Recombination with pERT87 (DXS164) in families with X-linked muscular dystrophy

Detection of dystrophin deletion carriers using FISH analysis

The detection of carrier status in female relatives of Duchenne/Becker muscular dystrophy patients is not always possible and this poses a problem in genetic counseling. We have developed a simple method that can be used in families in which affected males are characterized by the presence of a deletion within the dystrophin gene. PCR fragments, corresponding to the deleted regions are used as fluorescent probes for hybridization of peripheral lymphocytes nuclei of female relatives. The results obtained clearly demonstrate the feasibility of this method for detecting female DMD/BMD carriers.

Duchenne/Becker muscular dystrophy carrier detection using quantitative PCR and fluorescence-based strategies

Dystrophin gene deletions account for up to 68% of all Duchenne (DMD) and Becker (BMD) muscular dystrophy mutations. In affected males, these deletions can be detected easily using multiplex PCR tests which monitor for exon presence. In addition, quantitative dosage screening can discriminate female carriers. We previously analyzed multiplex PCR products by gel electrophoresis and quantitation of fluorescently labeled primers with the Gene Scanner in order to test carrier status. These multiplex PCR protocols detect DMD gene deletions adequately, but require up to 18 pairs of fluorochrome-labeled primers. We previously described two alternative fluorescent labeling strategies, each with approximately 1,000-fold greater sensitivity than ethidium bromide staining, which can be used to quantify the products of multiplex PCR. The first method uses the DNA intercalating thiazole orange dye TOTO-1 to stain PCR products after 20 cycles. In the second method, fluorescein-12,2'-dUTP is incorporated into products during PCR as a fluorescent tag for subsequent quantitative dosage studies. Both methods label all multiplexed exons including the 506 bp exon 48 fragment that is difficult to detect and quantify by standard ethidium bromide staining. Using this approach, we determined DMD/BMD carrier status in 24 unrelated families using a fluorescent fragment analyzer. Analysis of fluorochrome-labeled PCR products facilitates quantitative multiplex PCR for gene-dosage analysis.

Support of linkage of Gerstmann-Sträussler-Scheinker syndrome to the prion protein gene on chromosome 20p12-pter

Gerstmann-Sträussler-Scheinker syndrome (GSS) is a human transmissible spongiform encephalopathy recently linked to the human analog of the prion protein gene (PRNP) on chromosome 20p. We have studied a large German GSS family for linkage to PRNP and have obtained a peak lod score of 1.15 at a recombination fraction (theta) of 0.00. This result provides additional evidence that GSS is linked to a mutation in codon 102 of the PRNP gene. Combining our data with linkage data previously reported yields a peak lod score of 4.52 at theta = 0.0. No evidence for linkage heterogeneity was found in the combined data set.

Somatic mosaicism at the Duchenne locus

Results of testing a family for carrier status and prenatal diagnosis for Duchenne muscular dystrophy (DMD) are best explained by somatic mosaicism in the maternal grandfather. This genetic situation was identified using segregation analysis of intragenic DNA polymorphisms, a serum creatine phosphokinase assay, and physical examination of the patients. This event at the DMD locus represents one more potential source of error in carrier testing and prenatal diagnosis.

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.

Improved diagnosis of Duchenne/Becker muscular dystrophy

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Germinal mosaicism increases the recurrence risk for 'new' Duchenne muscular dystrophy mutations

In 288 Dutch and Belgian Duchenne and Becker muscular dystrophy families, the parental origin of 41 new deletion or duplication mutations was determined. Twenty seven of the new mutations occurred in the maternal X chromosome and nine in the grandmaternal and five in the grandpaternal X chromosome. The grandparental data are compatible with equal mutation rates for DMD in male and female X chromosomes. New mutations were defined by their presence in one or more progeny and absence in the lymphocytes of the mother or the grandparents. In one family a fraction of the maternal lymphocytes was found to carry the mutation, suggesting somatic mosaicism. In six cases out of 41, the mutation was transmitted more than once by a parent in whom the mutation was absent in lymphocytes, suggesting gonadal mosaicism as the explanation for the multiple transmission. Using our data for the recurrence of the mutations among the total of at risk haplotypes transmitted, we arrive at a recurrence risk of 14% for the at risk haplotype. The observation of this high risk of germinal mosaicism is crucially important for all physicians counselling females in DMD families. Recently, germinal mosaicism has been observed also in a number of other X linked and autosomal disorders. The implications and appropriate diagnostic precautions are discussed.

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.

Molecular biology of Duchenne and Becker's muscular dystrophy: clinical applications

Recent advances in molecular genetics have led to the isolation of the gene defective in patients with Duchenne and Becker's muscular dystrophy and the characterization of its protein product, dystrophin. In this communication, the developments culminating in the identification of the Duchenne muscular dystrophy locus are reviewed. The practical applications of this research and pitfalls that limit prenatal diagnosis and carrier detection are discussed.

