IMPLICATIONS OF THE NEW GENETICS FOR SCREENING FOR CYSTIC FIBROSIS

672

testing will need a sweat test for confirmation of the diagnosis. Small infants often produce inadequate amounts of sweat, so the diagnosis may be delayed for up to 2 months, by which time many of them already show evidence of lung

repeat

Screening for Disease IMPLICATIONS OF THE NEW GENETICS FOR SCREENING FOR CYSTIC FIBROSIS

J. A. DODGE Institute of Clinical

Science, Queen’s University of Belfast, Belfast BT12 6BJ

SINCE identification of chromosome 7 as the site of the gene mutation responsible for cystic fibrosis (CF) in 1985, rapid progress has been made in defining the precise locus of the gene.1 It is confidently predicted that the nature of the mutation will soon be known and that within 2 years there will be tests capable of identifying homozygotes and heterozygotes with total accuracy. How can such tests be used to the greatest benefit for individuals and society? Can the new technology be appplied to a whole population, and if so how can its value be measured? BENEFITS OF POPULATION SCREENING

There

few examples of population screening for but those that exist have been very successful. In Cyprus, the policy of screening couples for P-thalassaemia and, where appropriate, offering genetic counselling before marriage has reduced the birth rate of babies with p-thalassaemia by 95% over a decade.2 In the United States and Israel more than 300 000 Ashkenazi Jews were screened for carriers of Tay-Sachs disease. About 1 in 25 were carriers (as is the case for CF among Europeans). Prenatal diagnosis was offered to the 268 couples at risk and only 8 affected infants were born from 252 pregnancies monitored by amniocentesis in these families and another 562 monitored because of a previously affected child. In 1 of these 8 the prenatal test gave a false-negative result.3 are

heterozygotes,

disease.4 A new screening test based on DNA analysis would be 100% sensitive and specific, would be applicable even on the first day of life to cord blood samples, and would obviate the need for follow-up investigations or sweat tests (and hence the anxiety to parents while awaiting confirmation or exclusion of diagnosis).

Prenatal Diagnosis Two

The more reliable and restriction fragment length polymorphisms (RFLP) closely associated with the CF gene. By using a number of different gene probes, most families containing a living CF child, or one from whom a cell-line is available, can be informative. Analysis of fetal DNA from chorionic villus biopsy will indicate whether a fetus is affected. In the small proportion of non-informative families, prenatal diagnosis is based on the older and less specific test of amniotic fluid analysis for microvillar enzymes, which must be carried out between 13 and 16 weeks, by which time fetal quickening has occurred and the idea of screening becomes less acceptable to parents. A test based on precise DNA analysis of the CF gene or its product would be completely sensitive and specific, and be applicable to chorionic villus samples, which can be obtained as early as 8 weeks-although the biopsy procedure itself carries a risk of termination of up to 5%, which reduces with experience. more

techniques

recent

are

is based

in

use.

on

HETEROZYGOTE DETECTION

Neonatal

Screening The most satisfactory method of detecting CF in newborn babies is by measuring serum immunoreactive trypsin (IRT), but the test has a false-positive rate on first testing of 02-2%, and even those infants still positive on

born into families with no as to be expected from the in rate of 1 25. carrier around Any sizeable reduction in high the number of new cases of CF can thereafter come only from a programme of preconception or early pregnancy identification of heterozygotes followed by prenatal fetal diagnosis and selective termination of pregnancy if the parents wish. Should the parents opt to keep the baby identification of heterozygotes would help the health service to know which infants should be tested for CF at birth.

12. Helms C, Massanan R, Zeitler

20. Brown A,

PRESENT METHODS OF SCREENING FOR CF

R, et al. Nosocomial Legionnaires’ disease at university of Iowa hospitals. Clin Research 1982; 30: 777A. 13. Parry MF, Stampleman L, Hutchinson JH, Folta D, Steinberg MG, Krasnogor LJ. Waterborne Legionella bozemanii and nosocomial pneumonia in immunosuppressed patients. Ann Intern Med 1985; 103: 205-10. 14 Fischer-Hoch SP, Tobin JO’H, Nelson AM, et al. Investigation and control of an outbreak of Legionnaires’ disease in district general hospital. Lancet 1981; i: 932-36. 15. Helms CM, Massanari RM, Wenzel RP, Pfaller MA, Moyer NP, Hall N, Legionella Monitoring Committee Legionaires’ disease associated with hospital water system: 5 year progress report on continuous hyperchlorination. JAMA 1988; 259: 2423-28.

