Isolation of genes differentially expressed at the downstream anastomosis of prosthetic arterial grafts with use of mRNA differential display

Pathological Change and Whole Transcriptome Alternation Caused by ePTFE Implantation in Myocardium

Expanded polytetrafluoroethylene (ePTFE) is commonly used in cardiovascular surgery, but usually causes postoperation complications. Although great efforts have been done to relieve these complications or to understand their mechanism, there are no applicable strategies available and no understanding mechanisms, especially in the myocardium. Here, ePTFE membranes are implanted into the right ventricular outflow tract of rabbits, and the implant-related myocardium is dissected and analyzed by histology and transcriptome sequencing. ePTFE implantation causes myocardium inflammation and fibrosis. There are 1867 differently expressed mRNAs (DEmRNAs, 1107 upregulated and 760 downregulated) and 246 differently expressed lncRNAs (DElncRNAs, 110 upregulated and 136 downregulated) identified. Bioinformatic analysis indicates that the upregulated DEmRNAs and DElncRNAs are mainly involved in inflammatory, immune responses, and extracellular matrix remodeling, while the downregulated DEmRNAs and DElncRNAs are predominantly functioned in the metabolism and cardiac remodeling. Analysis of coexpression and regulatory relationship of DEmRNAs and DElncRNAs reveals that most DElncRNAs are trans-regulated on the relevant DEmRNAs. In conclusion, ePTFE implantation causes severe myocardial tissue damages and alters the transcriptome profiles of the myocardium. Such novel data may provide a landscape of mechanisms underlying the adverse reactions caused by ePTFE implantation and uncover new therapeutic targets for inhibiting the ePTFE-related complications.

Delivery of targeted gene therapies using a hybrid cryogel-coated prosthetic vascular graft

OBJECTIVES

The success of prosthetic vascular grafts in the management of peripheral arterial disease is frequently limited by the development of anastomotic neointimal hyperplasia (ANIH), with the host response to prosthetic grafts beginning soon after implantation. To address this, we combine a platform of polyethylene terephthalate (PET) fabric with an applied cryogel layer containing biologic agents to create a bioactive prosthetic graft system, with the ability to deliver therapeutics targeting modulators of the ANIH-associated transcriptome response, along with antithrombotic agents.

METHODS

Hybrid graft materials were synthesized by cryopolymerization of methacrylated alginate and heparin onto electrospun (ePET), knitted PET (kPET), or woven PET (wPET). Arg-Gly-Asp (RGD) peptides were added to increase cell adhesion. Scanning electron microscopy (SEM) was used to study the microstructure at 1 day, and 2, 4, and 8 weeks. Physical properties such as swelling ratio, pore connectivity, shape recovery, and stiffness were evaluated. Human aortic endothelial cell (HAoEC) adherence was visualized using confocal microscopy after 24 hours and proliferation was evaluated with a resazurin-based assay for 7 days. Confocal microscopy was used to assess delivery of adeno-associated virus (AAV-GFP) after incubation of hybrid grafts with HAoECs. Heparin activity of the materials was measured using an anti-Xa assay.

RESULTS

SEM demonstrated large interconnected pores throughout the entire structure for all graft types, with minimal degradation of the cryogel after 8 weeks. Hybrid materials showed a trend towards increased shape recovery, increased stiffness, decreased swelling ratio, and no difference in pore connectivity. HAoECs incorporated, adhered, and proliferated over 7 days on all materials. HAoECs were successfully transduced with AAV-GFP from the hybrid graft materials. Anti-Xa assay confirmed continued activity of heparin from all materials for over 7 days.

CONCLUSIONS

We have developed a bioactive prosthetic graft system with a cryogel coating capable of delivering biologic agents with antithrombotic activity. By applying the cryogel and selected agents onto PET prior to graft implantation, this study sets the stage for the system to be individualized and tailored to the patient, with bioengineering and targeted gene therapy strategies dovetailing to create an improved prosthetic graft adaptable to emerging knowledge and technologies.

