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All trans retinoic acid (ATRA) has revolutionized the therapy of acute promyelocytic leukemia (APL). Treatment of this leukemia with ATRA in combination with chemotherapy has resulted in complete remission rates >90 % and long-term remission rates above 80 %. Furthermore, the combination of ATRA and arsenic trioxide (ATO) was shown to be safe and effective in frontline treatment and, for patients with low and intermediate risk disease, possibly superior to the standard ATRA and anthracycline-based regimen. However, in spite of this tremendous progress, APL still remains associated with a high incidence of early death due to the frequent occurrence of an abrupt bleeding diathesis. This hemorrhagic syndrome more frequently develops in high-risk APL patients, currently defined as those exhibiting >10 × 10 9 /L WBC at presentation. In addition to high WBC count, other molecular and immunophenotypic features have been associated with high-risk APL. Among them, the expression in APL blasts of the stem/progenitor cell antigen CD34, the neural adhesion molecule (CD56), and the T cell antigen CD2 help to identify a subset of patients at higher risk of relapse and often the expression of these markers is associated with high WBC count. At the molecular level, the short PML/RARA isoform and FLT3 -internal tandem duplication (ITD) mutations have been associated with increased relapse risk. These observations indicate that extended immunophenotypic and molecular characterization of APL at diagnosis including evaluation of CD2, CD56, and CD34 antigens and of FLT3 mutations may help to better design risk-adapted treatment in this disease.

DNA methylation of tumor suppressor genes is a frequent mechanism of transcriptional silencing in cancer. The molecular mechanisms underlying the specificity of methylation are unknown. We report here that the leukemia-promoting PML-RAR fusion protein induces gene hypermethylation and silencing by recruiting DNA methyltransferases to target promoters and that hypermethylation contributes to its leukemogenic potential. Retinoic acid treatment induces promoter demethylation, gene reexpression, and reversion of the transformed phenotype. These results establish a mechanistic link between genetic and epigenetic changes during transformation and suggest that hypermethylation contributes to the early steps of carcinogenesis.

Recent discoveries have identified key molecular events in the pathogenesis of acute promyelocytic leukemia (APL), caused by chromosomal rearrangements of the transcription factor RAR (resulting in a fusion protein with the product of other cellular genes, such as PML). Oligomerization of RAR, through a self-association domain present in PML, imposes an altered interaction with transcriptional co-regulators (NCoR/SMRT). NCoR/SMRT are responsible for recruitment of histone deacetylases (HDACs), which is required for transcriptional repression of PML-RAR target genes, and for the transforming potential of the fusion protein. Oligomerization and altered recruitment of HDACs are also responsible for transformation by the fusion protein AML1-ETO, extending these mechanisms to other forms of acute myeloid leukemias (AMLs) and suggesting that HDAC is a common target for myeloid leukemias. Strikingly, AML1-ETO expression blocks retinoic acid (RA) signaling in hematopoietic cells, suggesting that interference with the RA pathway (genetically altered in APL) by HDAC recruitment may be a common theme in AMLs. Treatment of APLs with RA, and of other AMLs with RA plus HDAC inhibitors (HDACi), results in myeloid differentiation. Thus, activation of the RA signaling pathway and inhibition of HDAC activity might represent a general strategy for the differentiation treatment of myeloid leukemias.

