Neuropathy of haematopoietic stem cell niche is essential for myeloproliferative neoplasms.

Myeloproliferative neoplasms (MPNs) are diseases caused by mutations in the haematopoietic stem cell (HSC) compartment. Most MPN patients have a common acquired mutation of Janus kinase 2 (JAK2) gene in HSCs that renders this kinase constitutively active, leading to uncontrolled cell expansion. The bone marrow microenvironment might contribute to the clinical outcomes of this common event. We previously showed that bone marrow nestin(+) mesenchymal stem cells (MSCs) innervated by sympathetic nerve fibres regulate normal HSCs. Here we demonstrate that abrogation of this regulatory circuit is essential for MPN pathogenesis. Sympathetic nerve fibres, supporting Schwann cells and nestin(+) MSCs are consistently reduced in the bone marrow of MPN patients and mice expressing the human JAK2(V617F) mutation in HSCs. Unexpectedly, MSC reduction is not due to differentiation but is caused by bone marrow neural damage and Schwann cell death triggered by interleukin-1ß produced by mutant HSCs. In turn, in vivo depletion of nestin(+) cells or their production of CXCL12 expanded mutant HSC number and accelerated MPN progression. In contrast, administration of neuroprotective or sympathomimetic drugs prevented mutant HSC expansion. Treatment with ß3-adrenergic agonists that restored the sympathetic regulation of nestin(+) MSCs prevented the loss of these cells and blocked MPN progression by indirectly reducing the number of leukaemic stem cells. Our results demonstrate that mutant-HSC-driven niche damage critically contributes to disease manifestation in MPN and identify niche-forming MSCs and their neural regulation as promising therapeutic targets.

The hematopoietic stem-cell niche in health and leukemia.

Research in the last decade has shown that hematopoietic stem cells (HSCs) interact with and are modulated by a complex multicellular microenvironment in the bone marrow, which includes both the HSC progeny and multiple non-hematopoietic cell types. Intense work is gradually throwing light on the composition of the HSC niche and the molecular cues exchanged between its components, which has implications for HSC production, maintenance and expansion. In addition, it has become apparent that bidirectional interactions between leukemic cells and their niche play a previously unrecognized role in the initiation and development of hematological malignancies. Consequently, targeting of the malignant niche holds considerable promise for more specific antileukemic therapies. Here we summarize the latest insights into HSC niche biology and recent work showing multiple connections between hematological malignancy and alterations in the bone marrow microenvironment.

Vav3 collaborates with p190-BCR-ABL in lymphoid progenitor leukemogenesis, proliferation, and survival.

Despite the introduction of tyrosine kinase inhibitor therapy, the prognosis for p190-BCR-ABL(+) acute lymphoblastic leukemia remains poor. In the present study, we present the cellular and molecular roles of the Rho GTPase guanine nucleotide exchange factor Vav in lymphoid leukemogenesis and explore the roles of Vav proteins in BCR-ABL-dependent signaling. We show that genetic deficiency of the guanine nucleotide exchange factor Vav3 delays leukemogenesis by p190-BCR-ABL and phenocopies the effect of Rac2 deficiency, a downstream effector of Vav3. Compensatory up-regulation of expression and activation of Vav3 in Vav1/Vav2-deficient B-cell progenitors increases the transformation ability of p190-BCR-ABL. Vav3 deficiency induces apoptosis of murine and human leukemic lymphoid progenitors, decreases the activation of Rho GTPase family members and p21-activated kinase, and is associated with increased Bad phosphorylation and up-regulation of Bax, Bak, and Bik. Finally, Vav3 activation only partly depends on ABL TK activity, and Vav3 deficiency collaborates with tyrosine kinase inhibitors to inhibit CrkL activation and impair leukemogenesis in vitro and in vivo. We conclude that Vav3 represents a novel specific molecular leukemic effector for multitarget therapy in p190-BCR-ABL-expressing acute lymphoblastic leukemia.

Guanine nucleotide exchange factor Vav1 regulates perivascular homing and bone marrow retention of hematopoietic stem and progenitor cells.

