Cystic Fibrosis

Cystic Fibrosis

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Cystic Fibrosis Second edition

Edited by

Margaret E. Hodson MD MSC FRCP DmedEd Professor of Respiratory Medicine, Honorary Consultant Physician, Royal Brompton Hospital and Harefield NHS Trust, London, UK

Duncan M. Geddes MA MBBS MD FRCP Professor of Respiratory Medicine, Consultant Physician, Royal Brompton Hospital and Harefield NHS Trust, London, UK

A member of the Hodder Headline Group LONDON Co-published in the United States of America by Oxford University Press Inc., New York

First published in Great Britain in 1995 Second edition published in 2000 by Arnold, a member of the Hodder Headline Group, 338 Euston Road, London NW1 3BH http://www.arnoldpublishers.com Co-published in the United States of America by Oxford University Press Inc., 198 Madison Avenue, New York, NY 10016 Oxford is a registered trademark of Oxford University Press © 2000 Arnold All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronically or mechanically, including photocopying, recording or any information storage or retrieval system, without either prior permission in writing from the publisher or a licence permitting restricted copying. In the United Kingdom such licences are issued by the Copyright Licensing Agency: 90 Tottenham Court Road, London W1P 9HE. Whilst the advice and information in this book are believed to be true and accurate at the date of going to press, neither the authors nor the publisher can accept any legal responsibility or liability for any errors or omissions that may be made. In particular (but without limiting the generality of the preceding disclaimer) every effort has been made to check drug dosages; however, it is still possible that errors have been missed. Furthermore, dosage schedules are constantly being revised and new side-effects recognized. For these reasons the reader is strongly urged to consult the drug companies' printed instructions before administering any of the drugs recommended in this book. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloguing-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN 0 340 74208 9 1 2345678910 Commissioning Editor: Joanna Koster Project Editor: Catherine Barnes Production Editor: James Rabson Production Controller: lain McWilliams Project Manager: Alison Nick Typeset in 10/12 Minion by Phoenix Photosetting, Chatham, Kent Printed and bound in the United Kingdom by The Bath Press, Bath

