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The myelodysplastic syndromes (MDS) are clonal haemopathies characterized by pancytopenia, dysplastic haemopoiesis and a propensity towards leukaemic transformation ( Mufti & Galton, 1986). The French–American–British (FAB) classification, based on blood and marrow morphology ( Bennett et al, 1982 ), has been useful in identifying prognostically discrete subsets of MDS. In general, patients with advanced MDS subtypes such as refractory anaemia with excess blasts (RAEB) or RAEB in transformation (RAEB-t) have high rates of leukaemic evolution and a short median survival, whereas patients with refractory anaemia (RA) or refractory anaemia with ringed sideroblasts (RARS) are characterized by prolonged survival and <10% risk of leukaemic transformation.

Although it is now well established that MDS arises through the step-wise accumulation of genomic lesions within a haemopoietic stem cell, the precise mechanisms underlying MDS pathogenesis, and its heterogeneity and evolution to acute leukaemia, are poorly understood. Furthermore, the paradox of peripheral cytopenias despite most patients having a normo- or hypercellular marrow remains an enigma. Recently, it has been suggested that excessive programmed cell death or ‘apoptosis’ of marrow progenitors, at least in early phases of the disease, might explain this paradox ( Yoshida, 1993). Cell death might be appropriately triggered by activated T cells or marrow stroma in an attempt to eliminate the potentially harmful clone or inappropriately induced by abnormalities within the bone marrow microenvironment, such as a relative deficiency in haemopoietic growth factors. More likely, however, is the possibility that increased apoptosis arises from intrinsic defects within haemopoietic progenitors that lead to abnormalities in cell–cell or cell–stromal interaction, cell signalling or cell cycling. It is also conceivable that leukaemic evolution arises either through the development of immune tolerance or acquisition of genetic lesions that inhibit cell death and/or promote proliferation over and above apoptosis.

This paper outlines the apoptotic pathways, reviews evidence supporting its role in the pathogenesis of MDS and disease progression, and suggests possible molecular mechanisms whereby apoptosis in MDS might be dysregulated.

Fas-mediated apoptosis

  1. Top of page
  2. Fas-mediated apoptosis
  3. The Bcl-2 family
  4. Apoptosis and the cell cycle
  5. Evidence for increased apoptosis in MDS
  6. Increased apoptosis in MDS: the putative genetic lesions
  7. Leukaemic progression in MDS: role of p53
  8. Summary
  9. Acknowledgements
  10. References

Every eukaryotic cell possesses an inherent genetic programme triggering its own death ( Arends & Wyllie, 1991). This suicide programme or ‘apoptosis’ not only regulates tissue homeostasis by controlling cell numbers, but also allows the elimination of unwanted or potentially harmful cells. In haemopoietic cells, apoptosis is prevented by survival signals generated by haemopoietic growth factors and interleukins ( Williams et al, 1990 ), whereas fas ligand and tumour necrosis factor alpha (TNF-α) induce cell death. Growth factor deprivation, TNF-α– and fas ligand–receptor interaction trigger activation of a cascade of cysteine proteases or ‘caspases’ with homology to interleukin-1β-converting enzyme (ICE) which ultimately cleave proteins critical for cell viability.

Fas/APO-1 and TNF-α transmembrane receptor belong to the same receptor superfamily ( Nagata & Golstein, 1995) and share a homologous cytoplasmic protein domain. This domain is responsible for the transduction of death signals and has thus been termed the death domain (DD) ( Itoh & Nagata, 1993). Upon ligand binding, the transmembrane receptors trimerize ( Banner et al, 1993 ). This, in turn, alters the conformation of the DD so that it can associate with secondary signalling molecules. Whereas trimerization of TNF-αR triggers binding of TRADD (TNF-αR associated protein with a DD) ( Hsu et al, 1995 ), fas/ligand binding recruits association of FADD (fas associated protein with a DD) ( Chinnaiyan et al, 1995 ). A third adaptor protein, RIP (receptor interacting protein), binds to both TNF-αR and fas and appears to trigger a second apoptotic pathway ( Hsu et al, 1996 ). Once bound to fas/APO-1 receptor, FADD associates with caspase-8 (originally called FLICE or MACH) ( Muzio et al, 1996 ; Boldin et al, 1996 ), activating its protease domain. This triggers sequential activation of a cascade of caspases which subsequently cleave and inactivate proteins essential in maintaining DNA integrity such as the DNA repair enzyme poly (ADP-ribose) polymerase (PARP) ( Tewari et al, 1995 ; Enari et al, 1996 ) (Fig 1). In vitro studies have recently shown that caspase activation also requires release of cytochrome c from mitochondria ( Liu et al, 1996 ) and that this can be blocked by overexpression of the anti-apoptotic protein, Bcl-2 ( Yang et al, 1997 ). The Bcl-2 related protein, Bcl-XL also inhibits caspase activation and apoptosis by interacting with a nematode Caenorhabditis elegans CED-4 homologous protein recently identified as Apaf-1 ( Zou et al, 1997 ), which, when bound to Bcl-XL, associates with specific caspases such as caspase-8, maintaining them as a zymogen (inactive) ( Chinnaiyan et al, 1997 ) (Fig 2).