The Wellcome lecture, 1988. Muscular dystrophy: a time of hope

Duchenne muscular dystrophy (DMD) and its less severe allele Becker muscular dystrophy (BMD) are progressive muscle-wasting disorders of children. DMD is characterized by rapid loss of muscle fibres and the ensuing weakness results in lost mobility and eventual premature death. Despite extensive research for many years, the basic underlying biochemical defect has remained elusive. Here I try to demonstrate how the powerful techniques of molecular genetics can be used to gain a further understanding of this particular disorder and how, in principle, the techniques can be applied to the other 3000 human genetic disorders that are so far uncharacterized. Once the chromosomal map position of DMD was established, the locus that was being disrupted by mutation could be identified and the encoded protein product predicted from the nucleotide sequence of the RNA transcript. This has led to the identification of a previously uncharacterized protein named dystrophin. As the normal function of dystrophin is determined, more accurate clinical diagnosis of DMD and BMD should result and potential approaches to therapy should be designed.

Duchenne muscular dystrophy: detection of deletion carriers by spectrophotometric densitometry

DNA isolated from a family segregating a deletion in the Duchenne muscular dystrophy gene and control families was digested with restriction enzymes, Southern transferred, and probed with a radioactive dystrophin cDNA probe. The resulting autoradiographs were analyzed with a densitometric spectrophotometer to detect carriers of the deletion. The carrier status of females in the deletion pedigree was independently determined by genomic probes and confirmed by densitometry. In many Duchenne families, deletions will only be observed using cDNA probes which show few restriction fragment length polymorphisms (RFLPs). Such deletions would normally have to be detected using dosage gels. The spectrophotometric densitometry technique used by us does not require dosage gels, and avoids problems arising from non-informative meioses and cross-overs. It should be possible to screen every family with an exon deletion by spectrophotometric densitometry provided the presently available cDNA is suitably reduced to produce fewer bands on autoradiographs.

X-chromosome inactivation in the Wiskott-Aldrich syndrome: a marker for detection of the carrier state and identification of cell lineages expressing the gene defect

Recently developed techniques for the direct analysis of DNA have made possible the determination of patterns of cellular X-chromosome inactivation. These techniques provide a potential method for carrier detection for several X-linked human disorders in which obligate carriers show nonrandom X inactivation. By using restriction fragment length polymorphic (RFLP) gene-specific probes in conjunction with methylation-sensitive enzymes, we have characterized the patterns of X-chromosome inactivation in cell subsets from females belonging to 10 kindreds segregating for the X-linked immune deficiency disorder Wiskott-Aldrich syndrome (WAS). We show that selective inactivation of the X chromosome distinguishes obligate WAS carriers from noncarrier females and constitutes a valuable marker of the WAS carrier state. Selective inactivation phenomena were observed in the monocytes and T and B lymphocytes of obligate carriers, implying that the WAS gene defect is expressed in each of these cellular lineages. In conjunction with the use of linked DNA markers, RFLP-methylation analysis should render carrier detection feasible for the majority of females from WAS families. The results of such analyses also provide an initial step toward identifying the cellular level and molecular basis for WAS.

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 diagnosis and carrier detection by DNA studies in a Duchenne muscular dystrophy family with no living affected male

Restriction fragment length polymorphism studies and gene dosage analysis using the intragenic probes pERT87 were used to detect deletions in potential carriers in a family with Duchenne muscular dystrophy in which the only affected male was deceased. Two females were found to have inherited the paternal pERT87 alleles but not the maternal alleles, suggesting that they have inherited the pERT87 deletion from their mothers. The hybridization signals of pERT87 from these two females upon gene dosage analysis also suggested that they had a single copy of pERT87. The chorionic villi of a male fetus from one of these two females was found to be deleted for pERT87, suggesting that it was affected. This result confirmed the carrier status of the mother.

Prenatal diagnosis using deletion studies in Duchenne muscular dystrophy

Accurate carrier testing and prenatal diagnosis in Duchenne muscular dystrophy (DMD) families is facilitated when an Xp21 deletion is found to be segregating within a family. We discuss the results of the DNA testing in two families, one in which DNA from affected males was available for study and the other in which no DNA from an affected male was available. Factors complicating the counselling of DMD deletion families are outlined.

