Molecular Diagnostics and Therapy of Acute Myeloid Leukemia
Acute Myeloid Leukemia (AML) is the most frequent cause of acute leukemia affecting adults, its incidence increases steadily with age and has been correlated with DNA methylation aberration and inactivation of tumor suppressor genes. Recent epigenomic studies showed that DNA methylation abnormalities have been observed in age related acute myeloid leukemia and indicated the importance of DNA methylation analysis for AML diagnosis. An insight of the connection between DNA methylation and AML might help in development of novel molecular diagnostic and genomic therapies. Epigenetic drugs of two families namely the DNA-demethylating agents and inhibitors of histone deacetylase have emerged as the most promising compounds in this area and several pharmaceutical compounds have received approval for the treatment of specific leukaemia and lymphoma subtypes. In addition possible combination between molecular therapeutic approaches including the activation of certain signal transduction pathway(s) like the interleukins family and chromatin-remodeling events is feasible by the application of epigenetic drugs. This approach might be one of the potential promising solutions for the AML treatment. The present study reviewed recent advances in the research related to genomic and epigenomic of acute myeloid leukemia.
Received: November 05, 2011;
Accepted: February 02, 2012;
Published: March 20, 2012
Nowadays cancer is considered a major chronic disease leading to death; it
is only second to heart disease and viewed as an age related disease (Ahmed
et al., 2006; Dosay-Akbulut, 2006; Chatterjee,
2011; Gatoo et al., 2011; Singh
et al., 2011). Leukemia is neoplastic proliferation of leukocyte
precursor in the bone marrow. It could be classified to either lymphoid cells
or myeloid cells by the type of white blood cell affected and acute or chronic
according to the rate of cell growth. Acute leukemia develops rapidly and characterized
by an overgrowth of immature blood cells, while chronic leukemia comprises an
overgrowth of mature blood cells and develops slowly (Lowenberg
et al., 1999; Bolufer et al., 2006).
Furthermore, the two major groups of leukemia were classified into four subgroups,
namely: Acute Lymphoblastic Leukemia (ALL), Acute Myeloid Leukemia (AML), Chronic
Lymphoid Leukemia (CLL) and Chronic Myeloid Leukemia (CML). ALL is a severe
blood disorder in which abnormal leukocytes are identified as immature forms
of lymphocytes (Bassan et al., 2004; Bolufer
et al., 2006). AML is a cancer of the myeloid of blood cells and
characterized by the rapid growth of abnormal white blood cells in the bone
marrow, this specific nature of the disease leads to interference with the production
of normal blood cells (Pinto et al., 1998).
Implications of Cyclin-dependent kinases (CDKs), Cyclin-dependent kinase inhibitors
(CKIs), DNA methylation and other molecular events in the development of leukemia
have been investigated (Chan et al., 1995; Hirai
et al., 1995; Ibrahim, 2010a, 2012).
Taking into account the direct association of these factors in the development
of cancer, the objective of this study was to review the perspective of recent
advances in the research related to genomic and epigenomic of acute myeloid
leukemia in leukemia cancer research and their impact on the development of
new diagnostic and therapeutic approaches.
CLASSIFICATION AND DIAGNOSIS OF AML
AML development process in human is a multistage process where the chromosomal translocators in either the Hematopoietic Stem Cells (HSC) or in the progenitors induce the pre-leukemic state where successive mutational events lead to the AML development (Fig. 1).
AML patients show signs and symptoms of fatigue, hemorrhage, or infections
and fever due to decreases in red cells, platelets, or white cells, respectively.
AML main clinical findings are directly attributable to the leukemic infiltration
of the bone marrow, with resultant anemia, granulocytopenia, or thrombocytopenia,
with or without leukocytosis and leukemic infiltration of the various tissues,
including liver (hepatomegaly), spleen (splenomegaly) and lymph nodes (lymphadenopathy)
(Lowenberg et al., 1999). The most commonly used
method of AML classification is that developed by the French-American-British
(FAB) group which divides AML into eight distinct subtypes (M0, M1, M2, M,3,
M4, M5, M6 and M7 ) that differ with respect to the particular myeloid lineage
involved and the degree of leukemic-cell differentiation.
||Acute myeloid leukemia (AML) development process in human.
