The signalling cascade from initial stimulus to downstream effector — and where therapeutic intervention is possible at each node.
Cystic fibrosis (CF) is caused by mutations in the CFTR gene encoding an ATP-binding cassette (ABC) transporter that functions as a cAMP-regulated chloride and bicarbonate channel on epithelial cell surfaces. Over 2,000 CFTR variants have been identified, classified into functional categories: class I (premature stop codons, no protein produced), class II (protein misfolding and premature degradation — F508del, the most common mutation, accounting for approximately 70% of CF alleles), class III (gating defects — G551D, failure to open appropriately), class IV (conductance defects), and class V (reduced quantity). The downstream consequence of absent or dysfunctional CFTR is failure of chloride and water secretion, producing dehydrated, viscid mucus that obstructs airways, pancreatic ducts, and other epithelial surfaces.
CFTR modulator therapy has transformed CF care by addressing the underlying protein defect rather than managing downstream consequences. Potentiators (ivacaftor) bind to CFTR at the cell surface and increase the probability of channel opening (gate opening), restoring chloride conductance in class III gating mutations (G551D and others). For F508del (class II), misfolded CFTR must first be rescued from premature proteasomal degradation by corrector molecules that stabilise the protein and facilitate its trafficking to the cell surface. Single correctors (lumacaftor, tezacaftor) produced modest benefit in F508del homozygotes. The triple combination elexacaftor/tezacaftor/ivacaftor (Kaftrio), providing two complementary correctors plus a potentiator, produces approximately 10 percentage point improvements in ppFEV1 and transformative reductions in exacerbation rate and sweat chloride concentration.
Kaftrio is licensed in the UK for patients aged 2 years and above with at least one F508del allele, encompassing F508del/F508del homozygotes and F508del/MF (minimal function mutation) compound heterozygotes — collectively representing approximately 90% of the CF population. CFTR modulator eligibility is determined by individual CFTR genotype, and access is mediated through NHS England specialised commissioning pathways. Ongoing monitoring for hepatic enzyme elevation and lens opacities (relevant particularly for young children) is incorporated into surveillance protocols. Gene editing and mRNA-based therapies targeting the 10% of patients ineligible for current modulators represent the next frontier of CF disease modification.
Upstream blockade vs downstream blockade — understanding the distinction is critical for treatment selection and sequencing.
Clinically actionable insights for treatment selection and sequencing
Cystic fibrosis (CF) is caused by mutations in the CFTR gene encoding an ATP-binding cassette (ABC) transporter that functions as a cAMP-regulated chloride and bicarbonate channel on epithelial cell surfaces.
Over 2,000 CFTR variants have been identified, classified into functional categories: class I (premature stop codons, no protein produced), class II (protein misfolding and premature degradation — F508del, the most common mutation, accounting for approximately 70% of CF alleles), class III (gating defects — G551D, failure to open appropriately), class IV (conductance defects), and class V (reduced quantity).
The downstream consequence of absent or dysfunctional CFTR is failure of chloride and water secretion, producing dehydrated, viscid mucus that obstructs airways, pancreatic ducts, and other epithelial surfaces.
CFTR modulator therapy has transformed CF care by addressing the underlying protein defect rather than managing downstream consequences.
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