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

A Model for Human Situs Determination

Alterations of human laterality range from situs reversal or consensus isometry to isolated anomalies of the cardiac, respiratory, gastrointestinal, skeletal, and central nervous systems. A mechanism for human situs determination has been derived from the general model of Brown and Wolpert (1990) , with steps involving A-P axis, D-V axis, midline, global situs, and local situs specification. Comparison with Drosophila segmentation is supported by maternal transmission of certain human situs defects and mutation of appropriate Drosophila gene homologues in the human Waardenburg and Greig syndromes. Anteroposterior gradients in expression of vertebrate homeotic genes may relate to a proposed hierarchy of regional laterality decisions. Early alterations in A-P or D-V axis polarity would produce situs reversal in 100 of individuals, as observed in pure situs inversus, homozygous inv mice or manipulated Xenopus embryos. Later alterations would permit random right-left decisions and account for heterotaxy in 50 of affected individuals, as observed in poly asplenia or homozygous iv mice. Randomisation of brain asymmetry could explain why situs but not brain laterality may be reversed in humans, with forebrain situs reversal or isometry leading to brain anomalies. Homologues of Drosophila genes regulating axis polarity, heart and gut development are attractive candidates in human laterality disorders, but none is uniquely localised to the 6q14-q21 or9q32-q34, 7q22, 10q21-22, 11q13 or 11q25, 12q13, 13qter, or Xq24-q27.1 chromosome regions highlighted by heterotaxic patients or mutant mice.

Abnormal abdominal situs: what and how should we look for?

BACKGROUND

Prenatal management of abnormal abdominal situs is challenging since prognosis is highly variable depending on the associated malformations.

METHODS

The authors report on two cases of ambiguous abdominal situs. Prenatal management included specialized ultrasound examination of fetal anatomy and heart, amniocentesis for determination of the fetal karyotype and fetal abdominal MR.

RESULTS

In both cases, abnormal situs was associated with other abdominal anomalies such as azygous continuation, biliary atresia, bowel obstruction and polysplenia. Fetal MR was a useful additional tool in the assessment of fetal abdominal anatomy in both cases.

CONCLUSIONS

On the basis of these reports, the literature is reviewed and a prenatal management scheme is proposed.

Absent left superior vena cava with persistent right superior vena cava in visceroatrial situs inversus

We report an unusual anomaly of the upper body venous drainage in a patient with visceroatrial situs inversus. The patient had a persistent right superior vena cava draining into the left-sided right atrium by way of the coronary sinus, with the absence of the left superior vena cava associated with atrioventricular discordant levocardia, pulmonary atresia, and ventricular septal defect.

Heterotaxy syndrome -- asplenia and polysplenia as indicators of visceral malposition and complex congenital heart disease

Heterotaxy results from failure of the developing embryo to establish normal left-right asymmetry. Typical manifestations include abnormal symmetry and malposition of the thoraco-abdominal organs and vessels, complex congenital heart disease and extracardiac defects involving midline-associated structures. The spleen is almost always affected, and there is syndromic clustering of the malformations corresponding to the type of splenic abnormality present. This review outlines the embryologic and genetic background of the heterotaxy syndrome as well as the characteristic anatomic features, clinical manifestations, and diagnostic clues of its two main presentations with asplenia or polysplenia.

Genetics of human laterality disorders: insights from vertebrate model systems

Many internal organs in the vertebrate body are asymmetrically oriented along the left-right (L-R) body axis. Organ asymmetry and some components of the molecular signaling pathways that direct L-R development are highly conserved among vertebrate species. Although individuals with full reversal of organ L-R asymmetry (situs inversus totalis) are healthy, significant morbidity and mortality is associated with perturbations in laterality that result in discordant orientation of organ systems and complex congenital heart defects. In humans and other vertebrates, genetic alterations of L-R signaling pathways can result in a wide spectrum of laterality defects. In this review we categorize laterality defects in humans, mice, and zebrafish into specific classes based on altered patterns of asymmetric gene expression, organ situs defects, and midline phenotypes. We suggest that this classification system provides a conceptual framework to help consolidate the disparate laterality phenotypes reported in humans and vertebrate model organisms, thereby refining our understanding of the genetics of L-R development. This approach helps suggest candidate genes and genetic pathways that might be perturbed in human laterality disorders and improves diagnostic criteria.