Engraftment and maintenance of hematopoietic stem and progenitor cells (HSPC) depend on their ability to respond to extracellular signals from the bone marrow microenvironment, but the critical intracellular pathways integrating these signals remain poorly understood. Furthermore, recent studies provide contradictory evidence of the roles of vascular versus osteoblastic niche components in HSPC function. To address these questions and to dissect the complex upstream regulation of Rac GTPase activity in HSPC, we investigated the role of the hematopoietic-specific guanine nucleotide exchange factor Vav1 in HSPC localization and engraftment. Using intravital microscopy assays, we demonstrated that transplanted Vav1(-/-) HSPC showed impaired early localization near nestin(+) perivascular mesenchymal stem cells; only 6.25% of Vav1(-/-) HSPC versus 45.8% of wild-type HSPC were located less than 30 µm from a nestin(+) cell. Abnormal perivascular localization correlated with decreased retention of Vav1(-/-) HSPC in the bone marrow (44-60% reduction at 48 h posttransplant, compared with wild-type) and a very significant defect in short- and long-term engraftment in competitive and noncompetitive repopulation assays (<1.5% chimerism of Vav1(-/-) cells vs. 53-63% for wild-type cells). The engraftment defect of Vav1(-/-) HSPC was not related to alterations in proliferation, survival, or integrin-mediated adhesion. However, Vav1(-/-) HSPC showed impaired responses to SDF1α, including reduced in vitro migration in time-lapse microscopy assays, decreased circadian and pharmacologically induced mobilization in vivo, and dysregulated Rac/Cdc42 activation. These data suggest that Vav1 activity is required specifically for SDF1α-dependent perivascular homing of HSPC and suggest a critical role for this localization in retention and subsequent engraftment.

mTORC1-dependent and -independent regulation of stem cell renewal, differentiation, and mobilization.

The Tuberous Sclerosis Complex component, TSC1, functions as a tumor suppressor via its regulation of diverse cellular processes, particularly cell growth. TSC1 exists in a complex with TSC2 and functions primarily as a key negative regulator of mammalian target of rapamycin complex 1 (mTORC1) signaling and protein synthesis, although the TSC1/TSC2 complex also shows mTORC1-independent outputs to other pathways. Here, we explored the role of TSC1 in various aspects of stem cell biology and dissected the extent to which TSC1 functions are executed via mTORC1-dependent versus mTORC1-independent pathways. Using hematopoietic stem cells (HSCs) as a model system, we demonstrate that somatic deletion of TSC1 produces striking stem cell and derivative effector cell phenotypes characterized by increased HSC cell cycling, mobilization, marked progressive depletion, defective long-term repopulating potential, and hematopoietic lineage developmental aberrations. On the mechanistic level, we further establish that TSC1 regulation of HSC quiescence and long-term repopulating potential and hematopoietic lineage development is mediated through mTORC1 signaling. In contrast, TSC1 regulation of HSC mobilization is effected in an mTORC1-independent manner, and gene profiling and functional analyses reveals the actin-bundling protein FSCN1 as a key TSC1/TSC2 target in the regulation of HSC mobilization. Thus, TSC1 is a critical regulator of HSC self-renewal, mobilization, and multilineage development and executes these actions via both mTORC1-dependent and -independent pathways.

FIP1L1/PDGFRalpha synergizes with SCF to induce systemic mastocytosis in a murine model of chronic eosinophilic leukemia/hypereosinophilic syndrome.

Expression of the fusion gene FIP1-like 1/platelet-derived growth factor receptor alpha (FIP1L1/PDGFRalpha, F/P) and dysregulated c-kit tyrosine kinase activity are associated with systemic mastocytosis (SM) and chronic eosinophilic leukemia (CEL)/hypereosinophilic syndrome (HES). We analyzed SM development and pathogenesis in a murine CEL model induced by F/P in hematopoietic stem cells and progenitors (HSCs/Ps) and T-cell overexpression of IL-5 (F/P-positive CEL mice). These mice had more mast cell (MC) infiltration in the bone marrow (BM), spleen, skin, and small intestine than control mice that received a transplant of IL-5 transgenic HSCs/Ps. Moreover, intestinal MC infiltration induced by F/P expression was severely diminished, but not abolished, in mice injected with neutralizing anti-c-kit antibody, suggesting that endogenous stem cell factor (SCF)/c-kit interaction synergizes with F/P expression to induce SM. F/P-expressing BM HSCs/Ps showed proliferation and MC differentiation in vitro in the absence of cytokines. SCF stimulated greater migration of F/P-expressing MCs than mock vector-transduced MCs. F/P-expressing bone marrow-derived mast cells (BMMCs) survived longer than mock vector control BMMCs in cytokine-deprived conditions. The increased proliferation and survival correlated with increased SCF-induced Akt activation. In summary, F/P synergistically promotes MC development, activation, and survival in vivo and in vitro in response to SCF.

