Oncogene (1998) 16, 2029 ± 2032 1998 Stockton Press All rights reserved 0950 ± 9232/98 $12.00 http://www.stockton-press.co.uk/onc SHORT REPORT Bcr/Abl associated leukemogenesis in bcr null mutant mice Jan Willem Voncken1, Vesa Kaartinen, John Groen and Nora Heisterkamp Department of Pathology Ms#103, Section of Molecular Carcinogenesis and Childrens Hospital of Los Angeles Research Institute, 4650 Sunset Boulevard, Los Angeles, California 90027, USA The BCR gene contributes to Philadelphia-positive translocation with chromosome 9 in Philadelphia leukemogenesis via a number of discrete mechanisms, (Ph)-positive leukemia (de Klein et al., 1982). This one of which may be through interaction of its normal [t(9;22)(q34;q11)] translocation fuses BCR gene se- gene product with the Bcr/Abl oncoprotein. In the quences to the ABL proto-oncogene. Depending on current study this hypothesis was tested in vivo by where in the BCR locus the breakpoint occurs, a P190 introducing a Bcr/Abl P190 transgene into mice lacking or a P210 kDa chimeric Bcr/Abl oncoprotein is endogenous bcr protein. Our ®nding, that the P190 produced. At the molecular level, the BCR gene BCR/ABL oncogene is still capable of producing contributes to leukemogenesis through a number of leukemia in these mice with indistinguishable latency discrete mechanisms. Firstly, the BCR promoter and clinical pattern as in genetically matched counter- controls expression of both the remaining non- parts, rules out any signi®cant or major contribution of rearranged BCR allele and the BCR/ABL oncogene the bcr protein as a whole to leukemia development in in human leukemia. The observation that the these mice. physiological eects of bcr gene ablation in mice are found in those cell types also involved in Ph-positive Keywords: P190 Bcr/Abl; bcr null mutant; pre-B leukemia in man (Voncken et al., 1995a) most likely leukemia/lymphoma; Ph translocation pertains to dierential transcriptional regulation of the BCR gene in cells that have respiratory burst activity (granulocytes, macrophages and B-cells). Secondly, N-terminal Bcr sequences are directly The BCR gene product has been implicated in responsible for the activation of the Abl tyrosine regulation of the Rho family of small GTP-binding kinase in the chimeric Bcr/Abl gene products (Muller proteins Rho, Rac and Cdc42 through its exon 3 ± 10 et al., 1991; McWhirter et al., 1993; McWhirter and encoded GDP/GTP exchange factor (GEF) domain Wang, 1993, 1997). Thus, through its fusion to Abl, (Eva and Aaronson, 1985; Hart et al., 1991; Chuang et Bcr also contributes at a biochemical level to neoplastic al., 1995) and its C-terminal GTPase activating protein transformation of white blood cells. Finally, Bcr is able (GAP) activity (Diekmann et al., 1991; Hart et al., to form homotetramers through an N-terminal 1992). BcrGAP activity toward Rac and CDC42 is oligomerization domain constituted by amino acids currently the only well-understood biochemical prop- 28 ± 68 (McWhirter et al., 1993). Because both P210 erty of p160Bcr. By means of gene knock-out and P190 Bcr/Abl proteins retain this moiety, Bcr-Bcr/ technology, p160Bcr was found indispensable in vivo Abl heteromerization will occur in cells expressing both for regulation of respiratory burst activity in neutro- proteins. Indeed, Bcr consistently co-precipitates with phils through control of the cellular Rac-GTP/Rac- Bcr/Abl from Ph-positive cell lines and is a target for GDP pool during priming and activation (Voncken et the Bcr/Abl tyrosine kinase (Liu et al., 1993; Lu et al., al., 1995a). The Bcr protein also harbors a novel 1993; Pendergast et al., 1993; Puil et al., 1994). serine/threonine kinase activity (Stam et al., 1987; Li et Phosphorylation of tyrosine residues in the normal al., 1989; Maru and Witte 1991). BCR exon 1 Bcr protein by Bcr/Abl inhibits the Bcr serine/ sequences that encode this catalytic activity are unique threonine kinase encoded by BCR exon 1 and in the mammalian genome. Finally, Bcr carries several conversely, a serine-phosphorylated Bcr peptide from domains, including a peptide sequence bearing some exon 1 inhibits the Bcr/Abl tyrosine kinase activity (Liu homology to pleckstrin which potentially direct et al., 1996a,b). subcellular localization and interaction with other We have previously generated mice transgenic for cellular proteins. The biological signi®cance of these BCR/ABL P190 and P210 which develop leukemia/ domains and of the serine/threonine kinase domain in lymphoma (Heisterkamp et al., 1990; Voncken et al., vivo is as of yet unclear. 