P67L: a cystic fibrosis allele with mild effects found at high frequency in the Scottish population

Only three mutant cystic fibrosis (CF) alleles have to date been established as conferring a dominant mild effect on affected subjects who are compound heterozygotes. We now add a fourth, P67L, which occurs on about 1.4% of Scottish CF chromosomes. Among 13 patients (12 unrelated) with this allele, the average age at diagnosis was 22.5 +/- 11.3 years. None of the cases had consistently raised sweat chloride concentrations, the average value being 57 +/- 9 mmol/l; 77% of the patients were pancreatic sufficient. When compared to three other established mild CF alleles, R117H, A455E, and 3849 + 10kb C-T, a compound heterozygote for P67L has minimal disease and clinical suspicions are unlikely to be confirmed other than by DNA typing.

The incidence of cystic fibrosis in Scotland calculated from heterozygote frequencies

The incidence of cystic fibrosis (CF) has previously been calculated from epidemiological surveys and from neonatal screening. With the cloning of the CF gene it has become possible to derive incidence figures from heterozygote frequencies, provided that the distribution of mutant alleles among healthy carriers is the same as among affected people. We have estimated the allele frequencies for four CF mutations, AF508, G551D, G542X and R117H, in 14360 unselected women undergoing antenatal heterozygote screening. The proportion of R117H, an allele of known mild effect, was much greater for heterozygotes than for homozygotes. The incidence of CF was therefore calculated from the heterozygote frequencies of AF508, G551D and G542X in a larger cohort of 27 161 successively screened women. The point estimate for the incidence of CF in the Scottish population was 1 in 1984, with 95% confidence intervals of 1 in 1692 to 1 in 2336.

Antenatal screening for cystic fibrosis: a trial of the couple model

OBJECTIVE

To assess the delivery and acceptability of antenatal couple screening for cystic fibrosis. Carrier status was notified only when both members of a partnership had cystic fibrosis alleles and therefore a one in four risk of having an affected child.

DESIGN

Mouthwash samples were tested when both partners participated. Results were returned only to positive couples.

SETTING

Two large maternity hospitals in Edinburgh.

SUBJECTS

Screening was offered to all couples who booked at one of the two hospitals.

MAIN OUTCOME MEASURES

(a) The take up of screening, carriers and carrier couples identified, take up of prenatal diagnosis, and numbers of affected fetuses detected; (b) questionnaire measures of patient satisfaction and stress.

RESULTS

Screening was offered to 8536 couples. 714 (8.4%) were regarded as ineligible, usually because of late booking or absence of a partner. 1900 (24.3%) of the remainder declined screening. Among the 5922 screened couples, four tested positive--that is, both partners were cystic fibrosis heterozygotes. All four elected to have prenatal diagnosis. There were three terminations of pregnancy because of an affected fetus, one couple having two successive pregnancies with affected fetuses. The participation rate was 76% for eligible couples (5922/7822) and 69% for all couples (5922/8536). Only 89 screened couples (1.5%) requested information on individual carrier status. No anxiety was detected among a cohort of the screened population, and 99% of questioned participants expressed satisfaction with the concept of couple screening.

CONCLUSIONS

Antenatal couple screening is a satisfactory and acceptable way of screening for cystic fibrosis and has been adopted as routine in the two trial hospitals.

Mass screening for cystic fibrosis heterozygotes: two assay systems compared

Two different assay systems were used to detect mutant cystic fibrosis alleles in mouthwash samples from pregnant women attending antenatal clinics. In the first phase of the study, comprising 3110 consecutive samples, we compared the performance of an in-house system and a commercial amplification refractory mutation system (ARMS). Successful analyses were completed at first attempt in approximately 90% of samples, and the patient resampling rate was 1.1%. There were no false positives and only one detected false negative. In the second phase, comprising 3333 consecutive samples, only ARMS was used. Again, there were no false positives and the patient resampling rate dropped to 0.5%. On the basis of convenience and reduced operator time, we have elected to use ARMS for further screening for cystic fibrosis heterozygotes.

