Childhood and adult ALL: differences in epigenetic lesions associated with cell cycle genes

Cyclin-dependent kinase inhibitors in malignant hematopoiesis

The cell-cycle is a tightly orchestrated process where sequential steps guarantee cellular growth linked to a correct DNA replication. The entire cell division is controlled by cyclin-dependent kinases (CDKs). CDK activation is balanced by the activating cyclins and CDK inhibitors whose correct expression, accumulation and degradation schedule the time-flow through the cell cycle phases. Dysregulation of the cell cycle regulatory proteins causes the loss of a controlled cell division and is inevitably linked to neoplastic transformation. Due to their function as cell-cycle brakes, CDK inhibitors are considered as tumor suppressors. The CDK inhibitors p16^INK4a and p15^INK4b are among the most frequently altered genes in cancer, including hematopoietic malignancies. Aberrant cell cycle regulation in hematopoietic stem cells (HSCs) bears severe consequences on hematopoiesis and provokes hematological disorders with a broad array of symptoms. In this review, we focus on the importance and prevalence of deregulated CDK inhibitors in hematological malignancies.

Multiple functions of p27 in cell cycle, apoptosis, epigenetic modification and transcriptional regulation for the control of cell growth: A double-edged sword protein

The cell cycle is controlled by precise mechanisms to prevent malignancies such as cancer, and the cell needs these tight and advanced controls. Cyclin dependent kinase inhibitor p27 (also known as KIP₁) is a factor that inhibits the progression of the cell cycle by using specific molecular mechanisms. The inhibitory effect of p27 on the cell cycle is mediated by CDKs inhibition. Other important functions of p27 include cell proliferation, cell differentiation and apoptosis. Post- translational modification of p27 by phosphorylation and ubiquitination respectively regulates interaction between p27 and cyclin/CDK complex and degradation of p27. In this review, we focus on the multiple function of p27 in cell cycle regulation, apoptosis, epigenetic modifications and post- translational modification, and briefly discuss the mechanisms and factors that have important roles in p27 functions.

Impact of aberrant DNA methylation patterns including CYP1B1 methylation in adolescents and young adults with acute lymphocytic leukemia

Aberrant promoter DNA methylation is a well-described mechanism of leukemogenesis within hematologic malignancies, including acute lymphoblastic leukemia (ALL). However, the importance of methylation patterns among the adolescent and young adult (AYA) ALL population has not been well established. DNA methylation of 18 candidate genes in 33 AYA ALL patients was analyzed at diagnosis and during treatment, to evaluate the frequency and clinical relevance of aberrant methylation in an AYA population treated on a uniform therapeutic regimen. Of 16 informative genes, there was a median of 6 methylated genes per AYA ALL patient. Correlations were identified between increasing number of methylated genes with male sex (P = 0.04), increased white blood cell (WBC) count (P = 0.04) and increased bone-marrow blast percentage (P = 0.04). Increasing age was associated with EPHA5 methylation (P = 0.05). Overall, patients experienced favorable outcomes with median survival that was not reached. On univariate analysis, methylation of CYP1B1 was associated with worse overall survival (HR 10.7, 95% CI 1.3-87.6, P = 0.03), disease-free survival (HR 3.7, 95% CI 1.1-9.2, P = 0.04) and correlated with decreased CYP1B1 gene expression. A significant incidence of methylation within the AYA ALL population was identified, with increased methylation associated with distinct clinicopathologic features including male gender and elevated WBC count. Our results suggest aberrant methylation among AYA patients is frequent, and may provide a common pathogenic mechanism. The inferior outcome identified with methylation of the cytochrome p450 gene CYP1B1, an enzyme involved in drug metabolism and steroid synthesis, warrants further investigation.

