Apolipoprotein E gene polymorphism and the risk of intracerebral hemorrhage: a meta-analysis of epidemiologic studies

Background Studies investigating the association between the apolipoprotein E (APOE) gene polymorphism and the risk of intracerebral hemorrhage (ICH) have reported conflicting results. We here performed a meta-analysis based on the evidence currently available from the literature to make a more precise estimation of this relationship. Methods Published literature from the National Library of Medline and Embase databases were retrieved. Odds ratio (OR) and 95% confidence interval (CI) were calculated in fixed- or random-effects models when appropriate. Subgroup analyses were performed by race. Results This meta-analysis included 11 case–control studies, which included 1,238 ICH cases and 3,575 controls. The combined results based on all studies showed that ICH cases had a significantly higher frequency of APOE ϵ4 allele (OR= 1.42, 95% CI= 1.21,1.67, P<0.001). In the subgroup analysis by race, we also found that ICH cases had a significantly higher frequency of APOE ϵ4 allele in Asians (OR= 1.52, 95% CI= 1.20,1.93, P<0.001) and in Caucasians (OR= 1.34, 95% CI= 1.07,1.66, P=0.009). There was no significant relationship between APOE ϵ2 allele and the risk of ICH. Conclusion Our meta-analysis suggested that APOE ϵ4 allele was associated with a higher risk of ICH.


Introduction
Intracerebral hemorrhage (ICH) occurs at an annual incidence rate of 15 to 19 per 100,000 [1]. ICH accounts for approximately 15% of acute strokes in the United States and 22-35% in Asian populations [2][3][4][5]. ICH can be a devastating type of stroke, and the 30-day case mortality rate of ICH is 40% to 50%. [2,6]. A pooled prospective study found that the risk factors for ICH were older age, African-American ethnicity, hypertension, lower LDL-C, and lower triglycerides [7]. ICH has been shown to have important genetic and environmental risk factors.
Recently, some studies have been conducted to clarify the association between APOE gene polymorphisms and the risk of ICH [14][15][16][17][18][19][20][21][22][23][24]. However, previous studies investigating the association have reported conflicting results [25,26]. We here performed a meta-analysis based on the evidence currently available from the literature to make a more precise estimation of this relationship.

Literature search strategy
We used a detailed electronic search strategy in Medline and Embase from 1950 to the end of March 2013. Two authors independently searched the databases using following key words in all relevant combinations: 'cerebral' or 'intracerebral' or 'intracranial', 'hemorrhage', 'apolipoprotein E*' or ' ApoE*', 'polymorphism' or 'allele' or 'genotype' or 'variant'. The search was conducted without limitation on language. The reference lists of all retrieved publications were scrutinized for additional studies. If studies had partially overlapping subjects, the smaller dataset was excluded. If necessary, we attempted to contact the principal investigators of retrieved articles to require additional data.

Inclusion and exclusion criteria
The following criteria were used to include published studies: (i) independent epidemiological studies (for humans only); (ii) a clear description of APOE allele in ICH cases and controls; (iii) sufficient allele data were presented to calculate the odds ratio (OR) and 95% confidence interval (CI); Major reasons for exclusion of studies were (i) no control; (ii) not an original paper (e.g. review or letter etc.); (iii) duplicate publications.

Data extraction
Data were extracted by two authors independently, and disagreements were resolved by consensus. When a study did not explicitly report one or more of the requested data, we contacted the author of the study for additional details. The following data were extracted: the last name of the first author, publication year, country, study design, genotyping method, sample size and the results of studies.

Statistical analysis
All analyses were performed using STATA 11.0 (Stata-Corp LP, College Station, TX, USA). The Mantel-Haenszel method for fixed effects and the Der-Simonian-Laird method for random effects were used to estimate pooled OR and corresponding 95% CI. Meta-analysis heterogeneity was quantified by computing Cochrane's Q and corresponding P-value and I 2 (percent of effect size attributable to heterogeneity). We used fixed-effects methods if the result of the Q test was not significant. Otherwise, we calculated pooled estimates and confidence intervals assuming a random-effects model. Also, subgroup analyses were performed on the basis of race. In this study, P < 0.05 was considered statistically significant. Publication bias was assessed by visual inspection of funnel plots, the Begg's rank correlation method and the Egger's weighted regression method.

