Genetic Causation of Neointimal Hyperplasia in Hemodialysis Vascular Access Dysfunction

Abstract

The major cause of hemodialysis vascular access failure is venous stenosis resulting from neointimal hyperplasia. Genetic factors have been shown to be associated with cardiovascular disease (CVD) and peripheral vascular disease (PVD) in the general population. Genetic factors may also play an important role in vascular access stenosis and development of neointimal hyperplasia by affecting pathways that lead to inflammation, endothelial function, oxidative stress, and vascular smooth muscle proliferation. This review will discuss the role of genetics in understanding neointimal hyperplasia development in hemodialysis vascular access dysfunction and other disease processes with similar neointimal hyperplasia development such coronary artery disease and peripheral vascular disease.

I. Introduction

Hemodialysis vascular access dysfunction remains a leading cause of morbidity and hospitalizations in hemodialysis patients ^{1, 2}. The major cause of vascular access failure is venous stenosis as a result of neointimal hyperplasia within the per-anastomotic region (arteriovenous fistula) and graft-vein anastomosis (arteriovenous graft) ^3-5. Individual variability in the mechanistic response to vascular access maturation and development of vascular stenosis after arteriovenous (AV) access placement may be influenced by genetic susceptibility factors. Genetic factors have been shown to be associated with cardiovascular disease (CVD) and peripheral vascular disease (PVD) in the general population ^{6, 7}. Furthermore, genetic factors may also play an important role in vascular access stenosis and development of neointimal hyperplasia by affecting pathways that lead to inflammation, endothelial function, oxidative stress, and vascular smooth muscle proliferation ^{3, 5}.

The majority of genetic studies evaluating neointimal hyperplasia in vascular diseases have focused on CAD and PVD with very few from the dialysis access field. This review will discuss (1) the advances in the knowledge of neointimal hyperplasia development from genetic studies in CAD and PVD, (2) genetic studies to-date in hemodialysis vascular access dysfunction in arteriovenous fistulas (AVF) and grafts (AVG), and (3) studies evaluating the role of genetic factors with restenosis following angioplasty and in-stent restenosis in CVD and dialysis access.

II. Principles of Genetic Studies in Dialysis and Cardiovascular (CVD)-related Diseases

This section provides a general overview of the principles of genetic studies and provides basic information and terminology that will be used in subsequent sections which will describe studies performed in dialysis access and CVD-related diseases. This section is not intended to provide a comprehensive or updated review of genetics in medicine.

General Principles of Genetics in Medicine

Since the initiation of the human genome project, the potential of increased genetic knowledge to improve and advance human health has been widely supported ^8-11. Genetics is the study of single genes and their ultimate effects, while genomics is the study of not just single genes, but “large-scale, high-throughput molecular analysis of multiple genes, gene products, or regions of genetic material” ¹², including the function and interaction of all genes in the genome ¹³. There are wide variabilities in the prevalence of disease such as CVD and end-stage renal disease (ESRD) arising from differences in genetic factors and gene-environment interactions, and genomic technology has been utilized to better understand mechanisms and biology of these disease processes ^14-25. Potentially this genomic technology may provide a more precise approach for the identification of high-risk patients for CVD-related diseases and vascular access dysfunction in ESRD patients and the development of individual treatment strategies. The current genomic approaches used in CVD-related and kidney diseases that will be discussed in this review include single nucleotide-polymorphism (SNP) genotyping and gene expression analysis.

