Epithelial L7


Atypical Cystic Fibrosis (CF)

  1. Overview

    • Exploration of atypical cystic fibrosis, which presents milder symptoms compared to classical CF and occurs with fewer or absent mutations in the CFTR gene.

    • Affects about 1-2% of individuals from Northern Europe, often featuring only one or no mutations.

    • Discussion on potential modifier genes that could influence the expression of symptoms in atypical CF cases.

    • Symptoms manifestation is mutation-specific, meaning different mutations can result in varying clinical symptoms.

Epithelial Sodium Channel (ENaC) Mutations

  1. Objectives of Study

    • Describe observed ENaC mutations in patients.

    • Discuss data supporting changes in ENaC function among affected individuals.

    • Examine mechanisms altered by these mutations.

    • Utilize mouse models to assess the likely lung impact of identified mutations.

    • Aim: To evaluate the role of ENaC mutations in atypical CF.

  2. Clinical Findings

    • Atypical CF:

      • Mild symptoms with 1 or 0 CFTR mutations.

      • Diagnosis can reveal classical CF presenting severe symptoms with 2 CFTR mutations.

      • Symptoms are specific to the nature of the mutation.

    • Mutated Proteins in CF:

      • Hypotheses surrounding alternative gene mutations affecting patients with atypical CF.

  3. ENaC Gain of Function Mutations

    • These mutations lead to increased absorption of Na+, which results in the depletion of airway surface liquid (ASL) and the production of cystic fibrosis-like symptoms in patients.

    • Examples of mutations include SCNN1A (11 mutations), SCNN1B (7 mutations), and SCNN1G (8 mutations), screened across 45 patients, revealing additional mutations some with higher incidences than controls (31 patients screened).

Impact of Specific Mutations on ENaC Function

  1. Notable Variants

    • A range of mutations in the SCNN1A gene affecting channel function:

      • T663A-SCNN1A (Control)

      • F61L – SCNN1AA663

      • V114I – SCNN1AA663

      • L180L-R181W – SCNN1AA663

      • A334T – SCNN1AA663

      • W493R - multiple instances across backgrounds and variants.

    • Notably, protein levels remained similar across variants, questioning the expected impacts on ENaC function.

  2. Ionic Measurements

    • Reported electrical values in different conditions:

      • Normal: PD = -15.2 ± 6.9 mV, Δlow Cl- = -16.3 ± 9.2 mV, Δamiloride = 6.5 ± 2.6 mV.

      • Classical CF: PD = -31.1 ± 16.6 mV, Δlow Cl- = 0.08 ± 3.7 mV, Δamiloride = 19.4 ± 11.7 mV.

      • Clinical observation of W493R / F508del and W493R / E528E patients revealed critical values in PD and Δ conditions.

  3. Mouse Model Studies

    • Studies using mouse models demonstrate overexpression of SCNN1b, analyzing mucosal clearance and survival rates:

      • Measurements: Height of PCL (µm) and mucus clearance (mm/min) indicated significant changes.

      • Survival percentages tracked over a time period showed significant postnatal mortality in specific mutations.

Functional Mechanisms of ENaC

  1. Channel Activity

    • The mechanism involves Na+ ions flooding into the cells through open ENaC, leading to increased intracellular Na+ levels, where ENaC is retrieved via endocytosis, limiting sodium influx through feedback inhibition reliant on extracellular Na+ concentrations.

    • The aW493R mutation displayed altered feedback inhibition but no change in surface expression.

  2. Current Amplification

    • The study emphasizes on how cleaved ENaC by proteases leads to higher open probability (Po), resulting in active channels generating larger currents in contrast to uncleaved channels.

    • Systematically measured using the equation I = N.Po.g.(Vm-Ei) indicating adjustments in ion currents based on channel state.

  3. Protease Influence on Channels

    • Noticing the flickering behavior of channels in response to chymotrypsin highlighted activation of initially silent channels, showing variable Po based on enzymatic digestion effects.

    • Comparing mutant channels showed pre-cleavage conditions had inherently high Po, affecting post-cleavage observations.

Summary of Key Findings

  1. Clinical Observations

    • Presence of atypical CF patients who exhibit CF-like symptoms without the typical two CFTR mutations.

    • Identified ENaC mutations that lead to gain-of-function, explaining the associated symptoms.

    • Reports of loss-of-function mutations also observed which contribute to variability of symptoms.

    • Different mutations impact ion current elevation via changes in Po, leading to physiological ramifications for lung functionality.

  2. Study Reference

    • Rauh et al., 2010 detailed mutations in ENaC related to atypical cystic fibrosis as influencing channel open probability and reducing self-inhibition of Na+ influx. Journal of Physiology, 588.8, pages 1211-1225, DOI: 10.1113/jphysiol.2009.180224.

  3. Final Key Points

    • Inherent variability in mutations implies a substantial spectrum of symptoms related to atypical CF, underlying the importance of ENaC research in understanding this condition.