Imagine driving down a road and then suddenly seeing a stop sign obscured by an overhanging tree branch. You slam on the brakes but are unable to avoid a crash before reaching your true destination. Your response, world-renowned Israeli CF researcher Eitan Kerem recently explained to parents of Hopkins CF patients, is not unlike that of the molecular mechanism of a Class I mutation and its impact on CFTR protein production. The CFTR, like your car, is already en route to its destination – the cell membrane. But a premature stop codon characteristic of this Class I mutation screeches the protein to a halt, resulting in a sort of genetic fender bender.
“The CFTR has already been processed to find its way to the cell membrane,” Kerem said. “But if by mistake there is a premature stop sign, a truncated process is formed in which unstable mRNA is produced and the protein is not functional.”
Preventing that sudden stop and restoring the functionality of the CFTR protein has been the latest quest of Kerem, director of the Cystic Fibrosis Center at Hadassah Medical Center in Jerusalem. The principal investigator of many international multi-center clinical trials and author of key publications in the field, Kerem’s interests span all aspects of CF research, from the association between phenotype and genotype to prognostic factors for disease severity. Now his eye is on the development of new mutation-specific pharmacologic therapies to correct the basic defects in CFTR, or in his words “to see if we could impair the premature reading of the stop sign and produce a functioning CFTR.”
The good news, he reported, is that aminoglycoside antibiotics have been shown to suppress the premature termination codon by allowing the incorporation of an amino acid and, thus, normal translation of the CFTR. In one study, Kerem explained, patients were asked to take nose drops of the most potent of these aminoglycosides – gentamycin – or placebo. According to nasal potential difference measurements, gentamycin improved chloride transport, which surprised Kerem: “Having a high negative value is characteristic of CF. We usually don’t see chloride transport.”
Finding the appearance of CFTR in scraped nasal cells of patients who took gentamycin, Kerem was about to continue with these studies when PTC Therapeutics asked him to collaborate in an Israel/U.S. trial of PTC 124, also known as Ataluren, over two weeks. (In studies with CF mouse models, PTC 124 has been shown to prompt the appearance of functional CFTR in the apical cell membrane.) While not an aminoglycoside antibiotic, PTC 124 also targets premature stop signals – or so-called nonsense mutations – in the genetic code that disrupt protein production. Here the findings were mixed, with PTC 124 normalizing nasal potential difference and improving pulmonary function in the Israel patients but showing no effect in patients in the U.S. arm of the study. A third PTC 124 study in Belgium and France, Kerem added, showed improvement with PTC 124.
“We don’t know why the results in America were so negative while the results in Israel, Belgium and France were partially positive,” Kerem said.
To get at the answer, Kerem conducted a longer, three-month trial of the Israeli patients who had participated in the two-week PTC 124 trial. The results? Patients who received PTC 124 showed continued significant improvement in nasal potential difference but only slight improvement in pulmonary function. Kerem is now beginning a year-long PTC 124 trial with the goal of giving patients something more than normal nasal potential difference (see “Upcoming Clinical Trials,” page 1).
“We do not want patients to take drugs to show improvement in the nasal electrical gradient in the nose, we want them to feel better,” said Kerem. “We know PTC 124 does something, but how significant this is we do not yet know. This study may give us the answer."