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Figure 1. (caspase-8) and activates its protease domain. Activated caspase-8 triggers a caspase cascade and results in accumulation of CPP32 (caspase-3), which cleaves and inactivates death substrates. Bcl-XL suppresses apoptotic stimuli by recruiting CED-4, a protein that maintains some caspases as a zymogen, whereas Bcl-2 inhibits caspase-3 activation, possibly by inhibiting mitochondrial cytochrome c release.

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image

Figure 2. from inhibitory control.

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The Bcl-2 family

  1. Top of page
  2. Fas-mediated apoptosis
  3. The Bcl-2 family
  4. Apoptosis and the cell cycle
  5. Evidence for increased apoptosis in MDS
  6. Increased apoptosis in MDS: the putative genetic lesions
  7. Leukaemic progression in MDS: role of p53
  8. Summary
  9. Acknowledgements
  10. References

Bcl-2 and Bcl XL are members of a large family of proteins that can accelerate or inhibit apoptosis in response to a wide variety of extracellular stimuli and intracellular signals. They function as homodimers or heterodimers and it is the ratio of the anti- versus the pro-apoptotic proteins that determines a cell's susceptibility to death signals ( Yang & Korsmeyer, 1996). For example, when Bax is overexpressed, Bax homodimerization predominates and apoptotic cell death in response to a death trigger is accelerated. When Bcl-2 is overexpressed, however, it heterodimerizes with Bax and death is repressed ( Oltvai et al, 1993 ). The Bcl-2 family of proteins therefore act as gatekeepers for apoptotic signals.

Bcl-2 and related proteins are located on outer mitochondrial, nuclear and endoplasmic reticulum membranes ( Jacobson et al, 1993 ), and appear to function as transmembrane ion channels or pores ( Muchmore et al, 1996 ; Schendel et al, 1997 ), regulating intracellular calcium fluxes ( Lam et al, 1994 ) or possibly mitochondial cytochrome c release ( Minn et al, 1997 ). As well as their function as a channel protein, Bcl-2 and Bcl-XL act as ‘docker’ proteins ( Reed, 1997), determining the subcellular localization of other apoptosis regulators with which they interact. Thus, Bcl-2 targets the phosphatase calcineurin ( Shibasaki & McKeon, 1995) and protein complexes containing the tumour suppressor protein p53 ( Naumovski & Cleary, 1996) to mitochondrial membranes, whereas Bcl-XL sequesters CED-4/caspase-8 complexes (Fig 2).

Bcl-2 family members also interact with the oncogenes c-myc, R-ras and raf-1. C-myc is a transcription factor required for the transition from G1 to S phase of the cell cycle and can alternately promote cell division or cell death, depending on the cellular environment ( Harrington et al, 1994 ). When c-myc is overexpressed, cell death in response to an apoptotic stimulus is accelerated ( Evan et al, 1992 ). However, constitutive expression of Bcl-2 abrogates c-myc induced apoptosis ( Bissonnette et al, 1992 ). Bcl-2 also targets Raf-1 to mitochondrial membranes, where it cooperates with Bcl-2 in suppressing apoptosis partly by phosphorylating and inactivating the pro-apoptotic protein BAD ( Wang et al, 1996 ). The role of ras-mediated signal transduction in the regulation of apoptosis is particularly pertinent to MDS pathogenesis, since point mutations resulting in constitutively activated Ras protein are one of the most common genomic lesions in MDS and are linked to myeloid leukaemic transformation ( Paquette et al, 1993 ).