The problem of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a lethal X-linked muscular disorder. The biochemical defect remains unknown, but the gene responsible has been mapped to band Xp21. The gene has now been cloned in two laboratories solely from knowledge of its map location. L. M. Kunkel and his colleagues isolated genomic sequences (PERT 87) from within a large deletion causing DMD, whereas our group isolated genomic sequences (XJ) spanning the junction of an X-autosome translocation causing the disease. Chromosome walking by both groups has led to the isolation of over 400 kilobases of the PERT 87 and XJ region. Subclones of PERT 87 and XJ reveal restriction fragment length polymorphisms that segregate with the DMD gene in 95% of meioses, and fail to hybridize with DNA from about 8% of male patients. Selected subclones of PERT 87 and XJ contain exons that hybridize to muscle-derived complementary DNA (cDNA) clones. The cDNA clones detect a large (16 kilobase) message. Analysis of deletions, mutations and translocations suggests a DMD gene of between two million and three million base pairs. The clones obtained so far are useful for attempts to generate antibody against the gene product and for carrier identification and prenatal diagnosis.

Direct method for prenatal diagnosis and carrier detection in Duchenne/Becker muscular dystrophy using the entire dystrophin cDNA

DNA sequence polymorphisms (RFLPs) have been widely used as genetic markers for identification of the X chromosome that carries the mutation for Duchenne muscular dystrophy (DMD) in affected families, but serious limitations and pitfalls are associated with this approach [Darras et al., 1987]. The complementary DNA (cDNA) of the DMD gene has recently been isolated and shown to detect partial gene deletions in a large proportion of patients [Koenig et al., 1987]. Two prenatal studies are presented to illustrate how the unambiguous identification of deletion mutations by cDNA probes permits direct DNA-based diagnoses with high accuracy and in otherwise uninformative families. In a single proband family, DNA marker analysis had determined that the Xp21 chromosomal region present in the affected male was also carried by a male fetus in a subsequent pregnancy. Analysis of this family's DNA with probes covering the entire 14 kb cDNA revealed a small deletion in the affected male that was not present in the fetus nor in the mother. In the second family the fetus was a female deletion carrier identified by comparing intensities of restriction fragments. Since 1/3 of all DMD patients are thought to result from new mutations and most families have only single affected males, the cloned cDNA probes now available are likely to revolutionize DNA-based diagnostic studies in this disorder. More reliable, more rapid and less expensive than linkage studies with DNA polymorphisms, this method will be informative in the more than 50% of DMD/BMD cases that have deletion mutations.

Service experience using DNA analysis for genetic prediction in Duchenne muscular dystrophy

In August 1985 we instituted a carrier and prenatal testing service for Duchenne muscular dystrophy (DMD) using direct DNA analysis. The experience over the first nine months is described. We have analysed samples for RFLPs from 154 people including 53 women at risk of being DMD carriers from 37 families. We used the probes pERT87.8 (BstXI and TaqI polymorphisms), 87-15 (TaqI polymorphism), and pXJ1.1 (TaqI polymorphism). Forty-one of the women have had their risks altered. We found one deletion (pERT87-8) out of 23 DNA samples analysed from affected boys. We used a recombination fraction of 0.05 in risk calculations but did not detect any known crossovers. In nine of the families there is only an isolated case of DMD. In families where we have not been able to alter the risk of the women being a carrier (for example, because all brothers are dead), we have offered prenatal exclusion and have carried out one first trimester prenatal diagnosis on this basis. Lowering the risk of an affected fetus to less than 2.5% appears to be a satisfactory situation for many (most) of the women involved and seems to justify the introduction of genetic prediction based on single intragenic probes despite the 5% recombination frequency.

Long-range genomic map of the Duchenne muscular dystrophy (DMD) gene: isolation and use of J66 (DXS268), a distal intragenic marker

By cloning the endpoints of a DMD-associated deletion, we have "jumped" 1100 kb from pERT87-1 (DSX164) to a new locus designated J66 (DXS268), mapping distally within the Duchenne muscular dystrophy (DMD) gene. Both J66 and JBir are mapped by field-inversion gel electrophoresis and detect abnormal SfiI fragments in DMD patients and distal DMD-associated X; autosome translocations. Our long-range map extends the physical map of the DMD gene from 800 to 2000 kb (2 Mb) and increases the mapped portion of Xp21 to approximately 8 Mb. The position of the glycerol kinase gene and the adrenal hypoplasia locus are further confined to the region between J66 and the nearest distal probe L1-4. This region spans at least 1.5 Mb. The multiallelic J66 polymorphism has immediate application in the diagnosis of DMD and generally appears to be distal to DMD mutations.

Direct detection of more than 50% of the Duchenne muscular dystrophy mutations by field inversion gels

Duchenne muscular dystrophy (DMD) is an X-linked disorder affecting about 1 in 3,500 males. It is allelic with the milder Becker muscular dystrophy. The biochemical basis for both diseases is unknown and no effective treatment is available. Long-range physical mapping has shown that the DMD gene, localized in Xp21, is extremely large, exceeding 2 million base pairs. Until now, carrier detection and prenatal diagnosis has involved the use of linked restriction fragment length polymorphism markers which detect muscular dystrophy-associated deletions in about 10% of the cases. Field inversion gel electrophoresis (FIGE) allows the detection of structural rearrangements in 21 out of 39 of the DMD patients studied (54%), of which 14 (65%) were not detected by conventional methods. Large deletions seem to make up a much higher fraction of the DMD mutations than so far indicated by other methods. A region prone to deletion was located in the distal half of the gene. FIGE analysis could provide a valuable extension of information for carrier detection and prenatal diagnosis. The technique should be generally applicable to the study of diseases involving structural chromosomal rearrangements.

