A Single Let-7 MicroRNA Bypasses LIN28-Mediated Repression

• Human let-7a-3 and mouse let-7c-2 miRNAs escape LIN28 regulation
• Human let-7a-3 and mouse let-7c-2 pre-miRNAs are refractory to LIN28 binding
• LIN28 cold-shock domain interaction with let-7c-2 pre-miRNA loop is compromised
• A five-nucleotide substitution in pre-let-7c-2 loop restores LIN28 repression

SummaryLet-7 microRNAs (miRNAs) are critical regulators of animal development, stem cell differentiation, glucose metabolism, and tumorigenesis. Mammalian genomes contain 12 let-7 isoforms that suppress expression of a common set of target mRNAs. LIN28 proteins selectively block let-7 biogenesis in undifferentiated cells and in cancer. The current model for coordinate let-7 repression involves the LIN28 cold-shock domain (CSD) binding the terminal loop and the two CCHC-type zinc fingers recognizing a GGAG sequence motif in precursor let-7 (pre-let-7) RNAs. Here, we perform a systematic analysis of all let-7 miRNAs and find that a single let-7 family member, human let-7a-3 (and its murine ortholog let-7c-2), escapes LIN28-mediated regulation. Mechanistically, we find that the pre-let-7c-2 loop precludes LIN28A binding and regulation. These findings refine the current model of let-7 regulation by LIN28 proteins and have important implications for understanding the LIN28/let-7 axis in development and disease.

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