An atherosclerotic vascular disease that affects the blood vessels other than those in the coronary circulation, commonly known as the peripheral arterial disease (PAD) (Espinola-Klein, 2011). PAD is a complex disorder that is associated with multiple risk factors like ageing, smoking, diabetes mellitus, dyslipidemia, and hypertension (Krishna, Moxon and Golledge, 2015). However, there are reports showing the role of inflammation in the development and progression of atherosclerosis, eventually leading to PAD (Libby, Ridker, and Hansson, 2009).
Peripheral arterial disease (PAD)
Peripheral arterial disease or PAD is a medical condition caused by blockages of the arteries that provide blood flow to the arms or legs. The blockage in the arteries is due to the cholesterol plaque caused by atherosclerosis. However, some common symptoms of PAD are thigh or calf pain with exertion, also known as claudication, pain in legs and feet at rest, ulcer on the legs that do not heal, arm pain with exertion and different blood pressures in the right and left arms of more than 15 points.
Genetics of PAD
Genetic factors causing Peripheral arterial disease (PAD)
- Variation in the genetic code i.e. single-nucleotide polymorphisms SNPs: Polymorphism may alter a gene’s expression by altering the binding of its required transcription factors, impairing the stability or intracellular trafficking of its mRNA transcript, or limiting its ability to be translated into a functional protein. Therefore, PAD is generally caused by the polymorphism in the homocysteine pathway regulating enzyme MTHFR, the inflammatory cytokines IL-6 and the vascular adhesion molecule ICAM.
- Epigenetic DNA modifications: Epigenetic factors leading to PAD includes histone modification, chromatin remodelling, and DNA methylation. In addition, these factors induce structural modifications to the DNA molecule which then leads to the alteration in the expression of the genes. The gene becomes more or less amenable to transcription, hence causing the impairment in the function of the body leading to the disease (Yan, Matouk and Marsden, 2010).
- Other genetic factors causing PAD includes posttranscriptional regulation by noncoding RNAs that may alter expression of the gene products that are associated to vascular homeostasis, mitochondrial DNA variation and complex gene-by-environment interactions (Cooke and Wilson, 2010). Furthermore, these genetic alterations causing PAD to form a basis of the genetic biomarkers used in the diagnosis of the disease.
Genetic biomarker in PAD
A genetic marker is used to identify individuals who are at risk for Peripheral arterial disease (PAD) or those with PAD at risk for poorer outcomes. In addition, key molecules/pathways implicated in the pathogenesis of PAD are identified using these genetic determinants of PAD, thereby identifying new therapeutic targets for more definitive therapies. Therefore, the role of genetic markers is to modify the clinical outcomes of PAD as it has been seen that the recovery from the same degree of arterial injury is different between different genetic strains (Dokun et al., 2008). On the other hand, certain genetic markers may predispose to the development or progression of PAD. Genomic studies, however, identify specific gene-disease or gene-outcome associations, leading to the development of genetic biomarkers use a variety of approaches, including linkage studies, genome-wide association studies (GWAS), and candidate gene evaluation studies (Hazarika and Annex, 2017).
Approaches to identify specific genetic biomarkers
Detectable genes usually found in larger effect size and require relatively large sample size.
Candidate gene approach
|Description||Association between a specific genetic variant (i.e., an SNP) and a disease of interest to determine using of statistical analysis.
It is an individual case-based approach.
|The genome scanned for highly variable, pre-specified DNA markers (i.e., microsatellites). Some regions commonly present deceased members of the family due to links to a causative gene, which helps in mapping known as positional cloning.
It is a family-based approach.
|SNPs genotyped across the entire genome in subjects with and without a given disease.
SNPs with different frequencies between cases in comparison with controls remain associated with the disease (Kullo et al., 2006).
|Strengths||Genes with small effect sizes detected.
Do not require a large sample size.
|No previous knowledge of causative gene requires.
Scans the entire genome.
|Scans the entire genome.
Detects common variants with small effect size.
|Weaknesses||Pre-existing knowledge of the genes related to the disease is required.|
Factor II, V,VII; Fibrinogen; MTHFR; P2Y12 platelet receptor.
Interleukin-6; angiotensin-converting enzyme; CCR5 and CX3CR1 chemokine receptor; ICAM-1; Enos; etc.
1p31–causative gene remains unidentified
|9p21.3 locus correlates with PAD, CAD, and AAA; likely related to CDKN2A, CDKN2B and ANRIL expression (Helgadottir, 2008)
15q24 locus correlates with smoking burden and PAD; likely related to nACh receptor biology
Advantages of genetic biomarkers
A genetic marker for PAD could identify individuals at increased risk for PAD who may benefit from targeted therapies to delay or prevent the development of lower extremity atherosclerosis. However, genetic determinants of PAD may also uncover proteins implicated in the pathophysiology of lower extremity atherosclerosis, thereby identifying mechanisms for the development and progression of lower extremity atherosclerosis. Therefore, a better understanding of mechanisms of development and progression of lower extremity atherosclerosis may ultimately help identify new therapies for the prevention and treatment of PAD (Leeper, Kullo and Cooke, 2012).
PAD is an important and highly prevalent condition with a heritable component. A biomarker panel with high sensitivity and high specificity for PAD consists of biomarkers that circulate systemically but reflect the activity of local pathophysiologic processes. Furthermore, rapid advances in technology and bioinformatics drive genetic biomarker research. However, candidate gene approach and linkage studies help in designing studies with uncharacterized genetic determinants. On the other hand, genome-wide association studies depend on revealing genetic determinants. GWA studies help in collecting the accurate genotypic information in samples of sufficient size. The application of these technologies to PAD needs encouragement. Therefore, they yield useful genetic biomarkers helpful in diagnostics for PAD and provides novel insights into the pathophysiology of the disease. This eventually helps in designing new and efficient therapies for the treatment of the disease.
- Cooke, J. P. and Wilson, A. M. (2010) ‘Biomarkers of Peripheral Arterial Disease’, Journal of the American College of Cardiology, 55(19), pp. 2017–2023. doi: 10.1016/j.jacc.2009.08.090.
- Dokun, A. O. et al. (2008) ‘A Quantitative Trait Locus ( LSq-1 ) on Mouse Chromosome 7 Is Linked to the Absence of Tissue Loss After’, 117(9), p. 736447. doi: 10.1161/CIRCULATIONAHA.107.736447.
- Espinola-Klein, C. (2011) ‘Periphere arterielle Verschlusskrankheit’, Internist, 52(5), pp. 549–561. doi: 10.1007/s00108-011-2814-7.
- Hazarika, S. and Annex, B. H. (2017) ‘Biomarkers and genetics in peripheral artery disease’, Clinical Chemistry, 63(1), pp. 236–244. doi: 10.1373/clinchem.2016.263798.
- Helgadottir, A. (2008) ‘The same sequence variant on9p21 associates with myocardial infarction, abdominal aortic aneurysm and intracranial aneurysm’, Nat. Genet., 40(2), pp. 217–224. doi: 10.1038/ng.72.
- Krishna, S. M., Moxon, J. V. and Golledge, J. (2015) ‘A review of the pathophysiology and potential biomarkers for peripheral artery disease’, International Journal of Molecular Sciences, 16(5), pp. 11294–11322. doi: 10.3390/ijms160511294.
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- Yan, M. S.-C., Matouk, C. C. and Marsden, P. A. (2010) ‘Epigenetics of the vascular endothelium’, Journal of Applied Physiology, 109(3), pp. 916–926. doi: 10.1152/japplphysiol.00131.2010.