For children living with cystic fibrosis, pronouncing the name of their disease often poses a challenge. Since 1965, the term “65 roses” has been employed by children of all ages to describe their condition. What started off as a mispronunciation, has now been adopted as the emblem of cystic fibrosis, fronting fundraising campaigns worldwide. Cystic fibrosis has been embraced by the scientific research community and has become a paradigmatic single-gene disorder in Ireland, a country that claims the highest incidence of CF worldwide.

Although the youth affected by CF have chosen to simplify the name, it is crucial that the scientific community does not underestimate this disorder. Boasting almost 2000 identifiable mutations resulting in varying phenotypes, the CFTR gene has been a research giant, challenging both scientists and clinicians alike. However, despite massive advances in this field of study, significant gaps of knowledge and limitations in the management of CF remain. 

Introduction

Despite cystic fibrosis’ (CF) classification as a rare disease, CF is the most common life-threatening monogenic (single-gene) disorder in Caucasians. The estimated incidence of CF is 1 in 2500-4000 newborns and CF currently affects more than 70,000 individuals worldwide [1]. Notably, Ireland has the highest incidence (per head of population) of CF in the world. Approximately 1 in 19 Irish people are said to be carriers of CF [2] and expectedly, CF is deeply ingrained in the Irish psyche and culture, despite its “rare” status. 

Just like children with cystic fibrosis (CF) have simplified the name to “65 roses”, there is a tendency to oversimplify this deceptively complicated single-gene disorder. At a basic level, we know the underlying genetic cause of CF. Similarly, at a clinical level, we know the features that characterise CF, particularly advanced lung disease, which is the primary cause of mortality in people with CF. Between these two extremes, the exact mechanism in which loss of CFTR function results in the CF phenotype remains somewhat uncertain [3]. Therein lie the thorns of cystic fibrosis.

Nevertheless, recent progress in elucidating the pathophysiology of CF has laid strong foundations for bridging this gap and has formed the genesis for new treatments. To understand this link between the molecular defects at the core of CF and the progression of CF symptoms, we must first revise a few key fundamentals.

The estimated incidence of CF is 1 in 2500-4000 newborns and CF currently affects more than 70,000 individuals worldwide.

Back to basics

CF is an inherited autosomal recessive condition caused by mutations in the CFTR gene, located on chromosome 7. This gene encodes an ion channel expressed on epithelial cell membranes, which mediates sodium and bicarbonate transport. In addition to controlling chloride secretion, this channel regulates the function of other membrane transport proteins. Collectively, these protein channels play an important role in maintaining homeostasis by controlling the movement of water through the epithelium, which is particularly pertinent for mucous membranes. Hence, CFTR malfunction leads to fluid hyperabsorption and dehydration of the epithelial surface, producing thick, dehydrated secretions.

Over 1900 CFTR mutations have so far been described. F508del is the most common mutation with an allelic frequency of around 90% [4]. These mutations impact protein synthesis and have been classified into six functional classes. Class II, home to the infamous F508del mutation, comprises mutants that fail to traffic the channel to the cell surface, due to protein misfolding and premature degradation by the cell’s quality control system. However, the restriction of variants into a single class is problematic, as multiple processes can be affected by a single variant. For example, although F508del is categorised as a Class II mutation, in actuality, the effect this mutation has on CFTR synthesis and function, spans at least three functional classes [5]. Appreciation of the diversity of effects caused by a CFTR variant is important in the design of molecular treatments for CF, and represents just one thorn challenging the scientific community. 

From molecules to medicine – the biological consequences of CFTR mutations

CFTR is primarily present in the epithelial cells of the airways, intestine and in cells with exocrine and endocrine functions. In organs and tissues implicated in CF, absence or dysfunction of CTFR results in an ionic imbalance that leads to dehydrated mucous. Thus, CF pathogenesis is characterised by the build-up of a thick, sticky mucous in multiple organs, such as the lungs, sinuses, intestine, pancreas and reproductive organs [5].

