The clinical characteristics of NSCLC patients and cancer-free controls are given in this study (Table 1). The age of the NSCLC group and the cancer-free group was 58.51 ± 9.77 and 59.39 ± 9.48 years old, respectively, with no significant difference in age (P = 0.351) and age composition (P = 0.909) between the two groups. Moreover, there was no significant difference in the sex composition between the NSCLC group and the cancer-free group (P = 0.212). For smoking and drinking status, however, there were significantly more subjects that smoked (P = 0.004) or drank (P = 0.044) in the NSCLC group than in the cancer-free group. The tumour node metastasis stage (TNM stage) of NSCLC patients in this study included stage I and II (135 patients) and stage III and IV (70 patients).

The result of this study illustrated that there was a significantly higher relative expression of miR-21 in NSCLC patients than in cancer-free controls (Fig. 1A; NSCLC: 4.82 ± 2.33, Controls: 3.72 ± 1.84; t-test, t = 5.428, df = 420, P < 0.001).

Influences of miR-21 SNPs for expression of miR-21. A, Expression of miR-21 in NSCLC patients. B, Influences of rs1292037 for expression of miR-21. C, Influences of rs6504593 for expression of miR-21. Notes: SNP, single nucleotide polymorphism; NSCLC, non-small cell lung cancer

In this study, the influences of SNPs on the expression of miR-21 in NSCLC patients and cancer-free controls were further investigated. For rs1292037, the expression of miR-21 was the highest in individuals with CC genotype of the SNP in both NSCLC patients (Fig. 1B; Table 2; one-way ANOVA, F = 0.505, P = 0.01; Bonferroni test, PCC−TT = 0.011, PCC−TC = 0.038, PTT−TC = 0.702) and cancer-free controls (Fig. 1B; Table 2; one-way ANOVA, F = 11.605, P < 0.001; Bonferroni test, PCC−TT < 0.001, PCC−TC = 0.010, PTT−TC = 0.125). Similarly, for rs6504593, the expression of miR-21 was also the highest in individuals with the CC genotype of the SNP in both of NSCLC patients (Fig. 1C; Table 2; one-way ANOVA, F = 5.802, P = 0.004; Bonferroni test, PCC−TT = 0.039, PCC−TC = 0.037, PTT−TC = 0.995) and cancer-free controls (Fig. 1C; Table 2; one-way ANOVA, F = 5.362, P = 0.005; Bonferroni test, PCC−TT = 0.013, PCC−TC = 0.020, PTT−TC = 0.914).

The genotype frequencies of SNPs rs1292037 and rs6504593 were both in HWE for both of NSCLC (Table 3; rs1292037: PHWE = 0.364; rs6504593: PHWE = 0.142) and control (Table 3; rs1292037: PHWE = 0.617; rs6504593: PHWE = 0.287) groups. Therefore, the logistic regression analysis was performed to assess the associations of the allele and genotype of the SNPs with NSCLC susceptibility. For rs1292037, the C allele (Table 3; P = 0.036, OR = 1.341, 95% CI = 1.019–1.764) and CC genotype (Table 3; P = 0.033, OR = 1.859, 95% CI = 1.047–3.302; Pa = 0.030, ORa = 1.920, 95% CIa = 1.066–3.457) were significantly associated with increased risk of NSCLC. For rs6504593, the C allele (Table 3; P = 0.011, OR = 1.455, 95% CI = 1.091–1.941) and CC genotype (Table 3; P = 0.007, OR = 2.530, 95% CI = 1.273–5.030; Pa = 0.004, ORa = 2.782, 95% CIa = 1.374–5.630) were risk factors for NSCLC.

The further exploration of the impact of SNP-SNP interaction on NSCLC susceptibility illustrated that the interaction of the TT genotype of rs1292037 with the TC (Table 4; P = 0.006, OR = 2.847, 95% CI = 1.349–6.008) and CC genotype (Table 4; P = 0.013, OR = 4.067, 95% CI = 1.353–12.226) of rs6504593 was one of the risk factors of NSCLC. The interaction of the TC genotype of rs1292037 with the TT (Table 4; P = 0.006, OR = 2.588, 95% CI = 1.301–5.150), TC (Table 4; P = 0.045, OR = 2.030, 95% CI = 1.012–4.072) and CC (Table 4; P = 0.003, OR = 5.175, 95% CI = 1.783–15.032) genotype of rs6504593 was also found to be related to the increase of the risk of NSCLC. In addition, the interaction of the CC genotype of rs1292037 with the TT (Table 4; P = 0.018, OR = 2.958, 95% CI = 1.191–7.347), TC (Table 4; P = 0.002, OR = 3.796, 95% CI = 1.602–8.994) and CC (Table 4; P = 0.032, OR = 10.353, 95% CI = 1.079–99.378) genotype of rs6504593 was also a risk factor that related with the increase of NSCLC susceptibility.