Abdominal aortic aneurysm: a review of the genetic basis

The occurrence and development of abdominal aortic aneurysm may be related to the energy metabolism disorder and local inflammation

Background

The cellular mechanism of the formation of abdominal aortic aneurysm (AAA) is very complicated. A series of sophisticated events eventually led to significant pathological changes in the anatomical structure and function of the arterial wall and they are still not clear nowadays.

Methods

We pooled publicly available GEO datasets (GSE57691 and GSE47472) to get a comprehensive comparisons between normal tissues and AAA tissues to try to reveal molecular mechanism underlying the disease. Total 63 AAA samples and 18 normal tissue samples were compared and we fond that there were 784 significantly different gene (DEGs, threshold set as adjusted P < 0.05 and Log FC < 1) were identified. At the same time, we validate the possible signaling factor expression of AAA by comparing the normal tissue of the human body with the AAA tissue.

Results

In the pathway enrichment, we found that FOXP3 related signaling pathways, inflammation-related cytokine signaling pathways, interleukin-8-CXCR1 related signaling pathways and VEGFA and FGFR1 related signal pathway were significantly enrichmented. In Weighted gene co-expression network analysis (WGCNA), we found that the key hub genes were significantly related to lipid catabolic metabolism, which further verified the possibility that AAA might relate to energy metabolism disorders.

Conclusion

Based on the comprehensive analysis of previous high-throughput data and the validation of basic experiments, we found that the occurrence of AAA may be related to energy metabolism disorders and local inflammation.

C-reactive protein to albumin ratio provides important long-term prognostic information in patients undergoing endovascular abdominal aortic repair

BACKGROUND

The prognostic value of C-reactive protein/albumin ratio (CAR) is of import in cardiovascular diseases. Our aim was to evaluate the impact of the CAR in patients with asymptomatic abdominal aortic aneurysm (AAA) undergoing endovascular aneurysm repair (EVAR).

MATERIAL AND METHOD

We retrospectively evaluated 127 consecutive patients who underwent technically successful elective EVAR procedure between December 2014 and September 2020. The optimal CAR cut-off value was determined by using receiver operating characteristic (ROC) curve analysis. Based on the cut-off value, we investigated the association of CAR with long-term all-cause mortality.

RESULTS

32 (25.1%) of the patients experienced all-cause mortality during a mean 32.7 ± 21.7 months' follow-up. In the group with mortality, CAR was significantly higher than in the survivor group (4.63 (2.60-11.88) versus 1.63 (0.72-3.24), p < 0.001). Kaplan-Meier curves showed a higher incidence of all-cause mortality in patients with high CAR compared to patients with low CAR (log-rank test, p < 0.001). Multivariable Cox regression analysis revealed that glucose ≥ 110 mg/dL (HR: 2.740; 95% CI: 1.354-5.542; p = 0.005), creatinine ≥ 0.99 mg/dL (HR: 2.957, 95% CI: 1.282-6.819, p = 0.011) and CAR > 2.05 (HR: 8.190, 95% CI: 1.899-35.320, p = 0.005) were the independent predictors of mortality.

CONCLUSION

CAR was associated with a significant increase in postoperative long-term mortality in patients who underwent EVAR. Preoperatively calculated CAR can be used as an important prognostic factor.

Effect of Acupuncture on the p38 Signaling Pathway in Several Nervous System Diseases: A Systematic Review

Acupuncture is clinically used to treat various diseases and exerts positive local and systemic effects in several nervous system diseases. Advanced molecular and clinical studies have continually attempted to decipher the mechanisms underlying these effects of acupuncture. While a growing understanding of the pathophysiology underlying several nervous system diseases shows it to be related to inflammation and impair cell regeneration after ischemic events, the relationship between the therapeutic mechanism of acupuncture and the p38 MAPK signal pathway has yet to be elucidated. This review discusses the latest advancements in the identification of the effect of acupuncture on the p38 signaling pathway in several nervous system diseases. We electronically searched databases including PubMed, Embase, and the Cochrane Library from their inception to April 2020, using the following keywords alone or in various combinations: "acupuncture", "p38 MAPK pathway", "signaling", "stress response", "inflammation", "immune", "pain", "analgesic", "cerebral ischemic injury", "epilepsy", "Alzheimer's disease", "Parkinson's disease", "dementia", "degenerative", and "homeostasis". Manual acupuncture and electroacupuncture confer positive therapeutic effects by regulating proinflammatory cytokines, ion channels, scaffold proteins, and transcription factors including TRPV1/4, Nav, BDNF, and NADMR1; consequently, p38 regulates various phenomena including cell communication, remodeling, regeneration, and gene expression. In this review article, we found the most common acupoints for the relief of nervous system disorders including GV20, GV14, ST36, ST37, and LI4. Acupuncture exhibits dual regulatory functions of activating or inhibiting different p38 MAPK pathways, contributing to an overall improvement of clinical symptoms and function in several nervous system diseases.

