MicroRNA (miRNA)-binding site variants in 3' untranslated regions (3'UTRs) are a novel class of germ-line, functional mutations, which are now recognized as powerful biomarkers of human cancer risk and biology. The first mutation discovered in this class is the KRAS-variant, a let-7-binding site mutation in the 3'UTR of the KRAS oncogene. The KRAS-variant predicts increased cancer risk for certain populations, is a predictive biomarker of cancer treatment response across cancer types, leads to conserved tumor biology and elevated AKT signaling in KRAS-variant patient tumors, and was recently found to predict elevated TGF-β and immunosuppression in cancer patients. Based on the functional biology of the KRAS-variant in cancer patients, here we chose to investigate altered normal cellular biology in the presence of the KRAS-variant, through interrogation of an isogenic normal breast epithelial cell line model with and without the KRAS-variant. We find that KRAS-variant normal breast epithelial cells exhibit a mesenchymal phenotype, which appears to be due to numerous molecular changes, including miRNA dysregulation and autocrine pathway alterations, including elevated TGF-β, resulting in ZEB and SNAIL upregulation. Our findings support the hypothesis that the KRAS-variant has a fundamental biological impact on normal cellular biology, that is conserved in these patients when they develop cancer.

The KRAS-variant is a biologically functional, microRNA binding site variant, which predicts increased cancer risk especially for women. Because external exposures, such as chemotherapy, differentially impact the effect of this mutation, we evaluated the association of estrogen exposures, breast cancer (BC) risk and tumor biology in women with the KRAS-variant. Women with BC (n = 1712), the subset with the KRAS-variant (n = 286) and KRAS-variant unaffected controls (n = 80) were evaluated, and hormonal exposures, KRAS-variant status, and pathology were compared. The impact of estrogen withdrawal on transformation of isogenic normal breast cell lines with or without the KRAS-variant was studied. Finally, the association and presentation characteristics of the KRAS-variant and multiple primary breast cancer (MPBC) were evaluated. KRAS-variant BC patients were more likely to have ovarian removal pre-BC diagnosis than non-variant BC patients (p = 0.033). In addition, KRAS-variant BC patients also appeared to have a lower estrogen state than KRAS-variant unaffected controls, with a lower BMI (P < 0.001). Finally, hormone replacement therapy (HRT) discontinuation in KRAS-variant patients was associated with a diagnosis of triple negative BC (P < 0.001). Biologically confirming our clinical findings, acute estrogen withdrawal led to oncogenic transformation in KRAS-variant positive isogenic cell lines. Finally, KRAS-variant BC patients had greater than an 11-fold increased risk of presenting with MPBC compared to non-variant patients (45.39% vs 6.78%, OR 11.44 [3.42-37.87], P < 0.001). Thus, estrogen withdrawal and a low estrogen state appear to increase BC risk and to predict aggressive tumor biology in women with the KRAS-variant, who are also significantly more likely to present with multiple primary breast cancer.