Characterization of genetic deletions in Becker muscular dystrophy using monoclonal antibodies against a deletion-prone region of dystrophin

We have produced a new panel of 20 monoclonal antibodies (mAbs) against a region of the dystrophin protein corresponding to a deletion-prone region of the Duchenne muscular dystrophy gene (exons 45-50). We show that immuno-histochemistry or Western blotting with these "exon-specific" mAbs can provide a valuable addition to Southern blotting or PCR methods for the accurate identification of genetic deletions in Becker muscular dystrophy patients. The antibodies were mapped to the following exons: exon 45 (2 mAbs), exon 46 (6), exon 47 (1), exons 47/48 (4), exons 48-50 (6), and exon 50 (1). PCR amplification of single exons or groups of exons was used both to produce specific dystrophin immunogens and to map the mAbs obtained. PCR-mediated mutagenesis was also used to identify regions of dystrophin important for mAb binding. Because the mAbs can be used to characterize the dystrophin produced by individual muscle fibres, they will also be useful for studying "revertant" fibres in Duchenne muscle and for monitoring the results of myoblast therapy trials in MD patients with deletions in this region of the dystrophin gene.

Epitope mapping of recombinant antigens by transposon mutagenesis

We describe a method for generating a plasmid library expressing random truncations of a recombinant protein and for epitope mapping by screening the library with monoclonal antibodies. The key step is the random introduction of the transposon, Tn1000, which carries stop codons in all three reading frames, into a bacterial expression plasmid by using a simple bacterial mating procedure. Antibody-positive clones are then selected and the point of protein truncation is determined by sequencing the plasmid DNA at the point of transposon insertion. One advantage of the method is that no subcloning or in vitro manipulation of DNA is necessary.

Specificity and VH sequence of two monoclonal antibodies against the N-terminus of dystrophin

We have used a random library of 15-mer peptides expressed on phage to show that two monoclonal antibodies (mAbs) require only the first three amino acids of dystrophin (Leu-Trp-Trp) for binding. Since the mAbs recognize dystrophin in frozen muscle sections, the results suggest that this hydrophobic N-terminus of dystrophin is accessible to antibody in situ. Quantitative binding studies suggested minor differences in specificity between the two mAbs, so the Ig heavy-chain variable region (VH) sequences of the two hybridomas were determined by RT-PCR and cDNA sequencing. After elimination of PCR errors, the two cDNA sequences were found to be identical except for five somatic mutations which resulted in three amino acid changes in the second hypervariable region (CDR2). The results suggest that the two hybridomas originated from the same lymphocyte clone in a germinal centre of the spleen, but underwent different point mutations and subtype switches during clonal expansion to form blast cells.

Endovascular femoropopliteal bypass: a cadaveric study

OBJECTIVE

Patency rates of standard femoropopliteal bypass in infra-inguinal occlusive disease have yet to be matched by minimally invasive percutaneous procedures. We report a feasibility study of a less invasive endovascular femoropopliteal bypass technique.

METHODS

(1) groin exposure of femoral artery, (2) guidewire passage and mechanical dilatation of superficial femoral artery (SFA), (3) expandable helical cutter endarterectomy of SFA, (4) transluminal placement of PTFE graft, (5) graft balloon dilatation to shape and set distal interface and (6) end-to-end anastomosis of proximal graft to femoral artery. Development and testing was undertaken in 48 limbs of 26 fresh human cadavers. Limbs with no demonstrable SFA disease were excluded. Seventeen limbs had mild, diffuse disease. Three limbs had a single, short, tight stenosis. Seventeen limbs had multiple, high grade stenotic lesions 12-40 cm long (mean 24 cm). Eleven limbs had occlusive lesions, 8-38 cm long (mean 24 cm).

