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.     

Active cascade testing for carriers of cystic fibrosis gene.

OBJECTIVE--To examine the acceptability, practicability, efficiency, and application of active screening for carriers of the cystic fibrosis gene in the extended families of those in whom the disease is present (Cascade screening). DESIGN--Paediatricians and physicians provide details of their affected patients, pedigrees are drawn up, and relatives offered tests after initial contact by the affected nuclear families. Affected patients are genotyped in a laboratory with a special interest in the genetics of cystic fibrosis. SETTING--North Western health region. SUBJECTS--Relatives and partners of 607 people with cystic fibrosis. INTERVENTIONS--Genetic counselling by letter for people found to be carriers; formal genetic counselling and when indicated arrangements for prenatal diagnosis for couples discovered to be carriers. MAIN OUTCOME MEASURES--Number of carrier couples detected; action in pregnancy of detected carrier couples; extent of the uptake of screening by relatives. RESULTS--Of 1563 relatives or partners tested, 15 carrier couples were detected; of nine pregnancies undertaken by these 15, eight had prenatal tests and three terminated pregnancies. An average of 16 people per family have come forward for testing so far. CONCLUSIONS--Cascade screening for carriers of cystic fibrosis is well accepted by relatives, especially on the mother''s side of the family; it is 10 times more efficient in detecting carrier couples than unfocused screening. Detected carrier couples make practical use of the information in pregnancy. Active cascade screening for carriers is effective in cystic fibrosis and widespread application is recommended. These principles could be applied to other recessive disorders.     

Attitudes toward the prenatal diagnosis of cystic fibrosis: factors in decision making among affected families.

The objective of this study was to explore psychosocial factors underlying decisions about use of prenatal diagnosis for cystic fibrosis (CF), among parents of affected children. Anonymous survey questionnaires, supplemented by voluntary interviews, were used at 12 CF centers in six New England states, for a consecutive sample of families of minor children visiting CF centers during a 4-mo period. In all, 227 (71%) of 318 families responded. We hypothesized that attitudes toward utilization would be affected by (a) intentions to have children, (b) knowledge, (c) perception of risk, (d) the health of the child with CF, (e) expectations about the child's future, (f) attitudes toward abortion, (g) insurance, (h) genetic counseling, and (i) sociodemographic factors (including attendance at religious services). Of the 227 couples who responded, 69% were surgically sterile, over 45 years of age, widowed, or divorced, and 31% were at risk. Of 70 at-risk couples, 44% intended to have more children; of these, 77% had had or were considering CF prenatal diagnosis. Most families knew CF could be diagnosed prenatally; 20% would terminate for CF. Among intended prenatal diagnosis users, 44% would carry a fetus with CF to term, 28% would abort, and 28% were undecided. Stepwise logistic regression showed three variables significantly related to intentions to use prenatal diagnosis: (1) respondent's willingness to abort for CF (P less than .02, odds ratio 3.36), (2) respondent's siblings' approval of abortion for CF (P less than .03, odds ratio 2.99), and (3) respondent listed no accomplishments for the child with CF (P less than .09, odds ratio 3.01). The majority of affected families reject selective abortion for CF; many will curtail childbearing rather than use prenatal diagnosis.     

Screening for carriers of cystic fibrosis--a general practitioner's perspective.

The identification of the gene for cystic fibrosis has led to the possibility of population based screening for carriers of cystic fibrosis to identify couples at risk of having an affected child. Pilot studies have shown that screening is feasible and does not cause untoward anxiety, though the uptake of testing varies considerably with the setting and method of invitation. Screening offered at times when individuals (and health professionals) perceive it as directly relevant will probably gradually become established in the United Kingdom. This review examines the role of general practice in genetic carrier screening as exemplified by cystic fibrosis. General practice has a pivotal role from the beginning in providing individuals and couples with information, facilitating testing of patients'' relatives and of carriers identified by screening elsewhere (such as antenatal clinics), and offering testing in the context of reproduction. Screening for the cystic fibrosis gene will probably be followed by other genetic screening programmes.     

Heredity of progressive muscular dystrophy: Reflections on a case study

Abstract

Data on the prevalence of divorce and separation among parents of children with cystic fibrosis and other chronic diseases indicate that marital breakdown is no more prevalent among these couples than it is in a general population. For couples who attended genetic counseling clinics or had children with spina bifida or leukemia, the divorce rate is lower than the United States national average. For parents of children with cystic fibrosis, the divorce rate is the same as the national average. The high recurrence risk for cystic fibrosis may deter many parents from further reproduction. We speculate that the inability to plan more children may be the factor responsible for the higher prevalence of divorce among these parents compared to those of children with other chronic diseases.     