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Contents

Contributors 1

vii

Epidemiology

1

a Clinical epidemiology of cystic fibrosis

2

S. Walters

2

b The epidemiology of cystic fibrosis P. A. Lewis

13

Basic molecular genetics

27

G. Santis 3

Phenotype-genotype relationships

49

G. R. Cutting 4

5

Applied cell biology E. W. F. W. Alton and S. N. Smith

61

Microbiology of cystic fibrosis

83

N. H0iby and B. Frederiksen 6

Immunology of cystic fibrosis

109

G. Doring, G. Bellon and R. Knight 7

The pathology of cystic fibrosis M. N. Sheppard and A. G. Nicholson

141

8

Cardiopulmonary physiology A. Bush

157

Diagnostic methods

177

a Diagnosis

178

9

B. J. Rosenstein

10

b Screening D. J. H. Brock

189

Respiratory system a Pediatrics

203 204

S. G. Marshall, M. Rosenfeld and B. W. Ramsey b Adults

218

M. E. Hodson 11

Growth, development and nutrition J. M. Littlewood and S. P. Wolfe

243

vi Contents 12

Gastrointestinal and pancreatic disease in cystic fibrosis

261

A. G. F. Davidson 13

Liver and biliary disease in cystic fibrosis

289

D. Westaby 14

Reproductive and sexual health

301

S. M. Sawyer 15

Other organ systems

313

a Other organ systems

314

C. Koch and S. Lanng b Osteoporosis

329

S. L Elkin 16

Psychological aspects of cystic fibrosis

339

B. Lask 17

Nontransplant Surgery

349

D. M. Griffiths 18

Lung transplantation

361

B. P. Madden 19

Paramedical issues

375

a Physiotherapy

376

J. A. Pryor and B. A. Webber b Dietary treatment of cystic fibrosis

384

S. Poole, A. McAlweenie and F. Ashworth c Nursing

396

F. Duncan-Skingle and F. Foster d Cystic fibrosis home care

406

F. Duncan-Skingle and E. Bramwell e Social work

413

N. Cloutman f Occupational therapy

419

V. Otley Groom 20

Exercise and training for adults with cystic fibrosis

433

A. K. Webb and M. E. Dodd 21

Future prospects

449

M. Stern and D. M. Geddes Index

461

Contributors

E. W. F. W. Alton MA MD FRCP

M. Dodd MCSP

Professor of Gene Therapy and Respiratory Medicine,

Specialist Physiotherapy Clinician, Bradbury Cystic Fibrosis

Department of Gene Therapy, National Heart and Lung

Unit, North West Lung Centre, Wythenshawe Hospital,

Institute, Emmanuel Kaye Building, Manresa Road, London

Southmoor Road, South Manchester M23 9LT, UK

SW3 6LR, UK G. Doring PhD Dr rer nat. F. Ashworth BSc(Hons) SRD Chief Dietitian, Department of Nutrition and Dietetics, Royal Brompton Hospital and Harefield NHS Trust, Sydney Street,

Professor, Department of General and Environmental Hygiene, Hygiene-Institute, University of Tubingen, Wilhelmstrafs'e 31, D-72074 Tubingen, Germany

London SW3 6NP, UK G. Bellon MD Professor, Centre Hospitalier Lyon Sud, Service de Pediatrie, F-69310 Pierre Benite, France

F. Duncan-Skingle RGN NDNCert Hvcert Cystic Fibrosis Unit, Royal Brompton Hospital and Harefield NHS Trust, Sydney Street, London SW3 6NP, UK

E. C. Bramwell RGN

S. L Elkin MCSP MBBS MRCP

Department of Cystic Fibrosis, Royal Brompton Hospital and

Research Fellow, Department of Cystic Fibrosis, Royal

Harefield NHS Trust, Sydney Street, London SW3 6NP, UK

Brompton Hospital and Harefield NHS Trust, Sydney Street, London SW3 6NP, UK

D. J. H. Brock PHD FRCPE FRCPath FRSE Formerly Professor of Human Genetics, Human Genetics

F. Foster RGN RSCN

Unit, Molecular Medicine Centre, University of Edinburgh,

Formerly Royal Brompton Hospital and Harefield NHS Trust,

Western General Hospital, Edinburgh EG4 2XU, UK

Sydney Street, London SW3 6NP, UK

A. BUSh MD FRCP FRCPCH

Reader in Paediatric Respirology, Royal Brompton Hospital and Harefield NHS Trust, Sydney Street, London

SW3 6NP, UK

B. Frederiksen MD PHD Danish Cystic Fibrosis Center, Pediatrie Department G 5003, Rigshospitalet, Copenhagen, Denmark

N. Cloutman CQSW

D. M. Geddes MA MBBS MD FRCP

Social Worker, Department of Social Work, Royal Brompton

Professor of Respiratory Medicine, Consultant Physician,

Hospital and Harefield NHS Trust, Sydney Street, London

Royal Brompton Hospital and Harefield NHS Trust, Sydney

SW3 6NP, UK

Street, London SW3 6NP, UK

G. R. Cutting MD

D. M. Griffiths MCh FRCS

Center for Medical Genetics, CMSC 9-123, Johns Hopkins

Consultant Neonatal and Paediatric Surgeon, Wessex

University School of Medicine, 600 North Wolfe Street,

Regional Centre for Paediatric Surgery, Southampton General

Baltimore, Maryland 21287-3914, USA

Hospital, East Wing, Tremona Road, Southampton S016 6YD,

UK A. G. F. Davidson esc MD FRCPC Professor, Department of Pediatrics, Division of Biochemical

V. Otley Groom Dip COT SROT

Diseases, University of British Columbia and British

Head of Occupational Therapy, Department of Occupational

Columbia's Children's Hospital, 4480 Oak Street, Room 2066,

Therapy, Royal Brompton Hospital and Harefield NHS Trust,

Vancouver BC V6H 3V4, Canada

Sydney Street, London SW3 6NP, UK

viii Contributors

M. E. Hodson MD MSC FRCP Dmed Ed

J. A. Pryor MSC MBA FNZSP MCSP

Professor of Respiratory Medicine, Royal Brompton Hospital

Research Fellow Physiotherapy, Department of Cystic

and Harefield NHS Trust, Sydney Street, London SW3 6NP, UK

Fibrosis, Royal Brompton Hospital and Harefield NHS Trust, Sydney Street, London SW3 6NP, UK

