Myogenin is in an evolutionarily conserved linkage group on human chromosome 1q31-q41 and unlinked to other mapped muscle regulatory factor genes

Histological characteristics of musculus longissimus dorsi and their correlation with restriction fragment length polymorphism of the myogenin gene in Jinghua×Pietrain F(2) crossbred pigs

PCR-RFLP, periodic acid Schiff (PAS) and myosin heavy chain (MHC) immunohistochemistry were employed to survey the relationships between the genotypes of the myogenin gene (myog) and the histological characteristics of the muscle and other postmortem traits in the Jinghua×Pietrain (JP) F(2) crossbred pigs. The muscle fibers were classified into three groups according to their PAS-reactivity: PAS(-), PAS(+∼++), and PAS(+++) and accounted for approximately 16%, 74% and 10% of the total muscle, respectively. MHC immunohistochemistry was used to categorize muscle fibers into either slow or fast fibers; the proportions of slow and fast fibers were 6% and 94%, respectively. Two different cleavage patterns and three genotypes (AA, AB and BB) were identified and their frequencies were 0.4087, 0.4986 and 0.0928, respectively, for the three genotypes and 0.6580 and 0.3420, respectively, for cleavage patterns A and B. Different genotypes were variably associated with cross-sectional area (p=0.074), water-holding capacity (p=0.002), pH (p<0.001) and carcass temperature (p<0.001) of the loin muscle. Different genotypes showed marked correlation with the L value (p<0.001), a value (p=0.002), and b (p<0.001) of the Minolta meat color index. The genotypes were also significantly related to the cross-sectional area (p<0.001), diameter (p<0.001), aspect ratio (p<0.001) and the density (p<0.001) of muscle fibers from the longissimus dorsi muscle. The results also revealed that the genotypes showed no significant association with the circularity (p=0.132) or the percentage of intramuscular connective tissue (p=0.193).

MyoD distal regulatory region contains an SRF binding CArG element required for MyoD expression in skeletal myoblasts and during muscle regeneration

We show here that the distal regulatory region (DRR) of the mouse and human MyoD gene contains a conserved SRF binding CArG-like element. In electrophoretic mobility shift assays with myoblast nuclear extracts, this CArG sequence, although slightly divergent, bound two complexes containing, respectively, the transcription factor YY1 and SRF associated with the acetyltransferase CBP and members of C/EBP family. A single nucleotide mutation in the MyoD-CArG element suppressed binding of both SRF and YY1 complexes and abolished DRR enhancer activity in stably transfected myoblasts. This MyoD-CArG sequence is active in modulating endogeneous MyoD gene expression because microinjection of oligonucleotides corresponding to the MyoD-CArG sequence specifically and rapidly suppressed MyoD expression in myoblasts. In vivo, the expression of a transgenic construct comprising a minimal MyoD promoter fused to the DRR and beta-galactosidase was induced with the same kinetics as MyoD during mouse muscle regeneration. In contrast induction of this reporter was no longer seen in regenerating muscle from transgenic mice carrying a mutated DRR-CArG. These results show that an SRF binding CArG element present in MyoD gene DRR is involved in the control of MyoD gene expression in skeletal myoblasts and in mature muscle satellite cell activation during muscle regeneration.

Human bHLH transcription factor gene myogenin (MYOG): genomic sequence and negative mutation analysis in patients with severe congenital myopathies

The myogenin gene encodes an evolutionarily conserved basic helix-loop-helix transcription (bHLH) factor that is required for differentiation of skeletal muscle, and its homozygous deletion in mice results in perinatal death from respiratory failure due to the lack of muscle fibers. Since the histology of skeletal muscle in myogenin null mice is reminiscent of that found in severe congenital myopathy patients, many of whom also die of respiratory complications, we sought to test the hypothesis that an aberrant human myogenin (myf4) coding region could be associated with some congenital myopathy conditions. With PCR amplification, we found similarly sized PCR products for the three exons of the myogenin gene in DNA from 37 patient and 40 control individuals. In contrast to previously reported sequencing of human myogenin (myf4), we describe with automated sequencing several base differences in flanking and coding regions plus an additional 659 and 498 bp in the first and second introns, respectively, in all 37 patient and 40 control samples. We also find a variable length (CA)-dinucleotide repeat in the second intron, which may have utility as a marker for future linkage studies. In summary, no causative mutations were detected in the myogenin coding locus of genomic DNA from 37 patients with severe congenital myopathy.

