Prenatal testing for Duchenne and 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.

Dystrophin gene analysis and prenatal diagnosis of Duchenne muscular dystrophy in Russia

Of 126 families referred for counselling of Duchenne muscular dystrophy (DMD), DNA analysis has been suggested to 119 families with at least one affected child or with an affected close male relative of the woman at risk of being a DMD carrier. A large proportion (about 80 per cent) of the families were represented by sporadic cases (only one affected individual). By means of multiplex polymerase chain reactions with different sets of oligoprimers providing amplification of 10-11 different exons, altogether 49 dystrophin gene deletions were identified (41 per cent). Eighteen deletions clustered in the 5' 'hot spot' region of DMD cDNA and 36 in the distal half of the central rod domain around exons 43-53. An unusually high frequency (18 per cent) of deletions involving exons 17-19 was discovered. Large deletions extending through both 'hot spot' regions and thus occupying over 30-40 exons were recorded in five cases (10 per cent). Seventy-six of 94 families were found to be informative by RFLP analysis for intragenic or extragenetic DNA probes. Carrier status was ascertained in 20 and rejected in 32 female relatives in 40 DMD families. Eight DMD-affected fetuses were diagnosed prenatally by direct deletion testing or by RFLP analysis. Feasible interpopulation variations in the dystrophin gene deletion pattern are discussed. The prospects for more effective prenatal diagnosis and carrier detection in high-risk DMD families in Russia are briefly outlined.

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.

Genotype-phenotype correlation and germline mosaicism in DMD/BMD patients with deletions of the dystrophin gene

The molecular analysis of 127 DMD/BMD patients showed that 73 of them (57%) had deletions in the dystrophin gene. Two different methods were used in this study: (a) hybridization of HindIII-digested genomic DNA with nine cDNA probes corresponding to the entire 14kb cDNA of the DMD gene; and (b) simultaneous amplification of nine exons of the DMD gene (multiplex DNA amplification) by the polymerase chain reaction (PCR). When the deletion breakpoints of the intragenic deletions were analyzed with regard to their phenotypic consequences, nine patients were found to represent exceptions to the reading-frame hypothesis. Information regarding mental development was also available for 61 of the 73 deleted patients and for 34 of the 54 non-deleted ones. The proportion of mentally retarded patients was found to be similar in the two groups (deleted, 15%; non-deleted, 18%). Finally, in one family, a junction fragment present in the patient was not found in the peripheral blood DNA of the mother but was present in the sister, thus indicating germline mosaicism in the mother.

Accurate assessment of intragenic recombination frequency within the Duchenne muscular dystrophy gene

Polymorphic loci that lie at the two extremities of the Duchenne/Becker muscular dystrophy (DMD/BMD) gene have been used to estimate intragenic recombination rates. Multipoint linkage analysis of the CEPH panel of families suggests a total intragenic recombination frequency of nearly 0.12 (confidence intervals 0.041-0.226) over the genomic length of approximately 2 Mb.

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.

Detection of novel genetic markers by mismatch analysis

Chemical mismatch detection has been used to identify previously unknown genomic sequence variations that represent a new source of markers for genetic analysis. The approach detects all types of sequence changes, and therefore overcomes the limitation of restriction analysis, which identifies only a small fraction of the available sequence variations. Three new markers identified at the 3' end of the human dystrophin gene result from variable numbers of exact tandem repeats of 4bp (two examples) or 5bp (one example). None of these would have been detected as restriction fragment length polymorphisms by established procedures.

Screening for Duchenne muscular dystrophy

A programme was introduced in Wales to screen all 18 month old boys who were not yet walking for raised creatine kinase activity within the existing community developmental screening programme. During an 18 month period 25 229 such boys were identified of whom 19 930 (79%) had a Denver developmental screening test and 338 (1.7%) of these were not walking. Two hundred and five of those who did not walk (61%) had creatine kinase activity assayed and two cases of Duchenne muscular dystrophy were detected. We conclude that screening boys of 18 months who do not walk is worthwhile if the opportunity arises, but that a population based screening programme of this type is not justified as detection rates will be unacceptably low.

Pathogenic and nonpathogenic deletions in two families with Duchenne muscular dystrophy

Two deletions detected within the Duchenne/Becker muscular dystrophy (D/BMD) gene of normal male members of two DMD families were both independent, nonpathogenic deletions located in a large intron in the XJ region (DXS206) toward the 5' end of the gene [Burghes et al., 1987]. Investigation of the surrounding exons revealed no exon deletions or duplications. The simplest interpretation of these observations is that the deletions are entirely intronic and do not cause perturbation of the gene product, resulting in a normal phenotype. The disease phenotypes in the affected males in these two families are caused by exon deletions remote from this intron. Some caution is therefore indicated in using genomic deletions for clinical prediction.

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.

Applications of molecular genetics to gastrointestinal and liver diseases. I. Technical approaches

Recent developments in recombinant DNA techniques have allowed an understanding of the molecular genetics of many diseases, some affecting the gastrointestinal tract and liver. DNA probes which detect sequences within or near disease genes can be selected by direct approaches, if the gene product or primary gene function is known, or by indirect methods when the chromosomal location is known. Such probes have resulted in extensive family studies which can now define risks to family members of developing a genetic disease. The development of the polymerase chain reaction will also be of considerable use in clinical genetics and in the diagnosis of some infectious diseases. The techniques are summarized and examples of their use are given. A glossary of terms is also provided.

Rapid carrier and prenatal diagnosis of Duchenne and Becker muscular dystrophy

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Deletion screening of the Duchenne muscular dystrophy locus via multiplex DNA amplification

The application of recombinant DNA technology to prenatal diagnosis of many recessively inherited X-linked diseases is complicated by a high frequency of heterogeneous, new mutations (1). Partial gene deletions account for more than 50% of Duchenne muscular dystrophy (DMD) lesions, and approximately one-third of all cases result from a new mutation (2-5). We report the isolation and DNA sequence of several deletion prone exons from the human DMD gene. We also describe a rapid method capable of detecting the majority of deletions in the DMD gene. This procedure utilizes simultaneous genomic DNA amplification of multiple widely separated sequences and should permit deletion scanning at any hemizygous locus. We demonstrate the application of this multiplex reaction for prenatal and postnatal diagnosis of DMD.