SNP Genotyping

On average two unrelated individuals share more than 99.9% of their DNA sequences, but because more than 3 million base pairs constitute the human genome two unrelated humans differ at millions of base pairs ²⁶. A person’s “genotype” is the alleles present in an individual at the locus (loci) under consideration. Different versions in DNA sequence at a specific chromosomal location (locus) are called alleles, and when are found in more than 1% of the general population, the alleles constitute a genetic polymorphism ⁷. The most common type of polymorphisms are SNPs, DNA sequence variations when a single nucleotide in the genome sequence is altered. Other types of polymorphisms include minisatellite (insertion in tandem of multiple copies of a DNA sequence), microsatellite (extended stretches of DNA consisting of repeated units of DNA), and insertions/deletions (insertion or deletion of one or more nucleotides in one individual relative to another individual). Recent efforts have focused on evaluating gene polymorphisms with specific phenotypes (clinical presentation based on expression of a specific gene or genes or environmental factors) with clinical diseases ⁷. The two most common and complimentary approaches to used to study genetic sequence variations are the linkage approach and association strategies ²⁷.

Linkage approach evaluates families with a whole genome scan consisting of hundreds of anonymous markers to identify genetic loci that may be associated with the disease of interest and has been the dominant study design for investigation of the genetic basis of inherited disease²⁸. Linkage analysis searches for regions of the genome with a higher-than-expected number of shared alleles among affected individuals within a family and indicates that somewhere within this linked region there is a pre-disposing allele ²⁸. This approach has been successful to date in detecting genes for single-gene disorders with a large effect ²⁹.

The association approach evaluates the relationship of genetic variants, most commonly in unrelated individuals with the presence or absence of disease ³⁰. While linkage analysis is more powerful than association analysis for identifying rare high-risk alleles, association analysis is more powerful for detecting common disease alleles that confer modest disease risks ^{28, 30}. The scientific rationale for association studies is that common genetic variants with modest effects contribute to variation in complex disease in the general population and association studies have the ability to detect more modest genetic effects ^{12, 27, 31-33}.

Gene expression analysis

Evaluating changes in mRNA expression can be performed using several techniques such as Northern blotting, RNA differential display, and various PCR-based methods ¹². Assessment of mRNA expression quantitatively on a genomewide basis can be achieved with techniques such as serial analysis of gene expression and DNA microarrays. These techniques group genes into expression clusters (upregulated or downregulated) in disease states that can be recognized. Furthermore, gene expression analysis facilitates recognition of abnormally regulated gene clusters and identification of potential candidate genes for association testing and may suggest targets for therapy.

Strengths and Limitations of Genomic Studies

The strengths of genomic studies described above are their potential for gene discovery, identifying novel disease-associated regions, and generating new hypotheses for new testing ³⁴. Furthermore, this may lead to identifying new mechanisms of disease, expand the current knowledge of pathophysiologic processes, and lead to novel approaches for diagnosis, treatments, and prevention of disease.

Potential limitations in genomic studies is that the subdivision of a population into different groups with different marker allele frequencies and different disease prevalences can potentially lead to spurious associations ³⁵. In addition, particularly in genomewide association studies, potential false positive findings (type I errors), false negative findings (type II errors), are a concern because many of the findings do not reach the threshold to indicate statistical significance due to inadequate sample size. Furthermore, other limitations, particularly in nephrology research include problems with phenotype definition.

III. Genomic Studies and Neointimal Hyperplasia

This section will focus on genomic studies in both dialysis access (AVF and AVG) and CVD-related diseases (CAD and PVD) with an emphasis on neointimal hyperplasia. The section will be divided into four mechanistic areas: inflammation, endothelial function, oxidative stress, and vascular smooth muscle cell proliferation.

SNP Studies in Dialysis Access

Polymorphisms in Mediators of Inflammation

The role of inflammation in the development of atherosclerosis and cardiovascular events in ESRD has been previously well described ^36-39. Moreover, inflammation has been hypothesized as a potential process that leads to development of atherosclerosis, endothelial dysfunction, and neointimal hyperplasia ⁴⁰. Furthermore, stenotic lesions from AVFs have demonstrated high expression of cytokines ⁴¹. Inflammatory cytokine genes, and their promoter regions, have SNPs in the promoter regions that influence the rate and magnitude of cytokine production, and the development of neointimal hyperplasia ^{42, 43}.

