Complex MLL rearrangement in a patient with T-cell acute lymphoblastic leukemia

The Genetics and Mechanisms of T-Cell Acute Lymphoblastic Leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematologic malignancy derived from early T-cell progenitors. The recognition of clinical, genetic, transcriptional, and biological heterogeneity in this disease has already translated into new prognostic biomarkers, improved leukemia animal models, and emerging targeted therapies. This work reviews our current understanding of the molecular mechanisms of T-ALL.

The genetics and mechanisms of T cell acute lymphoblastic leukaemia

T cell acute lymphoblastic leukaemia (T-ALL) is an aggressive haematological malignancy derived from early T cell progenitors. In recent years genomic and transcriptomic studies have uncovered major oncogenic and tumour suppressor pathways involved in T-ALL transformation and identified distinct biological groups associated with prognosis. An increased understanding of T-ALL biology has already translated into new prognostic biomarkers and improved animal models of leukaemia and has opened opportunities for the development of targeted therapies for the treatment of this disease. In this Review we examine our current understanding of the molecular mechanisms of T-ALL and recent developments in the translation of these results to the clinic.

The NOTCH signaling pathway: role in the pathogenesis of T-cell acute lymphoblastic leukemia and implication for therapy

T-cell acute lymphoblastic leukemia/lymphoma (T-ALL) is characterized by aberrant activation of NOTCH1 in over 60% of T-ALL cases. The high prevalence of activating NOTCH1 mutations highlights the critical role of NOTCH signaling in the pathogenesis of this disease and has prompted the development of therapeutic approaches targeting the NOTCH signaling pathway. Small molecule gamma secretase inhibitors (GSIs) can effectively inhibit oncogenic NOTCH1 and are in clinical testing for the treatment of T-ALL. Treatment with GSIs and glucocorticoids are strongly synergistic and may overcome the gastrointestinal toxicity associated with systemic inhibition of the NOTCH pathway. In addition, emerging new anti-NOTCH1 therapies include selective inhibition of NOTCH1 with anti-NOTCH1 antibodies and stapled peptides targeting the NOTCH transcriptional complex in the nucleus.

Molecular pathogenesis and targeted therapies for NOTCH1-induced T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematologic tumor resulting from the malignant transformation of immature T-cell progenitors. Originally associated with a dismal prognosis, the outcome of T-ALL patients has improved remarkably over the last two decades as a result of the introduction of intensified chemotherapy protocols. However, these treatments are associated with significant acute and long-term toxicities, and the treatment of patients presenting with primary resistant disease or those relapsing after a transient response remains challenging. T-ALL is a genetically heterogeneous disease in which numerous chromosomal and genetic alterations cooperate to promote the aberrant proliferation and survival of leukemic lymphoblasts. However, the identification of activating mutations in the NOTCH1 gene in over 50% of T-ALL cases has come to define aberrant NOTCH signaling as a central player in this disease. Therefore, the NOTCH pathway represents an important potential therapeutic target. In this review, we will update our current understanding of the molecular basis of T-ALL, with a particular focus on the role of the NOTCH1 oncogene and the development of anti-NOTCH1 targeted therapies for the treatment of this disease.

Cancer genetics of epigenetic genes

The cancer epigenome is characterised by specific DNA methylation and chromatin modification patterns. The proteins that mediate these changes are encoded by the epigenetics genes here defined as: DNA methyltransferases (DNMT), methyl-CpG-binding domain (MBD) proteins, histone acetyltransferases (HAT), histone deacetylases (HDAC), histone methyltransferases (HMT) and histone demethylases. We review the evidence that these genes can be targeted by mutations and expression changes in human cancers.

Chromatin modifier enzymes, the histone code and cancer

In all organisms, cell proliferation is orchestrated by coordinated patterns of gene expression. Transcription results from the activity of the RNA polymerase machinery and depends on the ability of transcription activators and repressors to access chromatin at specific promoters. During the last decades, increasing evidence supports aberrant transcription regulation as contributing to the development of human cancers. In fact, transcription regulatory proteins are often identified in oncogenic chromosomal rearrangements and are overexpressed in a variety of malignancies. Most transcription regulators are large proteins, containing multiple structural and functional domains some with enzymatic activity. These activities modify the structure of the chromatin, occluding certain DNA regions and exposing others for interaction with the transcription machinery. Thus, chromatin modifiers represent an additional level of transcription regulation. In this review we focus on several families of transcription activators and repressors that catalyse histone post-translational modifications (acetylation, methylation, phosphorylation, ubiquitination and SUMOylation); and how these enzymatic activities might alter the correct cell proliferation program, leading to cancer.