Background The ZBTB16‐RARA fusion gene, resulting from the reciprocal translocation between ZBTB16 on chromosome 11 and RARA genes on chromosome 17 [t(11;17)(q23;q21)], is rarely observed in acute myeloid leukemia (AML), and accounts for about 1% of retinoic acid receptor‐α (RARA) rearrangements. AML with this rare translocation shows unusual bone marrow (BM) morphology, with intermediate aspects between acute promyelocytic leukemia (APL) and AML with maturation. Patients may have a high incidence of disseminated intravascular coagulation at diagnosis, are poorly responsive to all‐trans retinoic acid (ATRA) and arsenic tryoxyde, and are reported to have an overall poor prognosis. Aims The mutational profile of ZBTB16‐RARA rearranged AML has not been described so far. Materials and methods We performed targeted next‐generation sequencing of 24 myeloid genes in BM diagnostic samples from seven ZBTB16‐RARA+AML, 103 non‐RARA rearranged AML, and 46 APL. The seven ZBTB16‐RARA‐positive patients were then screened for additional mutations using whole exome sequencing (n = 3) or an extended cancer panel including 409 genes (n = 4). Results ZBTB16‐RARA+AML showed an intermediate number of mutations per patient and involvement of different genes, as compared to APL and other AMLs. In particular, we found a high incidence of ARID1A mutations in ZBTB16‐RARA+AML (five of seven cases, 71%). Mutations in ARID2 and SMARCA4, other tumor suppressor genes also belonging to SWI/SNF chromatin remodeling complexes, were also identified in one case (14%). Discussion and conclusion Our data suggest the association of mutations of the ARID1A gene and of the other members of the SWI/SNF chromatin remodeling complexes with ZBTB16‐RARA+AMLs, where they may support the peculiar disease phenotype. This is the first report showing the mutational landscape of the rare ZBTB16‐RARA rearranged acute myeloid leukemia (AML). We identified a high frequency of ARID1A mutations, suggesting the involvement of the SWI/SNF chromatin remodeling complexes in the clinical presentation, acute promyelocytic leukemia‐like morphology and all‐trans retinoic acid/arsenic trioxide treatment resistance of ZBTB16‐RARA rearranged AMLs.

All- trans retinoic acid (ATRA) plus arsenic trioxide was found to be noninferior to ATRA plus chemotherapy, and less toxic, in the treatment of acute promyelocytic leukemia. This is an early example of a curative therapy for acute leukemia without chemotherapy. Acute promyelocytic leukemia (APL) has become a highly curable disease with contemporary treatment, which consists of all- trans retinoic acid (ATRA) and anthracycline-based chemotherapy. 1 , 2 As reported in several large multicenter trials, this combination results in overall remission rates of up to 95% and cure rates now exceeding 80%. 3 – 11 Thus, the combination of ATRA and chemotherapy is currently considered the standard of care for newly diagnosed APL. 12 Arsenic trioxide is also highly effective in the treatment of APL. Early studies conducted in China and the United States showed that this agent can induce sustained molecular remission when used as . . .

PTEN ubiquitinylation in cancer The exclusion of the tumour suppressor PTEN from the cell nucleus has been linked with tumour progression; the mechanisms behind its aberrant localization are obscure, although ubiquitinylation of specific lysines in PTEN is known to regulate PTEN distribution between the cytoplasm and nucleus. Now Min Sup Song identify HAUSP, a deubiquitinating enzyme previously shown to act on the p53 tumour repressor, as the enzyme responsible for PTEN deubiquitination too. This activity is shown to regulate the cellular localization and function of PTEN. This role of HAUSP is antagonized by PML, another tumour suppressor. PML function is disrupted in promeyelocytic leukaemia, and drugs that are effective in treating this form of leukaemia are found to impinge on PTEN function, by affecting PML and HAUSP. Nuclear exclusion of the PTEN (phosphatase and tensin homologue deleted in chromosome 10) tumour suppressor has been associated with cancer progression 1 , 2 , 3 , 4 , 5 , 6 . However, the mechanisms leading to this aberrant PTEN localization in human cancers are currently unknown. We have previously reported that ubiquitinylation of PTEN at specific lysine residues regulates its nuclear–cytoplasmic partitioning 7 . Here we show that functional promyelocytic leukaemia protein (PML) nuclear bodies co-ordinate PTEN localization by opposing the action of a previously unknown PTEN-deubiquitinylating enzyme, herpesvirus-associated ubiquitin-specific protease (HAUSP, also known as USP7), and that the integrity of this molecular framework is required for PTEN to be able to enter the nucleus. We find that PTEN is aberrantly localized in acute promyelocytic leukaemia, in which PML function is disrupted by the PML–RARα fusion oncoprotein. Remarkably, treatment with drugs that trigger PML–RARα degradation, such as all- trans retinoic acid or arsenic trioxide, restore nuclear PTEN. We demonstrate that PML opposes the activity of HAUSP towards PTEN through a mechanism involving the adaptor protein DAXX (death domain-associated protein). In support of this paradigm, we show that HAUSP is overexpressed in human prostate cancer and is associated with PTEN nuclear exclusion. Thus, our results delineate a previously unknown PML–DAXX–HAUSP molecular network controlling PTEN deubiquitinylation and trafficking, which is perturbed by oncogenic cues in human cancer, in turn defining a new deubiquitinylation-dependent model for PTEN subcellular compartmentalization.