Studies of cytokine genes have demonstrated associations with inflammatory disease processes ^{42, 44}. In AVGs, TNF-α gene polymorphisms (G to A, position 308) have been associated with increased AVG thrombosis ⁴². In a recent study, 67 patients were genotyped and examined for associations of high (AA or GA) and low (GG) production TNF-α genotypes with AVG thrombosis and survival ⁴². Patients with a high-production TNF-α genotype had a significantly worse one and two year cumulative survival compared to low production genotypes ⁴². Moreover, patients with the A allele had nearly two times the thrombosis rate compared to the GG genotype ⁴². Another recent study evaluated genetic polymorphisms of IL-10 (G to A, position 1082) and TNF-α (G to A, position 308) in 75 hemodialysis patients with AVFs and AVGs with vascular access failure ⁴⁵. Patients with high producing genotype (GG > AG >AA) had higher levels of IL-10 levels, but there was no difference in genotypic frequency between patients with vascular access failures and those with functioning accesses. When examining the TNF-α genotypes, there was no difference in genotype frequency among patients with vascular access failure compared to those with functioning access. However, baseline TNF-α levels were much higher in the vascular access failure group compared to the functioning access group.

Transforming growth factor-β1 (TGF-β1) synthesis differs due to polymorphisms in the gene sequence encoding the signal sequence of the cytokine ⁴⁶. Combinations of SNPs in located at position +869 and + 915 result in high-producing, intermediate-producing, and low-producing of TGF-β1 ⁴⁷. In a study evaluating 120 patients who had undergone AVF placement for initiation of hemodialysis, AVF patency was in 62.4% vs 81.2% after 12 months for the intermediate compared to high producing TGF-β1genotypes ⁴⁸.

Polymorphisms in Mediators of Endothelial Function

The presence of uremia in ESRD could predispose patients to endothelial dysfunction ^{3, 39}. Veins have been demonstrated to produce less nitric oxide and prostacyclin compared to arteries ⁴⁹, which could predispose them to endothelial dysfunction and development of vascular access stenoses.

Elevated homocysteine levels have been shown to be a risk factor for arteriosclerosis, coronary artery disease, stroke, and venous thrombosis ^50-52. Blood levels are influenced by a polymorphism of the methylene tetrahydrofolate reductase (MTHFR) gene. MTHFR catalyzes the remethylation of homocysteine to methionine. Homocysteine causes endothelial dysfunction and injury through production of free radicals ^{53, 54}. Furthermore, homocysteine causes an increase in proliferative activity of vascular smooth muscle cells and a decrease in vascular endothelial cell activity ^{53, 54}. A polymorphism of the MTHFR genes (C to T) at position leads to a less active enzyme and increased production of homocysteine ⁵⁵. A recent study evaluated the role of MTHFR gene polymorphism AVF thrombosis ⁵⁶. 337 hemodialysis patients were examined for a MTHFR gene polymorphism and AVF thrombosis. Patients who developed AVF stenoses were more likely to have a TT versus CC genotype.

Nitric oxide (NO) has been reported to be a very potent regulator of vascular endothelial function ^{57, 58}. A recent study in AVGs evaluated the association between endothelial nitric oxide synthase (eNOS) gene intron 4 and thrombosis in AVGs ⁵⁹. 55 patients on maintenance hemodialysis via an AVG had blood drawn to identify genotypes (aa, bb, ab) eNOS gene intron 4 VNTR polymorphism. Patients were divided into two groups, those who experienced a thrombosis (Group I) in the first 12 months after AVG placement and those who did not (Group II). The frequency of the aa genotype in Group I was significantly higher than that in Group II. Furthermore, when evaluating primary patency, patients with the aa genotype had significantly lower primary patency rates than for the bb and ab genotype groupings.