Gene silencing in human aortic smooth muscle cells induced by PEI-siRNA complexes released from dip-coated electrospun poly(ethylene terephthalate) grafts

An excessive tissue response to prosthetic arterial graft material leads to intimal hyperplasia (IH), the leading cause of late graft failure. Seroma and abnormal capsule formation may also occur after prosthetic material implantation. The matricellular protein Thrombospondin-2 (TSP-2) has shown to be upregulated in response to biomaterial implantation. This study evaluates the uptake and release of small interfering RNA (siRNA) from unmodified and surface functionalized electrospun PET graft materials. ePET graft materials were synthesized using electrospinning technology. Subsets of the ePET materials were then chemically modified to create surface functional groups. Unmodified and surface-modified ePET grafts were dip-coated in siRNAs alone or siRNAs complexed with transfection reagents polyethyleneimine (PEI) or Lipofectamine RNAiMax. Further, control and TSP-2 siRNA-PEI complex treated ePET samples were placed onto a confluent layer of human aortic smooth muscle cells (AoSMCs). Complexation of all siRNAs with PEI led to a significant increase in adsorption to unmodified ePET. TSP-2 siRNA-PEI released from unmodified-ePET silenced TSP-2 in AoSMC. Regardless of the siRNA-PEI complex evaluated, AoSMC migrated into the ePET. siRNA-PEI complexes delivered to AoSMC from dip-coated ePET can result in gene knockdown. This methodology for siRNA delivery may improve the tissue response to vascular and other prosthetics.

Development of an RNA isolation procedure for the characterisation of human endothelial cell interactions with polyurethane cardiovascular bypass grafts

To date no reliable method has been developed for the isolation of RNA from cells seeded onto cylindrical vascular grafts. This study was performed in order to develop a reliable methodology for isolating RNA from cylindrical conduits made from poly(carbonate-urea)urethane (PU). Human umbilical vein EC were seeded onto PU vascular grafts and an Alamar blue assay performed to assess cell viability. Cells were prepared for RNA extraction by trypsinisation, cell scraping and direct application of cell lysis buffer. In all cases RNA was extracted using a "Qiagen RNeasy" kit. Alamar blue showed viable cells were present on all of the seeded PU vascular grafts. Levels of RNA extracted from the cells removed from the graft by the trypsinisation yielded 0.130 microg/microl, by scraping 0.078 microg/microl and by direct lysing 0.093 microg/microl of RNA, respectively. RTPCR was conducted successfully for GAPDH and TGF-beta1. Trypsinisation prior to RNA extraction provided the highest RNA yield and attained near complete cell removal ensuring that gene expression obtained was representative.

Temporal gene expression following prosthetic arterial grafting

BACKGROUND

Following prosthetic arterial grafting, cytokines and growth factors released within the perianastomotic tissues stimulate smooth muscle cell proliferation and matrix production. While much in vitro work has characterized this response, little understanding exists regarding the sequential up- and down-regulation of genes following prosthetic arterial grafting. This study evaluates temporal gene expression at the distal anastomosis of prosthetic arterial grafts using microarray analysis.

METHODS

Expanded polytetrafluoroethylene (ePTFE) carotid interposition grafts (n = 12) were surgically implanted into mongrel dogs. Distal anastomotic segments were harvested at 7, 14, 30, or 60 days. Contralateral carotid artery served as control. Total RNA was isolated from the anastomotic tissue and paired controls. Samples were probed with oligonucleotide microarrays consisting of approximately 10000 human genes to analyze differential gene expression at each time point.

RESULTS

Forty-nine genes were found to be up-regulated and 37 genes were found to be down-regulated at various time points. Six genes were found to be consistently up-regulated at all time intervals, including collagen type 1 alpha-1 and alpha-2, 80K-L protein (MARCKS), and osteopontin. Six genes were found to be consistently down-regulated, including smoothelin and tropomyosin 2. RT-PCR and immunohistochemistry confirmed the microarray data.

CONCLUSIONS

This study uses microarray analysis to identify genes that were temporally up- and down-regulated after prosthetic arterial grafting. Genes with similar patterns of expression have been identified, providing insights into related cellular pathways that may result in the formation of anastomotic intimal hyperplasia.

