David Bedwell and colleagues first proposed the idea of readthrough drugs to counter nonsense mutations in the 1990s. Now, they are already in use in some genetic disorders, and Bedwell is pursuing more powerful versions for cystic fibrosis. David Bedwell and colleagues first proposed the idea of readthrough drugs to counter nonsense mutations in the 1990s. Now, they are already in use in some genetic disorders, and Bedwell is pursuing more powerful versions for cystic fibrosis.

Attacking nonsense mutations in cystic fibrosis and a host of other diseases

November 05, 2018
By Matt Windsor
Readthrough drugs, first proposed by UAB researchers in the 1990s, are already helping patients with some genetic diseases caused by nonsense mutations. They could eventually help millions worldwide.

This is the third part of a series exploring breakthroughs in cystic fibrosis research that are paving the way for new treatments to help millions with other diseases, including COPD, asthma and more. Read part one, After cystic fibrosis 'miracle,' researchers are exploring ways to reach millions more, and part two, How did we get here? Cystic fibrosis drugs go from 0-90 percent effective in a few short years.

For the 5-10 percent of cystic fibrosis patients with “nonsense” mutations, the trouble isn’t defective or misfolded proteins — it’s no protein at all. Somewhere in their CFTR genes, a three-letter code that should tell the cell’s protein-building ribosomes to insert an amino acid has been accidentally changed into a premature “stop” sign. A one-letter switch can turn, for example, UAC (code for tyrosine) into UAA (which tells the ribosome, “stop, you’ve finished making this protein”). Because the protein is foreshortened and clearly abnormal, it is quickly destroyed by the cell’s quality-control processes.

The readthrough breakthrough

That’s why patients with nonsense mutations usually have a severe form of the disease, explains David Bedwell, Ph.D., associate director of the UAB Gregory Fleming James Cystic Fibrosis Research Center and chair of the Department of Biochemistry and Molecular Genetics in the UAB School of Medicine. In 1996, Bedwell and colleagues Marybeth Howard, Ph.D., and former CF Center director Raymond Frizzell, Ph.D., published a paper in Nature Medicine that proposed a way around the problem. Some antibiotic drugs, they showed, could encourage ribosomes to “read through” the premature stop signs and go on to make functional protein. Readthrough drugs, they proposed, could be a new treatment option for CF.

“Right after we published that, someone wrote a letter to the editor saying it would never work,” says Bedwell. It sounds like a “crazy idea,” he admits, but it is beginning to prove itself in patients. In Europe, a drug Bedwell has studied extensively, ataluren, is now used to treat Duchenne’s muscular dystrophy, and Bedwell is exploring promising results for ataluren in several other diseases, including Hurler syndrome, also known as mucopolysaccharidosis type 1. (Matt Might, Ph.D., through the UAB Precision Medicine Institute, is now attracting patients with many other genetic diseases caused by nonsense mutations; these diseases may also be future targets to readthrough therapies.)

This video from the Cystic Fibrosis Foundation explains what goes wrong in nonsense mutations and gives an overview of research efforts to correct them. Video courtesy Cystic Fibrosis Foundation.

Overriding the proofing function

The trick, Bedwell says, is to make the cell’s proofreading mechanisms a little more lenient. Occasionally, the protein-building ribosomes accidentally grab the wrong base as they work. Confronted with UAA (stop!), the ribosome might latch on to cytosine (C) instead of uracil (U), ending up with CAA (glutamine) instead. Now there’s no stop sign, and the ribosome keeps on building CFTR. (Even though CAA is the wrong amino acid, such a small variation doesn’t usually affect the protein’s function.) The ribosome’s proofing function normally catches and fixes these deviations from the written code, meaning that patients don’t get to benefit from the ribosome’s error. But the presence of a drug like ataluren seems to encourage the ribosome to ignore the mistake and go on making the protein.

Unfortunately, the process still relies on chance, and current readthrough drugs can’t boost protein production to the 30-35 percent levels needed to make a major difference in CF, Bedwell explains. So he and partners at UAB-affiliate Southern Research and the Cystic Fibrosis Foundation are seeking a better compound. The team has sifted through 750,000 options and is homing in on the 150-or-so most promising hits.

International effort

Because nonsense mutations are highly varied, it is important to have a range of samples to test potential new drugs. These will be available at UAB thanks to a multinational study led by George Solomon, M.D., a researcher in the CF Center and assistant professor in the Division of Pulmonary, Allergy and Critical Medicine. The RARE study is collecting a selection of tissue samples from patients with nonsense mutations in several countries to learn more about the biology of these mutations and to allow for next-generation cell-based tests, Solomon says.

A new drug could have uses far beyond CF, Bedwell points out. Up to 11 percent of all genetic diseases are thought to be caused by nonsense mutations, he says. “These drugs could help millions worldwide.”

Continue on to part four, Clinical trial for one: the promise of patient-derived assays