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Cystic Fibrosis

There are at least 60,000 Cystic Fibrosis (CF) sufferers worldwide; this the most common life-shortening single gene disorder in the Caucasian population. Birth prevalence is about 1 in 2,400, equivalent to 300 new cases in the UK each year. CF is characterised by an excessive accumulation of thick mucus in the respiratory system and digestive tract and the main clinical features include progressive lung disease and pancreatic enzyme deficiency. Life expectancy has increased from five years in 1964 to about 31 years today. This has mainly been because of improved treatment.

It has been known for a long time that CF was a genetic disorder: a couple comprising two carriers of the CF gene has a one in four risk of having a child that has CF. More recently, scientists showed that a protein called the transmembrane conductance regulator (CFTR) is responsible for CF. Around 1,000 mutations in the CFTR gene have been identified, although not all of these cause the disease.

Early research on bacteria

The story of how the genetic mutations lead to CF began even before researchers were intentionally studying the disease: an example of how research in one field can, by accident, be crucial in another. In 1986, MRC-funded researchers in Dundee (who then moved to Oxford), led by Chris Higgins, published a paper in Nature describing a class of bacterial membrane transport proteins – the ATP-binding cassette (ABC) superfamily of transport systems1. These mediate the transport of molecules across the cell membrane. The discovery became very important and relevant to CF, because the CFTR, which regulates the transport of chloride ions across cell membranes, and hence the stickiness of the mucus, was later found to belong to this family of proteins.

A marker for the disease

Before the CFTR protein was characterised, MRC researchers at the MRC Clinical and Population Cytogenetics Unit in Edinburgh began looking for markers for CF, with the aim of finding the gene that causes the condition. They ended up finding a protein that is not the underlying cause of the disease – it is not mutant in CF – but is interesting because it accumulates in CF as a result of inflammation.

The team, led by Veronica van Heyningen, published a paper in Nature in 1987, describing the mapping of two genes for this protein, which was called the CF antigen (containing proteins S100A8 and S100A9), to chromosome 12. Since then, this protein was found to be important in the study of other inflammatory diseases, and even tumours, as a marker of disease severity. There is some evidence that the presence of the CFTR gene mutation affects the inflammatory response in the lung and the CF antigen is a marker for this. More recently, the CF antigen has been useful for determining whether gene therapy has worked.

The cause

In 1989, a consortium of US and Canadian researchers found the CF gene itself on chromosome 73. This allowed MRC researchers in Edinburgh – David Porteous’s group at the Human Genetics Unit – to make a mouse model of CF. This was the first animal model that mimicked many of the features of the disease. The identification of the gene also offered the opportunity of testing for the condition and for identifying carriers by screening for the mutant gene, which was investigated by MRC-funded researchers at the University of Edinburgh, led by David Brock. They proposed ‘couple screening’: screening pregnant women, and consequently their partners, for carrier status (using a mouth swab), followed by an optional genetic test of the amniotic fluid cells if both parents were found to be carriers. They also explored ‘cascade testing’, which screens relatives of a person affected by CF.

Antenatal carrier screening was introduced at a few trial hospitals in Scotland and there was a research trial and a subsequent service offered in and around Edinburgh until 2005. However, the service was halted due to technical issues and in order to wait for clarification of the national policy position. There were also concerns about the impact on the antenatal programme of the Scotland-wide newborn screening programme for CF, which was introduced in 2003.

Currently, the UK National Screening Committee (NSC) recommends that neonatal screening for CF should be offered for all newborn babies. This tests a blood spot for a protein produced by the pancreas, followed by a confirmatory DNA test in babies with raised levels. It is in place in Scotland, Wales and Northern Ireland and is due to be introduced in England by April 2007. The purpose is to identify babies early and hence start treatment at the appropriate time. Meanwhile, the NSC is continuing to discuss whether antenatal screening for carrier status should be offered. The committee recognises that more research is needed before further policy advice is given, and it wants to explore further the implications of having antenatal and neonatal screening in place simultaneously.

Gene therapy

Gene therapy is the insertion of genes into a person’s cells or tissues to treat a disease – it became a possibility as a result of cloning the CF gene. It was pioneered in the mid-1990s by MRC researchers in Edinburgh, Oxford and London. With support from the pharmaceutical industry, they attempted the first clinical trial using DNA plasmids and liposomes – balls of lipids that naturally stick to the surface of cells and enter them. However, the challenge was in the efficiency of delivery of the DNA – a normal copy of the CFTR gene – into the lungs of CF patients, and further work was required.

The CF Trust, the UK’s charity for CF, followed on from the MRC by funding a gene therapy consortium initiative that brought together the three gene therapy trial groups in the UK. The consortium will shortly begin a new clinical trial, following improvements to the method achieved in the laboratory. However, liposomes are generally less effective ‘vectors’ for gene therapy than viruses, which naturally invade cells and enter them. Scientists can harness this property by putting a copy of the therapeutic gene into the virus that then carries it into the cell, but there are safety issues. The MRC is currently providing funding for researchers at the Institute of Child Health to develop an enhanced viral vector that is better at delivering genes than liposomes, and is also safe.

Treating symptoms of CF

Until gene therapy works, the best way to manage CF is by helping the body’s defence system cope with the disease, and research in this area has been responsible for raising the life expectancy of people with CF.

Researchers at the MRC Immunochemistry Unit in Oxford have been developing novel therapeutics – lung surfactant proteins – for the treatment of lung inflammation and infection which is experienced by CF patients. CF patients naturally have lower levels of these therapeutic proteins so are more vulnerable to infections, allergic responses and inflammation. The team has produced ‘recombinant’ surfactant proteins, produced by genetically-modified bacteria, and is currently seeking a commercial partner to carry out clinical trials.

The MRC has identified respiratory diseases in general, including CF research, as a priority area. We have issued a special request for grant proposals in this area for projects aimed at increasing our understanding of the mechanisms of respiratory disease that underpin the development of new diagnosis and therapeutic approaches.

References

1. Higgins et al. (1986). A family of related ATP-binding subunits coupled to many distinct biological processes in bacteria. Nature, 323, 448

2. Dorin et al. (1987). A clue to the basic defect in cystic fibrosis from cloning the CF antigen gene. Nature, 326, 614

3. Riordan et al. (1989). Identification of the cystic fibrosis gene: cloning and characterization of complementary DNA. Science, 245, 1066

MRC, December 2006

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