The identification of novel wound-healing genes through differential display

Effective methods to identify novel genes in complicated dynamic tissue processes are needed in molecular biology research. Traditional techniques primarily target known genes and are inefficient in the pursuit of unknown genes. Here we describe the use of a modified differential display polymerase chain reaction (DD-PCR) protocol for the identification of genes differentially expressed in wound healing. Full-thickness dorsal wounds were made on 35 adult rats, followed by wound harvest at 12 hours, 24 hours, 3 days, 5 days, 7 days, 10 days, and 14 days after injury. Modified DD-PCR was performed and gene fragments displaying definite changes during wound healing were cloned and sequenced. Gene fragments from DD-PCR were compared with available gene bank database sequences. Specific primer PCR was used to confirm DD-PCR expression patterns. As a result, over 1000 gene fragments were amplified by DD-PCR, 35 of which demonstrated distinct differences during repair. Cloning and sequencing of 13 of these gene fragments revealed that some were homologous to several characterized genes with previously unsuspected roles in repair, whereas others were completely novel genes with no known function. Specific primer PCR further confirmed expression of six of these 13 gene fragments. Only one of the 13 cloned fragments, later identified as interleukin-1beta, had well-recognized associations with tissue injury. Other fragments corresponded to various genes involved in cellular processes such as differentiation, proliferation, exocytosis, and myofibril assembly. No prior studies have linked them to wound healing. We have demonstrated that modified DD-PCR can be used to effectively identify novel genes differentially expressed during repair. Because DD-PCR allows for the simultaneous amplification of multiple arbitrary transcripts, it is a powerful genetic screening tool for complicated dynamic tissue processes, particularly when multiple, limited-sized samples are involved.

Altered ubiquitin/proteasome expression in anastomotic intimal hyperplasia

OBJECTIVE

Anastomotic intimal hyperplasia remains a leading mechanism of prosthetic arterial graft failure. Recent studies using messenger RNA differential display have demonstrated altered proteasome gene expression at the anastomoses in an expanded polytetrafluoroethylene canine carotid model. However, this technique is technically limited because of a paucity of available hyperplastic tissue at early time periods after arterial injury. Microarray gene chip technology offers a new and sensitive technique to assay early gene expression, requiring far less tissue for analysis. The purpose of this study was to screen for altered proteasome gene expression at 48 hours and 14 days after prosthetic arterial grafting.

METHODS

Expanded polytetrafluoroethylene grafts (6-mm diameter, n = 9) were implanted into 25-kg mongrel dogs. The normal intervening carotid artery was used as control. At 48 hours and 14 days, RNA was extracted from the perianastomotic tissue and compared with RNA from the control carotid. Messenger RNA was then hybridized to microarray genomes screening for differential gene expression.

RESULTS

Two 26S proteasome genes and five ubiquitin pathway genes were significantly underexpressed at 48 hours, among several hundred significantly expressed clones. The two 26S proteasome genes were 26S proteasomal subunit p55 (0.26), and 26S proteasomal subunit p40.5 (0.13). The underexpressed ubiquitin genes included ubiquitin (0.31), Nedd-4-like ubiquitin-protein ligase (0.30), ubiquitin conjugating enzyme UbcH2 (0.25), putative ubiquitin C-terminal hydrolase UHX1 (0.11), and ubiquitin-conjugating enzyme UbcH7 (0.12). At 14 days, six ubiquitin genes were underexpressed, and 17 26S proteasome genes were significantly downregulated.

CONCLUSIONS

This study shows decreased expression of the ubiquitin/proteasome pathway 48 hours after graft implantation and similar diminished expression patterns after 14 days. This early and sustained underexpression after arterial bypass may lead to altered cell cycle control and matrix protein signaling, contributing to the unregulated proliferation of smooth muscle cells and extracellular matrix in anastomotic intimal hyperplasia after prosthetic arterial grafting.

Relationship of the 20S proteasome and the proteasome activator PA28 to atherosclerosis and intimal hyperplasia in the human vascular system

Down-regulation of the proteasome activator PA28 results in abnormal proteasome activation and has been implicated in the development of intimal hyperplasia (IH) in animal models. Demonstration of proteasome and PA28 expression has not yet been documented in the human vascular system. This study sought to define the distribution of the 20S proteasome and its activator PA28 in human vessels and determine the relationship between the expression of the proteasome and PA28 and the development of atherosclerosis and IH. Vascular biopsies were obtained from 70 patients at the time of surgery, were snap frozen and sectioned in 5-micron sections, and prepared using standard histological techniques. The immunoperoxidase technique was used to identify 20S proteasome and PA28 expression in diseased and normal human arteries and veins as well as in patent bypass grafts with and without IH. Expression was graded by a blinded pathologist (scale: 1-4). Repeat quantification of the immunopositive cells was also performed. Expression of 20S proteasome and PA28 was identified in all vascular tissues examined. The proteins were identified predominately within the cytoplasm of vascular smooth muscle cells and endothelial cells. PA28 was more intensely expressed in quiescent regions of the vessel wall as compared to areas undergoing active proliferation and remodeling. PA28-mediated activation of the proteasome may be necessary to maintain normal cellular homeostasis and prevent excessive cellular proliferation in the human vascular system. Abnormalities of proteasome activation may have a significant role in the development of atherosclerosis and IH.

