RETROVIRAL INTEGRATION AT THE EVI-2 LOCUS IN BXH-2 MYELOID-LEUKEMIA CELL-LINES DISRUPTS NF1 EXPRESSION WITHOUT CHANGES IN STEADY-STATE RAS-GTP LEVELS

Approximately 15% of BXH-2 myeloid leukemias harbor proviral integrati ons at the Evi-2 common viral integration site. Evi-2 is located withi n a large intron of the Nf1 tumor suppressor gene, raising the possibi lity that proviral integration at Evi-2 predisposes mice to myeloid tu mor development by disrupting Nf1 expression. This hypothesis is suppo rted by data suggesting that mutations in the human NF1 gene are causa lly associated with the development of juvenile chronic myelogenous le ukemia (K. M. Shannon, P. O'Connell, G. A. Martin, D. Paderanga, K. Ol son, P. Dinndorf, and P. McCormick, N. Engl. J. Med. 330:597-601, 1994 ) and mouse studies showing that aged mice, heterozygous for a germ li ne Nf1 mutation, develop myeloid leukemia with loss of the wild-type N f1 allele (T. Jacks, T. S. Shih, E. M. Schmitt, R. T. Bronson, A. Bern ards, and R. A. Weinberg, Nat. Genet. 7:353-361, 1994). To determine i f viral integration at Evi-2 disrupts Nf1 expression, we derived a ser ies of BXH-2 myeloid leukemia cell fines with or without viral integra tions at Evi-2. In all cell lines examined, viral integration at Evi-2 resulted in the production of only truncated Nf1 transcripts and no s table, full-length neurofibromin, Although neurofibromin is a GTPase-a ctivating protein (GAP) for p21(ras) proteins, its loss in the BXH-2 l eukemic cell lines was not correlated with an increased steady-state l evel of p21(ras) bound to GTP. These data suggest that neurofibromin i s not the sole mediator of Pas-GAP activity in myeloid cells and may h ave a GAP-independent function in myeloid cells.