Molecular Genetics of Cystic Fibrosis

DESCRIPTION (provided by applicant): Cystic fibrosis (CF) is a life-limiting disorder of fluid and electrolyte transport affecting 70,000 individuals worldwide. Patients with CF carry loss of function mutations in each CF Transmembrane conductance Regulator (CFTR) gene. CFTR encodes a cAMP-activated chloride channel that is critical for proper hydration of mucous secretions in the pulmonary airways and pancreatic ducts and maintaining the correct concentration of chloride in sweat. Recently, treatment of CF has taken a major step forward with successful clinical trials of Kalydeco (VX-770), a potentiator compound that increases the function of CFTR bearing the G551D mutation (~2-3% of CF alleles) leading to reduced sweat chloride concentration ([Cl-]) and improved lung function measurements. The success of Kalydeco has fostered the development of numerous other compounds that can correct misfolded forms of mutant CFTR ("correctors") and additional compounds that "potentiate" CFTR that does not conduct chloride at wildtype levels. We now stand at the threshold of being able to treat CF at its root cause; however, there are substantial challenges in delivering molecular therapy to all CF patients. First, there are many different missense and in frame deletion mutations in CFTR (n=816) that a clinical trial for each mutation is not feasible. Second, we don't know if transient increase in CFTR function translates into long-term improvement in lung disease, the major cause of mortality in CF. Third, as lung disease in CF proceeds over many years, we need to gauge effectiveness of CFTR-directed treatment using 'short-term' clinical endpoints. Key preliminary observations suggest that we can address each challenge. CF-causing missense and in frame deletion mutations (n = 47) clustered into groups according to their effect on folding and/or chloride conduction of CFTR appear to correspond to corrector or potentiator responsiveness. Analysis of ~23,000 patients in the CFTR2 database (cftr2.org) revealed that CFTR chloride channel function (as % of wildtype) transformed to a logarithmic scale correlates with cross-sectional lung function (r = 0.43; p = 0.002) and with sweat [Cl-] (r = 0.80; p= 1.3 x 10-11). The overall goal of this application is to inform treatment of CF with CFTR-directed therapies by advancing our understanding of the relationship between CFTR function and clinical manifestations. This goal will be achieved by 1) Classifying CF-causing mutations according to their effect on folding and/or conduction to assess their likely response to current and future CFTR-directed therapeutics; 2) Determining the extent to which longitudinal lung function correlates with in vitro CFTR molecular defects and 3) Determining if longitudinal measures of lung function correlate with in vivo CFTR function as measured by sweat [Cl-]. Upon completion of these aims, we will have mined the CFTR2 database and correlated phenotype data with in vitro properties of CFTR mutants to maximize the number of CF patients eligible for CFTR-directed therapy, to estimate the benefit of molecular therapy on long-term lung function in CF patients and to validate the use of sweat gland function to monitor the effectiveness of CFTR-directed treatment.

DESCRIPTION (provided by applicant): Cystic fibrosis (CF) is a life-limiting disorder of fluid and electrolyte transport affecting 70,000 individuals worldwide. Patients with CF carry loss of function mutations in each CF Transmembrane conductance Regulator (CFTR) gene. CFTR encodes a cAMP-activated chloride channel that is critical for proper hydration of mucous secretions in the pulmonary airways and pancreatic ducts and maintaining the correct concentration of chloride in sweat. Recently, treatment of CF has taken a major step forward with successful clinical trials of Kalydeco (VX-770), a potentiator compound that increases the function of CFTR bearing the G551D mutation (~2-3% of CF alleles) leading to reduced sweat chloride concentration ([Cl-]) and improved lung function measurements. The success of Kalydeco has fostered the development of numerous other compounds that can correct misfolded forms of mutant CFTR ("correctors") and additional compounds that "potentiate" CFTR that does not conduct chloride at wildtype levels. We now stand at the threshold of being able to treat CF at its root cause; however, there are substantial challenges in delivering molecular therapy to all CF patients. First, there are many different missense and in frame deletion mutations in CFTR (n=816) that a clinical trial for each mutation is not feasible. Second, we don't know if transient increase in CFTR function translates into long-term improvement in lung disease, the major cause of mortality in CF. Third, as lung disease in CF proceeds over many years, we need to gauge effectiveness of CFTR-directed treatment using 'short-term' clinical endpoints. Key preliminary observations suggest that we can address each challenge. CF-causing missense and in frame deletion mutations (n = 47) clustered into groups according to their effect on folding and/or chloride conduction of CFTR appear to correspond to corrector or potentiator responsiveness. Analysis of ~23,000 patients in the CFTR2 database (cftr2.org) revealed that CFTR chloride channel function (as % of wildtype) transformed to a logarithmic scale correlates with cross-sectional lung function (r = 0.43; p = 0.002) and with sweat [Cl-] (r = 0.80; p= 1.3 x 10-11). The overall goal of this application is to inform treatment of CF with CFTR-directed therapies by advancing our understanding of the relationship between CFTR function and clinical manifestations. This goal will be achieved by 1) Classifying CF-causing mutations according to their effect on folding and/or conduction to assess their likely response to current and future CFTR-directed therapeutics; 2) Determining the extent to which longitudinal lung function correlates with in vitro CFTR molecular defects and 3) Determining if longitudinal measures of lung function correlate with in vivo CFTR function as measured by sweat [Cl-]. Upon completion of these aims, we will have mined the CFTR2 database and correlated phenotype data with in vitro properties of CFTR mutants to maximize the number of CF patients eligible for CFTR-directed therapy, to estimate the benefit of molecular therapy on long-term lung function in CF patients and to validate the use of sweat gland function to monitor the effectiveness of CFTR-directed treatment.