Therapeutic development in cystic fibrosis (CF), a fatal pediatric lung disease affecting 1 in 3500 newborns annually, is rapidly advancing. “Although exciting progress has been made,” said Children’s of Alabama pediatric pulmonologist William T. Harris, M.D., an associate scientist at the Gregory Fleming James CF Research Center at the University of Alabama at Birmingham (UAB) who treats patients at Children’s of Alabama, “we are just midstream.”
Where previous advances focused on the downstream consequences of disease, such as malnutrition, chronic infection and mucopurulent secretion, recent drug developments target genetic mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) protein itself. Dr. Harris’ research focuses on improving the efficacy of these agents, called CFTR modulators.
One key therapeutic clue is to identify why certain children with exactly the same CFTR genotype have widely different disease trajectories. Dr. Harris has studied the mechanisms behind this disease disparity and has discovered the mechanism through which transforming growth factor beta (TGF-β), a leading gene modifier of CF lung disease severity, inhibits CFTR functional expression. Dr. Harris now targets this mediator of disease progression as a therapeutic opportunity to optimize CFTR modulator response.
His research discovered that small, non-coding nucleotide sequences called microRNA (miRNA) regulate CFTR function. miRNA diminish gene expression by degrading the gene transcript or inhibiting protein translation. In CF, TGF-β stimulates miR-145 expression, which binds to and degrades the CFTR gene transcript. This prevents protein expression and diminishes channel function. Both TGF-β and miR-145 are markedly increased in CF lungs and airway epithelia, posing a significant barrier to effective intervention.
Introducing miR-145 antagonists to airway epithelia reverses TGF-β suppression of CFTR and potentiates CFTR modulator response. However, TGF-β signaling and miR-145 activity are involved in multiple functions throughout the body, raising concern for off-target consequences. “As CF outcomes improve, the tolerance for side effects becomes very low,” he said. “Thus, the therapeutic intervention must be highly specific with a clearly defined target that only blocks the effects on CFTR.”
Antisense oligonucleotides (ASOs) offer that option. These short nucleic acid sequences bind to specific molecules of RNA, regulating expression of the gene. The FDA has already approved a handful of ASOs to treat congenital pediatric diseases such as Duchenne muscular dystrophy and spinal muscular atrophy.
“ASOs are appealing in CF because they can be delivered directly to the lungs (via inhalation) to bypass systemic side effects,” Dr. Harris said. He is partnering with Ionis Pharmaceuticals, a leader in the development of ASOs, to test an ASO that prevents miR-145 binding to the CFTR transcript. This approach is called target site blockade (TSB).
“TSB offers a nuanced strategy to address the problem of CFTR inhibition without interrupting TGF-β/miR-145 availability for other regulatory processes,” he explained. “I expect oligotherapeutics to benefit CF patients across genotype and improve next-generation therapeutics whether that be small molecule correctors or evolving gene editing strategies.”
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