Cellular Mechanisms of Aortic Aneurysm Formation

Aortic aneurysms are a common vascular disease in Western populations that can involve virtually any portion of the aorta. Abdominal aortic aneurysms are much more common than thoracic aortic aneurysms and combined they account for >25 000 deaths in the United States annually. Although thoracic and abdominal aortic aneurysms share some common characteristics, including the gross anatomic appearance, alterations in extracellular matrix, and loss of smooth muscle cells, they are distinct diseases. In recent years, advances in genetic analysis, robust molecular tools, and increased availability of animal models have greatly enhanced our knowledge of the pathophysiology of aortic aneurysms. This review examines the various proposed cellular mechanisms responsible for aortic aneurysm formation and identifies opportunities for future studies.

TIMP‑3 suppresses the proliferation and migration of SMCs from the aortic neck of atherosclerotic AAA in rabbits, via decreased MMP‑2 and MMP‑9 activity, and reduced TNF‑α expression

The present study investigated the role of tissue inhibitor of matrix metalloproteinase‑3 (TIMP‑3) in regulating the proliferation, migration, apoptosis and activity of matrix metalloproteinase (MMP)‑2 and ‑9, during the development of an atherosclerotic abdominal artery aneurysm (AAA). Experiments were conducted using rabbit AAA neck (NA) smooth muscle cells (SMCs), to investigate the potential for TIMP‑3 to be used as a novel stent coating in preventing aortic dilation adjacent to the AAA. The atherosclerotic AAA model was induced in New Zealand white rabbits via a 6‑week high‑cholesterol diet, followed by incubation of the targeted aortic region with elastase. SMCs were isolated from the aorta adjacent to the aneurysm 30 days after AAA model induction, and stimulated with 3, 10, 30 or 100 ng/ml TIMP‑3. Cell proliferation was investigated using Cell Counting Kit‑8 reagent, migration was examined using a Boyden chamber assay and apoptotic rate was analyzed using the Annexin V‑fluorescein isothiocyanate Apoptosis Detection kit. Gelatin zymography and ELISA were used to measure the activity of MMP‑2 and MMP‑9, and the expression of tumor necrosis factor‑α (TNF‑α), respectively. Analysis of cell proliferation indicated that 10, 30 and 100 ng/ml TIMP‑3 reduced cell viability. Cell migration was decreased by 10, 30 and 100 ng/ml TIMP‑3. MMP‑2 activity was inhibited by 10, 30 and 100 ng/ml TIMP‑3, and MMP‑9 activity was suppressed by 30 and 100 ng/ml TIMP‑3. The protein levels of secreted TNF‑α were reduced by 10, 30 and 100 ng/ml TIMP‑3. The present study demonstrated the ability of 30 and 100 ng/ml TIMP‑3 to attenuate migration and proliferation, and to inhibit the activity of MMP‑2, MMP‑9 and TNF‑α secretion of NA SMCs. In conclusion, TIMP‑3 may be considered a potential therapeutic drug for use in a novel drug‑eluting stent, to attenuate the progressive dilation of the aortic NA.

