Bouncing back from elastin deficiency.

Large arteries are comprised of vascular smooth muscle cells (SMCs) embedded within a complex, cell-derived extracellular matrix. Collagen and elastic fibers, the major constituents of the vascular matrix, are secreted and assembled by SMCs and confer tensile and elastic properties. In the medial layer of elastic arteries, elastin forms concentric fenestrated lamellar layers that intercalate with alternating rings of SMCs to form functional lamellar units.1 In the aorta, elastic fibers represent the largest component of the extracellular matrix, contributing up to 50% of aortic dry weight.2 A series of elegant reports has demonstrated a critical role for elastin in the regulation of vascular morphogenesis in mice. Elastin (eln)-null mice die shortly after birth because of aortic obstruction by SMC proliferation.3 Heterozygous mice (eln / ) are viable but produce 50% less elastin mRNA and protein these animals are hypertensive, exhibit thinner elastic lamellae, more lamellar units, decreased aortic compliance, and mild cardiac hypertrophy compared with eln / mice.4 Extensive experimental studies have revealed that elevated arterial pressure is an adaptation to maintain cardiac output and tissue perfusion in spite of vessel stiffness,5 whereas the increase in lamellar unit number is an adaptation to normalize wall stress.6 In this issue of Circulation Research, Hirano et al report the phenotypic rescue of elastin-deficient mice by generation of a humanized elastin mouse.7 Using a bacterial artificial chromosome encoding the entire human elastin gene (hBAC), they engineered mice to express functional human elastin (ELN) under the control of its native promoter. Several transgenic founder lines demonstrated at least 1 functional ELN insert and were capable of producing human elastin mRNA. Spatial and temporal expression of the human ELN transgene was similar to endogenous mouse elastin. Moreover, the hBAC mRNA product was appropriately spliced, and the protein was correctly secreted, assembled, and incorporated into mouse elastic fibers. At first glance, it may seem surprising that human elastin can substitute for mouse elastin, considering the differences in exon splicing and the lower than average amino acid conservation between species. In retrospect, however, Hirano et al might have expected the 2 proteins to be interchangeable, because it is primarily the structure of the elastin protein that is important for function. The alternating hydrophobic and crosslinking domains of elastin are conserved throughout vertebrate evolution,8 and it is this repetitive domain structure that promotes the self-assembly of elastin into fibrillar structures,9 provides elastomeric properties,10,11 and stabilizes elastin to withstand repeated cycles of extension and recoil.12 Transgenic expression of human elastin prevented lethality in the eln-null mouse, although the rescued animals expressed only 30% of normal elastin levels. In accordance with the lower elastin content, these animals exhibited a more severe cardiovascular phenotype than eln / mice, evidenced by stiffer vessels, higher blood pressure, and greater cardiac hypertrophy. Introduction of the hBAC into the eln / mice increased elastin content by 40%, and this resulted in a decrease in lamellar unit number and arterial pressure to levels measured in eln / mice, and partially restored vascular compliance. Taken together, these findings suggest a direct relationship between the amount of elastin produced (mouse and human combined) and the severity of the cardiovascular defect in mice. Thus, these mice might provide an elegant system for teasing apart the different thresholds for elastin expression which lead to specific abnormalities in elastic tissues. Indeed, these authors have also used this model to investigate elastin-dosing effects on lung development and susceptibility to smoke-induced emphysema.13 In humans, ELN deficiency has been attributed to genetic diseases (reviewed by Milewicz et al14). However no relationship has been established between elastin protein levels and cardiovascular phenotype, although it likely exists based on the wide spectrum of cardiovascular disease severity in patients with elastin deficiencies. Some people hemizygous for ELN can survive into adulthood with little or no cardiovascular problems, whereas in others, the vascular system is severely compromised even before birth.15 There are clearly other factors, most likely a combination of genetic and environmental, that influence the severity of elastin-related arteriopathy in humans. Population-specific differences in exon splicing and elastin deposition have recently been identified,16 and 3 quantitative trait loci affecting elastin production have been mapped in rats.17 Future research in humans and rodents will focus on identifying these important modifier genes, because they will be crucial to implementing successful genetic therapies. Although certain physiological parameters were improved in the hBAC-null animals, other key functions of elastin were not examined in the current study. Studies of eln / mice have revealed that elastin is required to maintain SMC quiescence and circumferential orientation in vivo.3 These studies are consistent with in vitro findings that insoluble The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Department of Laboratory Medicine and Pathobiology (P.J.A., M.P.B.), Institute of Medical Science (P.J.A., L.R.O., M.P.B.), and Department of Molecular and Medical Genetics (L.R.O.), University of Toronto, Ontario, Canada. Correspondence to Michelle P. Bendeck, Professor, Career Investigator, Heart and Stroke Foundation of Ontario, Department of Laboratory Medicine and Pathobiology, University of Toronto, Medical Sciences Building, Room 6213, 1 King’s College Circle, Toronto, Ontario, Canada M5S 1A8. E-mail [email protected] (Circ Res. 2007;101:439-440.) © 2007 American Heart Association, Inc.