The application of linkage analysis to genetic counselling in families with Duchenne or Becker muscular dystrophy

Service provision of complex mutation analysis: a technical and economic appraisal using dystrophin point mutation analysis as an example

Duchenne muscular dystrophy (DMD) results from mutations in the dystrophin gene. One-third of cases arise from point mutations, which are heterogeneous and difficult to detect. The aims of this study of dystrophin point mutation analysis were to assess its technical feasibility in a routine diagnostic laboratory, and to estimate its costs and clinical benefits. The methods used were a laboratory based study using reverse transcription-polymerase chain reaction (RT-PCR) and a protein truncation test, and a mathematical model to estimate costs and clinical benefits. None of the cases analyzed had an identifiable dystrophin deletion or duplication. They were 12 males affected with DMD and two obligate female carriers; two female carriers of known dystrophin point mutations were also analyzed. Point mutations were detected in six out of 12 males, but in none of the female carriers. Assuming a sensitivity of 50% the model predicts significant clinical benefits of point mutation analysis over linkage analysis, including a reduction in the number of prenatal diagnoses (by 0.77 per family), terminations of pregnancy (by 0.18 per family), and terminations of unaffected fetuses (by 0.16 per family). The mean cost of point mutation analysis to prevent the termination of an unaffected fetus is 6220 US dollars.

Use of dystrophin genomic and cDNA probes for solving difficulties in carrier detection and prenatal diagnosis of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) results from mutations in the X-linked gene coding for the muscular protein dystrophin. The isolation of genomic and cDNA probes for this gene has greatly facilitated the detection of DMD carriers, which previously relied mainly on measurements of serum creatine kinase (CK), and has enabled prenatal diagnosis of this disease. However, the relatively large size of the gene and the high frequency of recombination and mutation events within the dystrophin locus continue to pose difficulties in the genetic counselling and prenatal diagnosis of DMD, and render the conclusions of molecular analysis less clear cut. This communication presents examples of two such difficulties: the distinction between sporadic and inherited cases in families with a single patient and normal CK levels in all females, and the distinction between mutant and normal dystrophin alleles in families in which the patients have died. The combined use of genomic and cDNA probes allows one to make these distinctions. An additional complicating factor, gonadal mosaicism, is demonstrated.

Research studies in applied human genetics: a quantitative analysis and critical review of recent literature

To determine the extent to which speculations about the social, legal, and ethical implications of genetic counseling, screening, and prenatal diagnosis are being studied empirically, the substantive contents of major genetics (N = 5) and obstetrics journals (N = 2) from the years 1985-1989 were reviewed. Among the approximately 9,000 articles published, only 58 containing relevant substantive data could be identified. Data collected in a single study were reported in more than one article in at least ten cases so that these articles actually represent only 45 distinct studies. Most described investigations of the attitudes and reactions of individuals or couples who had had or been referred for genetic counseling or prenatal diagnosis. These observational studies generally employed study-specific questionnaires, many of which were apparently self-administered by respondents, to obtain data. This survey and analysis of the recent literature suggests that despite frequent editorials and other commentaries underlining the problematic nature of developments in medical genetics and calling for their investigation, the "gate-keepers" to this service continue to pay scant attention to these issues in their reported research. Innovative and interdisciplinary studies that will provide information to close the many gaps in our understanding of the consequences of developments in applied human genetics are recommended for the future.

Dystrophin-related muscular dystrophies

The gene for the locus involved in Duchenne and Becker muscular dystrophies has been cloned and subject to intense analysis. The protein product of the locus is called dystrophin, and it has been shown to be associated with the muscle fiber membrane. The new knowledge of the molecular genetics of these disorders is being applied rapidly in clinical practice. Carrier detection and prenatal diagnosis have been revolutionized by the use of probes for the gene. These probes are also being employed to clarify cases where conventional clinical examination results in equivocal diagnoses. It is suggested that the disorders characterized by dystrophin abnormalities should be called dystrophin-related muscular dystrophies (DRMD). There are mouse and dog models for DRMD and these are being used to explore therapeutic strategies for treating DRMD patients.

Duchenne muscular dystrophy in Wales: impact of DNA linkage analysis and cDNA deletion screening

A register of families with Duchenne muscular dystrophy (DMD) has been maintained in Wales since 1973. Since 1986 we have attempted to refine carrier status, and when necessary offer prenatal diagnosis, for those at significant risk by using intragenic probes. cDNA probes were included from the beginning of 1988. Thirty-four (30%) of the 115 women tested were assigned a risk of carrying the DMD gene of less than 5%. Thirty-three (29%) of the women at 5% or greater risk are now able to have prenatal diagnosis using a molecular deletion; such deletions were detected in 50% of affected boys. The remaining women could have prenatal diagnosis using a linked intragenic probe with an error rate varying between 0.25% and 9%. In 19 cases DNA samples from DMD boys who were dead at the time of analysis were used, indicating that it is essential to bank DNA from all males affected by DMD. We conclude that a large proportion of women at risk of carrying the DMD gene can now be helped by DNA studies.

Manifesting carrier of Becker muscular dystrophy (BMD): clinical and recombinant DNA studies

We studied a Becker muscular dystrophy (BMD) family with a manifesting carrier. Proximal muscle weakness, pseudohypertrophy of the calves, significantly elevated serum creatine kinase and dystrophic alterations in the muscle biopsy were the characteristic phenotypical features of this manifesting carrier. The recombinant DNA study showed a recombinant chromosome with a crossover between pERT 87-8 and pERT J-Bir in the manifesting carrier. However, the proximal part of the short arm of her X chromosome was identical to a non-manifesting carrier (her sister) and to her affected brother. For this reason, we assumed the BMD mutation was proximal to the crossover. The dystrophin cDNA probes showed no deletion of DMD/BMD gene.