Recent studies have shown that a Klotho gene mutation (G-395A) is associated with endothelial dysfunction, artherosclerosis, and thrombosis. ^60-62. A recent study evaluated polymorphisms (G-395A) in the klotho gene in 126 patients and looked at three genotypes (GG, GA, and AA). Patients with an A allele required significantly more early intervention for thrombosis compared to those patients who were non-carriers of the A allele ⁶³.

Polymorphisms in Mediators of Oxidative Stress

Oxidative stress is defined as a disturbance in the equilibrium between antioxidants and pro-oxidants, with increased levels of pro-oxidants resulting in tissue damage ⁶⁴. Studies in experimental models of vascular injury have demonstrated that an increase in oxidative stress, through reactive oxygen species, results in an increase in vascular stenosis ⁶⁵.

Heme oxygenase 1 (HO-1) is a rate limiting enzyme involved in the heme degradation to carbon monoxide (CO), free iron and biliverdin ⁶⁶. This enzyme has been recognized as a playing a crucial role in oxidative stress, platelet activation, and vascular smooth muscle cell proliferation and neointimal hyperplasia formation through the effects of its reaction product, CO ⁶⁷. Transcription of the HO-1 gene is regulated by the length polymorphism of a dinulceotide GT repeat in the promoter region of the HO-1 gene ⁶⁷. A recent study evaluated whether the length polymorphism of dinucleotide GT repeats in the HO-1 gene promoter region predicts AVF survival in hemodialysis patients ⁶⁶. This study enrolled 603 hemodialysis patients. Among these patients 425 did not experience an AVF failure with 178 patients experiencing an episode of AVF failure. GT repeats were assigned in the following manner: (1) GT repeats ≥ 30 were classified as an L (long) allele class, and (2) GT repeats < 30 were classified as an S (short) allele class. L/L genotypes were defined as both alleles ≥ 30 GT repeats, L/S genotypes as one allele ≥ 30 GT repeats and the other < 30, and S/S genotypes as both alleles < 30 GT repeats. Significant associations were found between AVF failure and the L/L and L/S genotype (compared to S/S genotype).

Matrix metalloproteinases (MMPs) have been associated with smooth muscle cell migration and degradation of extracellular matrix ^{68, 69}. Gene polymorphisms of MMPs have been associated with CAD ^70-73, myocardial infarction ^{74, 75}, restenosis after angioplasty ^{76, 77}, and carotid intima-media thickness ⁷⁸. A recent study of evaluated gene polymorphisms in MMPs (MMP-1, MMP-2, and MMP-9, and TIMP-1 and TIMP-2) and reported that specific genotypes of MMP-1, MMP-3, and MMP-9 were associated with higher frequencies of AVF failure⁷⁹.

Polymorphisms in Mediators of Vascular Smooth Muscle Proliferation

Dialysis access stenosis is histologically characterized by neointimal hyperplasia ^{3, 80, 81}. Vascular smooth muscle cell proliferation, migration, and extracellular matrix synthesis are important processes leading to development of neointimal hyperplasia ³. Polymorphisms in genes that regulate vascular smooth muscle cell proliferation may play a role in development of neointimal hyperplasia.

In experimental models of vascular injury, neointimal hyperplasia has been shown to be stimulated by the renin-angiotensin system ⁸² and blockade of neointimal hyperplasia occurs using angiotensin-converting (ACE) enzyme inhibitors in arterial and venous models of CAD ^{83, 84}. Polymorphisms involving insertion of base pair sequences (II genotype) have been associated with lower ACE levels compared with deletion (DD) or heterogenous genotypes (I/D). Polymorphisms (I/D) in the ACE gene have been shown to be associated with AVG thrombosis in a small study ⁸⁵. Another study evaluating 137 patients with creation of new AVF showed no difference in AVF survival by genotype (II, I/D, or DD) ⁸⁶. Studies evaluating the restenosis after coronary angioplasty also showed no difference by ACE genotype ⁸⁷.