Bcl-2 also appears to play a role in cell cycle regulation. In transgenic mice, Bcl-2 overexpression maintains T cells in G0 and suppresses transcription of delayed early cytokines by binding to the transcription factor NFAT (nuclear factor of activated T cells) complexed with calcineurin ( Linette et al, 1996 ; Shibasaki et al, 1997 ).

Apoptosis and the cell cycle

  1. Top of page
  2. Fas-mediated apoptosis
  3. The Bcl-2 family
  4. Apoptosis and the cell cycle
  5. Evidence for increased apoptosis in MDS
  6. Increased apoptosis in MDS: the putative genetic lesions
  7. Leukaemic progression in MDS: role of p53
  8. Summary
  9. Acknowledgements
  10. References

The cell cycle ensures that a cell stimulated to divide successfully completes DNA replication, repair and chromosomal segregation in a stepwise fashion. Orderly progression through the cell cycle is dependent upon specific protein kinase complexes which enable the cell to pass through several regulatory transition points. These complexes consist of a cyclin-dependent kinase (Cdk) and a regulatory cyclin molecule. Kinase activity is regulated by phosphorylation and dephosphorylation executed by phosphatases and the cyclin-dependent-kinase inhibitors (CdkI) (reviewed by Grana & Reddy, 1995). The transition from G1 to S phase is controlled by cyclins D and E and their corresponding kinases Cdk 2, 4 and 6 ( Sherr, 1993). A major substrate for the G1/S kinases is the retinoblastoma protein (Rb) ( Kato et al, 1993 ). Early in G1 when kinase activity is low, Rb remains hypophosphorylated. In this state it associates with the transcription factor E2F, inhibiting its transcriptional activity ( Johnson et al, 1993 ). As kinase activity increases with G1 progression, Rb becomes phosphorylated and dissociates from E2F. E2F is then free to activate the genes necessary for G1/S transition ( Helin et al, 1993 ) (Fig 3). Aberrant expression of the cyclins, their kinases or the kinase inhibitors indicates that the cell cycle is defective or that conditions are inappropriate to propagate cell proliferation and should therefore be aborted by inducing G1 arrest or by triggering apoptosis.

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Figure 3. Fig 3. Regulation of G1/S transition. In early G1 phase of the cell cycle, retinoblastoma protein (Rb) is unphosphorylated and binds E2F, preventing its transcriptional activity. As cyclin dependent kinase (Cdk) activity increases with G1 progression, Rb becomes phosphorylated and dissociates from E2F, which subsequently activates genes necessary for G1/S transition. Dephosphorylation of Rb by phosphatases during S phase allows its re-association with E2F.

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In addition to cyclin/Cdk regulation, there are several cell cycle checkpoints which ensure that DNA replication or repair has been successfully completed before the cell can enter the next phase. A major checkpoint involves the tumour supressor protein p53. Following DNA damage, p53 accumulates and arrests cells in mid to late G1, so that DNA can be repaired before cells have committed themselves to DNA replication ( Lin et al, 1992 ). Alternatively, in the face of overwhelming genomic damage, p53 can trigger cell death. p53 both activates and represses gene transcription. Genes activated by p53 include the CDK inhibitor, p21waf-1/cip-1 ( El-Deiry et al, 1993 ), the human analogue of the mouse double minute-2, mdm-2, which binds and abrogates p53 activity ( Barak et al, 1993 ), the apoptosis promoter Bax ( Miyashita et al, 1994 ) and possibly fas/APO-1 ( Owen-Schaub et al, 1995 ). In contrast, p53 appears to repress transcription of Bcl-2 ( Miyashita et al, 1994 ).

