Genetic mapping near the myd locus on mouse chromosome 8

The role of defective glycosylation in congenital muscular dystrophy

The dystrophin glycoprotein complex (DGC) is an assembly of proteins spanning the sarcolemma of skeletal muscle cells. Defects in the DGC appear to play critical roles in several muscular dystrophies due to disruption of basement membrane organization. O -mannosyl oligosaccharides on alpha-dystroglycan, a major extracellular component of the DGC, are essential for normal binding of alpha-dystroglycan to ligands (such as laminin) in the extracellular matrix and subsequent signal transmission to actin in the cytoskeleton of the muscle cell. Muscle-Eye-Brain disease (MEB) and Walker-Warburg Syndrome (WWS) have mutations in genes encoding glycosyltransferases needed for O -mannosyl oligosaccharide synthesis. Myodystrophic myd mice and humans with Fukuyama Congenital Muscular Dystrophy (FCMD), congenital muscular dystrophy due to defective fukutin-related protein (FKRP) and MDC1D have mutations in putative glycosyltransferases. These human congenital muscular dystrophies and the myd mouse are associated with defective glycosylation of alpha-dystroglycan. It is expected other congenital muscular dystrophies will prove to have mutations in genes involved in glycosylation.

Mutation of Large, which encodes a putative glycosyltransferase, in an animal model of muscular dystrophy

The myodystrophy (myd) mutation arose spontaneously and has an autosomal recessive mode of inheritance. Homozygous mutant mice display a severe, progressive muscular dystrophy. Using a positional cloning approach, we identified the causative mutation in myd as a deletion within the Large gene, which encodes a putative glycosyltransferase with two predicted catalytic domains. By immunoblotting, the alpha-subunit of dystroglycan, a key muscle membrane protein, is abnormal in myd mice. This aberrant protein might represent altered glycosylation of the protein and contribute to the muscular dystrophy phenotype. Our results are discussed in the light of recent reports describing mutations in other glycosyltransferase genes in several forms of human muscular dystrophy.

Cloning and characterization of the human factor XI gene promoter: transcription factor hepatocyte nuclear factor 4alpha (HNF-4alpha ) is required for hepatocyte-specific expression of factor XI

Factor XI is the zymogen of a plasma protease produced primarily in liver that is required for normal blood coagulation. We cloned approximately 2600 base pairs of the human factor XI gene upstream of exon one, identified transcription start sites, and conducted a functional analysis. Luciferase reporter assays demonstrate that the 381 base pairs upstream of exon one are sufficient for maximum promoter activity in HepG2 hepatocellular carcinoma cells. The removal of 19 base pairs between -381 and -363 results in a nearly complete loss of promoter activity. This region contains the sequence ACTTTG, a motif required for binding of the transcription factor hepatocyte nuclear factor 4alpha (HNF-4alpha) to the promoters of several genes. Gel mobility shift assays using HepG2 or rat hepatocyte nuclear extract confirm HNF-4alpha binds between bp -375 and -360. Scrambling the ACTTTG motif completely abolishes promoter activity in luciferase assays. The factor XI promoter functions poorly when transfected into HeLa carcinoma cells, and gel mobility shift experiments with HeLa nuclear extracts demonstrate no HNF-4alpha binding to the ACTTTG sequence. When a rat HNF-4alpha expression construct is co-transfected into HeLa cells, factor XI promoter activity is enhanced approximately 10-fold. We conclude that HNF-4alpha is required for hepatocyte-specific expression of factor XI.

Mutant glycosyltransferase and altered glycosylation of alpha-dystroglycan in the myodystrophy mouse

Spontaneous and engineered mouse mutants have facilitated our understanding of the pathogenesis of muscular dystrophy and they provide models for the development of therapeutic approaches. The mouse myodystrophy (myd) mutation produces an autosomal recessive, neuromuscular phenotype. Homozygotes have an abnormal gait, show abnormal posturing when suspended by the tail and are smaller than littermate controls. Serum creatine kinase is elevated and muscle histology is typical of a progressive myopathy with focal areas of acute necrosis and clusters of regenerating fibers. Additional aspects of the phenotype include sensorineural deafness, reduced lifespan and decreased reproductive fitness. The myd mutation maps to mouse chromosome 8 at approximately 33 centimorgans (cM) (refs. 2, 4-7). Here we show that the gene mutated in myd encodes a glycosyltransferase, Large. The human homolog of this gene (LARGE) maps to chromosome 22q. In myd, an intragenic deletion of exons 4-7 causes a frameshift in the resultant mRNA and a premature termination codon before the first of the two catalytic domains. On immunoblots, a monoclonal antibody to alpha-dystroglycan (a component of the dystrophin-associated glycoprotein complex) shows reduced binding in myd, which we attribute to altered glycosylation of this protein. We speculate that abnormal post-translational modification of alpha-dystroglycan may contribute to the myd phenotype.