Gene Expression Profiling Studies in CVD-related Diseases

Atherosclerosis in CVD-related disease such as CAD and PVD has many pathological similarities to venous stenosis in dialysis access dysfunction. Knowledge of the crucial genes and gene variants could provide the knowledge necessary to better understand pathogenesis and physiology and disease mechanisms of vascular stenosis. Gene expression profiling utilizing mRNA extracted from human tissue with varying degrees of atherosclerosis has been recently studied, identifying potential genes involved in the process of atherosclerosis development. Such studies to-date have not been performed in dialysis access stenosis. This next section will describe recent studies and results from CAD and PVD studies utilizing gene expression profiling techniques.

Gene Expression Profiling of Atherosclerosis

A recent study evaluated human aorta samples with varying degrees of atherosclerosis ²³. Genes such as apo E, osteopontin, and olr1, which have previously been associated with atherosclerosis ^88-93, were upregulated ²³. New genes, not previously linked to atherosclerosis, such as capg, gm2, mmp9, and ccrl2, were also reported in this study ²³. MMP9 and ccrl2 (whose gene product serves as a receptor for monocyte chemotactic protein 1) have both been previously implicated in atheroma progression and neointima hyperplasia ^94-98.

Smooth muscle cells have been hypothesized to play a central role in the pathogenesis of neointimal hyperplasia development in CVD ⁹⁹. A recent experimental study compared smooth muscle cells from the neointima formed 4 weeks after aortic grafting compared to normal aorta ¹⁰⁰. This study found 13 upregulated genes in the neointima (compared to control), 7 encoding matrix proteins and 2 encoding inducers of matrix proteins ¹⁰⁰. Notable genes that were downregulated included the regulator of G-protein signaling-5 and nonmusclemyosin heavy chain-B ¹⁰⁰. Surprisingly, TGF-β, osteopontin, and elastin were not detected in the neointima as frequently reported in other studies ^101-103. This may suggest that graft neointima may have different characteristics compared to other forms of neointima.

Gene Expression Profiling of Peripheral Vascular Disease

PVD also shares characteristics similar to CAD in that the pathology of vascular stenosis also involves progressive atherosclerosis. A recent study evaluated femoral artery samples from patients with advanced and intermediate PVD lesion and normal samples using gene expression profiling, utilizing Affymetrix microarray platforms ¹⁰⁴. 116 genes overlapped (68 upregulated and 48 downregulated) between the intermediate and advanced femoral artery lesions ¹⁰⁴. In both of these lesions immune/inflammatory genes were significantly upregulated ¹⁰⁴.

IV. Genetics of Restenosis after Angioplasty and In-Stent Restenosis

Percutaneous transluminal angioplasty (PTA) and stenting has revolutioned the treatment of CAD as well as dialysis access stenosis. However, restenosis, in part due to neointimal hyperplasia, has been the Achilles heel following following PTA in both disease processes, limiting long-term success. This section will discuss the basic pathophysiology of restenosis following PTA and the role of genetics in restenosis, primary focusing on CAD (as the literature in dialysis access is limited), but has broad mechanistic and clinical applicability to dialysis access stenosis.

Pathophysiology of Restenosis Following Percutaneous Transluminal Angioplasty

The pathophysiology of restenosis is complex and still remains poorly understood. The majority of experimental studies evaluating restenosis have been performed in CAD, as models in dialysis access are lacking at this time. The current evidence suggest that restenosis is a maladaptive response of the vessel to trauma during angioplasty which results in thrombosis, inflammation, cellular proliferation, and extracellular matrix production ^{105, 106}. The loss of luminal size following PTA and stenosis can be influenced by three major factors: (1) elastic recoil ^107-109, (2) progressive neointimal hyperplasia ^110-113, and (3) adverse remodeling ^114-116.

Elastic recoil is a dynamic and progressive process that occurs immediately after PTA and results in reduction of luminal diameter ¹⁰⁵. Elastic recoil has been demonstrated to account for almost 50% of lumen loss during angioplasty^{108, 109}. Stent implantation after PTA reduces recoil, but recoil still persists after stent placement ¹¹⁷.

