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EDITORIALS

Cystic fibrosis: prospects forscreening and therapy

Cystic fibrosis (CF) is one of the commonest inheriteddiseases, affecting approximately 1 in 2500 live birthsin Caucasian populations. Identification last year ofthe gene mutated in CF patients1.2 (named cysticfibrosis transmembrane conductance regulator,CFTR) has not only enabled prenatal diagnosis bydirect analysis of the mutation3 but also encouragedresearchers to contemplate population carrier

screening and the possibility of "cure" of patients bysomatic gene therapy. The physiology and genetics ofthe disease have lately been reviewed,4,5 and an earliereditorial (Dec 16, p 1433) discussed the biochemicaldefect underlying CF.The coding regions of the CFTR gene predict a

protein (1480 aminoacids, molecular mass

approximately 170 kD) which is probably a memberof a large family of eukaryotic membrane-boundtransport molecules.2°6 These proteins have two

hydrophobic domains. The decrease in fluid secretionand accumulation of dehydrated mucus that is themost severe symptom of CF is almost certainly due toaltered chloride transport across epithelia (as observedin the diagnostic sweat test).The only mutation in the CFTR gene identified so

far is a deletion of three base-pairs resulting in theremoval of the phenylalanine residue at position 508 ofthe mature protein (hence its name F5o8).2,7 Thisaminoacid is within a prospective ATP-bindingdomain, and its deletion may prevent nucleotide

binding or changes in CFTR conformation that areessential for normal function.

Identification of the CFTR gene makes it possibleto understand the control of ion transport, and the

study of the CFTR protein in its normal and mutantforms should provide a molecular basis for bettertherapy. A possible method is the creation of a mousemodel for CF by homologous recombination ofmutant genes into the mouse analogue of CFTR.8 Ifthis can be achieved, it would allow greaterunderstanding of the progress of the disease andprovide a safe means of testing new treatments. Theultimate form of treatment for a genetic disease isreplacement of the mutated gene by a normalcopy-gene therapy.9 This contentious approach has

been proposed for the treatment of the

haemoglobinopathies, in which normal genescontained in retroviral vectors are introduced into thebone marrow. The most severely affected organ in CFpatients is the lung, and since drugs can easily beadministered by aerosol it may be possible to

introduce normal CFTR genes, or protein, into thestem cells that differentiate into airway tissue.

This approach requires the identification of the celltypes within the lung that normally express CFTRand the refinement of methods for introducing DNAto these cells. Now that the CF gene has been

pinpointed it is possible to determine its tissue-typeand cell-type specific expression. DNA linked topolylysine can be taken in and expressed by eukaryoticcells both in vitro and in vivo. 10 This method obviatesthe need for retroviral vectors, in which non-specificexpression can be difficult to control. If DNA can beintroduced regularly via aerosol, there will be no needfor integration of the normal gene into host DNA.Exciting prospects may be in the offing.A more immediate way of tackling CF has also been

presented by cloning of the gene and identification ofthe major mutation-population screening. If theunaffected heterozygous carriers of the mutant gene (1 1in 25 of the population) can be identified, diseaseprevention becomes possible by selective terminationafter prenatal diagnosis in 1 in 4 at-risk pregnancies(when both partners carry the mutant gene). Thisissue has prompted heated exchanges in

correspondence columns lately. 11-13 It is argued thatscreening for CF heterozygotes should wait until moreof the mutations have been identified and theindividuals being tested have been sufficiently welleducated to understand the results.7,11-13The frequency of .F 508 on CF chromosomes varies

between ethnic groups. In Scotland, this mutation ispresent on approximately 75 % of CF chromosomes,14whereas in Spanish and Italian populations it

comprises only 49% and 43% of mutations,respectively. 15 If one assumes that the Scottish

population is broadly representative of the UK as awhole, 3% of the population would be positive whentested for the deletion. In 3% of these cases their

partner will also test positive for the deletion (ie,[1/25 x 3/4]2 =1/1100 couples). Unfortunately, theremainder will have had their risk of having a CF child

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increased to 1 in 400 and will therefore require carefulcounselling and attitude assessment to determinewhether this status is acceptable. The effort now beingexpended worldwide on identification of othermutations should reduce this proportion in the nearfuture.

Meanwhile, it seems prudent to instigate pilotstudies to determine the best point at which to screen(eg, antenatal, family planning, or general practiceclinic) and to assess how people react to being toldtheir genetic status. Assuming an uptake of 85%among pregnant women (the most likely target group)and 90% among their partners, and detection of 75%of CF mutations, the incidence of CF among thosetested could be reduced by over 40%. Since anyscreening programme will take several years to

become fully operational, pilot schemes should beintroduced in the very near future. It is important thatthe mistakes made in the introduction of screening forsickle-cell anaemia are not repeated and that lessonsare learnt from the successful screening for

p-thalassaemia in Mediterranean populations.16Detection of heterozygotes carrying the D F 508mutation is facilitated by the use of polyacrylamide gelelectrophoresis, which obviates the need for

radioisotopes .’-7,111 Alternatively, the amplificationrefractory mutation system (ARMS)l9 may proveuseful for analysis of this mutation.Thus cloning of the CF gene gives hope for a

greater understanding of the disease, for moreeffective treatment regimens in the long term, andmost importantly, for heterozygote screening. Carrierscreening should be freely available for two mainreasons: (a) it gives prospective parents the chance tomake an informed choice, and (b) the reduction in theincidence of CF in the population will free resourcesfor the CF patients alive today. In the UK, screeningfor the A F508 mutation alone would identify 120 CFfetuses each year out of approximately 700 000

pregnancies in the country as a whole.