Matrix metalloproteinase family polymorphisms and the risk of aortic aneurysmal diseases: A systematic review and meta-analysis

It has been suggested that matrix metalloproteinase (MMP) polymorphisms are associated with the pathogenesis of aortic aneurysmal diseases. In this study, we conducted a systematic review with an update meta-analysis to investigate the relationship between MMP family polymorphisms and aortic aneurysmal diseases. We systematically reviewed 24 polymorphisms in 8 MMP genes related to the risk of abdominal aortic aneurysm (AAA), thoracic AA or thoracic aortic dissection (TAD). A total of 19 case-control studies with 15 highly studied MMP polymorphisms were included in our meta-analysis. Our results suggested that MMP2rs243865, MMP3rs3025058, MMP13rs2252070 polymorphisms were significantly associated with AAA risk, MMP2rs11643630, MMP8rs11225395 polymorphisms were correlated with TAD risk, and MMP9rs3918242 under the dominant model could increase AAA risk in hospital-based subgroup. No associations with aortic aneurysmal diseases were identified for other polymorphisms assessed in our meta-analysis. In summary, some studied MMP polymorphisms associated with the risk of aortic aneurysmal diseases are potential predictive biomarkers for the clinical application. Moreover, other MMP polymorphisms with limited studies but relevant to aortic aneurysmal formation and progression need further prospective and large investigations to confirm results.

Serum High-Sensitive C-Reactive Protein Level and CRP Genetic Polymorphisms Are Associated with Abdominal Aortic Aneurysm

BACKGROUND

Abdominal aortic aneurysm (AAA) development involves an inflammatory process with a potential genetic background. C-reactive protein (CRP) is an acute phase protein and was elevated in patients with AAA. The aim of this study was to investigate the association among serum high-sensitive CRP (hsCRP) concentration, its CRP genetic polymorphisms, and AAA.

METHODS

Serum hsCRP concentrations and abdominal aorta diameters were measured, and correlation analysis between them was performed in 155 unrelated participants with AAA and 310 non-AAA controls. Tagging single nucleotide polymorphisms (SNPs) in the CRP gene (rs1417938, rs1130864, rs1205, rs1800947) were identified via HapMap. Stratification analysis was performed to evaluate the effects of SNPs on the concentration of serum hsCRP. The association between 4 SNPs and AAA was assessed by unconditional logistic regressions.

RESULTS

Elevated serum hsCRP level was found to be an independent risk factor for AAA (odds ratio [OR] = 3.91, 95% confidence interval [CI]: 2.45, 6.23) after adjustment for confounding factors. Concentrations of serum hsCRP were significant different (P = 0.01) in 4 subgroups derived from participants with abdominal aorta diameter <20 mm, 20-29 mm, 30-54 mm, and ≥55 mm. Stratification analysis revealed there was significant high frequency of elevated hsCRP levels in subjects carrying rs1205-CC genotype compared with those carrying rs1205-TT or CT genotypes (P = 0.004, OR = 2.31, 95% CI: 1.30, 4.11), suggesting that the genotype CC of rs1205 was associated with higher serum hsCRP levels. However, the frequency of rs1205-CC in AAA patients (15.3%) was similar to control subjects (17.6%), and we could not confirm rs1205-CC was the genetic risk factor of AAA (OR = 1.18, 95% CI: 0.69, 2.01). Moreover, we found another CRP polymorphism rs1417938-TT had a significantly higher likelihood of AAA than the AT genotype (OR = 2.07, 95% CI: 1.06, 4.03) for the first time, indicating there was perhaps a role for rs14117938-T polymorphism that correlates with AAA.

CONCLUSIONS

Serum hsCRP may be related to the presence of AAA and abdominal aorta diameter. Genetic polymorphisms in CRP gene could influence the concentration of serum hsCRP and the likelihood of AAA, but the causal relationship between AAA and CRP should be demonstrated further.

Meta-analysis of Outcomes Following Aneurysm Repair in Patients with Synchronous Intra-abdominal Malignancy

OBJECTIVES

The management of concomitant intra-abdominal malignancy (IAM) and abdominal aortic aneurysm (AAA) remains a challenge, even though malignancy is common in an elderly population. By means of systematic review and meta-analysis, the aim was to investigate outcomes in patients undergoing open (OAR) or endovascular AAA repair (EVAR) that have a concomitant malignancy.

METHODS

A systematic literature review was performed (Medline and EMBASE databases) to identify all series reporting outcomes of AAA repair (OAR or EVAR) in patients with concomitant IAM. Meta-analysis was applied to assess mortality and major morbidity at 30 days and long term.