Heterotaxy with left atrial isomerism in a patient with deletion 18p

We report on a female infant with partial deletion of the short arm of chromosome 18 (del 18p) and heterotaxy with left atrial isomerism. Congenital heart defect (CHD) is found in 10% of the literature reports. Interestingly, situs abnormalities have been diagnosed in four patients with del 18p, including ours. This finding could imply that a locus or loci involved in the development of normal body situs lies within this chromosomal region. Del 18p must be consid- ered when evaluating a patient with phenotypic anomalies and CHD in lateralization defects.

Heterotaxy: associated conditions and hospital-based prevalence in newborns

PURPOSE

To provide insight into the possible etiology and prevalence of heterotaxy, we studied conditions associated with heterotaxy in a consecutive hospital population of newborns.

METHODS

From 1972 to March, 1999 (except February 16, 1972 to December 31, 1978), 58 cases of heterotaxy were ascertained from a cohort of 201,084 births in the ongoing Active Malformation Surveillance Program at the Brigham and Women's Hospital. This registry includes livebirths, stillbirths, and elective abortions. Prevalence among nontransfers (i.e., patients whose mothers had planned delivery at this hospital) was calculated as approximately 1 per 10,000 total births (20 of 201,084).

RESULTS

We analyzed a total of 58 patients consisting of 20 (34%) nontransfers and 38 (66%) transfers. Patients were categorized by spleen status as having asplenia (7 nontransfers, 25 total), polysplenia (8, 20), right spleen (4, 11), normal left (0, 1), and unknown (1, 0). Among the 20 nontransfer and 59 total heterotaxy patients, the following associated medical conditions were present: chromosome abnormality (1 nontransfer, 2 total), suspected Mendelian or chromosome microdeletion disorder (1 nontransfer, 6 total), and maternal insulin-dependent diabetes mellitus (1 nontransfer, 2 total). There were 6 twins (1 member each from 6 twin pairs including 1 dizygous, 4 monozygous, 1 conjoined; 2 were nontransfers). An associated condition occurred in 5 (25%) nontransfer and 16 (28%) total patients, or among 10 of 53 singleton births (19%).

CONCLUSIONS

Although most cases of heterotaxy in this series were sporadic events, an associated condition was present in about one-fourth of the cases. Not all of these conditions would be considered causative etiologies. Based on this small series alone, maternal insulin-dependent diabetes cannot be viewed as a risk factor for heterotaxy. However, the specific association of diabetes with polysplenia with/without left atrial isomerism is noteworthy, and adds weight to animal and epidemiologic case-control data.

Microdeletion of chromosome sub-band 2q37.3 in two patients with abnormal situs viscerum

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.

Mutation of the mouse hepatocyte nuclear factor/forkhead homologue 4 gene results in an absence of cilia and random left-right asymmetry

Winged helix transcription factors play important roles in cellular differentiation and cell-specific gene expression. To define the role of the winged helix factor hepatocyte nuclear factor/forkhead homologue (HFH)-4, a targeted mutation was created in the mouse hfh-4 gene. No expression of HFH-4 was detected in hfh-4(-)/- mice by RNA blot analysis, in situ hybridization, or RT-PCR. hfh-4(-)/- mice were noted to have abnormalities of organ situs consistent with random determination of left-right asymmetry. In addition, a complete absence of cilia was noted in hfh-4(-)/- mice. The hfh-4 gene is thus essential for nonrandom determination of left-right asymmetry and development of ciliated cells. Homozygous mutant mice also exhibited prenatal and postnatal growth failure, perinatal lethality and, in some cases, hydrocephalus. RT-PCR revealed an absence of left-right dynein (lrd) expression in the embryonic lungs of hfh-4(-)/- mice, suggesting that HFH-4 may act by regulating expression of members of the dynein family of genes. The abnormalities in ciliary development and organ situs in hfh-4(-)/- mice are similar to those observed in human congenital syndromes such as Kartagener syndrome. Targeted mutation of hfh-4 thus provides a model for elucidating the mechanisms regulating ciliary development and determination of left-right asymmetry.

De novo balanced translocation (6;18)(q21;q21.3 or q22) [corrected] in a patient with heterotaxia

We report on a sporadic case of heterotaxia with a de novo chromosome structural abnormality. The patient had inversely located heart (dextrocardia), stomach, duodenum, and cecum. In addition, she had cerebral atrophy, hypertelorism with telecanthus, infraorbital skin furrows, ear-lobe grooves, prominent maxilla and teeth, large carp mouth, short fifth fingers with limited flexion, generalized hypotonicity, and severe psychomotor retardation. High-resolution chromosome banding analysis demonstrated an apparently balanced translocation: 46,XX,t(6;18)(q21;q21.3). It is hypothesized that both heterotaxia and the chromosomal abnormality in the patient are causally related and a putative situs determining gene has been disrupted by the chromosome break, i.e., a position effect or a cryptic deletion at around the breakpoints. The translocation in our patient may be a good source for positional cloning of the gene.

