CFTR: The CF Gene and Its Regulation in Physiology and Disease

Abstract

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) gene, which encodes an atypical transporter that functions as a chloride channel. The regulation of both the gene and the protein is highly complex, involving many novel regulatory mechanisms: these include differential usage of transcription start sites and upstream exons; alternative splicing and the production of unique polypeptide isoforms; and post‐transcriptional influences on transcript stability and protein levels. Together they mediate basal and tissue‐specific CFTR expression in epithelial and nonepithelial tissues, such as the heart, under both normal physiological conditions and in the disease state.

Key Concepts:

  • Cystic fibrosis (CF) is one of the most well‐known and commonest monogenetic disorders in the Caucasian population.

  • Nearly 2000 mutations have been identified in the CFTR gene.

  • The CFTR gene, also known as ABCC7, contains 27 exons and encodes a chloride channel that is unique within the ABC transporter superfamily.

  • Despite CF being primarily a disease of epithelial cells, CFTR expression has also been identified in nonepithelial cells, for example, hypothalamic neurons and cardiomyocytes.

  • Although relatively widespread, CFTR expression shows tissue‐specific regulatory mechanisms that act at multiple levels, including transcription, post‐transcription and translation.

  • Collectively, these control pathways yield distinct, tightly regulated transcripts that encode unique CFTR protein variants.

  • The complexity of CFTR expression may be the main determining factor against the production of an effective cure for CF.

  • Further study of these parallel tissue‐specific regulatory mechanisms, especially in organs that appear to be protected in CF, may provide insights into CF pathology and potential disease treatments.

Keywords: cystic fibrosis; CFTR; chloride channel; ABC transporter; epithelium; heart; cardiac hypertrophy

Figure 1.

A topographical model of CFTR. The 2D structure showing the presence of 12 transmembrane (TM) domains (yellow) that are generally arranged into two sets of six TMs (TMI–TMVI and TMVII–TMXII) to form a pore through which chloride ions (Cl) flow from an intracellular to extracellular location. Also shown are two nucleotide‐binding domains (NBD1 and NBD2) (red) and a large R‐domain (blue), all located between intracellular amino‐ and carboxyl‐termini (N and C). The CFTR protein also contains sites that are structurally and functionally important, such as the presence of glycosylation (brown) between TMVII and TMVIII, positively (green circle) and negatively (orange circle) charged amino acids in the R‐domain, entry sites for adenosine triphosphate (ATP) (purple circle) within each NBD and phosphorylation sites for protein kinase C (PKC) (grey diamond) and protein kinase A (PKA) (brown square). The relative location for each exon–exon boundary (i.e. splice junction) (red line) of the CFTR gene is superimposed onto the CFTR protein structure. Note that the literature makes reference to the CFTR gene containing either 24 or 27 exons, a discrepancy due to some authors describing three exonic subdivisions (i.e. exon 6*: 6a and 6b; exon 14*: 14a and 14b; and exon 17*: 17a and 17b). The major CF mutation (ΔF508) in exon 10 (dark blue) and exon 5 (pink), the latter found to be alternatively‐spliced in the cardiac CFTR isoform, are highlighted.

Figure 2.

Post‐transcriptional regulation of an mRNA transcript containing an upstream ORF (uORF) under steady state conditions. (a) For transcripts that are devoid of uORFs (e.g. high CFTR expression in the duodenum), the 40S ribosomal subunit (light blue) scans from the 5′‐m7GpppG (cap) structure along the mRNA in a 5′‐end to 3′‐end direction. On encountering the main AUG (mAUG) codon, the 60S ribosomal subunit (dark blue) interacts with the 40S ribosomal subunit and translation of the main ORF (mORF) (green) is initiated. Protein synthesis continues until a stop codon (UAA, UAG or UGA) is encountered and the translation complex disassembles. (b) For transcripts containing a uORF (red) in the 5′‐UTR, translation will initiate at an upstream AUG (uAUG) codon until a stop codon is encountered. Subsequently, a small peptide is produced from the uORF sequence with the release of the 60S ribosomal subunit. If the downstream sequence adjacent to the stop codon of the uORF contains a stable stem‐loop RNA secondary structure (pink), the 40S ribosomal subunit is more likely to disengage from the transcript. As a result, the density of ribosomes present over the mORF is significantly less, which leaves the transcript more susceptible to nuclease attack. In practice, the 40S ribosomal subunit might not completely disengage from the mRNA after encountering the uORF and may reinitiate translation at the mAUG, although at a reduced rate. Collectively, these processes result in less stable transcripts and a lower rate of protein production. (c) Partial genomic structure of the rabbit cftr gene (up to exon 6 only) showing alternative splicing (black lines) in transcripts containing ‘traditional’ exon 1 (red), three novel upstream exons (exon −1A, green; exon −1B, blue; and exon −1C, pink) and exon 5 (orange). TSSs (black vertical line) are indicated for both heart (H) and duodenal (D) tissues in normal adults (A, subscript), normal embryos (E, subscript) and adult cardiac hypertrophy (H, subscript). In‐frame mATG codons are indicated and many uORFs are shown as grey boxes beneath each cftr exon.

Figure 3.

CFTR gene expression and chloride channel activity in the heart. (a) The normal cardiac action potential (AP) (blue) consists of a number of distinct phases that begin with rapid depolarisation, followed by early repolarisation that reaches a plateau, before a final repolarisation stage that restores the current to the resting membrane potential. The time taken to proceed through these phases is known as the cardiac AP duration (APD). On stimulating chloride channels, such as those encoded by the CFTR gene, the APD greatly shortens (red). (b) Schematic representation of the heart to illustrate the movement of deoxygenated (blue arrow) and oxygenated (red arrow) blood during the cardiac cycle. Black arrows indicate the locations of epicardial and endocardial surfaces as well as the apical and basal regions of the heart. LA, left atrium; LV, left ventricle; RA, right atrium; and RV, right ventricle. (c) Representation of CFTR expression in the normal and the hypertrophied heart, showing an epicardial (high) to endocardial (low) gradient of CFTR mRNA transcripts across the left ventricular wall that is lost in cardiac hypertrophy.

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Further Reading

Guggino WB (1994) Chloride channels. In: Current Topics in Membranes, vol. 42. San Diego: Academic Press.

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Web Links

Cystic Fibrosis Mutation Database (US). http://www.genet.sickkids.on.ca/cftr/Home.html

Cystic Fibrosis Foundation (US). http://www.cff.org/

Cystic Fibrosis Trust (UK). http://www.cftrust.org.uk

Online Mendelian Inheritance in Man (OMIM) entry for CF. http://www.omim.org/entry/219700

Online Mendelian Inheritance in Man (OMIM) entry for CFTR. http://www.omim.org/entry/602421

Wikipedia entry for CF. http://en.wikipedia.org/wiki/Cystic_fibrosis

Wikipedia entry for CFTR. http://en.wikipedia.org/wiki/Cystic_fibrosis_transmembrane_conductance_regulator

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Davies, Wayne IL(Dec 2013) CFTR: The CF Gene and Its Regulation in Physiology and Disease. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0022929]