Carrier detection and prenatal diagnosis in X linked muscular dystrophy using restriction fragment length polymorphisms

Carrier testing of children for two X-linked diseases: A retrospective study of comprehension of the test results and social and psychological significance of the testing

OBJECTIVE

To evaluate long-term consequences of genetic carrier testing performed in childhood in terms of awareness and comprehension of the test result, and the social and psychological significance of such testing.

STUDY DESIGN

The families of 66 young females who had been tested for carriership during childhood between 1984 and 1988 were approached. Of the 66 families, 23 young females in families affected by Duchenne muscular dystrophy (DMD), 23 young females in families affected by hemophilia A (HA), and their mothers participated in our study. We used a questionnaire including multiple-choice and open-ended questions.

RESULTS

Of the young female participants tested in the families affected by DMD or HA, 65% knew their test results. Only 65% of DMD mothers and 78% of HA mothers remembered correctly the test results of their daughters. The majority (83%) of the young females tested sought no genetic counseling when reaching adulthood. The reason for this was not determined. Most (78%) reported that the test result had not influenced their lives, whereas some felt relieved to know they had not been carriers. Talking about hereditary disease in the family and between friends was open, and results of the carrier test had usually been told to friends.

CONCLUSION

Carrier testing was in most cases correctly understood and the matter openly discussed. Our results do not suggest that testing in childhood had caused serious harm to the young individuals tested. On the other hand, we found no obvious benefits from this early testing.

Carrier testing of children for two X-linked diseases: a retrospective evaluation of experience and satisfaction of subjects and their mothers

Carrier testing of children for inherited disease that will not affect the health of the children themselves but of their future children is generally regarded as problematic. In this retrospective study, we determined how young women had experienced genetic carrier testing when they were children. The families of 66 young females who had been tested for carriership during childhood between 1984 and 1988, were approached. Of them, 23 young females in families affected by Duchenne muscular dystrophy, and 23 young females in families affected by hemophilia A, and their mothers, participated in our study. We used a questionnaire including multiple-choice and open-ended questions. We recorded general attitudes to testing, satisfaction with testing, degree of trust in test results, making decisions regarding testing, privacy, and opinions about age at testing. Thirty-five out of 46 of the young women tested (76%) were satisfied with carrier testing in childhood. However, the young women in whom the test results had been uncertain were statistically more often unsatisfied with the testing than those who had been found or not found to be carriers (p = 0.002). In each group, the opinions of mothers were parallel to those of their daughters. Seventy-eight percent of daughters regarded carrier testing as a family matter in which parents can make a decision. About half of those tested recalled that they had been allowed to participate in decision-making in a satisfying way. Thirty-nine out of 46 (85%) of the young women tested, and 33/46 (72%) of the mothers, suggested that carrier testing should be performed in childhood or during teenage years.

Carrier testing of children for two X linked diseases in a family based setting: a retrospective long term psychosocial evaluation

The question of whether genetic carrier testing should be performed on children has been the subject of much debate. However, one important element has been lacking from this debate. There has been practically no knowledge of how those tested in childhood have experienced carrier testing. Twenty three subjects in families affected by Duchenne muscular dystrophy and 23 in families affected by haemophilia A, all of whom had been tested during childhood for carriership in the Department of Medical Genetics, University of Helsinki, from 1984 to 1988, participated in our study. We investigated long term psychosocial consequences of carrier testing in childhood. A questionnaire relating to sociodemographic background and life situation was used, together with assessment of health related quality of life (HRQOL) using the RAND 36 item Health Survey 1.0 (RAND). RAND results showed that the emotional, social, and physical well being of the young female subjects was not statistically different from those of control female subjects at a similar age. We also found no statistically significant differences in means in any RAND dimension (p<0.146) between carriers, non-carriers, and a group in which carrier status was uncertain. However, two out of seven carriers reported that they were worried and three that they were slightly worried about the test result. Four out of 22 young female subjects in the uncertain group reported being worried and 11 reported being slightly worried.

Carrier detection in Duchenne and Becker muscular dystrophy Argentine families

In order to offer carrier detection, genetic counseling, and prenatal diagnosis to families with Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) in our country, segregation analysis of highly polymorphic short tandem repeats (STR) (dC-dA)n: (dG-dT)n loci was utilized. The risks to females of 15 DMD BMD families (9 familial and 6 sporadic) were evaluated on STR, pedigree and serum creatine kinase (SCK) data. From the 36 females at risk of being carriers (not including 8 obligate carriers), results of STR analysis were compatible with carrier status in 7 and not compatible in 20. In 9 females, no information regarding carriership was derived from the STR analysis. Prenatal diagnosis is now possible on the carrier females. Previously identified deletions in the central part of the gene were confirmed by STR analysis in 3 families. Five new alleles were identified in Argentine individuals; allele frequencies differed from those of North American people. Results derived from this study are useful for carrier detection and genetic counseling in DMD/BMD. One case of probable mosaicism in an unaffected father was detected on a pedigree basis in a family with DMD patients.

