MUSCLE HISTOLOGY IN CARRIERS OF DUCHENNE MUSCULAR DYSTROPHY

Advances in Dystrophinopathy Diagnosis and Therapy

Dystrophinopathies are x-linked muscular disorders which emerge from mutations in the Dystrophin gene, including Duchenne and Becker muscular dystrophy, and dilated cardiomyopathy. However, Duchenne muscular dystrophy interconnects with bone loss and osteoporosis, which are exacerbated by glucocorticoids therapy. Procedures for diagnosing dystrophinopathies include creatine kinase assay, haplotype analysis, Southern blot analysis, immunological analysis, multiplex PCR, multiplex ligation-dependent probe amplification, Sanger DNA sequencing, and next generation DNA sequencing. Pharmacological therapy for dystrophinopathies comprises glucocorticoids (prednisone, prednisolone, and deflazacort), vamorolone, and ataluren. However, angiotensin-converting enzyme (ACE) inhibitors, angiotensin receptor blockers (ARBs), and β-blockers are the first-line to prevent dilated cardiomyopathy in dystrophinopathy patients. Duchenne muscular dystrophy gene therapy strategies involve gene transfer, exon skipping, exon reframing, and CRISPR gene editing. Eteplirsen, an antisense-oligonucleotide drug for skipping exon 51 from the Dystrophin gene, is available on the market, which may help up to 14% of Duchenne muscular dystrophy patients. There are various FDA-approved exon skipping drugs including ExonDys-51 for exon 51, VyonDys-53 and Viltolarsen for exon 53 and AmonDys-45 for exon 45 skipping. Other antisense oligonucleotide drugs in the pipeline include casimersen for exon 45, suvodirsen for exon 51, and golodirsen for exon 53 skipping. Advances in the diagnosis and therapy of dystrophinopathies offer new perspectives for their early discovery and care.

Neurogenetic motor disorders

Advances in the field of neurogenetics have practical applications in rapid diagnosis on blood and body fluids to extract DNA, obviating the need for invasive investigations. The ability to obtain a presymptomatic diagnosis through genetic screening and biomarkers can be a guide to life-saving disease-modifying therapy or enzyme replacement therapy to compensate for the deficient disease-causing enzyme. The benefits of a comprehensive neurogenetic evaluation extend to family members in whom identification of the causal gene defect ensures carrier detection and at-risk counseling for future generations. This chapter explores the many facets of the neurogenetic evaluation in adult and pediatric motor disorders as a primer for later chapters in this volume and a roadmap for the future applications of genetics in neurology.

Outdated risk assessment in a family with Duchenne dystrophy: Implications for duty to reassess

Carrier risk assessment for Duchenne muscular dystrophy (DMD) is necessary to counsel women at risks of developing cardiomyopathy and having a child with DMD. Comprehensive molecular testing for dystrophin gene mutations has only been available since 2003¹; women counseled earlier have outdated risk assessments. We present a 5-generation family in whom results of familial mutation testing for DMD newly identified 10 obligate carriers and 28 women at risk to be carriers for DMD.

Reassessing carrier status for dystrophinopathies

The cloning of the DMD gene, and the identifications of mutations in it as the cause of Duchenne muscular dystrophy (DMD), makes a compelling story that is aptly told elsewhere.¹ The locus-the largest in the human genome-consists of 79 exons, distributed over 2.5 million nucleotides on the X chromosome, which are assembled into a complementary DNA (cDNA) of around 14 kb encoding the predominant muscle isoform of the dystrophin protein.² The size of the gene, and the number of exons, had historically made mutation analysis challenging. For more than a decade, the standard clinical assay was a multiplex PCR test that amplified sequences from a limited number of exons; nevertheless, because it included exons within the deletion hotspots of the gene, this method could confirm the presence of mutations in up to 98% of boys with exonic deletions.^3,4.