Demand for DNA probe testing in three genetic centres in Britain (August 1986 to July 1987)

We report a preliminary analysis of the data collected during the first year of the evaluation of clinical genetics in the context of DNA probes in three genetic centres, to show the pattern of the demand for genetic services in the three centres and the services used in meeting that demand. The analysis includes information on 10,185 persons from 2852 families. The results are presented according to mode of inheritance and according to the most common disorders for which DNA probes have been used in the three centres. The results indicate that the use of DNA probes is now a major element of activity in genetic departments, and that as long as indirect DNA probe testing is the predominant manner of using recombinant technology, the clinical input will be an important element of the costs, probably more so than that of the DNA laboratories, as a large number of family members needs to be tested. In most cases centres have concentrated activity on DNA testing for common and severe genetic disorders. However, there are disorders, such as familial hypercholesterolaemia, which have not been part of the established pattern of services. Conversely, a relatively high number of families have been studied for some disorders of very low incidence. This suggests that the number of DNA laboratories should be limited. The precise arrangements will need to be established. With such services the distribution of DNA testing facilities for different disorders can be controlled to limit duplication. The model followed in Scotland based on collaboration between centres is worth considering. We have detected very large differences in take up rate of services within and between regions. Although many factors may contribute to these differences, ease of access and lay and professional awareness are probably the most important. This is supported by the fact that more patients from the same or neighbouring DHAs attend the genetic centre than from those further away. We also concluded that published guidelines for clinicians in general on the uses of DNA probes, the type of families that could benefit, and the centres to which referrals should be sent would be very useful in increasing coverage and maximising the effectiveness of the services. Since this may increase demand, this educative tool should be coordinated and agreed by the Departments of Health with all the genetic departments and centres in the country.

Molecular deletions in the Duchenne/Becker muscular dystrophy gene

To gain further information relating to the frequency, position and size of DNA deletions in the Duchenne/Becker muscular dystrophy (D/BMD) gene region, and to detect any correlation of these deletions with phenotype, a large clinic-based population of DMD and BMD patients has been investigated using 13 cloned intragenic sequences. Our of 263 separate patients studied, 75 showed a deletion of at least one locus (28.5%). These represented 25.6% (55/215) of DMD patients and 41.7% (20/48) of BMD patients, suggesting that the milder phenotype is more often likely to be due to a deletion. The deletions range from 6 kilobases (kb) to greater than 1000 kb in size. The distribution of deletions across the gene region shows at least one region (detected by P20) prone to deletion mutations in both DMD and BMD patients. There is no simple correlation of position or extent of deletions with DMD or BMD, although deletion of a specific region towards the 5' end of the gene may be more often associated with a milder phenotype. Apparently similar deletions can give rise to phenotypes differing significantly in severity, presumably indicating further complexities in the molecular or cellular pathology.

Possibilities and limitation of prenatal diagnosis and carrier determination for Duchenne and Becker muscular dystrophy using cDNA probes

Two cDNA probes, cf23a and cf56a, identify deletions of selected exons in about 50% of our DMD/BMD patients. We have estimated the most likely order of the 11 exons detectable with both probes with respect to the different extensions of the deletions. In one of our BMD pedigrees, the observed deletion could be traced in the affected males through three generations. This result shows that with the use of cDNA probes detecting deletions, the only risk of error in genomic prenatal diagnosis is the general high frequency of new mutations for DMD/BMD. This is important progress in diagnosis compared to the 2 to 5% risk of misdiagnosis because of crossing over events using conventional linkage analysis with bridging or intragenic probes. The first prenatal diagnosis of an unaffected fetus of a woman who is a DMD carrier according to ultrasound examination is described. In one of our DMD males, the cDNA probe cf56a detects a deletion breakpoint. His sister also shows the altered band and is therefore a DMD carrier, while his mother has a totally normal band pattern. The interpretation of this observation could be either germline mosaicism or two identical new mutations. The identification of deletion breakpoints is a new diagnostic strategy, especially for carrier determination, which excludes misdiagnosis owing to crossing over events and the problems of dosage estimation. It is, however, limited by the low frequency of breakpoints detectable with cDNA probes. Therefore, the generation of new intron probes in this region is an important goal.

Prenatal testing for Duchenne and Becker muscular dystrophy

DNA studies were undertaken following 53 requests from pregnant women at risk for Duchenne and Becker muscular dystrophy, including 32 in whom there was only 1 affected individual in the family (sporadic cases). The DNA restriction fragment length polymorphisms were informative in 51 of the 53 cases. In 10 of 25 pregnancies with male fetuses the risk to the fetus was reduced to 5% or less. Referral of possible carriers before onset of pregnancy is strongly advisable on both medical and economic grounds. The banking of DNA from affected individuals for future use in the estimation of risks to their relatives should be encouraged.

DNA deletions in mild and severe Becker muscular dystrophy

The DNA of 33 patients diagnosed as suffering from Becker muscular dystrophy (BMD) has been probed with cloned DNA sequences from Xp21, known to reveal DNA deletions in patients suffering from the more severe Duchenne muscular dystrophy (DMD). Two BMD cases showed clear deletions. A third case gave aberrant band sizes, which further analysis showed to be caused by a small deletion. This suggests that deletions in DXS164 occur approximately as frequently in BMD as they do in DMD. Of the two cases showing large deletions, one is at the severe end of the Becker clinical spectrum, whilst the other is a classical Becker-type dystrophy. The fact that loci defined by probes commonly deleted in classical DMD patients are also deleted in BMD patients of varying severity is strong additional evidence that these disorders are allelic, and further justifies the use of probes with defined linkage relationships to DMD also being used for counselling in BMD families.