Evidence for increased apoptosis in MDS

  1. Top of page
  2. Fas-mediated apoptosis
  3. The Bcl-2 family
  4. Apoptosis and the cell cycle
  5. Evidence for increased apoptosis in MDS
  6. Increased apoptosis in MDS: the putative genetic lesions
  7. Leukaemic progression in MDS: role of p53
  8. Summary
  9. Acknowledgements
  10. References

Histological evidence of increased intramedullary haemopoietic cell apoptosis in MDS patients has been demonstrated by morphology, in vitro staining with apoptotic markers and flow cytometry in a number of studies ( Clark & Lampert, 1990; Raza et al, 1995 ; Rajapaksa et al, 1996 ). Raza et al (1995 ), using in situ end-labelling (ISEL) to identify DNA strand breaks generated during apoptosis, demonstrated that >50% MDS patients had excessive trilineage intramedullary cell death. Moreover, using a double labelling technique, they found that a large proportion of S-phase cells were simultaneously undergoing apoptosis. More recently, Rajapaksa et al (1996 ), by flow cytometric quantification of the sub-G1 (subdiploid) fraction of DNA within bone marrow cells, also demonstrated elevated levels of apoptosis in MDS patients compared to normal controls, whereas in acute myeloid leukaemia (AML) marrow, apoptosis was reduced. In contrast to Raza et al (1996b ), who found that programmed cell death was maximal in advanced MDS and was restricted to more differentiated CD34 cells, Rajapaksa et al (1996 ) showed that apoptosis was most prominent in the early MDS subtypes, RA and RARS, whereas the proportion of apoptotic cells in advanced MDS more closely approximated that seen in AML. Moreover, in MDS marrow, apoptosis was largely restricted to CD34+ cells. In our laboratory, using flow cytometric detection of fluorescein-labelled annexin V, which specifically binds to phosphatidylserine exposed on the outer membrane of apoptotic cells, we have observed increased apoptosis in patients with MDS compared to normal controls and AML patients. In accordance with other recent studies, excessive cell death was largely restricted to the early MDS subtypes, RA, RARS and RAEB (median % apoptosis CD34+ cells: normal BM, 18.6% (3.4–32.4%); MDS RA-RAEB, 62.8% (20.8–92.2%); RAEB-t and MDS-AML, 9.8% (2.1–30.2%) and predominantly affected CD34+ cells ( Greenberg et al, 1994 ; Tsoplou & Zoumbos, 1996; Bouscary et al, 1997 ; Bogdanovic et al, 1997 ; Parker et al, 1997 ) (Fig 4).

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Figure 4. -PE mAb and Annexin V–FITC. Cell analysis was performed on an EPICS XL flow cytometer (Coulter).

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Although there is now much evidence that increased haemopoietic progenitor cell apoptosis contributes to the ineffective haemopoiesis and peripheral cytopenias characteristic of early MDS, and progression to AML might result from clones that have escaped apoptotic control, as yet, neither the molecular defects giving rise to increased apoptosis, nor the mechanisms resulting in leukaemic progression have been elucidated.

Increased apoptosis in MDS: the putative genetic lesions

  1. Top of page
  2. Fas-mediated apoptosis
  3. The Bcl-2 family
  4. Apoptosis and the cell cycle
  5. Evidence for increased apoptosis in MDS
  6. Increased apoptosis in MDS: the putative genetic lesions
  7. Leukaemic progression in MDS: role of p53
  8. Summary
  9. Acknowledgements
  10. References

Bone marrow microenvironment

Bone marrow stroma regulates haemopoiesis through production of soluble or transmembrane cytokines and by direct contact mediated by adhesion molecules or ‘integrins’. Studies demonstrating defective bone marrow stroma in patients with aplastic anaemia ( Marsh et al, 1991 ) prompted investigations into the role of the bone marrow microenvironment in the pathogenesis of MDS and its particular role in triggering apoptosis. Although early studies found no consistent functional defect of MDS stroma ( Coutinho et al, 1990 ), Raza et al (1995 ) demonstrated excessive apoptosis of MDS stromal cells as well as haemopoietic progenitors. More recently, Silverman et al (1997 ), using long-term bone marrow cultures (LTBMC), demonstrated decreased or absent stromal growth in >80% patients with MDS. Furthermore, MDS stroma was less able to support normal haemopoiesis.

