Linkage studies in Duchenne and Becker muscular dystrophies

Further impairment of muscle phosphate kinetics by lengthening exercise in DMD/BMD carriers. An in vivo 31P-NMR spectroscopy study

We used phosphorus magnetic resonance spectroscopy (31P-MRS) to study the effect of exercise-induced muscle injury in the calf muscle of 7 DMD/BMD carriers and 6 non-carrier females. All subjects performed 50-80 lengthening contractions with the right calf muscles. 48 h after lengthening exercise non-carriers showed increased sensitivity to pressure in their gastrocnemius accompanied by increased T2 relaxation times and by elevated Pi/PCr ratios at rest. DMD/BMD carriers did not show any effect of lengthening exercise on these measurements. In-magnet exercise revealed in all carriers a reduced initial rate of Pi recovery and an increased time to fully recovery the resting value of intracellular pH. Lengthening exercise further decreased the initial rate of Pi recovery. Non-carriers did not show any variation attributable to lengthening exercise either during in-magnet work or during recovery from exercise. We found that lengthening exercise contractions causes: (1) less muscle injury in carriers compared to non-carriers, (2) even slower rate of Pi recovery, but (3) no effect on Pi recovery in non-carriers. The use of lengthening exercise and measurements of Pi recovery may be a useful method to evaluate the disease process in DMD/BMD.

31P-NMR spectroscopy of skeletal muscle in Becker dystrophy and DMD/BMD carriers. Altered rate of phosphate transport

Muscle energy metabolism was studied by phosphorus nuclear magnetic resonance spectroscopy (31P-NMR) in 6 patients with Becker dystrophy, and in 24 female DMD/BMD carriers (n = 18) and non-carriers (n = 6). At rest all patients showed a high Pi/PCr ratio due to low PCr and high Pi contents, and a high intracellular IpH. 31P-NMR of carriers and non-carriers did not differ from controls. In patients and carriers in-magnet exercise revealed a reduced ability to perform work and Pi/PCr ratios higher than controls for comparable relative levels of steady-state work. Post-exercise Pi recovery was found abnormal in patients and in carriers. The 31P-NMR abnormalities found in the working muscle of both BMD patients and female DMD/BMD carriers indicate a defect of phosphate metabolism that, be it primary or secondary, reflects a deficit of energy metabolism.

Muscle energy metabolism in female DMD/BMD carriers: a 31P-MR spectroscopy study

31P nuclear magnetic resonance spectroscopy (31P-MRS) was used to investigate the energy metabolism of the gastrocnemius muscle in 16 DMD/BMD female carriers. No significant difference was found with the controls in the resting muscle, while all carriers showed a decreased ability to perform work, and a higher Pi/PCr ratio for comparable relative levels of steady-state work. The rate of postexercise recovery of PCr/Pi ratio was lower in all carriers. The decreased rate of PCr/Pi recovery appears to be mainly due to a decreased rate of Pi recovery. Our findings show abnormal muscle energy metabolism in DMD/BMD female carriers.

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.

Canine X-linked muscular dystrophy. An animal model of Duchenne muscular dystrophy: clinical studies

The progression of clinical disease and serum creatine kinase (CK) levels in canine X-linked muscular dystrophy (CXMD) was studied in 7 dogs from birth to 12-14 months and in 18 dogs at varying intervals from birth to 8 weeks. One affected male was studied from age 3.5 to 6 years, and all pups were descendants of this dog. A lethal neonatal form was recognized in some pups. In the more typical form, clinical signs of stunting, weakness and gait abnormalities were evident by 6-9 weeks and were progressive, leading to marked muscle atrophy, fibrosis and contractures by 6 months. Serum CK levels were markedly elevated, such that affected pups could be identified by 1 week. CK values increased until 6-8 weeks, then plateaued at approx. 100 times normal. Affected females and beagle-cross dogs were less severely affected than large breed-cross dogs. In the 2 adult dogs with cardiac insufficiency CK levels had decreased to 5-15 times normal. These studies show that CXMD and Duchenne muscular dystrophy have striking phenotypic as well as genotypic similarities. In addition, these studies of CXMD suggest that in females and in smaller dogs the same genetic defect results in a less severe clinical disease.