RESULTS

We successfully completed the procedure in 39 (81%) limbs. We failed to complete the procedure in nine limbs; four from failed guidewire passage, four from vessel avulsion, and one from graft deployment failure. Histology confirmed endarterectomy cleavage in the standard plane. Angiography and explants demonstrated a patent graft and popliteal artery, and smooth distal graft/arterial interface with no obvious defects in 24 (62%) cases. Defects included combinations of: contrast extravasation/reflux, graft malpositioned/incorrectly sized, distal graft fold, and distal intimal flap.

CONCLUSION

Endovascular femoropopliteal bypass is feasible and warrants further studies for possible clinical application.

A novel dystrophin isoform is required for normal retinal electrophysiology

Dystrophin is present in the outer plexiform layer of the retina and is required for normal retinal function as measured by electroretinography. We describe the identification of a novel isoform of dystrophin (Dp260) present in the mouse retina. The unique 5' terminus of the mRNA originates from a newly identified exon and is spliced in frame to exon 30 of the Duchenne muscular dystrophy (DMD) gene. The retinal isoform of dystrophin has 13 novel amino acids as its N-terminus followed by most of the dystrophin rod domain and the cysteine-rich C-terminal domains. Analysis of mouse tissues indicated this isoform of dystrophin is expressed in retina, brain and cardiac tissue. Comparison of retinal electrophysiology in mdx and mdxCv3 mouse suggests that Dp260 is required for normal retinal function.

The N-terminal half of dystrophin is protected from proteolysis in situ

Using a panel of "exon-specific" monoclonal antibodies, we have examined the products of degradation of dystrophin by endogenous proteases in post-mortem human muscle. Four main sites of dystrophin digestion were identified, all of them in the C-terminal half of the molecule. Two of them correspond to "hinges" in the central rod region and a third in the C-terminal domain follows the dystroglycan binding site. The results support the Koenig and Kunkel model for the tertiary structure of dystrophin (J. Biol. Chem. 265 (1990) 4560-4566), but suggest that much of the N-terminal half of dystrophin is protected from proteolysis, possibly by interaction with the sub-sarcolemmal cytoskeleton. Although the results seem inconsistent with an anti-parallel dimer model of dystrophin in which hinge 2 and hinge 3 are close together, possible ways of reconciling them with such a model are also considered.

Characterization of revertant muscle fibers in Duchenne muscular dystrophy, using exon-specific monoclonal antibodies against dystrophin

Most Duchenne muscular dystrophy (DMD) patients have genetic deletions or point mutations in the dystrophin gene that alter the reading frame of dystrophin mRNA. This causes early termination of translation, and no dystrophin (or, less commonly, a truncated N-terminal dystrophin fragment) is produced. In many DMD patients, however, a small proportion of muscle fibers show strong dystrophin staining, and these "revertant fibers" are thought to arise by a mechanism that restores the reading frame. Exon-specific monoclonal antibodies (mAbs) have now been used to determine, for the first time, which exons are removed, in order to correct the reading frame in individual muscle fibers. Thus, 15 revertant fibers in a DMD patient with a frameshift deletion of exon 45 were shown to correct the frameshift by the additional deletion of exon 44 (or perhaps exon 46 in some fibers) from the dystrophin mRNA, but not by larger deletions. This result was consistent with reverse transcription (RT)-PCR and sequencing of a minor dystrophin mRNA with an exon 43/46 junction in this biopsy. In a DMD patient with a frameshift deletion of exons 42 and 43, however, larger deletions than the minimum necessary were used to correct the frameshift. In this patient, who produces a half-size N-terminal dystrophin fragment in all fibers, we were able to show that the revertant dystrophin replaces the truncated dystrophin in revertant-fiber sarcolemma. The results are consistent with somatic mutations in revertant-fiber nuclei, which result in removal of additional exons from dystrophin mRNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha-actinin in nemaline bodies in congenital nemaline myopathy: immunological confirmation by light and electron microscopy

