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Selection for c-myc integration sites in polyclonal T-cell lymphomas.
Dana R Broussard1, Jennifer A Mertz, M Lozano
1Section of Molecular Genetics and Microbiology, Institute for Cellular and Molecular Biology, The University of Texas at Austin, 100 W. 24th Street, Austin, TX 78712, USA.
Journal of Virology
|February 12, 2002
Summary
Type B leukemogenic virus (TBLV) causes T-cell lymphomas by inserting near the c-myc gene. Surprisingly, strong viral enhancers promoting c-myc overexpression were not always selected in vivo, suggesting a role for modest overexpression in preventing apoptosis.
Area of Science:
- Virology
- Oncology
- Molecular Biology
Background:
- Type B leukemogenic virus (TBLV) induces T-cell lymphomas in mice.
- TBLV-induced lymphomas show c-myc overexpression but limited detectable viral integrations near c-myc.
- Tumorigenesis mechanisms involving retroviral integrations and oncogene deregulation are complex.
Purpose of the Study:
- To investigate the mechanism by which TBLV induces T-cell lymphomas.
- To determine the role of TBLV integrations near the c-myc gene in lymphomagenesis.
- To understand the selection pressures driving tumor cell growth in vivo.
Main Methods:
- Polymerase chain reaction (PCR) to detect TBLV integrations near c-myc.
- Southern and Northern blotting to analyze viral integrations and c-myc RNA expression.
- Reporter gene assays (Renilla luciferase) to quantify transcriptional effects of TBLV long terminal repeat (LTR) enhancers on c-myc.
- In vivo tumor passage and semiquantitative PCR to assess selection of specific viral integrations.
Main Results:
- PCR detected TBLV insertions near c-myc in at least 30% of lymphomas, with some tumors being polyclonal.
- TBLV long terminal repeat (LTR) enhancers, particularly those with three repeats, significantly upregulated c-myc transcription in reporter assays.
- Surprisingly, viral integrations leading to maximal c-myc overexpression in vitro were often not selected in vivo; modest overexpression appeared favored.
- Selection for clonal growth in vivo favored specific integrations, with some four-repeat enhancers being selected during tumor passage.
Conclusions:
- TBLV can induce T-cell lymphomas through insertions near c-myc, leading to its overexpression.
- The number of enhancer repeats in the TBLV LTR influences c-myc upregulation.
- In vivo tumor growth selection may favor cells with moderate c-myc overexpression to prevent apoptosis, rather than maximal overexpression.