Comparative Efficacy of Intratracheal Adeno-Associated Virus Administration to Newborn Rats

Novel Lung Tropic Adeno-Associated Virus Capsids for Therapeutic Gene Delivery

Efforts to identify mutations that underlie inherited genetic diseases combined with strides in the development of gene therapy vectors over the last three decades have culminated in the approval of several adeno-associated virus (AAV)-based gene therapies. Genetic diseases that manifest in the lung such as cystic fibrosis (CF) and surfactant deficiencies, however, have so far proven to be elusive targets. Early clinical trials in CF using AAV serotype 2 (AAV2) achieved safety, but not efficacy endpoints; however, importantly, these studies provided critical information on barriers that need to be surmounted to translate AAV lung gene therapy toward clinical success. Bolstered with an improved understanding of AAV biology and more clinically relevant lung models, next-generation molecular biology and bioinformatics approaches have given rise to novel AAV capsid variants that offer improvements in transduction efficiency, immunological profile, and the ability to circumvent physical barriers in the lung such as mucus. This review discusses the principal limiting barriers to clinical success in lung gene therapy and focuses on novel engineered AAV capsid variants that have been developed to overcome those challenges.

Targeted Gene Delivery through the Respiratory System: Rationale for Intratracheal Gene Transfer

Advances in DNA- and RNA-based technologies have made gene therapy suitable for many lung diseases, especially those that are hereditary. The main objective of gene therapy is to deliver an adequate amount of gene construct to the intended target cell, achieve stable transduction in target cells, and to produce a clinically therapeutic effect. This review focuses on the cellular organization in the normal lung and how gene therapy targets the specific cell types that are affected by pulmonary disorders caused by genetic mutations. Furthermore, it examines the pulmonary barriers that can compromise the absorption and transduction of viral vectors and genetic agents by the lung. Finally, it discusses the advantages and limitations of direct intra-tracheal gene delivery with different viral vectors in small and large animal models and in clinical trials.

Candidate diseases for prenatal gene therapy

Prenatal gene therapy aims to deliver genes to cells and tissues early in prenatal life, allowing correction of a genetic defect, before irreparable tissue damage has occurred. In contrast to postnatal gene therapy, prenatal application may target genes to a large population of stem cells, and the smaller fetal size allows a higher vector to target cell ratio to be achieved. Early gestation delivery may allow the development of immune tolerance to the transgenic protein, which would facilitate postnatal repeat vector administration if needed. Moreover, early delivery would avoid anti-vector immune responses which are often acquired in postnatal life. The NIH Recombinant DNA Advisory Committee considered that a candidate disease for prenatal gene therapy should pose serious morbidity and mortality risks to the fetus or neonate, and not have any effective postnatal treatment. Prenatal gene therapy would therefore be appropriate for life-threatening disorders, in which prenatal gene delivery maintains a clear advantage over cell transplantation or postnatal gene therapy. If deemed safer and more efficacious, prenatal gene therapy may be applicable for nonlethal conditions if adult gene transfer is unlikely to be of benefit. Many candidate diseases will be inherited congenital disorders such as thalassaemia or lysosomal storage disorders. However, obstetric conditions such as fetal growth restriction may also be treated using a targeted gene therapy approach. In each disease, the condition must be diagnosed prenatally, either via antenatal screening and prenatal diagnosis, for example, in the case of hemophilias, or by ultrasound assessment of the fetus, for example, congenital diaphragmatic hernia. In this chapter, we describe some examples of the candidate diseases and discuss how a prenatal gene therapy approach might work.

Efficient gene transfer into the mouse lung by fetal intratracheal injection of rAAV2/6.2

Fetal gene therapy is one of the possible new therapeutic strategies for congenital or perinatal diseases with high mortality or morbidity. We developed a novel delivery strategy to inject directly into the fetal mouse trachea. Intratracheal (i.t.) injection at embryonic day 18 (E18) was more efficient in targeting the fetal lung than conventional intra-amniotic (i.a.) delivery. Viral vectors derived from adeno-associated virus serotype 6.2, with tropism for the airway epithelium and not earlier tested in the fetal mouse lung, were injected into the fetal trachea. Bioluminescence (BL) imaging (BLI) was combined with magnetic resonance (MR) imaging (MRI) for noninvasive and accurate localization of transgene expression in vivo. Histological analysis for β-galactosidase (β-gal) revealed 17.5% of epithelial cells transduced in the conducting airways and 1.5% in the alveolar cells. Stable gene expression was observed up to 1 month after injection. This study demonstrates that direct injection of rAAV2/6.2 in the fetal mouse trachea is superior to i.a. delivery for transducing the lung. Second, as stable gene transfer was detected up to 1 postnatal month, this approach may be useful to evaluate fetal gene therapy for pulmonary diseases such as cystic fibrosis, requiring both substantial numbers of transduced cells as well as prolonged gene expression to obtain a stable phenotypic effect.

Analysis of adeno-associated virus progenitor cell transduction in mouse lung

Although recombinant adeno-associated virus (rAAV) has been widely used in lung gene therapy approaches, it remains unclear to what extent commonly used AAV serotypes transduce adult progenitors in the lung. In this study, we evaluated the life span and proliferative capacity of rAAV1-, 2-, and 5-transduced airway cells in mouse lung, using a LacZ-CRE reporter transgenic model and Cre-expressing rAAV. In this model, the expression of CRE recombinase led to permanent genetic marking of transduced cells and their descendants with LacZ. To investigate whether the rAAV-transduced cells included airway progenitors, we injured the airways of rAAV-infected mice with Naphthalene, while simultaneously labeling with 5-bromodeoxyuridine (BrdU) to identify slow-cycling progenitor/stem cells that entered the cell cycle and retained label. Both rAAV5 and rAAV1 vectors were capable of transducing a subset of long-lived Clara cells and alveolar type II (ATII) cells that retained nucleotide label and proliferated following lung injury. Importantly, rAAV1 and 5 appeared to preferentially transduce conducting airway epithelial progenitors that had the capacity to clonally expand, both in culture and in vivo following lung injury. These studies suggest that rAAV may be a useful vector for gene targeting of airway stem/progenitor cells.