Cardiac structure and function in female carriers of a canine model of Duchenne muscular dystrophy

This investigation tested the hypothesis that carriers of golden retriever muscular dystrophy (GRMD), a genetically homologous condition of Duchenne muscular dystrophy (DMD), have quantifiable abnormalities in myocardial function, structure, or cardiac rhythm. Eleven GRMD carriers and four matched controls had cardiac evaluations and postmortem examinations. 24-h ECG Holter monitoring disclosed ventricular ectopy in 10 of 11 carriers and 2 of 4 controls. Conventional echocardiography failed to demonstrate significant differences between carriers and controls in systolic function. All carriers had multifocal, minimal to marked myofiber necrosis, fibrosis, mineralization, inflammation, and/or fatty change in their hearts. Immunohistochemistry revealed a mosaic dystrophin deficiency in scattered cardiac myofibers in all carriers. No controls had cardiac histologic lesions; all had uniform dystrophin staining. Despite cardiac mosaic dystrophin expression and degenerative cardiac lesions, GRMD carriers at up to 3 years of age could not be distinguished statistically from normal controls by echocardiography or 24-h Holter monitoring.

Quantification of normal dystrophin mRNA following myoblast transplantation in mdx mice

A mutagenesis RT-PCR method was used to detect normal dystrophin mRNA following the injection of normal myoblasts in mdx mice using two immunosuppressors. A specific sequence of the dystrophin mRNA (257 bp) was amplified by RT-PCR from the muscle total RNA. MaeIII digestion of the amplified products allowed us to distinguish the normal messenger of dystrophin from the dystrophic one and to establish the percentage of normal and of dystrophic (mdx) dystrophin mRNA. Normal dystrophin mRNA was detected using this technique in mdx muscles transplanted with histocompatible normal myoblasts. For this type of transplantation, no significant difference in the percentage of normal dystrophin mRNA was observed between immunosuppressed mice and those not immunosuppressed. No normal dystrophin mRNA was, however, observed in mdx mice following histoincompatible normal myoblast transplantation without immunosuppression. When such transplantations were done in mice immunosuppressed with cyclosporine or FK-506, normal dystrophin mRNA accounted for 31% and 36% of the total dystrophin mRNA, respectively. In fact, one animal immunosuppressed with FK-506 expressed as much as 57% of normal dystrophin mRNA. These results thus show that FK-506 makes it possible to restore dystrophin expression to a level comparable to that observed in DMD carriers that are usually asymptomatic.

Mosaic expression of dystrophin in symptomatic carriers of Duchenne's muscular dystrophy

A deficiency of the protein dystrophin is known to be the cause of Duchenne's muscular dystrophy. To examine the expression of dystrophin in symptomatic female carriers of this X-linked recessive disorder, we performed immunohistochemical studies on muscle-biopsy specimens from three such carriers, using an antiserum raised against a synthetic peptide fragment of dystrophin. In all three carriers, most individual muscle fibers reacted either strongly or not at all to the antiserum for dystrophin; only 2 to 8 percent of fibers showed partial immunostaining. This mosaic staining pattern was present on both cross-sectional and longitudinal muscle specimens. Although the mosaic pattern was seen in all fiber types, more than 80 percent of type 2B and 2C fibers from two of the carriers did not react with the antiserum. Similar studies in nine normal subjects showed consistently strong staining of all muscle fibers. No muscle fibers from 31 patients with Duchenne's muscular dystrophy reacted with the antiserum. We conclude that symptomatic carriers of Duchenne's muscular dystrophy can be identified by a distinct mosaic pattern in the immunohistochemical staining of the surface membrane of skeletal-muscle specimens. This finding may have practical implications for genetic counseling, although it remains to be shown whether the same staining pattern will be found in muscle specimens from asymptomatic carriers of Duchenne's muscular dystrophy.

