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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. [ABSTRACT FROM PUBLISHER]

Two Japanese-Brazilian siblings with type 2C limb girdle muscular dystrophy showed a maternal 521-T deletion in exon 6 and a larger paternal deletion of exon 6 in the gamma-sarcoglycan gene. One sib was ambulant at 29 years of age, whereas the other sib was confined to a wheelchair at the age of 12. Sarcoglycan staining of the muscle was reduced in both siblings but it did not correlate with the observed variability of the clinical severity., (Copyright 2000 John Wiley & Sons, Inc.)

Multiple epiphyseal dysplasia (MED) is a degenerative cartilage condition shown in some cases to be caused by mutations in genes encoding cartilage oligomeric matrix protein or type IX collagen. We studied a family with autosomal dominant MED affecting predominantly the knee joints and a mild proximal myopathy. Genetic linkage to the COL9A3 locus on chromosome 20q13.3 was established with a peak log(10) odds ratio for linkage score of 3.87 for markers D20S93 and D20S164. Reverse transcription-PCR performed on the muscle biopsy revealed aberrant mRNA lacking exon 3, which predicted a protein lacking 12 amino acids from the COL3 domain of alpha3(IX) collagen. Direct sequencing of genomic DNA confirmed the presence of a splice acceptor mutation in intron 2 of the COL9A3 gene (intervening sequence 2, G-A, -1) only in affected family members. By electron microscopy, chondrocytes from epiphyseal cartilage exhibited dilated rough endoplasmic reticulum containing linear lamellae of alternating electron-dense and electron-lucent material, reflecting abnormal processing of mutant protein. Type IX collagen chains appeared normal in size and quantity but showed defective cross-linking by Western blotting. The novel phenotype of MED and mild myopathy is likely caused by a dominant-negative effect of the exon 3-skipping mutation in the COL9A3 gene. Patients with MED and a waddling gait but minimal radiographic hip involvement should be evaluated for a primary myopathy and a mutation in type IX collagen.

Dystrophin-related and dystrophin-associated proteins (DAPs) are thought to play an important role in the stability and maintenance of the plasma membrane during muscle contraction and relaxation. Studies conducted on the electric organ of Torpedo californica have shown that some of the DAPs are also involved in the formation and maintenance of neuromuscular junctions (NMJs). In addition, dystrophin and several DAPs have been shown to be the primary genetic defect in a number of phenotypically similar muscular dystrophies. We previously reported the identification and characterization of human dystrobrevin, a protein which is unique in being both homologous to dystrophin and a dystrophin-associated protein. Here we describe the genomic organization of the human dystrobrevin gene. It is encoded by 23 exons spanning at least 180 kb of chromosome 18q12. Three different C-termini of dystrobrevin are generated by the mutually exclusive mRNA splicing of three exons. Two alternatively spliced exons (exons 11A and 12) are utilized exclusively in striated muscles. A comparison between the genomic organization of dystrophin and human dystrobrevin shows that the two genes have significant similarities in their genomic structure, implying an ancestral or evolutionary relationship. Based on intronic sequence, a primer set was designed to specifically amplify each exon of dystrobrevin to screen for mutations by SSCP in patients with neuromuscular diseases for which dystrobrevin could be a candidate.

We report here the complete cloning and sequencing of human microtubule associated protein 1B (MAP1B). Comparisons to mouse and partial rat MAP1B sequence indicate that this gene is extremely well conserved, with 91 and 90% identity, respectively. The entire human MAP1B genomic region has been isolated and the genomic organization determined. The gene includes seven exons, and the third exon contains sequence not represented in mouse or rat MAP1B. This sequence, labeled 3A, is present at the 5' end of an alternative transcript that is expressed at approximately 1/10th the level of the full-length transcript. By comparisons of human MAP1B with the sequence databases, we have identified a MAP1B-related gene that is probably the human homologue of rat MAP1A. This gene is expressed at high levels in brain and spinal cord and much lower levels in muscle and maps to the long arm of human chromosome 15.