Mass screening for cystic fibrosis heterozygotes: two assay systems compared

Two different assay systems were used to detect mutant cystic fibrosis alleles in mouthwash samples from pregnant women attending antenatal clinics. In the first phase of the study, comprising 3110 consecutive samples, we compared the performance of an in-house system and a commercial amplification refractory mutation system (ARMS). Successful analyses were completed at first attempt in approximately 90% of samples, and the patient resampling rate was 1.1%. There were no false positives and only one detected false negative. In the second phase, comprising 3333 consecutive samples, only ARMS was used. Again, there were no false positives and the patient resampling rate dropped to 0.5%. On the basis of convenience and reduced operator time, we have elected to use ARMS for further screening for cystic fibrosis heterozygotes.

Prenatal cystic fibrosis carrier screening: factors in a woman's decision to decline testing

Among 2207 women eligible to be screened for cystic fibrosis (CF) carrier status during pregnancy, 325 (15 per cent) declined to be tested. Of these, 260 (80 per cent) answered a questionnaire soliciting their reasons for not participating. The main factor was opposition to termination of pregnancy, with 43 per cent being against termination for any reason and another 11 per cent against termination of a CF fetus. Other reasons given were partner's disapproval or non-participation (10 per cent), perceived risk of a CF child being low (7 per cent), the error rate of the test (6 per cent), and the generation of unacceptable levels of anxiety (5 per cent). Eleven women (4 per cent) said that they did not wish to be tested during pregnancy, but only six of these would have accepted screening at another time.

Prenatal screening for cystic fibrosis: attitudes and responses of participants

A screening programme to detect cystic fibrosis heterozygotes has been running in the antenatal clinics of a major Edinburgh maternity hospital for more than 2 years. A questionnaire was used to assess participants' knowledge of the genetics of the disorder and their attitudes to being screened. The respondents were 64 female heterozygotes and 63 of their non-heterozygous male partners, 101 female controls and 100 male controls. Although the two groups of controls received far less direct information than the carriers and their partners, all four groups were well informed about the genetics of cystic fibrosis and the significance of being a gene carrier. A majority of each group felt that adequate information had been given in the information leaflet, that they understood the purpose of screening and that they were glad to have participated. There was a consensus that CF carrier testing should be routinely offered to pregnant women, and also that it should be available in family planning clinics and GP health centres, but not in schools.

Prenatal screening for cystic fibrosis: psychological effects on carriers and their partners

This study aimed to assess the psychological impact of screening for cystic fibrosis (CF) carrier status in a population of pregnant women. A cohort of 1798 women, who accepted the offer of testing before 18 weeks of pregnancy, filled in a self administered questionnaire seeking information on their perceived risk of carrier status and their emotional response, as well as a general health questionnaire (GHQ). Sixty-four women identified as CF carriers had partners who received a negative test result. This group and their partners were assessed, together with selected controls, on four further occasions: (1) on receiving the carrier's positive test result; (2) on receiving the partner's negative test result; (3) six weeks later; (4) six weeks after delivery. The instruments used were the GHQ and the Symptom Rating Test (SRT). When compared to control subjects, carriers showed a significant increase in generalised psychological disturbance which could be attributed specifically to symptoms of anxiety and depression during the period (average four days) that they awaited their partner's test result. On receiving a partner's negative test result, the carriers returned to control levels and maintained this equilibrium. Although there was no significant difference in generalised psychological disturbance between partners and their selected controls, partners did become significantly more anxious and manifested feelings of inadequacy while awaiting their own test result. Both male partners and male control subjects were more likely to become anxious if their partner was distressed.