N. H0iby MD Dr Med sd Professor and Chairman, Department of Clinical

B. W. Ramsey MD

Microbiology, Rigshospitalet, University of Copenhagen,

Professor, Department of Pediatrics, Director, Therapeutic

Juliane Maries Vej 22 DK-2100, Copenhagen, Denmark

Development Network, Children's Hospital and Regional

R. Knight PhD MBBS

Seattle WA 98105-0371, USA

Medical Center, 4800 Sand Point Way N.E., P.O. Box 5371,

Imperial College, National Heart and Lung Institute, Emmanuel Kaye Building, Manresa Road, London SW3 6LR, UK

M. Rosenfeld MD MPH Assistant Professor, Department of Pediatrics, Children's

C. Koch MD Dr Med Sci

Professor, Department of Pediatrics and Danish CF Center, Rigshospitalet, 5003, GGK, Blegdamsvej 9, DK-2100 Copenhagen 0, Denmark S. Lanng Dr Med sci Department of Pediatrics and Danish CF Center, Rigshospitalet, 5003, GGK, Blegdamsvej 9, DK-2100 Copenhagen 0, Denmark

Hospital and Regional Medical Center, 4800 Sand Point Way N.E., Seattle WA 98105-0371, USA B. J. Rosenstein MD Professor of Pediatrics, Johns Hopkins University, Director, Cystic Fibrosis Center, Johns Hopkins Hospital, 315 Park, Baltimore, Maryland 21287-2533, USA G. Santis MD FRCP Senior Lecturer in Respiratory Medicine, Department of Respiratory Medicine and Allergy, Thomas Guy House, Guy's

B. Lask FRCPsych FRCPCH MPhil MBBS

Hospital, St. Thomas' Street, London SE1 9RT, UK

Consultant Psychiatrist, Department of Psychiatry, St. George's Hospital Medical School, London SW17 ORE, UK

S. M. Sawyer MBBS MD FRACP Associate Professor, Department of Respiratory Medicine and

P. A. Lewis PhD Ceng

Center for Adolescent Health, Royal Children's Hospital,

Senior Lecturer, Public Health Group, School of Postgraduate

Parkville 3052, Victoria, Australia

Medicine, University of Bath, Bath BA2 7AY, UK M. N. Sheppard MD FRCPath J. M. Littlewood OBE MD FRCP FRCPE FRCPCH OCH

Consultant Histopathologist and Senior Lecturer, Department

Regional Paediatric Cystic Fibrosis Unit, St. James University

of Pathology, Royal Brompton Hospital and Harefield NHS

Hospital, Leeds LS9 7TF, UK

Trust, Sydney Street, London SW3 6NP, UK

A. McAlweenie BSC SRD

S. N. Smith CEd BA PhD

Formerly Dietetics Department, Royal Brompton Hospital

Department of Gene Therapy, National Heart and Lung Institute,

and Harefield NHS Trust, Sydney Street, London SW3 6NP, UK

Emmanuel Kaye Building, Manresa Road, London SW3 6LR, UK

B. P. Madden MD MSC FRCPI FRCP

M. Stern MBCHB pho MRCP

Consultant Cardiothoracic and Transplant Physician,

Senior Lecturer, Department of Gene Therapy, National Heart

Cardiothoracic Unit, St. George's Hospital, Blackshaw Road,

and Lung Institute, Emmanuel Kaye Building, Manresa Road,

London SW17 OQT, UK

London SW3 6LR, UK

S. G. Marshall MD

S. Walters BSC MB FRCP FFPHM

Associate Professor, Department of Pediatrics, Children's

Senior Lecturer, Department of Public Health and

Hospital and Regional Medical Center 4800 Sand Point Way

Epidemiology, University of Birmingham Medical School,

N.E., P.O. Box 5371, Seattle WA 98105-0371, USA

Edgbaston, Birmingham B15 2TT, UK

A. G. Nicholson MD MRCPath

A. K. Webb FRCP

Consultant in Histopathology, Department of Pathology,

Consultant Physician, Clinical Director of Bradbury Cystic

Royal Brompton Hospital and Harefield NHS Trust, Sydney

Fibrosis Unit, North West Lung Centre, Wythenshawe

Street, London SW3 6NP, UK

Hospital, Southmoor Road, South Manchester M23 9LT, UK

S. Poole BSC SRD

B. A. Webber FCSP DSc(Hons)

Formerly Dietetics Department, Royal Brompton Hospital

Formerly Physiotherapy Department, Royal Brompton

and Harefield NHS Trust, Sydney Street, London SW3 6NP, UK

Hospital, Sydney Street, London SW3 6NP, UK

Contributors ix D. Westaby MA FRCP Consultant Physician and Gastroenterologist, Chelsea and Westminster Hospital, 369 Fulham Road, London SW10 9NH, UK

S. P. Wolfe BSC SRD Regional Paediatric Cystic Fibrosis Unit, St. James University Hospital, Leeds LS9 7TF, UK

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1 Epidemiology

1a Clinical epidemiology of cystic fibrosis

S. Walters Introduction Diagnosis Clinical features Social and demographic features in adults Prognosis Provision of medical care Treatment of cystic fibrosis

.