Evidence demonstrating that apoptotic cells are often found in clusters in MDS marrow ( Bogdanovic et al, 1997 ) supports a role for cytokines in triggering excessive cell death. Several studies have detected elevated levels of TNF-α mRNA and protein in bone marrow mononuclear cells of MDS patients compared to normal controls ( Raza et al, 1996b ; Gersuk et al, 1996 ). Raza et al (1996b ) also demonstrated that the level of TNF-α protein was directly related to the degree of programmed cell death in individual patients and was specifically distributed around apoptotic cells. Furthermore, inhibition of TNF-α using pentoxifylline or anti-TNF-α monoclonal antibody resulted in a reduction in marrow apoptosis, enhanced in vitro haemopoietic colony growth and a favourable cytogenetic and/or haematological response in a proportion of patients ( Raza et al, 1996a ; Gersuk et al, 1996 ). Increased serum TNF-α levels in MDS patients have also been reported and have been correlated with both the degree of anaemia ( Verhoef et al, 1992 ) and poor response to recombinant erythropoietin ( Musto et al, 1994 ). It is unclear, however, whether increased TNF-α expression in MDS results from an intrinsic defect within the bone marrow microenvironment or is secreted by ‘normal’ stromal cells in an attempt to eliminate the abnormal MDS clone. Alternatively, TNF-α might be secreted in an autocrine fashion from a malignant subclone. It is also feasible that elevated TNF-α levels reflect increased scavenger macrophage activity and merely represent an effect and not a cause of excessive apoptosis.

Other authors have found altered levels of regulatory cytokines such as interleukin-1-beta (IL1-β), interleukin-6, interleukin-8, stem cell factor (SCF) and granulocyte-macrophage colony stimulating factor (GM-CSF), in the serum or marrow of MDS patients ( Maurer et al, 1993 ; Bowen et al, 1993 ; Visani et al, 1993 ). Indeed, successful treatment with granulocyte colony stimulating factor (G-CSF) and erythropoietin to reverse cytopenias in MDS patients is associated with a significant reduction in the percentage of apoptotic cells ( Hellstrom-Lindberg et al, 1997 ).

Fas expression

Dysfunction of fas-signalling pathways has been implicated in the pathogenesis of various disorders including aplastic anaemia ( Maciejewski et al, 1995b ) and a rare disorder designated autoimmune lymphoproliferative syndrome (ALPS) in which a single deleterious fas gene mutation is always observed ( Fisher et al, 1995 ; Rieux-Laucat et al, 1995 ). Several investigators have demonstrated increased fas antigen and/or ligand expression on CD34+ cells in MDS marrow ( Gersuk et al, 1996 ; Bouscary et al, 1997 ). Although these studies showed no correlation between fas expression and the degree of marrow apoptosis, peripheral blood cytopenias or karyotype, levels of fas mRNA and protein were higher in early MDS compared to more advanced subtypes, suggesting that leukaemic blasts lose expression of this antigen as the disease progresses. Furthermore, whereas functional anti-fas monoclonal antibody repressed in vitro colony growth in MDS patients ( Bouscary et al, 1997 ), inhibition of fas-mediated signalling using a fas-immunoglobulin fusion protein resulted in enhanced in vitro colony formation ( Gersuk et al, 1996 ). The mechanism whereby fas may be up-regulated in myelodysplasia is unknown; however, recent evidence demonstrating that TNF-α and interferon-gamma (IFN-γ) can increase fas expression in normal haemopoietic progenitors ( Maciewski et al, 1995a ) further supports the hypothesis that excessive apoptosis in MDS is mediated by cytokines. Levels of fas expression in MDS clones could have important clinical and therapeutic implications. Indeed, fas/fas-ligand signalling pathways have been implicated in mediating doxorubicin-induced apoptosis in leukaemic blasts ( Friesen et al, 1996 ) and disruption of fas signalling has been associated with multidrug resistance in leukaemia cell lines ( Landowski et al, 1997 ) and poor response to chemotherapy in AML ( Min et al, 1996 ).

