A large spectrum of mutations within the ELN
locus has been identified as being responsible for nonsyndromic SVAS.1, 4, 5, 9, 11, 12, 13
Reduction of ELN
expression has been reported in the skin fibroblast and aortic smooth muscle cells of SVAS patients, with premature truncation mutations in ELN
resulting in haploinsufficiency of elastin.5, 10
In an attempt to molecularly characterize our SVAS patient group, we performed a mutation screening of the ELN
gene. We detected seven novel mutations, of which five result in PTC and two affect the natural splicing of the ELN
Consistent with previous studies, our results of mRNA expression analysis of two identified PTC mutations showed that the functional haploinsufficiency of ELN, through nonsense-mediated degradation of mRNA, is one of the primary pathogenic mechanisms leading to SVAS. Even if not formally proven in this study, the other three PTC mutations are also probably targeted by NMD.
Interestingly, in one of the familial cases, the same change was also detected in one of the asymptomatic parents. This was not surprising because it has been reported that disease severity within SVAS families varies from asymptomatic carriers of the mutation to individuals who die in infancy from severe cardiac disease.1
Variable expressivity and reduced penetrance of elastin arteriopathy are observed in both WBS and nonsyndromic SVAS.14
This variability is typical of diseases associated with haploinsufciency, in which the genetic background is expected to have a major modifying effect.4
To date, only few ELN
gene alterations reported in SVAS patients are missense mutations.1, 5, 12, 15
In our group of SVAS patients, we found two point mutations; they induce aberrant and/or additional splicing and are predicted to lead to truncated proteins as well.
We analyzed the effect of the c.2044+5G>C mutation in fibroblast culture and found an aberrant transcript that is predicted to produce a significant amount of abnormal tropoelastin lacking the domains encoded by the last six exons of the gene. The missing part contains several essential hydrophilic cross-linking domains and the well-conserved C-terminus region characterized by two cysteine residues forming a disulfide bond and the positively charged RKRK sequence. It has been shown that the disruption of the disulfide bond dramatically reduces the ability of tropoelastin to assemble and be included in developing fibers, as it does the removal of the C-terminal region.16
Therefore, we can speculate that the mutant tropoelastin, if secreted from the cell, would likely impair the tropoelastin from assembling in a dominant-negative manner, resulting in a deleterious effect on the normal elastogenesis in those patients. A definitive resolution of this issue would require arterial smooth muscle in order to investigate whether these mutations yield abnormal elastin and/or normal elastogenesis, but this kind of tissue is not available.
Finally, we did not find any ELN gene abnormality with our approach in the remaining sporadic and familial patients with clinically diagnosed SVAS included in the study. Familial cases were not investigated for linkage analysis because of the small number of family members available for analysis. DHPLC detection mutation rate and mutations in the regulatory regions of the gene might be the possible causes for the lack of ELN mutations in those patients. Alternatively, mutations in another gene could also cause SVAS. In this regard, further studies are required.
In summary, this study illustrates the importance of conducting mutation screening of the ELN gene among patients with vascular abnormalities, particularly SVAS and pulmonary stenosis, so that new mutations can be identified and characterized. Our findings also reinforce the idea that haploinsufficiency at elastin is the main cause of these vasculopathies, hence, therapeutic strategies should be directed toward the compensation of this pathogenic mechanism.