ACUTE LYMPHOBALSTIC LEUKEMIA
Affecting about 4,000 patients each year in the United States, acute
lymphoblastic leukemia (ALL) is the most common malignancy in children 108. It is now curable in more
than 80% of children, but the overall
cure rate is only 40% in adult ALL,
although much progress has been achieved during the past two decades 108. Most of the current knowledge
of the biology and treatment of ALL originates from studies of children, which
is also the emphasis of the following dissuasion unless stated otherwise.
GENETICS
Molecular analysis has contributed greatly to our understanding
of the pathogenesis and prognosis of ALL. Although the frequency of particular
genetic subtypes differs in children and adults, the general mechanisms
underlying the induction of ALL are similar. Chromosomal abnormalities
represent a major genetic alteration in leukemia patients. They include the
aberrant expression of proto-oncogenes, chromosomal translocations that create
fusion genes encoding active kinases and altered transcription factors, and hyperdiploidy
involving more than 50 chromosomes.
A number of leukemia subtypes have been defined based on their genetic
make up. These subtypes include B
lineage leukemias that contain t(12;21)[TEL-AML1],
t(9;22)[BCR-ABL], t(1;19)[E2A-PBX1], rearrangements in the MLL
gene on chromosome 11, or a hyperdiploid karyotype and T lineage leukemias
(T-ALL)109. The most common
translocation found in childhood B-precursor ALL is the t(12;21)(p13;q22),
which causes TEL-AML1 translocations, presenting in approximately 25% of
childhood ALL 110. Another common chromosomal
aberration found in B-precursor ALL is the presence of more than 46 chromosomes
(hyperdiploid ALL) 109. Mechanism of leukemogenesis
in hyperdiploid ALL is unknown while activating mutations in the receptor
tyrosine kinase FLT3 were identified in approximately 20% of hyperdiploid ALL 111,112. The t(1;19)(q23;p13) encoding
the E2A-PBX fusion protein is present in about 6% of all B-precursor ALLs and
in 25% of cases with a preB immunophenotype 113,114. E2A contains a bHLH domain through
which to bind sequence specific DNA binding and dimerization, and it plays a
critical role in lymphocyte development 115. Loss of E2A function and
dysregulation of HOX by PBX1 may contribute to leukemogenesis in this subtype
of ALL 116. t(9;22)[BCR-ABL] is more common in adult ALL
while is only detected in 4% childhood ALL 110.
Transcription factor genes, such as bHLH genes MYC 117, TAL1(SCL) 118, and LYL1 119, are the preferred targets of
chromosomal translocations in the acute T-cell leukemias 110. When rearranged near
enhancers within the TCRβ or α/δ-chain locus, these regulatory
genes become active, and their protein products inappropriately enhance target
gene transcription. In addition to genes encoding bHLH proteins, additional
classes of regulatory genes are rearranged near TCR loci, including those
encoding the proteins LMO1 and LMO2 within the cysteine-rich LIM family 120,121. Mechanisms of leukemia
transformation by LIM proteins is unclear, but LMO1 showed transformation
capability in the transgenic mice model and LMO2 can bind to the bHLH protein
TAL1 in vitro 122,123. HOX11 and HOX11L2 are the
two major HOX genes that are inappropriately placed under the control of TCR
loci 110. About 20% of childhood T-ALL
patients demonstrated a HOX11L2 gene translocation by fluorescence in situ
hybridization 124. NOTCH1 is a gene that
normally encodes a transmembrane receptor that is involved in the regulation of
normal T-cell development and may other tissues during embryologic development.
NOTCH-1 activation by truncation had previously been shown in a rare t(7;9)
T-cell ALL 125 and the same truncated
fragment was shown to induce T-cell ALL in mouse models 126,127. It’s suggested that specific
mutations in sequences encoding both the heterodimerization and PEST domains of
NOTCH1 exist in over 50% of primary
patient T-cell ALL samples 128.
These genetic alterations contribute to the leukemic transformation
of hematopoietic stem cells or their committed progenitors by changing cellular
functions. They alter key regulatory processes by maintaining or enhancing an
unlimited capacity for self-renewal, subverting the controls of normal
proliferation, blocking differentiation, and promoting resistance to death
signals (apoptosis).