A TaqMan low-density array to predict outcome in advanced Hodgkin's lymphoma using paraffin-embedded samples.

PURPOSE: Despite major advances in the treatment of classic Hodgkin's lymphoma (cHL), approximately 30% of patients in advanced stages may eventually die as result of the disease, and current methods to predict prognosis are rather unreliable. Thus, the application of robust techniques for the identification of biomarkers associated with treatment response is essential if new predictive tools are to be developed. EXPERIMENTAL DESIGN: We used gene expression data from advanced cHL patients to identify transcriptional patterns from the tumoral cells and their nonneoplastic microenvironment, associated with lack of maintained treatment response. Gene-Set Enrichment Analysis was used to identify functional pathways associated with unfavorable outcome that were significantly enriched in either the Hodgkin's and Reed-Sternberg cells (regulation of the G2-M checkpoint, chaperones, histone modification, and signaling pathways) or the reactive cell microenvironment (mainly represented by specific T-cell populations and macrophage activation markers). RESULTS: To explore the pathways identified previously, we used a series of 52 formalin-fixed paraffin-embedded advanced cHL samples and designed a real-time PCR-based low-density array that included the most relevant genes. A large majority of the samples (82.7%) and all selected genes were analyzed successfully with this approach. CONCLUSIONS: The results of this assay can be combined in a single risk score integrating these biological pathways associated with treatment response and eventually used in a larger series to develop a new molecular outcome predictor for advanced cHL.

Bone Marrow Mesenchymal Stem Cells Support Acute Myeloid Leukemia Bioenergetics and Enhance Antioxidant Defense and Escape from Chemotherapy.

Like normal hematopoietic stem cells, leukemic stem cells depend on their bone marrow (BM) microenvironment for survival, but the underlying mechanisms remain largely unknown. We have studied the contribution of nestin+ BM mesenchymal stem cells (BMSCs) to MLL-AF9-driven acute myeloid leukemia (AML) development and chemoresistance in vivo. Unlike bulk stroma, nestin+ BMSC numbers are not reduced in AML, but their function changes to support AML cells, at the expense of non-mutated hematopoietic stem cells (HSCs). Nestin+ cell depletion delays leukemogenesis in primary AML mice and selectively decreases AML, but not normal, cells in chimeric mice. Nestin+ BMSCs support survival and chemotherapy relapse of AML through increased oxidative phosphorylation, tricarboxylic acid (TCA) cycle activity, and glutathione (GSH)-mediated antioxidant defense. Therefore, AML cells co-opt energy sources and antioxidant defense mechanisms from BMSCs to survive chemotherapy.

A high-throughput study in melanoma identifies epithelial-mesenchymal transition as a major determinant of metastasis.

Metastatic disease is the primary cause of death in cutaneous malignant melanoma (CMM) patients. To understand the mechanisms of CMM metastasis and identify potential predictive markers, we analyzed gene-expression profiles of 34 vertical growth phase melanoma cases using cDNA microarrays. All patients had a minimum follow-up of 36 months. Twenty-one cases developed nodal metastatic disease and 13 did not. Comparison of gene expression profiling of metastatic and nonmetastatic melanoma cases identified 243 genes with a >2-fold differential expression ratio and a false discovery rate of <0.2 (206 up-regulated and 37 down-regulated). This set of genes included molecules involved in cell cycle and apoptosis regulation, epithelial-mesenchymal transition (EMT), signal transduction, nucleic acid binding and transcription, protein synthesis and degradation, metabolism, and a specific group of melanoma- and neural-related proteins. Validation of these expression data in an independent series of melanomas using tissue microarrays confirmed that the expression of a set of proteins included in the EMT group (N-cadherin, osteopontin, and SPARC/osteonectin) were significantly associated with metastasis development. Our results suggest that EMT-related genes contribute to the promotion of the metastatic phenotype in primary CMM by supporting specific adhesive, invasive, and migratory properties. These data give a better understanding of the biology of this aggressive tumor and may provide new prognostic and patient stratification markers in addition to potential therapeutic targets.

Somatic hypermutation signature in B-cell low-grade lymphomas.