Quantitative synthesis
The combined results based on all studies showed that ICH cases had a significantly higher frequency of APOE ε4 allele (OR= 1.42, 95% CI= 1.21,1.67, P<0.001) (Figure 1) ( Table 3). In the subgroup analysis by race, we also found   (Figure 1) ( Table 3). There was no significant relationship between APOE ε2 allele and the risk of ICH ( Figure 2) ( Table 3).

Heterogeneity analysis and publication bias
Statistical heterogeneity was not found among studies in overall comparisons by using the Q statistic (Table 3). Publication bias was not found by the Begg's rank correlation method (Figure 3) or Egger weighted regression method ( Figure 4).

Discussion
There is evidence for a role of genetic factors in the development of ICH. Studies investigating the association between genetic polymorphisms and ICH risk are being reported with rapidly increasing frequency. Endoglin gene polymorphism was a risk factor for sporadic ICH [37]. A comparative study that angiotensin converting enzyme (ACE) gene DD homozygosity of the I/D polymorphism in  intron 16 is an independent risk factor for ICH in a Polish population [38]. A case-control study suggested that the beta1-tubulin Q43P polymorphism could be associated with ICH in men from southern Spain [39]. A populationbased prospective nested case-control study found that estrogen receptor alpha gene polymorphisms were associated with first-ever ICH, particularly in combination with hypertension [40]. A case-control study found that glutathione peroxidase 1 C593T polymorphism was associated with lobar ICH in a Polish population [41]. A case-control study suggested that the rs2228048 of TGFBR2 gene may be associated with development of ICH in Korean population [42]. A study suggested that the rs17222919 of ALOX5AP may be associated with the development of ICH in Korean population [43].
The APOE gene polymorphisms are associated with many other diseases. A meta-analysis showed that APOE ε4 allele appeared to be associated with a higher prevalence of dementia in Parkinson disease [44]. A meta-analysis suggested that the APOE ε4 isoform was a genetic factor that might influence the age at onset of temporal lobe epilepsy [45]. A meta-analysis showed that the APOE ε4 allele was associated with an increased risk of developing hypertension [46]. A metaanalysis found that the APOE ε4 allele was associated with a moderately increased risk for progression from mild cognitive impairment to Alzheimer's disease-type dementia [47]. Prevalence of APOE ε4 alleles was significantly higher in patients with coronary artery disease than controls [48].  The exact mechanism of the association between APOE polymorphism and the risk of ICH remains unclear. APOE plays a critical role in redistributing lipids among central nervous system cells for normal lipid homeostasis [49], repairing injured neurons [50], maintaining synaptodendritic connections [51], neurite outgrowth [52], synaptic plasticity [53], mitochondrial resistance to oxidative stress [54], and glucose use by neurons and glial cells [25]. Compared with ε3/ε3, ε4 allele-containing genotypes are associated with increased total cholesterol levels [13]. It appears that the ε4 allele enhances amyloid deposition in blood vessels [55]. Thus, one might expect ε4 carriers to have increased susceptibility to ICH, especially in a lobar location. Furthermore, APOE ε4 allele was also associated with an increased risk of developing hypertension [46], which may be the reason that APOE ε4 allele was associated with a higher risk of ICH.
Several limitations of our meta-analysis should be noted. First of all, meta-analysis is powerful but also controversial-controversial because several conditions are critical to a sound meta-analysis, and small violations of those conditions can lead to misleading results [56]. Second, relatively small sample size of studies in overall comparisons was observed in this meta-analysis. The results of small meta-analyses should be regarded with caution, even if the P value shows extreme statistical significance [57]. Thirdly, because of the lack of individual patient data, we could not perform an adjustment estimate. In spite of these limitations, our meta-analysis also had some advantages. First, the major strengths of the meta-analysis are that we used a comprehensive searching strategy based on computer-assisted and manual searching which allowed the eligible studies to be included as far as possible. Second, no heterogeneity or publication bias was found, which leads to a possibly robust result.
In conclusion, our meta-analysis suggested that APOE ε4 allele was associated with a higher risk of ICH. Future studies will be required to clarify the biological implications of our findings.