Mucociliary Escalator: Out of Order

Although CF affects several organs, the most significant clinical manifestations are seen in the lungs and airways. In bronchial tissue, the CFTR channel is found in submucosal glands and ciliated epithelial cells. The ciliated cell is the workhorse of the mucociliary escalator, a protective mechanism which clears excess secretions from our lungs. CF patients have a defective or absent CFTR and as such lack the ability to sufficiently hydrate the airway surface layer: an important mucous layer that lines the airway tract. The dehydrated mucous layer draws water from the protective coating on the cilia. Eventually, the cilia collapse as a result of the aberrant osmotic gradient and the mucociliary escalator ceases. Mucous stasis eventually leads to airway plugging, chronic bacterial infection, inflammation and airway tissue damage, in the form of bronchiectasis [6].

As mucociliary clearance is an important defence mechanism against pathogens and dust particles, its reduction in CF patients leads to chronic infections by a restricted group of pathogens: pseudomonas aeruginosa, a hallmark of CF which is found in 80% of patients by the age of 18 years [1].

Intestinal changes

The most serious acute complication of the CF intestinal phenotype is the obstruction of the terminal ileum or proximal large intestine, referred to as meconium ileus. Additionally, abnormal GI microbiota and inflammation may contribute to mucosal damage and ulceration. A new endoscopy technique relying on a swallowed capsule (PillCam) reported 63% of patients investigated had various lesions, including ulcers [7].

CFTR and the Pancreas

In healthy individuals, the human pancreatic ductal epithelium secretes 1-2L/day of alkaline fluid, which flushes digestive enzymes into the duodenum alkalinising acidic chyme. Decreased CFTR function leads to a lower fluid volume and increased acidity, precipitating protein rich secretions that plug smaller ducts.  This results in progressive damage to the exocrine tissue of the pancreas. A lack of digestive enzymes leads to malabsorption, and in its most severe form, pancreatic insufficiency.

Regarding CF-related diabetes (CFRD), the aetiology is complex and the involvement of CFTR in the pathophysiology remains a contentious issue. While the exact mechanism of CFRD continues to be debated, it is widely accepted that expression of abnormal CFTR channels in the exocrine part of the pancreas, is to blame for endocrine dysfunction and this may be due to reduced blood flow to islet cells [8]. Regardless of the aetiology, abnormal glucose regulation adds to the complexity of this “simple” monogenic disorder. 

Treatment limitations

CF is a complicated disorder for several reasons.  As noted previously, almost 2000 CFTR gene mutations have so far been reported. This is further complicated by the presence of “complex alleles” i.e. those containing more than one CFTR mutation. Secondly, the CFTR genotype is often a poor predictor of the full spectrum of clinical phenotypes and multi-systemic consequences. Thirdly, an increasing number of CFTR mutations are associated with isolated disease characteristics such as chronic pancreatitis, chronic sinusitis, disseminated bronchiectasis or male infertility; the distinction between these CFTR-opathies and CF is not always straightforward. Finally, it has been proposed that CFTR plays several other roles in the cell but it remains unclear whether correction of the primary function of CFTR will also restore these additional “secondary” functions. 

Playing it safe – conventional therapy limitations

Conventional therapies for CF include chest physiotherapy, antibiotics, mucolytics and nutritional supplementation. These therapies are chiefly destined to manage CF symptoms and disease complications. Failure to treat the underlying molecular defect is a major limitation of this approach. These therapies are costly and time consuming, further limiting their success in combatting CF. Antimicrobial resistance and compliance issues are two omnipresent challenges in CF treatment.

Antimicrobial resistance

Despite improvements in outcomes, many CF patients still die from pulmonary complications. Treatment of bacterial lung infection remains one of the primary goals of CF care. Management of these infections involves complicated antibiotic regimes. Antimicrobial resistance poses a continually evolving limitation to this conventional therapy. Known pathogens such as P. aeruginosa and B. cepacia continue to affect CF disease progression and over the last decade MRSA has demonstrated a notable increase in prevalence, increasing from 4% in 1999 to 25.7% in 2010. Recent research has demonstrated that chronic MRSA infection in CF is an independent risk factor for death, not just a marker of disease severity or end of life in individuals with CF [9]. Given this affirmation of the clinical significance of MRSA pulmonary infection in CF, tackling increasingly resistant pathogens with dwindling new antimicrobial options represents a limitation of conventional therapy. 

