New study finds compound that may improve response to existing CF drug in kids with rare CF mutations

Currently there are just two drugs available to treat patients with cystic fibrosis (CF). While this is a huge step forward in the treatment of the disease, unfortunately more than half the total CF patient population do not benefit from these drugs due to their specific genetic mutation.

CF is caused by mutations in the gene known as CFTR. In Canada, approximately one in 3,600 children are born with this fatal genetic disease that primarily affects the lungs and digestive system.

While we know that CF is caused by mutations in the CFTR gene, there are many different mutations within that gene that could be responsible for the disease, some more rare than others. Researchers at The Hospital for Sick Children (SickKids) have been working to determine how these already-approved drugs may also benefit kids with rare CFTR mutations.

In a new study published in EMBO Molecular Medicine, the SickKids team identified a new investigational “amplifier” compound that enhances the response of the drug, Orkambi®, in tissues from patients with a rare CFTR mutation. Orkambi® is a combination treatment that was developed for patients with the most common mutation: ΔF508

To learn more about the impact of their findings, we sat down with SickKids’ Senior Scientist, Dr. Christine Bear and her team. Contributing team members include Drs. Regis Pomes, Felix Ratjen, Theo Moraes, Tanja Gonska and Steven Molinski. Dr. Steven Molinski is the first author of this paper and recent PhD graduate.

What is the significance of your findings?

This study is significant in that we describe new cell-based strategies to understand the response of rare CF-causing mutations to existing drugs and provide a platform to test new therapies in the future.  

More specifically, our study provides the first evidence for the effectiveness of a new investigational cystic fibrosis 'amplifier' compound in enhancing the response of Orkambi®, in tissue from patients with a rare CFTR mutation.

To do this we studied a rare CF‐causing mutation: c.3700 A>G (ΔI1234_R1239), and showed that this variant exhibited several defects at the protein level which could be partially repaired using Orkambi® in laboratory cell lines. While some of the defects could be repaired in the cell lines, when tested on the nasal cells from patients with this mutation, we found the effect was minimal. Because of this we focused on amplifier compounds that may boost the effect of Orkambi®.

We were able to recreate the effect of this mutation in a new lung cell line using a gene-editing tool, CRISPR/Cas9, wherein the CFTR gene was edited to bear the specific rare mutation. First, we found that the investigational amplifier compound was effective in boosting the “rescue” effect of the approved drug Orkambi® in this lung cell line and then validated this amplification response in nasal cells from the particular individual affected with this rare CF mutation.

Were you surprised by the results?

Interestingly, the rare mutation that we studied: c.3700 A>G (ΔI1234_R1239), exhibited several defects that are comparable to the major mutation, ΔF508, and so we initially predicted that drugs designed for ΔF508 would also be effective for this rare variant. However, we soon learned using patient-specific tissues that this rare mutation had additional unique defects, and predicted that the existing therapy (Orkambi®) alone may not be sufficient.  Our findings prompted testing a novel, investigational intervention aimed at enhancing CFTR protein abundance. This "amplification effect" boosted the effect of Orkambi® in our patient-specific tissue models, pointing to potentially new therapeutic options for individuals with this mutation.

How did you come to these results? What are the strengths of your approach?

We used a multidisciplinary approach in our study, including computer simulations, drug testing in standard cell lines and novel genetically-engineered (CRISPR/Cas9) cell lines, as well as validation of drug combinations in patient tissue. The main strength of our approach is that we could comprehensively interrogate and thus understand the molecular defects exhibited by this rare CF mutation in generic laboratory cell lines, in an edited lung cell line and in tissues from individuals. These same tissues showed that combination of an investigational compound (an “amplifier”) may be necessary in order to boost the response of the approved CF drug -Orkambi®, in individuals with this mutation.

What are the next steps for this research?

By studying rare CF-causing mutations, like the one described in this report, we are learning more about how to “repair” various defects in CFTR protein to achieve our goal of identifying drugs that have the potential to become cost-effective therapies that will enhance the quality of life for all individuals living with CF.

We will continue using our multidisciplinary approach for novel, mutation-specific drug development which would lead to personalized medicine for individuals with other rare CF mutations.

The study was funded by Cystic Fibrosis Canada, SickKids Foundation and Canadian Institutes of Health Research (CIHR) and Al Qamra Holding Group. It is an example of how SickKids is contributing to making Ontario Healthier, Wealthier and Smarter http://healthierwealthiersmarter.ca/