BACKGROUND: Immunoglobulin gene somatic hypermutation is a biologically relevant and clinically useful prognostic factor in different types of low-grade B-cell lymphomas, including chronic lymphocytic leukemia, mantle cell lymphoma and splenic marginal zone lymphoma. DESIGN AND METHODS: With the aim of identifying surrogate markers of somatic hypermutation, a combined investigation of IgV(H) mutational status and expression profiles of 93 samples from patients with small B-cell lymphoma was performed. RESULTS: The analysis identified an somatic hypermutation signature of genes involved in the regulation of gene transcription, DNA repair and replication, and chromosome maintenance. Eight of these genes were subjected to protein analysis using tissue microarrays, for a set of 118 cases. We found a clear link between RAD51C and CDK7 protein expression and somatic hypermutation status, in that positive expression of either marker was significantly associated with a mutated status (p<0.003). We also found that positive expression of TFDP1 and POLA was significantly associated with ongoing somatic hypermutation (p<0.001). To assess the potential clinical applicability of these somatic hypermutation markers, we studied a series of cases of mantle cell lymphoma included in a tissue microarray. The expression of RCC1 and CDK7, separately and together, was found to be significantly associated with longer overall survival. CONCLUSIONS: An somatic hypermutation signature has been identified for different types of small B-cell lymphoma. This has a potential mechanistic and diagnostic value.

Inactivation of the lamin A/C gene by CpG island promoter hypermethylation in hematologic malignancies, and its association with poor survival in nodal diffuse large B-cell lymphoma.

PURPOSE: Lamins support the nuclear envelope and provide anchorage sites for chromatin, but they are also involved in DNA synthesis, transcription, and apoptosis. Although the lack of expression of A-type lamins in lymphoma and leukemia has been reported, the mechanism was unknown. We investigated the possible role of CpG island hypermethylation in lamin A/C silencing and its prognostic relevance. PATIENTS AND METHODS: The promoter CpG island methylation status of the lamin A/C gene, encoding the A-type lamins, was analyzed by bisulfite genomic sequencing and methylation-specific polymerase chain reaction in human cancer cell lines (n = 74; from 17 tumor types), and primary leukemias (n = 60) and lymphomas (n = 80). Lamin A/C expression was determined by reverse-transcription polymerase chain reaction, Western blot, immunohistochemistry, and immunofluorescence. RESULTS: seven (50%) of 14 leukemia- and five (42%) of 13 lymphoma cell lines. The presence of hypermethylation was associated with the loss of gene expression while a demethylating agent restored expression. In primary malignancies, lamin A/C hypermethylation was present in 18% (nine of 50) of acute lymphoblastic leukemias and 34% (14 of 41) of nodal diffuse large B-cell lymphomas. The presence of lamin A/C hypermethylation in nodal diffuse large B-cell lymphomas correlated strongly with a decrease in failure-free survival (Kaplan-Meier, P = .0001) and overall survival (Kaplan-Meier, P = .0005). CONCLUSION: Epigenetic silencing of the lamin A/C gene by CpG island promoter hypermethylation is responsible for the loss of expression of A-type lamins in leukemias and lymphomas. The finding that lamin A/C hypermethylation is associated with poor outcome in diffuse large B-cell lymphomas suggests important clinical implications.

Regulation of hematopoietic progenitors by estrogens as a basis for new antileukemic strategies.

We recently reported that estrogens regulate survival, proliferation, and self-renewal of hematopoietic stem cells and progenitors via estrogen receptor-α activation. Through its proapoptotic effect on malignant progenitors, tamoxifen treatment blocks the development of JAK2 (V617F) -induced myeloproliferative neoplasms in mice and sensitizes MLL-AF9-induced leukemias to chemotherapy, without detrimental effects on normal hematopoiesis.

Bone marrow stem cells: current and emerging concepts.

The interactions of stromal cells with hematopoietic cells in the bone marrow have long been a subject of research, but only recently have technologies allowed us to dissect them at the stem cell level. On the other hand, limitations of these technical tools might explain numerous discrepancies in this field. It is becoming increasingly clear that mesenchymal stem cells (MSCs) represent an important component of the hematopoietic stem cell (HSC) niche in the bone marrow. However, there is heterogeneity among HSCs, and many putatively different mesenchymal progenitors identified in the bone marrow using Cre recombinase-driven mouse lines seem to exhibit HSC niche properties. Development of better reporter lines has demonstrated that some of these Cre lines do not always specifically mark the expected cells. Also, characterization of different cell populations has often been partial, and issues of redundancy and compensation might explain apparently contradictory results. Recognizing and overcoming these limitations, while also clearly defining the distinctions between subgroups of mesenchymal cells, will be essential to advance the field.

