MLL—a gene of interest

The mixed lineage leukemia gene, MLL, has been a gene of interest to clinical and basic scientists alike for more than a decade. This gene, located in chromosome band 11q23, is frequently rearranged by translocations, and less often by inversions, in de novo acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML) of infants, children, and adults, as well as secondary AML after treatment with topoisomerase II inhibitors. MLL is remarkable in that it is involved in rearrangements with more than 40 different partners. It has also been shown to undergo internal rearrangement by partial tandem duplication in a subset of adults with AML with normal cytogenetics or trisomy 11.¹ MLL-rearranged leukemias are often aggressive, and they generally carry a worse prognosis than their MLL-unrearranged counterparts. Thus, from a clinical perspective, MLL-rearranged leukemias are both intriguing and challenging.

MLL is also fascinating from a molecular biology standpoint. Originally identified by cloning human chromosome band 11q23 breakpoints, MLL was shown to be the mammalian homolog of Drosophila trithorax. Under normal circumstances, MLL encodes a histone methyltransferase that has been reported to assemble a supercomplex of proteins involved in transcriptional regulation.²  As a result of chromosomal rearrangements in acute leukemias, MLL is fused in-frame with a wide variety of nuclear and cytoplasmic proteins, and it undergoes oncogenic activation by multiple mechanisms, including acquisition of transcriptional effector domains (following fusion with nuclear transcription factors, cofactors, or chromatin remodeling proteins) and dimerization.^3,4  Perhaps the best understood targets of normal and chimeric MLL proteins are the clustered homeobox or HOX genes⁵ ; however, gene expression analyses suggest that a number of other potentially important target genes exist in MLL-rearranged leukemias. Investigations aimed at unraveling the complex and varied molecular mechanisms underlying MLL-rearranged leukemias, with the goal of identifying potential therapeutic targets, are ongoing.

With the application of fluorescence-based cytogenetic technologies, a new subgroup of patients with myeloid malignancies, overrepresentation of band 11q23, and increased MLL copy number has recently been identified. In this issue of Blood, Poppe and colleagues (page 229) present novel data suggesting an etiologic role for MLL gain of function in AML and myelo-dysplastic syndromes (MDSs) with amplification, as opposed to rearrangement, of 11q23. Selected for the study were 31 cytogenetically well-characterized patient samples, and one cell line, with 11q23 gain. Fluorescence in situ hybridization (FISH) was performed to define the copy numbers of MLL, DDX6, FLI1, and ETS1 across a 10-megabase (Mb) region of the 11q23 amplicon; all 4 genes were retained in all cases. Based upon the number of copies of MLL and the results of routine cytogenetic analyses, patients were divided into 3 groups with low (3-5 copies), intermediate (5-10 copies), and high (> 10 copies) levels of 11q23 gains. Expression levels of 6 selected 11q23 oncogenes (MLL, DDX6, CBL, ETS1, PLZF, and FLI1) were then compared with 11q23 genomic status, and highly significant expression differences were consistently identified only for MLL and DDX6. Functionality of the MLL transcript was indirectly demonstrated by up-regulation of HOXA9 gene expression in samples with high-level MLL amplification. Expression of 5 mixed lineage leukemia-associated genes, PROML1, ADAM10, NKG2D, ITPA, and MEIS1, was also strongly up-regulated in samples displaying high-level 11q23 amplification. Finally, the investigators showed significantly increased MLL expression in a separate unselected series of MDS samples without MLL rearrangement. Taken together, the data indicate that MLL is a key target of 11q23 overrepresentation, MLL overexpression results in gain of MLL function, and the transcriptional events associated with MLL amplification are similar to those seen with MLL rearrangement. This is the first report of the functional consequences of 11q23 overrepresentation in myeloid malignancies, and it suggests that patients with these diseases might also benefit from therapy targeted for MLL-rearranged acute leukemias.