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Rationale: MyomiRs miR-499, miR-208a and miR-208b direct cardiac myosin gene expression. Sequence complementarity between miRs and their mRNA targets determines miR effects, but the functional consequences of human myomiR sequence variants are unknown.

Objective: To identify and investigate mutations in human myomiRs in order to better understand how and to what extent naturally-occurring sequence variation can impact miR-mRNA targeting and end-organ function.

Methods and Results: Screening of [almost equal to]2,600 individual DNAs for myomiR sequence variants identified a rare mutation of miR-499, u17c in the 3' end, well outside the seed region thought to determine target recognition. In vitro luciferase reporter analysis showed that the 3' miR-499 mutation altered suppression of a subset of artificial and natural mRNA targets. Cardiac-specific transgenic expression was used to compare consequences of wild-type and mutant miR-499. Both wild-type and mutant miR-499 induced heart failure in mice, but miR-499 c17 misdirected recruitment of a subset of miR-499 target mRNAs to cardiomyocyte RNA-induced silencing complexes, altering steady-state cardiac mRNA and protein make-up and favorably impacting cardiac function. In vitro analysis of miR-499 target site mutations and modeling of binding energies revealed abnormal miR-mRNA duplex configurations induced by the c17 mutation.

Conclusions: A naturally occurring miR-499 mutation outside the critical seed sequence modifies mRNA targeting and end-organ function. This first description of in vivo effects from a natural human miR mutation outside the seed sequence supports comprehensive studies of individual phenotypes or disease-modification conferred by miR mutations.

(C) 2012 American Heart Association, Inc.