Taspase1: a threonine aspartase required for cleavage of MLL and proper HOX gene expression

New insight into the molecular mechanisms of MLL-associated leukemia

Rearrangements of the MLL gene (ALL1, HRX, and Hrtx) located at chromosome band 11q23 are commonly involved in adult and pediatric cases of primary acute leukemias and also found in cases of therapy-related secondary leukemias. Studies on mouse models of MLL translocation and cell lines containing MLL rearrangements showed that the MLL gene linked chromosomal rearrangements to cellular differentiation and tumor tropism. Moreover, recent structural/functional studies on MLL and aberrant MLL proteins provided new clues and suggested that different mechanisms might be included in leukemogenesis by MLL rearrangements. The connection between these different mechanisms will help us understand globally how aberrant MLL oncogenes affect the normal cellular processes at molecular level.

MLL: a histone methyltransferase disrupted in leukemia

Rearrangements of the MLL gene, which is located at chromosome 11q23, are associated with aggressive acute leukemias in both children and adults. MLL regulates Hox gene expression through direct promoter binding and histone modification. MLL rearrangements occurring in leukemia include MLL fusion genes, partial tandem duplications of MLL and MLL amplification. MLL fusions and amplification upregulate Hox expression, apparently resulting in a block of hematopoietic differentiation. Future therapies for MLL-associated leukemia might involve blocking Hox gene upregulation by using fusion proteins or inhibiting the activity of Hox proteins themselves.

Mutations of RAI1, a PHD-containing protein, in nondeletion patients with Smith-Magenis syndrome

Smith-Magenis syndrome (SMS) is a mental retardation/multiple congenital anomalies disorder associated with a heterozygous approximately 4-Mb deletion in 17p11.2. Patients with SMS show variability in clinical phenotype despite a common deletion found in >75-80% of patients. Recently, point mutations in the retinoic acid induced 1 (RAI1) gene, which lies within the SMS critical interval, were identified in three patients with many SMS features in whom no deletion was detected. It is not clear if the entire SMS phenotype can be accounted for by RAI1 haploinsufficiency, nor has the precise function of RAI1 been delineated. We report two novel RAI1 mutations, one frameshift and one nonsense allele, in nondeletion SMS patients. Comparisons of the clinical features in these two patients, three of the previously reported RAI1 point mutation cases, and the patients with a common deletion suggest that the majority of the clinical features in SMS result from RAI1 mutation, although phenotypic variability exists even among the individuals with RAI1 point mutations. Bioinformatics analyses of RAI1 and comparative genomics between human and mouse orthologues revealed a zinc finger-like plant homeo domain (PHD) at the carboxyl terminus that is conserved in the trithorax group of chromatin-based transcription regulators. These findings suggest RAI1 is involved in transcriptional control through a multi-protein complex whose function may be altered in individuals with SMS.

Interaction of AF4 wild-type and AF4.MLL fusion protein with SIAH proteins: indication for t(4;11) pathobiology?

The human AF4 (ALL-1 fused gene on chromosome 4) gene (4q11) is recurrently involved in reciprocal translocations to the MLL (mixed lineage leukemia) gene (11q23), correlated with high-risk acute lymphoblastic leukemia (ALL) in infants and early childhood. The t(4;11) translocation is one of the most frequent MLL translocations known today. In general, MLL translocations are the result of an illegitimate recombination process leading to reciprocal fusions of unrelated translocation partner (TP) genes with the MLL gene. Owing to the constant presence of the derivative (11) product, it was hypothesised that only MLL.TP fusion genes are responsible for the leukemogenic process. This concept has been successfully tested for some known MLL fusions, while other MLL fusions failed. Here, we demonstrate growth-transforming potential of AF4 wild-type and the AF4.MLL fusion protein. The underlying oncogenic mechanism involves the two E3 ubiquitin ligases SIAH1 and SIAH2, the N-terminal portion of AF4 and the protection of the AF4.MLL fusion protein against proteosomal degradation.

