Germline Genetic Variation Disturbing The Let-7/LIN28 Double-negative Feedback Loop Alters Breast Cancer Susceptibility

Previous studies have shown that let-7 post-transcriptionally represses the expression of LIN28, and LIN28, in turn, blocks the maturation of let-7, forming a double-negative feedback loop. In this study, we investigated the effect of germline genetic variants on regulating the homeostatic status of the let-7/LIN28 loop and the influence of these variations on breast cancer risk. We demonstrated that the single nucleotide polymorphism (SNP), rs3811463, located near the binding site of let-7 in LIN28 may lead to a differential regulation of let-7 binding to LIN28 by altering the local mRNA secondary structure, thus disturbing the feedback loop between these two factors. Moreover, normal breast tissue harboring the rs3811463-C allele had significantly lower let-7 levels and higher LIN28 protein levels relative to the normal breast tissue harboring the wild-type T allele. Because the in vitro and ex vivo experiments consistently suggested that LIN28 modified breast cancer susceptibility, we subsequently evaluated the relationship between common SNPs found in LIN28 and breast cancer in a stepwise manner. The first hospital-based association study (n= 2,300) demonstrated that two SNPs (including rs3811463) significantly associated with breast cancer risk. The C allele of the rs3811463 SNP corresponded to an increased risk with an odds ratio (OR) of 1.25 (P=0.0091), which was successfully replicated in another independent study (n=1,156) with community-based controls. The combined P-value of the two studies was 8.0×10-5. Furthermore, a soft agar assay indicated that LIN28 was able to transform MCF-10A cells, which is a normal breast epithelial cell line. Taken together, our study demonstrated that host genetic variation disturbing the regulation of the let-7/LIN28 double-negative feedback loop can alter breast cancer risk.