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The development of resistance to chemotherapy is a major cause of relapse in acute lymphoblastic leukemia (ALL). Though several mechanisms associated with drug resistance have been studied in detail, the role of carbohydrate modification remains unexplored

Dasatinib is a potent dual Abl/Src inhibitor approved for treatment of Philadelphia chromosome-positive (Ph-positive) leukemias. At a once-daily dose and a relatively short half-life of 3-5 h, tyrosine kinase inhibition is not sustained. However, transient inhibition of K562 leukemia cells with a high-dose pulse of dasatinib or long-term treatment with a lower dose was reported to irreversibly induce apoptosis. Here, the effect of dasatinib on treatment of Bcr/Abl-positive acute lymphoblastic leukemia (ALL) cells was evaluated in the presence of stromal support. Dasatinib eradicated Bcr/Abl ALL cells, caused significant apoptosis and eliminated tyrosine phosphorylation on Bcr/Abl, Src, Crkl and Stat-5. However, treatment of mouse ALL cells with lower doses of dasatinib over an extended period of time allowed the emergence of viable drug-resistant cells. Interestingly, dasatinib treatment increased cell-surface expression of CXCR4, which is important for survival of B-lineage cells, but this did not promote survival. Combined treatment of cells with dasatinib and a CXCR4 inhibitor resulted in enhanced cell death. These results do not support the concept that long-term treatment with low-dose dasatinib monotherapy will be effective in causing irreversible apoptosis in Ph-positive ALL, but suggest that combined treatment with dasatinib and drugs such as AMD3100 may be effective.

The BCR gene is implicated in the development of Ph-positive leukemia through its fusion with the nonreceptor tyrosine kinase gene ABL. The normal 160 kDa Bcr protein has several functional domains, and recently one specific role for Bcr was established in the regulation of respiratory burst activity in white blood cells. Bcr expression levels are relatively constant throughout mouse development until adulthood in brain and in hematopoietic tissues, a pattern that is distinctly different from that of the functionally related n-chimerin gene. In the present study, RNA in situ hybridization was used to explore the normal cellular function of Bcr in rodent brain and hematopoietic organs. The data pinpoint the high bcr expression in the brain to the hippocampal pyramidal cell layer and the dentate gyrus, and to the piriform cortex and the olfactory nuclei, reflecting a potentially interesting function for Bcr in these highly specialized brain regions.

Philadelphia chromosome-positive leukemias result from the fusion of the BCR and ABL genes, which generates a functional chimeric molecule. The Abr protein is very similar to Bcr but lacks a structural domain which may influence its biological regulatory capabilities. Both Abr and Bcr have a GTPase-activating protein (GAP) domain similar to those found in other proteins that stimulate GTP hydrolysis by members of the Rho family of GTP-binding proteins, as well as a region of homology with the guanine nucleotide dissociation-stimulating domain of the DBL oncogene product. We purified as recombinant fusion proteins the GAP- and Dbl-homology domains of both Abr and Bcr. The Dbl-homology domains of Bcr and Abr were active in stimulating GTP binding to CDC42Hs, RhoA, Rac1, and Rac2 (rank order, CDC42Hs > RhoA >Racl = Rac2) but were inactive toward Rap1A and Ha-Ras. Both Bcr and Abr acted as GAPs for Rac1, Rac2, and CDC42Hs but were inactive toward RhoA, Rap1A, and Ha-Ras. Each individual domain bound in a noncompetitive manner to GTP-binding protein substrates. These data suggest the multifunctional Bcr and Abr proteins might interact simultaneously and/or sequentially with members of the Rho family to regulate and coordinate cellular signaling.

The closely related small GTP-binding proteins Rac1, Rac2, and Rac3 are part of a larger Rho subfamily of Ras proteins. Because disruption of Ras signaling pathways is relevant to the pathogenesis of a wide variety of cancers, it is important to clearly define the structural and functional characteristics of the participating proteins and their encoding genes. Rho subfamily members are involved in a range of signal transduction pathways relevant to cell growth, differentiation, motility, and stress, and Rac proteins are now recognised as a necessary component of Ras-mediated cellular transformation. We previously mapped RAC3 to chromosome band 17q23→ q25, a region that contains a number of candidate tumour suppressor genes. Because of its oncogenic potential, we have now further refined the location of this gene. Here we confirm that RAC3 maps to chromosome band 17q25.3 and further show that it maps some distance telomeric of a well-characterised minimal breast and ovarian candidate tumour suppressor gene region, BROV. The genomic structure of RAC3, including exon and intron boundaries, is also presented. Copyright © 2000 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]