To elucidate the protein composition of the nemaline bodies present in the muscle fibres of patients with congenital nemaline myopathy (CNM), we studied muscle biopsies with monoclonal antibodies against alpha-actinin and desmin in combination with a modified Gomori trichrome method. Electron microscopy of immunolabelled resin embedded sections was used for cytochemical localisation of alpha-actinin and desmin. Light microscopy of sections immunolabelled for alpha-actinin showed a cross-striation of the muscle fibres corresponding to the Z band pattern, focal thickening of the Z bands and additional reactivity with a granular pattern corresponding to the presence of nemaline bodies. Labelling of desmin did not show a similar pattern. Electron microscopy confirmed the presence of alpha-actinin in the nemaline bodies and Z bands, whereas desmin was only found in intermediate filaments around the Z bands. Western blots showed single, sharp alpha-actinin bands indistinguishable from normal. Our results provide direct evidence for the presence of alpha-actinin in nemaline bodies and a lack of quantitative or qualitative differences between the alpha-actinin of normal and CNM muscle.

Full-length and short forms of utrophin, the dystrophin-related protein

All previous studies of the localization of utrophin (the dystrophin-related protein) in muscle and other tissues have been performed only with antibodies against the C-terminal region of the protein. Since several short forms of dystrophin, the apo-dystrophins, are produced from the 3' end of the dystrophin gene, there is a possibility that similar short forms of utrophin exist and that these could be responsible for some of the many different localizations of 'utrophin' in muscle. We have produced a new panel of 15 mAbs against the N-terminal region of utrophin and we have used it together with mAbs against the C-terminal region to show that full-length utrophin is present at neuromuscular junctions, in nerves, blood vessels and capillaries in normal muscle and in the sarcolemma of patients with muscular dystrophy and dermatomyositis. However, two of the 15 mAbs also recognised rat/mouse utrophin and both of these detected an additional 62 kDa protein on Western blots of rat C6 glioma cells. This potential 62 kDa 'apo-utrophin' was not detected in human cerebral cortex, in rat Schwannoma cells nor in any of the non-nerve cells and tissues tested.

Screening donor blood for malaria by polymerase chain reaction

In countries where malaria is endemic, its transmission is a hazard of blood transfusion. The microscopical and immunological methods in current use for malaria diagnosis are unsatisfactory for low levels of parasitaemia in blood donations. The polymerase chain reaction (PCR) can be 100-fold more sensitive than thick blood film examination when appropriate primers are used and can detect and distinguish Plasmodium falciparum and P. vivax in a single tube. A study of 1506 blood donations in Ho Chi Minh City (3 of which were positive) suggests that PCR can provide an effective screen for P. falciparum under local conditions. Studies in a region of Viet Nam where malaria is common showed that PCR detects many more cases of low-level parasitaemia (19/30) than thick blood films (4/30).

Apo-dystrophin-1 and apo-dystrophin-2, products of the Duchenne muscular dystrophy locus: expression during mouse embryogenesis and in cultured cell lines

Two promoters in the distal half of the Duchenne Muscular Dystrophy gene drive transcription of mRNAs which have novel first exons and encode the shortened forms of dystrophin, apo-dystrophin-1 (Dp71) and apo-dystrophin-2 (Dp116). Apo-dystrophin-1 has a G + C rich promoter and is expressed in a wide range of cell types, whilst apo-dystrophin-2 is confined to peripheral nerve and brain. We have isolated and sequenced the unique 5' exon of rat apo-dystrophin-2 mRNA. Conceptual translation of this sequence indicates that apo-dystrophin-2 contains a unique 23 amino acid terminal peptide. Using specific probes derived from sequences at the 5' ends of apo-dystrophin-1 and apo-dystrophin-2 we have determined the expression of these two mRNAs during mouse embryonic development by RNA in situ hybridization. In contrast to full-length dystrophin, neither of these short dystrophin transcripts appear before organogenesis is well established. Apo-dystrophin-1 mRNA is detected in midline cells of the ventral neural tube and later, in the ependymal cells lining the ventricles of the brain. These results suggest that apo-dystrophin-1 mRNA is associated with glial cells in the CNS. Apo-dystrophin-1 transcripts are also abundant in the teeth primordia throughout their development. In contrast apo-dystrophin-2 mRNA is largely undetectable during development, although transcripts are seen in the newborn brain. Western blots of late human fetal tissue extracts confirm that apo-dystrophin-2 is most abundant in brain and analysis of RNA and protein in cultured cell lines reveal expression of apo-dystrophin-1 and apo-dystrophin-2 in glioma cells.

