ASSOCIATION BETWEEN CANDIDATE GENE POLYMORPHISMS AND ASTHMA SEVERITY IN ADULTS. A BAYESIAN ANALYSIS.

Introduction Different genes are associated with categorical classifications of asthma severity. However, continuous measures of disease severity should be used to catch the heterogeneity of asthma phenotypes and to increase the power in association studies. Furthermore, due to the complex pathological processes involved in asthma, statistical models that simultaneously test the effect of all variants have a better performance and they gain power in identifying phenotypes associated to genetic variants, as compared to statistical models that consider a single variant at a time. AimsThis thesis is aimed at (i) providing continuous measures of asthma severity (Study-line 1) and (ii) evaluating the association between disease severity and single nucleotide polymorphisms (SNPs) in candidate gene regions (Study-line 2), in adult patients with ever asthma from the general population.Data from the Gene Environment Interactions in Respiratory Diseases study (GEIRD, 2007-2010), which is an Italian, population-based, (multi)case-control study on respiratory health, were used.Study-line 1Respiratory symptoms, anti-asthmatic treatment and lung function were measured on 520 cases of asthma (aged 20-64) in seven Italian centres (Ancona, Pavia, Salerno, Sassari, Terni, Torino and Verona). The variables that represent the same dimension of asthma severity were identified through an exploratory factor analysis (EFA) and were summarized through a multiple factor analysis (MFA). Only respiratory symptoms and anti-asthmatic treatment were summarized in a continuous score (STS). STS ranges from 0 (no symptoms/treatment) to 10 (maximum symptom frequency and treatment intensity). STS was positively correlated with the Global INitiative for Asthma (GINA) classification of asthma severity computed on the 137 cases with a doctor's diagnosis (Spearman’s coefficient=0.91, p-value<0.0001) (concurrent validity). Furthermore, using a cohort of 1,097 European asthmatics (ECRHS II study; 1999/2002), increasing SST levels at baseline (1991/1993) were positively associated with long-term outcomes (hospitalization and lost workdays for breathing problems, asthma attack frequency and use of asthma controllers) (predictive validity). Finally, the STS scores computed from the GEIRD and ECRHS II data were comparable (Pearson’s coefficient=0.97, p-value<0.00001) (replication analysis). Study-line 2At present, in the GEIRD study, genetic data have been collected in the Verona centre only. Therefore, only 326 subjects (aged 20-64) with ever asthma were evaluated in this analysis. A panel of 236 tag-SNPs, which are representative of 53 candidate gene regions with a previous indication of a possible association with asthma/COPD/rhinitis, was genotyped by using a custom GoldenGate Genotyping Assay and fulfilled the quality check. Measures of asthma severity are: the Symptom frequency and anti-asthmatic Treatment intensity Score (STS), which was developed in the study-line 1, and pre-bronchodilator FEV1% predicted. The association of SNPs with STS and FEV1% predicted was evaluated by using Bayesian Least Absolute Shrinkage and Selection Operator (LASSO) zero-inflated Poisson and linear regression models, respectively. These models allow the simultaneous selection of genetic variants and the estimation of their association with outcomes, adjusting for the effect of gender, body mass index and smoking habits. SNP rs2427829 in FCER1A was the only SNP associated with FEV1% predicted (mean = -0.323, credible interval = [-4.533, -0.268]). There were no SNPs associated with the risk of having symptomatic asthma (i.e. STS>0 vs STS=0), whereas four SNPs were associated with an increased STS in symptomatic asthma (i.e. E[STS] if STS>0): rs573122 in FCER1A (mean = 0.166, credible interval = [0.020, 0.320]), rs676750 in CHML (mean = -0.248, credible interval = [-0.434, -0.072]), rs4334089 in VDR (mean = -0.255, credible interval = [-0.456, -0.059]) and rs11080344 in NOS2 (mean = -0.224, credible interval = [-0.390, -0.064]).ConclusionsSTS is a valid and replicable measure of asthma severity in adults, which could be used in epidemiological studies. An application of this score in a genetic association analysis shows that five SNPs (in FCER1A, CHML, VDR and NOS2) could play a role in disease severity among adults with ever asthma.