1907
J Physiol 592.9 (2014) pp 1907–1908
PERSPECTIVES
The G551D CFTR chloride channel spurs the development of personalized medicine Horst Fischer Children’s Hospital Oakland Research Institute, 5700 Martin Luther King Jr Way, Oakland, CA 94609, USA
The Journal of Physiology
Email: [email protected]
The cystic fibrosis (CF)-causing mutation, G551D, of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl– channel was identified early on as the mutant most likely to be responsive to treatment. This mutation-specific line of thought progressed towards the currently proposed general approach of genotype-specific personalized medicine for CF. After years of drug development, G551D CFTR is now the first mutant that is targeted by a selective drug available to CF patients carrying this mutation (Accurso et al. 2010). The many described CFTR mutations that cause CF are usually classified by their cell biological fates. One class of CFTR mutations results in defective regulation (i.e. activation) such that channels remain essentially closed. G551D CFTR is the commonest of these regulation mutations with an incidence of 2.5% to 5% in the CF patient population. Admittedly, this is only a small fraction of all CF patients, but over the years G551D CFTR has served as a model to demonstrate the idea that the function of a defective CFTR Cl– channel can be restored to clinically meaningful levels. G551D CFTR is special because the channel protein is processed normally inside the cell and is targeted properly to the apical cell membrane of Cl– -secreting epithelia. It just does not open very well and thus cannot perform its normal function (which is epithelial Cl– and bicarbonate secretion). This characteristic contrasts greatly with the commonest mutation in the CF population, F508del, which is largely degraded intracellularly and does not reach the cell membrane in any reasonable quantity. Thus, F508del CFTR was considered as a difficult target because it required the correction of a poorly understood intracellular trafficking machinery. G551D CFTR, on the other hand, was propelled into the limelight of drug developers, channel
kineticists, and eventually clinicians by its proper membrane localization. Despite its rather low occurrence in the CF patient population and its poor function in the membrane, G551D CFTR was considered a manageable target. Not surprisingly, G551D CFTR was the first mutant that could be corrected pharmacologically in CF patients (Illek et al. 1999) pointing the way to a mutation-specific, personalized CF treatment. Many years of further drug development eventually identified the first effective medicine that specifically targets G551D CFTR, which represents the first genotype-specific treatment of CF patients (Accurso et al. 2010). The G551D CFTR Cl– channel has an exceedingly small open probability of less than 1% of normal wild-type CFTR. Recording G551D CFTR currents in a laboratory involves a lot of staring at flat lines. Nevertheless, G551D CFTR can open. Its infrequent and short-lived openings provide the opportunity to step in and stabilize the open state in order to increase the open probability of the channel. This approach has worked for CF drug development, and it appeared reasonable to assume that a molecular biology approach (which can be considered as a much more specific intervention) would be of similar utility. In this issue of The Journal of Physiology, Sheppard and colleagues (Xu et al. 2014) used a clever approach of engineering second-site mutations into G551D CFTR. Previously, these second-site mutations had been shown to increase the open probability of F508del CFTR by increasing both the frequency of opening and the open time of the channel to levels of wild-type CFTR (Roxo-Rosa et al. 2006). The two second-site mutations (also called revertant mutations) used in both the previous and the current study were G550E and 4RK (which by itself is a combination of four arginine-to-lysine mutations, R29K, R516K, R555K and R766K). Both had been shown to increase profoundly the open probability of the relatively frail F508del CFTR channel. Thus, in the current study the authors probably reasoned that this might be helpful for G551D CFTR as well. The results were surprising, though. In G551D CFTR, the second-site mutations resulted in marginal changes in open probability, which were largely caused
C 2014 The Authors. The Journal of Physiology C 2014 The Physiological Society