Bcl-2 family

Bcl-2 was originally identified as the deregulated oncogene in t(14;18) lymphomas ( Tsujimoto et al, 1985 ). Since then, aberrant expression of Bcl-2 and related proteins has been demonstrated in a diversity of human cancers. In AML, Bcl-2 overexpression is associated with CD34 positivity, relapsed disease, autonomous in vitro growth, resistance to chemotherapy, and reduced overall survival ( Delia et al, 1992 ; Campos et al, 1993 ; Bradbury et al, 1997 ). Moreover, Stoetzer et al (1996 ) have shown that the ratio of anti-apoptotic Bcl-2 to pro-apoptotic Bax is significantly correlated with response to chemotherapy, with no patient with a ratio greater than 1 achieving complete remission. Using two-colour flow cytometric analysis of fixed, permeabilized cells stained with CD34-FITC and antibodies to Bax, Bad (pro-apoptotic), Bcl-2 and Bcl-X (anti-apoptotic), we have observed that leukaemic progression appears to be associated with a reduced ratio of pro- versus anti-apoptotic Bcl-2-related proteins (Bax and Bad: Bcl-2 and Bcl-x ratio: MDS RA-RAEB, median 3.25 (1.6–9.42); RAEB-t/MDS-AML, median 1.67 (0.4–2.89); de novo AML, median 0.51 (0.03–1.29)) ( Parker et al, 1997 ) (Fig 5). Furthermore, Rajapaksa et al (1996 ) detected a reduced c-myc:Bcl-2 ratio as MDS evolved towards AML. The association between overexpression of Bcl-2 and/or Bcl-XL and resistance to cytotoxic agents ( Kuhl et al, 1997 ) offers a potential therapeutic tool to improve outcome following chemotherapy in MDS and AML. Indeed, the differentiation agent all-trans-retinoic acid (ATRA), especially in combination with G-CSF, both increases sensitivity of AML blasts to cytarabine and reduces Bcl-2 expression ( Hu et al, 1996 ).

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Figure 5. Fig 5. Two-colour cytometric analysis of Bcl-2 family members in BM CD34+ cells. Cells were stained with mouse monoclonal anti-CD34-FITC and then fixed and permeabilized prior to incubating with unconjugated intracellular antibody and secondarily labelled with PE-conjugated rabbit anti-mouse antibody. Negative controls were performed by incubating cells with isotype-specific antibodies (IMC).

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Cell cycle abnormalities

The importance of orderly progression through the cell cycle in maintaining normal tissue homeostasis is reflected by the frequency at which genetic lesions in human cancers target cell cycle regulatory elements. Cyclin D1, which regulates Rb phosphorylation, is overexpressed in many human cancers as a result of gene translocation or amplification ( Jiang et al, 1993 ), whereas inactivation of Rb is frequently seen in solid tumours ( Harbour et al, 1988 ; Jiang et al, 1993 ). Aberrant expression of the transcription factor E2F, the apparent target of the Rb pathway, has not yet been reported in human neoplasms, although gene amplification and translocation has been detected in several well-established human leukaemia cell lines ( Saito et al, 1995 ). In AML, low or absent Rb expression has been detected in 20–30% cases and is associated with autonomous in vitro colony growth, poor response to chemotherapy and reduced overall survival ( Kornblau et al, 1994 ; Zhu et al, 1994 ). In MDS, however, altered expression of the cyclins, Rb or E2F are uncommon ( Preudhomme et al, 1994 ). Recently, the DNA binding protein PUR-alpha (PURα), which is involved in gene transcription and cell cycle control, has been found to bind specifically to the hypophosphorylated form of Rb ( Johnson et al, 1995 ). Deletion of at least one PURα allele, located on the long arm of chromosome 5 ( Ma et al, 1995 ), has been detected in 100% patients with cytogenetically visible 5q deletions. Moreover, changes in the PURα gene have been found in a high proportion of MDS patients, even in the absence of visible 5q deletions ( Lezon-Geyda et al, 1997 ).

There has been growing interest in the cyclin-dependent kinase inhibitors (CdkI) as potential tumour suppressor genes (reviewed by Hirama & Koeffler, 1995). In haematological neoplasms, p15INK4B and p16INK4A gene inactivation, as a result of methylation or deletion, is most commonly seen in lymphoid malignancies, especially adult T-cell leukaemia (ATL) ( Hatta et al, 1995 ). p15INK4B/p16INK4A inactivation, although observed in myeloid cell lines, is rarely seen in myeloid leukaemias and MDS ( Nakamaki et al, 1995 ). Likewise, inactivation of genes encoding the CdkIs p18, p19, p21WAF1 and p27Kip1 appears to be exceedingly rare in human cancer and has not yet been observed in MDS ( Shiohara et al, 1994 ; Ponce-Castaneda et al, 1995 ; Nakamaki et al, 1997 ).

