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Regulation of viral and cellular oncogene expression by cytosine methylation
Abstract:
Mink cells morphologically transformed by either Snyder-Theilen feline sarcoma virus (ST-FeSV) or Abelson murine leukemia virus (Abelson-MuLV) exhibit relatively high rates of reversion to the nontransformed phenotype. The proviral DNAs are conserved within the revertant lines and have not undergone changes in integration sites due to translocations or other genomic rearrangements. In contrast, expression of well-defined viral-encoded transforming proteins is blocked and elevated levels of phosphotyrosine characteristic of the parental transformed cells are reduced to control levels. Loss of the transformed phenotype is associated with increased cytosine methylation of proviral DNA sequences while levels of methylation resume control levels upon spontaneous retransformation of revertant clones. Following molecular cloning, and transfection to Rat-2 cells, ST-FeSV proviral DNAs from revertant and transformed cells induced similar numbers of transformed foci. Cytosine methylation sites involved in regulation of expression of the major ST-FeSV encoded transforming protein have been localized within the proviral DNA itself rather than in adjacent cellular flanking sequences. In contrast to the v-fes proviral DNA, c-fes, the cellular homolog of the ST-FeSV acquired transforming sequences, is highly methylated in cytosine residues in both transformed and revertant clones. These findings demonstrate regulation of viral oncogene-mediated transformation by cytosine methylation and suggest that expression of cellular homologs of viral oncogenes, such as c-fes, are also subject to regulation at this level.
Insights
Cellular transformation by oncogenic viruses can be reversed through increased DNA methylation, which silences viral oncogenes. This epigenetic regulation also impacts cellular oncogene expression.
Area of Science:
- Molecular Biology
- Epigenetics
- Virology
Background:
- Mink cells transformed by feline sarcoma virus (ST-FeSV) or Abelson murine leukemia virus (Abelson-MuLV) frequently revert to a non-transformed state.
- Proviral DNA remains integrated without genomic rearrangement, but viral oncogene expression is suppressed.
Purpose of the Study:
- To investigate the epigenetic mechanisms regulating the reversion and retransformation of virus-transformed cells.
- To determine the role of DNA methylation in controlling viral oncogene expression and cellular homolog regulation.
Main Methods:
- Analysis of proviral DNA integration and methylation patterns in transformed and reverted mink cells.
- Molecular cloning and transfection of proviral DNA into Rat-2 cells to assess transforming potential.
- Comparison of methylation levels in viral oncogenes (v-fes) and their cellular homologs (c-fes).
Main Results:
- Reversion to a non-transformed phenotype correlates with increased cytosine methylation of proviral DNA.
- Spontaneous retransformation is associated with decreased methylation levels.
- Methylation sites regulating ST-FeSV oncogene expression are within the proviral DNA.
- Cellular homolog c-fes is highly methylated in both transformed and reverted cells.
Conclusions:
- Cytosine methylation epigenetically regulates viral oncogene expression and mediates cellular transformation reversal.
- Cellular homologs of viral oncogenes, like c-fes, are also subject to epigenetic regulation by DNA methylation.