The basic characteristics of study subjects
A total of 468 subjects including 238 HAPE patients (220 males and 18 females) and 230 healthy individuals (213 males and 17 females) were enrolled in this study. The mean age and standard deviation of the cases and the controls were 32.35 ± 10.78 and 33.45 ± 9.05 years, respectively. As shown in Table 1, There was no significant difference between the case and control groups in regards to the distribution of gender and age (P = 1.000 and 0.236, respectively).
Basic information and preliminary statistics of the selected SNPs
The basic information of the four SNPs was presented in Table 2. We genotyped the four SNPs (rs3093193, rs12459936, rs3093144 and rs3093110) and the genotyping success rates of the four SNPs were > 95.0% among the HAPE cases and the controls. The genotype distribution of of all SNPs in the control groups were in accordance with Hardy-Weinberg Equilibrium (HWE) (P > 0.05). The differences in allele frequency between cases and controls were compared by Chi-squared test, and whether the P value was less than 0.05 to assess the association with the risk of developing HAPE. The minor allele of each SNP as a risk factor was compared to the wild-type (major) allele. Statistical analysis showed that the allele frequency of rs3093193 was significantly different between the healthy control group and the HAPE patients, and rs3093193 was significantly correlated with the reduced risk of HAPE (OR = 0.70, 95% CI = 0.52 - 0.93, P = 0.014). However, the other three SNPs (rs12459936, rs3093144 and rs3093110) were not significantly associated with HAPE risk in the allele model (P = 0.099, P = 0.252 and 0.050, respectively).
Associations between genotype frequencies and HAPE susceptibility
Multiple inheritance models (dominant, recessive, additive, and codominant models) were applied for analyzing the association between each SNP and HAPE risk in Table 3. Among the four CYP4F2 polymorphisms, rs3093193 was found to reduce HAPE risk under dominant model (C/G-G/G vs. GG: OR = 0.66, 95% CI = 0.46 - 0.95, P = 0.027) and log-additive model (OR = 0.70, 95% CI = 0.52 - 0.94, P = 0.017). As for polymorphism rs12459936, an increased risk of HAPE was found in the codominant model (C/T vs. C/C: OR = 2.08, 95% CI = 1.33-3.26, P = 0.001) and the dominant model (C/T-T/T vs. C/C: OR = 1.88, 95% CI = 1.23 - 2.88, P = 0.003). The other two SNPs (rs3093144 and rs3093110) were not related to HAPE susceptibility (P > 0.05).
Stratification analysis of CYP4F2 polymorphisms and HAPE risk
Stratified analysis regarding the impact of CYP4F2 polymorphisms on HAPE according to age are displayed in Table 4. The results indicated that rs3093193 was correlated with a decreased HAPE risk at age ≤ 32 years under the allele model (OR = 0.63, 95% CI = 0.42 - 0.95, P = 0.028), codominant model (G/G vs. C/C: OR = 0.28, 95% CI = 0.10 - 0.85, P = 0.024) and recessive model (G/G vs. C/C-C/G: OR = 0.30, 95% CI = 0.10 - 0.87, P = 0.027). Rs12459936 was related to enhance HAPE risk at age ≤ 32 years under the allele model (OR = 0.63, 95% CI = 0.42 - 0.95, P = 0.028), codominant model (C/T vs. C/C: OR = 2.92, 95% CI = 1.52 - 5.63, P = 0.001; T/T vs. C/C: OR = 2.30, 95% CI = 1.10 - 4.83, P = 0.027), dominant model (C/T-T/T vs. C/C: OR = 2.69, 95% CI = 1.45 - 4.96, P = 0.002) and log-additive model (OR = 1.52, 95% CI = 1.05 - 2.19, P = 0.026). Conversely, two loci (Rs3093193 and rs3093110) of BDNF gene were obversed decreased HAPE risk at age > 32. Rs3093193 was correlated with a decreased HAPE risk at age > 32 years under the codominant model (C/T-T/T vs. C/C: OR = 0.57, 95% CI = 0.33 - 0.99, P = 0.046). Rs3093110 was associated with a decreased HAPE risk at age > 32 years under the allele model (OR = 0.63, 95% CI = 0.42 - 0.95, P = 0.028), codominant model (A/G vs. A/A: OR = 0.42, 95% CI = 0.21 - 0.83, P = 0.013), dominant model (A/G-G/G vs. A/A: OR = 0.44, 95% CI = 0.23 - 0.86, P = 0.016) and log-additive model (OR = 0.52, 95% CI = 0.29 - 0.95, P = 0.034).
Furthermore, we conducted another stratified analysis of gender adjusted for age as shown in Table 5. In males, rs3093193 was correlated with a decreased risk of HAPE in males under under the allele model (OR = 0.67, 95% CI = 0.50 - 0.91, P = 0.010), codominant model (C/G vs. C/C: OR = 0.65, 95% CI = 0.44 - 0.97, P = 0.036), dominant model (C/G-G/G vs. C/C: OR = 0.62, 95% CI = 0.42 - 0.91, P = 0.015) and log-additive model (OR = 0.67, 95% CI = 0.50 - 0.91, P = 0.011). Rs12459936 was related to improved risk of LC in the codominant model (C/T vs. C/C: OR = 2.12, 95% CI = 1.33 - 3.37, P = 0.002) and dominant model (C/T-T/T vs. C/C: OR = 1.94, 95% CI = 1.25 - 3.02, P = 0.003). Rs3093110 was related to enhance HAPE risk in the allele model (OR = 0.61, 95% CI = 0.40 - 0.94, P = 0.026) and log-additive model (OR = 0.63, 95% CI = 0.41 - 0.96, P = 0.034). No significant correlation between any genotypes among the SNPs rs3093144 and HAPE risk were observed.
Associations between haplotype analyses and HAPE risk
Finally, haplotype-based association and linkage disequilibrium (LD) study were conducted to show the association between CYP4F2 haplotype and risk of HAPE. Among the subpopulation (age ≤ 32 years), three SNPs were found to exist in one LD block (rs3093193, rs12459936 and rs3093144) in CYP4F2 gene (Fig.1). The distributions of different haplotypes of CYP4F2 gene in both the patients of HAPE and controls, and its association with HAPE risk were presented in Table 6. The results showed that the haplotype CTC was found to prominently increase the risk of HAPE among age ≤ 32 years under unconditional logistic regression analysis adjusted for age and gender (OR = 1.52, 95% CI = 1.05-2.19, p = 0.026). Other haplotypes did not display the correlativity.