Normal and dystrophin-deficient muscle fibers in carriers of the gene for Duchenne muscular dystrophy

Dystrophin is the gene product that is affected in Duchenne muscular dystrophy (DMD). Antibodies against dystrophin were used to study the protein in muscle fibers of carriers of the gene. The results showed that DMD carriers have normal and dystrophin-deficient fibers. Dystrophin immunohistochemistry may be helpful for the detection of DMD carriers.

Computed tomographic findings in manifesting carriers of Duchenne muscular dystrophy

Clinical and computed tomographic (CT) findings in 3 manifesting carriers of Duchenne muscular dystrophy are reported. CT proved to be an important adjunct to the clinical examination: in all our 3 cases a decrease in density was found in various non-paretic muscles.

Duchenne muscular dystrophy: pathogenetic aspects and genetic prevention

Duchenne muscular dystrophy (DMD) is the most common sex linked lethal disease in man (one case in about 4000 male live births). The patients are wheelchair bound around the age of 8-10 years and usually die before the age of 20 years. The mutation rate, estimated by different methods and from different population studies, is in the order of 7 X 10(-5), which is higher than for any other X-linked genetic disease. Moreover, unlike other X linked diseases such as hemophilia A or Lesh-Nyhan's disease, there seems to be no sex difference for the mutation rates in DMD. Several observations of DMD in girls bearing X-autosomal translocations and linkage studies on two X chromosomal DNA restriction fragment length polymorphisms indicate that the DMD locus is situated on the short arm of the X chromosome, between Xp11 and Xp22. It may be of considerable length, and perhaps consisting of actively coding and non-active intervening DNA sequences. Thus unequal crossing over during meiosis in females could theoretically account for a considerable proportion of new mutations. However, there is no structurally or functionally abnormal protein known that might represent the primary gene product, nor has any pathogenetic mechanism leading to the observed biochemical and histological alterations been elucidated. Among the numerous pathogenetic concepts the hypothesis of a structural or/and functional defect of the muscular plasma membrane is still the most attractive. It would explain both the excess of muscular constituents found in serum of patients and carriers, such as creatine kinase (CK), as well as the excessive calcium uptake by dystrophic muscle fibres, which, prior to necrosis, could lead to hypercontraction, rupture of myofilaments in adjacent sarcomeres and by excessive Ca uptake to mitochondrial damage causing crucial energy loss. The results of studies on structural and functional membrane abnormalities in cells other than muscle tissue, e.g., erythrocytes, lymphocytes and cultured fibroblasts, indicate that the DMD mutation is probably demonstrable in these tissues. However, most of the findings are still difficult to reproduce or even controversial. DMD is an incurable disease; therefore most effort, in research as well as in practical medicine, is concentrated upon its prevention.(ABSTRACT TRUNCATED AT 400 WORDS)

The nuclear-cytoplasmic relationship in 'mosaic' skeletal muscle fibers from mouse chimaeras

In mouse chimaeras, individual skeletal muscle fibers typically contain populations of myonuclei derived from both cell lines. This 'mosaic' circumstance has provided an opportunity to investigate directly whether the mammalian myofiber syncytium is functionally subdivided into territories, each preferentially influenced by products encoded by the local myonucleus, or whether the multiple nuclei direct the synthesis of products that achieve a uniform distribution throughout the fiber. Chimaeras were produced in which one cell line was derived from an embryo homozygous for gpi-1a, whereas the other was homozygous for the gpi-1b; each allele specifies electrophoretically distinguishable isozymes of the cytosolic enzyme glucosephosphate isomerase (GPI-1). Microtechniques capable of measuring the proportion of each isozyme expressed within small samples of individual muscle fibers have been established, permitting the comparison of the relative quantitative distributions of the GPI-1 isozyme types along the length of individual chimaera fibers. From individual mosaic fibers, all samples yielded identical isozyme profiles, demonstrating that GPI-1 is not sequestered adjacent to the nucleus directing its synthesis; rather, it achieves a homogeneous distribution throughout the mosaic syncytium. The GPI-1 gene locus encodes only the GPI-1 monomer, whereas the functional enzyme detected in our analysis is a dimer that results from the aggregation of monomers in the cytoplasm. The quantitative distribution of dimer types within each mosaic fiber was consistent with random aggregation amongst all monomers represented in the final isozyme pattern, a result requiring that monomers or earlier precursors were mixed in the myofiber cytoplasm prior to assembly of the enzymatically active dimer. Thus, both the final distribution of enzyme dimers within fibers and the patterns of monomer aggregation suggest that there are no subdivisions related to the spatial separation of the genotypically distinct myonuclei within mosaic muscle fibers.