Role of p53 family members p73 and p63 in human hematological malignancies

p53, mutated in over half of human cancers and about 13% of all hematological malignancies, maintains genomic integrity and triggers cellular senescence and apoptosis of damaged cells. In contrast to p53, the homologs p73 and p63 play critical roles in development of the central nervous system and skin/limbs, respectively. Moreover, dependent on the context they can exert tumor suppressor activities that cooperate with p53. Unlike p53, p73 and p63 are rarely mutated in cancers. Instead, up-regulation of the anti-apoptotic dominant-negative ΔNp73 and ΔNp63 isoforms is the most frequent abnormality in solid cancers. In hematological malignancies the most frequent p73 defect is promoter methylation and loss of expression, associated with unfavorable clinical outcomes. This suggests an essential tumor suppressor role of p73 in blood cells, also supported by genetic mouse models. Many therapeutic approaches aiming to restore p73 activity are currently being investigated. In contrast, the most frequent p63 abnormality is protein overexpression, associated with higher disease grade and poorer prognosis. Surprisingly, although available data are still scarce, the emerging picture is up-regulation of transactivation-competent TAp63 isoforms, suggesting a tumor-promoting role in this context.

Glucocorticoid and proteasome inhibitor impact on the leukemic lymphoblast: multiple, diverse signals converging on a few key downstream regulators

Twenty years ago a proteasome inhibitor was suggested as therapy for glucocorticoid-resistant multiple myeloma, a disease that involves terminally differentiated B cells. Since then, research has proven that it has utility on a number of tumors resistant to chemotherapy. Hematologic malignancy, however, often involves lesser differentiated cells, which have a high potential to modulate their intrinsic machinery and thereby activate alternative rescue pathways. A corresponding multiplicity of therapies is not always practical. One approach to conditions with heterogeneous physiology is to identify key biochemical mediators, thereby reducing the number of treatment targets. Results from several ongoing studies indicate convergence of genomically diverse signal pathways to a limited number of key downstream regulators of apoptosis. Convergence of pathways can be exploited to address the problem of genetic heterogeneity in acute leukemia: this would mean treating multiple molecular aberrations with fewer drugs and enhanced therapeutic benefit.

p57(Kip2) and cancer: time for a critical appraisal

p57(Kip2) is a cyclin-dependent kinase inhibitor belonging to the Cip/Kip family, which also includes p21(Cip1) and p27(Kip1). So far, p57(Kip2) is the least-studied Cip/Kip protein, and for a long time its relevance has been related mainly to its unique role in embryogenesis. Moreover, genetic and molecular studies on animal models and patients with Beckwith-Wiedemann syndrome have shown that alterations in CDKN1C (the p57(Kip2) encoding gene) have functional relevance in the pathogenesis of this disease. Recently, a number of investigations have identified and characterized heretofore unexpected roles for p57(Kip2). The protein appears to be critically involved in initial steps of cell and tissue differentiation, and particularly in neuronal development and erythropoiesis. Intriguingly, p27(Kip1), the Cip/Kip member that is most homologous to p57(Kip2), is primarily involved in the process of cell cycle exit. p57(Kip2) also plays a critical role in controlling cytoskeletal organization and cell migration through its interaction with LIMK-1. Furthermore, p57(Kip2) appears to modulate genome expression. Finally, accumulating evidence indicates that p57(Kip2) protein is frequently downregulated in different types of human epithelial and nonepithelial cancers as a consequence of genetic and epigenetic events. In summary, the emerging picture is that several aspects of p57(Kip2)'s functions are only poorly clarified. This review represents an appraisal of the data available on the p57(Kip2) gene and protein structure, and its role in human physiology and pathology. We particularly focus our attention on p57(Kip2) changes in cancers and pharmacological approaches for modulating p57(Kip2) levels.

p57KIP2: "Kip"ing the cell under control

p57(KIP2) is an imprinted gene located at the chromosomal locus 11p15.5. It is a cyclin-dependent kinase inhibitor belonging to the CIP/KIP family, which includes additionally p21(CIP1/WAF1) and p27(KIP1). It is the least studied CIP/KIP member and has a unique role in embryogenesis. p57(KIP2) regulates the cell cycle, although novel functions have been attributed to this protein including cytoskeletal organization. Molecular analysis of animal models and patients with Beckwith-Wiedemann Syndrome have shown its nodal implication in the pathogenesis of this syndrome. p57(KIP2) is frequently down-regulated in many common human malignancies through several mechanisms, denoting its anti-oncogenic function. This review is a thorough analysis of data available on p57(KIP2), in relation to p21(CIP1/WAF1) and p27(KIP1), on gene and protein structure, its transcriptional and translational regulation, and its role in human physiology and pathology, focusing on cancer development.

