Shah N P, Witte O N, Denny C T
Department of Microbiology and Molecular Genetics, University of California-Los Angeles 90024.
Mol Cell Biol. 1991 Apr;11(4):1854-60. doi: 10.1128/mcb.11.4.1854-1860.1991.
The t(9;22) Philadelphia chromosome translocation fuses 5' regulatory and coding sequences of the BCR gene to the c-ABL proto-oncogene. This results in the formation of hybrid BCR-ABL mRNAs and proteins. The shift in ABL transcriptional control to the BCR promoter may play a role in cellular transformation mediated by this rearrangement. We have functionally localized the BCR promoter to a region 1 kb 5' of BCR exon 1 coding sequences by using a chloramphenicol acetyltransferase reporter gene assay. Nucleotide sequence analysis of this region revealed many consensus binding sequences for transcription factor SP1 as well as two potential CCAAT box binding factor sites and one putative helix-loop-helix transcription factor binding site. No TATA-like or "initiator" element sequences were found. Because of low steady-state levels of BCR mRNA and the high GC content (78%) of the promoter region, definitive mapping of transcription start sites required artificial amplification of BCR promoter-directed transcripts. Overexpression from the BCR promoter in a COS cell system was effective in demonstrating multiple transcription initiation sites. In order to assess the effects of chromosomal translocation on the transcriptional control of the BCR gene, we determined S1 nuclease protection patterns of poly(A)+ RNA from tumor cell lines. No differences were observed in the locations and levels of BCR transcription initiation sites between those lines that harbored the t(9;22) translocation and those that did not. This demonstrates that BCR promoter function remains intact in spite of genomic rearrangement. The BCR promoter is structurally similar to the ABL promoters. Together, this suggests that the structural fusion of BCR-ABL and not its transcriptional deregulation is primarily responsible for the transforming effect of the t(9;22) translocation.
9;22 费城染色体易位将 BCR 基因的 5' 调控序列和编码序列与 c-ABL 原癌基因融合。这导致形成杂交 BCR-ABL mRNA 和蛋白质。ABL 转录控制向 BCR 启动子的转变可能在这种重排介导的细胞转化中起作用。我们通过使用氯霉素乙酰转移酶报告基因测定法在功能上将 BCR 启动子定位到 BCR 外显子 1 编码序列 5' 方向 1 kb 的区域。该区域的核苷酸序列分析揭示了许多转录因子 SP1 的共有结合序列以及两个潜在的 CCAAT 盒结合因子位点和一个推定的螺旋-环-螺旋转录因子结合位点。未发现类似 TATA 或“起始子”元件序列。由于 BCR mRNA 的稳态水平较低以及启动子区域的高 GC 含量（78%），转录起始位点的精确定位需要人工扩增 BCR 启动子指导的转录本。在 COS 细胞系统中由 BCR 启动子过度表达有效地证明了多个转录起始位点。为了评估染色体易位对 BCR 基因转录控制的影响，我们测定了来自肿瘤细胞系的 poly(A)+ RNA 的 S1 核酸酶保护模式。在携带 9;22 易位的细胞系和未携带该易位的细胞系之间，未观察到 BCR 转录起始位点的位置和水平存在差异。这表明尽管基因组发生重排，BCR 启动子功能仍然完整。BCR 启动子在结构上与 ABL 启动子相似。总之，这表明 BCR-ABL 的结构融合而非其转录失调是 9;22 易位转化作用的主要原因。
