Type B leukemogenic virus has a T-cell-specific enhancer that binds AML-1

Area of Science:

• Virology
• Molecular Biology
• Oncology

Background:

• Type B leukemogenic virus (TBLV) causes T-cell tumors in mice.
• TBLV's long terminal repeats (LTRs) differ from MMTV due to deletions and triplications.
• The LTR triplication may function as a viral enhancer element.

Purpose of the Study:

• To investigate if the TBLV LTR triplication acts as a viral enhancer.
• To identify transcription factors responsible for TBLV enhancer activity.
• To understand the role of these factors in T-cell leukemogenesis.

Main Methods:

• Insertion of the TBLV LTR triplication into a reporter construct (luciferase).
• Reporter gene assays in various cell lines (T-cells, fibroblasts, B-cells, mammary cells).
• Site-directed mutagenesis of the MMTV LTR to mimic TBLV deletion.
• Gel shift assays to analyze transcription factor binding (AML-1, NF-A, NF-B).
• Overexpression studies of AML-1 in mammary cell lines.

Main Results:

• The TBLV LTR triplication demonstrated enhancer activity, particularly in T-cells.
• Enhancer function was orientation-independent and cell-type specific.
• A specific mutation (556M) significantly reduced TBLV enhancer activity (20-fold).
• This mutation diminished binding of AML-1 (RUNX1) and other factors (NF-A, NF-B).
• AML-1 overexpression increased TBLV LTR expression (30-fold) in mammary cells.

Conclusions:

• The TBLV LTR triplication functions as a cell-type specific enhancer, primarily in T-cells.
• AML-1 (RUNX1) binding is essential for optimal TBLV enhancer activity.
• AML-1, potentially with other factors, plays a critical role in TBLV-mediated leukemogenesis.