The t(1;19) translocation that characterizes 25% of pediatric pre-B cell acute lymphoblastic
leukemias (
pre-B ALL) produces a chimeric gene, joining 5' sequences that encode a transcriptional activator domain of E2A with 3' sequences that, in part, encode a homeo box domain of a new gene called pbx1. Two E2A-pbx1 transcripts have been cloned. They encode the putative fusion
proteins,
p85(E2A-Pbx1) and
p77(E2A-Pbx1), which differ in Pbx1 sequences alone, containing unique carboxyl termini whose sequences diverge after the Pbx1 homeo box. In this study, an antiserum to Pbx1 was used to investigate the identity and abundance of E2A-Pbx1 fusion
proteins in both the
pre-B ALL cell line, 697, and in cryopreserved leukemic bone marrow cells, obtained from six children with t(1;19)-positive
pre-B ALL. Five species of E2A-Pbx1
proteins were identified in all cells containing t(1;19), two of which were indistinguishable from in vitro-translated
p85(E2A-Pbx1) and
p77(E2A-Pbx1). To assess the
biological properties of
p85(E2A-Pbx1) and
p77(E2A-Pbx1) in fibroblasts, the cDNAs encoding these
proteins were cloned into retroviral vectors, and each was introduced into NIH-3T3 cells. Both
p85(E2A-Pbx1) and
p77(E2A-Pbx1) are localized in the nucleus, and expression of either resulted in malignant conversion of NIH-3T3 cells as assayed by
tumor formation in nude mice. When scored by focus formation, density-independent growth, and growth in
agar assays,
p77(E2A-Pbx1) was a much more potent transforming
protein than was
p85(E2A-Pbx1). Because subtle mutations in
p85(E2A-Pbx1) converted its transforming activity into that of
p77(E2A-Pbx1), we suggest that a sequence within the unique carboxyl terminus of
p85(E2A-Pbx1) serves to negatively regulate its biochemical activity.