TREATMENT
Since ALL is a heterogeneous disease, its treatment is more often
individualized according to genotype, phenotype and risk factors 108. Mature B-cell ALL is the
only subtype that is treated with short-term intensive chemotherapy 129,130. For all other ALL patients,
three stages of treatment are included: the initial remission-induction
therapy followed by intensification (or consolidation) therapy and
continuation treatment to eliminate residual leukemia 108. The remission-induction
therapy includes the administration of a glucocorticoid (prednisone,
prednisolone, or dexamethasone), vincristine, and at least one other
agent (usually asparaginase, an anthracycline, or both). This
treatment phase eradicates more than 99% of the initial burden of leukemia cells and
restores normal hematopoiesis and a normal performance status. When
normal hematopoiesis is restored, patients then receive intensification therapy
which includes high-dose methotrexate with mercaptopurine, high-dose
asparaginase given for an extended period, and reinduction
treatment. Allogeneic transplantation is the ultimate form of treatment intensification
and has been indicated to benefit certain very-high-risk pediatric and adult
patients, such as those with BCR-ABL+ ALL or those with a
poor initial response to treatment 131,132. A combination of
methotrexate administered weekly and mercaptopurine given daily
forms the most common continuation regimens. Although as many as two-thirds of
childhood cases may be curable with only 12 months of treatment, it
is not possible to reliably identify this subgroup prospectively 133. Therefore, ALL patients
generally receive prolonged continuation therapy to ensure maximum cure rate 134. Therapy directly acting at
the central nervous system is critical for treatment success, since it is often
the place harboring residual leukemia cells after chemotherapy, thereby
contributing to relapse 135.
Biological differences in leukemogenesis between adult and
childhood ALL contribute to the differential prognosis. Childhood and adult ALL
differ markedly in the prevalences of various cytogenetic abnormalities. For
example, Philadelphia
chromosome (Ph)-positive ALL, a high-risk cytogenetic subset, accounts for one
quarter of adult ALL cases but occurs in less than 5% of children. Similarly,
ETV6/RUNX1 (TEL-AML1) fusion and hyperdiploidy, both of which are good risk
genetic features, together comprise about 50% of childhood ALL, but only about
10% of adult ALL 136. Age influences the prognostic
effect of the same genetic lesions. Among patients with t (9;22),
children one to nine years of age have a better prognosis than
adolescents with the same disease 137, who in turn fare
better than adults 138,139. Among patients with MLL-AF4
fusion, infants fare considerably worse than older children, and
adults have an especially poor outcome 138,140. In T-cell ALL, the
presence of t (11;19) with MLL-ENL fusion and overexpression of
the HOX11 gene confer a good prognosis.
GLUCOCORTICOID INDUCED CELL DEATH
Glucocorticoids (GC) such as dexamethasone (Dex) induce cell
apoptosis and cell cycle arrest in thymocytes and some leukemia (Fig.1.3).
Although the mechanism of GC-induced cell death is still elusive, the integrity
of the GC signaling pathway, including DNA binding of the GR and subsequent
transcriptional regulation of specific genes, appears to be important for its
pro-apoptosis effect. The gene for the
GR is located on chromosome 5 (5q31) and it consists of nine exons encoding
three characteristic domains of the GC protein 141. The N-terminal domain (NTD) contains a
transactivation domain (AF-1) that is involved in transcriptional activation of
target genes 142,143. The DNA binding domain (DBD) is in the
middle, and it consists of two highly conserved zinc finger domains essential
for binding to the glucocorticoid response element (GRE) sequences of regulated
genes. The C-terminal of the GR protein contains the ligand binding domain
(HBD) that is also required for binding heat-shock proteins and GR dimerization
GC-induced apoptosis depends on sufficient levels of GR and
subsequent alterations in gene expression. However, basal level GR expression
is not enough to mediate GC-induced apoptosis, and positive autoregulation is a
necessary component of this process in leukemia cell lines 144. Although non-genomic actions
of the GR are not excluded, most data suggest that GC induced apoptosis is
linked to de novo gene expression. The target genes whose
transactivation or transrepression initiates apoptosis remain unclear. Expression
profiling suggested that GCs modulate the expression of distinct sets of genes,
rather than causing generalized transcriptional alterations 145-149. Expression of many components
in the extrinsic and intrinsic death pathways are altered in a pro-apoptotic
manner upon treatment of sensitive lymphoid cells with GCs 143.
