Genes encoding transcription factors are the targets of specific chromosomal translocations in human leukemic cells, but the mechanisms by which they promote aberrant growth and differentiation are still largely undefined. This proposal focuses on hepatic leukemia factor (HLF), a newly identified sequence-specific DNA-binding protein of the basic-region/leucine-zipper (bZip) superfamily. A t(17;19)(q22;pl3) chromosomal translocation in childhood B-lineage acute lymphoblastic leukemia (ALL) disrupts the HLF gene at the breakpoint on chromosome 17 and the E2A gene at the breakpoint on chromosome 19. The resulting fusion gene (E2A-HLF) produces chimeric proteins that retain the amino-terminal transcriptional activation domain of E2A, but not its basic helix-loop-helix domain, which is replaced by the bZip DNA-binding and dimerization domain of the HLF protein. An attractive hypothesis to account for productive E2A-HLF gene rearrangements in cases of early B-lineage ALL is that the chimeric proteins contribute to malignant transformation and ensure maintenance of the leukemic state. The proposed research will be conducted on two distinct but complementary aspects of the roles of HLF and E2A-HLF proteins in the pathogenesis of childhood acute leukemia. First, the ability of E2A-HLF and HLF proteins to regulate the expression of reporter genes in constructs containing cis-acting DNA sequences that mediate HLF binding will be tested in lymphoid and myeloid leukemia cell lines. These studies will also assess the ability of HLF to heterodimerize with other bZip proteins, in order to determine whether homo- or heterodimers of normal or chimeric HLF proteins activate the expression of critical target genes in leukemic cells. Concurrently, the oncogenic potential and spectrum of progenitor cells susceptible to the transforming effects of these proteins will be determined in murine bone marrow cells infected with retroviral vectors carrying E2A-HLF or HLF cDNAs. Functional domains of chimeric E2A-HLF proteins will be tested separately, to determine how this hybrid protein can subvert the transcriptional programs that normally control the growth and differentiation of hematopoietic cells. Improved understanding of the involvement of these transcription factors in human acute leukemias should yield important information about the regulatory circuits governing normal hematopoiesis and may suggest new approaches for the diagnosis and treatment of hematopoietic malignancies.
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