2 2 2 4 7

9 11 12

1b The epidemiology of cystic fibrosis P. A. Lewis Introduction Incidence of the cystic fibrosis phenotype (CFP) Life-span analyses for populations with CF phenotypes Assessing the effectiveness of healthcare uling international differences in life span The epidemiology of the CFTK gene Conclusion References

13. 13 15 17 20 21 22 22

1a Clinical epidemiology of cystic fibrosis S. WALTERS

INTRODUCTION Epidemiology may be defined as the study of the distribution and determinants of disease frequency in human populations1. Clinical epidemiology applies epidemiological principles to a clinical population, i.e. to a population already known to have a particular disease. An analogous definition might therefore be that clinical epidemiology is the study of the distribution of disease manifestations and determinants of disease outcome in a clinical population. Clinical epidemiology is the basic science underpinning the practice of evidence-based medicine. Therefore a knowledge of the clinical epidemiology of cystic fibrosis, and in particular a knowledge of risk factors for development of certain disease manifestations, and factors influencing overall prognosis, will be of value to all clinicians involved in the care of cystic fibrosis patients. This chapter examines and describes the clinical epidemiology of cystic fibrosis. It includes a description of clinical features and prognostic indicators, social and demographic features of the population, and finally some information about the application of clinical epidemiology to evidence-based practice in cystic fibrosis care. This chapter could not have been written without the high-quality information that has been collected over the years by both the United States and Canadian patient data registries.

DIAGNOSIS Although genotype analysis is readily available, rare mutations are so many and the phenotype so variable that diagnosis is usually made either on clinical grounds or on the basis of a screening program. In 1996, 6.2 per cent of newly diagnosed patients in the USA were diagnosed by neonatal screening2. Regardless of whether the case presents clinically or through screening, diagnosis is

confirmed by the sweat test; 98 per cent of patients in the USA had sweat sodium or chloride greater than 61 mEq/L, with a mean sweat chloride of 101.7 mEq/L (standard deviation 18.91 mEq/L). Of the 2 per cent who were sweat-test negative, 75 per cent were diagnosed on the basis of two known mutations, 1 per cent on transepithelial potential difference, and 24 per cent on clinical manifestation2. Age at diagnosis In the USA, 70 per cent of all cystic fibrosis (CF) patients were diagnosed before their first birthday2, and 90 per cent before their eighth birthday. The proportion diagnosed before 1 year was similar, although slightly lower, in series from New Zealand (61 per cent)3 and Ireland (55 per cent)4. However, late diagnosis continues to be made, with occasional cases being diagnosed as late as the seventh decade of life (Fig. 1.1). Late diagnosis (after 16 years of age) is associated with a milder clinical syndrome than seen in patients diagnosed as children5. This includes better lung function and nutritional status, and a lower prevalence of colonization by Pseudomonas aeruginosa. Late-diagnosed patients usually represent the mild end of the clinical spectrum presented by cystic fibrosis. This has very important implications when considering studies looking at the long-term benefits of neonatal screening for cystic fibrosis. Neonatal screening Intuitively, it seems reasonable to assume that early diagnosis of patients with cystic fibrosis before the onset of chronic bacterial colonization of the respiratory tract, and before the onset of clinical malnutrition, would lead to an improved clinical outcome. However, there is little good evidence that long-term outcome is improved as a result of neonatal screening.

Clinical epidemiology 3

Fig. 1.1

Age at diagnosis of cystic fibrosis. (Data from US CF Foundation Patient Data Registry, 1996.)