Adult T-biphenotypic acute leukaemia: clinical and biological features and outcome

Biphenotypic acute leukaemia with T-lymphoid and myeloid markers is rare and poorly documented. In the Leucemie Aigue Lymphoblastique de l'Adulte (LALA) prospective trial (LALA 94) of treatment for adult acute lymphoblastic leukaemia (ALL), seven patients (0.86%) had T-biphenotypic forms. The clinical and biological characteristics and outcome of these seven patients are reported here. The patients' median age was 35 years. At diagnosis, all had a tumoural syndrome and five had a mediastinal mass. In all the cases, leukaemic cells expressed myeloid and lymphoid markers. Two patients (28%) entered complete remission (CR) after induction chemotherapy. Four of the five remaining and assessable patients entered CR after designed salvage chemotherapy with mitoxantrone and high-dose cytosine arabinoside. Three patients are currently in CR. Three patients died, from treatment toxicity in two cases and progressive disease in one case. One patient relapsed 6 months after allogeneic bone marrow transplantation and is still alive. Thus, biphenotypic T-acute leukaemia is clinically frequently associated with mediastinal involvement and the response to conventional chemotherapy used in ALL is poor. However, sustained CR can be achieved by salvage chemotherapy combining an intercalating agent with high-dose cytosine arabinoside, as used in acute myeloid leukaemia.

Dimerization of MLL fusion proteins immortalizes hematopoietic cells

MLL fusion proteins are leukemogenic, but their mechanism is unclear. Induced dimerization of a truncated MLL immortalizes bone marrow and imposes a reversible block on myeloid differentiation associated with upregulation of Hox a7, a9, and Meis1. Both dimerized MLL and exon-duplicated MLL are potent transcriptional activators, suggesting a link between dimerization and partial tandem duplication of DNA binding domains of MLL. Dimerized MLL binds with higher affinity than undimerized MLL to a CpG island within the Hox a9 locus. However, MLL-AF9 is not dimerized in vivo. The data support a model in which either MLL dimerization/exon duplication or fusion to a transcriptional activator results in Hox gene upregulation and ultimately transformation.

Double jeopardy from a single translocation: deletions of the derivative chromosome 9 in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is characterized by formation of a BCR-ABL fusion gene, usually as a consequence of the Philadelphia (Ph) translocation between chromosomes 9 and 22. Recently the development of new fluorescence in-situ hybridization (FISH) techniques has allowed identification of unexpected deletions of the reciprocal translocation product, the derivative chromosome 9, in 10% to 15% of patients with CML. These deletions are large, span the translocation breakpoint, and occur at the same time as the Ph translocation. Such deletions therefore give rise to previously unsuspected molecular heterogeneity from the very beginning of this disease, and there is mounting evidence for similar deletions associated with other translocations. Several studies have demonstrated that CML patients who carry derivative chromosome 9 deletions exhibit a more rapid progression to blast crisis and a shorter survival. Deletion status is independent of, and more powerful than, the Sokal and Hasford/European prognostic scoring systems. The poor prognosis associated with deletions is seen in patients treated with hydroxyurea or interferon, and preliminary evidence suggests that patients with deletions may also have a worse outcome than nondeleted patients following stem cell transplantation or treatment with imatinib. Poor outcome cannot be attributed to loss of the reciprocal ABL-BCR fusion gene expression alone, and is likely to reflect loss of one or more critical genes within the deleted region. The molecular heterogeneity associated with the Philadelphia translocation provides a new paradigm with potential relevance to all malignancies associated with reciprocal chromosomal translocations and/or fusion gene formation.

The kappa B transcriptional enhancer motif and signal sequences of V(D)J recombination are targets for the zinc finger protein HIVEP3/KRC: a site selection amplification binding study

BACKGROUND

The ZAS family is composed of proteins that regulate transcription via specific gene regulatory elements. The amino-DNA binding domain (ZAS-N) and the carboxyl-DNA binding domain (ZAS-C) of a representative family member, named kappaB DNA binding and recognition component (KRC), were expressed as fusion proteins and their target DNA sequences were elucidated by site selection amplification binding assays, followed by cloning and DNA sequencing. The fusion proteins-selected DNA sequences were analyzed by the MEME and MAST computer programs to obtain consensus motifs and DNA elements bound by the ZAS domains.

RESULTS

Both fusion proteins selected sequences that were similar to the kappaB motif or the canonical elements of the V(D)J recombination signal sequences (RSS) from a pool of degenerate oligonucleotides. Specifically, the ZAS-N domain selected sequences similar to the canonical RSS nonamer, while ZAS-C domain selected sequences similar to the canonical RSS heptamer. In addition, both KRC fusion proteins selected oligonucleoties with sequences identical to heptamer and nonamer sequences within endogenous RSS.

CONCLUSIONS

The RSS are cis-acting DNA motifs which are essential for V(D)J recombination of antigen receptor genes. Due to its specific binding affinity for RSS and kappaB-like transcription enhancer motifs, we hypothesize that KRC may be involved in the regulation of V(D)J recombination.