This process is a multistage process where the chromosomal translocators
in either the hematopoietic stem cells (HSC) or in the progenitors induce
the pre-leukemic state where successive mutational events lead to the AML
development. The AML phenotype is propagated and maintained in the leukemic
stem cells (LSC) sub-population due to AML cells bearing the necessary mutations
involved in the pathogenesis process
The more frequent chromosomal abnormalities shown in AML patients were t (8;
21), t (6; 9), inv (16), del (11q) and t (9; 11), respectively (Harrison,
Several methods have been used for diagnosis of AML; these methods include
morphologic examination of blasts in bone marrow or blood and cytogenetic abnormalities
(Betz and Hess, 2010). Cytogenetic analysis revealed
that more than 50% AML patients have an abnormal karyotype (Grimwade
and Hills, 2009). This type of analysis was also conducted on other types
of cancers, e.g., malignant gliomas (Kandavelu and Vanisree,
2011). Recent study indicated the importance of molecular analysis based
on various techniques, such as molecular analysis of protein (Hamad
et al., 2009) or Polymerase Chain Reaction (PCR) for leukemia diagnosis
(Ibrahim et al., 2009, 2010;
Saleh et al., 2010), these might have become
an essential part of the diagnostic panel for acute leukemia. A new molecular
method was used for detection of Arg72Pro that has been extensively genotyped
for association with a wide variety of cancers. The method is based on the tetra-primer
amplification refractory mutation system-polymerase chain reaction (ARMS-PCR),
with a single PCR to discriminate both alleles (Lajin and
Alachkar, 2011). In addition, it was suggested that cytomorphology, cytogenetics,
fluorescence in situ hybridization (FISH) and immuno-phenotyping with
multi-parameter flow cytometry (MFC) need to be applied for diagnosis (Moosavi
et al., 2008; Bacher et al., 2009).
GENES INVOLVED IN SIGNAL TRANSDUCTION OF LEUKEMIA
Protein phosphorylation is an important mechanism in signal transduction pathways;
specific enzymes perform this function, these are known as kinases which regulate
the essential and vital routes linked to various cell growth activities including
cell growth and proliferation (Broekman et al., 2011).
This mechanism is controlled by Cyclin-Dependent Kinases (CDKs) proteins which
are well conserved serine/threonine kinases; they have a very low kinase activity
and require the binding to cyclins for their activation (Connell-Crowley
et al., 1993; Morgan, 1997). There is another
group of proteins called cyclin-dependent kinase Inhibitors (CKIs), these are
regulatory proteins which prevent the phosphorylation of targets by CDK/cyclin
involved in cell cycle (Nigg, 1995; Elledge
et al., 1996; Bruggeman and van Lohuizen, 2006).
Research in this field provided information about CDKs and CKIs role in controlling
cell cycle and development of cancer. It have been shown in several forms of
cancer, CKIs such as p16 and p27 are mutated (Serrano et
al., 1993; Kamb et al., 1994; Nobori
et al., 1994). On the other hand, there are indications that they
are degraded in several types of cancer; thus low levels of p27 are correlated
with poor clinical prognosis (Porter et al., 1997).
Other studies have shown that there are two families of CKIs. The first family
includes the INK4 proteins (inhibitors of CDK4), so named for their ability
to specifically inhibit the catalytic subunits of CDK4 and CDK6. This family
includes four proteins, p16INK4a, p15INK4b, p18INK4c and p19INK4d (Serrano
et al., 1993; Hannon and Beach, 1994; Guan
et al., 1994; Chan et al., 1995; Hirai
et al., 1995). The four proteins (p15, p16, p18 and p19) which are
associated with the development of AML are shown in Table 1.