Genomic characterization of the lysophosphatidic acid receptor gene, lp(A2)/Edg4, and identification of a frameshift mutation in a previously characterized cDNA

To understand the regulation, evolution, and genetics of lp(A2)/Edg4, a second lysophosphatidic acid receptor gene, we characterized its complete cDNA sequence, genomic structure, and chromosomal location. The full-length mouse transcript sequence was determined using rapid amplification of cDNA ends. Southern blot and restriction fragment length polymorphism segregation analyses revealed that the mouse gene was present as a single copy and located at the middle of Chromosome 8 near the mutations for myodystrophy (myd) and "kidney-anemia-testes" (kat). This region is syntenic with human chromosome 19p12, where the human genomic clone containing the lp(A2) gene (EDG4) was mapped. Sequence analysis of genomic clones demonstrated that both mouse and human transcripts were encoded by three exons, with an intron separating the coding region for transmembrane domain VI. Reverse transcriptase-PCR demonstrated that the three exons were spliced in all mouse tissues shown to express the transcript. Finally, in a comparison of all human lp(A2) sequences present in the database, we identified several sequence variants in multiple tumors. One such variant (a G deletion) in the initially characterized Edg4 cDNA clone (derived from an ovarian tumor) results in a frameshift mutation near the 3' end of the coding region. In addition to increasing our understanding of the mechanisms underlying lysophosphatidic acid signaling and lysophospholipid receptor gene evolution, these results have important implications regarding the genomic targeting and oncogenic potential of lp(A2).

BSMAP, a novel protein expressed specifically in the brain whose gene is localized on chromosome 19p12

Using the sequence of cosmids derived from chromosome 19p12, we have identified a gene encoding a novel protein, BSMAP (brain-specific membrane-anchored protein) and cloned cDNA encoding the full-length open reading frame. Northern blot analysis revealed that BSMAP mRNA is preferentially expressed at a high level in the brain. BSMAP has a putative transmembrane domain and is predicted to be a type-I membrane glycoprotein. Genomic sequence analysis revealed that the gene encoding BSMAP consists of eight exons spanning approximately 8 kb and lies 6 kb away from the gene encoding CLF-1 in a reverse orientation. Although no candidate genetic disorders were found to map either to this precise region of chromosome 19 or to the syntenic region of the mouse genome, the highly specific expression of BSMAP mRNA suggests a role for the protein in CNS function.

The mouse homolog of FRG1, a candidate gene for FSHD, maps proximal to the myodystrophy mutation on chromosome 8

The human autosomal dominant neuromuscular disorder facioscapulohumeral muscular dystrophy (FSHD) is associated with deletions within a complex tandem DNA repeat (D4Z4) on Chromosome (Chr) 4q35. The molecular mechanism underlying this association of FSHD with DNA rearrangements is unknown, and, thus far, no gene has been identified within the repeat. We isolated a gene mapping 100 kb proximal to D4Z4 (FSHD Region Gene 1:FRG1), but were unable to detect any alterations in total or allele-specific mRNA levels of FRG1 in FSHD patients. Human Chr 4q35 exhibits synteny homology with the region of mouse Chr 8 containing the gene for the myodystrophy mutation (myd), a possible mouse homolog of FSHD. We report the cloning of the mouse gene (Frg1) and show that it maps to mouse Chr 8. Using a cross segregating the myd mutation and the European Collaborative Interspecific Backcross, we showed that Frg1 maps proximal to the myd locus and to the Clc3 and Ant1 genes.

Chromosomal mapping of five highly conserved murine homologues of the Drosophila RING finger gene seven-in-absentia

Seven-in-absentia (sina) is epistatic to all other known genes in the sevenless-ras signaling pathway, which mediates R7 photoreceptor formation in the Drosophila eye. The murine genome contains several closely related sina homologues (Siah1A-D, Siah2) that are also likely to participate in ras signaling. As part of a genetic and biochemical analysis of the mammalian Siah genes, we have used gene-specific probes to map the chromosomal positions of each family member. Here we report their chromosomal positions in relation to a number of known mouse mutations and also describe an analysis of the human Siah genes. By comparing the complexity of the Siah genes in these two mammalian species we have gained further insight into which members of this murine multigene family are likely to be functional.