Association of nitric oxide synthase gene polymorphism with asthma: A systematic review and meta‐analysis

Abstract Introduction This study examines the associations between asthma and nitric oxide (NO) synthase (NOS) gene polymorphisms. Methods After a systematic literature search in electronic databases, studies were selected based on eligibility criteria. Data were extracted from research articles and were synthesized and tabulated. Where a particular polymorphism data were reported by multiple studies, meta‐analyses of odds ratios were performed, or odds ratios reported by individual studies were pooled. Results Twenty studies (4450 asthma patients and 5306 non‐asthmatic individuals) were identified. Many studies did not find any association between CCTTT repeat polymorphism in NOS2 gene and asthma. However, a study reported that pretreatment mean exhaled NO levels in asthmatics were found to be significantly higher in genotypes with higher number of CCTTT repeats. Also, alleles with