Risks of fetal cystic fibrosis based on linkage disequilibrium data

Summary First-trimester prenatal diagnosis of cystic fibrosis depends on tissues being available from a previous affected child for determination of the phase relationship of DNA markers. If no such tissues are available, it is possible to estimate the risks of a couple producing an affected child from the distribution of haplotypes showing linkage disequilibrium with the cystic fibrosis gene. We have calculated all the fetal risk subsets from the various parental haplotype combinations for the restriction fragment length polymorphisms identified by the KM. 19/PstI and XV-2c/TaqI systems. We conclude that only in a limited number of parental combinations are the fetal risks sufficiently high or sufficiently low to be used in prenatal diagnosis.     

Determination of haplotypes of DNA markers linked to the mucoviscidosis locus: detection of heterozygotes

Linked DNA probes have been used in three families presenting an affected child with cystic fibrosis. The strategy used for the determination of haplotypes associated with parental normal and mutated genes is presented as well as its application to the detection of cystic fibrosis carriers among healthy children.     

Reproductive patterns and thalassaemia major

This study investigated the effect of the diagnosis of transfusion-dependent homozygous beta-thalassemia on subsequent parental reproductive patterns in 44 families in New South Wales. The results indicate a shift over time from parental risk-taking (either consciously or in ignorance of the implications of the diagnosis) to premature curtailment of reproduction to the likelihood of attaining birth expectations through antenatal diagnosis. 67% of families with both of their 1st 2 children affected by thalassemia major had additional children, compared with 38% of those where the 1st child was affected and the 2nd child was unaffected and 37% of families where the 1st child was unaffected and the 2nd child was affected. Overall, 48% of mothers of children with thalassemia major had as many children as they had expected at the time of marriage, 13% had more, and 39% had fewer children. The mothers of older children were less likely to have had the planned number of births than those of children born more recently. The value of antenatal diagnosis to couples at risk of thalassemia appears to have been to enable them to meet or almost meet their birth expectations.     

Isolation of a further anonymous informative DNA sequence from chromosome seven closely linked to cystic fibrosis.

A library prepared from flow-sorted chromosomes was used to isolate single-copy sequences from chromosome seven. One such sequence 7C22 has been shown to be polymorphic for an EcoRI restriction site and to be informative for the study of CF in approximately 35% of matings. The segregation of the 7C22 alleles was followed through nineteen informative families with more than one child affected by cystic fibrosis. We report that the locus for 7C22 is linked to the locus for cystic fibrosis at a recombination fraction of 0.045. This marker will prove useful in improving the accuracy and informativeness of prenatal diagnosis and in constructing a fine genetic map around the cystic fibrosis gene.     

Use of linkage disequilibrium data in prenatal diagnosis of cystic fibrosis

Summary Parents at risk of bearing a child with cystic fibrosis, and who have no living affected child, often use prenatal diagnosis based on microvillar enzyme assay in second-trimester amniotic fluid samples. If enzyme levels are abnormal and the pregnancy is terminated, it is possible in principle to use the fetal tissues to establish the phase relationship of linked DNA markers for a subsequent first-trimester prenatal diagnosis. However, the probability of a fetus being affected after an abnormal microvillar enzyme test may be no greater than 80%. The strong linkage disequilibrium between haplotypes at the D7S23 locus and the cystic fibrosis gene may be used to increase this probability. If fetal tissues are homozygous for the 6.6-kb band defined by pKM.19 and PstI and also homozygous for the 2.1-kb band with pXV-2c and TaqI, the chance of being affected increases from 80% to between 95% and 97%. We regard this as being sufficiently certain for use in phase determination.     

Reproductive attitudes of couples having a child with cystic fibrosis in Saguenay-Lac-Saint-Jean (Quebec, Canada)

Cystic fibrosis (CF) has a high incidence (1/936 live births) and carrier rate (1/15 inhabitants) in Saguenay-Lac-Saint-Jean. One objective of a major enquiry among several subsets of individuals from this high-risk population for CF was to evaluate the reproductive behaviour of couples with a CF child attending the comprehensive CF clinic in Chicoutimi. The knowledge of the recurrence risk resulted in deciding against further progeny or in reducing the number of children. More reliable contraception methods after the birth of the CF child, but not prenatal diagnosis, were used. Although a minority of parents with a CF child would abort a CF foetus, they apparently started viewing pregnancy interruption for CF after prenatal diagnosis as an acceptable reproductive option.     