Apolipoprotein J inhibits the migration, adhesion, and proliferation of vascular smooth muscle cells

PURPOSE

Apolipoprotein J (ApoJ) is expressed during tissue injury and remodeling and has been implicated in vascular smooth muscle cell (VSMC) differentiation. Recently, the gene for ApoJ was identified as upregulated in distal anastomotic intimal hyperplasia after prosthetic arterial grafting. In this study we investigate the effect of ApoJ on VSMC migration, adhesion, proliferation, and gene expression.

METHODS

To study the effect of ApoJ on cell migration, we used a microchemotaxis chamber with an intervening 8-microm pore semipermeable polycarbonate membrane. Assays were performed with ApoJ alone (1-50 microg/mL) or in combination with platelet-derived growth factor homodimer bb (PDGF-bb; 10 ng/mL) or 2% fetal bovine serum (FBS; n = 8) in the lower wells. The influence of extracellular matrix interactions on ApoJ and chemotaxis was studied in a similar way, except membranes were collagen coated. Furthermore, cells were exposed to ApoJ 15 minutes before or after seeding on the coated membrane (n = 10). The influence of ApoJ on cell adhesion to collagen was assessed with cell exposure to ApoJ 15 minutes before or after seeding on a collagen-coated membrane (n = 10). Migration and adhesion were quantified by counting the number of cells per three independent high-power fields with light microscopy. The effect of ApoJ in 0.4% FBS with or without PDGF-bb on VSMC proliferation (n = 12) was assessed by means of [Methyl-(3)H] thymidine incorporation. The transcriptional profile of VSMCs in 2% FBS exposed to ApoJ and a control for 24 hours was analyzed with an oligonucleotide microarray containing 12,560 genes.

RESULTS

ApoJ alone was not chemotactic for VSMCs. Without collagen, ApoJ decreased the migration of VSMCs toward 2% FBS by 96% or more starting at 10 microg/mL (P < .05) and toward PDGF-bb by 60.9% or more starting at 25 microg/mL (P < .05) compared with the control. When collagen was introduced, ApoJ (25 microg/mL) decreased migration toward 2% FBS by 64% (P < .01) and toward PDGF-bb by 67.5% (P < .01) and decreased adhesion by 26.8% (P < .01) only when VSMCs in solution were exposed to ApoJ before placement on collagen. ApoJ did not induce VSMC proliferation. ApoJ alone decreased VSMC thymidine incorporation by 41.1% at 25 microg/mL (P < .05). ApoJ decreased thymidine incorporation of PDGF-bb stimulated VSMCs by 42.8% at 50 microg/mL (P < .05). Interleukin-8 and endothelin-1 were demonstrated by means of the microarray to be differentially expressed more than twofold in VSMCs that were exposed to ApoJ.

CONCLUSION

ApoJ is a potent inhibitor of VSMC migration, adhesion, and proliferation. Its genetic targets are linked to cell senescence and differentiation. Therefore, ApoJ may play a role, in part, in modulating the VSMC response to injury.

The mechanical behavior of vascular grafts: a review

The development of intimal hyperplasia (IH) near the anastomosis of a vascular graft to artery is directly related to changes in the wall shear rate distribution. Mismatch in compliance and diameter at the end-to-end anastomosis of a compliant artery and rigid graft cause shear rate disturbances that may induce intimal hyperplasia and ultimately graft failure. The principal strategy being developed to prevent IH is based on the design and fabrication of compliant synthetic or innovative tissue-engineered grafts with viscoelastic properties that mirror those of the human artery. The goal of this review is to discuss how mechanical properties including compliance mismatch, diameter mismatch, Young's modulus and impedance phase angle affect graft failure due to intimal hyperplasia.