1. Rommens JM, lanuzzi MC, Kerem B, et al. Identification of the cysticfibrosis gene: chromosome walking and jumping. Science 1989; 245:1059-65.

2. Riordan JR, Rommens JM, Kerem B, et al. Identification of the cysticfibrosis gene: cloning and characterization of complementary DNA.Science 1989; 245: 1066-73.

3. McIntosh I, Raeburn JA, Curtis A, Brock DJH. First trimester prenataldiagnosis of cystic fibrosis by direct gene probing. Lancet 1989; ii:972-73.

4. Boat TF, Welsh MJ, Beaudet AL. Cystic fibrosis. In: Scriver CR,Beaudet AL, Sly WS, Valle D, eds. The metabolic basis of inheriteddisease, 6th ed. New York: McGraw-Hill, 1989: 2649-80.

5. Goodfellow PN, ed. Cystic fibrosis. Oxford: Oxford University Press,1989.

6. Higgins C. Protein joins transport family. Nature 1989; 341: 103.7. Kerem B, Rommens JM, Buchanan JA, et al. Identification of the cystic

fibrosis gene: genetic analysis. Science 1989; 245: 1074-80.8. Cappechi MR. The new mouse genetics: altering the genome by gene

targetting. Trends Genet 1989; 5: 70-76.9. Danielson H. Gene therapy in man. Lancet 1988; i: 1271-72.

10. Wu CH, Wilson JM, Wu GY. Targeting genes: delivery and persistentexpression of a foreign gene driven by mammalian regulatory elementsin vivo. J Biol Chem 1989; 264: 16985-87.

11. Stewart AD. Screening for cystic fibrosis. Nature 1989; 341: 696.12. Ten Kate LP. Carrier screening in CF. Nature 1989; 342: 131.13. Roderick PJ, Chapple J. Screening for cytstic fibrosis. Lancet 1989; ii:

1403-04.14. McIntosh I, Lorenzo M-Z, Brock DJH. Frequency of &Dgr;F508 mutation

on cystic fibrosis chromosomes in UK. Lancet 1989; ii: 1404-05.15. Estivill X, Chillon M, Casals T, et al. &Dgr;F508 gene deletion in cystic

fibrosis in Southern Europe. Lancet 1989; ii: 1404.16. The haemoglobinopathies in Europe: combined report on two WHO

meetings. Copenhagen: WHO Regional Office for Europe, 1988.17. Mathew CG, Roberts RG, Harris A, et al. Rapid screening for &Dgr;F508

deletion in cystic fibrosis. Lancet 1989; ii: 1346.18. Schieffer H, Verlind E, Penninga D, et al. Rapid screening for &Dgr;F508

deletion in cystic fibrosis. Lancet 1989; ii: 1345-46.19. Newton CR, Heptinstall LE, Summers C, et al. Amplification refractory

mutation system for prenatal diagnosis and carrier assessment in cysticfibrosis. Lancet 1989; ii: 1481-83.

Organ donors in the UK—gettingthe numbers right

During the past five years services for patients withend-stage renal failure in the UK have improvedconsiderably. By extending the age limits to includethe very young and the old, acceptance rates have risensteadily in 1987, 50 new patients per million of thepopulation (pmp) were taken on for treatment thatyear. 1988 saw a further rise, with some areas, such asWales, accepting over 70 new patients pmp fortreatment.2 Kidney transplant survival rates have alsoimproved to the extent that approximately 80% offirst cadaveric kidney transplants can now be expectedto function for at least a year. Yet there has been no

improvement in the supply of cadaveric organs fortransplantation. In 1981, Jennett estimated that therewere 4000 brainstem deaths annually in the UK,3 andit had been assumed that if sufficient effort was madeto identify these individuals as organ donors therewould be enough kidneys to supply the dialysispopulation. However, the annual rate of kidneytransplantation only increased from 1443 to 1575between 1984 and 1988 and transplant activity during1989 was only marginally better. The waiting list fortransplantation continues to increase, and now standsat 3684.The 1988 figure represents just 29 kidney

transplants a year pmp, which is less than half the raterequired to match the annual accrual of patients withrenal failure. Dr Gore and her colleaguess have latelyreported the results of an audit that was carried out in278 intensive care units in England. Of 2853 deathsrecorded during the first quarter of 1989, only 148patients (5%) became organ donors-ie, 12 donors or24 kidneys pmp.

Realisation that the supply of cadaver kidneys might havereached a ceiling has prompted much debate, since, if true,the long-term objectives for the management of patientswith renal failure may have to be revised (and recosted). Asymposium on organ donation was held in November at theDepartment of Health, and further discussions took place ata similar symposium in Cardiff, and at the autumn meetingof the British Transplantation Society. At the symposia,some preliminary and as yet unpublished findings werepresented from an audit of hospital deaths that had just beeP