All- trans retinoic acid binds to, inhibits and induces degradation of the active form of the prolyl isomerase Pin1, thereby turning off and on a variety of Pin1 substrate oncogenes and tumor suppressors, respectively. A common key regulator of oncogenic signaling pathways in multiple tumor types is the unique isomerase Pin1. However, available Pin1 inhibitors lack the required specificity and potency for inhibiting Pin1 function in vivo . By using mechanism-based screening, here we find that all- trans retinoic acid (ATRA)—a therapy for acute promyelocytic leukemia (APL) that is considered the first example of targeted therapy in cancer, but whose drug target remains elusive—inhibits and degrades active Pin1 selectively in cancer cells by directly binding to the substrate phosphate- and proline-binding pockets in the Pin1 active site. ATRA-induced Pin1 ablation degrades the protein encoded by the fusion oncogene PML–RARA and treats APL in APL cell and animal models as well as in human patients. ATRA-induced Pin1 ablation also potently inhibits triple-negative breast cancer cell growth in human cells and in animal models by acting on many Pin1 substrate oncogenes and tumor suppressors. Thus, ATRA simultaneously blocks multiple Pin1-regulated cancer-driving pathways, an attractive property for treating aggressive and drug-resistant tumors.

Acute promyelocytic leukemia (APL) is a subtype of acute leukemia characterized by a unique t(15;17) translocation generating the PML/RARA fusion gene and hybrid oncoprotein. Besides its critical role in leukemogenesis, this genetic aberration serves as a disease-specific biomarker for rapid diagnosis and monitoring of minimal residual disease (MRD). Moreover, PML/RARA is specifically targeted by All-trans retinoic acid (ATRA) and arsenic trioxide (ATO), two agents that synergistically act to induce degradation of the oncoprotein. Large clinical studies including two randomized trials conducted in newly diagnosed APL patients have shown that the ATRA–ATO combination is superior to conventional ATRA and chemotherapy both in terms of efficacy and safety. Preliminary studies using oral formulations of arsenic and ATRA suggest that oral arsenic is as effective and manageable as intravenous ATO. Following early retrospective studies indicating the prognostic relevance of PML/RARA monitoring, several prospective studies were conducted in large cohorts of APL patients enrolled in clinical trials with the aim of better assessing the prognostic value of longitudinal PCR testing. The results consistently showed that molecular remission (defined as negativization of the PCR test for PML/RARA) correlates with a significantly decreased risk of relapse, whereas persistence of PCR positivity for PML/RARA after consolidation or conversion from negative to positive during follow-up is strongly associated with hematologic relapse. Based on these data, various groups started using pre-emptive salvage therapy for patients who persisted PCR-positive after frontline consolidation or converted from negative to positive PCR during follow-up. Finally, several expert panels have recommended that molecular remission should be considered a therapeutic objective in APL, and molecular response has been adopted as a study endpoint in modern clinical trials.

The acute promyelocytic leukemia (PML)-retinoic acid receptor α (RAR α ) fusion product recruits histone deacetylase (HDAC) and DNA methyltransferase (DNMT) activities on retinoic acid (RA)-target promoters causing their silencing and differentiation block. RA treatment induces epigenetic modifications at its target loci and restores myeloid differentiation of APL blasts. Using RA-sensitive and RA-resistant APL cell lines and primary blasts, we addressed the functional relevance of the aberrant methylation status at the RA-target promoter RAR β 2 and the mechanism by which methylation is reversed by RA. RA decreased DNMT expression and activity, which correlated with demethylation at specific sites on RAR β 2 promoter/exon-1, and the ability of APL blasts to differentiate in vitro and in vivo . None of these events occurred in an RA-resistant APL cell line containing a PML-RAR α defective for ligand binding. The specific contribution of the HDAC and DNMT pathways to the response of APL cells to RA was also tested by inhibiting these enzymatic activities with TSA and/or 5-azacytidine. In RA-responsive and RA-resistant APL blasts, TSA and 5-azacytidine induced specific changes on the chromatin state at RA-target sites, increased the RA effect on promoter activity, endogenous RA-target gene expression and differentiation. These results extend the rationale for chromatin-targeted treatment in APL and RA-resistant leukemias.