**7. Gene correction in the lung**

In a recent study, nuclease-encoding chemically modified mRNA (nec-mRNA) was described as a novel vehicle for delivering genome-editing components directly to the lung [37]. Using a murine model of SP-B deficiency, nec-mRNA-encoded ZFNs were able to demonstrate the first report of life-prolonging gene correction specifically within lung tissue.

In the transgenic mouse model of SP-B deficiency, SP-B cDNA is under the control of a Tetracycline-inducible promoter [58]. Administration of doxycycline allows SP-B to be expressed at wild-type levels. If doxycycline is removed, SP-B expression drops and mice begin to exhibit phenotypic changes similar to those seen in humans with the disease: thickened alveolar walls, heavy cellular infiltration, increased macrophages and neutrophils, interstitial edema, congestion, augmented cytokines in the lavage, a significant drop in lung function, and acute onset of respiratory distress leading to death within days [59,60].

In order to demonstrate the value of nec-mRNA for lung-based genome-editing applications, this report utilized HDR to insert a constitutive CAG promoter immediately upstream of the SP-B cDNA. The resulting doxycycline-independent expression was able to significantly prolong the life of treated mice [37].

While this study was not performed in a humanized mouse model, the approach was able to demonstrate that extra-chromosomal nec-mRNA is capable of transducing airway epithelial cells, expressing genome-editing reagents, and achieving HDR rates sufficient for therapeutic levels of protein expression. Main findings from the study include:

