3.1 Methylation Levels of HTR2A Significantly Elevated in RA Patients
We examined the methylation status of HTR2A cg15692052 in HC, AS, OA, gout, SS, RA, PSA, SLE, DM. We detected seven CG sites, including cg15692052_75, cg15692052_125, cg15692052_143, cg15692052_149, cg15692052_167, cg15692052_185, and cg15692052_187.
We compared the methylation level differences among other groups compared to the healthy control group. The results showed: Excluding the OA group, the methylation levels of cg15692052_75/125/143 were significantly increased (P < 0.05); the methylation level of cg15692052_149 was significantly elevated in all groups except for the OA and PSA groups (P < 0.05); the methylation level of cg15692052_163 was significantly higher in all groups except for the OA, SS, and Gout groups (P < 0.05);the methylation levels of cg15692052_185/187 were significantly elevated in the RA, SLE, DM groups (P < 0.05);the average methylation level was significantly higher in all groups except for the OA and SS groups (P < 0.05) (Fig. 1C-J).
On further examining the methylation differences of HTR2A between HC and various serological subtypes of RA, the results showed: Compared to the normal group, the methylation levels of the seven sites of cg15692052 and the average methylation level were significantly increased in all four RA subtypes (RF-negative RA patients, RF/CCP double positive, RF/CCP double negative, CCP single negative) (P < 0.05); Compared to RF/CCP double negative RA patients or RF/CCP double positive RA patients, the methylation level of cg15692052_75 in CCP single negative RA patients was significantly increased (P < 0.05); Compared to RF-negative RA patients or RF/CCP double positive RA patients, the methylation levels of cg15692052_125/143/149 and the average methylation level in CCP single negative RA patients were significantly increased (P < 0.05); Compared to RF/CCP double positive RA patients, the methylation levels of cg15692052_167/185 in CCP single negative RA patients were significantly increased (P < 0.05), and the methylation level of cg15692052_187 in RF/CCP double negative RA patients was significantly increased (P < 0.05) (Fig. 1K-Q).
3.2 Changes in Haplotype Methylation Proportion of HTR2A
We compared the changes in the haplotype proportion of HTR2A cg15692052 methylation in other groups compared to normal individuals. The results showed that CCCCCCC was significantly increased in the Gout, PSA, RA, SLE, and DM groups (P < 0.05); TTCCCCC was significantly decreased in the RA, SLE, and DM groups (P < 0.05); TTTTTTT was significantly decreased in the AS, PSA, RA, SLE, and DM groups (P < 0.05); CCCTCCC was significantly increased in the PSA, RA, and DM groups (P < 0.05); CTCCCCC was significantly increased in the RA group (P < 0.05); TTTTTCC was significantly decreased in the RA and DM groups (P < 0.05); CCTCCCC was significantly increased in the RA, OA, Gout, and DM groups (P < 0.05); TTTTTCCC was significantly decreased in the Gout, RA, SLE, and DM groups (P < 0.05); TTTTTTC was significantly decreased in the OA, RA, SLE, and DM groups (P < 0.05); CCCCCCT was significantly increased in the RA, SLE, and DM groups (P < 0.05); CCCCCTC was significantly increased in the PSA, RA, and DM groups (P < 0.05) (Fig. 2A-K).
