the deletion; 84% of the 27 CF chromosomes in phase with an XV2c/KM 19 haplotype belong to the 1/2 haplo- type and have the AF508 mutation. The observed ...
407 the deletion; 84% of the 27 CF chromosomes in phase with an XV2c/KM 19 haplotype belong to the 1/2 haplotype and have the AF508 mutation. The observed strong association between the AF mutation and the KM19 allele 2 is obviously mainly responsible for the prevalence of the XV2c/KM19 1/2 haplotype in the deletion cases. The difference in association between the XV2c and KM19 RFLP alleles and AF is probably a result of the closer localization of KM19 to the CF gene in comparison with the XV2c probe. It remains to be seen if the somewhat lower incidence of the AF mutation in the CF chromosomes analysed in this study, when compared with the original data of Kerem et al. (1989b), represents a true biological difference or an effect of chance. The seriousness of the disease, the relatively high incidence and ease of detection of the AF mutation, and the foreseeable discovery and diagnostic use of other mutations causing CF call for strategies and pilot studies to investigate the acceptance by the public of heterozygote screening systems for CF in different countries.
quency of AF508. We analyzed differences in the ages at which the disease is diagnosed between patients who are homozygous for AF508 and the other patients. We tested 190 CF chromosomes for the presence or absence of the most common mutation AF508. We studied the chromosomes from 93 patients with non-consanguineous parents. One hundred and seventy eight chromosomes were derived from 89 unrelated patients and 6 chromosomes were from 2 pairs of affected first cousins. The remaining 6 chromosomes were from unrelated obligate carriers. All individuals were of Dutch origin. The patients' ages at the time of initial DNA analysis ranged between 6 months and 33 years (mean 9, median 12 years). In the families with multiple affected offspring, we included the age of the first diagnosed patient in this calculation. The methods used for DNA analysis are described in the Summary Table at the end of the study. Wilcoxon's rank sum test was applied to compare the distributions of the ages at diagnosis between AF508 homozygotes and other patients.
Results and discussion The mutation AF508 o n D u t c h cystic f i b r o s i s c h r o m o s o m e s : f r e q u e n c y a n d r e l a t i o n to p a t i e n t s a g e at d i a g n o s i s D. J. J. Halley 1, H.J. Veeze 2, L. A. Sandkuyl ~, E. Wesby-van Swaay 1, N. H. M. van Damme ~, W. H. D e e l e n 1, J. E. Witte,1 and M. F. Niermeijer 1 1Department of Clinical Genetics, Erasmus University, P.O. Box 1738, NL-3000 DR Rotterdam, The Netherlands 2Department of Pediatrics, SubdivisionGastroenterology, Sophia Children'sHospital, P.O. Box 70029, NL-3000 LL Rotterdam, The Netherlands
Summary. We tested 190 chromosomes from Dutch cystic fibrosis (CF) patients and carriers for the presence or absence of the major CF mutation AF508. This mutation was found on 77% of the Dutch CF chromosomes. We observed a significant difference in the distribution of the ages at diagnosis between homozygotes for AF508 and the other patients. AF508 homozygotes tend to be identified as patients at neonatal or infantile age. The age at diagnosis of patients with at least one unknown allele, on the other hand, ranged between neonatal and young adult age.
The overall frequency of AF508 in our sample of 190 Dutch CF chromosomes was 77.4% (Table 1). This implies that 60% of Dutch CF patients are homozygous for AF508. An equal proportion of prenatal diagnoses in pregnancies at a 1 in 4 risk may now be made by direct mutation analysis (Halley et al. 1989; McIntosh et al. 1989b; Scheffer et al. 1989). The obvious advantages of mutation analysis are speed and reliability. A major restriction of indirect analysis is the requirement of DNA from an index patient. Prenatal diagnosis is now also accessible to 60% of the (Dutch) parents with a deceased affected child in the absence of DNA to establish linkage phase. Mutation analysis can detect 77% of CF carriers among Dutch individuals with no family history of CF. The majority of the chromosomes were haplotyped for the XV2c/TaqI and KM19/PstI polymorphisms (Table 1).