Special communicationthe critical role of mechanical forces in blood vessel development, physiology and pathology

The following extended abstracts were presented at the Research Initiatives in Vascular Disease Conference, Movers and Shakers in the Vascular Tree-Hemodynamic and Biomechanical Factors in Blood Vessel Pathology, sponsored by The Lifeline Foundation and the Cardiovascular & Interventional Radiology Research and Educational Foundation; jointly sponsored by the International Society for Cardiovascular Surgery, North American Chapter, The Society for Vascular Surgery, and The Society of Cardiovascular and Interventional Radiology; in cooperation with the National Institutes of Health-National Heart, Lung &Blood Institute on Mar 11-12, 1999, in Bethesda, Md.

Interferon gamma up-regulates a novel protein in vascular smooth muscle cells

PURPOSE

By means of the technique of messenger RNA (mRNA) differential display, we previously isolated a partial DNA clone found to be down-regulated at the polytetrafluoroethylene (PTFE) hyperplastic arterial anastomosis compared with the normal artery. The partial DNA gene sequence was found to be homologous with interferon gamma up-regulated protein (IGUP) first found in human psoriatic keratinocytes. We cloned the entire IGUP gene from human vascular smooth muscle cells (VSMCs) to determine its regulation by gamma interferon (gamma-IFN) and other cytokines in cultured human VSMCs.

METHODS

By means of polymerase chain reaction, the IGUP gene was amplified from a QUICK-Clone complementary DNA human aorta kit using 5' and 3' oligonucleotide primers to the known IGUP sequence. Immunohistocytochemistry studies compared normal artery and distal anastomotic IH. Human VSMCs were stimulated with 1000 U/mL of gamma-IFN, 5 ng/mL of platelet-derived growth factor BB (PDGF-BB), 3. 2 ng/mL basic fibroblast growth factor, 3.3 ng/mL transforming growth factor beta(TGF-beta), 10 ng/mL of vascular endothelial growth factor, and 10% fetal bovine serum (FBS) for zero, 24, 48 and 72 hours. Western blot analysis of lysates of the stimulated VSMCs was performed to determine up-regulation of IGUP.

RESULTS

DNA sequencing confirmed the cloning of the entire coding region of the IGUP gene with 100% homology to the known IGUP DNA sequence. There was strong expression of IGUP in quiescent VSMCs and marked reduction of expression of IGUP in proliferating smooth muscle cells. gamma-IFN was the only cytokine, of the cytokines evaluated, to up-regulate production of IGUP in VSMCs.

CONCLUSION

IGUP is a novel protein in VSMCs found to be down-regulated in areas of anastomotic IH, as compared with a normal artery. We have now shown IGUP to be up-regulated only by gamma-IFN in human VSMCs. IGUP may, therefore, be the intermediary for the known gamma-IFN inhibition of human VSMC proliferation.

Identification of multiple genes with altered expression at the distal anastomosis of healing polytetrafluoroethylene grafts

PURPOSE

Anastomotic intimal hyperplasia remains a significant cause of delayed prosthetic arterial graft failure. Prior studies have identified several genes with altered expression within the hyperplastic region at the downstream polytetrafluoroethylene arterial anastomosis as compared with normal arteries. The purpose of the current study was to determine the sequence of early gene-related events at the distal anastomosis of an in vivo prosthetic arterial graft model. Messenger RNA (mRNA) differential display was used to screen for alterations in gene expression between anastomotic sites and control arterial segments.

METHODS

Six carotid interposition 6-mm expanded polytetrafluoroethylene grafts were placed in mongrel dogs, with the intervening carotid artery segment serving as the baseline control. Five days after graft implantation, the distal anastomotic artery segments were harvested and total RNA was isolated from both the intervening normal arteries and anastomotic segments. Differential mRNA display was used to identify candidate complementary DNA (cDNA) clones with expression that differed in anastomotic segments as compared with normal intervening arteries. Northern blot analysis confirmed alteration of gene expression. The cDNA clones were sequenced, and gene databases were searched. Novel sequences were used as probes for screening human cDNA libraries.