Further examination of the differences in the haplotype methylation proportion of HTR2A between HC and various serological subtypes of RA revealed that compared to HC, CCCCCCC/CCCTCCC was significantly increased in all four subtypes of RA (RF single-negative RA patients, RF/CCP double-positive, RF/CCP double-negative, CCP single-negative) (P < 0.05); CCCCCCC was significantly increased in CCP single-negative RA patients compared to RF single-negative, RF/CCP double-negative, and RF/CCP double-positive RA patients (P < 0.05); TCCCCCC was significantly increased in RF/CCP double-negative patients compared to the HC group, RF single-negative or RF/CCP double-positive RA patients (P < 0.05); TTCCCCC was significantly decreased in RF single-negative, RF/CCP double-positive, and CCP single-negative RA patients compared to the HC group (P < 0.05); TTCCCCC was significantly increased in RF/CCP double-positive and RF/CCP double-negative RA patients compared to RF single-negative RA patients (P < 0.05); TTCCCCC was significantly decreased in CCP single-negative RA patients compared to RF/CCP double-positive RA patients (P = 0.004); TTCCCCC was significantly increased in RF/CCP double-negative RA patients compared to CCP single-negative RA patients (P = 0.002); TTTTTTT/TTTTTCC was significantly decreased in all four subtypes of RA (RF single-negative RA patients, RF/CCP double-positive, RF/CCP double-negative, CCP single-negative) compared to HC (P < 0.05); CTCCCCC/CCCCCTC was significantly increased in RF/CCP double-positive, CCP single-negative, and RF/CCP double-negative RA patients compared to HC (P < 0.05); TTTTTCCC was significantly decreased in RF single-negative, RF/CCP double-positive, and CCP single-negative RA patients compared to HC (P < 0.05); TTTTTCCC was significantly increased in RF single-negative, RF/CCP double-positive, and RF/CCP double-negative RA patients compared to CCP single-negative RA patients (P < 0.05); TTTTTTC was significantly decreased in RF/CCP double-positive, RF/CCP double-negative, and CCP single-negative RA patients compared to HC (P < 0.05); TTTTTTC was significantly increased in RF single-negative and RF/CCP double-positive RA patients compared to CCP single-negative RA patients (P < 0.05); TTTTTTC was significantly decreased in RF/CCP double-negative RA patients compared to RF single-negative RA patients (P = 0.028); CCCCCT was significantly increased in CCP single-negative and RF/CCP double-positive RA patients compared to HC (P < 0.05) (Fig. 2L-V).
3.3 Correlation between HTR2A Methylation Levels and Common Clinical Indicators in RA Patients
We further investigated the correlation between the methylation levels of HTR2A cg15692052 at individual sites and the average methylation level with common clinical indicators in RA patients, including gender, age, height, weight, ESR, CRP, RF, CCP, presence of hypertension, and presence of interstitial lung disease. The results showed: cg15692052_75 was significantly positively correlated with ESR, CRP, and the presence of interstitial lung disease (r = 0.15, 0.22, and 0.10, P = 0.002, P < 0.001, and P = 0.044); cg15692052_125 was significantly positively correlated with gender, ESR, CRP, and the presence of interstitial lung disease (r = 0.11, 0.13, 0.20, and 0.15, P = 0.021, 0.010, P < 0.001, and P = 0.003); cg15692052_143 was significantly positively correlated with gender, ESR, CRP, and the presence of interstitial lung disease (r = 0.12, 0.10, 0.18, and 0.11, P = 0.013, 0.039, P < 0.001, and P = 0.025);cg15692052_149 was significantly positively correlated with gender, CRP, and the presence of interstitial lung disease (r = 0.10, 0.17, and 0.13, P = 0.041, 0.001, and 0.008); cg15692052_167 was significantly positively correlated with gender, ESR, CRP, and the presence of interstitial lung disease (r = 0.13, 0.12, 0.19, and 0.13, P = 0.010, 0.019, P < 0.001, and P = 0.009); cg15692052_185 was significantly positively correlated with gender, CRP, and the presence of interstitial lung disease (r = 0.14, 0.15, and 0.13, P = 0.005, 0.003, and 0.009); cg15692052_187 was significantly positively correlated with gender, ESR, CRP, and the presence of interstitial lung disease (r = 0.17, 0.10, 0.16, and 0.19, P = 0.001, 0.049, 0.001, and P < 0.001); the average methylation level was significantly positively correlated with gender, ESR, CRP, and the presence of interstitial lung disease (r = 0.12, 0.12, 0.20, and 0.13, P = 0.015, 0.012, P < 0.001, and P = 0.008) (Fig. 3A).