Allele frequencies of intragenic, and 5' and 3' markers of the dystrophin gene in Japanese families afflicted with Duchenne or Becker muscular dystrophy

Using the polymerase chain reaction method (PCR), we examined the allele frequencies and heterozygosities of 7 polymorphic sites (pERT87, and CA polymorphisms in the 5' and 3' regions) of the dystrophin gene in 20 Japanese Duchenne muscular dystrophy and Becker muscular dystrophy (DMD or BMD) families consisting of 36 males, including 23 cases of DMD and BMD, and 28 females. The allele frequencies of three primer and enzyme sets in the pERT87 locus were well comparable to those in the previously reported Japanese female cases but different from in other countries. The frequencies of 5' markers of the dystrophin gene in Japanes were different from the reported Caucasian frequencies. As for 5'DYS-I and 5'DYS-II, the numbers of alleles in our cases were less than in Caucasians, and the heterozygosities of all three markers (5'DYS-I, II and III) were lower than in Caucasians. However, the 3'CA polymorphisms showed almost the same frequencies and heterozygosities as in Caucasians. All of our females showed a heterozygous pattern for at least one locus, with the combination of the seven markers. The usefulness of linkage analysis involving PCR methods with these intragenic, and 5' and 3' markers of the dystrophin gene in the carrier and prenatal diagnosis of DMD and BMD was confirmed by the successful prenatal diagnoses in 15 fetuses, the exception being one case considered to have a new mutation.

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.

Analysis of RFLPs and DNA deletions in the Chinese Duchenne muscular dystrophy gene

Sixty-nine unrelated Chinese DMD patients were studied with a series of genomic and cDNA probes. Analysis of 13 polymorphic sites showed that pERT87-1, 87-8, 87-15, and XJ probes gave favourable allele frequencies in the Chinese population, and nearly 90% of the DMD families in this study were informative for prenatal diagnosis and carrier detection using these four polymorphic markers. Nine out of 69 (13%) were also found to have gene deletions using a panel of genomic probes. However, when using cDNA probes, deletions were found in 56.5% of the patients. The deletions were concentrated in the areas of probes 7 and 8, giving a proportion of about 80% of all deleted patients in this study. All these results provide valuable information for planning prenatal diagnosis programmes for DMD in China.

Hemophilia B: diagnostic value of RFLP analysis in 19 of the 20 known Finnish families

With the aim of determining the usefulness of RFLP analysis in carrier detection and prenatal diagnosis, we studied all available members including 40 patients, 30 obligate carriers and 39 women at risk belonging to 19 out of a total of 20 Finnish hemophilia B families. The allele frequencies of the three intragenic polymorphisms studied (TaqI, XmnI and DdeI) did not differ significantly from those reported in other Caucasian populations. A considerable degree of linkage disequilibrium between the three polymorphisms was observed. Carriership evaluated in 39 females at risk led to exclusion in 14 while carriership was established in 5. The proportion of women who by pedigree analysis had a carriership risk between 10% and 90% could be reduced from 97% to 51% by RFLP analysis. Prenatal diagnosis using an intragenic polymorphism could potentially be offered to 69% of hemophilia carriers. DNAs from 19 unrelated patients were screened for mutations using a full-length cDNA probe, but no abnormal hybridization patterns were observed. Our results indicate that RFLP segregation analysis provides a useful method of carrier detection and prenatal diagnosis in hemophilia B.

Genetic counselling in Duchenne and Becker muscular dystrophy is problematic when carrier studies give controversial results

DNA-analysis with flanking and intragenic markers gave confusing results in 7 out of 74 (9.5%) Finnish families with Duchenne or Becker muscular dystrophy. In five families a sister or maternal aunt of the patient had elevated serum creatine kinase (CK) activity, although DNA-analysis indicated a low risk for carriership. In one family the two affected brothers had different pERT87 alleles. In one family the intragenic deletion found in a patient was not present in his mother, who was an obligatory carrier. Deletions were detected with cDNA probes in the probands in five of the families, but the controversy regarding carriership still remained. It is necessary to combine all available data from pedigree analysis, CK measurements, and DNA studies whenever carrier studies are performed, but it appears that major problems in counselling and prenatal diagnosis will still remain in a proportion of the families.

