Silencing a gene involved in the processing of the mutant CFTR protein in cystic fibrosis (CF) improved folding and increased the amount of functional protein that reached the plasma membrane — reaching clinically relevant levels when used in combination with the modulator drug VX-809 (lumacaftor). The results, from experiments in a laboratory setting, showed that targeting modifier genes may be a promising approach to reduce symptoms of CF.
The most common of the roughly 2,000 gene mutations that can cause CF is a deletion of the amino acid phenylalanine on the 508 position in the chloride channel gene, termed ΔF508-CFTR. This mutation causes the CFTR protein to misfold, and the protein is mostly degraded before reaching the cell membrane. Proteins that escape the degradation process are nevertheless unstable, and removed from the membrane and degraded.
A number of approaches have been tried to rescue the faulty folding of mutant CFTR, including the use of modifier drugs, aiding the protein to fold properly. But these attempts have resulted in only modest improvement.
Several proteins are involved in the processing of the CFTR protein once it’s transcribed from the gene, and a research group from McGill University, Canada, figured it might be possible to target those proteins to improve the function of mutant CFTR.
In an earlier study, the team screened the yeast genome to identify such modifier genes. Using the yeast equivalent of the ΔF508-CFTR mutation as a model, researchers identified a number of genes interacting with the mutant. These genes are evolutionarily conserved, meaning they most likely function in the same way in humans.
In the new study, “Ribosomal Stalk Protein Silencing Partially Corrects the ΔF508-CFTR Functional Expression Defect,“ published in the journal PLOS Biology, researchers showed that when these modifier genes were silenced, more CFTR protein reached the cell membrane in cultured human bronchial epithelial cells. But only one of the genes, called RPL12, could demonstrate a proportional increase in function of the chloride channel, and further experiments confirmed that silencing RPL12 improved folding efficiency and rendered the mutant CFTR more stable, resulting in a threefold increase in protein density in the cell membrane.
Silencing RPL12 had no effect on the capacity of other mutated proteins to reach the cell membrane, nor did it affect the protein processing of non-mutated CFTR, highlighting the selectivity of the approach.
RPL12 is known to slow the process of translation from gene to protein, and study findings indicated that slowing the translation process by other means also improved the folding and stability of the mutant CFTR, and resulted in more functional protein reaching the cell membrane.
Combining gene silencing with VX-809 resulted in an increase of functional CFTR to about 50 percent of non-mutated levels, indicating that the drug and the gene silencing approach use different mechanisms to improve the characteristics of the mutant protein. Such an increase is also clinically relevant.