The MLL (mixed-lineage leukemia) gene is involved in many chromosomal translocations associated with acute myeloid and lymphoid leukemia. We previously identified a transcriptional repression domain in MLL, which contains a region with homology to DNA methyltransferase. In chromosomal translocations, the MLL repression domain is retained in the leukemogenic fusion protein and is required for transforming activity of MLL fusion proteins. We explored the mechanism of action of the MLL repression domain. T he MLL (mixed-lineage leukemia) gene located on chromosome band 11q23 is involved in many chromosomal translocations associated with acute leukemia (1-5). MLL is involved in translocations with Ͼ40 different genes, and breakpoints in MLL fall in an 8.3-kb breakpoint cluster region (refs. 6 and 7; see Fig. 6). The 430-kDa MLL protein is cleaved specifically into amino and carboxyl terminal peptides, which associate with each other (8-10). Domains of MLL include AT hooks, repression and activation domains, plant homeodomain (PHD) fingers, and a SET [Su(var)3-9, enhancer of zeste, and trithorax] domain, which was shown recently to have histone methyltransferase activity (10, 11). Recent murine models of MLL leukemia, including one from our lab, have confirmed that the aminoterminal portion of MLL fused in-frame to the partner gene is critical for leukemogenesis (12, 13). These fusions retain the AT hooks and the repression domain of MLL but lose the PHD, activation, and SET domains. The MLL repression domain initially was defined by using a reporter gene system (14) and was shown to be critical in the context of an MLL fusion for bone marrow transformation in vitro (15). Recently, this region of MLL also was shown to bind nonmethylated CpG DNA in vitro (16). Only by understanding how the MLL protein, including the repression domain in the amino-terminal portion of MLL, normally performs its regulatory functions can one infer how the MLL fusion proteins lead to hematopoietic cell transformation and leukemia development. In this respect, it will be important to characterize more extensively the interaction between the MLL repression domain and corepressors, and to assess the significance of these interactions.It was shown recently that DNA methyltransferase 1 (DNMT1) binds to histone deacetylase 1 (HDAC1), and this activity maps immediately adjacent to the region of sequence similarity between DNMT1 and MLL (17). The region of similarity, the cysteine-rich CXXC domain, is highly conserved among a small group of proteins, including DNMT1, MLL, and methyl-CpG-binding protein 1 (MBD1͞PCM1) (5,14,(18)(19)(20). Two regions of MLL, the CXXC domain (RD1) and the adjacent region (RD2), behave independently as transcriptional repressors in a reporter gene system (14). Although it is unknown how HDAC1 mediates the repression function of DNMT1, several possibilities exist. HDACs are believed to repress transcription by recruiting repressor complexes (21) or by removing acetyl groups from core histone tails in chromatin; hypoace...

The PHD fingers of the human MLL and Drosophila trx proteins have strong amino acid sequence conservation but their function is unknown. We have determined that these fingers mediate homodimerization and binding of MLL to Cyp33, a nuclear cyclophilin. These two proteins interact in vitro and in vivo in mammalian cells and colocalize at specific nuclear subdomains. Overexpression of the Cyp33 protein in leukemia cells results in altered expression of HOX genes that are targets for regulation by MLL. These alterations are suppressed by cyclosporine and are not observed in cell lines that express a mutant MLL protein without PHD fingers. These results suggest that binding of Cyp33 to MLL modulates its effects on the expression of target genes.

A new family of cyclophilins with an RNA recognition motif (RRM) has members in vertebrates, roundworms and flatworms. We have identified a Drosophilacyclophilin, Dcyp33, with a high degree of amino acid sequence identity and similarity with other members of the family. Dcyp33 interacts through its RRM domain with the third PHD finger of trithorax. This interaction is conserved in the human homologues of these proteins, Cyp33 and MLL. Over expression of Dcyp33 in DrosophilaSL1 cells results in down-regulation of AbdominalB Hoxgene expression, mirroring the effect of human Cyp33 on the expression of human HOXgenes.