Purification and characterization of human recombinant IgE-Fc fragments that bind to the human high affinity IgE receptor

The Fc-region of immunoglobulin E (IgE) comprising C epsilon 2, C epsilon 3, and C epsilon 4 domains is sufficient for binding to the alpha chain of the high affinity IgE-Fc receptor (Fc epsilon RI alpha). In order to identify the smallest Fc fragment capable of binding to the Fc epsilon RI alpha with high affinity, various regions of the IgE-Fc molecule were expressed in COS cells and investigated for their ability to bind Fc epsilon RI alpha. The smallest fragment that showed Fc epsilon RI alpha binding activity spans amino acids 329-547 and lacks the entire C epsilon 2 domain. Two active fragments, viz. Fc epsilon(315-547) (containing Cys328 which is responsible for interchain S-S bonding) and Fc epsilon(329-547), have been overexpressed in CHO cells and purified to homogeneity. The purified proteins bind to the Fc epsilon RI alpha with high affinity, similar to native IgE. SDS-polyacrylamide gel electrophoresis analyses indicate that Fc epsilon(315-547) is an S-S-linked dimer of apparent molecular mass of 68 kDa. Fc epsilon(329-547) appears on SDS-gel as three distinct bands at approximately 32 kDa, both under reducing and nonreducing conditions. However, size exclusion chromatography and analytical ultracentrifugation studies suggest that Fc epsilon(329-547) also remains associated as a dimer. The presence of N-linked glycosylation was detected in both proteins. The deglycosylated form of Fc epsilon(315-547) was isolated after Endo F/N-glycosidase F digestion and demonstrated to have binding activity comparable to that of the mock-digested protein. These results suggest that the presence of N-linked sugars is not necessary for Fc epsilon RI alpha binding. Both proteins blocked the release of histamine from RBL cells expressing human Fc epsilon RI alpha in a dose-dependent manner. The availability of these recombinant IgE-Fc proteins will be helpful in elucidating the key epitopes essential for the binding of IgE to its high affinity receptor.

A preliminary trial of couple screening for cystic fibrosis: designing an appropriate information leaflet

An information leaflet, inviting participation in an antenatal screening trial for cystic fibrosis, was sent to 388 couples together with the pregnant woman's first clinic appointment. The leaflet pointed out that couples would be treated as a unit and that further action would be taken only if both partners were found to carry mutant alleles. Participants and non-participants were also asked to fill in a questionnaire eliciting their views on the leaflet. Three hundred and twelve (80%) questionnaires were returned and 253 (65%) couples elected to be screened. More than 90% of respondents found the leaflet easy to understand, although about 10% wanted more information on cystic fibrosis. The main reason for entering the trial was to avoid the birth of an affected child, and the main reason for non-entry was opposition to termination of pregnancy. There was little anxiety about the prospect of being screened. However, more than a third of couples mis-identified their risk of both carrying a CF gene, despite the figure of 1 in 600 being explicitly stated in the leaflet.

Prenatal screening for cystic fibrosis

Screening for carriers of CF (cystic fibrosis) is now possible but the best way of delivering such a service is unknown. In one model 4348 women attending antenatal clinics in an Edinburgh maternity hospital were invited to participate in a trial of prenatal screening. Mouthwash samples were tested for six CF alleles (85% of mutant genes) and when a woman was found to be a CF carrier her partner was also tested. Heterozygous couples were offered prenatal diagnosis. 609 (14%) women declined to enter the trial and another 574 (13%) were not screened, usually because of late booking. Among the remaining 3165 women there were 111 carriers of a CF gene (1 in 29). 4 of these 111 had carrier partners and these couples opted for prenatal diagnosis, the 1 pregnancy with an affected fetus being terminated. The psychological impact of screening was assessed by the general health questionnaire. There was a significant increase in stress at the time of the test result among women identified as carriers. However, this disappeared when their male partners tested normal and did not reappear later in the pregnancy. By providing time for couples to discuss the possibility of screening and by offering the test at a point (the antenatal booking clinic) at which most pregnant women are seen, this approach has advantages, provided that counselling is readily available.