In CAD, balloon inflation during angioplasty fractures the atherosclerotic plaque, causing endothelial damage, and triggers platelet adhesion and activation of a plethora of vasoactive mediators which promote smooth muscle cell proliferation ^{111, 118}. Moreover, the angioplasty procedure itself results in endothelial denudation and loss of antithrombotic factors such as nitric oxide, prostacyclin, and tissue plasminogen activator, further exacerbating the platelet adhesion and aggregation process ¹⁰⁵. Subsequently, smooth muscle cell proliferation and migration leads to neointimal hyperplasia development, which has also been shown in clinical studies in hemodialysis AVFs ¹¹³.

The vascular remodeling process observed during atherosclerosis and following vein-graft (and dialysis access) placement and restenosis following PTA is distinctly different. Recent data suggests that the remodeling that occurs following angioplasty is associated with adventitial thickening and scar contraction ^{105, 115, 119}. It remains unclear whether neointimal thickening or vascular remodeling plays the most important factor in restenosis. However, a previous study utilizing intravascular ultrasound imaging following PTA demonstrated that that luminal diameter was largely due to the magnitude of vessel wall remodeling rather than neointimal hyperplasia formation ¹¹⁶.

In-stent Restenosis

Endovascular stents reduce restenosis by providing a rigid luminal scaffolding for the vessel. In-stent restenosis reflects the cascade of molecular and cellular events that are occurring in the vessel wall. Stenting induces localized injury to the vessel wall, which releases a host of thrombogenic, vasoactive, and mitogenic mediators that cause re-narrowing at the injured site ¹²⁰. Similar to restenosis following angioplasty, the three main processes that lead to in-stent restenosis are elastic recoil, neointimal hyperplasia, and vascular remodeling ¹²⁰. However, neointimal hyperplasia is the major factor for in-stent restenosis ¹²⁰.

Genetic Studies and Vascular Restenosis Following Angioplasty and Stenting

Restenosis is a multifactorial biological process resulting from complex interactions between extrinsic and intrinsic factors of predisposition ¹²¹. Intrinsic predisposition to restenosis may involve genetic factors. To date, there are no genetic studies in dialysis access evaluating the role of polymorphisms with restenosis following angioplasty and stenting. The next section will discuss studies of candidate genes involved in the pathogenesis of restenosis flowing PTA and in-stent restenosis in CAD.

Candidate Genes of the Renin-Angiotensin Systemand Restenosis after PTA and In-Stent Restenosis

Angiotensin converting enzyme (ACE) has been reported to be involved in vascular stenosis, vasoconstriction, and smooth muscle proliferation, all relevant to the pathophysiology of neointimal hyperplasia development ¹²². While initial studies reported an association between the I/D ACE polymorphism and restenosis after PTA in coronary arteries¹²³, subsequent studies could not confirm these results ^124-127. However, more recent association studies in patients with coronary stenting showed a positive relationship with between ACE I/D polymorphism and restenosis ^{128, 129}. This relationship, particularly in patients with in-stent restenosis, is likely due to the fact that neointimal hyperplasia is the primary mechanism of restenosis after stenting ¹¹⁶.

Candidate Gene Polymorphisms of Endothelial Function and Restenosis after PTA and In-Stent Restenosis

Impaired production of nitric oxide (NO) promotes proliferation of vascular smooth muscle cells, and subsequently may induce neointimal hyperplasia development leading to restenosis after PTA and in-stent restenosis ¹³⁰. Therefore, endothelial nitric oxide synthase gene (eNOS) may be an ideal candidate gene to evaluate for restenosis after PTA and in-stent restenosis ¹²². Several studies in CAD have reported that Glu298Asp polymorphism of the eNOS gene has been associated with coronary vasospasm and myocardial infarction ^{131, 132}. Furthermore, several studies have recently reported associations between gene polymorphisms of eNOS (Glu298A, -786T>C, 894 G/T) and in-stent restenosis ^133-135. There are no current studies to date evaluating the role of these polymorphisms in restenosis after PTA.