DNA METHYLATION, CANCER AND AML
Epigenomics deals with molecular elements and mechanisms influencing gene expression
(Heindel et al., 2006; Guil
and Esteller, 2009). Three types of epigenomic mechanisms have been identified
in the humans genomic DNA; these are associated with gene expression and
implicated in development, diseases and other biological phenomena (Ibrahim,
2010a, b, 2012).
||The four proteins of cyclin-dependent kinase inhibitors (CKIs)
associated with the development of Acute Myeloid Leukemia (AML), references
are shown in the text
The three mechanisms are DNA methylation, histone modifications and RNA interference
(Heindel et al., 2006; Holliday,
2006; Suzuki and Yoshino, 2008; Taby
and Issa, 2010). Genomic DNA methylation is the product of methylation of
cytosine and formation of 5-methylcytosine (Strathdee and
Brown, 2002). DNA methylation profile of human genome is called methylome
(Lister and Ecker, 2009). Molecular analysis demonstrated
that 3-4% of all cytosine molecules are methylated, yet approximately 0.75-1%
5-methylcytosine makes up of all nucleotide bases in the DNA of normal human
tissue (Esteller and Herman, 2002; Esteller,
2008; Fraga et al., 2002). It is worth mentioning
an important characteristic of genomic DNA, that is the presence of CpG islands,
which are stretches of CpG dinucleotides and are generally unmethylated but
these regions are prone to DNA methylation (Bird, 1992).
Accumulating research works have provided increasing support for the association
between of DNA methylation with various malignancies (Ibrahim,
2010a, b; Ibrahim and Makkiya,
2011; Schulz and Goering, 2011; Ibrahim,
2012). Nowadays it is accepted that changes in the normal DNA methylation
patterns are the most common molecular lesions of genomic DNA of cancer cell.
There are two types of DNA methylation aberrations; these are either alterations
in the global hypomethylation which lead to oncogene activation and chromosomal
rearrangement or hypermethylation of CG islands of the promoter regions of tumor
suppressor genes (Nguyen et al., 2001; Liu
et al., 2003). It is worth mentioning that hypermethylation of the
promoter-associated CpG islands of suppressor genes will cause loss of gene
expression, especially when this methylation affects tumor suppressor genes.
Evidence for this came from the study of two such genes, VHL (the gene mutated
in Von Hippel-Lindau disease) and CDKN2A (which encodes p16INK4A, a gene commonly
mutated in many types of cancer) where, in a subset of tumors, dense promoter
methylation was described, in association with loss of gene expression (which
could be partially restored using methylation inhibitors), absence of coding
region mutations and tumor-specific patterns of methylation (Kuerbitz
et al., 1999).
An interesting investigation was carried out on the breast cancer cell lines
to investigate the relationship between promoter methylation (assessed by methylation-specific
PCR, bisulfite sequencing and 5-aza-2'deoxycytidine treatment) and the DNA methyltransferase
machinery (total DNMT activity and expression of DNMT1, DNMT3a and DNMT3b proteins)
(Roll et al., 2008). The study revealed two groups
of cell lines that possessed distinct methylation signatures, hypermethylator
cell lines and low-frequency methylator cell lines.
As indicated earlier, leukaemia has traditionally been viewed as a genetic
disease; however recent investigation has shown that defects in the epigenomic
DNA methylation also play an important role. In this respect a recent study
illustrated that digestion of genomic DNA of normal individuals with HpaII (which
cleaves the sequence CCGG only if the internal cytosine residue is unmethylated)
and MspI (which cleaves the same sequence regardless of methylation) showed
lower degree of differences in RAPD-DNA patterns, indicating normal methylation
patterns. However, different picture was shown when studying RAPD-DNA bands
profile of CML genomic DNA following digestion with restriction HpaII and MspI,
clear variations in bands patterns were observed when using selected RAPD primers
to amplify digested genomic DNA by two enzymes (MspI and HpaII) for normal individuals
and CML patients (Ibrahim, 2010a).
GENOMIC MUTATIONS OF TUMOR SUPPRESSOR GENES
Tumor suppressor genes can reduce the probability of transforming normal human
cell into tumor cell (Yeo, 1999; Sherr,
2004; Ibrahim, 2010a). It is well known that mutations,
chromosomal translocations and DNA methylation aberration can inactivate tumor
suppressor genes thus leading to activation of oncogenes without altering the
primary sequence of DNA (Robertson and Wolffe, 2000;
Jones and Baylin, 2007; Esteller,
2008). Accordingly, some tumor suppressor genes have the ability to stop
the cell from multiplying until the DNA damage is repaired; however, once tumor
suppressor genes do not function correctly, the cells with DNA damage continue
to divide and can accumulate further DNA damage that can eventually lead to
the formation of a cancer cell (Macleod, 2000; Hussain
et al., 2001). Literatures provided information on the role of number
of tumor-suppressor genes. Mutations in Rb suppressor gene are associated with
retinoblastoma, a serious cancer of the retina that occurs in early childhood.
