Theo Dingermann

New insights to the MLL recombinome of acute leukemias

Chromosomal rearrangements of the human MLL gene are associated with high-risk pediatric, adult and therapy-associated acute leukemias. These patients need to be identified, treated appropriately and minimal residual disease was monitored by quantitative PCR techniques. Genomic DNA was isolated from individual acute leukemia patients to identify and characterize chromosomal rearrangements involving the human MLL gene. A total of 760 MLL-rearranged biopsy samples obtained from 384 pediatric and 376 adult leukemia patients were characterized at the molecular level. The distribution of MLL breakpoints for clinical subtypes (acute lymphoblastic leukemia, acute myeloid leukemia, pediatric and adult) and fused translocation partner genes (TPGs) will be presented, including novel MLL fusion genes. Combined data of our study and recently published data revealed 104 different MLL rearrangements of which 64 TPGs are now characterized on the molecular level. Nine TPGs seem to be predominantly involved in genetic recombinations of MLL: AFF1/AF4, MLLT3/AF9, MLLT1/ENL, MLLT10/AF10, MLLT4/AF6, ELL, EPS15/AF1P, MLLT6/AF17 and SEPT6, respectively. Moreover, we describe for the first time the genetic network of reciprocal MLL gene fusions deriving from complex rearrangements.

Hyperforin is a dual inhibitor of cyclooxygenase-1 and 5-lipoxygenase.

The acylphloroglucinol derivative hyperforin is the major lipophilic constituent in the herb Hypericum perforatum (St. Johns wort). The aim of the present study was to investigate if hyperforin as well as extracts of H. perforatum can suppresses the activities of 5-lipoxygenase (5-LO) and cyclooxygenases (COX), key enzymes in the formation of proinflammatory eicosanoids from arachidonic acid (AA). In freshly isolated human polymorphonuclear leukocytes stimulated with Ca(2+) ionophore A23187, hyperforin inhibited 5-LO product formation with IC(50) values of about 1-2 microM, in the absence or presence of exogenous AA (20 microM), respectively, being almost equipotent to the well-documented 5-LO inhibitor zileuton (IC(50) = 0.5-1 microM). Experiments with purified human 5-LO demonstrate that hyperforin is a direct 5-LO inhibitor (IC(50) approximately 90 nM), acting in an uncompetitive fashion. In thrombin- or ionophore-stimulated human platelets, hyperforin suppressed COX-1 product (12(S)-hydroxyheptadecatrienoic acid) formation with an IC(50) of 0.3 and 3 microM, respectively, being about 3- to 18-fold more potent than aspirin. At similar concentrations, hyperforin suppressed COX-1 activity in platelets in presence of exogenous AA (20 microM) as well as in cell-free systems. Hyperforin could not interfere with COX-2 product formation and did not significantly inhibit 12- or 15-LO in platelets or leukocytes, respectively. We conclude that hyperforin acts as a dual inhibitor of 5-LO and COX-1 in intact cells as well as on the catalytic activity of the crude enzymes, suggesting therapeutic potential in inflammatory and allergic diseases connected to eicosanoids.

The AF4.MLL fusion protein is capable of inducing ALL in mice without requirement of MLL.AF4.

The chromosomal translocation t(4;11)(q21;q23) is the most frequent genetic aberration of the human MLL gene, resulting in high-risk acute lymphoblastic leukemia (ALL). To elucidate the leukemogenic potential of the fusion proteins MLL.AF4 and AF4.MLL, Lin(-)/Sca1(+) purified cells (LSPCs) were retrovirally transduced with either both fusion genes or with MLL.AF4 or AF4.MLL alone. Recipients of AF4.MLL- or double-transduced LSPCs developed pro-B ALL, B/T biphenotypic acute leukemia, or mixed lineage leukemia. Transplantation of MLL.AF4- or mock-transduced LSPCs did not result in disease development during an observation period of 13 months. These findings indicate that the expression of the AF4.MLL fusion protein is capable of inducing acute lymphoblastic leukemia even in the absence of the MLL.AF4 fusion protein. In view of recent findings, these results may imply that t(4;11) leukemia is based on 2 oncoproteins, providing an explanation for the very early onset of disease in humans.

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.

Transcription linked to recombination : a gene-internal promoter coincides with the recombination hot spot II of the human MLL gene

The MLL gene is frequently involved in chromosomal translocations associated with high-risk acute leukaemia. Infant and therapy-related acute leukaemia patients display chromosomal breakpoints preferentially clustered in the telomeric portion of the MLL breakpoint cluster region (SCII). Here, we demonstrate that SCII colocalizes with a gene-internal promoter element in the mouse and human MLL gene, respectively. The mRNA generated encodes an N-terminally truncated version of MLL that still exhibits many functional regions, including the C-terminal SET-domain. Etoposide-induced DNA double-strand breaks colocalize with the binding site of RNA polymerase II and the transcription initiation region, but not with a nearby Topo II consensus sequence. Thus, the observed genomic instability of the human MLL gene is presumably linked to transcriptional processes. The consequences of this novel finding for the creation of chromosomal translocations, the biology of the MLL protein and for MLL-mediated acute leukaemia are discussed.

