Germline Genetic Variants Disturbing the <em>Let-7</em>/LIN28 Double-Negative Feedback Loop Alter Breast Cancer Susceptibility

Previous studies have shown that let-7 can repress the post-transcriptional translation of LIN28, and LIN28 in turn could block the maturation of let-7, forming a double-negative feedback loop. In this study, we investigated the effect of germline genetic variants on regulation of the homeostasis of the let-7/LIN28 loop and breast cancer risk. We initially demonstrated that the T/C variants of rs3811463, a single nucleotide polymorphism (SNP) located near the let-7 binding site in LIN28, could lead to differential regulation of LIN28 by let-7. Specifically, the C allele of rs3811463 weakened let-7–induced repression of LIN28 mRNA, resulting in increased production of LIN28 protein, which could in turn down-regulate the level of mature let-7. This effect was then validated at the tissue level in that the normal breast tissue of individuals with the rs3811463-TC genotype expressed significantly lower levels of let-7 and higher levels of LIN28 protein than those individuals with the rs3811463-TT genotype. Because previous in vitro and ex vivo experiments have consistently suggested that LIN28 could promote cellular transformation, we then systematically evaluated the relationship between rs3811463 as well as other common LIN28 SNPs and the risk of breast cancer in a stepwise manner. The first hospital-based association study (n = 2,300) demonstrated that two SNPs were significantly associated with breast cancer risk, one of which was rs3811463, while the other was rs6697410. The C allele of the rs3811463 SNP corresponded to an increased risk of breast cancer with an odds ratio (OR) of 1.25 (P = 0.0091), which was successfully replicated in a second independent study (n = 1,156) with community-based controls. The combined P-value of the two studies was 8.0×10−5. Taken together, our study demonstrates that host genetic variants could disturb the regulation of the let-7/LIN28 double-negative feedback loop and alter breast cancer risk.既往研究证实，let-7可抑制LIN28的转录后翻译，而LIN28反过来又可阻断let-7的成熟，二者形成双负反馈环（double-negative feedback loop）。本研究探讨了生殖系遗传变异对let-7/LIN28环稳态调控及乳腺癌风险的影响。本研究首先证实，位于LIN28基因let-7结合位点附近的单核苷酸多态性（single nucleotide polymorphism, SNP）rs3811463的T/C变异可导致let-7对LIN28的调控出现差异。具体而言，rs3811463的C等位基因可削弱let-7介导的LIN28 mRNA抑制作用，导致LIN28蛋白表达量升高，进而下调成熟let-7的水平。该效应在组织层面得到了验证：与携带rs3811463-TT基因型的个体相比，携带rs3811463-TC基因型个体的正常乳腺组织中let-7表达水平显著更低，而LIN28蛋白表达水平显著更高。鉴于既往体外（in vitro）和离体（ex vivo）实验均证实LIN28可促进细胞转化，本研究随后以逐步分析的方式系统评估了rs3811463及其他常见LIN28相关SNP与乳腺癌风险的关联。第一项以医院为基础的关联研究（样本量n=2300）显示，有两个SNP与乳腺癌风险显著相关，分别为rs3811463和rs6697410。rs3811463的C等位基因与乳腺癌风险升高相关，比值比（odds ratio, OR）为1.25（P=0.0091），该结果在第二项以社区人群为对照的独立研究（样本量n=1156）中得到了成功验证。两项研究的合并P值为8.0×10⁻⁵。综上，本研究证实宿主遗传变异可干扰let-7/LIN28双负反馈环的调控，并改变乳腺癌的发病风险。