In utero administration of a gene-editing technology in a mouse model has been able to correct a mutation that causes fatal respiratory failure in the animals within hours of birth if left untreated. For now, the work is proof of concept, and survival rates in treated animals remained low. But the fact that prenatal gene editing was able to prevent an otherwise lethal disease at all has encouraging implications.

The study was published in the April 17, 2019, issue of Science Translational Medicine.

Congenital genetic lung diseases such as inherited surfactant protein syndromes, cystic fibrosis and alpha-1 antitrypsin deficiency present upon birth, late childhood or early adulthood, with a shortened life expectancy and no definitive treatment options.

In fact, there more than 1,000 inborn errors of metabolism caused by monogenic mutations that lead to the buildup of toxic metabolic intermediates. In utero CRISPR gene editing has garnered a lot of interest recently as a potentially useful tool for treating those genetic diseases before or shortly after birth.

In their experiments, the team used an adeno-associated virus to deliver CRISPR/Cas9 gene-editing reagents that were able to correct a point mutation in the surfactant protein C (SFTPC).

SFTPC causes lung dysfunction, with varying degrees of severity depending on the exact mutation. Consequences in humans range from death in infancy to an increased risk of fibrosis in adulthood. In the mouse model studied by principal investigator William Peranteau and his team, the mutation is lethal within hours after birth.

"This is potentially exciting for families that are carrying fetuses with congenital origin problems, but it is important to stress that this is only just the first proof-of-concept study of many studies that need to be done," Peranteau, attending surgeon in the Division of Pediatric General, Thoracic and Fetal Surgery at Children's Hospital of Philadelphia, told BioWorld.

Peranteau emphasized that there are some innate qualities of the fetus that may allow for those treatments to be more efficient and more effective. First, the fetus is small. So when delivering gene-editing material, it becomes possible to maximize the dose per weight of the recipient. Secondly, the immune system of the fetus is not mature, so there is a reduced risk for immunological rejection of the therapy. Lastly, progenitor cells are more prevalent and accessible in the fetus.

Inhaled gene therapy, in a manner of speaking

An engineered adenovirus was injected into the amniotic sac at 16 days gestation, which is analogous to the third trimester in humans, as a mouse's gestation period is just 20 days. Injection of the viral vector containing the gene-editing technology into the amniotic sac was not performed until nearly the end of gestation, because that is when fetal breathing movements start. Peranteau explained that, "for lack of a better term, the mice inhale the amniotic fluid, thus targeting the epithelial cells lining the airways of the lung for insertion of the vector DNA."

Investigators were able to demonstrate efficient editing of lung epithelial cells before birth, and they were able to ameliorate the disease model. Alveolar epithelial cells and airway secretory cells lining lung airways cells showed the highest percentage of editing. Ideally, clinicians would like to edit a progenitor cell so gene edits would be effective for a long time if not for the life of the patient, and there was some evidence of editing of progenitor cells in the current study.

While typically untreated mice with this mutation die from respiratory failure within hours of birth, some mice treated with in utero gene editing survived for several months. The investigators are currently looking at longer time points.

The survival rate of gene-edited mice was still extremely low – only roughly 5% of treated animals survived for a week after birth. However, that was partly due to the limitations of the experimental technique.

The high mortality rate in mice "is partly due to their small size, and this particular technical limitation would not be an issue in larger animals, including humans," the authors wrote. Peranteau also noted that intratracheal delivery would be a possible option in the future.

Peranteau elaborated on the potential significance of his team's work, "For a limited number of congenital abnormalities, you can do an intervention, but the majority of families with congenital genetic abnormalities only benefit from the counseling and the management of their baby after they are born. Because of my clinical experience, overarching research goals are to explore novel prenatal treatments for congenital genetic abnormalities, whether that is by replacement gene therapy, prenatal gene editing, which has become garnered a lot of interest recently, or in utero stem cell transplantation for diseases like sickle cell disease and thalassemia. These are all ongoing areas of research."