Cystic Fibrosis of the Pancreas—Diagnosis by Sodium Electrode Sweat Tests

One of the most difficult and unreproducible procedures in clinical laboratories has been the measurement of electrolytes in sweat. The iontophoresis techniques for the diagnosis of cystic fibrosis of the pancreas, which are widely used, are fraught with difficulties.Measurement of sweat electrolytes with sodium or chloride electrodes is gradually replacing the iontophoresis methods. A simple modification of the sodium electrode technique was used for diagnosis in 11 cystic fibrosis patients and in 260 normal children. The values obtained with the sodium electrode clearly separated the normal children from those with cystic fibrosis.     

Cystic fibrosis typing with DNA probes: experience of a screening laboratory

Summary A sample of 125 individuals from 37 British cystic fibrosis (CF) families with at least one living affected child were typed with probes for restriction fragment length polymorphisms (RFLPs) known to be linked to the CF gene. These probes were MetD, MetH, pJ3.11 and 7C22. Using this combination of probes, 30 out of the 37 families were sufficiently informative to enable prenatal diagnosis of the disease. Linkage analysis has also proved to be useful in excluding CF in two cases where diagnosis of the disease was equivocal in the sibling of an affected child.     

Gène CFTR et infertilité masculine en 2001 Conseil génétique, diagnostic prénatal, diagnostic pré-implantatoire

As the vas deferens is also absent in the majority of CF (cystic fibrosis) males, it has been proposed that CBAVD (Congenital Bilateral Absence of Vas Deferens) males may present an incomplete or mild form of CF. Many studies using more extensive mutation analysis have confirmed the role of CFTR gene defects: 80% of CBAVD patients carry one or two mutations. Each patient with a diagnosis of CBAVD should also be examined for pulmonary and pancreatic signs, and sweat tests should be performed. In couples with CBAVD linked to CFTR mutations, the risk of having children with CF or infertility is increased if the female is also a carrier. The woman should be screened for the most frequent CFTR mutations according to her ethnic background. After screening for 80% of the mutations responsible for CF, the residual risk of being a carrier with negative screening is: Z=h(1?a)/(1?ah)=1/120 considering a carrier frequency of 1/25 in the general population. In the case of positive screening, antenatal diagnosis by chorionic villus sampling may be proposed. However, in some situations it is difficult to predict the phenotypic consequences for the child, particularly when a severe transmutation of a variable allele is identified. As these couples require medically-assisted reproduction techniques, pre-implantation genetic diagnosis appears to be more appropriate than antenatal diagnosis. Only embryos that inherit the non-mutated maternal CFTR allele are replaced in the uterus. Examination of childre born to couples with CBAVD is mandatory: immunoreactive trypsinogen assay at 3 days of age, sweat test at 3 months and clinical examination, especially looking for signs of CF. Identification of CFTR mutations in a CBAVD patient has important consequences for his family. Each sibling has a 50% risk of being a carrier and a 25% risk of inheriting the same genotype. The genetic counsellor must inform these siblings about the possible risk of having CF children if they carry CFTR mutations and if their partner is also a carrier.     

Social Effects of Genetic Counselling

In a follow-up study of 104 subjects referred for genetic counselling between 1965 and 1969 all were at risk of having children with a variety of serious genetic disorders. Most subjects were in social classes III and IV, were married, in their late 20s, and already had an affected child. Sixty-three per cent. were referred by hospital consultants, 27% by their general practitioners, and 10% were self-referrals. All of those counselled appeared to have appreciated the genetic implications, although four overestimated the risks and 11 underestimated the risks.Of those at high risk (greater than 1 in 10) of having an affected child 10 out of 55 couples “planned” further pregnancies despite the risks. In two this was because they had been unable to adopt a child, in four because they had no living children and the disorders in question usually resulted in stillbirth or death in infancy so that the “burden” of an affected child would be of relatively short duration, and one mother had had antenatal diagnosis and selective abortion. Most of the couples in the low-risk group (less than 1 in 20) were reassured and planned further pregnancies.     

Cystic fibrosis carrier population screening in the primary care setting.

To determine the receptivity of prenatal care providers and their patients to carrier testing for cystic fibrosis (CF), we offered free carrier screening, followed by genetic counseling of carriers, to all prenatal care providers in Rochester, NY, for all their female patients of reproductive age, pregnant or not. Of 124 prenatal care providers, only 37 elected to participate, but many of these offered screening only to pregnant women. The acceptance rate among pregnant women was approximately 57%. The most common reasons for accepting screening were to obtain reassurance (50.7%) and to avoid having a child with CF (27.8 %). The most common reasons for declining screening were not intending to terminate a pregnancy for CF (32.4%) and believing that the chance of having a CF child was very low (32.2%). Compared with decliners, acceptors were more likely to have no children, regarded having a child with CF as more serious, believed themselves more susceptible to having such a child, knew more about CF, would be more likely to terminate a pregnancy if the fetus were shown to have CF, and more strongly supported offering CF screening to women of reproductive age. Of 4,879 women on whom results were obtained, 124 were found to be carriers. Of these 124 carriers, the partners of 106 were tested. Of the five at-risk couples, four requested prenatal diagnosis and one requested neonatal diagnosis. No woman found to be a carrier whose partner tested negative requested prenatal diagnosis. Except for the imperfect knowledge of those testing negative, none of the adverse outcomes predicted for CF carrier testing in the general population were observed in this study.     

