EXPRESSION OF THE ZN FINGER GENE, EVI-1, IN ACUTE PROMYELOCYTIC LEUKEMIA

The EVI-1 gene encodes a Zn finger, DNA binding protein previously det ected in some acute myelogenous leukemias (AML) and myelodysplasias (M DS), but not in normal marrow or cord blood cells. Experimental studie s suggest EVI-1 blocks cellular differentiation by binding to GATA-1 o r other specific DNA sequences controlling gene expression, and may be involved in the pathogenesis of some AMLs. To further define potentia l roles for EVI-1 in leukemia pathogenesis, we studied its regulation in acute promyelocytic leukemias (APL). Seven of 11 APL cases expresse d EVI-1 RNA detected by RNA PCR at diagnosis, and expression was detec ted in two additional cases after treatment with all-trans retinoic ac id (ATRA). Two of four cases studied at relapse also expressed EVI-1 R NA. To investigate regulation of EVI-1 expression In APL, we examined its expression in the NB4, APL cell line, NB4 cells did not express EV I-1 under basal conditions, but expressed EVI-1 after ATRA-induced dif ferentiation. When NB4 cells were exposed to ATRA and transferred to c ultures with N,N'-hexamethylene-bis-acetamide (HMBA), differentiation occurred but EVI-1 RNA was not detected, indicating that EVI-1 express ion was not required for terminal, NB4 differentiation. ATRA-resistant NB4 cells were obtained by continuous culture in gradually increasing concentrations of ATRA. These cells did not express markers of differ entiation but continued to express EVI-1 for several weeks even after ATRA withdrawal. To assess whether expression of the APL PML-RAR alpha fusion gene alone was sufficient for ATRA induction of EVI-1, the PML -RAR alpha gene cDNA was expressed in U937 histiocytic lymphoma cells. ATRA treatment of PML-RAR alpha-transfected or control U937 cells did not induce EVI-1 expression. In conclusion, this study demonstrates t he EVI-1 gene is consistently expressed in APL cells either constituti vely or after ATRA treatment. ATRA represents the first biologically a ctive agent shown to specifically regulate EVI-1 expression in blood c ells, In contrast to previous studies in AML and MDS, the pattern of E VI-1 expression suggests it may facilitate rather than inhibit myeloid differentiation during ATRA treatment. However, effects of EVI-1 expr ession ape likely to be complex, and expression in ATRA-resistant APL cells may indicate multiple roles for this gene.