Gene deletions in X-linked muscular dystrophy

Of the approximately 170 families with X-linked muscular dystrophy of the Duchenne (DMD) and Becker (BMD) type in Finland, we have studied 90 unrelated patients for intragenic deletions by using the cDNA probes described by Koenig et al. Forty-five patients (50%) had molecular deletions of one or several of the 65 exon-containing HindIII fragments. In six deletion cases junction fragments of altered size were seen. Thirty-eight (84%) of the 45 deletions were detected using only two (1-2a and 8) of the six cDNA subclones. Using a wheelchair age of 12 years to distinguish between DMD and BMD, we found that the proportions of patients with deletions were similar. Deletions were equally common in familial and sporadic disease. BMD was more commonly caused by deletions in the 5' end of the gene than was DMD. In at least three instances deletions of similar type resulted in diseases of similar severity. Of 14 patients with mental retardation seven had deletions; six of these comprised exons contained in probe 8. We conclude that cDNA hybridization studies provide a powerful diagnostic tool in DMD and BMD and that they promise to produce better insights into molecular-clinical correlations.

A genetic linkage map of five marker loci in and around the Duchenne muscular dystrophy locus

Linkage analysis of five marker loci in and around the Duchenne muscular dystrophy (DMD) locus, DXS84, DXS206, DXS164, DXS270, and DXS28, was conducted with 499 families. Overall, the best multipoint distances were found to be DXS84-3.7 +/- 0.6 cM-DXS206-1.0 +/- 0.4 cM-DXS164-1.9 +/- 0.6 cM-DXS270-12.0 +/- 1.1 cM-DXS28. A comparison of this linkage map with the established physical map suggests the presence of hot spots for recombination in the DMD locus.

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.

Molecular analysis of muscular dystrophy

It is now possible to map almost any disease locus to a chromosomal region in the human genome by family studies with restriction fragment length polymorphisms. Duchenne and Becker muscular dystrophies have been shown to be localized within the same small region of Xp21 on the human X chromosome. Myotonic dystrophy has been localized to a region close to the centromere of chromosome 19. Technologies are now available to identify candidate genes for the diseases. Autosomal recessive muscular dystrophies are more difficult to study, but even these will be amenable to analysis in the very near future. The next decade should witness some exciting advances in the molecular analysis and clinical management of human muscular dystrophies.

Microdeletions in patients with X-linked muscular dystrophy: molecular-clinical correlations

The DNA from 68 patients with X-linked (Duchenne and Becker) muscular dystrophy belonging to 49 unrelated families was analyzed for microdeletions using 13 closely linked or gene-specific DNA-markers. Fourteen patients from eight families showed a deletion involving a least one of the markers used, giving a deletion frequency of 16%. The proportion of families with deletions was 36% in the Becker and 8% in the Duchenne form of the disease. With one exception, the extent of the deletion was different in different families. All living, affected males from the same family carried the same deletion. The extent or the localization of the deletion did not correlate with clinical features such as severity of disease or mental retardation.

Familial deletion in Becker type muscular dystrophy within the pXJ region

A family of an isolated patient with Becker muscular dystrophy has been investigated by DNA analysis. Southern blotting and hybridization were performed with six probes (C7, pERT87.15, pERT87.1, pXJ1.1, pXJ2.3, 754) mapping in the Xp21 region. A deletion within the pXJ region was demonstrated in the proband, his mother and all three sisters. The segregation pattern for the restriction fragment length polymorphisms (RFLPs) observed with the pXJ probes as well as with pERT87.15, pERT87.1 and 754 probes indicates that the deletion is of grandpaternal origin.

Linked polymorphic DNA markers in the prediction of X-linked muscular dystrophy

Ten polymorphic DNA markers, including gene specific markers of loci DXS164 and DXS206, were tested for allele frequencies, degree of heterozygosity and linkage in 34 Finnish families with X-linked muscular dystrophy. With the exception of the BamHI RFLP of DXS164 subclone pERT87-15, allele frequencies and the degree of heterozygosity failed to show any significant deviation from the data published elsewhere. We document a high degree of linkage disequilibrium between several RFLPs belonging to locus DXS164. Our linkage data include one recombination between DMD and DXS164 enabling a tentative location of the mutation site distal to DXS164. The maximum lod score for linkage between the disease locus and DX164 was 7.828 at a recombination fraction of 0.02. According to our data DXS28 and DXS43 may be located further away from the disease locus than previously thought. We use only gene specific markers for genetic counselling. Excluding deletions, 97.1% of women were heterozygous for at least one such marker. A diagnostic procedure in which useful information can be obtained in over 90% of all diagnostic situations, using only four filters, is proposed.

Diagnosis of genetic disease using recombinant DNA. Supplement

Recombinant DNA methodology has greatly increased our knowledge of the molecular pathology of the human genome at the same time as providing the means to diagnose inherited disease as the DNA level. We present here a list of recent reports of both direct and indirect analysis of human inherited disease which is intended to serve as a guide to current molecular genetic approaches to diagnostic medicine.