RESULTS

The literature review identified 36 series (543 patients) and the majority (18 series) reported on patients with colorectal malignancy and AAA. Mean weighted mortality for OAR at 30 days was 11% (95% CI: 6.6% to 17.9%); none of the EVAR patients died peri-operatively. The weighted 30-day major complication rate for EVAR was 20.4% (10.0-37.4%) and for OAR it was 15.4% (7.0-30.8%). Most patients had their AAA and malignancy treated non-simultaneously (56.6%, 95% CI, 42.1-70.1%). In the EVAR cohort, three patients (4.6%) died at last follow-up (range 24-64 months). In the OAR cohort 23 (10.6%) had died at last follow up (range from 4 to 73 months).

CONCLUSION

In this meta-analysis, OAR was associated with significant peri-operative mortality in patients with an IAM. EVAR should be the first-line modality of AAA repair. The majority of patients were not treated simultaneously for the two pathologies, but further investigation is necessary to define the optimal timing for each procedure and malignancy.

Identification of key genes associated with the human abdominal aortic aneurysm based on the gene expression profile

The present study was aimed at screening the key genes associated with abdominal aortic aneurysm (AAA) in the neck, and to investigate the molecular mechanism underlying the development of AAA. The gene expression profile, GSE47472, including 14 AAA neck samples and eight donor controls, was downloaded from the Gene Expression Omnibus database. The total AAA samples were grouped into two types to avoid bias. Differentially expressed genes (DEGs) were screened in patients with AAA and subsequently compared with donor controls using linear models for microarray data, or the Limma package in R, followed by gene ontology enrichment analysis. Furthermore, a protein-protein interaction (PPI) network based on the DEGs was constructed to detect highly connected regions using a Cytoscape plugin. In total, 388 DEGs in the AAA samples were identified. These DEGs were predominantly associated with limb development, including embryonic limb development and appendage development. Nuclear receptor co-repressor 1 (NCOR1), histone 4 (H4), E2F transcription factor 4 (E2F4) and hepatocyte nuclear factor 4α (HNF4A) were the four transcription factors associated with AAA. Furthermore, HNF4A indirectly interacted with the other three transcription factors. Additionally, six clusters were selected from the PPI network. The DEG screening process and the construction of an interaction network enabled an understanding of the mechanism of AAA to be gleaned. HNF4A may exert an important role in AAA development through its interactions with the three other transcription factors (E2F4, NCOR1 and H4), and the mechanism of this coordinated regulation of the transcription factors in AAA may provide a suitable target for the development of therapeutic intervention strategies.

Analysis of multiple genetic polymorphisms in aggressive-growing and slow-growing abdominal aortic aneurysms

BACKGROUND

The natural history of abdominal aortic aneurysms (AAAs) suggests that some remain slow in growth rate whereas many develop a more accelerated growth rate and reach a threshold for intervention. We hypothesized that different mechanisms are responsible for AAAs that remain slow growing and never become actionable vs the aggressive AAAs that require intervention and may be reflected by distinct associations with genetic polymorphisms.

METHODS

AAA growth rate was determined from serial imaging data in 168 control and 141 AAA patients with ultrasound or computed tomography imaging studies covering ∼5 years. Genetic polymorphisms all previously reported as showing a significant correlation with AAA with functional effects on the expression or function were determined by analysis of the genomic DNA, including angiotensin 1 receptor (rs5186), interleukin-10 (IL-10; rs1800896), methyl-tetrahydrofolate reductase (rs1801133), low-density lipoprotein receptor-related protein 1 (LRP1; rs1466535), angiotensin-converting enzyme (rs1799752), and several matrix metalloproteinase 9 (MMP-9) single nucleotide polymorphisms.

RESULTS

Of the AAA patients, 81 were classified as slow AAA growth rate (<3.25 mm/y) vs 60 with aggressive AAA growth rate (>3.25 mm/y, those presenting with a rupture, or those with maximal aortic diameter >5.5 cm [male] or >5.0 cm [female]). Discriminating confounds between the groups were identified by logistic regression. Analyses identified MMP-9 p-2502 single nucleotide polymorphism (odds ratio [OR], 0.54; 95% confidence interval [CI], 0.31-0.94; P = .029) as a significant confound discriminating between control vs slow-growth AAA, MMP-9 D165N (OR, 0.49; 95% CI, 0.26-0.95; P = .035) and LRP1 (OR, 4.99; 95% CI, 1.13-22.1; P = .034) between control vs aggressive-growth AAAs, and methyltetrahydrofolate reductase (OR, 2.99; 95% CI, 1.01-8.86; P = .048), MMP-9 p-2502 (OR, 2.19; 95% CI, 1.05-4.58; P = .037), and LRP1 (OR, 4.96; 95% CI, 1.03-23.9; P = .046) as the statistically significant confounds distinguishing slow-growth AAAs vs aggressive-growth AAAs.

