Premenopausal serum midkine levels and risk of estrogen receptor positive breast cancer: a prospective, nested case-control study.

[BACKGROUND] Midkine is a heparin-binding growth factor that is overexpressed in most human malignancies, including breast cancer. While elevated midkine levels have been associated with tumor progression and aging, its role as a predictive biomarker for breast cancer risk in healthy individuals remains unclear. We previously showed that higher midkine expression in estrogen receptor-positive (ER +) breast cancer in younger (< 55) women is associated with shorter disease-free survival. We investigated whether serum midkine levels in premenopausal women are associated with subsequent risk of ER + breast cancer.

[METHODS] We conducted a prospective, nested case-control study within the New York University Women's Health Study (NYUWHS). Serum midkine levels were measured in baseline blood samples from 249 premenopausal women who developed ER + breast cancer more than 10 years after blood collection and 249 matched controls. Conditional logistic regression models were used to estimate odds ratios (ORs) and 95% confidence intervals (CIs) across quartiles and continuous midkine levels, adjusting for key breast cancer risk factors.

[RESULTS] Higher circulating midkine levels were associated with a marginally statistically significant lower risk of ER + breast cancer. Compared to the lowest quartile, women in the highest quartile had an OR of 0.55 (95% CI: 0.30-0.99; P for trend = 0.10). A doubling in midkine was associated with a 34% reduction in risk (OR = 0.66; 95% CI: 0.42-1.02). The inverse association was generally consistent across subgroups.

[CONCLUSION] These findings suggest that higher baseline serum midkine levels in premenopausal women are associated with a reduced long-term risk of ER + breast cancer. This challenges prior assumptions about midkine's uniformly pro-tumorigenic role and suggests it may be a context-dependent biomarker in breast cancer development.