RESULTS

Approximately 7000 mRNA species were screened, and 26 candidate clones were obtained. Northern blot analysis showed altered gene expression in 10 (38%) of the clones, undetectable signals in 13 (50%), and nonregulation in 3 (12%). Seven clones with 92% homology at the nucleotide level to human alpha1 (III) procollagen gene and novel sequence were expressed only at the distal anastomosis. A clone representing apolipoprotein J and a novel sequence had increased expression at the distal anastomosis of 364% +/- 236% and 156% +/- 47%, respectively (mean percentage, control +/- standard deviation).

CONCLUSIONS

These studies identified genes with expressions that increased or were exclusive to the distal anastomosis of healing prosthetic arterial grafts in an in vivo prosthetic arterial graft model. Type III collagen may contribute significantly to the composition of the extracellular matrix associated with intimal hyperplasia by increasing lesion volume. Apolipoprotein J, through its association with proteases, may modulate some of the matrix changes seen early after grafting.

Studies on effects of Lp(a) lipoprotein on gene expression in endothelial cells in vitro

The reason(s) for the atherogenic properties of Lp(a) lipoprotein is still unclear, and several mechanisms have been studied. Alterations in gene expression in endothelial cells (ECs) could be important with respect to risk for coronary heart disease (CHD). We have tested the effects of Lp(a) lipoprotein or the apolipoprotein of Lp(a) lipoprotein (apo(a)) on cultured human umbilical vein endothelial cells (HUVECs) with respect to: (1) the level of endothelin-1 (ET-1) mRNA; (2) release of ET-1 into the culture medium; (3) plasminogen activator inhibitor-1 (PAI-1) secretion into the culture medium and; (4) total gene expression in HUVECs, examined by a polymerase chain reaction (PCR)-based technique, differential display-reverse transcription-PCR (DD-RT-PCR). Lp(a) lipoprotein reduced the level of ET-1 mRNA as well as the release of ET-1. The reduction of ET-1 in the medium was even more pronounced when HUVECs were incubated with apo(a), but we found no effect of apo(a) on ET-1 mRNA level. Neither Lp(a) lipoprotein nor apo(a) had a significant influence on PAI-1 secretion. DD-RT-PCR revealed 11 fragments that could represent differences between cells exposed or not exposed to Lp(a) lipoprotein. Following subcloning and sequencing, 18 sequences that differed between exposed and unexposed cultures were obtained. Four of the subcloned fragments have up to now been used as a probe for northern blot analyses, and one fragment was confirmed to be regulated by Lp(a) lipoprotein. In conclusion, Lp(a) lipoprotein is shown to control ET-1 mRNA levels and the function of at least one more gene, the nature of which is unknown.

Vascular endothelial growth factor expression in canine peripheral vein bypass grafts

PURPOSE

Autologous veins used as arterial bypass grafts undergo initial loss of the endothelial cell (EC) lining, which is followed by reendothelialization. We characterized the expression of the EC-specific angiogenic mitogen, vascular endothelial growth factor (VEGF), in vascular grafts to help elucidate the molecular and cellular events after bypass procedures.

METHODS

Cephalic vein-femoral artery interposition grafts were placed in mongrel dogs. Vein grafts and arteries were harvested at either 48 hours or 4 weeks after bypass, the total RNA was isolated, and the VEGF mRNA expression was evaluated by Northern blot analysis. Tissue segments from each time period were evaluated by immunohistochemical analysis using anti-VEGF antibodies.

RESULTS

VEGF mRNA expression in vein grafts as compared with control veins was increased 2.5-fold 48 hours after bypass grafting (p = 0.02) but returned to initial control levels in grafts removed at 4 weeks. Distal arterial segments, which included the anastomotic site without attached vein graft, had a 21.4-fold increase in VEGF expression at 48 hours (p = 0.02) and a 6.6-fold increase at 4 weeks (p < 0.01) as compared with control arterial segments. Vessels subjected to arteriotomy or ischemia alone also demonstrated increased VEGF expression. Immunohistochemical analysis revealed VEGF protein within ECs and smooth muscle cells of the venous bypass graft, with maximal levels observed within intimal hyperplasia at the arterial anastomosis.

CONCLUSIONS

After arterial reconstruction procedures using venous conduits, VEGF is significantly increased at 48 hours in the vein graft and arterial anastomosis. VEGF expression in the vein graft normalizes within 4 weeks but remains significantly elevated in the adjacent arterial segment. Increased VEGF production after arterial grafting may facilitate reendothelialization, thus partially accounting for optimal patency rates achieved with autologous vein grafts.