Francesca Saluzzo
Daniela Maestro
INTRODUCTION
Cystic Fibrosis (CF) is the most common lethal genetic disease in Caucasians.
CF is a systemic disease, although the extent of clinical manifestations is highly heterogeneous in distinct organs.
CF is caused by mutations in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene. More than 1900 mutations have been identified, the commonest being the F508 deletion leading to a misfolded mutant protein that is entrapped in the ER and is prematurely degraded, thus failing to reach the plasma membrane (PM) of the epithelial cells.
CF-associated bronchopulmonary disease is the principal cause of morbidity and mortality, leading to airway obstruction by viscous mucus and chronic inflammation with recurrent bacterial infections, mainly by Pseudomonas aeruginosa. Other prevalent features of CF are insufficiency of the exocrine pancreas, increased electrolytes in sweat and male infertility.
Centro Fibrosi Cistica Regione Piemonte e Valle d'Aosta
CURRENT THERAPY
To date, the clinical management of CF focuses on treating CF symptoms, yet fails to address the primary cause of CF, namely the loss-of-function of CFTR. Therapeutic strategies aimed at correcting the CFTR defect (“CFTR-repair”) have recently emerged.
Among the CFTR repairing molecules, the potentiator VX-770 revealed high efficacy in reducing sweat chloride concentrations, the hallmark of CF, and improving lung function in patients harboring the rare G551D CFTR mutation, present in only 4% of CF population. Even if new studies now extend its use to some others gating mutations, unfortunately such a treatment is not effective in patients carrying the most common F508del-CFTR mutant.
Thus, alternative approaches should be envisaged to overcome CFTR defect.
Targeting a genetic defect: cystic fibrosis transmembrane conductance regulator modulator in cystic fibrosis 2013
Ivacaftor and Cystic Fibrosis
Ivacaftor
The cost of ivacaftor is $311,000 per year, which an editorial in JAMA called "exorbitant", particularly because of the support by the National Institutes of Health and Cystic Fibrosis Foundation in its development. Vertex said it would make the drug available free to patients in the United States with no insurance and a household income of under $150,000
AUTOPHAGY IN CF
Recently, alternative approaches have been proposed, based on the concept that an impressive dysregulation of proteostasis takes place in CF cells. Indeed, disabled autophagy characterizes bronchial epithelial cells of human and mouse CF lungs. This because, the defective function of CFTR leads to increased levels of reactive oxygen species (ROS) that induced sustained expression and activation of the enzyme Tissue Transglutaminase (TG2) coupled with enzyme activation. TG2 is a master regulator of cell environment, so, the inappropriately high TG2 activation cross-links several TG2 substrates, among which the protein BECN1, a master regulator of autophagy and of autophagosome formation. As a consequence, BECN1 is sequestered with intracellular aggregates, resulting in defective autophagy with accumulation of SQSTM1, a major autophagic substrate.
Cystic fibrosis: a disorder with defective autophagy. 2011
AN AUTOPHAGY-RESTORING APPROACH
Notably, rescuing autophagy, either by genetic manipulations in vitro and in vivo, or by means of pharmacological compound, is possible to reduce lung inflammation. Moreover, such an autophagy-restoring approach results allows F508del-CFTR to traffic to and reside at the plasma membrane of airway epithelial cells.
In this study they demonstrate that Cysteamine, a well-known inhibitor of TG2, and the catalase SOD mimetic EUK-134, are able to prevent BECN1 crosslinking by TG2, thus favoring F508del-CFTR trafficking and stabilizing the “rescued” protein in the plasma membrane for 24 h after drug washout. In these experiments the authors use a multifaceted biochemical and functional approach by surface biotinylation and membrane fractionation, as well as by the analysis of iodide efflux to monitor CFTR function. Moreover, they used an approach of gene silencing in vivo in F508del-CFTR mice through which they show that direct genetic depletion of SQSTM1 can mimic the effect of Cysteamine both in vitro and in vivo.
The effects of Cysteamine on CFTR stability after rescue rely on the capability to reduce the accumulation of SQSTM1 level at the PM. Indeed a further work, belonging to the same research group, demonstrates that the ubiquitin binding domain of SQSTM1 is responsible for the CFTR disposal, through impairing the endosomal trafficking and compromising CFTR recycling, thus targeting CFTR to the lysosomes for degradation. As a proof of this concept, the authors show that the enforced overexpression of SQSTM1 lacking the UBA domain, prevents CFTR disposal and favors CFTR residence at the PM after rescue.
Targeting the Intracellular Environment in Cystic Fibrosis: Restoring Autophagy as a Novel Strategy to Circumvent the CFTR Defect. 2013
Disease-relevant proteostasis regulation of cystic fibrosis transmembrane conductance regulator. 2013
Cysteamine and Cystic Fibrosis
CONCLUSION
The results can open a new scenario in CF pathogenesis and treatment. Indeed, they can address drug discovery programs toward the identification of new classes of pharmacological compounds capable of restoring autophagy as, subsequently stabilize F508del CFTR at the PM. Notably, this new strategy could enable the beneficial action of CFTR potentiators (i.e. chemical compounds that impact on CFTR gating), even on F508del-CFTR. Accordingly, the authors demonstrate that the administration of the CFTR potentiator Genistein to F508del-CFTR homozygous mice, can sustain the beneficial effects of Cysteamine after withdrawal. Therefore, this study may provide the rationale and the proof-of-concept for the design of new pharmacological approach for CF patients.
Targeting autophagy as a novel strategy for facilitating the therapeutic action of potentiators on ΔF508 cystic fibrosis transmembrane conductance regulator. 2013
Towards a rational combination therapy of cystic fibrosis: How cystamine restores the stability of mutant CFTR. 2013