The extrinsic pathway is initiated by death receptors of the
tumor necrosis factor (TNF) receptor superfamily such as CD95 (APO-1/Fas) or
TNF-related apoptosis-inducing ligand (TRAIL) 150. In this pathway, caspase-8
activates downstream effector caspases such as caspase 3 through either mitochondria-dependent
or mitochondria-independent pathways 151. In the former, Bid, a
pro-apoptotic member of the Bcl-2 family, is activated and triggers cytochrome
c release from mitochondria. Cytochrome c binds APAF-1 and then activates
caspase-9, which in turn activates effector caspases. In the
mitochondria-independent pathway, caspase-8 directly cleaves and activates
caspase-3, bypassing mitochondria and cytochrome c release. GC-induced
thymocyte apoptosis was unaffected in Bid-deficient mice, indicating that the
death receptor pathway doesn’t play an essential role in GC-mediated cell death
152.
The intrinsic and mitochondria-mediated pathway responds to
intracellular signals such as GC. This leads to release of pro-apoptotic molecules
upon depolarization of the mitochondrial membrane potential 153. The apoptotic response is
tightly regulated by the interaction between pro- and anti-apoptotic Bcl-2
family members. Pro-apoptotic factors include Bim, Bid, Bad and Puma, which activate
Bax and Bak, and anti-apoptotic Bcl-2
family members consist of Bcl-2 and Bcl-xL, which bind and
neutralize their pro-apoptotic counterparts 153-155. Following formation of pores
in the outer mitochondrial membrane by Bax and Bak, cytochrome c and other
factors such as Smac/Diabolo are released into the cytosol. Cytochrome c
triggers caspase-3 activation through formation of the cytochrome
c/Apaf-1/caspase-9-containing apoptosome, which finally leads to apoptosis 156. Smac/Diabolo promotes
caspase activation through neutralizing the inhibitory effects of IAPs 157. Thus, apoptosis is tightly
controlled by the balance between anti- and pro-apoptotic Bcl-2 family members
together with the caspase-inhibitory IAP molecules.
The mitochondrial apoptosis pathways have been shown to be
important for GC-induced apoptosis 158. GC-treatment induces loss of
mitochondrial membrane potential in thymocytes and leukemic T cells, which is prevented by caspase-9 deficiency 159-162. Another essential principle
for GC-induced mitochondrial apoptosis may be induction of proapoptotic 163,164 and repression of
antiapoptotic 165,166 Bcl-2 family proteins,
leading to transcriptional deregulation of the Bcl-2 rheostat. For example, Bim-xL, a proapoptotic member of
the Bcl-2 family, was noted to be induced by GCs at 20 hours and is thought to
precipitate apoptosis 167. GC-regulated proapoptotic and
BH3-only proteins Bim and Puma have been shown to be key initiators of
GC-induced apoptosis in vivo 54. Bcl-2-deficient or Bcl-XL-deficient
mice display lymphoid apoptosis in vivo and enhanced cell death of
thymocytes in vitro after GC-treatment 155,168.
A number of signaling pathways are known to modulate the complex
mechanism of GC-induced apoptosis. It is likely that the balance between pro-survival
and pro-apoptotic signaling pathways determines the ultimate fate of the cell. These
include GC-mediated repression of the proto-oncogene c-myc or induction of IκB,
an inhibitor of the survival transcription factor NF-κB 169,170. NF-κB is a heterodimeric
transcription factor for more than 100 genes including cytokines, cytokine
receptors, chemotactic proteins, and adhesion molecules 171,172, and is involved in the regulation of
apoptosis 173,174. In human leukemic T cells, GCs induce
synthesis of IκBα, which causes the retention of NF-κB in the cytoplasm and correlates with the
induction of apoptosis 175. GR can also directly repress NF-κB activity 176. Another example of how GC-induced apoptosis
is regulated by crosstalk with other signaling pathways involves protein kinase
C (PKC). PKC includes several subfamilies of enzymes including Ca2+-dependent
PKC and Ca2+-independent PKC. It has been shown that the Ca2+-independent
PKC subfamily is involved in induction of apoptosis, whereas activation of Ca2+-dependent
PKC is capable of inhibiting GC-induced apoptosis 177. It has been shown that sustained expression
of the protooncogene c-myc provides protection against GC-induced cell death in
the human leukemia cell line CEM-C7, and the downregulation of c-myc
accompanies induction of apoptosis by GCs 178. However, recently it was found that repression
of c-myc is essential for cell cycle arrest in the G1 phase, but is not
required for GC-induced apoptosis 179. GC-mediated cell death may also
involve GR-dependent repression of MAP kinase phosphatase-1 (MKP-1) and
subsequent activation of the proapoptotic JNK pathway, which can be inhibited
by Rapamycin 180,181.