Antibiotic irrigation and conservative surgery for major aortic graft infection

PURPOSE

Traditional surgical treatment for panprosthetic aortic graft infection entails radical excision of the graft, aortic stump closure, and extraanatomic revascularization of the lower limbs. This carries an early mortality rate of 24% to 45%. Amputation rates range from 11% to 37%. Multiple operations and prolonged hospital stay are usual. We have developed a more conservative management technique with the aim of improving outcome.

METHODS

We describe an innovative method of treating the condition with prolonged, high-dose, local antibiotic irrigation therapy, systemic antibiotic treatment, surgical debridement, and graft conservation in a prospectively studied series of 10 patients.

RESULTS

The actual 30-day patient survival rate is 90%, the 1-year survival rate is 80%, and the 4-year survival rate is 67%. Two patients died because of graft infection, and the third died, uninfected, of an unrelated cause. No limbs have been amputated. Only two patients required a second operation. Mean postoperative hospital stay was 32 days. The seven survivors have been closely followed up with regular computed tomography or indium scanning and clinical examination and appear to be free from infection at a mean of 61 months after cessation of irrigation therapy.

CONCLUSION

The technique appears to represent a significant improvement in the management of this major complication of vascular surgery.

An abdominal aortic aneurysm screening programme for all males over the age of 50 years

Previously published ultrasound screening programmes for abdominal aortic aneurysm (AAA) have concentrated on males in the 65 to 75 year age range, suggesting this as the most cost-effective cohort to target. In this unique study we have broadened the criteria for screening. General practitioners in one health district were approached to supply details of all males aged 50 years and over to be offered aortic ultrasound scanning. Over a period of 18 months, 4145 individuals were asked to attend and 3030 (73%) have attended. Attendance rates were: between 50 and 64 years, 73%; between 65 and 79 years, 75%; for 80 years and over, 64%--significantly less (p = 0.01-0.001) than the other two age bands. Aortic dilatation (diameter > or = 2.5 cm) was found in 6.3% of the 50 to 64 year age group, 16.8% of the 65 to 79 year age group, and 23.3% of the 80 years and over age group. An established aneurysm (> or = 4.6 cm) was found in 0.3%--6 individuals (50-64 years), 2.5% (65-79 years) and 4.1% (> or = 80 years). The results suggest that aortic screening may be worthwhile extending to a wider age band. By focusing follow-up, this should give greater value for younger men in terms of community productivity and allows for selective intervention in the elderly.

Interleukin-11 expression in donor bone marrow cells improves hematological reconstitution in lethally irradiated recipient mice

Lethally irradiated mice were transplanted with syngeneic bone marrow cells infected with a recombinant retrovirus vector containing the human interleukin-11 (hIL-11) cDNA under the control of the human cytomegalovirus (CMV) immediate early promoter. The hIL-11 RNA transcript from the vector was detected in the spleen and bone marrow of the recipient mice, and hIL-11 protein accumulated in their serum. The hematological reconstitution of these mice was compared with recipient mice rescued with bone marrow infected with the parental retrovirus vector not containing the hIL-11 cDNA. The hIL-11-expressing mice had an accelerated recovery of circulating platelets and red and white blood cells. Three months after the transplantation, bone marrow was harvested from the mice and used to rescue other lethally irradiated recipients. The hIL-11 mRNA and protein were also detected in these secondary recipients, and the mice showed improved hematological reconstitution relative to a control group. No abnormal cell proliferation or other histopathology was observed in the hIL-11-expressing mice.