Candidate Gene Polymorphisms of the Inflammatory System and Restenosis after PTA and In-Stent Restenosis

The pathogenesis of restenosis following PTA and stent placement involves an inflammatory component ^{136, 137}. Gene polymorphisms of interleukin-1 receptor antagonist have been shown to be associated with in-stent restenosis and restenosis after PTA in CAD ^{138, 139}. Other polymorphisms on the interleukin-1 gene and polymorphisms of common inflammatory mediators such as TNF-α and interleukin-10 have failed to demonstrate associations with in-stent restenosis or restenosis after PTA ^138-140.

Candidate Gene Polymorphisms of Oxidative Stress Mediators and Restenosis after PTA and In-Stent Restenosis

Matrix metalloproteinases (MMPs) have been associated with smooth muscle cell migration and degradation of extracellular matrix and play a role in vascular remodeling and neointimal hyperplasia formation in CVD ^{68, 69}. Gene polymorphisms studies of MMP3 (with alleles designated 5A or 6A) have shown that patients with 6A6A MMP3 genotype were more suspectible to restenosis after PTA, but not in-stent-restenosis ⁷⁶. This suggest that MMP3 genotype may play an important role in remodeling after angioplasty ¹²².

Heme oxygenase-1 (HO-1) is a rate-limiting enzyme in heme degradation, leading to production of free iron, biliverdin, and carbon monoxide (CO) and CO exerts significant antioxidant, antiproliferative, and anti-inflammatory effects in the vascular system, thereby influencing neointimal hyperplasia formation ^141-144. The length polymorphism of the GT repeat in HO-1, which controls the level of gene transcription, has been associated with stenosis after stenting and restenosis after angioplasty in CAD ^{145, 146}.

V. The Future of Hemodialysis Vascular Access Research Utilizing Genomic Technology

The impact of genetic variability on development of neointimal hyperplasia has been more extensively studied in CAD and PVD. However, in dialysis access, these studies are currently lacking. Future studies in a large population evaluating candidate genes in predicting both short and long outcomes in AVF and AVG are overdue in dialysis access. Furthermore, given the recent development of high-density genotyping arrays, more novel approaches utilizing whole genome association studies have provided opportunities to evaluate the association of genetic variants with common disease such as CAD. There is an urgent need for these studies to be performed in dialysis access.

Recent studies in CAD and PAD have directly evaluated diseased and normal tissue, where neointimal hyperplasia forms, to determine global gene expression patterns using microarray analysis ^{23, 104}. These studies have allowed for identification of novel genes targets to study the pathophysiology of neointimal hyperplasia formation. Again, in dialysis access, these studies have yet to be performed, as the pathophysiology and mechanisms of neointimal hyperplasia development remains poorly understood.

Looking into the future, one could envision SNPs and gene expression profiling, in patients receiving a new dialysis access, as a pre-operative diagnostic tool to assess the probability of a patient developing certain types of neointimal hyperplasia and vascular access dysfunction and restenosis after angioplasty.

VI. Conclusions

At the present time, there are few if any therapies in dialysis access to treat neointimal hyperplasia that develops after AVF and AVG placement. This is in part due to the limited understanding of the pathophysiology and mechanisms of neointimal hyperplasia formation. Unlike the fields of CAD and PVD, genetic and genomic studies in dialysis access are very limited to-date. Knowledge of the crucial genes and gene variants will not only provide novel insights into the mechanisms (i.e. oxidative stress, inflammation, endothelial dysfunction, etc.) and pathophysiology of neointimal hyperplasia development in dialysis access stenosis, but will also allow for generation of genomic patient phenotypes that will provide the detail necessary for improving diagnosis and prognosis of dialysis access dysfunction, and how individual patients will respond to interventions and future therapies.