Composite Hodgkin lymphoma and mantle cell lymphoma: two clonally unrelated tumors.

Association of Hodgkin lymphoma and non-Hodgkin lymphoma is rare and, specifically, the combination of Hodgkin lymphoma and mantle cell lymphoma has not been previously described. Here we describe composite mantle cell lymphoma and Hodgkin lymphoma affecting the spleen in one case and the eyelid and cervical lymph nodes in a second. In both, nodules of classical Hodgkin lymphoma were intermixed with diffuse or nodular areas of typical mantle cell lymphoma. Immunohistochemical and molecular analyses confirmed cyclin D1 overexpression secondary to the translocation t(11;14) in the small mantle cell lymphoma component; with CD30, CD15, and EBV expression in the Hodgkin and Reed-Sternberg cells. Finally, clonal analysis of rearranged immunoglobulin genes performed on microdissected Hodgkin and Reed-Sternberg and mantle cell lymphoma cells provided definite evidence of separate clonal origins of the two tumors in the patients. These EBV-positive, clonally unrelated tumors seem to represent true composite neoplasms, in contrast to cases showing merely clonal progression.

Estrogen signaling selectively induces apoptosis of hematopoietic progenitors and myeloid neoplasms without harming steady-state hematopoiesis.

Estrogens are potent regulators of mature hematopoietic cells; however, their effects on primitive and malignant hematopoietic cells remain unclear. Using genetic and pharmacological approaches, we observed differential expression and function of estrogen receptors (ERs) in hematopoietic stem cell (HSC) and progenitor subsets. ERα activation with the selective ER modulator (SERM) tamoxifen induced apoptosis in short-term HSCs and multipotent progenitors. In contrast, tamoxifen induced proliferation of quiescent long-term HSCs, altered the expression of self-renewal genes, and compromised hematopoietic reconstitution after myelotoxic stress, which was reversible. In mice, tamoxifen treatment blocked development of JAK2(V617F)-induced myeloproliferative neoplasm in vivo, induced apoptosis of human JAK2(V617F+) HSPCs in a xenograft model, and sensitized MLL-AF9(+) leukemias to chemotherapy. Apoptosis was selectively observed in mutant cells, and tamoxifen treatment only had a minor impact on steady-state hematopoiesis in disease-free animals. Together, these results uncover specific regulation of hematopoietic progenitors by estrogens and potential antileukemic properties of SERMs.

Self-renewing human bone marrow mesenspheres promote hematopoietic stem cell expansion.

Strategies for expanding hematopoietic stem cells (HSCs) include coculture with cells that recapitulate their natural microenvironment, such as bone marrow stromal stem/progenitor cells (BMSCs). Plastic-adherent BMSCs may be insufficient to preserve primitive HSCs. Here, we describe a method of isolating and culturing human BMSCs as nonadherent mesenchymal spheres. Human mesenspheres were derived from CD45- CD31- CD71- CD146+ CD105+ nestin+ cells but could also be simply grown from fetal and adult BM CD45--enriched cells. Human mesenspheres robustly differentiated into mesenchymal lineages. In culture conditions where they displayed a relatively undifferentiated phenotype, with decreased adherence to plastic and increased self-renewal, they promoted enhanced expansion of cord blood CD34+ cells through secreted soluble factors. Expanded HSCs were serially transplantable in immunodeficient mice and significantly increased long-term human hematopoietic engraftment. These results pave the way for culture techniques that preserve the self-renewal of human BMSCs and their ability to support functional HSCs.

Tumor microenvironment and mitotic checkpoint are key factors in the outcome of classic Hodgkin lymphoma.

Around 20% to 30% of patients with Hodgkin lymphoma (HL) do not benefit from standard therapies and finally succumb to their disease. The factors that influence the outcome of HL have not been elucidated, underscoring the demand for the identification of biologic risk factors and new therapeutic targets. We analyzed the gene expression profiles of samples from 29 patients with advanced classic HL treated with standard therapy and compared the expression profiles of patients with favorable and unfavorable clinical outcome. Using supervised methods, we identified 145 genes associated with outcome, which were grouped into 4 signatures representing genes expressed by either the tumoral cells (genes involved in the regulation of mitosis and cell growth/apoptosis) or the tumor microenvironment. The relationship between the expression of 8 representative genes and survival was successfully validated in an independent series of 235 patients by quantification of protein expression levels on tissue microarrays. Analysis of centrosomes and mitotic checkpoint confirmed the existence of an abnormal transition through mitosis in HL cells. Therefore, genes related to tumor microenvironment, cell growth/apoptosis, and regulation of mitosis are associated with treatment response and outcome of patients with HL.