Defects in the determination of left-right asymmetry

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Asplenia syndrome in a child with a balanced reciprocal translocation of chromosomes 11 and 20 [46,XX,t(11;20)(q13.1;q13.13)]

We present a 6-year-old girl with a balanced 11;20 translocation [46,XX,t(11;20)(q13.1;q13.13)pat], asplenia, pulmonic stenosis, Hirschsprung disease, minor anomalies, and mental retardation. This case represents the second report of an individual with situs abnormalities and a balanced chromosome rearrangement involving a breakpoint at 11q13. Polymerase chain reaction (PCR) analysis of microsatellite markers excluded uniparental disomy for chromosomes 11 and 20. Segregation analysis of markers in the 11q13 region in the proposita and her phenotypically normal carrier sibs did not show a unique combination of maternal and paternal alleles in the patient. We discuss several possible explanations for the simultaneous occurrence of situs abnormalities and a balanced 11;20 translocation. These include (1) chance, (2) a further chromosome rearrangement in the patient, (3) gene disruption and random situs determination, and (4) gene disruption plus transmission of a recessive or imprinted allele from the mother.

Asplenia syndrome and isolated total anomalous pulmonary venous connection in siblings

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.

Homeobox genes and the patterning of skin diseases

Fundamental to our understanding of skin diseases and their presentation is an understanding of the pattern of their development. When we have established the molecular basis of their development we will be in a much better position to control and treat such diseases. The homeobox genes are a family of regulatory proteins that influence pattern formation at many levels. Their presence in skin implicates them in this important role. It seems highly likely that they will be shown to be fundamental to the development of the patterns used in diagnosing skin diseases.

Mutational risks in females: genomic imprinting and maternal molecules

Genetic mechanisms for selective mutagenesis in female mammals might include alterations of genomic imprinting, maternally derived molecules, mitochondrial DNA or sex chromosome loci. None of these mechanisms provides an obvious explanation for the higher mutational rates observed for certain mutagens in mouse female pronuclei, but the association of DNA methylation with maternal genomic imprinting is an enticing avenue for research. Further characterization of the extent and homology of genomic imprinting among mammals is required before its relevance to mutagenesis can be determined. The existence of maternal effect mutations in mammals merits evaluation but is not yet proven. The relevance of mitochondrial DNA to female-specific mutagenesis will be greatest in multi-generational studies.

Sex ratios of affected and transmitting members of multiple case families with neural tube defects

In order to study the genetic aspects of the relation between neural tube defects and sex, we selected families with at least two closely related affected members. The sex ratios of both affected and normal transmitting persons were determined in these multiple case families. Our results indicate that there is a relation between the position of the lesion in the spine and sex. Furthermore, the affected persons in one family show significant concordance for sex as shown by the analysis of families with just two affected members. To our surprise, the group of normal transmitters appears to consist of significantly more females than males. This is in contrast to similar families with non-syndromic cleft lip +/- palate, where males predominate both among affected persons and normal transmitters. Finally, affected females most often inherited the predisposition to a neural tube defect from their mother. The possible role of inherited factors is discussed.

Genomics of human dysmorphogenesis

A survey of Mendelian Inheritance in Man emphasizes the large Mendelian contribution to human dysmorphogenesis and contrasts single gene conditions with chromosomal disorders. There were 1761 conditions that involved altered morphogenesis (49% of disease entries), including 1040 multiple defect syndromes and 721 inherited single birth defects. Premature death (36-57%), mental retardation (20-59%), and growth retardation (37-59%) are more frequent in autosomal recessive or X-linked syndromes, while predisposition to tumorigenesis was more common in dominant (16%) than recessive (3.4%) syndromes. Comparison of the Mendelian conditions with 100 chromosomal disorders showed a strikingly similar spectrum of malformation, with skeletal, craniofacial, eye, epidermal, and neuromuscular systems being most frequently affected. Chromosomal syndromes average 10.6 systems affected per disorder, in contrast to 3.55 for Mendelian syndromes, and pleiotropy does correlate weakly with aneuploid segment length. Genomic understanding of these relationships is still primitive, with 74 of 1609 (4.6%) autosomal conditions and 43 of 152 (29%) X-linked conditions mapped to specific chromosomal regions. The societal toll of human dysmorphogenesis and the evident progress with X-linked disorders provide a powerful rationale for the Human Genome Project.