Serum myoglobin in muscular dystrophy and carrier detection

Serum myoglobin was measured by radioimmunoassay in 143 patients with various muscle diseases including 55 boys with Duchenne dystrophy, 56 carriers of the Duchenne dystrophy gene, 8 carriers of the Becker dystrophy gene, 60 first-degree relatives of patients with sporadic (non-genetically transmitted) muscle diseases and 85 normal controls. A significant difference (P less than 0.001) was found between the serum myoglobin levels in normal control men (x = 31.7 range 10-70 microgram/l) and those in women (x = 17.2 range 4-27 microgram/l) but no difference was found between the controls and relatives of patients with muscle disease. Up to the age of 60 years, no correlation was found between age and serum myoglobin levels in controls. However, levels in boys with Duchenne dystrophy were found to increase slightly up to the age of 10 years and to decrease thereafter. No correlation was found between serum myoglobin and creatine kinase activity in these boys but in Duchenne carriers correlation was close (P less than 0.001). Eighteen of 23 definite and 13 of 33 possible carriers of Duchenne dystrophy had myoglobin concentrations above the equivalent normal range. Of those carriers with elevated myoglobin levels, 7 definite and 4 possible carriers had normal serum creatine kinase activity. This was equivalent to an improvement in "detection" of 31% and 12% in these groups of carriers. Of the 8 Becker dystrophy carriers, 6 had elevated myoglobin but all had normal creatine kinase levels. It appears that measurement of serum myoglobin as well as creatine kinase activity may be of great benefit to carrier detection programmers, but the need to establish accurately the extent of normal variation in women is emphasized.

Needle muscle biopsy for carrier detection in Duchenne muscular dystrophy. Part 1. Light microscopy--histology, histochemistry and quantitation

Needle muscle biopsies from 14 genetically proven carriers of Duchenne muscular dystrophy (DMD), 14 possible carriers with elevated serum creatine kinase activity (SCK) and 37 possible carriers with normal SCK were compared with similar samples from 10 healthy volunteer females with the aim of identifying subtle changes that might aid carrier detection. Qualitative differences were similar to those previously reported and included variation in fibre size, an increase in the number of internal nuclei and fragmentation of the myofibrillar network. By visual assessment, only 11% of the biopsies were classified as unequivocally abnormal but quantitation of fibre type proportions, fibre diameters and the number of internal nuclei showed that 71% of definite carriers, 86% of possible carriers with elevated SCK and 78% of possible carriers with normal SCK had at least one feature that was significantly different from controls. The commonest abnormalities were an increase in the proportion of type 1 fibres, an alteration in the mean diameter ratio of type 1:type 2 fibres, an increase in the fibre diameter variation coefficient and an increase in the number of internal nuclei. The changes in definite carriers were not related to elevated SCK and several with normal SCK showed morphological differences. It was concluded that quantitation of morphological features is essential for a full appraisal of biopsies from DMD carriers and that this approach is a useful aid to carrier detection.