Leukaemic progression in MDS: role of p53

  1. Top of page
  2. Fas-mediated apoptosis
  3. The Bcl-2 family
  4. Apoptosis and the cell cycle
  5. Evidence for increased apoptosis in MDS
  6. Increased apoptosis in MDS: the putative genetic lesions
  7. Leukaemic progression in MDS: role of p53
  8. Summary
  9. Acknowledgements
  10. References

The role of wild-type p53 in eliminating genetically altered cells makes it a suitable candidate as mediator of apoptosis in early MDS. Moreover, acquisition of lesions within the p53 machinery, which would allow the illegitimate survival of neoplastic clones, could be responsible for the leukaemic evolution witnessed in some MDS patients. Indeed, in chronic myeloid leukaemia, mutation or deletion of the p53 gene, located on the short arm of chromosome 17, is frequently observed in myeloid blast crisis but rarely seen in the chronic phase of the disease ( Mashal et al, 1990 ). p53 gene aberrations have been observed in 5–15% patients with de novo MDS or AML ( Jonveaux et al, 1991 ; Sugimoto et al, 1993 ; Wattel et al, 1994 ) and up to 38% cases of therapy-related MDS/AML ( Ben-Yehuda et al, 1996 ). p53 mutations in MDS are usually associated with loss of the wild-type allele ( Lai et al, 1995 ) and are frequently accompanied by additional complex cytogenetic abnormalities often involving chromosomes 5 and 7 ( Sugimoto et al, 1993 ; Lai et al, 1995 ; Kitagawa et al, 1994 ). In MDS and AML patients with visible deletions of the short arm of chromosome 17, 69% harbour p53 mutations ( Lai et al, 1995 ). Such cases are characterized by a specific form of dysgranulopoiesis, reduced marrow apoptosis, poor response to chemotherapy and reduced overall survival ( Wattel et al, 1994 ). The fact that p53 mutations are almost exclusively seen in advanced MDS FAB subtypes ( Sugimoto et al, 1993 ), and appear to predict leukaemic evolution ( Kitagawa et al, 1994 ), suggests that disruption of p53-mediated apoptosis might, at least in some cases, be responsible for disease progression in MDS.

Overexpression of MDM-2, which normally negatively regulates p53, can also be transforming. Certainly, MDM-2 overexpression through gene amplification has been found in both tumourigenic mouse cell lines and in up to 30% sarcomas ( Fakharzadeh et al, 1991 ; Oliner et al, 1992 ) and can overcome wild-type p53-mediated suppression of transformed cellular growth ( Finlay, 1993). Although MDM-2 gene alterations have not been detected in haematological malignancies, overexpression of MDM-2 has been observed in 12–54% cases of AML and has been associated with unfavourable karyotype, poor response to chemotherapy and short survival ( Bueso-Ramos et al, 1993 ; Preudhomme et al, 1993 ; Quesnel et al, 1994 ). However, evidence for a role of MDM-2 aberrations in leukaemic evolution in MDS is, at present, lacking ( Preudhomme et al, 1993 ; Quesnel et al, 1994 ).

Summary

  1. Top of page
  2. Fas-mediated apoptosis
  3. The Bcl-2 family
  4. Apoptosis and the cell cycle
  5. Evidence for increased apoptosis in MDS
  6. Increased apoptosis in MDS: the putative genetic lesions
  7. Leukaemic progression in MDS: role of p53
  8. Summary
  9. Acknowledgements
  10. References

Recent evidence suggests that excessive intramedullary apoptosis of haemopoietic progenitors contributes to the ineffective haemopoiesis and peripheral blood cytopenias characteristic of early MDS. Most studies also demonstrate that leukaemic evolution is associated with a reduction in programmed cell death, thereby allowing illegitimate survival and expansion of a neoplastic clone. As yet, however, there has been little progress in the quest to identify the molecular mechanisms underlying deregulated apoptosis in MDS. A clearer understanding of the stem cell abnormality, the sequence of events involved in disease evolution and the role of the apoptotic machinery in these events is sure to lead to a more objective basis for the design of novel and effective therapeutic strategies.

References

  1. Top of page
  2. Fas-mediated apoptosis
  3. The Bcl-2 family
  4. Apoptosis and the cell cycle
  5. Evidence for increased apoptosis in MDS
  6. Increased apoptosis in MDS: the putative genetic lesions
  7. Leukaemic progression in MDS: role of p53
  8. Summary
  9. Acknowledgements
  10. References
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