Recurring chromosomal translocations involving chromosome 11, band q23, have been observed in acute lymphoid leukemias and especially in acute myeloid leukemias. We recently showed that breakpoints in four 11q23 translocations, t(4;11)(q21;q23), t(6;11)(q27;q23), t(9;11)(p22;q23), and t(11;19)(q23;p13.3), were contained within a yeast artificial chromosome done bearing the CD3D and CD3G gene loci. We have identified within the CD3 yeast artificial chromosome a transcription unit that spans the breakpoint junctions of the 4;11, 9;11, and 11;19 translocations, and we describe two other, related transcripts that are upregulated in the RS4;11 cell line. We have named this gene MLL (myeloid/lymphoid, or mixed-fineage, leukemia).

Translocations involving chromosome band 11q23, found in acute lymphoid and myeloid leukemia, disrupt the MLL gene. This gene encodes a putative transcription factor with homology to the zinc fingers and other domains of the Drosophila trithorax gene product and to the "AT-hook" motif of hig mobility group proteins. To map potential transcriptional activation or repression doains of the MLL protein, yeast GAL4 DNA-binding domain and MLL hybrid protein-expressing plaids were cotransfected with chioramphenicol acetyltransferase reporter plasmids in a transient transfection system. We found that MLL contains a strong activation domain and a repression domain. The former, located telomeric (3') to the breakpoint region, activated transcription 18-fold to >200-fold, depending on the promoter and cell line used for transfection. A repression domain that repressed transcription 4-fold was located centromeric (5') to the breakpoint region of MLL. The MLL AT-hook domain protein was expressed in bacteria and was utilized in a gel mobility shift assay to assess DNA-binding activity. The MLL AT-hook domain could bind cruciform DNA, recognizing structure rather than sequence of the target DNA. In translocations involving MLL, loss of an activation domain with retention of a repression domain and a DNA-binding domain on the der(11) chromosome could alter the expression of downstream target genes, suggesting a potential mechanism of action for MLL in leukemia.

A BSTR ACTThe recurring translocation t(11;16)-(q23;p13.3) has been documented only in cases of acute leukemia or myelodysplasia secondary to therapy with drugs targeting DNA topoisomerase II. We show that the MLL gene is fused to the gene that codes for CBP (CREB-binding protein), the protein that binds specifically to the DNAbinding protein CREB (cAMP response element-binding protein) in this translocation. MLL is fused in-frame to a different exon of CBP in two patients producing chimeric proteins containing the AT-hooks, methyltransferase homology domain, and transcriptional repression domain of MLL fused to the CREB binding domain or to the bromodomain of CBP. Both fusion products retain the histone acetyltransferase domain of CBP and may lead to leukemia by promoting histone acetylation of genomic regions targeted by the MLL AT-hooks, leading to transcriptional deregulation via aberrant chromatin organization. CBP is the first partner gene of MLL containing well defined structural and functional motifs that provide unique insights into the potential mechanisms by which these translocations contribute to leukemogenesis.The t(11;16)(q23;p13.3) is a rare recurring translocation that has been described in 11 patients to date (1). All of these patients have therapy-related acute leukemia of myeloid or lymphoid phenotype, or myelodysplasia, after exposure to DNA topoisomerase II inhibitors (anthracyclines or epipodophyllotoxins) for treatment of a primary malignancy.MLL (also called ALL1, Htrx, and HRX; refs. 2-6), which is located on chromosomal band 11q23, is involved in translocations with at least 40 different partner genes (7-9). These translocations result in acute leukemia, either lymphoblastic or myeloid͞monocytic, with a close correlation between the specific translocation and a particular leukemia phenotype. MLL also is involved in translocations that occur secondary to therapy of a primary malignant disease with drugs that target DNA topoisomerase II and result in therapy-related acute myeloid leukemia or acute lymphoblastic leukemia (10-14). The t(11;16)(q23;p13.3) occurs only in therapy-related leukemia or myelodysplasia, in contrast to other MLL translocations such as the t(9;11), t(4;11), or t(11;19), which are seen primarily in de novo leukemia with no more than about 5-10% having leukemia occurring after treatment.MLL codes for a very large protein, with a predicted molecular mass of 431 kDa (4-6). The protein contains several domains identified by homology to other proteins or by functional analysis. Three AT-hook DNA-binding domains near the amino terminus also are found in the high-mobility group proteins HMG-I(Y) (15). MLL contains a region of homology to mammalian DNA methyltransferases, transcriptional activation and repression domains, and a cysteine-rich region that forms three C4HC3 zinc fingers [plant homeodomain (PHD) or leukemia-associated-protein domains] (16-21). The PHD domain and the SET [Su(var)3-9 enhancer of zeste, and trithorax] domain at the carboxyl terminus are the regions...