X-linked recessive (Duchenne) muscular dystrophy (DMD) and purine metabolism: effects of oral allopurinol and adenylate

Data are presented which suggest that Duchenne muscular dystrophy (DMD) may have some origin in a severe deficiency of total muscle adenine nucleotides. Using double-blind techniques, this possibility was tested in 16 DMD patients by giving oral allopurinol, a synthetic inhibitor of the purine catabolic enzyme xanthine oxidase. Sublingual procaine adenylate was also briefly tested. Instances of clinical improvement quickly occurred which were statistically significant; they were accompanied by a significant increase in physical strength. These improvements have been maintained for more than 6 mo by administration of a small amount of allopurinol daily. Procaine adenylate had little effect. These results support the above view of DMD and seem to indicate that existing purines, retained and recycled after allopurinol, can sustain such improvement, and that additional adenylate is unnecessary.

Quantitative EMG and histological carrier detection of Duchenne muscular dystrophy

Seventy-nine women known as, or suspected to be, carriers of the Duchenne type of muscular dystrophy were examined. The 15 known carriers had an estimation of the CPK serum level and a manual quantitative EMG, which gave the high detection rate of 93%. The 64 suspected carriers had CPK determination and quantitative EMG, or CPK and muscle biopsy, and the value of each technic is discussed. The problem of giving a reassuring answer to women considered to be possible carriers on genetic criteria, but who are not really carriers, is solved if the results of all three tests are negative.

Carrier detection in X-linked recessive (Duchenne) muscular dystrophy: pyruvate kinase isoenzymes and creatine phosphokinase in serum and blood cells

Allosterism allows individual assay of both isoenzymes, one abundant in muscle, of pyruvate kinase (PK), recently reported superior to serum creatine phosphokinase (CPK) in detecting patients with and female carriers of X-linked recessive (Duchenne) muscular dystrophy (DMD). Extensive comparative studies did not support these findings and confirmed the marked superiority of CPK over rariants of PK or other enzymes in sensitivity, stability and convenience. Deducting the adenylate kinase increment (AKI) further refined the CPK assay, eliminating the effect of haemolysis in diagnosis and enabling studies of blood cell content. Both leucocytes and erythrocytes liberated PK and lactate dehydrogenase (LDH) after brief chilling or disruption. Only erythrocytes showed a CPK content, however, constantly adjusted to match that of serum as if by free cell membrane passage, but less accomodating to a sudden large influx of CPK than of LDH, where an apparent buffering effect could account for differences in clinical response.

Spectrum of function of neutrophils from carriers of sex-linked chronic granulomatous disease

Bactericidal and metabolic activities were compared for polymorphonuclear leukocytes from normal controls, patients with sex-linked chronic granulomatous disease, five obligate carriers, and six potential carriers of CGD. Bacteria surviving at one hour were quantitated in a standardized assay which employed 1.25 Staphylococcus aureus per neutrophil. Heat-killed bacteria or a chemical agent, phorbol myristate acetate, were used to stimulate increases in utilization of oxygen, oxidation of [1--14C] glucose, and reduction of neotetrazolium chloride by PMN. The results demonstrate that PMN from the individual obligate carriers of CGD have a broad spectrum of functional capabilities. Neutrophils from one obligate carrier performed in the above in vitro tests and others on a par with normal control cells, whereas the PMN of others displayed deficiencies nearly as profound as those of the affected CGD patients. The observations parallel the broad range of phenotypic expression observed in heterozygotic carriers of other sex-linked recessive disorders as a result of random inactivation of the X chromosome. Although predictable from the current concept of random X inactivation, the spectrum has not been previously demonstrated for carriers of sex-linked recessive CGD and thus has important implications for the detection and counseling of carriers of CGD.

