New mdx mutation disrupts expression of muscle and nonmuscle isoforms of dystrophin.

The dystrophin gene encodes several tissue-specific protein isoforms that are generated by alternative splicing and by transcription from at least three separate promoters. We have characterized the mutation in a new strain of mdx mice that results in aberrant splicing of both the 14 and 4.8 kilobase dystrophin mRNAs and disrupts expression of the muscle and brain 427K and nonmuscle 70K isoforms of dystrophin. In contrast, we have determined that expression of the 70K isoform is normal in the original mdx mutant. We have cloned the unique 5' exon of the murine 4.8 kb mRNA and have analysed the tissue distribution and aberrant splicing of this transcript in the mdx3Cv mutant. This new mdx mutant will provide an improved model system for functional studies of the dystrophin C-terminus in muscle and nonmuscle tissues.

PCR analysis of muscular dystrophy in mdx mice.

PCR amplification has enabled a variety of studies to be performed on the murine dystrophin transcripts. Figure 7.12 displays a summary of the features of the murine dystrophin mRNA that have been described in this article. The location of the mutation in the original mdx mouse is indicated, as are the different spliced forms of the dystrophin transcript. Also shown are the location of various PCR primer binding sites that were used to deduce the alternative splicing pattern of the gene. It is likely that conventional cloning efforts aimed at identifying the variety of dystrophin spliced forms would have taken years to perform, particularly since several of the isoforms are expressed at levels significantly below the estimated 0.02% of total mRNA that dystrophin represents in skeletal muscle (Hoffman et al., 1987a, b). Amplification of dystrophin mRNA simplifies scanning methods for the identification of DNA sequence variations. Attempts to re-isolate and sequence the 14 kb cDNA to determine the mutation in separate strains of mdx mice are not likely to be time or cost effective. PCR enables these types of questions to be answered in a relatively short period of time, and similar types of analyses can be applied to human DMD tissues. Knowledge of the transcript diversity displayed by the dystrophin gene will enable the role of these separate isoforms to be addressed. Despite considerable effort by a variety of laboratories over the last five years, the precise functional role played by dystrophin remains unclear, and it can only be assumed that the separate isoforms act to modulate the functional role of dystrophin in separate tissues or in response to differing physiological states. PCR amplification of the dystrophin isoforms has enabled the variable regions of the transcript to be subcloned (Bies et al., 1992). These clones have been used to reintroduce the variable regions into full-length mini-gene expression vectors, which are currently being tested for functional activity through the generation of transgenic mdx mice. The transgenic mice can be easily identified through the PCR-ASO assays described in this article, and the reverse transcriptase PCR assays will enable a detailed analysis of the expression pattern of the introduced mini-genes. It is hoped that such analyses will further attempts to determine the feasibility of using gene therapy as a treatment for DMD/BMD.

Differential expression of dystrophin isoforms in strains of mdx mice with different mutations.

Mutations in the dystrophin gene are responsible for Duchenne and Becker muscular dystrophy (DMD/BMD). Studies of dystrophin expression and function have benefited from use of the mdx mouse, an animal model for DMD/BMD. Here we characterized mutations in three additional strains of mdx mice, the mdx2cv, mdx4cv and mdx5cv alleles. The mutation in the mdx2cv mouse was found to be a single base change in the splice acceptor sequence of dystrophin intron 42. This mutation leads to a complex pattern of aberrant splicing that generates multiple transcripts, none of which preserve the normal open reading frame. In the mdx5cv allele, the dystrophin mRNA contains a 53 bp deletion of sequences from exon 10. Analysis of the genomic DNA uncovered a single A to T transversion in exon 10. Although this base change does not alter the encoded amino acid, a new splice donor was created (GTGAG) that generates a frameshifting deletion in the processed mRNA. In the mdx4cv allele, direct sequencing revealed a C to T transition in exon 53, creating an ochre codon (CAA to TAA). The differential location of these mutations relative to the seven known dystrophin promoters results in a series of mdx mouse mutants that differ in their repertoire of isoform expression, such that these mice should be useful for studies of dystrophin expression and function. The mdx4cv and mdx5cv strains may be of additional use in gene transfer studies due to their low frequency of mutation reversion.

Human and murine dystrophin mRNA transcripts are differentially expressed during skeletal muscle, heart, and brain development.