The reasons for this include the lack of suitable randomized studies, the majority employing either pseudorandomization, or using historical or geographic controls, or no controls at all. In those studies where randomization has been employed, there is a fundamental difficulty in interpretation of results because of the problem that those diagnosed late in life have milder disease. This means that the screened cohort will include the majority of patients, including those with very mild disease, whereas the control cohort will include, at least initially, only those with more severe disease. Therefore reported differences in clinical outcome might be explained by this bias. Finally, studies with randomized design have been let down by failure to analyze by intention-to-screen (i.e. analyze the groups as originally randomized); this usually means including those missed by screening (false negatives) in the group diagnosed clinically. This would tend to enhance clinical differences between the groups. In a pseudo-randomized study in the UK, which was not analyzed by intention-to-screen, there were few reported clinical benefits, the main one being reduced time in hospital in the screened cohort in the first year of life. However, this could be explained by the differential rates of inclusion of mild cases in the screened and unscreened cohorts under the age of 1 year6. Longerterm results analyzed by intention-to-screen are

required. The study with the most robust design and analysis comes from Wisconsin, USA. This study has recently reported results of 10-year follow-up, demonstrating small, but significant differences in favor of the screened cohort for nutritional parameters, especially in those with pancreatic insufficiency and homozygous for DF5087. It should be remembered that there are other benefits from running a screening program among neonates that have not been considered in published reports. This includes the ability to offer prenatal diagnosis to parents of infants detected by screening, the reduction of stress among parents seeking a diagnosis for their sick child, rapid determination of population prevalence in developing countries or areas where this is unknown, and the ability to research early development of respiratory and nutritional abnormalities in CF patients before clinical symptoms develop.

Clinical presentation The most common clinical presentation of CF remains acute or persistent respiratory symptoms, appearing in 51 per cent of all cases diagnosed in the USA2. Other common clinical features on presentation in this report were failure to thrive or malnutrition (43 per cent),

4 Epidemiology

Fig. 1.2 Clinical features suggesting the diagnosis of CF. These modes of clinical presentation are not mutually exclusive, and therefore the total is greater than 100 per cent. (Data from the CF Foundation US Clinical Patient Database, 1996.)

steatorrhea or abnormal stools (35 per cent), and meconium ileus or intestinal obstruction (19.1 per cent). Clinical presentations of US patients are summarized in Fig. 1.2. It is interesting to note that although overall only 3.5 per cent were diagnosed by screening (prenatal or neonatal), in 1996, this method of presentation comprised 9.1 per cent of newly diagnosed patients2. Genotype as a means of presentation was 1.5 per cent overall, but 5.8 per cent of newly diagnosed patients in 1996. This suggests that the number of patients with CF diagnosed before the onset of any clinical symptoms is increasing. Congenital bilateral absence of the vas deferens (CBAVD) is a clinical syndrome recently described in which there is a relatively high prevalence of CFTR (CF transmembrane conductance regulator) mutations, with 14.5 per cent being homozygous for CFTR mutations, 48.1 per cent being heterozygous, and 37.4 per cent having no CFTR abnormalities8. A high proportion of patients with CBAVD homozygous for CFTR mutations have abnormal sweat chloride9, and a few have some clinical symptoms suggestive of CF. The suggestion has been made that CBAVD patients who are homozygous for CFTR mutations represent the mildest form of CF.

CLINICAL FEATURES Lung function The US Patient Data Registry2 recorded that, in 1996, the mean forced expired volume (FEV,) was 72.3 per cent predicted (SD 27.5 per cent) and the mean FVC was 84.5

Fig. 1.3 Lung function (per cent predicted) in the US CF clinical population. (Data from US Patient Data Registry, 1996.)

per cent predicted (SD 23.3 per cent)2. The distribution of per cent predicted FEV, and FVC percentiles in the US CF clinical population is summarized in Fig. 1.3. FEV, and FVC by age are shown in Fig. 1.4. These diagrams are difficult to interpret because of age-cohort effects. They represent all CF patients in the US Patient Data Registry, and therefore consist of several different birth cohorts, not all of which were exposed to current treatment practices throughout their life. In addition, there is a survivor effect, meaning that in each cohort, lung function can only be measured in those remaining alive. As age increases, the proportion of each birth cohort that remains as survivors decreases, until those left become the extreme survivors of that birth cohort, and therefore very atypical of the original group. Difficulties with cohort and survivor effects mean that it is not possible to predict decline in lung function among a cohort of existing patients from these current lung-function measurements. However, Fig. 1.4 demonstrates the pattern that might be expected in a population of patients which develop a progressive predominantly obstructive respiratory function defect, followed by censoring of patients with a severe defect from the population either by death or transplantation. Increase in mean lung-function parameters due to censoring are offset by decline among the remaining members of the cohort. Various cohort studies of CF patients have demonstrated a lung-function decline in individual adult patients of approximately 3-5 per cent predicted per annum in FEV1. Figure 1.3 demonstrates that the majority of patients at any point in time have good lung function, with only a minority at any single point in time suffering from severe respiratory function defects. Table 1.1 shows that the majority of those with moderate to severe lung disease are in the older age groups. Lung function is related to other aspects of clinical

