presentation of heart lesions. From a genetic viewpoint, there is a basic defect--the bulboventricular heart--which can be considered congenital. The other malformations can be considered formes frustes of the defect type.

CONCLUSIONS

The iv gene is a developmental gene that affects basic developmental mechanisms. In this regard, heart lesions may not be the primary result of the abnormal gene activity but rather are secondary to defective interactions during cardiac development.

Critical period of rat development when sidedness of asymmetric body structures is determined

We recently reported that rat embryos cultured from the presomite stage in a medium containing the alpha-1 adrenergic agonist, phenylephrine, have a high incidence of situs inversus. In the present study, we have determined more precisely the critical period of development when situs inversus is induced. Rat embryos were harvested at 8 AM on Day 9 of gestation (plug day = Day 0), and divided into different stages of development, namely, early, mid, and late primitive streak stages and early, mid, and late neural plate stages. They then were cultured in rotating bottles to which phenylephrine, 0.5 mM, was added for various durations. After 49 hr of culture, embryos were examined for general morphology including sidedness of the bulboventricular loop, tail, and chorioallantoic placenta. Phenylephrine increased the incidence of situs inversus above control when administered throughout culture from either the early neural plate stage or before, and when administered for 4 hr or more from the early neural plate stage. This increase was significant even at the mid and late primitive streak stages when the control incidence was high. Our results suggest that sidedness of asymmetric body structures is determined during the early neural plate stage. This period is well before the 6-8-somite stage when morphological signs of body asymmetry first appear.

Situs inversus in the developing mouse: proteins affected by the iv mutation (genocopy) and the teratogen retinoic acid (phenocopy)

To decipher genes that are important in the determination of laterality, we compared two-dimensional protein gels from wild-type C57BL/6J mice and C57BL/6J mice that carried the iv mutation, which confers random determination of visceral situs. To span the time period(s) during which laterality determination occurs, we compared computer-analyzed two-dimensional protein gels from wild-type mouse embryos and iv/iv mouse embryos at 7.5, 8.0, and 8.5 days post-coitum. One polypeptide that was expressed only on day 8.0 of development and only in wild-type embryos represents a particular candidate for determination of laterality. Day 8.5 postcoitum represents the earliest time in murine development that laterality is manifest. Two-dimensional gels were compared from 8.5 day embryos that were C57BL/6J wild-type, C57BL/6J iv/iv, or C57BL/6J wild-type and exposed to the teratogen retinoic acid late on day 7. Reproducible alterations of protein synthesis were observed in both the iv genocopy and retinoic acid phenocopy, yielding abnormal laterality determination. The intersection of these peptide changes identifies a protein likely to play a role in the determination of laterality.

Balanced translocation 12/13 and situs abnormalities: homology of early pattern formation in man and lower organisms?

Studies in "lower" organisms have identified a set of homologous sequences expressed in oocytes and early embryos that is critical for pattern formation. Mutations in such genes may exhibit maternal effect--they cause abnormalities in the fetus only when present in the mother. We report on a mother and child with identical, apparently balanced translocations having the breakpoints 12q13.1 and 13p13. The fetus had multiple anomalies including bilateral trilobar lungs, complex heart defect, malrotation of the gut, and asplenia, while the mother was entirely normal. Several hypotheses are advanced to explain this variable expression including transection of a gene with maternal effect--lateral asymmetry in the fetus is influenced by the maternal genotype. This explanation would account for the higher transmission of congenital heart disease to offspring by affected females noted in several studies. The human counterparts of 2 loci (int-1 and HOX 3) involved in Drosophila early pattern formation are located near the translocation breakpoint 12q13.1. If one of these genes is responsible for situs abnormality, then university of positional code (but not of embryologic mechanism) is suggested for higher metazoans.

A linkage map of distal mouse chromosome 12

To refine the linkage map of distal mouse Chromosome 12, we have identified DNA restriction fragment variants associated with a creatine kinase gene (Ck-3), the Akt proto-oncogene, an Abelson proviral integration site (D12N1), and the immunoglobulin heavy chain VH3609 variable region family (Igh-V36). The patterns of inheritance of these markers in backcross progeny and recombinant inbred mouse strains allowed their localization with respect to previously mapped genes to yield the linkage map: Aat-15.8 cM-Ck-3-0.9 cM-(Crip, Akt, Igh-C)-0.3 cM-(D12N1, Igh-V). This map confirms genetically the localization of the Igh-V gene complex distal to Igh-C on the chromosome. It differs from previous maps in placing D12N1 distal to Igh-C, and in suggesting that the Igh-V gene complex spans less than one centiMorgan (cM). Other DNA sequence variants detected with the creatine kinase probe allowed definition of four additional genetic loci: Ck-1 near Lmyc-1 on Chromosome 4; Ck-2 between Upg-1 and Hprt-ps1 (D17Rp10) on distal Chromosome 17; Ck-4 near Mpmv-17 and Mls-3 on Chromosome 16; and Ck-5 near Hba on Chromosome 11.