CONCLUSIONS

Logistic regression identified different genetic confounds for the slow-growth and aggressive-growth AAAs, indicating a potential for different genetic influences on AAAs of distinct aggressiveness. Future logistic regression studies investigating for potential genetic or clinical confounds for this disease should take into account the growth rate and size of the AAA to better identify confounds likely to be associated with aggressive AAAs likely to require intervention.

Genetics of abdominal aortic aneurysm

PURPOSE OF REVIEW

Family history is a risk factor for abdominal aortic aneurysm (AAA), suggesting that genetic factors play an important role in AAA development, growth and rupture. Identification of these factors could improve understanding of the AAA pathogenesis and be useful to identify at risk individuals.

RECENT FINDINGS

Many approaches are used to examine genetic determinants of AAA, including genome-wide association studies (GWAS) and DNA linkage studies. Two recent GWAS have identified genetic markers associated with an increased risk of AAA located within the genes for DAB2 interacting protein (DAB2IP) and low density lipoprotein receptor-related protein 1 (LRP1). In addition, a marker on 9p21 associated with other vascular diseases is also strongly associated with AAA. The exact means by which these genes currently control AAA risk is not clear; however, in support of these findings, mice with vascular smooth muscle cell deficiency of Lrp1 are prone to aneurysm development. Further current work is concentrated on other molecular mechanisms relevant in AAA pathogenesis, including noncoding RNAs such as microRNAs.

SUMMARY

Current studies assessing genetic mechanisms for AAA have significant potential to identify novel mechanisms involved in AAA pathogenesis of high relevance to better clinical management of the disease.

C-reactive protein polymorphism rs3091244 is associated with abdominal aortic aneurysm

BACKGROUND

Abdominal aortic aneurysm (AAA) formation involves an inflammatory process with a strong genetic background. C-reactive protein (CRP) regulates inflammation and is elevated in patients with AAA. The aim of this study was to investigate the association of the triallelic (C, A, and T alleles) rs3091244 functional CRP single nucleotide polymorphism (SNP) with AAA.

METHODS

This was a case-control study involving two independent populations: 351 AAA patients (mean aortic diameter, 6.25 ± 1.47 cm) and 391 controls (mean diameter, 2.4 ± 0.2 cm) were recruited from Greece (the main cohort); and 371 patients (mean diameter, 5.4 ± 1.0 cm) and 362 controls (mean diameter, 2.4 ± 0.6 cm) were recruited from the United Kingdom (replication cohort). The frequency of the functional triallelic (C, T, and A alleles) rs3091244 polymorphism was analyzed in univariate and adjusted (for cardiovascular risk factors) analyses, assuming that the rare T and A alleles have similar functional properties (pooled analysis for T and A). Three groups were constructed: group A included those with the rare T and A alleles (genotypes TT, AA, and TA), group B included heterozygotes for the C allele (CT, CA), and group C included C allele homozygotes (CC, reference genotype). Finally, meta-analysis of the two populations was performed together with previously reported results.

RESULTS

Genotype distributions differed significantly between cases and controls in both cohorts (P < .001 and P = .001). Adjusted analysis (for all aneurysm-related risk-factors) showed an estimated odds ratio of 4.88 (95% confidence interval [CI], 2.96-8.04) for SNP group A and 2.38 (95% CI, 1.69-3.36) for SNP group B (P < .001 in both cases) in the initial cohort and 2.07 (95% CI, 1.33-3.21) for SNP group A and 1.70 (95% CI, 1.21-2.39) for SNP group B (P = .001 and .002) in the replication cohort. The SNP group A patients among the main cohort also had higher CRP levels (median, 26; interquartile range, 17-52 mg/L vs median, 4; interquartile range, 4-12 mg/L; P < .001). Aneurysms >5.5 cm were significantly more frequent among the SNP groups A and B compared with C allele homozygotes both in the main and the replication cohorts (P < .001 and P = .001, respectively). Meta-analysis of the two populations with previously reported results showed a positive association between minor-allele and aneurysm presence with an odds ratio of 1.47 (95% CI, 1.01-2.14; I(2) = 83.1%; P = .04).