Dystrophin transcripts were shown to be alternatively spliced in a pattern characteristic of both tissue type and developmental stage. Multiple novel spliced forms of dystrophin mRNA were identified in murine brain tissue, skeletal and cardiac muscle, diaphragm, and human cardiac Purkinje fibers. The transcript diversity was greatest in adult, non-skeletal muscle tissues. Sequence analysis revealed that four tandem exons of the murine gene are differentially spliced in at least 11 separate patterns to generate distinct isoforms. Two of these forms were observed in all tissues examined, while several others were uniquely observed in cardiac muscle and brain. Cardiac Purkinje fibers express an isoform primarily observed in brain tissue. Several spliced transcripts were observed only in postnatal development. Differential utilization of a fifth exon results in two mRNA splice forms that encode separate embryonic and adult C-termini of dystrophin. Comparison of murine with human dystrophin mRNAs showed that similar isoform expression patterns exist across species. These observations suggest that functionally distinct isoforms of the dystrophin protein are expressed in separate tissues and at different stages of development. These isoforms may be of significance in understanding the various tissue-specific effects produced by dystrophin gene mutations in Duchenne and Becker muscular dystrophy patients.

Characterization of dystrophin and utrophin diversity in the mouse.

Utrophin is a 400 kDa autosomal homolog of dystrophin and a component of the submembranous cytoskeleton. While multiple dystrophin isoforms have been identified along with alternatively spliced products, to date only two different mRNA species of utrophin have been identified. To determine the degree of evolutionary conservation between dystrophin and utrophin isoforms, we have compared their expression patterns in adult mice. Northern blot analysis of multiple adult tissues confirmed that only two major sizes of transcripts are produced from each gene: 13 and 5.5 kb from utrophin and 14 and 4.8 kb from dystrophin. However, western blot analysis detected several putative short utrophin isoforms that may be homologs of the dystrophin isoforms Dp140, Dp116 and Dp71. We also identified an alternatively spliced utrophin transcript that lacks the equivalent of the alternatively spliced dystrophin exon 71. Finally, we demonstrated that the C-terminal domain of utrophin targeted to neuromuscular junctions in normal mice, but localized to the sarcolemma efficiently only in the absence of dystrophin. Our results provide further evidence for a common evolutionary origin of the utrophin and dystrophin genes.

Overexpression of dystrophin in transgenic mdx mice eliminates dystrophic symptoms without toxicity.

Duchenne and Becker muscular dystrophy (DMD and BMD) are X-linked recessive diseases caused by defective expression of dystrophin. The mdx mouse, an animal model for DMD, has a mutation that eliminates expression of the 427K muscle and brain isoforms of dystrophin. Although these animals do not display overt muscle weakness or impaired movement, the diaphragm muscle of the mdx mouse is severely affected and shows progressive myofibre degeneration and fibrosis which closely resembles the human disease. Here we explore the feasibility of gene therapy for DMD by examining the potential of a full-length dystrophin transgene to correct dystrophic symptoms in mdx mice. We find that expression of dystrophin in muscles of transgenic mdx mice eliminates the morphological and immunohistological symptoms of muscular dystrophy. In addition, overexpression of dystrophin prevents the development of the abnormal mechanical properties associated with dystrophic muscle without causing deleterious side effects. Our results provide functional evidence for the feasibility of gene therapy for DMD.

Expression of full-length and truncated dystrophin mini-genes in transgenic mdx mice.

Duchenne and Becker muscular dystrophy are caused by defects in the dystrophin gene, and are candidates for treatment by gene therapy. We have shown previously that overexpression of a full-length dystrophin cDNA prevents the development of dystrophic symptoms in mdx mice. We show here that this functional correction can be achieved by expressing the full-length muscle isoform at a lower level than is present in control animals. Gene therapy for DMD may necessitate the use of truncated dystrophin mini-genes to accommodate the limited cloning capacity of current-generation viral delivery vectors. We have constructed both murine and human mini-genes deleted for exons 17-48, and have demonstrated that expression of either mini-gene can almost completely prevent the development of dystrophic symptoms in transgenic mdx mice. These results suggest that viral-mediated expression of moderate levels of a truncated dystrophin could be an effective treatment for DMD.