**5. Targeting miRNA as new putative therapeutic tool**

### **5.1. Could miRNAs help in improving CF treatment?**

A recent work demonstrated that miR-138 mimics might restore CFTR-Phe508del expression and functional chloride transport. However, the authors stressed that miR-138 mimics may also have undesired effects, because miR-138 targets SIN3A, a highly conserved transcriptional repressor that regulates many genes [67]. Another anti-miRNA agent has been exploited as inhibitor of miR-509-3p, which is involved in the regulation of the *CFTR* gene. Recently, we reported that miRNA function can be blocked by targeting the *CFTR* gene with blockers. We designed blockers to prevent the binding of several miRNAs specifically to the 3'UTR-*CFTR* and tested them in well-differentiated primary human nasal epithelial cells from healthy individuals and patients with CF carrying the p.Phe508del *CFTR* mutation. These molecules rescued CFTR chloride channel activity by increasing *CFTR* mRNA and protein levels. This is in agreement with previous studies showing that complementation of just 6–10% of CFTR transcripts leads to the production of enough CFTR to maintain normal chloride transport in epithelia [75]. These data are supported by findings that the presence of a naturally occurring sequence variation in the *CFTR* promoter, in cis of a severe mutation, increases transcription. This allows the production of enough CFTR protein to reach the apical membrane cells and partially restore CFTR channel function, thus leading to a moderate CF phenotype despite the presence of a severe disease-causing mutation [76]. Similarly, stabilization of p.Phe508del CFTR protein has been associated with increased p.Phe508del CFTR channel activity [77].

### **5.2. Assays and molecules**

As depicted in Figure 5Ad, inhibitors or target-site blocker oligonucleotides have been previously used to restore CFTR expression. Tests have been performed by incorporating inhibitors that induce degradation of the targeted endogenous miRNA or with oligonucleoti‐ des that block miRNA binding to the 3'UTR of *CFTR* in cell lines, primary cultures and reconstituted epithelium