Leukemia proto-oncoprotein MLL forms a SET1-like histone methyltransferase complex with menin to regulate Hox gene expression

MLL (for mixed-lineage leukemia) is a proto-oncogene that is mutated in a variety of human leukemias. Its product, a homolog of Drosophila melanogaster trithorax, displays intrinsic histone methyltransferase activity and functions genetically to maintain embryonic Hox gene expression. Here we report the biochemical purification of MLL and demonstrate that it associates with a cohort of proteins shared with the yeast and human SET1 histone methyltransferase complexes, including a homolog of Ash2, another Trx-G group protein. Two other members of the novel MLL complex identified here are host cell factor 1 (HCF-1), a transcriptional coregulator, and the related HCF-2, both of which specifically interact with a conserved binding motif in the MLL(N) (p300) subunit of MLL and provide a potential mechanism for regulating its antagonistic transcriptional properties. Menin, a product of the MEN1 tumor suppressor gene, is also a component of the 1-MDa MLL complex. Abrogation of menin expression phenocopies loss of MLL and reveals a critical role for menin in the maintenance of Hox gene expression. Oncogenic mutant forms of MLL retain an ability to interact with menin but not other identified complex components. These studies link the menin tumor suppressor protein with the MLL histone methyltransferase machinery, with implications for Hox gene expression in development and leukemia pathogenesis.

Autocatalytic Cleavage of the EMR2 Receptor Occurs at a Conserved G Protein-coupled Receptor Proteolytic Site Motif

Post-translational cleavage at the G protein-coupled receptor proteolytic site (GPS) has been demonstrated in many class B2 G protein-coupled receptors as well as other cell surface proteins such as polycystin-1. However, the mechanism of the GPS proteolysis has never been elucidated. Here we have characterized the cleavage of the human EMR2 receptor and identified the molecular mechanism of the proteolytic process at the GPS. Proteolysis at the highly conserved His-Leu downward arrow Ser(518) cleavage site can occur inside the endoplasmic reticulum compartment, resulting in two protein subunits that associate noncovalently as a heterodimer. Site-directed mutagenesis of the P(+1) cleavage site (Ser(518)) shows an absolute requirement of a Ser, Thr, or Cys residue for efficient proteolysis. Substitution of the P(-2) His residue to other amino acids produces slow processing precursor proteins, which spontaneously hydrolyze in a defined cell-free system. Further biochemical characterization indicates that the GPS proteolysis is mediated by an autocatalytic intramolecular reaction similar to that employed by the N-terminal nucleophile hydrolases, which are known to activate themselves by self-catalyzed cis-proteolysis. We propose here that the autoproteolytic cleavage of EMR2 represents a paradigm for the other GPS motif-containing proteins and suggest that these GPS proteins belong to a cell surface receptor subfamily of N-terminal nucleophile hydrolases.

ALL-1/MLL1, a homologue of Drosophila TRITHORAX, modifies chromatin and is directly involved in infant acute leukaemia

Rearrangements of the ALL-1/MLL1 gene underlie the majority of infant acute leukaemias, as well as of therapy-related leukaemias developing in cancer patients treated with inhibitors of topoisomerase II, such as VP16 and doxorubicin. The rearrangements fuse ALL-1 to any of >50 partner genes or to itself. Here, we describe the unique features of ALL-1-associated leukaemias, and recent progress in understanding molecular mechanisms involved in the activity of the ALL-1 protein and of its Drosophila homologue TRITHORAX.

The diverse superfamily of lysine acetyltransferases and their roles in leukemia and other diseases

Acetylation of the epsilon-amino group of lysine residues, or N(epsilon)-lysine acetylation, is an important post-translational modification known to occur in histones, transcription factors and other proteins. Since 1995, dozens of proteins have been discovered to possess intrinsic lysine acetyltransferase activity. Although most of these enzymes were first identified as histone acetyltransferases and then tested for activities towards other proteins, acetyltransferases only modifying non-histone proteins have also been identified. Lysine acetyltransferases form different groups, three of which are Gcn5/PCAF, p300/CBP and MYST proteins. While members of the former two groups mainly function as transcriptional co-activators, emerging evidence suggests that MYST proteins, such as Esa1, Sas2, MOF, TIP60, MOZ and MORF, have diverse roles in various nuclear processes. Aberrant lysine acetylation has been implicated in oncogenesis. The genes for p300, CBP, MOZ and MORF are rearranged in recurrent leukemia-associated chromosomal abnormalities. Consistent with their roles in leukemogenesis, these acetyltransferases interact with Runx1 (or AML1), one of the most frequent targets of chromosomal translocations in leukemia. Therefore, the diverse superfamily of lysine acetyltransferases executes an acetylation program that is important for different cellular processes and perturbation of such a program may cause the development of cancer and other diseases.

Proteolytic processing in development and leukemogenesis

There are now numerous examples in the hematopoietic system of genes that are critical for normal hematopoietic development, but when mutated, rearranged, or overexpressed, contribute to leukemogenesis. Two papers in this issue of Cell provide a fascinating twist on this paradigm, and suggest that proteolytic processing of certain of these genes plays an important role both in development and in leukemogenesis. These findings also suggest the possibility that proteases may be therapeutic targets in leukemia.