Hearing impairment due to mutations suggests both loss and gain of function effects.

The microRNA miR-96 is important for hearing, as point mutations in humans and mice result in dominant progressive hearing loss. is expressed in sensory cells along with and , but the roles of these closely-linked microRNAs are as yet unknown. Here we analyse mice carrying null alleles of , and of and together to investigate their roles in hearing. We found that / heterozygous mice had normal hearing and homozygotes were completely deaf with abnormal hair cell stereocilia bundles and reduced numbers of inner hair cell synapses at four weeks old. knockout mice developed normal hearing then exhibited progressive hearing loss. Our transcriptional analyses revealed significant changes in a range of other genes, but surprisingly there were fewer genes with altered expression in the organ of Corti of null mice compared with our previous findings in mutants, which have a point mutation in the miR-96 seed region. This suggests the more severe phenotype of mutants compared with / mutants, including progressive hearing loss in heterozygotes, is likely to be mediated by the gain of novel target genes in addition to the loss of its normal targets. We propose three mechanisms of action of mutant miRNAs; loss of targets that are normally completely repressed, loss of targets whose transcription is normally buffered by the miRNA, and gain of novel targets. Any of these mechanisms could lead to a partial loss of a robust cellular identity and consequent dysfunction.