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MicroRNAs (miRNAs) are small, non-coding, single-stranded RNAs between 18-22 nucleotides long that regulate gene expression. Expression of miRNAs is altered in tumor compared to normal tissue; there is some evidence that these changes may be reflected in the serum of cancer cases compared to healthy individuals. This has yet to be examined in a prospective study where samples are collected before diagnosis.    Methods  We used Affymetrix arrays to examine serum miRNA expression profiles in 410 participants in the Sister Study, a prospective cohort study of 50,884 women. All women in the cohort had never been diagnosed with breast cancer at the time of enrollment. We compared global miRNA expression patterns in 205 women who subsequently developed breast cancer and 205 women who remained breast cancer-free. In addition within the case group we examined the association of miRNA expression in serum with different tumor characteristics, including hormone status (ER, PR, and HER-2) and lymph node status.    Results  Overall, 414 of 1,105 of the human miRNAs on the chip were expressed above background levels in 50 or more women. When the average expression among controls was compared to cases using conditional logistic regression, 21 miRNAs were found to be differentially expressed (P≤.05). Using qRT-PCR on a small, independent sample of 5 cases and 5 controls we verified overexpression of the 3 highest expressing miRNAs among cases, miR-18a, miR-181a, and miR-222; the differences were not statistically significant in this small set. The 21 differentially expressed miRNAs are known to target at least 82 genes; using the gene list for pathway analysis we found enrichment of genes involved in cancer-related processes. In a separate case-case analyses restricted to the 21 miRNAs, we found 7 miRNAs with differential expression for women whose breast tumors differed by HER-2 expression, and 10 miRNAs with differential expression by nodal status.    Conclusions  miRNA levels in serum show a number of small differences between women who later develop cancer versus those who remain cancer-free.

Serum microRNA expression as an early marker for breast cancer risk in prospectively collected samples from the Sister Study cohort