Genetics of cystic fibrosis: CFTR mutation classifications toward genotype-based CF therapies

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Abstract

Cystic fibrosis (CF) is an autosomal recessive disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which encodes an epithelial anion channel. Since the identification of the disease in 1938 and up until 2012, CF patients have been treated exclusively with medications aimed at bettering their respiratory, digestive, inflammatory and infectious symptoms. The identification of the CFTR gene in 1989 gave hopes of rapidly finding a cure for the disease, for which over 1950 mutations have been identified. Since 2012, recent approaches have enabled the identification of small molecules targeting either the CFTR protein directly or its key processing steps, giving rise to novel promising therapeutic tools.

This review presents the current CFTR mutation classifications according to their clinical consequences and to their effect on the structure and function of the CFTR channel. How these classifications are essential in the establishment of mutation-targeted therapeutic strategies is then discussed. The future of CFTR-targeted treatment lies in combinatory therapies that will enable CF patients to receive a customized treatment. This article is part of a Directed Issue entitled: Cystic Fibrosis: From o-mics to cell biology, physiology, and therapeutic advances.

Introduction

Mutations disrupting the function of the cystic fibrosis transmembrane conductance regulator (CFTR) gene, also referred to as ABCC7, cause cystic fibrosis (CF), a severe autosomal recessive disease. CF affects 1 in 2500–4500 newborns among the Caucasian population. All other ethnic groups are affected to a lesser extent. In 1989, Francis Collins, Lap-Chee Tsui and John R. Riordan (Kerem et al., 1989) identified the CFTR gene and its most frequent mutation, F508del, as being the cause of CF. The CFTR gene is located on the long arm of chromosome 7 (7q31.2) and its structure comprises 27 exons, spanning over 190 kb (Fig. 1A). After splicing of the large introns, the CFTR mRNA is 6.5 kb and all of its exons are necessary to produce a functional CFTR protein.

Since 1990, the laboratories dedicated to CF genetic diagnosis and research worldwide have been grouped into the international Cystic Fibrosis Genetic Analysis Consortium (CFGAC). This consortium identifies and describes mutations of the CFTR gene. Inputs are then deposited on the CF mutation database “CFTR1” (http://www.genet.sickkids.on.ca/cftr/) for public access. Since the discovery of the CFTR gene, extensive sequencing enabled the identification of over 1950 different mutations. Recently, a second CF mutation database “CFTR2” for “Clinical and Functional Translation of CFTR”, has been created (http://www.cftr2.org/). It gathers up-to-date information about newly discovered CFTR gene mutations, classifies them as disease-causing, neutral or mutation of varying clinical consequences, and provides clinical information about specific mutation combinations.

To better our understanding of CF pathogenesis, as well as to facilitate diagnosis and choice of treatment, two classification systems for CFTR mutations have been created, one according to clinical consequences, the other according to CFTR structure–function assessment. Currently available CF therapies mainly treat the symptoms of the disease, and have increased the mean life expectancy of CF patients from 5 years in the 1970s to about 35–40 years of age today. To address the underlying cause of the disease, recent approaches have enabled the identification of small molecules targeting either the CFTR protein directly, or its key processing steps. These novel therapeutic tools correct specific defects responsible for CFTR protein loss-of-function and need to be adapted to each patient's genotype. This highlights the importance of thoroughly characterizing each patient at the molecular level.

This review first presents the clinical spectrum of cystic fibrosis, as well as the two existing classification systems for CFTR mutations. How CFTR mutations are assigned to CF patients is then described. The use of this essential information in the development of genotype-based therapeutic strategies is then discussed.

Section snippets

The clinical spectrum of CF

The first description of the disease was done in 1938 by Dorothy Hansine Andersen, describing an abnormal pancreas, which presented with cysts and fibrosis (Andersen, 1938). The clinical spectrum of CF has since greatly expanded, giving rise to diagnoses of classic and non-classic CF presented in Fig. 2 (De Boeck et al., 2006, Farrell et al., 2008).

Although classic CF most often presents with a severe multi-organ phenotype, and non-classic CF with milder single-organ phenotypes, this is not

Current mutation-targeted therapeutic strategies

As our understanding of the genetics of CF grows, and as technological advances render molecular diagnostic tools more powerful, many CF researchers worldwide now focus on the development of drugs that aim to treat CF by targeting the underlying cause of the disease. These drugs include molecules inducing readthrough of premature termination codons, potentiator molecules, as well as corrector molecules, presented hereafter.

Conclusion

CF is a complex disease with a broader clinical and genetic spectrum than previously thought. This is making CF diagnosis (classic vs. non-classic CF), carrier screening and prenatal screening decisions more and more difficult.

New technologies and research approaches currently enable CF researchers to work at developing drugs that target the dysfunction of the CFTR protein directly. Our knowledge of the existing CFTR mutations and their consequences, both at the structure-function level (how

Acknowledgments

The authors thank Dr. Fabrice Lejeune for the critical reading of this manuscript and helpful discussions. The authors thank the French association Vaincre la Mucoviscidose for its continuous support.

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    This article is part of a Directed Issue entitled: Cystic Fibrosis: From o-mics to cell biology, physiology, and therapeutic advances.

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