CONCLUSIONS

The rare T and A alleles were significantly related with AAA presence in both populations and correlated with higher CRP levels and AAA diameter.

Standardization of outcome measures in clinical trials of pharmacological treatment for abdominal aortic aneurysm

An abdominal aortic aneurysm (AAA) is a common aortic wall disease with an increased prevalence in the elderly population (4-8% for those aged >65 years). Many AAAs are slow growing and remain insidious. Current standard of care for patients with small AAAs (<49 mm) is surveillance, with interventional therapy (open surgical repair or endovascular aneurysm repair) recommended for large (>50-55 mm), rapidly growing (>10 mm/year) or symptomatic AAAs. Although open surgical repair or endovascular aneurysm repair are effective, significant short- and long-term postoperative morbidity and mortality occurs. Currently, there is no pharmacological treatment specific for AAA; the need for the development of targeted pharmacological therapies based on clinically relevant and feasible outcomes acceptable to the medical community, regulatory agencies and third-party payers is high. A consensus on such end points will be critical to accelerating the development of pharmacological agents to prevent formation, arrest the expansion and reduce the rupture risk of AAA.

Anemia is Associated With Mortality Following Endovascular Repair of Abdominal Aortic Aneurysm

Aim: The aim of this study was to compare midterm mortality between anemic and nonanemic patients undergoing endovascular repair of abdominal aortic aneurysm and to assess a correlation with markers of inflammation. Methods: Anemia was defined as hemoglobin <13 (men) and <12 g/dL (women). The impact of anemia and inflammatory markers on mortality was assessed using Kaplan-Meier curves and Cox regression. Results: A total of 224 patients (12 females [5.36%]; age: 69.73 ± 8.72 years) were included; 102 (45.53%) were anemic. Median follow-up was 17 months (interquartile range: 7-25 months). Nine patients died (1.79%; 8 anemic vs 1 nonanemic). Survival was lower for patients with anemia (log-rank, P = .01). White blood cell count and C-reactive protein (CRP) differed significantly ( P < .001 and P = .01). Anemia and CRP were associated with decreased survival (Cox regression, P = .01, hazard ratio [HR]: 0.35, 95% confidence interval: 0.14-0.84 and P = .002, HR: 1.18, 95% CI: 1.06-1.31). Conclusion: Patients with anemia had decreased survival over the midterm; inflammatory markers were higher among this group.

Dynamic molecular processes mediate cellular mechanotransduction

Cellular responses to mechanical forces are crucial in embryonic development and adult physiology, and are involved in numerous diseases, including atherosclerosis, hypertension, osteoporosis, muscular dystrophy, myopathies and cancer. These responses are mediated by load-bearing subcellular structures, such as the plasma membrane, cell-adhesion complexes and the cytoskeleton. Recent work has demonstrated that these structures are dynamic, undergoing assembly, disassembly and movement, even when ostensibly stable. An emerging insight is that transduction of forces into biochemical signals occurs within the context of these processes. This framework helps to explain how forces of varying strengths or dynamic characteristics regulate distinct signalling pathways.

Genomic DNA pooling strategy for next-generation sequencing-based rare variant discovery in abdominal aortic aneurysm regions of interest-challenges and limitations

The costs and efforts for sample preparation of hundreds of individuals, their genomic enrichment for regions of interest, and sufficient deep sequencing bring a significant burden to next-generation sequencing-based experiments. We investigated whether pooling of samples at the level of genomic DNA would be a more versatile strategy for lowering the costs and efforts for common disease-associated rare variant detection in candidate genes or associated loci in a substantial patient cohort. We performed a pilot experiment using five pools of 20 abdominal aortic aneurysm (AAA) patients that were enriched on separate microarrays for the reported 9p21.3 associated locus and 42 additional AAA candidate genes, and sequenced on the SOLiD platform. Here, we discuss challenges and limitations connected to this approach and show that the high number of novel variants detected per pool and allele frequency deviations to the usually highly false positive cut-off region for variant detection in non-pooled samples can be limiting factors for successful variant prioritization and confirmation. We conclude that barcode indexing of individual samples before pooling followed by a multiplexed enrichment strategy should be preferred for detection of rare genetic variants in larger sample sets rather than a genomic DNA pooling strategy.