Expression of the 43 kDa dystrophin-associated glycoprotein in human neuromuscular disease

The expression of the 43 kDa dystrophin-associated glycoprotein (43DAG) has been studied using immunohistochemical labelling with a monoclonal antibody, MANDAG-1, and compared with immunolabelling for dystrophin and the dystrophin-related protein, utrophin, in normal muscle and in muscle from 50 patients with neuromuscular disease. 43DAG and dystrophin were expressed in vascular smooth muscle and at the sarcolemma of normal muscle fibres, with increased labelling at neuromuscular and myotendinous junctions. 43DAG expression was reduced in Duchenne and Becker dystrophies with patchy labelling, more intense around presumptive satellite cells. In Duchenne dystrophy, there was increased 43DAG expression in "revertant" fibres. In Becker dystrophy, 43DAG expression was more extensive around individual fibres, showed more interfibre variation and was more closely related to the intensity of immunolabelling for both dystrophin and utrophin than in Duchenne dystrophy. In other neuromuscular diseases, including congenital muscular dystrophy, no abnormalities of 43DAG expression were identified. The results suggest that in the absence of dystrophin, 43DAG is synthesized but is not stabilized in the sarcolemma. Stability is greater in Becker dystrophy but a normal dystrophin molecule appears to be required for the complete and stable membrane integration of 43DAG. Utrophin may confer some additional stability to the membrane integration of 43DAG but this is incomplete where dystrophin is absent or abnormal.

Expression of the transcripts initiated in the 62nd intron of the dystrophin gene

The pattern of expression of two distal transcripts initiated in the 62nd intron of the dystrophin gene was investigated under different circumstances; (i) during the development of different rat tissues these transcripts and Dp71, a protein encoded by one of them, increased with brain development and decreased with muscle development; (ii) in cultured glial and neuronal cells, the distal promoter was coactivated with tissue-specific upstream promoters, the muscle-type promoter in glial cells and the brain-type promoter in neuronal cells, which suggests that activity of the upstream promoter does not interfere with activity of the distal promoter; (iii) in lymphoblasts of DMD patients with various deletions of the dystrophin gene, the most distal of which included the 56th intron, the production of the distal transcript was not perturbed.

Molecular cloning of murine FLT and FLT4

A wide range of growth and differentiation processes are regulated by the signalling of receptor tyrosine kinases (RTKs). We have developed a nested polymerase chain reaction (PCR) procedure with degenerate primers, and used it to identify RTKs expressed in murine fetal thymus. A novel RTK, called FLT4, and the murine homologue of FLT were found, and their PCR fragment sequences were used to isolate larger cDNA clones spanning the complete coding regions of these receptors. FLT4 was found to contain an extracellular region similar to the corresponding sequences of FLT and Flk-1, containing seven immunoglobulin domains.

Dystrophin-related protein, utrophin, in normal and dystrophic human fetal skeletal muscle

Dystrophin is the product of the Duchenne muscular dystrophy (DMD) gene. Dystrophin-related protein (utrophin), an autosomal homologue of dystrophin, was studied in skeletal muscle from normal fetuses aged 9-26 weeks and one stillbirth of 41 weeks' gestation, and compared with low- and high-risk DMD fetuses aged 9-20 weeks. Utrophin was present at the sarcolemma from before 9 weeks' gestation, although there was variability in intensity both within and between myotubes. Sarcolemmal immunolabelling became more uniform, and levels of utrophin increased to a maximum at approximately 17-18 weeks. Levels then declined, until by 26 weeks sarcolemmal labelling was negligible and levels were similar to adult control muscle. By 41 weeks there was virtually no sarcolemmal labelling, although immunolabelling of capillaries was bright. Similar results were obtained with normal and DMD fetal muscle. Utrophin is therefore expressed in the presence and absence of dystrophin and down-regulated before birth in normal fetal muscle fibres. Samples were not available to determine whether or when, utrophin levels decline in DMD fetal muscle. On Western blots, utrophin was shown to have a smaller relative molecular mass than adult dystrophin, but similar to the fetal isoform. Blood vessels were brightly immunolabelled at all ages, although utrophin immunolabelling of peripheral nerves increased with gestational age.

