By identifying a naturally occuring antibiotic peptide in human lung fluid, scientists at the University of Pennsylvania Medical Center have illuminated the link between the genetic defect known as cystic fibrosis and the chronic lung infections in cystic-fibrosis patients. Their research indicates that the antibiotic peptide — human beta-defensin 1, or HBD-1 — is rendered inactive by the high salt levels that exist in the lungs of cystic-fibrosis patients. As a result, the bacteria responsible for the infections settle in, often with fatal results
“It’s often true in biological research that a mutation, such as the
defective cystic-fibrosis gene, helps delineate a normal process,” says
Dr. James M. Wilson, director of the Institute for Human Gene Therapy at Penn and co-author of a recent article in the journal Cell.
“We know the air is thick with bacteria, which we inhale constantly —
why, then, don’t we become infected? We think it may be because of the
actions of this molecule.”
Defensins are a group of recently discovered biological chemicals that
have been shown to protect a wide range of epithelial cells from
infection. HBD-1 is the only known human beta-defensin, but Wilson and
his colleagues were inspired by data involving frogs in Iowa and kidney
patients in Germany. Research on the former showed that frogs have a
natural antibiotic that allows their skin to heal following laboratory
incisions. The German research revealed a human beta — defensin-HBD-1
— in the filtered blood of dialysis patients.
Wilson then made the speculative leap that a natural antibiotic might be
present in lung fluid, too. He was right, but he also discovered that
the peptide is salt-sensitive: it kills harmful bacteria in normal lung
fluid but not in fluid high in salt, the kind found in cystic-fibrosis
patients.
Since cystic fibrosis is caused by a defect in the gene responsible for a protein called cystic fibrosis transmembrane regulator (CFTR), which helps control salt balance, Wilson reasons that if enough CFTR were inserted into lung cells, the salt-balance of the fluid would be restored and the HBD-1 would be able to kill bacteria. Other avenues of therapy are possible, too.