Genetic screening for the next decade: application of present and new technologies.

Molecular genetic technology is diffusing from the research laboratory to the clinical laboratory, where it has already begun to influence prenatal diagnosis and counseling. In the very near future, this technology will be applied more generally, using population-based screening strategies. Pilot programs are beginning to evaluate the technical feasibility and efficacy of recombinant DNA techniques for newborn screening follow-up. DNA-based population screening is being considered for heterozygous carriers of an autosomal recessive disorder such as cystic fibrosis in order to identify carrier couples at risk of having an affected child. We will review the current DNA methodologies in the context of three genetic disorders: sickle-cell disease, Duchenne muscular dystrophy, and cystic fibrosis. We will then consider the requirements for implementation of these new technologies. We will conclude that implementation will require two key factors: machines and people. Machines are required to automate molecular genetic procedures, which are currently personnel-intensive, so that the expense can be reduced and the procedures made more cost-effective. The people who are required are health professionals knowledgeable in the clinical aspects of the target disorders, as well as in the DNA laboratory testing. These professionals will be able to facilitate sample acquisition and information exchange among the laboratory, the primary health care provider, and the families requesting consultation.     

An Algerian child homozygous for the M470V polymorphism and for a deletion of two nucleotides in exon 10 of the CFTR gene, shows severe cystic fibrosis symptoms.

When screening for the presence of major cystic fibrosis mutations in Algerian cystic fibrosis families by heteroduplex formation, aberrant heteroduplexes were observed for exon 10 in one family. Here we describe the clinical and molecular findings in a severely affected child of this family, homozygous for the 1609delCA and for the M470V polymorphism.     

Homogeneity of cystic fibrosis in Italy.

In 12 unrelated Italian cystic fibrosis (CF) families the frequencies of four DNA polymorphisms closely linked to the CF gene on chromosome 7 were quite similar to those reported for other population samples. Among the 23 affected children from the 12 families, only one recombinant occurred between the CF gene and the met locus, thus confirming the hypothesis of genetic homogeneity of CF previously suggested by the analysis of consanguineous marriages among 624 couples of CF parents. Chi-square test of association indicates a possible linkage disequilibrium between the CF gene and the DNA polymorphism that is most informative in our sample (pmetH TaqI).     

Neonatal screening for cystic fibrosis using immunoreactive trypsinogen and direct gene analysis: four years' experience.

OBJECTIVE--To assess the performance and impact of a two tier neonatal screening programme for cystic fibrosis based on an initial estimation of immunoreactive trypsinogen followed by direct gene analysis. DESIGN--Four year prospective study of two tier screening strategy. First tier: immunoreactive trypsinogen measured in dried blood spot samples from neonates aged 3-5 days. Second tier: direct gene analysis of cystic fibrosis mutations (delta F508, delta I506, G551D, G542X, and R553X) in samples with immunoreactive trypsinogen concentrations in highest 1% and in all neonates with meconium ileus or family history of cystic fibrosis. SETTING--South Australian Neonatal Screening Programme, Adelaide. SUBJECTS--All 88,752 neonates born in South Australia between December 1989 and December 1993. INTERVENTIONS--Neonates with two identifiable mutations were referred directly for clinical assessment and confirmatory sweat test; infants with only one identifiable mutation were recalled for sweat test at age 3-4 weeks. Parents of neonates identified as carriers of cystic fibrosis mutation were counselled and offered genetic testing. MAIN OUTCOME MEASURES--Identification of all children with cystic fibrosis in the screened population. RESULTS--Of 1004 (1.13%) neonates with immunoreactive trypsinogen > or = 99th centile, 912 (90.8%) had no identifiable mutation. 23 neonates were homozygotes or compound heterozygotes; 69 carried one identifiable mutation, of whom six had positive sweat tests. Median age at clinical assessment for the 29 neonates with cystic fibrosis was 3 weeks; six had meconium ileus and two had affected siblings. 63 neonates were identified as carriers of a cystic fibrosis mutation. Extra laboratory costs for measuring immunoreactive trypsinogen and direct gene analysis were $A1.50 per neonate screened. CONCLUSION--This strategy results in early and accurate diagnosis of cystic fibrosis and performs better than screening strategies based on immunoreactive trypsinogen measurement alone.