Rubin C M, Carrino J J, Dickler M N, Leibowitz D, Smith S D, Westbrook C A
Department of Medicine, University of Chicago, IL 60637.
Proc Natl Acad Sci U S A. 1988 Apr;85(8):2795-9. doi: 10.1073/pnas.85.8.2795.
Philadelphia chromosome-positive acute lymphoblastic leukemia occurs in two molecular forms, those with and those without rearrangement of the breakpoint cluster region on chromosome 22. The molecular abnormality in the former group is similar to that found in chronic myelogenous leukemia. To characterize the abnormality in the breakpoint cluster region-unrearranged form, we have mapped a 9;22 translocation from the Philadelphia chromosome-positive acute lymphoblastic leukemia cell line SUP-B13 by using pulsed-field gel electrophoresis and have cloned the DNA at the translocation junctions. We demonstrate a BCR-ABL fusion gene on the Philadelphia chromosome. The breakpoint on chromosome 9 is within ABL between exons Ia and II, and the breakpoint on chromosome 22 is approximately equal to 50 kilobases upstream of a breakpoint cluster region in an intron of the BCR gene. This upstream BCR breakpoint leads to inclusion of fewer BCR sequences in the fusion gene, compared with the BCR-ABL fusion gene of chronic myelogenous leukemia. Consequently, the associated mRNA and protein are smaller. The exons from ABL are the same. Analysis of leukemic cells from four other patients with breakpoint cluster region-unrearranged Philadelphia chromosome-positive acute lymphoblastic leukemia revealed a rearrangement on chromosome 22 close to the breakpoint in SUP-B13 in only one patient. These data indicate that breakpoints do not cluster tightly in this region but are scattered, possibly in a large intron. Given the large size of BCR and the heterogeneity in breakpoint location, detection of BCR rearrangement by standard Southern blot analysis is difficult. Pulsed-field gel electrophoresis should allow detection at the DNA level in every patient and thus will permit clinical correlation of the breakpoint location with prognosis.
费城染色体阳性急性淋巴细胞白血病有两种分子形式，即22号染色体上断裂簇区域有重排和无重排的形式。前一组的分子异常与慢性粒细胞白血病中发现的相似。为了表征断裂簇区域未重排形式的异常，我们使用脉冲场凝胶电泳对费城染色体阳性急性淋巴细胞白血病细胞系SUP-B13中的9;22易位进行了定位，并克隆了易位连接处的DNA。我们在费城染色体上证实了一个BCR-ABL融合基因。9号染色体上的断点位于ABL基因第1a外显子和第2外显子之间，22号染色体上的断点大约在BCR基因内含子中一个断裂簇区域上游50千碱基处。与慢性粒细胞白血病的BCR-ABL融合基因相比，这个上游BCR断点导致融合基因中包含的BCR序列更少。因此，相关的mRNA和蛋白质更小。来自ABL的外显子相同。对另外四名断裂簇区域未重排的费城染色体阳性急性淋巴细胞白血病患者的白血病细胞分析显示，只有一名患者的22号染色体上有一个靠近SUP-B13中断点的重排。这些数据表明，断点在该区域并不紧密聚集，而是分散的，可能在一个大的内含子中。鉴于BCR的大小和断点位置的异质性，通过标准Southern印迹分析检测BCR重排很困难。脉冲场凝胶电泳应该能够在每个患者的DNA水平上进行检测，从而使断点位置与预后的临床相关性成为可能。