Myerowitz RL, Pasculle AW, Dowling JN, et al. Opportunistic lung infection due to ’Pittsburgh pneumonia agent.’ N Engl J Med 1979; 301: 953-58. 17. Rogers BH, Donowitz GR, Walker GK, Harding SA, Sande MA. Opportunistic pneumonia: a clinicopathological study of five cases by an unidentified acid-fast bacterium N Engl J Med 1979; 301: 959-61. 18. Tobin JO’H, Dunnill MS, French M, et al. Legionnaires’ disease in a transplant unit:

16.

isolation of the causative agent from shower baths. Lancet 1980; ii: 118-21. 19 Cordes LG, Wiesenthal AM, Gorman GW, et al. Isolation of Legionella pneumophila from hospital shower heads. Ann Intern Med 1981; 94: 195-97.

Most infants with CF

are

previous history of the disease,

Yu V L, Magnussen MH, Vickers RM, Garrity GM, Elder EM. Isolation of Pittsburgh pneumonia agent from a hospital shower. Appl Env Microbiol 1982; 43: 725-26.

21. Muder RR, Yu VL, Woo AH. Mode of transmission of Legionella pneumophila. Arch Intern Med 1986; 146: 1607-11. Antopol SC, Ellner PD. Susceptibility of Legionella pneumophila to ultraviolet radiation. Appl Environ Microbiol 1979; 38: 347-48. 23. Vickers RM, Brown A, Garrity GM. Dye-containing buffered charcoal-yeast extract medium for differentiation of members of the family legionellaceae. J Clin Microbiol 1981; 13: 380-82. 24. Edelstein PH. Control of Legionella in hospitals. J Hosp Infect 1986; 8: 109-15 25. Muraca P, Stout JE, Yu VL. Comparative assessment of chlorine, heat, ozone, and UV light for killing Legionella pneumophila within a model plumbing system Appl

22.

Environ Microbiol 1987; 53: 447-53. 26. Knudson GB. Photoreactivation of UV-irradiated

Legionella pneumophila and other Legionella species. Appl Environ Microbiol 1985; 44: 975-80.

27. Yamamoto

H, Urakami I, Nakano K, Ikedo M, Yabuuchi E Effects of Flonlizer, ultraviolet sterilizer, on Legionella species inhabiting cooling tower water. Microbiol Immunol 1987; 31: 745-52. 28. Colbourne JS, Pratt DJ, Smith MG, Fisher-Hoch SP, Harper D Water fittings as sources of Legionella pneumophila in a hospital plumbing system. Lancet 1984, r 210-13.

673 DEMAND FOR AND DESIRABILITY OF SCREENING PROGRAMMES

Cystic fibrosis fulfils most of the WHO criteria for genetic disorders that justify neonatal screening programmes,5 with the arguable reservations that (i) we await conclusive evidence that screening is "cost-effective" and (ii) existing treatment is not curative. A recent report from New South Wales has claimed that the financial cost of the neonatal screening programme was more than offset by the savings in hospital admissions for affected infants during the first 2 years of life. Other authors have reported a beneficial effect on clinical outcome from very early diagnosis and treatment.7 Clinicians are aware of this benefit, and parents of affected children strongly favour neonatal screening to avoid the emotional and physical costs of delayed diagnosis.8-10 Parents requesting prenatal diagnosis at present are those with previous experience of CF in the family. However, until recently, the less-than-total accuracy of diagnostic tests based on amniotic fluid analysis, combined with the late stage at which they can be carried out, has discouraged many from embarking on further pregnancies, or, if pregnant, from requesting prenatal diagnosis. Ethical decisions concerning termination of affected pregnancies will have to be made by parents and their medical advisers at an early stage and a general consideration of the ethics of termination is outside the scope of this paper, but a few points may be relevant. Since, unlike the case with anencephaly, there is some treatment, albeit only palliative, for CF, the decisions involve a cost-benefit assessment in emotional, physical, or financial terms for the individual, the family, and society. The financial costs can be computed more easily than the physical and emotional costs. The availability of a DNAbased test to a wider population-ie, those with a family history of CF but a less than 1 in 4 empirical risk of having an affected fetus-will highlight the need for expert advice on the nature and implications of CF if parents requesting the test are to make a truly informed decision about the outcome. Experience gained from prenatal screening programmes for spina bifida may help in formulating policy on patient education. It is unlikely that a general population survey at this time would reveal a widespread desire by individuals to know their carrier status. It would be interesting to know the views of parents of CF children who found out that they were carriers only when they had an affected child-although it is difficult for such parents to be objective. The reluctance of individuals to fmd out whether they are carriers reflects on public education in genetics generally and CF in particular. Stigmatisation of heterozygotes, whether real or perceived, would disappear with universal understanding that we are all heterozygotes for a number of serious conditions, and that at least 2 million persons in the UK are heterozygous for CF. An individual is more likely to be a CF carrier than to have AB rhesus negative blood, and no more responsible for it. Parents or prospective parents have an obvious right to choose or to opt out of a mass heterozygote screening programme, but they also deserve information on which to base that choice. The cystic fibrosis child deserves to be identified as early as possible, and this will become more important if treatment capable of reversing or controlling the intracellular metabolic abnormality becomes available. Present treatment is able to prolong life and health. If parents withhold permission for heterozygote screening and

