Age at first birth and lung cancer: a two-sample Mendelian randomization study

BACKGROUND: Growing evidence suggests that female reproductive factors, like age at first birth (AFB), may play a potential role in the progression of lung cancer (LC). However, previous studies are susceptible to confounding factors, inadequate attention to variation by histology or reverse causality. Few studies have comprehensively evaluated their association and the causal effect remains unclear. METHODS: We aimed to determine whether AFB is causally correlated with the risk of LC, by means of utilizing aggregated data from the large genome-wide association studies conducted on AFB (251,151 individuals) and data of LC from International Lung and Cancer Consortium (ILCCO, 11,348 cases and 15,861 controls). We used 10 AFB-related single nucleotide polymorphisms as instrument variables and applied several two-sample Mendelian randomization (MR) methods. Secondary results according to different histological subtypes of lung cancer were also implemented. RESULTS: Conventional inverse-variance weighted method indicated that genetic predisposition towards number unit (1 year) increase of AFB was associated with a 18% lower risk of LC [odds ratio (OR) =0.82, 95% confidence interval (CI): 0.69–0.97; P=0.029]. When results were examined by histotypes, an inverse association was observed between genetically predisposed number unit (1 year) increase of AFB and lung adenocarcinoma (OR =0.75, 95% CI: 0.59–0.97, P=0.017) but not with squamous cell lung cancer (OR =0.77, 95% CI: 0.57–1.05, P=0.103). The results demonstrated no association between number unit decrease of AFB and LC. Pleiotropy was not presented through sensitivity analyses including MR pleiotropy residual sum and outlier test (P=0.412). Genetic predisposition towards older AFB was additionally associated with longer years of schooling (OR =1.12, 95% CI: 1.08–1.16, P<0.001), lower body mass index (OR =0.93, 95% CI: 0.88–0.98, P=0.004) and less alcohol consumption (OR =0.99, 95% CI: 0.99–1.00, P=0.004). CONCLUSIONS: Our study suggested that older AFB was a causal protective factor in the progression of LC. Further studies elucidating the potential mechanisms are needed.