Genetic heterogeneity in Duchenne dystrophy

We evaluated 173 patients in the Clinical Investigation of Duchenne Dystrophy study to determine if patients from the same kindred were more alike clinically than patients within the study population as a whole. A high intrafamilial correlation was noted for age-adjusted muscle strength scores. While noninherited factors could, in part, explain the results, it seems likely that genetic heterogeneity may contribute to the observed similarity within Duchenne dystrophy kindreds.

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.

The impact of DNA analysis on fetal diagnosis

There has been rapid progress in mapping disease-specific gene loci to particular chromosomal regions and in cloning the relevant genes or DNA sequences that can act as genetic markers. These advances will have an impact on fetal diagnosis of monogenic disorders for a number of reasons, the most important being the ability to use chorionic villus DNA taken in the first trimester to make a fetal diagnosis, no matter how tissue-specific the gene defect. Diagnosis based on analysis of the protein gene product requires a tissue that is expressing that gene. In a few disorders the mutation within the gene can be detected directly, but more often an approach, termed gene tracking, will be required to predict the genotype of the fetus. This requires blood samples from a few family members and the analysis must be carried out prior to the chorion sampling and ideally before the pregnancy. This initial family study is required to determine whether gene tracking is possible in that particular family. A number of potential problems, both technical and in clinical management, highlight the need for close collaboration between clinical geneticists, molecular geneticists and obstetricians.

Complete cloning of the Duchenne muscular dystrophy (DMD) cDNA and preliminary genomic organization of the DMD gene in normal and affected individuals

The 14 kb human Duchenne muscular dystrophy (DMD) cDNA corresponding to a complete representation of the fetal skeletal muscle transcript has been cloned. The DMD transcript is formed by at least 60 exons which have been mapped relative to various reference points within Xp21. The first half of the DMD transcript is formed by a minimum of 33 exons spanning nearly 1000 kb, and the remaining portion has at least 27 exons that may spread over a similar distance. The DNA isolated from 104 DMD boys was tested with the cDNA for detection of deletions and 53 patients exhibit deletion mutations. The majority of deletions are concentrated in a single genomic segment corresponding to only 2 kb of the transcript.

Molecular heterogeneity of translocations associated with muscular dystrophy

Individual translocation chromosomes from six girls suffering from Duchenne or Becker muscular dystrophy (DMD or BMD) have been isolated in human-mouse somatic cell hybrids. DNA prepared from these hybrids was probed with sequences physically close to the locus; these include a junction fragment from the site of the X:21 translocation (pXJ1) and subclones from the pERT 87 (DXS164) region which are absent in a minority of male DMD patients. Both sets of sequences mapped within the area defined by the translocation breakpoints, confirming their close proximity to the DMD and BMD loci. Furthermore, the X chromosome breakpoints of the translocations can be divided into three categories depending upon their position in relation to the sequences recognised by pXJ1 and pERT 87. The genomic target disrupted by the translocations examined here is a minimum of 176 kb.

Localisation of Xp21 meiotic exchange points in Duchenne muscular dystrophy families

The inheritance of Duchenne muscular dystrophy in 25 families was studied with 13 X chromosome specific cloned DNA fragments from 10 loci in and surrounding Xp21. When multiple probes were informative, the meiotic exchange points for each meiosis were located in individual families. Neither genetic nor physical evidence indicates an unusually high recombination rate across Xp21 in these 25 families.

Isolation of candidate cDNAs for portions of the Duchenne muscular dystrophy gene

Duchenne muscular dystrophy (DMD) and the less severe Becker muscular dystrophy (BMD) are human X-linked muscle-wasting disorders that have been localized to the band Xp21 by genetic linkage analysis and cytologically detectable abnormalities. A cloned DNA segment, DXS164 (or pERT87), has been shown to detect deletions in the DNA of unrelated DMD and BMD males. Here we present the nucleotide sequence of two highly conserved DNA fragments from the DXS164 locus and their homologous sequences from the mouse X chromosome. One of the human conserved segments hybridized to a large transcript in RNA isolated from human fetal skeletal muscle and was used to isolate cDNA clones which cover approximately 10% of this transcript. The cDNA clones map to Xp21 and hybridize with a minimum of eight small regions that span 130 kilobases (kb) of the DXS164 locus. These expressed sequences are candidates for portions of the gene responsible for both DMD and BMD.