neonatal screening programme exists, are they then depriving their child of his best chance of survival? If only one partner refuses heterozygote testing then he or she would be depriving the other partner of important information and perhaps depriving the child of his best chance of diagnosis and treatment. If a heterozygote diagnosis system were provided, parents could then hardly blame the health services for late diagnosis. no mass

IMPLICATIONS FOR THE HEALTH SERVICE

Scientists and health care providers have responsibilities only to educate parents but also to advise the health services about the prospects for cure or control, and to provide information on the financial, emotional, and physical burden of CF which administrators can use in formulating health service priorities. Ultimately, administrative decisions about introducing or maintaining screening services are likely to be made on a cost-benefit basis. The financial advantages of a neonatal screening test based on the genetic defect are unknown but the unit cost of the test is likely to depend on its uptake. It would also eliminate the need for sweat tests since the neonatal screening test would be diagnostic. It might even be possible to combine neonatal screening for CF with a genetic test for phenylketonuria, thereby eliminating some of the current costs of the phenylketonuria screening programme. Such calculations could make a neonatal test a very attractive financial proposition. Improvement in survival of CF patients means that there is a substantial increase in the CF adult population. In Britain, where treatment for adult CF patients costs about 10 000 per annum, or c 100 000 per individual on the assumption of a life expectancy of 10 years after reaching the age of 15, the net increase in the CF adult population of about 100 a year over the past decade has meant that the cost of treatment of CF is increasing by about 1 million annually. Shrinkage of the CF population by screening of the entire population of child-bearing age could be very cost-effective. Such a programme might also make universal neonatal screening unnecessary; only infants born to known carriers would need to be screened. In countries such as the UK, where there is a great deal of population movement, screening for heterozygotes would have to be carried out on a national scale if it were to succeed as a once-and-for-all exercise-although it could be piloted in a region where the population is relatively static. According to the evidence from associated polymorphisms, all cases of CF probably arose from a single ancestor and new spontaneous mutations are very unlikely.1 Further generations of heterozygotes would therefore be the offspring of those identifiable in a single universal investigation, and would be a small selected population to whom future tests could be applied at relatively little cost. To ensure the success of a population screening programme what the public needs is general and specific education on the implications of general disease and the possibilities for its control. The successful screening programmes for 0-thalassaemia and Tay Sachs disease were both preceded by an intensive educational campaign-the populations concerned were linked and defined by geographical or ethnic criteria, and awareness of the nature of the diseases for which they were being screened was high. not

674

on-site instrument, known as CORRTEX, which, the US Government insists, is necessary for the effective verification of a treaty signed in 1974 but never ratified. This, the Threshold Test Ban Treaty, limits the size of tests to 150 kilotons (about 10 times the size of the Hiroshima

to test an

International Physicians for the Prevention of Nuclear War COULD WE SAFELY NEGOTIATE A TREATY BANNING ALL NUCLEAR TESTS?

bomb). MONITORING OF TESTS-OR THEIR ABSENCE

JEREMY

LEGGETT

Department of Geology, Imperial College of Science and Technology, London SW7 2BP