by an increased frequency of opening but these were somewhat diminished by concurrent reductions of open times. These observations were quite different from the effects of the second-site mutations found previously in F508del CFTR, where open times were greatly increased and the frequency of opening was increased by the G550E second-site mutation but not by 4RK. Without going into a number of additional subtle differences between the two CFTR mutants, these observations support one strong conclusion: inserting defined molecular changes into two different CFTR mutants results in largely different functional outcomes. By extension, this is probably also true for the effect of a specific drug on different CFTR mutations. These findings are at the heart of the current thinking of personalized, mutation-specific medicine for CF: different CFTR mutations represent different drug targets. Current drug-development techniques generate highly selective drugs, which are less likely to bind to related targets and further support the notion of mutation-specific drugs. Much of this is speculation, as there is currently just one approved drug available for one CFTR mutation (which is G551D), but the current study from the Sheppard laboratory supports the notion that different CFTR mutations behave quite differently when exposed to identical structural changes introduced by site-directed mutagenesis (Xu et al. 2014), or, by extension, when using highly selective drugs. References Accurso FJ, Rowe SM, Clancy J, Boyle MP, Dunitz JM, Durie PR, Sagel SD, Hornick DB, Konstan MW & Donaldson SH (2010). Effect of VX-770 in persons with cystic fibrosis and the G551D-CFTR mutation. N Engl J Med 363, 1991–2003. Illek B, Zhang L, Lewis NC, Moss RB, Dong J-Y & Fischer H (1999). Defective function of the cystic fibrosis-causing mutation G551D is recovered by genistein. Am J Physiol Cell Physiol 277, C833–C839. Roxo-Rosa M, Xu Z, Schmidt A, Neto M, Cai Z, Soares CM, Sheppard DN & Amaral MD (2006). Revertant mutants G550E and 4RK rescue cystic fibrosis mutants in the first nucleotide-binding domain of CFTR by different mechanisms. Proc Natl Acad Sci U S A 103, 17891–17896.
DOI: 10.1113/jphysiol.2014.274464
1908 Xu Z, Pissarra LS, Farinha CM, Liu J, Cai Z, Thibodeau PH, Amaral MD & Sheppard DN (2014). Revertant mutants modify, but do not rescue, the gating defect of the cystic fibrosis mutant G551D-CFTR. J Physiol 592, 1931–1947.
Perspectives
J Physiol 592.9
Additional information
Funding
Competing interests
The author is supported by Cystic Fibrosis Research Inc., Grant no. 12-001.
None declared.
C 2014 The Authors. The Journal of Physiology C 2014 The Physiological Society
J Physiol 592.9 (2014) pp 1907–1908
PERSPECTIVES
The G551D CFTR chloride channel spurs the development of personalized medicine Horst Fischer Children’s Hospital Oakland Research Institute, 5700 Martin Luther King Jr Way, Oakland, CA 94609, USA
The Journal of Physiology
Email: [email protected]
The cystic fibrosis (CF)-causing mutation, G551D, of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl– channel was identified early on as the mutant most likely to be responsive to treatment. This mutation-specific line of thought progressed towards the currently proposed general approach of genotype-specific personalized medicine for CF. After years of drug development, G551D CFTR is now the first mutant that is targeted by a selective drug available to CF patients carrying this mutation (Accurso et al. 2010). The many described CFTR mutations that cause CF are usually classified by their cell biological fates. One class of CFTR mutations results in defective regulation (i.e. activation) such that channels remain essentially closed. G551D CFTR is the commonest of these regulation mutations with an incidence of 2.5% to 5% in the CF patient population. Admittedly, this is only a small fraction of all CF patients, but over the years G551D CFTR has served as a model to demonstrate the idea that the function of a defective CFTR Cl– channel can be restored to clinically meaningful levels. G551D CFTR is special because the channel protein is processed normally inside the cell and is targeted properly to the apical cell membrane of Cl– -secreting epithelia. It just does not open very well and thus cannot perform its normal function (which is epithelial Cl– and bicarbonate secretion). This characteristic contrasts greatly with the commonest mutation in the CF population, F508del, which is largely degraded intracellularly and does not reach the cell membrane in any reasonable quantity. Thus, F508del CFTR was considered as a difficult target because it required the correction of a poorly understood intracellular trafficking machinery. G551D CFTR, on the other hand, was propelled into the limelight of drug developers, channel