Analysis of the mouse Splotch-delayed mutation indicates that the Pax-3 paired domain can influence homeodomain DNA-binding activity

The murine Pax-3 protein contains two DNA-binding domains, a paired domain and a homeodomain, and alterations in the Pax-3 gene are responsible for the neural tube defects observed in the Splotch (Sp) mouse mutant. Of five Sp alleles, Splotch-delayed (Spd) is the only one that encodes a full-length Pax-3 protein, containing a single glycine-to-arginine substitution within the paired domain. To better understand the consequence of this mutation on Pax-3 function, we have analyzed the DNA-binding properties of wild-type and Spd Pax-3, using oligonucleotides that bind primarily to the paired domain (e5) or exclusively to the homeodomain (P2). Wild-type Pax-3 was found to bind e5 in a specific manner. In contrast, the Spd mutation reduced binding of Pax-3 to e5 17-fold, revealing a defect in DNA binding by the paired domain. Surprisingly, the Spd mutation also drastically reduced the homeodomain-specific binding to P2 by 21-fold when compared with the wild-type protein. Interestingly, a deletion which removes the Spd mutation was found to restore P2-binding activity, suggesting that within the full-length Pax-3 protein, the paired domain and homeodomain may interact. We conclude, therefore, that the Spd mutation is phenotyically expressed in vitro by a defect in the DNA-binding properties of Pax-3. Furthermore, it is apparent that the paired domain and homeodomain of Pax-3 do not function as independent domains, since a mutation in the former impairs the DNA-binding activity of the latter.

The gene coding for the alpha 1 subunit of the skeletal dihydropyridine receptor (Cchl1a3 = mdg) maps to mouse chromosome 1 and human 1q32

Using both chromosomal in situ hybridization and molecular techniques, we report the genetic localization of the gene coding for the alpha 1 subunit of the skeletal slow Ca2+ current channel/DHP receptor gene (Cchl1a3) on human Chromosome (Chr) 1 (1q31-1q32 region) and on mouse Chr 1 (region (F-G)). On the basis of single-strand conformation polymorphism (SSCP-PCR) analysis in an interspecific backcross, we have determined that the Cchl1a3 = mdg (muscular dysgenesis) locus is very closely linked to the myogenin (Myog) locus.

Muscle deficiency and neonatal death in mice with a targeted mutation in the myogenin gene

Myogenin is a muscle-specific transcription factor that can induce myogenesis in a variety of cell types in tissue culture. To test myogenin's role in vivo, mice homozygous for a targeted mutation in the myogenin gene were generated. These mice survive fetal development but die immediately after birth and show a severe reduction of all skeletal muscle. Myogenin-mutant mice differ from mice carrying mutations in genes for the related myogenic factors Myf5 and MyoD, which have no muscle defects. Myogenin is therefore essential for the development of functional skeletal muscle.

Myogenes and myotubes

Investigation of the congenital myopathies has been limited by a lack of knowledge regarding basic mechanisms involved in normal myogenesis of human muscle and the relative rarity of these diseases. A newly recognized family of regulatory genes has been shown to be necessary for myogenesis to proceed to formation of normal mature muscle. It is likely that investigation of patients with one or more types of structural myopathy may show that abnormalities of the regulatory basic helix-loop-helix (bHLH) genes may be responsible for disease.

Splotch (Sp2H), a mutation affecting development of the mouse neural tube, shows a deletion within the paired homeodomain of Pax-3

The molecular basis of the mouse mutation splotch (Sp), which is associated with spina bifida and exencephaly, was analyzed at three of its alleles, Sp, Sp2H, and Spr. We mapped the paired box gene Pax-3 within the Inha to Akp3 interval, near or at the Sp locus on chromosome 1, and found Pax-3 to be deleted in heterozygous Spr/+ mice. Analysis of genomic DNA and cDNA clones constructed from Sp2H/Sp2H embryos identified a deletion of 32 nucleotides in the Pax-3 mRNA transcript and gene. This deletion maps within the paired homeodomain of PAX-3 and is predicted to create a truncated protein as a result of a newly created termination codon at the deletion breakpoint. Our study provides evidence for a causal link between deletion of the paired homeodomain of Pax-3 and the Sp2H mutation, and infers that Pax-3 plays a key role in normal neural development.