3.4 Correlation between HTR2A Haplotypes and Common Clinical Indicators in RA Patients
Given the differences in the proportion of HTR2A cg15692052 methylation haplotypes, we further analyzed their correlation with common clinical indicators in RA. The results showed that CCCCCCC was significantly positively correlated with ESR and CRP (r = 0.13 and 0.21, P = 0.001 and P < 0.001), and significantly negatively correlated with age (r=-0.10, P = 0.047). TCCCCCC was significantly positively correlated with Gender, age, and the presence of interstitial lung disease (r = 0.12, 0.11, and 0.18, P = 0.012, 0.022, and P < 0.001), and significantly negatively correlated with RF (r=-0.11, P = 0.028). TTCCCCC was significantly positively correlated with age (r = 0.15, P = 0.003) and significantly negatively correlated with CRP (r=-0.13, P = 0.007). TTTTTTT was significantly negatively correlated with Gender, CRP, and the presence of interstitial lung disease (r=-0.17, -0.15, and − 0.14, P = 0.001, 0.002, and 0.006). TTTTTCC was significantly negatively correlated with ESR, CRP, and the presence of interstitial lung disease (r=-0.12, -0.20, and − 0.13, P = 0.012, P < 0.001, and P = 0.011). TCCTCCC was significantly positively correlated with age and weight (r = 0.12 and 0.11, P = 0.013 and 0.024). TTTTCCC was significantly negatively correlated with ESR, CRP, the presence of hypertension, and the presence of interstitial lung disease (r=-0.14, -0.21, -0.10, and − 0.13, P = 0.004, P < 0.001, P = 0.049, and 0.008). TTTTTTC was significantly negatively correlated with CRP (r=-0.13, P = 0.007). CCCCCTC was significantly negatively correlated with height (r=-0.11, P = 0.025) (Fig. 3B).
3.5 Methylation Level of HTR2A as an Auxiliary Diagnostic Marker for RA
We further assessed whether the methylation level of HTR2A could serve as a diagnostic biomarker for RA and its subtypes. We combined LASSO and random forest methods to jointly screen for important variables, and then used Logistic, random forest, and Xgboost methods to construct clinical prediction models, with all patients except RA serving as the control group(FigureS1). The group of CCP single-negative RA patients was excluded from subsequent analysis due to insufficient sample size. For distinguishing between RA and non-RA patients, the variables included in the model identified by LASSO and random forest were age, CRP, ESR, height, Gender, and cg15692052_185. The validation set AUCs of the models constructed by RF, Xgboost, and Logistic were 0.757/0.734/0.672, with F1 scores of 0.632/0.745/0.516(Fig. 4 and TableS2). For distinguishing between RF/CCP double-negative RA and non-RF/CCP double-negative RA patients, the variables included in the model were CCP, RF, CRP, weight, cg15692052_143, age, cg15692052_187, Gender, and height. The validation set AUCs of the models constructed by RF, Xgboost, and Logistic were 0.912/0.966/0.825, with F1 scores of 0.969/0.994/0.948(Fig. 4 and TableS2). For distinguishing between RF/CCP double-positive RA and non-RF/CCP double-positive RA patients, the variables included in the model were CCP, RF, CRP, Gender, cg15692052_187, age, and cg15692052_185. The validation set AUCs of the models constructed by RF, Xgboost, and Logistic were 0.832/0.846/0.714, with F1 scores of 0.712/0.826/0.574. For distinguishing between RF single-negative RA and non-RF single-negative RA patients, the variables included in the model were CCP, RF, ESR, Gender, cg15692052_185, age, and cg15692052_75. The validation set AUCs of the models constructed by RF, Xgboost, and Logistic were 0.928/0.923/0.932, with F1 scores of 0.939/0.984/0.930 (Fig. 4 and TableS2).