Outcome of fetuses diagnosed with atrioventricular septal defect

Wnt/β-catenin and sonic hedgehog pathways interact in the regulation of the development of the dorsal mesenchymal protrusion

BACKGROUND

The dorsal mesenchymal protrusion (DMP) is a second heart field (SHF) derived tissue involved in cardiac septation. Molecular mechanisms controlling SHF/DMP development include the Bone Morphogenetic Protein and Wnt/β-catenin signaling pathways. Reduced expression of components in these pathways leads to inhibition of proliferation of the SHF/DMP precursor population and failure of the DMP to develop. While the Sonic Hedgehog (Shh) pathway has also been demonstrated to be critically important for SHF/DMP development and atrioventricular septation, its role in the regulation of SHF proliferation is contentious.

RESULTS

Tissue-specific deletion of the Shh receptor Smoothened from the SHF resulted in compromised DMP formation and atrioventricular septal defects (AVSDs). Immunohistochemical analysis at critical stages of DMP development showed significant proliferation defect as well as reduction in levels of the Wnt/β-catenin pathway-intermediates β-catenin, Lef1, and Axin2. To determine whether the defects seen in the conditional Smoothened knock-out mouse could be attributed to reduced Wnt/β-catenin signaling, LiCl, a pharmacological activator of this Wnt/β-catenin pathway, was administered. This resulted in restoration of proliferation and partial rescue of the AVSD phenotype.

CONCLUSIONS

The data presented suggest that the Wnt/β-catenin pathway interact with the Shh pathway in the regulation of SHF/DMP-precursor proliferation and, hence, the development of the DMP.

Complete atrioventricular canal

Complete atrioventricular canal (CAVC), also referred to as complete atrioventricular septal defect, is characterised by an ostium primum atrial septal defect, a common atrioventricular valve and a variable deficiency of the ventricular septum inflow. CAVC is an uncommon congenital heart disease, accounting for about 3% of cardiac malformations. Atrioventricular canal occurs in two out of every 10,000 live births. Both sexes are equally affected and a striking association with Down syndrome was found. Depending on the morphology of the superior leaflet of the common atrioventricular valve, 3 types of CAVC have been delineated (type A, B and C, according to Rastelli's classification). CAVC results in a significant interatrial and interventricular systemic-to-pulmonary shunt, thus inducing right ventricular pressure and volume overload and pulmonary hypertension. It becomes symptomatic in infancy due to congestive heart failure and failure to thrive. Diagnosis of CAVC might be suspected from electrocardiographic and chest X-ray findings. Echocardiography confirms it and gives anatomical details. Over time, pulmonary hypertension becomes irreversible, thus precluding the surgical therapy. This is the reason why cardiac catheterisation is not mandatory in infants (less than 6 months) but is indicated in older patients if irreversible pulmonary hypertension is suspected. Medical treatment (digitalis, diuretics, vasodilators) plays a role only as a bridge toward surgery, usually performed between the 3rd and 6th month of life.

Cell type-specific over-expression of chromosome 21 genes in fibroblasts and fetal hearts with trisomy 21

Background

Down syndrome (DS) is caused by trisomy 21 (+21), but the aberrations in gene expression resulting from this chromosomal aneuploidy are not yet completely understood.

Methods

We used oligonucleotide microarrays to survey mRNA expression in early- and late-passage control and +21 fibroblasts and mid-gestation fetal hearts. We supplemented this analysis with northern blotting, western blotting, real-time RT-PCR, and immunohistochemistry.

Results

We found chromosome 21 genes consistently over-represented among the genes over-expressed in the +21 samples. However, these sets of over-expressed genes differed across the three cell/tissue types. The chromosome 21 gene MX1 was strongly over-expressed (mean 16-fold) in senescent +21 fibroblasts, a result verified by northern and western blotting. MX1 is an interferon target gene, and its mRNA was induced by interferons present in +21 fibroblast conditioned medium, suggesting an autocrine loop for its over-expression. By immunohistochemistry the p78^MX1 protein was induced in lesional tissue of alopecia areata, an autoimmune disorder associated with DS. We found strong over-expression of the purine biosynthesis gene GART (mean 3-fold) in fetal hearts with +21 and verified this result by northern blotting and real-time RT-PCR.

Conclusion

Different subsets of chromosome 21 genes are over-expressed in different cell types with +21, and for some genes this over-expression is non-linear (>1.5X). Hyperactive interferon signaling is a candidate pathway for cell senescence and autoimmune disorders in DS, and abnormal purine metabolism should be investigated for a potential role in cardiac defects.

Poor sensitivity of routine fetal anomaly ultrasound screening for antenatal detection of atrioventricular septal defect

OBJECTIVE

To report the antenatal detection rate in a consecutive series of liveborn infants with atrioventricular septal defect (AVSD).

DESIGN

Review and analysis of referrals for detailed fetal echocardiography and postnatal diagnosis of AVSD.

SETTING

Tertiary referral centre for congenital heart disease centre with data prospectively collected between 1996 to 2001.

RESULTS

92 consecutively liveborn infants with AVSDs were identified of which 27 (29%) were detected by routine obstetric antenatal ultrasound. The antenatal diagnosis rate was worse for liveborn infants with trisomy 21 (12 of 49 (25%) v 15 of 43 (35%) chromosomally normal children) and for infants with AVSD without other structural heart disease (18 of 74 (24%) v 9 of 18 (50%) infants with associated structural heart disease).

CONCLUSION

Despite the potential ability of fetal ultrasound to detect AVSDs, the antenatal diagnosis rate is poor. This is particularly true for infants with trisomy 21 and is of importance when counselling parents with an apparently normal fetal ultrasound scan.

Prenatal diagnosis and management of fetal cardiovascular malformations

Screening for fetal cardiovascular malformations is widely performed. Its accuracy is not yet satisfactory, but better training of ultrasonographers and extension from the four-chamber view to the study of the outflow tract are probably clues to an improvement. The main impact of prenatal diagnosis is still the termination of pregnancy for severe malformations and for those associated with chromosomal or extracardiac anomalies. There is now evidence that prenatal diagnosis improves perinatal morbidity or mortality for some malformations. New information about the molecular genetic basis of congenital heart disease will help in management and counselling.