Clinical epidemiology 5

Fig. 1.4 Mean per cent predicted FFV1 by age in the US CF clinical population. (Data from the US Patient Data Registry, 1996.)

status in CF (Table 1.2). However, it is difficult to determine from cross-sectional data whether these complications of CF arose because of declining lung function or were the cause of it.

Respiratory infection The organism most frequently reported in sputum culture from cystic fibrosis patients is Pseudomonas aeruginosa. This is true in most reported case series. In the US, 60 per cent of patients had P. aeruginosa in their sputum or other respiratory cultures2. However, the prevalence of

P. aeruginosa infection varies between countries, and between treatment centers within countries. In Canada, for example, the isolation frequency for P. aeruginosa was 48 per cent in 1995, but varied between treatment centers from 25 per cent to 52 per cent11. The reported prevalence in New Zealand was 44 per cent3, and in adults in France was 62 per cent12 and in Ireland, 69 per cent4. Likewise, although the overall prevalence of Burkholderia cepacia infection was only slightly higher in Canada than in the US (9.2 per cent in Canada, 3.6 per cent in the US), the prevalence varied from 2 per cent to 21 per cent in different treatment centers within Canada. This is likely to represent both differences in the distribution of risk factors for colonization for these organisms, and differences in policies to limit cross-infection operating in the different treatment centers. The frequency of all organisms isolated in US patients in 1996 is shown in Fig. 1.5. Of the factors that affect infection rates with different microorganisms, one of the most important is age. The prevalence of culture-positivity for Pseudomonas aeruginosa, Burkholderia cepacia, Aspergillus spp. and mycobacteria increase with age. The prevalence of culture-positivity for Haemophilus influenzae falls with age. These patterns for the most frequently isolated organisms are shown in Fig. 1.6. Other suggested risk factors for colonization with Pseudomonas aeruginosa include genotype (certain genotypes are associated with lower colonization rates), sex13 (females may be colonized younger than males), pancreatic insufficient phenotype, and nosocomial

Table 1.1 Severity of lungfunction defect by age (data from US Patient Data Registry, 19962) Severity of FEV, defect

Normal (> 90% predicted) Mild (70-89% predicted) Moderate (40-69% predicted) Severe (T mutation in the CFTR gene. Hum. Mol. Genet., 1,281-282. 98. Ferec, C., Audrezet, M.P., Mercier, B. et al. (1992) Systematic screening for mutations in the cystic fibrosis gene: new implications for carrier detection. Nature Gew?f.,1,188-191. 99. Fanen, P., Ghanem, N., Vidaud, M. et al. (1992) Molecular characterisation of cystic fibrosis: 16 novel mutations identified analysis of the whole cystic fibrosis transmembrane conductance regulator (CFTR) coding regions and splice site junctions. Genomics, 13,770-776. 100. Gasparini, P., Nunes, V., Savoia, A. et al. The search for Southern European cystic fibrosis mutations: identification of two new mutations, four variants, and intronic sequences. Genomics, 10,193-200. 101. Farrall, M., Law, H.-Y., Rodeck,C.H.tf 0/.(1986) Firsttrimester prenatal diagnosis of cystic fibrosis with linked DNA probes. Lancet, i, 1402-1404. 102. Chehab, F.F., Johnson, J., Louie, E. et al. (1991) A dimorphic 4-bp repeat in the cystic fibrosis gene is in absolute linkage disequilibrium with the AF508 mutation: implications for prenatal diagnosis and mutation origin. Am.J. Hum. Genet., 48,223-226. 103. Tsui, L.-C. (1990) Population analysis of the major mutation in cystic fibrosis. Hum. Genet., 85,391-392. 104. Sereth, H., Shoshani, T., Bashan, N. and Kerem, H.-S. (1992) The selective advantage hypothesis for CF

46 Basic molecular genetics

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