Studies on the carrier state in X-linked recessive (Duchenne) muscular dystrophy

Observations in 12 normal women and 12 female carriers of X-linked recessive Duchenne muscular dystrophy (DMD), of whom 4 had symptoms and 8 had none, were compared between all 4 groups and with those in 2 DMD boys, one active and one crippled. Carrier symptoms were readily ascertained by systematic examination. Measurement of both lower legs in all 24 women showed neither calf enlargement nor asymmetry in carriers beyond normal variation. Two DMD carrier daughters were noted of the same DMD carrier mother but by different fathers. Whole body counting showed the biological half-life of previously administered 86Rb to be much reduced in DMD, but no differences were found between normal women and any group of carriers. The test is thus valueless for carrier detection, and reasons are given why it should be so. Simultaneous measurement of total body K+, with subsequent determination by isotope dilution of total body water as 3H2O space and extracellular water as NH4 82Br space, showed increased intracellular water and reduced intracellular K+ concentration in all carriers, as if due to osmotic causes, with actual loss of muscle mass and slight diminution of serum K+ in the 4 symptomatic carriers only. Because of diurnal and other variations, the means and standard deviations for six serum enzymes from six fortnightly assays in all subjects were used to measure precise individual status. Their coefficients of variation were abnormal only in symptomatic carriers and ambulant DMD, easily overtexed by even accustomed exertion. This is shown to support previous propositions on the pathogenesis of DMD and the escape of muscle cell content into the circulation.

Serum creatine kinase studies in the detection of carriers of Duchenne dystrophy

A number of methods which might improve the detection of carriers of Duchenne muscular dystrophy, based on the estimation of serum creatine kinase (CK), have been tried in an attempt to improve the 70% detection rate obtained with random samples. One series of experiments involved controlled exercise on a bicycle ergometer, the second series was based on sampling of muscle venous blood after controlled ischaemic exercise, and a third series of experiments was carried out to see whether the rate of enzyme inactivation was different in carriers of Duchenne dystrophy compared with controls. These methods were not found to improve the carrier detection rate based upon serum CK estimation.

Histochemically demonstrable fibre abnormalities in normal skeletal muscle and in muscle from carriers of Duchenne muscular dystrophy

Deltoid muscle was removed at motor point biopsy from 10 female relatives of patients with Duchenne muscular dystrophy and from seven others, with no evidence of neuromuscular disease. Transverse cryostat sections of the muscle from each case were stained for reduced diphosphopyridine nucleotide diaphorase and it was found that all contained varying numbers of degenerating type 1 fibres. The percentage of abnormal fibres in the type 1 fibre population was then calculated for each case and it was found that the muscles from the patients with dystrophic relatives contained considerably higher percentages of abnormal fibres, which also showed more severe degeneration, than did the muscles from the normal cases. There was no absolute correlation between serum creatine kinase levels and degree of pathological change, though patients with the most severe changes in their muscles had abnormally high serum creatine kinase levels. It is suggested that histochemical studies could be a useful addition to the present diagnostic tests for the carrier state in Duchenne muscular dystrophy.

Biochemical studies on haemoglobin variants of the irus macaque

Haemolysates from 202 M . irus , imported mainly from Thailand and Vietnam, were examined by starch-gel electrophoresis. In addition to the normal haemoglobin, Hb-A mi , two major haemoglobin variants designated as Hb-P mi and Hb-Q mi and two minor components were found. Hb-P mi , which occurred in 12% of the sample, forms molecular aggregates, especially when released from the red cell. Peptide analysis showed that it differs from Hb-A mi in the absence of peptides α .TP III and α -Tp IV. Sera from animals with this haemoglobin in their red cells show two haeme-positive bands in addition to the usual single haptoglobin band; this pattern can be produced in the sera of some animals which do not possess it, by addition of Hb-P mi . Hb-Q mi , which occurred in 24% of the animals, migrates anodally to Hb-A mi at alkaline pH and does not form aggregates. It is found in two ranges of concentration when present with Hb-A mi . It was shown by recombination experiments to have normal β Ami -chains. The sample was polymorphic for a minor component which was shown to have normal β Ami chains. Some animals have two major haemoglobins and also this minor component and therefore possesses three different non- β -chains. It is suggested that the minor component is the product of a mutated duplicate of the α -locus. The population genetics of these variant haemoglobins and the possible selective role of simian malaria are discussed.