Modulated binding of SATB1, a matrix attachment region protein, to the AT-rich sequence flanking the major breakpoint region of BCL2

The t(14,18) chromosomal translocation that occurs in human follicular lymp homa constitutively activates the BCL2 gene and disrupts control of apoptos is. Interestingly, 70% of the t(14,18) translocations are confined to three 15-bp clusters positioned within a 150-bp region (major breakpoint region or [MBR]) in the untranslated portion of terminal exon 3, We analyzed DNA-p rotein interactions in the MBR, as these may play some role in targeting th e translocation to this region. An 87-bp segment (87MBR) immediately 3' to breakpoint cluster 3 was essential for DNA-protein interaction monitored wi th mobility shift assays. We further delineated a core binding region withi n 87MBR: a 33-bp, very AT-rich sequence highly conserved between the human and mouse BCL2 gene (37MBR). We have purified and identified one of the cor e factors as the matrix attachment region (MAR) binding protein, SATB1, whi ch is known to bind to AT-rich sequences with a high propensity to unwind. Additional factors in nuclear extracts, which we have not yet characterized further, increased SATB1 affinity for the 37MBR target four- to fivefold. Specific binding activity within 37MBR displayed cell cycle regulation in J urkat T cells, while levels of SATB1 remained constant throughout the cell cycle, Finally, we demonstrated in vivo binding of SATB1 to the MBR, strong ly suggesting the BCL2 major breakpoint region is a MAR. We discuss the pot ential consequences of our observations for both MBR fragility and regulato ry function.