kineticists, and eventually clinicians by its proper membrane localization. Despite its rather low occurrence in the CF patient population and its poor function in the membrane, G551D CFTR was considered a manageable target. Not surprisingly, G551D CFTR was the first mutant that could be corrected pharmacologically in CF patients (Illek et al. 1999) pointing the way to a mutation-specific, personalized CF treatment. Many years of further drug development eventually identified the first effective medicine that specifically targets G551D CFTR, which represents the first genotype-specific treatment of CF patients (Accurso et al. 2010). The G551D CFTR Cl– channel has an exceedingly small open probability of less than 1% of normal wild-type CFTR. Recording G551D CFTR currents in a laboratory involves a lot of staring at flat lines. Nevertheless, G551D CFTR can open. Its infrequent and short-lived openings provide the opportunity to step in and stabilize the open state in order to increase the open probability of the channel. This approach has worked for CF drug development, and it appeared reasonable to assume that a molecular biology approach (which can be considered as a much more specific intervention) would be of similar utility. In this issue of The Journal of Physiology, Sheppard and colleagues (Xu et al. 2014) used a clever approach of engineering second-site mutations into G551D CFTR. Previously, these second-site mutations had been shown to increase the open probability of F508del CFTR by increasing both the frequency of opening and the open time of the channel to levels of wild-type CFTR (Roxo-Rosa et al. 2006). The two second-site mutations (also called revertant mutations) used in both the previous and the current study were G550E and 4RK (which by itself is a combination of four arginine-to-lysine mutations, R29K, R516K, R555K and R766K). Both had been shown to increase profoundly the open probability of the relatively frail F508del CFTR channel. Thus, in the current study the authors probably reasoned that this might be helpful for G551D CFTR as well. The results were surprising, though. In G551D CFTR, the second-site mutations resulted in marginal changes in open probability, which were largely caused
C 2014 The Authors. The Journal of Physiology C 2014 The Physiological Society
by an increased frequency of opening but these were somewhat diminished by concurrent reductions of open times. These observations were quite different from the effects of the second-site mutations found previously in F508del CFTR, where open times were greatly increased and the frequency of opening was increased by the G550E second-site mutation but not by 4RK. Without going into a number of additional subtle differences between the two CFTR mutants, these observations support one strong conclusion: inserting defined molecular changes into two different CFTR mutants results in largely different functional outcomes. By extension, this is probably also true for the effect of a specific drug on different CFTR mutations. These findings are at the heart of the current thinking of personalized, mutation-specific medicine for CF: different CFTR mutations represent different drug targets. Current drug-development techniques generate highly selective drugs, which are less likely to bind to related targets and further support the notion of mutation-specific drugs. Much of this is speculation, as there is currently just one approved drug available for one CFTR mutation (which is G551D), but the current study from the Sheppard laboratory supports the notion that different CFTR mutations behave quite differently when exposed to identical structural changes introduced by site-directed mutagenesis (Xu et al. 2014), or, by extension, when using highly selective drugs. References Accurso FJ, Rowe SM, Clancy J, Boyle MP, Dunitz JM, Durie PR, Sagel SD, Hornick DB, Konstan MW & Donaldson SH (2010). Effect of VX-770 in persons with cystic fibrosis and the G551D-CFTR mutation. N Engl J Med 363, 1991–2003. Illek B, Zhang L, Lewis NC, Moss RB, Dong J-Y & Fischer H (1999). Defective function of the cystic fibrosis-causing mutation G551D is recovered by genistein. Am J Physiol Cell Physiol 277, C833–C839. Roxo-Rosa M, Xu Z, Schmidt A, Neto M, Cai Z, Soares CM, Sheppard DN & Amaral MD (2006). Revertant mutants G550E and 4RK rescue cystic fibrosis mutants in the first nucleotide-binding domain of CFTR by different mechanisms. Proc Natl Acad Sci U S A 103, 17891–17896.
DOI: 10.1113/jphysiol.2014.274464
1908 Xu Z, Pissarra LS, Farinha CM, Liu J, Cai Z, Thibodeau PH, Amaral MD & Sheppard DN (2014). Revertant mutants modify, but do not rescue, the gating defect of the cystic fibrosis mutant G551D-CFTR. J Physiol 592, 1931–1947.
Perspectives
J Physiol 592.9
Additional information
Funding
Competing interests
The author is supported by Cystic Fibrosis Research Inc., Grant no. 12-001.
None declared.
C 2014 The Authors. The Journal of Physiology C 2014 The Physiological Society