1992, 1995b). In a separate study, we have eliminated BCR was ®rst identi®ed as a gene encompassing the the cellular bcr function in mice through gene targeting breakpoint cluster region, a region on human (Voncken et al., 1995a). The availability of both mouse chromosome 22 which undergoes a reciprocal models for Philadelphia-positive leukemia and a bcr mutant lacking bcr protein allowed us to test the hypothesis that the Bcr protein as a whole contributes to leukemia through its interaction with Bcr/Abl. Correspondence: N Heisterkamp Leukemogenesis in the BCR/ABL P190 transgenic 1 Present address: The Netherlands Cancer Institute, Division of mice is usually accompanied by the development of Molecular Carcinogenesis, H-2, Plesmanlaan 121, 1066 CX lymphomas which largely consist of malignant Amsterdam, The Netherlands Received 10 September 1997; revised 19 November 1997; accepted 19 lymphoblasts. The bcr protein is expressed in most November 1997 normal tissues but the highest expression levels are Leukemia in bcr null mutants JW Voncken et al 2030 found in brain (Figure 1, lane 4). In the lymphomas, row involvement and asymmetric lymph node swelling, the endogenous p160bcr is expressed at a very high level, paralysis) developed. Soft tissue invasion was compar- which is comparable to that in brain (Figure 1 compare able in BCR/ABL-bcr(7/7) and in (+/+) animals. lanes 1 ± 3 with lane 4) and exceeds that found in bone We also introduced a BCR/ABL P210 transgene (1994- marrow of disease-free animals (not shown). The bcr line; Voncken et al., 1995) into the bcr(7/7) back- protein detected in these lymphoblasts migrates as a ground and obtained leukemic animals, indicating that double band (Figure 1). Fibroblasts transfected with the P210 protein does not require the presence of Bcr/Abl but not non-transfected controls also show a endogenous bcr protein for leukemogenesis (not double band of endogenous bcr protein (not shown). shown). This indicates that, similar to the CML cell lines, the Our ®nding, that the P190 BCR/ABL oncogene is endogenous bcr interacts with the Bcr/Abl protein and still capable of producing leukemia in transgenic mice becomes tyrosine phosphorylated in our transgenic when bred into the bcr(7/7) background, with mouse model (Liu et al., 1996a). indistinguishable latency and clinical pattern as in In earlier investigations, the development of genetically matched (+/+) P190 BCR/ABL counter- leukemia was studied in the P190 BCR/ABL trans- parts, rules out any signi®cant or major contribution of genic mouse line 623, which was of a B6 CBA genetic the bcr protein as a whole to the oncogenic process. It background (Voncken et al., 1992). Transgenic 623 remains formally possible that some of the enzymatic ospring died reproducibly of pre-B cell acute activity exhibited by the endogenous bcr protein is lymphoblastic leukemia (ALL) or lymphoblastic leukemia lymphoma; 50% mortality was approxi- mately at 10 weeks. The bcr(7/7) genotype was established in an 129Sv background. Introduction of a 129Sv chromosomes into the B6 CBA P190 transgenic background resulted in a signi®cantly increased latency of BCR/ABL-mediated leukemia development both in female and male wild types (Figure 2, compare open diamonds with small dots). There was no signi®cant dierence in tumor incidence, nor latency to development of leukemia between genetically matched BCR/ABL-bcr(+/+), BCR/ABL-bcr(7/7) and BCR/ABL-bcr(+/7) ani- mals of either sex. Fifty percent mortality for all `129Sv-background' transgenic female genotypes was about 22 weeks (Figure 2a) whereas for males this was at more than 1 year (Figure 2b). The dierence between males and females may be caused by possible hormone-responsive elements in the transgenic P190 construct, e.g., in the MT promoter segment. Gross examination of terminally ill animals revealed b comparable pathology, independent of the genetic status of the bcr locus. The type of neoplasia that developed was uniform, regardless of genotype: typically, either ALL (high peripheral white blood cell count, splenomegaly, limited lymph node and bone marrow involvement, hind limb paralysis) or lympho- blastic leukemia/lymphoma (splenomegaly, bone mar- 1 2 3 4 Figure 2 Survival curves of BCR/ABL P190 transgenics. The BCR/ABL transgenes were bred into the bcr(7/7) background by mating double transgenic (TG) males of a B6CBA background to bcr(7/7) females of a hybrid B6CBA/129Sv background. Heterozygous bcr(+/7) transgenic animals were subsequently bred to each other to obtain, with exception of the bcr null mutation, genetically matched wild type/TG and null mutant/TG Figure 1 Western blot analysis of p160bcr in lymphomas of BCR/ animals. (a) Females. TG P190 bcr null mutants [(7/7), n=15; ABL P190 transgenic mice. Lanes 1 ± 3 contain lysates from three open circles], TG bcr heterozygotes [(7/+), n=19, open squares] dierent terminally ill animals, lane 4 a brain lysate from a and TG wild types [(+/+), n=42; open diamonds] are compared. disease-free wild type animal. The arrows point to the bcr TG wild type animals of a B6CBA background [(+/+), n=115; doublets in the lymphomas and to the single bcr band in brain. 20 small dots] are also shown.