Adherence to complicated CF medical regimens

Between the use of inhaled mucolytics, inhaled antibiotics, airway clearance, nutritional enzymes, supplements and equipment maintenance, many individuals with CF spend hours each day on their treatment regimens. Adherence to these complicated CF medication regimens is only approximately 50% [9]. Recently, a YouTube video of a six year-old boy taking his daily CF medications went viral, amassing almost one million views in less than one week. His daily pill count was 45 [10]. Current research has highlighted both the challenge of adherence to these complicated CF treatments and the resultant impact on clinical outcomes. Two studies have demonstrated that poorer adherence to inhaled tobramycin was associated with an increased risk of hospitalisation and increased health care costs [11].

Why taking it personally matters

In recent times, CF has become the ‘poster-child’ for personalised medicine. It is evident that symptom management, rather than correction of the underpinning molecular defect, is the overarching limitation of conventional therapies. Effective treatment of CF at the molecular level requires restoration of CFTR function in affected tissues. New personalised therapies are under development, targeting either the dysfunctional gene or protein. This new era shines a beacon of hope for curative strategies, rather than focussing on end-stage disease management of affected organs. 

Protein therapy

Protein therapies are aimed at correcting the dysfunction of CFTR, targeting specific mutation classes to personalise the treatment. One of the major limitations of these therapies is the vast range of mutations in the CFTR gene, and the heterogeneity of their effects. The pharmaceutical lumacaftor which allows CFTR to be expressed at the cell surface, was developed to tackle F508del. However, phase II clinical trials indicated that restoring CFTR expression was not sufficient, and CFTR function had to be promoted by a potentiator. Consequently, a clinical trial that combined lumacaftor, with the potentiator ivacaftor showed significant improvements [9]. Thus Orkambi, a lumacaftor/ivacaftor combination recently licensed by the European Medicines Agency was born. The identification of the effects of each individual mutation and the creation of multiple targeted drugs are essential to effectively treat CF. 

Gene therapy

One hopes that in the future, gene therapy will be the gold standard treatment for patients with CF. The concept of gene therapy in CF involves adding a correct sequence of CFTR in a way that is incorporated into the patient’s cells and is able to circumvent the CFTR mutation.  Vector options include both viral and synthetic. Unfortunately, the viral vectors studied for gene transfer induced an excessive inflammatory response or showed poor efficacy in CF. Therefore, weakly immunogenic synthetic vectors are an attractive alternative. Currently, a phase III clinical trial of an inhaled gene therapeutic is ongoing in the UK with a synthetic lipid vector but no results have been published to date. 

Deliverance

Another limitation of gene therapy in CF is that thus far, agents have been delivered only to the lung (by inhalation) and hence would have no benefit for other organs. Another challenge posed by this thorny rose is access to the site of delivery, as the thick respiratory mucous may prevent a favourable bioavailability. Chronic administration may also be an issue, even with a low immunogenic system [9].

Cost

When the drug Kalydeco (ivacaftor) came to market, the Irish government was caught in a precarious position. With a price tag of €234,000 per patient per year the National Centre for Pharmacoeconomics (NCPE) initially recommended against its sanction for use. However, after coming under fire from patient advocacy groups, the government negotiated a reduced price with the manufacturing company and Kalydeco has since been approved. 

Presently the government is facing an equally precarious position as Orkambi has recently been rejected by the HSE and NCPE as being “unjustifiably expensive and not cost effective” [13], although negotiations with the pharmaceutical company Vertex are ongoing. With the availability of new innovative therapies for CF on the rise, this situation is almost certainly to be repeated. 

Conclusion

Despite being a simple monogenic disease, CF has provided many lessons of complexity to the field of biomedical science. Cloaked in the guise of apparent simplicity, CF is indeed the monogenic disease paradigm, greatly contributing to the advancement of both scientific research and clinical practice.

As Jack Riordan, who together with Lap-Chee Tsui and Francis Collins made the original CFTR discovery, stated in a recent publication for the 20th anniversary of the discovery “the disease has contributed much more to science than science has contributed to the disease” [1]. 

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