MicroRNAs (miRNAs) are a class of noncoding small RNAs that regulate gene expression by base pairing with target mRNAs at the 3′-terminal untranslated regions (3′-UTRs), leading to mRNA cleavage or translational repression. Single-nucleotide polymorphisms (SNPs) located at miRNA-binding sites (miRNA-binding SNPs) are likely to affect the expression of the miRNA target and may contribute to the susceptibility of humans to common diseases. We herein performed a genome-wide analysis of SNPs located in the miRNA-binding sites of the 3′-UTR of various human genes. We found that miRNA-binding SNPs are negatively selected in respect to SNP distribution between the miRNA-binding ‘seed’ sequence and the entire 3′-UTR sequence. Furthermore, we comprehensively defined the expression of each miRNA-binding SNP in cancers versus normal tissues through mining EST databases. Interestingly, we found that some miRNA-binding SNPs exhibit significant different allele frequencies between the human cancer EST libraries and the dbSNP database. More importantly, using human cancer specimens against the dbSNP database for case-control association studies, we found that twelve miRNA-binding SNPs indeed display an aberrant allele frequency in human cancers. Hence, SNPs located in miRNA-binding sites affect miRNA target expression and function, and are potentially associated with cancers.

MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression at the posttranscriptional level to lead to mRNA degradation or repressed protein production. The expression of miRNA is deregulated in many types of cancers. To determine whether genetic alterations in miRNA genes are associated with cancers, we have systematically screened sequence variations in several hundred human miRNAs from >100 human tumor tissues and 20 cancer cell lines. We identified 8 new single-nucleotide polymorphisms (SNPs) and 14 novel mutations (or very rare SNPs) that specifically present in human cancers. These mutations/SNPs are distributed in the regions of pri-, pre- and even mature miRNAs, respectively. Importantly, whereas most of the mutations did not exert detectable effects on miRNA function, a G --> A mutation at 19 nt downstream of miRNA let-7e led to a significant reduction of its expression in vivo, indicating that miRNA mutation could contribute to tumorigenesis. These data suggest that further screening for genetic variations in miRNA genes from a wide variety of human cancers should increase the discovery and identification of molecular diagnostic and therapeutic targets and complement the mutation analysis of consensus coding sequences in human cancers.

Regulated expression of transgenes in mammals is an important technique in both functional genomic studies and clinical applications. Here we describe a regulated gene expression system for mammals, based on coumarin-switched dimerization of the bacterial DNA gyrase B subunit (GyrB). The transactivator was constructed by fusing the GyrB activator to the bacterial lambda repressor-binding domain. The antibiotic coumermycin in nanomolar concentrations activated the transgene through binding of the homodimerized chimeric transactivator to the lambda operator located upstream of a minipromoter. More significantly, addition of novobiocin, an antagonist of coumermycin, promptly switched off expression of the gene by abolishing coumermycin-induced dimerization of the transactivator. Site-directed mutagenesis of the lambda repressor-binding domain resulted in significant reduction of basal expression levels and an induction reaching four orders of magnitude in stably transfected 293A cells in response to coumermycin. The capability of this inducible system for tightly regulated gene expression was demonstrated by the ready generation of stable cell lines inducibly expressing the proapoptotic bax gene in mammalian cells. Hence, this novel coumarin switch-on/switch-off system should broaden the utility of regulated gene expression, particularly when rapid on/off interchange is required.

Use of RNA interference (RNAi) as a reverse genetics tool for silencing genes in mammalian cells is achieved by in vitro transfection of small interfering RNAs (siRNAs). For a target gene, several siRNAs must be designed according to the empirical rules. We demonstrated that functional short hairpin RNAs (shRNAs) could be synthesized in Escherichia coli and delivered directly via bacterial invasion to the near entirety of a mammalian cell population to trigger RNAi. Furthermore, using a luciferase-target gene transcript, we identified effective shRNAs and siRNAs from RNAi libraries delivered conveniently through bacterial invasion in 96-well plates without need for preparation, purification and transfection of shRNAs. Notably, several of the most highly effective shRNAs and siRNAs identified do not fit the empirical rules commonly used for siRNA design, suggesting that this approach is a powerful tool for RNAi research, and could be used complementarily to the empirical rules for RNAi applications.