Splenic marginal zone lymphoma: proposal of new diagnostic and prognostic markers identified after tissue and cDNA microarray analysis.

Splenic marginal zone lymphoma (SMZL) is a newly recognized lymphoma type whose precise molecular pathogenesis is still essentially unknown. This hampers differential diagnosis with other small B-cell malignancies. With the aim of characterizing this tumor more comprehensively, and of identifying new diagnostic and prognostic markers, we performed cDNA microarray expression profiling and tissue microarray (TMA) immunohistochemical studies in a relatively large series of 44 SMZLs. The results were related to immunoglobulin heavy chain variable region (IgV(H)) mutational status and clinical outcome. SMZLs display a largely homogenous signature, implying the existence of a single molecular entity. Of the genes deregulated in SMZLs, special mention may be made of the genes involved in B-cell receptor (BCR) signaling, tumor necrosis factor (TNF) signaling and nuclear factor-kappaB (NF-kappaB) activation, such as SYK, BTK, BIRC3, TRAF3, and LTB. Other genes observed were SELL and LPXN, which were highly expressed in spleen, and lymphoma oncogenes, such as ARHH and TCL1. In contrast, the genes CAV1, CAV2, and GNG11 located in 7q31, a commonly deleted area, were down-regulated in the entire series. A comparison with the genes comprising the signature of other small B-cell lymphomas identified 3 genes whose expression distinguishes SMZL, namely ILF1, SENATAXIN, and CD40. Shorter survival was associated with CD38 expression, naive IgV(H) genes, and the expression of a set of NF-kappaB pathway genes, including TRAF5, REL, and PKCA.

Epigenetic differences arise during the lifetime of monozygotic twins.

Monozygous twins share a common genotype. However, most monozygotic twin pairs are not identical; several types of phenotypic discordance may be observed, such as differences in susceptibilities to disease and a wide range of anthropomorphic features. There are several possible explanations for these observations, but one is the existence of epigenetic differences. To address this issue, we examined the global and locus-specific differences in DNA methylation and histone acetylation of a large cohort of monozygotic twins. We found that, although twins are epigenetically indistinguishable during the early years of life, older monozygous twins exhibited remarkable differences in their overall content and genomic distribution of 5-methylcytosine DNA and histone acetylation, affecting their gene-expression portrait. These findings indicate how an appreciation of epigenetics is missing from our understanding of how different phenotypes can be originated from the same genotype.

Cell cycle deregulation in B-cell lymphomas.

Disruption of the physiologic balance between cell proliferation and death is a universal feature of all cancers. In general terms, human B-cell lymphomas can be subdivided into 2 main groups, low- and high-growth fraction lymphomas, according to the mechanisms through which this imbalance is achieved. Most types of low-growth fraction lymphomas are initiated by molecular events resulting in the inhibition of apoptosis, such as translocations affecting BCL2, in follicular lymphoma, or BCL10 and API2/MLT1, in mucosa-associated lymphoid tissue (MALT) lymphomas. This results in cell accumulation as a consequence of prolonged cell survival. In contrast, high-growth fraction lymphomas are characterized by an enhanced proliferative activity, as a result of the deregulation of oncogenes with cell cycle regulatory functions, such as BCL6, in large B-cell lymphoma, or c-myc, in Burkitt lymphoma. Low- and high-growth fraction lymphomas are both able to accumulate other alterations in cell cycle regulation, most frequently involving tumor suppressor genes such as p16(INK4a), p53, and p27(KIP1). As a consequence, these tumors behave as highly aggressive lymphomas. The simultaneous inactivation of several of these regulators confers increased aggressivity and proliferative advantage to tumoral cells. In this review we discuss our current knowledge of the alterations in each of these pathways, with special emphasis on the deregulation of cell cycle progression, in an attempt to integrate the available information within a global model that describes the contribution of these molecular changes to the genesis and progression of B-cell lymphomas.