Matrisome and Immune Pathways Contribute to Extreme Vascular Outcomes in Williams-Beuren Syndrome

BACKGROUND

Supravalvar aortic stenosis (SVAS) is a characteristic feature of Williams-Beuren syndrome (WBS). Its severity varies: ~20% of people with Williams-Beuren syndrome have SVAS requiring surgical intervention, whereas ~35% have no appreciable SVAS. The remaining individuals have SVAS of intermediate severity. Little is known about genetic modifiers that contribute to this variability.

METHODS AND RESULTS

We performed genome sequencing on 473 individuals with Williams-Beuren syndrome and developed strategies for modifier discovery in this rare disease population. Approaches include extreme phenotyping and nonsynonymous variant prioritization, followed by gene set enrichment and pathway-level association tests. We next used GTEx v8 and proteomic data sets to verify expression of candidate modifiers in relevant tissues. Finally, we evaluated overlap between the genes/pathways identified here and those ascertained through larger aortic disease/trait genome-wide association studies. We show that SVAS severity in Williams-Beuren syndrome is associated with increased frequency of common and rarer variants in matrisome and immune pathways. Two implicated matrisome genes (ACAN and LTBP4) were uniquely expressed in the aorta. Many genes in the identified pathways were previously reported in genome-wide association studies for aneurysm, bicuspid aortic valve, or aortic size.

CONCLUSIONS

Smaller sample sizes in rare disease studies necessitate new approaches to detect modifiers. Our strategies identified variation in matrisome and immune pathways that are associated with SVAS severity. These findings suggest that, like other aortopathies, SVAS may be influenced by the balance of synthesis and degradation of matrisome proteins. Leveraging multiomic data and results from larger aorta-focused genome-wide association studies may accelerate modifier discovery for rare aortopathies like SVAS.

Novel mutation in ELN gene causes cardiac abnormalities and inguinal hernia: case report

BACKGROUND

Elastin-driven genetic diseases are a group of complex diseases driven by elastin protein insufficiency and dominant-negative production of aberrant protein, including supravalvular aortic stenosis (SVAS) and autosomal dominant cutis laxa. Here, a Chinese boy with a novel nonsense mutation in the ELN gene is reported.

CASE PRESENTATION

We report a 1-year-old boy who presented with exercise intolerance, weight growth restriction with age, a 1-year history of heart murmur, and inguinal hernia. Gene sequencing revealed a novel nonsense mutation in the ELN gene (c.757 C > T (p.Gln253Ter), NM_000501.4). Due to severe branch pulmonary artery stenosis, the reconstruction of the branch pulmonary artery with autologous pericardium was performed. The inguinal hernia repair was performed 3 months postoperatively. After six months of outpatient follow-up, the child recovered well, gained weight with age, and had no special clinical symptoms.

CONCLUSION

We identified a de novo nonsense mutation in the ELN gene leading to mild SVAS and severe branch pulmonary artery stenosis. A new phenotype of inguinal hernia was also needed to be considered for possible association with the ELN gene. Still, further confirmation will be necessary.

Domains 16 and 17 of tropoelastin in elastic fibre formation

Naturally occurring mutations are useful in identifying domains that are important for protein function. We studied a mutation in the elastin gene, 800-3G>C, a common disease allele for SVAS (supravalvular aortic stenosis). We showed in primary skin fibroblasts from two different SVAS families that this mutation causes skipping of exons 16-17 and results in a stable mRNA. Tropoelastin lacking domains 16-17 (Delta16-17) was synthesized efficiently and secreted by transfected retinal pigment epithelium cells, but showed the deficient deposition into the extracellular matrix compared with normal as demonstrated by immunofluorescent staining and desmosine assays. Solid-phase binding assays indicated normal molecular interaction of Delta16-17 with fibrillin-1 and fibulin-5. However, self-association of Delta16-17 was diminished as shown by an elevated coacervation temperature. Moreover, negative staining electron microscopy confirmed that Delta16-17 was deficient in forming fibrillar polymers. Domain 16 has high homology with domain 30, which can form a beta-sheet structure facilitating fibre formation. Taken together, we conclude that domains 16-17 are important for self-association of tropoelastin and elastic fibre formation. This study is the first to discover that domains of elastin play an essential role in elastic fibre formation by facilitating homotypic interactions.

Novel mutations in the human elastin gene (ELN) causing isolated supravalvular aortic stenosis

Supravalvular aortic stenosis (SVAS), an inherited vascular disease, is caused by mutations in the elastin gene (ELN). Our aim was to identify novel mutations of ELN and to determine the expression of ELN in patients with SVAS. For screening mutations in ELN, we performed PCR-directed sequence analysis with genomic DNA isolated from SVAS patients and control subjects. Expression of ELN at the mRNA and protein levels were assessed by real-time PCR and Western blot analyses, respectively, using primary skin fibroblast cultures established from SVAS patients and control subjects. We identified two novel mutations of ELN, G297_A308del and Q700X, in two unrelated Korean patients with isolated SVAS. G297_A308del occurred de novo while Q700X was derived maternally. In the patient with G297_A308, elastin expression was not significantly altered at the mRNA level, but was reduced to approximately 50% of the normal control at the protein level. The elastin expression levels in the patient with Q700X were reduced to <50% of the normal controls at both the mRNA and protein levels. Our findings confirm that functional haploinsufficiency of elastin is responsible for the pathogenesis associated with isolated SVAS across different ethnic backgrounds.