Transcriptional properties of human NANOG1 and NANOG2 in acute leukemic cells

Transcripts of NANOG and OCT4 have been recently identified in human t(4;11) leukemia and in a model system expressing both t(4;11) fusion proteins. Moreover, downstream target genes of NANOG/OCT4/SOX2 were shown to be transcriptionally activated. However, the NANOG1 gene belongs to a gene family, including a gene tandem duplication (named NANOG2 or NANOGP1) and several pseudogenes (NANOGP2-P11). Thus, it was unclear which of the NANOG family members were transcribed in t(4;11) leukemia cells. 5′-RACE experiments revealed novel 5′-exons of NANOG1 and NANOG2, which could give rise to the expression of two different NANOG1 and three different NANOG2 protein variants. Moreover, a novel PCR-based method was established that allows distinguishing between transcripts deriving from NANOG1, NANOG2 and all other NANOG pseudogenes (P2–P11). By applying this method, we were able to demonstrate that human hematopoietic stem cells and different leukemic cells transcribe NANOG2. Furthermore, we functionally tested NANOG1 and NANOG2 protein variants by recombinant expression in 293 cells. These studies revealed that NANOG1 and NANOG2 protein variants are functionally equivalent and activate a regulatory circuit that activates specific stem cell genes. Therefore, we pose the hypothesis that the transcriptional activation of NANOG2 represents a ‘gain-of-stem cell function’ in acute leukemia.

Two independent gene signatures in pediatric t(4;11) acute lymphoblastic leukemia patients

Objective:  Gene expression profiles become increasingly more important for diagnostic procedures, allowing clinical predictions including treatment response and outcome. However, the establishment of specific and robust gene signatures from microarray data sets requires the analysis of large numbers of patients and the application of complex biostatistical algorithms. Especially in case of rare diseases and due to these constrains, diagnostic centers with limited access to patients or bioinformatic resources are excluded from implementing these new technologies.

MLL-mediated transcriptional gene regulation investigated by gene expression profiling

The human mixed lineage leukemia (MLL) gene is involved in about 50 different chromosomal translocations, associated with the disease phenotype of acute leukemia. However, the normal function of MLL is less understood. Homozygous knockouts of murine Mll were embryonal lethal, while heterozygous disruption led to aberrant hox gene expression associated with skeletal malformations, growth retardation, and impaired hematopoiesis. To understand MLL functions on the molecular level, gene expression profiling experiments were performed with a pair of murine cell lines (MLL+/+ and MLL−/−). Microarray hybridization experiments revealed 197 potential target genes that are differentially expressed, providing new and important clues about MLL functions.

The heterodimerization domains of MLL—FYRN and FYRC—are potential target structures in t(4;11) leukemia

The chromosomal translocation t(4;11)(q21;q23) is a frequent genetic aberration of the mixed lineage leukemia (MLL) gene, predominantly associated with high-risk acute lymphoblastic leukemia (ALL) in pediatric patients. Previous studies demonstrated that mice transplanted with hematopoietic cells expressing the AF4–MLL fusion protein develop proB ALL. The AF4–MLL oncoprotein becomes activated by Taspase1-mediated hydrolysis, which subsequently leads to a heterodimer of the cleavage products AF4–MLL·N and MLL·C. This protein–protein interaction is due to the FYRN and FYRC interaction domains present in both protein fragments. Heterodimerization subsequently induces high-molecular-weight protein complex formation that is protected against SIAH1/2-mediated polyubiquitinylation. Here, we attempted to selectively block this initial heterodimerization step, aiming to prevent the oncogenic activation of the AF4–MLL multiprotein complex. The minimal interaction interface was experimentally defined first in a bacterial two-hybrid system, and then in mammalian cells by using a biosensor assay. Expression of the FYRC domain, or smaller portions thereof, resulted in the inhibition of heterodimer formation, and blocked AF4–MLL multiprotein complex formation with subsequent destruction of the AF4–MLL oncoprotein. Thus, it is in principle possible to specifically target the AF4–MLL protein. This knowledge can now be exploited to design inhibitory decoys in order to destroy the AF4–MLL oncoprotein.

An Interstitial Deletion at 3p21.3 Results in the Genetic Fusion of MLH1 and ITGA9 in a Lynch Syndrome Family

Purpose: Germline mutations in DNA mismatch repair genes, mainly MLH1 or MSH2, have been shown to predispose with high penetrance for the development of the clinical phenotype of hereditary nonpolyposis colorectal cancer (Lynch syndrome). Here, we describe the discovery and first functional characterization of a novel germline MLH1 mutant allele. Experimental Design: A large kindred including 54 potential carriers was investigated at the molecular level by using different types of PCR experiments, gene cloning, transfection studies, Western blot experiments, and mismatch repair assays to identify and characterize a novel MLH1 mutant allele. Twenty-two of 54 putative carriers developed colon cancer or other tumors, including breast cancer. Results: The identified MLH1 mutant allele emerged from an interstitial deletion on chromosome 3p21.3, leading to an in-frame fusion of MLH1 (exons 1-11) with ITGA9 (integrin α 9; exons 17-28). The deleted area has a size of about 400 kb; codes for LRRFIP2 (leucine-rich repeat in flightless interaction protein 2), GOLGA4 (Golgi autoantigen, golgin subfamily a, 4), and C3orf35/APRG1 (chromosome 3 open reading frame 35/AP20 region protein 1); and partly disrupts the AP20 region implicated in major epithelial malignancies. Tumor cells lost their second MLH1 allele. The MLH1•ITGA9 fusion protein provides no capability for DNA mismatch repair. Murine fibroblasts, expressing a doxycycline-inducible MLH1•ITGA9 fusion gene, exhibit a loss–of–contact inhibition phenotype. Conclusions: This is the first description of a functional gene fusion of the human MLH1 gene, resulting in the loss of mismatch repair capabilities. The MLH1•ITGA9 fusion allele, together with deletions of the AP20 region, presumably defines a novel subclass of Lynch syndrome patients, which results in an extended tumor spectrum known from hereditary nonpolyposis colorectal cancer and Muir-Torre syndrome patients.