GLUCOCORTICOID RESISTANCE
The mechanism of GC resistance in some human leukemia and
lymphoma cells is poorly understood. Cancer cells can employ multiple
strategies that ultimately evade apoptosis following chemotherapy. General
mechanisms of resistance include decreased drug uptake, increased drug efflux,
alterations in the drug target, drug metabolism, repair of DNA damage, cell
cycle checkpoint mediators, and changes in downstream mediators of the
apoptotic pathway 182.
Since the integrity of the GC signaling pathway is critical for
GC induced apoptosis, disruption of any step of this pathway could lead to
resistance to GC. With supporting experimental evidence, many underlying
reasons for GC-resistance have been suggested and include a low number of GR,
mutations of the GR, expression of different GR splice variants, different
phosphorylation patterns of GR, multidrug resistance by overexpression of
P-glycoprotein, increased levels of glutathione and glutathione S-transferase,
abnormal expression of GR binding proteins, dysregulation of transcription
factors such as NF-κB, AP-1, dysregulation of GR target genes such as c-myc,
autoregulation of GR itself, or dysregulation of members of the apoptosis
pathway, such as anti-apoptotic expression of Bcl-2 family proteins 158. Depending on molecular
components affected in the GC induced signaling pathway, the mechanisms of GC
resistance may be grouped into ‘upstream’ and ‘downstream’ levels 183. The ‘upstream mechanisms’
concern the GC, GR and GR-associated proteins that control its function and the
‘downstream mechanisms’ interfere with individual GC responses, such as
transcriptional expression and induction of apoptosis.
Overexpression of the mdr-1 gene-encoded P-glycoproteins, which
are ATP-binding cassette (ABC) transporters that pump various drugs out of the cell, has been
suggested to account for cross-resistance to GC resulting after exposure to
combination chemotherapy 184-186. GR expression levels have
been correlated to GC sensitivity in several experimental systems. Since GR is
a target gene of GC signaling, GR auto-induction but not the basal GR
expression was shown to be critical for GC-induced apoptosis 187,188. As a transcriptional factor, GR recruits a
number of cofactors such as SRC-1, TIF2/GRIP1, CBP/p300, NcoR and SMRT required
for its gene regulatory activities 189,190. Mutation or abnormal
expression of these cofactors compromises transcriptional activities of GR and
therefore reduces sensitivity to GC. Inefficient expression of GC target genes
might contribute to GC resistance. A number of such genes, including c-myc 191,192, IkB 175,193, and c-jun 187,194, have been indicated to be
required for GC induced apoptosis in some leukemia cell lines. However, their
role in GC resistance is still controversial based on reports from different
research groups.
GC resistance might also result from deregulation of the
apoptotic effector machinery, which leads to an imbalance of pro-apoptosis and
anti-apoptosis forces in the cells. Upregulation of cell survival signals inhibits
GC-induced apoptosis, resulting in resistance to GC-induced apoptosis in
leukemia therapy 195. Activation of MAPK kinase
(MEK) and extracellular signal-regulated kinase (ERK) antagonizes GC-induced
apoptosis in CD4+ T cells 196. Also, other protein tyrosine
kinases are deregulated in hematological malignancies 197, which in turn activate the
Ras/Raf/MEK, NF-κB, PI3-K/AKT and β-catenin survival pathways. AKT inhibitors
increased the sensitivity of a follicular lymphoma cell line to GC-induced
apoptosis by inducing Bad translocation to the mitochondria 198.
Together, resistance towards GC-induced apoptosis may concern
defects in the GR itself, GR binding partners, dysregulation of GR target genes
and transcription factors, or it may be due to activity of general resistance
mechanisms, defects in the apoptosis pathway, or a shift of the balance of
cellular signaling pathways to anti-apoptotic signaling. In line with the
latter case, it is a consistent finding that GC sometimes show paradox
regulations, such as regulation of some apoptosis genes in a pro-apoptotic
manner in situations where GC promote survival and vice versa 199. Thus, pro- and
anti-apoptotic signaling may occur in parallel within a single cell but the
balance between survival and suicide may shift to pro-apoptotic signals upon a
strong apoptosis signal and vice versa.
Post Comment
No comments