A COMPLETE ban on nuclear testing would bring to an end the qualitative nuclear arms race. If signed-and abided by-a Comprehensive Test Ban Treaty (CTBT) would mean that the nuclear weapons on the drawing-boards in weapons laboratories today could not be brought to fruition. Such weapons include highly accurate warheads, earthpenetrators, and other devices which can be used in a nuclear preemptive strike. But could those who sign the treaty be sure that other signatories were not cheating? To use arms-control parlance, could a CTBT be verified? The Partial Test Ban Treaty, the 25th anniversary of which fell on Aug 10, 1988, banned nuclear tests in the atmosphere, underwater, and in space. Since it was signed, however, testing underground has continued apace. In September, 1987, the US and Soviet Governments opened negotiations on nuclear weapons testing. The mandate for negotiations commits the two superpowers to proceed "stage by stage" to "the ultimate objective of a complete cessation of nuclear testing." But the word "ultimate" commits no-one to a timescale, and the US Government policy, spelt out on many occasions in the past few years, is that some level of nuclear testing will always be necessary as long as the US relies on nuclear weapons for deterrence. The present Administration is unique among US Governments of the nuclear age in not supporting the principle of a CTBT. The Soviet Union favours attempts to negotiate a CTBT; and its 18-month self-imposed moratorium on nuclear testing from August, 1985, to February, 1987, was aimed at encouraging the US and the UK to reopen CTB talks (last held in 1977-80). But the moratorium went unreciprocated, and the Soviet Union has now accepted the "step-by-step" approach favoured by the US.! In August, as the first step, Soviet scientists went to Nevada to measure a US nuclear test. American scientists are soon to travel to Kazakhstan to measure a Soviet nuclear

These joint verification experiments (JVE’s) are designed to improve measurement of the size of nuclear explosions of between 50 and 150 kilotons. They are not relevant to a comprehensive test ban, but they are designed test.

M, Scambler PJ. A candidate for the cystic fibrosis locus isolated by selection for methylation-free islands. Nature 1987; 326: 840-45. Report of WHO European; Mediterranean Working Group on Haemoglobinopathies: The haemoglobinopathies in Europe. Copenhagen. WHO European Regional Office (in press) Kaback M. Summary of worldwide Tay-Sachs disease screening and detection. Los Angeles University of California, 1981 Wilken B, Towns SJ, Mellis CM. Diagnostic delay in cystic fibrosis: lessons from newborn screening. Arch Dis Childh 1983; 58: 863-66. Wilson JMG, Junger G Principles and practice of screening for disease. Geneva: World Health Organisation, 1968

1. Estivill X, Farrall 2

3 4

5

Many experts believe that the size of nuclear tests can be monitored just as accurately from afar as CORRTEX is able to do at the site of the explosion. Explosions set up seismic waves in the planet’s outer shell, which can be recorded by sensitive instruments (seismometers). The Office of Technology Assessment (OTA), an independent scientific body set up with bipartisan support to advise the US Congress, has recently completed a lengthy study of nuclear test-ban verification.2 Reviewing evidence, classified and unclassified, from governmental and non-governmental experts, it concludes that seismometers outside the Soviet Union could detect any explosion in hard rock in the Soviet Union down to a level of 1 kiloton. The Soviet Union has agreed, if there is a test ban, to install seismometers inside its territory. Furthermore, the OTA report concludes that if several types of seismic wave are measured, monitoring explosions at a distance of thousands of kilometres is just as accurate as using CORRTEX at the Soviet test site. For these reasons, the President of the US Arms Control Association has described the JVEs as "a needlessly complex and circuitous assault on a largely manufactured problem."3 But how low a level of clandestine testing could seismology be relied on to uncover? This is a critical question when the options of a comprehensive test ban or a threshold test ban are weighed. Parties to a treaty have to bear in mind, firstly, that explosions must be distinguished from other seismic events such as earthquakes and, secondly, that an attempt might be made to evade the test ban by detonating a bomb in a huge cavern, a technique known as decoupling, so as to dampen the seismic waves. The conclusions of the OTA on possible evasion by the Soviet Union are instructive. The Soviet Union could attempt to evade a test ban only with a small bomb-up to 10 kilotons at most. And that would require the use of a cavern some 90 metres in diameter at a depth of around 1 km, a feat which could be attempted only in salt deposits. This enterprise would be a daunting prospect-trying to hide, in geologically favourable areas, the preparation and use of the cavern from surveillance by photographic and signalsintelligence satellites. The report assesses what might be monitorable satisfactorily by the US if the Soviet Union tried to run the evasion gauntlet, concluding that "most experts agree that a high quality network of internal (seismic) stations combined

6. Wilken B, Chalmers G. Reduced morbidity in patients with cystic fibrosis detected by

neonatal

screening. Lancet 1985; ii: 1319-21. JE, Meerman GJ, Martyn A, ten Kate LP, Knol K. Screening for cystic fibrosis. A comparative study. Acta Paediatr Scand 1987, 76: 209-14. 8. Standing C. The treatment of our son with cystic fibrosis J R Soc Med 1987; 80 (suppl 15) 2-4. 9. Grant J. Our expenences of cystic fibrosis and what we expect from the paediatrician J R Soc Med 1987; 80: 5-6. 10. Wilkinson J. The experience and expectations of parents of a child with cystic fibrosis J R Soc Med 1987, 80: 7-9. 7. Dankert-Roelse