The distinctive biology of cancer in adolescents and young adults

One explanation for the relative lack of progress in treating cancer in adolescents and young adults is that the biology of malignant diseases in this age group is different than in younger and older persons, not only in the spectrum of cancers but also within individual cancer types and within the patient (host). Molecular, epidemiological and therapeutic outcome comparisons offer clues to this distinctiveness in most of the common cancers of adolescents and young adults. Translational and clinical research should not assume that the biology of cancers and patients is the same as in other age groups, and treatment strategies should be tailored to the differences.

In vitro functional alterations in the hematopoietic system of adult patients with acute lymphoblastic leukemia

In previous studies, we have demonstrated that progenitor cell-enriched marrow cell populations from patients with myeloid leukemia - including both acute (AML) and chronic (CML) - show severe functional alterations when cultured in stroma-free liquid cultures supplemented with stimulatory cytokines. In trying to expand our characterization of the biology of leukemic cells, in the present study we have used a similar approach and analyzed the in vitro growth of equivalent cell populations from patients with acute lymphoblastic leukemia (ALL). ALL marrow cell populations -enriched for hematopoietic progenitors by means of a negative selection procedure- were assessed for their proliferation and expansion potentials, in liquid cultures supplemented with a mixture of early- and late-acting recombinant stimulatory cytokines, throughout a 25-day culture period. ALL cells, although capable of responding to the stimulatory signals provided by hematopoietic stimulators, showed deficient proliferation potentials (reduced capacity to generate more nucleated cells), as compared with their normal counterparts. The capacity to generate myeloid and erythroid progenitors was also significantly reduced in ALL cultures. Interestingly, the functional alterations observed in ALL cultures (i.e., deficient proliferation and expansion potentials) were more pronounced in those from Ph+ patients than in those from Ph- patients. This study indicates that bone marrow cell populations - enriched for hematopoietic progenitor cells - from ALL patients possess deficient proliferation and expansion potentials in vitro, and that such functional alterations are more severe when cells are derived from Ph+ patients, as compared to their Ph- counterparts.

The role of p73 in hematological malignancies

The P73 gene is a homologue of the P53 tumor suppressor. Owing to its structural similarity with p53, p73 was originally considered to have tumor suppressor function. However, the discovery of N-terminal truncated isoforms with oncogenic properties showed a 'two in one' structure of its product, p73 protein. The full-length variants are strong inducers of apoptosis, whereas the truncated isoforms inhibit proapoptotic activity of p53 and the full-length p73. Thus, p73 is involved in the regulation of cell cycle, cell death and development. Moreover, it plays a role in carcinogenesis and controls tumor sensitivity to treatment. p73 is commonly expressed in tumor cells in hematological malignancies. Overexpression of p73 protein and aberrant expression of its particular isoforms, with very low frequency of P73 hypermethylation or mutations, were found in malignant myeloproliferations, including acute myeloblastic leukemia. In contrast, hypermethylation and subsequent inactivation of the P73 gene are the most common findings in malignant lymphoproliferative disorders, especially acute lymphoblastic leukemia (ALL) and non-Hodgkin's lymphomas. Assessment of P73 methylation may provide important prognostic information, as was confirmed in patients with ALL. This review summarizes some aspects of p73 biology with particular reference to its possible pathogenetic role and prognostic significance in hematological malignancies.

Downregulation of DBC1 expression in acute lymphoblastic leukaemia is mediated by aberrant methylation of its promoter

The DBC1 gene is a potential tumour suppressor gene that is commonly hypermethylated in epithelial cancers. We studied the role of promoter hypermethylation in the regulation of DBC1 in acute lymphoblastic leukaemia (ALL) cell lines and 170 ALL patients at diagnosis. Abnormal methylation of DBC1 was observed in all ALL cell lines and in 17% of ALL patients. Moreover, DBC1 methylation was associated with decreased DBC1 expression, while treatment of ALL cells with 5-Aza-2'-deoxycytidine resulted in demethylation of the promoter and upregulation of DBC1 expression. Fluorescence in situ hybridisation identified the deletion of one allele of DBC1 in some ALL cell lines, which indicated that the lack of DBC1 expression was due to deletion of one allele and methylation of the other. In conclusion, these results demonstrate, for the first time, that the expression of DBC1 is downregulated in a percentage of patients with ALL due to the hypermethylation of its promoter and/or gene deletion.