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Chimeric and molecular genetic analysis of myelin-deficient (shiverer and mld) mutant mice
K Mikoshiba1, H Okano, J Aruga
1Institute for Protein Research, Osaka University, Japan.
Abstract:
The shiverer and myelin-deficient (mld) mutants are two allelic mutations. Both are characterized by hypomyelination in the CNS and deficient expression of the MBP gene. Chimeric analysis of the pathogenesis of shiverer showed that shiverer mutation acts intrinsic to the oligodendrocyte, which is the only cell type expressing the MBP gene in the CNS. Molecular genetic studies by several groups demonstrated that shiverer is a deletion mutation in the MBP gene. Consequently, no MBP is produced in the shiverer mutant. In mld, however, partial expression of the MBP gene was observed. Interestingly, MBP was expressed in a mosaic fashion in the CNS of mld mice. Molecular genetic studies revealed that the mld mutant has duplicated MBP genes in tandem on a distal part (E2-4) of chromosome 18q. The reduced expression was based on the level of mRNA. A large portion is inverted in the upstream copy, and an intact copy is located downstream. We showed in mld mutants that antisense RNA corresponding to the inverted segment is transcribed, and it forms RNA duplex, with the RNA transcribed from the normal gene located downstream.
Insights
The myelin-deficient (mld) mouse model exhibits hypomyelination due to duplicated myelin basic protein (MBP) genes. Antisense RNA from an inverted gene copy interferes with MBP expression, causing neurological defects.
Area of Science:
- Neuroscience
- Genetics
- Molecular Biology
Background:
- Shiverer and myelin-deficient (mld) are allelic mutations causing central nervous system (CNS) hypomyelination and reduced myelin basic protein (MBP) gene expression.
- Shiverer mutation is a deletion in the MBP gene, leading to a complete lack of MBP production.
- Oligodendrocytes are the sole CNS cell type expressing the MBP gene, and shiverer mutation acts intrinsically within these cells.
Purpose of the Study:
- To investigate the molecular basis of myelin deficiency in the mld mutant.
- To understand the mechanism of MBP gene dysregulation in the mld mouse model.
- To elucidate the role of gene duplication and antisense RNA in hypomyelination.
Main Methods:
- Chimeric analysis to determine the cell-intrinsic nature of the shiverer mutation.
- Molecular genetic studies to identify mutations in shiverer and mld.
- Analysis of MBP gene duplication and mRNA expression levels in mld mutants.
- Detection and characterization of antisense RNA in mld mice.
Main Results:
- Shiverer mutation is a deletion in the MBP gene, resulting in no MBP production.
- Mld mutants possess tandemly duplicated MBP genes on chromosome 18q, with one copy inverted.
- MBP expression in mld CNS is mosaic and reduced at the mRNA level.
- Antisense RNA transcribed from the inverted MBP gene copy forms duplexes with sense RNA from the intact copy, inhibiting expression.
Conclusions:
- The shiverer mutation causes a complete loss of MBP due to gene deletion.
- The mld mutation leads to hypomyelination through a novel mechanism involving gene duplication and antisense RNA interference.
- Understanding these distinct genetic mechanisms provides insight into myelin development and oligodendrocyte function.
