Evaluating the causal impact of reproductive factors on breast cancer risk: a multivariable mendelian randomization approach

Background Observational evidence proposes a protective effect of having children and an early age at first birth on the development of breast cancer, however the causality of this association remains uncertain. In this study we assess whether these reproductive factors impact breast cancer risk independently of age at menarche, age at menopause, adiposity measures and other reproductive factors that have been identified as being causally related to or genetically correlated with the reproductive factors of interest. Methods We used genetic data from UK Biobank (273,238 women) for reproductive factors, age at menarche and menopause, and adiposity measures, and the Breast Cancer Association Consortium for risk of overall, estrogen receptor (ER) positive and negative breast cancer as well as breast cancer subtypes. We applied univariable and multivariable Mendelian randomization (MR) to estimate direct effects of ever parous status, ages at first birth and last birth, and number of births on breast cancer risk. Results We found limited evidence of an effect of age at first birth on overall or ER positive breast cancer risk in either the univariable or multivariable analyses. While the univariable analysis revealed an effect of later age at first birth decreasing ER negative breast cancer risk (Odds ratio (OR): 0.76, 95% confidence interval:0.61-0.95 per standard deviation (SD) increase in age at first birth), this effect attenuated with separate adjustment for age at menarche and menopause (e.g., OR 0.83, 0.62-1.06 per SD increase in age at first birth, adjusted for age at menarche). In addition, we found evidence for an effect of later age at first birth on decreased human epidermal growth factor receptor 2 enriched breast cancer risk but only with adjustment for number of births (OR 0.28 (0.11-0.57) per SD increase in age at first birth). We found little evidence for direct effects of ever parous status, age at last birth or number of births on breast cancer risk, however, analyses of ever parous status and age at last birth were limited by weak instruments in the multivariable analysis. Conclusions This study found minimal evidence of a protective effect of earlier age at first birth on breast cancer risk, while identifying some evidence for an adverse effect on ER negative breast cancer risk. However, multivariable MR of ever parous status and age at last birth is limited by weak instruments which might be improved in future studies with larger sample sizes and when additional genetic variants related to reproductive factors are identified. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement All authors work in a unit that receives funding from the University of Bristol and the UK Medical Research Council (MRC) (MC\_UU\_00011/1, MC\_UU\_00011/5, MC\_UU\_00011/6). C.P., G.C.S., L.D.H., A.F., and R.C.R. are members of the Menarche, Menstruation, Menopause and Mental Health (4M) consortium, which was established with support from the GW4 Alliance. C.P. is supported by a Wellcome Trust PhD studentship in Molecular, Genetic and Lifecourse Epidemiology (108902/B/15/Z). L.D.H. is supported by Career Development Awards from the UK MRC (MR/M020894/1). R.C.R. is supported by the CRUK-funded Integrative Cancer Epidemiology Programme (C18281/A19169).B.L.-L. is funded by a Vice-Chancellor's Research Fellowship from the University of Bristol and acknowledges support from the Academy of Medical Sciences/Wellcome Trust/the Government Department of Business, Energy and Industrial Strategy/British Heart Foundation/Diabetes UK Springboard Award (SBF003/1170), Elizabeth Blackwell Institute for Health Research (University of Bristol), and Wellcome Trust Institutional Strategic Support Fund (204813/Z/16/Z). ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: UK Biobank received ethical approval from the North West Multi-Centre Research Ethics Committee (REC reference: 16/NW/0274) and was conducted in accordance with the principles of the Declaration of Helsinki. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes The availability of all data analysed in this study has been referenced throughout the manuscript and supplementary materials. GWAS summary statistics for breast cancer risk outcomes were obtained from