New insights into elastin and vascular disease

Elastin is synthesized and secreted by vascular smooth muscle cells and is the major extracellular matrix component deposited in the arterial wall. When last reviewed by this journal in 1994, the link between elastin and a rare occlusive vascular disease had just been established. Since that time, it has become increasingly clear that elastin is a critical autocrine factor that maintains vascular homeostasis through a combination of biomechanical support and biologic signaling. This review examines the complexity of elastin-smooth muscle cell interactions, and how new insights may impact understanding of the pathogenesis and treatment of vascular disease.

Connection between elastin haploinsufficiency and increased cell proliferation in patients with supravalvular aortic stenosis and Williams-Beuren syndrome

To elucidate the pathomechanism leading to obstructive vascular disease in patients with elastin deficiency, we compared both elastogenesis and proliferation rate of cultured aortic smooth-muscle cells (SMCs) and skin fibroblasts from five healthy control subjects, four patients with isolated supravalvular aortic stenosis (SVAS), and five patients with Williams-Beuren syndrome (WBS). Mutations were determined in each patient with SVAS and in each patient with WBS. Three mutations found in patients with SVAS were shown to result in null alleles. RNA blot hybridization, immunostaining, and metabolic labeling experiments demonstrated that SVAS cells and WBS cells have reduced elastin mRNA levels and that they consequently deposit low amounts of insoluble elastin. Although SVAS cells laid down approximately 50% of the elastin made by normal cells, WBS cells deposited only 15% of the elastin made by normal cells. The observed difference in elastin-gene expression was not caused by a difference in the stability of elastin mRNA in SVAS cells compared with WBS cells, but it did indicate that gene-interaction effects may contribute to the complex phenotype observed in patients with WBS. Abnormally low levels of elastin deposition in SVAS cells and in WBS cells were found to coincide with an increase in proliferation rate, which could be reversed by addition of exogenous insoluble elastin. We conclude that insoluble elastin is an important regulator of cellular proliferation. Thus, the reduced net deposition of insoluble elastin in arterial walls of patients with either SVAS or WBS leads to the increased proliferation of arterial SMCs. This results in the formation of multilayer thickening of the tunica media of large arteries and, consequently, in the development of hyperplastic intimal lesions leading to segmental arterial occlusion.

[Role of elastin in the development of vascular function. Knock-out study of the elastin gene in mice]

The elastic fibres endow extensible tissues with resiliency, such as in blood vessels, heart, skin and lung. Elastic fibres are made of microfibrils, and mainly elastin (90%) which provides the fibre with elasticity. Beside the biomechanical role of elastin, a close correlation between elastin and elastic fibre network disorganisation and vascular smooth muscle cell (VSMC) growth disregulation has been known for several years through the description and study of several human or animal polyfeatured or obstructive vascular diseases, such as supravalvular aortic stenosis (SVAS) and Williams syndrome (WS), both related to heterozygous mutations or deletion in the elastin gene. The study of mice knock-out for the elastin gene (homozygous or heterozygous) leads to think that elastin should now be seen as an important elastic component providing extensible tissues with resiliency, as well as a major developmental regulator of VSMC life cycle and smooth muscle tissue organisation. Further developments in the area of preventive therapy of SVAS, WS or other inherited muscular disorders are likely to arise from these results.

Effects of aortic stenosis on renal renin, angiotensin receptor, endothelin and NOS gene expression in rats

BACKGROUND/AIMS

Published data regarding the effects of common cardiovascular diseases, i.e. aortic stenosis on renal regulation of major vasoconstrictive (renin, endothelins) and vasodilatory systems (NO) are controversial. Therefore we aimed to evaluate the effects of chronic aortic stenosis on the renal renin-angiotensin, endothelin and NO systems.

METHODS

Experimental supravalvular aortic stenosis was induced by using silver clips with a 0.6 mm internal diameter on the ascending aorta of weanling rats. Renal endothelin-1 (ET-1), endothelin-3 (ET-3), renin, AT(1a), AT(1b), eNOS, and bNOS gene expression were assessed by RNase protection assay.

RESULTS

Renal renin gene expression increased twofold in rats with aortic stenosis. In contrast, renal ET-1, ET-3, eNOS, bNOS, and AT(1a), AT(1b) gene expression were unchanged in rats with aortic stenosis.

CONCLUSION

Our study demonstrates that in rats with severe experimental supravalvular aortic stenosis only renal renin gene expression is stimulated. This contrasts with severe heart failure where endothelins and NO synthases are also upregulated. Different patterns of regulation of renal vasoactive mediators may be of importance for the extent of the renal impairment associated with aortic stenosis, and may be correlated with the severity of congestive heart failure.