Monoclonal antibodies against the muscle-specific N-terminus of dystrophin: characterization of dystrophin in a muscular dystrophy patient with a frameshift deletion of exons 3-7

The first three exons of the human muscle dystrophin gene were expressed as a beta-galactosidase fusion protein. This protein was then used to prepare two monoclonal antibodies (mAbs) which react with native dystrophin on frozen muscle sections and with denatured dystrophin on western blots but which do not cross-react with the dystrophin-related protein, utrophin. Both mAbs recognized dystrophin in muscular dystrophy (MD) patients with deletions of exon 3, and further mapping with 11 overlapping synthetic peptides showed that they both recognize an epitope encoded by the muscle-specific exon 1. Neither mAb recognizes the brain dystrophin isoform, confirming the prediction from mRNA data that this has a different N-terminus. One Becker MD patient with a frameshift deletion of exons 3-7 is shown to produce dystrophin which reacts with the N-terminal mAbs, as well as with mAbs which bind on the C-terminal side of the deletion. The data suggest that transcription begins at the normal muscle dystrophin promoter and that the normal reading frame is restored after the deletion. A number of mechanisms have been proposed for restoration of the reading frame after deletion of exons 3-7, but those which predict dystrophin with an abnormal N-terminus do not appear to be major mechanisms in this patient.

Use of epitope libraries to identify exon-specific monoclonal antibodies for characterization of altered dystrophins in muscular dystrophy

The majority of mutations in Xp21-linked muscular dystrophy (MD) can be identified by PCR or Southern blotting, as deletions or duplications of groups of exons in the dystrophin gene, but it is not always possible to predict how much altered dystrophin, if any, will be produced. Use of exon-specific monoclonal antibodies (mAbs) on muscle biopsies from MD patients can, in principle, provide information on both the amount of altered dystrophin produced and, when dystrophin is present, the nature of the genetic deletion or point mutation. For this purpose, mAbs which recognize regions of dystrophin encoded by known exons and whose binding is unaffected by the absence of adjacent exons are required. To map mAbs to specific exons, random "libraries" of expressed dystrophin fragments were created by cloning DNAseI digestion fragments of a 4.3-kb dystrophin cDNA into a pTEX expression vector. The libraries were then used to locate the epitopes recognized by 48 mAbs to fragments of 25-60 amino acids within the 1,434-amino-acid dystrophin fragment used to produce the antibodies. This is sufficiently detailed to allow further refinement by using synthetic peptides and, in many cases, to identify the exon in the DMD (Duchenne MD) gene which encodes the epitope. To illustrate their use in dystrophin analysis, a Duchenne patient with a frameshift deletion of exons 42 and 43 makes a truncated dystrophin encoded by exons 1-41, and we now show that this can be detected in the sarcolemma by mAbs up to and including those specific for exon 41 epitopes but not by mAbs specific for exon 43 or later epitopes.

A quantitative ELISA for dystrophin

A novel approach to the quantitation of the muscular dystrophy protein, dystrophin, in muscle extracts is described. The two-site ELISA uses two monoclonal antibodies against dystrophin epitopes which lie close together in the rod domain of the dystrophin molecule in order to minimize the effects of dystrophin degradation. Dystrophin is assayed in its native form by extracting with non-ionic detergents and avoiding the use of SDS.