BACKGROUND: Reproductive factors are well known risk factors for breast cancer; however, little is known about how genetic variants in hormonal pathways interact with that relationship. METHODS: One thousand one hundred thirty nine cases of breast cancer in women and 1322 frequency-matched controls were compared. Genetic variants in hormonal pathways (identified in the Kyoto Encyclopedia of Genes and Genomes) were screened according to their relationship with breast cancer using the Cochran-Armitage statistic. Information on reproductive factors was obtained using a face-to-face questionnaire. The interaction among the selected genetic variants and reproductive factors was tested with logistic regression. RESULTS: Concerning C allele in rs2229712, compared to nulliparity in non-carriers the ORs for 1-2 and > 2 deliveries were 0.48 (0.28-0.81) and 0.34 (0.19-0.59), and in C carriers they were 0.92 (0.42-1.98) and 0.71 (0.31-1.61). Similar results were found in women carrying the C allele in rs1269851. Carriers of Allele T in rs35652107 and allele C in rs6018027 had the delivery number effect more pronounced. CONCLUSIONS: The number of deliveries had a dose-response protective effect on breast cancer; women carrying C allele in rs2229712 did not benefit from this protective effect.

This work was partially funded by the “Accion Transversal del Cancer”, approved by the Spanish Ministry Council on the 11th October 2007; The Instituto de Salud Carlos III-FEDER [PI08/1770, PI08/0533, PI08/1359, PI09/00773-Cantabria, PI09/01286-León, PI09/01903-Valencia, PI09/02078-Huelva, PI09/01662-Granada, PI11/01403, PI11/01889-FEDER, PI11/00226, PI11/01810, PI11/02213, PI12/00488, PI12/00265, PI12/01270, PI12/00715, PI12/00150, PI14/01219, PI14/0613, PI15/00069, PI15/00914, PI15/01032]; The Fundación Marqués de Valdecilla [API 10/09]; The ICGC International Cancer Genome Consortium CLL (The ICGC CLL-Genome Project is funded by Spanish Ministerio de Economía y Competitividad (MINECO) through the Instituto de Salud Carlos III (ISCIII) and Red Temática de Investigación del Cáncer (RTICC) del ISCIII (RD12/0036/0036)); The Junta de Castilla y León [LE22A10–2]; The Consejería de Salud of the Junta de Andalucía [2009-S0143]; The Conselleria de Sanitat of the Generalitat Valenciana [AP_061/10]; The Recercaixa [2010ACUP 00310]; The Regional Government of the Basque Country; The Consejería de Sanidad de la Región de Murcia; The European Commission [grants FOOD-CT-2006-036224-HIWATE]; The Spanish Association Against Cancer (AECC) Scientific Foundation; The Catalan Government DURSI [grant 2014SGR647]; The Fundación Caja de Ahorros de Asturias; and the University of Oviedo.