Table 1. Frequencyof AF508on Dutch CF chromosomes. 1, Larger allele; 2, smaller allele; [ ], phase unknown AF508
Other mutation
3 123 1
8 14 6 2
-
1 1 1 3
3 4
Introduction and methods We examined whether the higher frequency of the "high risk" haplotype on Dutch cystic fibrosis (CF) chromosomes (Maciejko et al. 1989) as compared with the data of Kerem et al. (1989b) is accompanied by a higher fre-
2 11 Total
147 (77.4%)
-
7 43 (22.6%)
Haplotype [XV2c, KM19] 11 12 21 22 [1 1 []2 1[] 2[] [][1 Not done
408
Table 2. Age at diagnosis for AF508 homozygotes and other CF patients AF508 homozygotes < 1month 1- 2months 2-12months 1- 5 years 6-10 years 11-15 years 16-20 years Total
Others a
8 2 4 9 0 0 0
2 0 4 2 4 2 3
23
17
a Fourteen patients had one AF508 allele, three had two unknown mutations In agreement with the report by Kerem et al. (1989b), AF508 was found almost exclusively against the background of the [XV2c] 1, [KM19] 2 haplotype. The same haplotype was also present on about half of the phaseknown chromosomes carrying mutations other than AF508 (14/30, Table 1). A 1 2 haplotype frequency of 87% was calculated for the present sample of Dutch CF chromosomes (data not shown). A parameter that may serve as an indicator for the overall clinical severity is the age at diagnosis. So far, we have been able to obtain reliable information on the age at diagnosis of 40 patients. All but 3 of these patients were diagnosed in one clinical center, the Sophia Children's Hospital, Rotterdam. Twenty three patients were homozygous for the AF508 mutation. Fourteen cases had been diagnosed as CF patients during the first year of life and 10 of these diagnoses were made within 2 months after birth but not through neonatal screening (Table 2). No patient from this group had been diagnosed after the age of 4. The other category consisted of 14 compound heterozygotes with one AF508 allele and 3 patients with two unknown alleles. More than 50% of the patients from the latter category had been identified after 6 years of age (Table 2). We examined whether there is a significant difference in the distributions of the ages at diagnosis between the AF508 homozygotes and the other patients using Wilcoxon's rank sum test. This showed a significant difference ( T = 3.03, P < 0.001). We are currently supplementing the data on the age of diagnosis with specific clinical and physiological features.
Acknowledgements. We thank the patients and their relatives who made their DNA available for analysis. We gratefully acknowledge the help and support of Professor H. Neijens, Dr. M. Sinaasappel (Sophia Children's Hospital), Dr. B.A. Oostra (Department of Cell Biology and Genetics), Dr. F. J. Los and Professor H. Galjaard (Department of Clinical Genetics). This work was supported in part by the Dutch Liver Gut Foundation (H.J.V.).
M u t a t i o n analysis at the cystic fibrosis locus in the British p o p u l a t i o n Ann Harris, Frances Beards, and Christopher Mathew Paediatric Research Unit, Division of Medical and Molecular Genetics, United Medical and Dental Schools of Guy's and St. Thomas's Hospitals, 8th Floor Guy's Hospital Tower, London Bridge, London SE1 9RT, UK
Summary. The cystic fibrosis (CF) population of South East England seems to reflect a similar distribution in frequency of the AF508 mutation and its haplotype association as has been observed in the North American population.
Introduction and methods The British Caucasian population resident in South East England has undergone extensive genetic analysis at the cystic fibrosis (CF) locus (Harris et al. 1988; Harris 1990). Following the isolation of the CF gene and definition of the major disease-causing mutation (Rommens et al. 1989; Riordan et al. 1989; Kerem et al. 1989b), this population in South East England has been analysed further. The methods used for D N A analysis are described in the Summary Table at the end of the study.
Results and discussion Information on the presence or absence of the AF508 mutation in each CF gene has been correlated with the haplotypes for the closely linked D N A markers XV2c and KM19 on the same chromosome. It has been known for some time that these two markers are in strong linkage disequilibrium with the CF locus (Estivill et al. 1987a, b). In our population, 86% of CF chromosomes carry the B haplotype that has allele 1 for XV2c in combination with allele 2 for KM19 (Table 1). Of these B haplotype chromosomes, 82% (102/124) carry the AF508 deletion
Table 1. Haplotypes for XV2c and KM19 in CF carriers Haplotype XV2c/KM19
CF (%)
Non-CF (%)
Total
A
1 1 1 2
C
2 1
D
2 2
55 (37.9) 24 (16.6) 49 (33.8) 17 (11.7)
60
B
5 (3.4) 126 (86.3) 9 (6.2) 6 (4.1) 146
145
150 58 23 291