To identify CA repeats in genomic sequences which had been previously subcloned into plasmids, we performed PCR using a (CA)n primer and a flanking vector primer on the genomic inserts. By incorporation of a restriction enzyme site into the (CA)n primer, we have been able to subclone the genomic DNA so that the sequence flanking the CA repeat is readily determined. Primers can then be designed to amplify across the CA repeat in patient DNA samples. Application of this technique to genomic DNAs surrounding the upstream "brain" promoter of the dystrophin gene has led to the discovery of four new CA repeats. Three of these repeats are highly polymorphic, with PICs ranging from .586 to .768. The location of these markers at the extreme 5' terminus of the dystrophin gene, together with their high degree of polymorphism and ease of assay, makes them ideal for linkage analysis in families with Duchenne muscular dystrophy.

Twenty-nine deletion breakpoints were mapped in 220 kb of the DXS164 locus relative to potential exons of the Duchenne and Becker muscular dystrophy gene. Four deletion junction fragments were isolated to acquire outlying Xp21 loci on both the terminal and centromere side of the DXS164 locus. The junction loci were used for chromosome walking, searches for DNA polymorphisms, and mapping against deletion and translocation breakpoints. Forty-four unrelated deletions were analyzed using the junction loci as hybridization probes to map the endpoints between cloned Xp21 loci. DNA polymorphisms from the DXS164 and junction loci were used to follow the segregation of a mutation in a family that represents a recombinant. Both the physical and genetic data point to a very large size for this X-linked muscular dystrophy locus.

The Duchenne muscular dystrophy gene cloning is presented along with evidence which indicates that the gene locus encoding the Duchenne muscular dystrophy (DMD) transcript is extremely large. Partial nucleotide sequence of the cDNA from mouse and humans has been used to predict the amino acid sequence for a part of the DMD protein. The protein is highly conserved between the two mammals and might have structural characteristics similar to other muscle-specific proteins.

A portion of the Duchenne muscular dystrophy (DMD) gene transcript from human fetal skeletal muscle and mouse adult heart was sequenced, representing approximately 25 percent of the total, 14-kb DMD transcript. The nucleic acid and predicted amino acid sequences from the two species are nearly 90 percent homologous. The amino acid sequence that is predicted from this portion of the DMD gene indicates that the protein product might serve a structural role in muscle, but the abundance and tissue distribution of the messenger RNA suggests that the DMD protein is not nebulin.

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.

A protocol for the rapid cloning of many DNA fragments from an amplified genomic region is described. The procedure is based on a modification of the phenol-emulsion reassociation technique (PERT) previously used to clone DNA fragments missing from a chromosomal deletion [Kunkel et al., Proc. Natl. Acad. Sci. USA 82 (1985) 4778-4782]. The procedure was used to construct recombinant libraries in the plasmid pBR322 which were highly enriched for amplified sequences from two neuroblastoma cell lines, CHP-126 and IMR-32. Many new amplified DNA fragments were isolated from these libraries, indicating that the PERT methodology should be of general use in isolating amplified DNA from other cell lines and tumors.

Dystrophin is the protein product of the Duchenne/Becker muscular dystrophy locus. It has a relative molecular mass of 427,000 and is encoded by a large RNA transcript processed from more than 65 exons spread over two million base pairs of the human X chromosome. We have used the polymerase chain reaction to see whether any of these exons are used alternatively in the different tissues that express dystrophin. As reported for rat dystrophin, we find that the first exons of the human dystrophin transcript is different in brain and muscle, indicating that dystrophin expression could be differentially regulated in these tissues by usage of distinct promoters. The 3' end of the dystrophin transcript can be alternatively spliced to create numerous isoforms differing at their carboxyl domains; this is the only domain of dystrophin that does not share any similarity with the related cytoskeletal alpha-actinins. These alternative transcripts yield dystrophin molecules which may interact with different proteins of the tissues expressing dystrophin.