A MicroRNA Profile in Fmr1 Knockout Mice Reveals MicroRNA Expression Alterations with Possible Roles in Fragile X Syndrome.

Fragile X syndrome (FXS), a common form of inherited mental retardation, is caused by a loss of expression of the fragile X mental retardation protein (FMRP). FMRP is involved in brain functions by interacting with mRNAs and microRNAs (miRNAs) that selectively control gene expression at translational level. However, little is known about the role of FMRP in regulating miRNA expression. Here, we found a development-dependant dynamic expression of Fmr1 gene (encoding FMRP) in mouse hippocampus with a small peak at postnatal day 7 (P7). MiRNA microarray analysis showed that the levels of 38 miRNAs showed a significant increase with about 15 ~ 250-folds and the levels of 26 miRNAs showed a significant decrease with only about 2 ~ 4-folds in the hippocampus of P7 Fmr1 knockout (KO) mice. The qRT-PCR assay showed that nine of the most increased miRNAs (>100-folds in microarrays) increased about 40 ~ 70-folds and their pre-miRNAs increased about 5 ~ 10-folds, but no significant difference in their pri-miRNA levels was observed, suggesting that the alterations of partial miRNAs are an indirect consequence of FMRP lacking. We further demonstrated that a set of protein-coding mRNAs, potentially targeted by the nine miRNAs, were down-regulated in the hippocampus of Fmr1 KO mice. Finally, luciferase assays demonstrated that miR-34b, miR-340, and miR-148a could down-regulate the reporter gene expression by interacting with the Met 3' UTR. Taken together, these findings suggest that the miRNA expression alterations resulted from the absence of FMRP might contribute to molecular pathology of FXS.