Italian experience regarding the prevention of Duchenne and Becker muscular dystrophies

The indirect approach to carrier detection and prenatal diagnosis of Duchenne and Becker muscular dystrophies based on the study of DNA polymorphisms closely linked to this gene has been followed by five Italian laboratories in the study of 106 pedigrees. Out of 354 women studied up to 1 May 1987, 147 were identified as carriers because of pedigree information and/or of increased creatine phosphokinase (CPK) values. Of the remaining 207, 184 could be assigned to three arbitrarily defined risk categories (low, intermediate and high) using linkage analysis. This disaggregation of women at risk is clearly more useful than that defined before DNA analysis, in which the same 184 women could be assigned only to the low or intermediate risk categories. Prenatal diagnosis was theoretically possible in 90% of carrier women, and was actually performed in 14 pregnancies, which led to the identification of four affected male foetuses, one also having Down syndrome.

Binding of Ricinus communis I lectin to developing dystrophic muscle in human fetus

In previous studies it was shown that a D-galactose-specific lectin, Ricinus communis I (RCA I), does not bind to the plasma membrane of muscle fibres from patients with Duchenne muscular dystrophy (DMD) in contrast to normal muscle. We have now studied RCA I binding to the membranes of developing human fetal muscle in fetuses at 95% risk of DMD (n = 6) and normal controls (n = 5) with a developmental range of 12-20 weeks of gestation. The results were compared to the membrane appearance with conventional ultrastructure. Binding of RCA I to the muscle basement membrane was consistently strong from the early stages of myogenesis, such as in fusing myoblasts/myocytes. RCA I binding to the plasma membrane was weak but detectable in both DMD and normal fetuses at 12-14 weeks of gestation. Both the normal and diseased condition showed an increase of RCA I labelling of the muscle plasma membrane at 15-17 weeks and strong labelling at 18-20 weeks of gestation. No difference was observed in the RCA I localization of normal and diseased human fetal muscle plasma membrane. It is concluded that (a) the plasma membrane in developing fetal muscle undergoes a maturation process between 12 and 20 weeks gestational age leading to an increase in expression of RCA I binding carbohydrate moieties; and (b) that the absence of RCA I binding glycoprotein in mature DMD muscle plasma membrane reflects a change acquired during the course of disease.

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.

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.

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.

A small deletion in the Duchenne/Becker muscular dystrophy locus--a functionally important region?

A DNA deletion in a patient with Becker muscular dystrophy (BMD) has been delineated by restriction endonuclease mapping. The deletion is unusually small, removing six kilobases (kb) of DNA distal to pERT 87-1 (DXS164). This region has previously been shown to contain an exon of a candidate gene which, when defective, causes Duchenne muscular dystrophy (DMD) or Becker muscular dystrophy. Removal of this exon and surrounding DNA is apparently sufficient, in this case, to cause a BMD phenotype. The occurrence of this deletion in DXS164 would appear to confirm that this region is part of the BMD locus. Many DMD patients have deletions in and around this region, adding further evidence for the allelic nature of the two disorders. This fortuitous deletion may identify a functionally important domain of the protein product in terms of the severity of phenotype manifested.

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.

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.

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

A total of 278 families of probands with Duchenne or Becker muscular dystrophy has been ascertained and offered genetic counselling. Linkage studies have been performed in these families using polymorphic DNA markers identifying loci linked to Duchenne and Becker muscular dystrophy. The clinical features of the probands are discussed: there was marked intrafamilial resemblance in the severity of the disease. We estimate that a complete study of potential carriers in these families would require analysis of samples from approximately 1400 subjects. The results of linkage studies tended to move women's carrier risk estimates (based on CK and pedigree data) towards the extremes of the risk categories, providing a more definitive risk estimate for 81% of the women who were previously in the middle range of carrier risk probabilities. About 70% of the families had only one affected member. Linkage analysis altered carrier risk estimates in 95% of sisters and aunts of index cases, but only affected estimates of the mother's carrier risks in about 11% of isolated cases. Even where linkage studies were not helpful in elucidating carrier risks, information could usually be obtained for use in prenatal diagnosis if required. We have assessed the attitudes to pregnancy and prenatal diagnosis of women at risk of being carriers of Duchenne or Becker muscular dystrophy and report 17 pregnancies in these women.