11q23 translocations split the "AT-hook" cruciform DNA-binding region and the transcriptional repression domain from the activation domain of the mixed-lineage leukemia (MLL) gene.

Abstract

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.

MLL: How complex does it get?

“…Using GAL4-MLL chimeras, it was shown that this region of the MLL protein contains a transcriptional activation domain [Zeleznik-Le et al, 1994]. This activation domain binds to the CREB-binding protein (CBP), a known regulator of gene expression [Ernst et al, 2001].…”

“…MLL contains three AT-hook motifs which have DNA binding activity, specific for AT-rich DNA [Zeleznik-Le et al, 1994]. These domains do not recognize a specific sequence but rather a structure of cruciform or bent DNA.…”

Isolation and characterization of a Pufferfish MLL (Mixed lineage leukemia)-like gene (fMll) reveals evolutionary conservation in vertebrate genes related to Drosophila trithorax

“…Within the fMll`domain 3', we were also able to identify smaller blocks of higher identity (Figure 4). One of these, Block 5 (Figures 3 and 4), overlaps with the minimal transactivation domain (MTAD) identi®ed in vitro (Zeleznik-Le et al, 1994;Prasad et al, 1995). Two small motifs of 7 amino acids located at the carboxy-terminal end of the region encoded by exon 25a (Block 4a: QVDGADD) and at the amino-terminal end of exon 25b (Block 4b: QLDGVDD) in fMll are 100% identical to those in MLL and Mll (Figures 3 and 4).…”

“…In the remaining exon junctions, identi®cation of appropriate splice junctions and similarity with the human protein and gene structure were utilised to predict the exon positions. b Domains are discussed in the text, b1, b2 etc refer to Block 1, Block 2 etc.`repressor' refers to the region described as a transcriptional repressor segment (Zeleznik-Le et al, 1994;Prasad et al, 1995). All other abbreviations are as in Figure 3.…”

MLL is fused to CBP, a histone acetyltransferase, in therapy-related acute myeloid leukemia with a t(11;16)(q23;p13.3)

“…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)(17)(18)(19)(20)(21). The PHD domain and the SET [Su(var)3-9 enhancer of zeste, and trithorax] domain at the carboxyl terminus are the regions most conserved with the Drosophila trithorax (trx) protein.…”