BACKGROUND: Many women carry male cells of presumed fetal origin-so-called male-origin microchimerism (MOM)-in their circulation and tissues. Studies have found reduced risks of hormone dependent cancers, including breast and ovarian cancer, among MOM-positive women. The aim of this study was to investigate the association between MOM and endometrial cancer.

METHODS: We designed a prospective case-cohort study including 76 cases and 505 controls from the Diet, Cancer and Health cohort aged 50-64 years and cancer-free at enrolment in 1993-1997. We analyzed blood samples for the presence of Y-chromosome (DYS14). We examined the association between MOM and endometrial cancer in weighted Cox regression models. As a negative control outcome, we studied the association between MOM and injuries to test for spurious associations.

RESULTS: We detected MOM in 65.9% controls and 54.0% cases. While we observed no overall association between MOM and endometrial cancer (HR=0.73, 95% CI: 0.47-1.15), we found a borderline significantly reduced rate of Type 1 endometrial cancer (HR=0.66, 95% CI: 0.39-1.00), but not other types of endometrial cancers (HR=1.00, 95% CI: 0.35-2.90). The reduced rate was not modified by hormonal exposure (P = 0.79). We found no association between MOM and risk of injuries (HR=0.96, 95% CI: 95% CI: 0.78-1.21).

CONCLUSIONS: Our study suggests that MOM is inversely associated with Type 1 endometrial cancer, without evidence of an interaction with hormonal exposure. We encourage future research to confirm our findings.