In addition to retinoblastomas, mutations in the Rb gene have been detected
in osteosarcomas, bladder carcinomas, small-cell lung carcinomas, prostate carcinomas,
breast carcinomas, some types of leukemia and cervical carcinomas (Krug
et al., 2002). Mutations in the tumor suppressor gene p53 are found
in a large proportion of human hematological malignancies and other cancers;
increased amounts of cellular p53 protein after DNA damage have been associated
with cell-cycle arrest and programmed cell death (apoptosis) and mutations or
losses of p53 have been resulted in development of cancer (Velculescu
and El-Deiry, 1996; Foulkes, 2007).
MOLECULAR AND DNA METHYLATION THERAPY
Several recent approaches have been reported for possible therapy of various
types of cancers (Saeed et al., 2005; Saxena
and Moorthy, 2007; Mohan et al., 2008; Hafidh
et al., 2009). One of these approaches is molecular utilization of
interleukins in cancer therapy; the interleukins (ILs) represent another large
family of cytokines, with at least 25 different constituent members (IL-1 to
IL-25) having been characterized thus far. The immune stimulatory activity of
IL-2 has proved beneficial in the treatment of some cancer types. Further studies
have shown additional cancer types, most notably ovarian and bladder cancer,
non-Hodgkins lymphoma and acute myeloid leukemia, to be at least partially
responsive to IL-2 treatment (Walsh, 2003a,b).
Moreover, there is an increasing trend toward utilization of an important fundamental
distinction between epigenetics and genetics in new novel ways of cancer treatment;
this difference is the potential reversible property of DNA methylation and
histone alterations. Thus, it has been suggested the possibility of using this
phenomena for cancer treatment. Reforming DNA methylation alterations in the
methylome of cancers patients is part of new approach of epigenomic therapy
of cancer (Esteller, 2008; Jain
et al., 2009). The results in this field showed the possibility of
amending the aberrated DNA methylation pattern of the affected CG regions of
cancers genomic DNAs by using specific drugs. Accordingly, the therapeutic
approach is to adjust the aberrated DNA methylation status of hypermethylated
tumor-suppressor genes to normal state (Ibrahim, 2010a,
b). One of the essential differences between human cancer
genetics and epigenetics is that DNA methylation and histone modification changes
are reversible under the right circumstances. Thus, epigenetic alterations are
one of the weakest points in the defenses of the cancer cell, because those
hypermethylated tumour-suppressor genes in their long sleep can
be awoken and reactivated with the right drug regimens and exert their normal
growth-inhibitory functions. Two families of epigenetic drugs, DNA-demethylating
agents and inhibitors of histone deacetylase, have emerged as the most promising
compounds in this area and several pharmaceutical compounds have received approval
for the treatment of specific leukemia and lymphoma subtypes (Fandy
et al., 2007; Hellebrekers et al., 2007;
Abujamra et al., 2010; Chen
et al., 2010; Mani and Herceg, 2010; Martinet
and Bertrand, 2011). The successful story in these malignant disorders needs
now to be translated to epithelial solid tumours.
Genomic and epigenomic molecular technologies can help in introducing new approaches for classification, diagnosis and therapies of cancers. AML like several other cancers are apparently epigenetic diseases of reverse nature making them target to specific medical drugs. Thus there is potential to develop new therapeutic tactic using combination of available molecular treatment approaches by interfering into the signal transduction pathway and DNA methylation, this might be the best treatment modality with future perspectives. Current direct or indirect molecular treatment of AML might increase the patients bad prognosis. It is expected that further understanding of the functional molecular mechanisms of this disease is necessary for effective, successful design of therapeutic approaches leading to successful cure of the disease.
We would like to thank the Princess Sumaya University for Technology, Qatar University, Baghdad University and University of Würzburg for supporting the research related to this publication.
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