The ALL) gene, located at chromosome band 11q23, is involved in acute leukemia through a series of chromosome translocations and fusion to a variety of genes, most frequently to AF4 and AF9. The fused genes encode chimeric proteins. Because the Drosophila homologue ofALL), trithorax, is a positive regulator of homeotic genes and acts at the level of transcription, it is conceivable that alterations in ALL) transcriptional activity may underlie its action in malignant transformation. To Chromosome 1 1q23 abnormalities are associated with 5-10% of human acute leukemias, in particular in children under the age of 1 year, and in secondary leukemias (1). These leukemias have a lymphoid, myeloid, or lymphoid-myeloid phenotype and uniformly carry a poor prognosis. More than 20 different 11q23 translocations have been identified by now (2, 3). In nearly all of these aberrations, the ALLI gene (also called HRX, MLL, or HTRX) is involved (4-7). ALLI is thought to be the human homologue of the Drosophila trithorax gene, which acts as a positive regulator of the homeotic gene complexes Antennapedia and Bithorax.11q23 chromosome translocations cleave ALL1 within the breakpoint cluster region and fuse it to a partner gene, resulting in production of fused RNAs and presumably of chimeric proteins (4, 5). The chimeric proteins containing the N terminus of ALL1 are thought to be the critical products of the translocations (for review, see ref. 8). Presently, 10 partner genes have been cloned (8-13).The mechanism(s) by which the chimeric ALL1 proteins act to trigger neoplasm is not known. Two models were proposed: the first (5) emphasized gain of function and suggested that the altered protein (specifically ALL1 /ENL) is a hybrid transcription factor in which the DNA binding domain of ALL1The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.localizes the hypothetical transactivation domain of ENL to regulatory sites of target genes. The second (14) suggested that the cleavage of ALL1 protein results in loss of its function and that the partner polypeptide augments this loss. Resolution between the two models would necessitate developing biological and biochemical assays to measure transcriptional regulatory activity of the normal ALL1 protein, of the normal partner proteins, and of the chimeric species. As a first step in this direction, we looked for domains with transcriptional activation or repression potential within ALL1, within the AF4 and AF9 partner proteins (presumed to be nuclear proteins), and within the AF17 partner polypeptide, which contains a dimerization motif. A similar approach to investigate the ALL1 protein was recently reported (15). MATERIALS AND METHODSPlasmids. Effector plasmid vectors were either RSV-GAL4 (16) or pGALM (17). The former contains a GAL4 DNA binding domain (amino acids 1-147) flanked by the Rous sarcoma virus (RSV) promoter and ...

The MDMX gene product is related to the MDM2 oncoprotein, both of which interact with the p53 tumor suppressor. We have identified a novel transcript of the MDMX gene that is expressed in a variety of cell lines, and in particular, in growing and transformed cells. This transcript is identical to the published sequence yet it has a short internal deletion of 68 base pairs. This deletion produces a shift in the reading frame after codon 114, resulting in the inclusion of a stop codon at amino acid residue 127 (full-length MDMX is 489 residues). This truncated MDMX protein is termed MDMX-S ("short form"), represents only the p53-binding domain, and appears to bind p53 better than full-length MDMX. The MDMX-S protein can be detected in cell extracts and when overexpressed is much more effective than MDMX at inhibiting p53-mediated transcriptional activation and induction of apoptosis. Since MDMX-S lacks the central and carboxyl-terminal regions contained within full-length MDMX, it is likely to play a key role in the regulation of cell proliferation and apoptosis in a way distinct from MDMX.The p53 tumor suppressor protein plays an important role in regulating movement through a number of cell cycle checkpoints (1-3, 38). The activity of p53 is in turn modulated through the action of the MDM2 protein which is amplified in a variety of tumors (4 -6). MDM2 can associate with p53 (7) and directly inhibit p53's ability to activate transcription of target genes, such as p21. MDM2 performs this function in a number of ways. First, the amino-terminal domain MDM2 binds tightly to the transcriptional transactivation domain of p53 (4 -6, 8, 9). Through this interaction, MDM2 blocks the ability of p53 to activate transcription of specific target genes by repressing the formation of the preinitiation complex mediated through the TFIIE and TATA-binding protein subunits of RNA polymerase II (10, 11). The second blocking activity of MDM2 is that it targets p53 for proteolytic degradation by the ubiquitin-proteasome pathway (12)(13)(14)40). Third, MDM2 also functions to shuttle p53 out of the nucleus and into the cytoplasm, via a nuclear export signal located within the MDM2 protein (33).While MDM2 was first identified as a protein that inhibits the action of p53 it has recently been shown to affect other components of the G 1 /S transition. MDM2 was found to associate with both the E2F1 transcription factor (15) and the retinoblastoma tumor suppressor (pRb) 1 (16). The interaction with pRb was found to be repressive, in that the association of MDM2 with the carboxyl-terminal domain of pRb inhibited the growth regulatory function of pRb (16). The interaction with E2F1, over the amino terminus of MDM2 and the carboxyl terminus of E2F1, was stimulatory in the sense that MDM2 enhanced the ability of E2F1 to activate target gene expression (15). Thus MDM2 appears to regulate at least three of the important players in the G 1 /S transition: p53, pRb, and E2F1. It has therefore been concluded that MDM2 functions in the fashion of a domi...

No abstract

No abstract