This article is mentioned in:

1. Introduction

Leukemia is a malignancy of the hematopoietic system characterized by diffuse replacement of the bone marrow by neoplastic cells (1). Acute lymphoid leukemia (ALL) and acute myeloid leukemia (AML) are oncohematologic diseases in which the process of differentiation and limited proliferation that characterizes normal hematopoiesis is altered and replaced by a malignant clonal expansion of immature hematopoietic cells (blasts) in the bone marrow or peripheral blood (2–5).

Epidemiological studies indicate that the annual incidence rates of childhood ALL vary worldwide between one and four new cases per 100,000 children younger than 15 years, with a peak incidence at approximately 2–5 years of age (6–8), whereas AML has been observed with an incidence of 3.7 per 100,000 persons and an age-dependent mortality of 2.7 to nearly 18 per 100,000 persons (9).

Abnormalities in chromosome number as well as structural rearrangements (translocations) are detected in 60–80% of patients with ALL, whereas the remaining 20–40% have a normal karyotype (9–14). In addition to those with a normal karyotype, t(12;21)(p13;q22);TEL/AML1 (ETV6-RUNX1), t(9;22)(q34,q11); BCR/ABL (BCR-ABL1), t(4;11) (q21;q23); MLL/AF4 (KMT2A/AFF1), t(1;19)(q23;p13); E2A/PBX1 (TCF3-PBX1), are among the most common cytogenetic subtypes in ALL (10–12,14), and have been incorporated in the World Health Organization (WHO) classification as the criteria for subclassification of acute leukemia (15).

ALL can be distinguished from AML using morphologic, immunohistochemical, and immunologic methods (16). The study by Golub et al showed that the gene expression profiles can discriminate the ALL from AML (17). However, the precise genes and pathways that exert critical control over determination of lineage fate during leukemia development remain unclear (18).

Hematopoiesis is a highly regulated process of the differentiation, proliferation hematopoietic stem cells give rise to all the blood lineages: the myeloid lineage, which comprises neutrophils, eosinophils, basophils, monocytes, macrophages, megakaryocytes, platelets and erythrocytes; and the lymphoid lineage, which includes T and B cells (19,20).

The process is modulated by a series of molecular events, and there is increasing evidence that miRNAs have important roles in modulating hematopoietic process by targeting the expression of transcription factors and genes that are involved in the regulation of cell proliferation, metabolism, and apoptosis (21). Aberrant expression of many different miRNAs has been observed in several cancers, including hematological malignancies. Furthermore, approximately 50% of miRNAs are located at fragile sites and genomic regions in the human genome associated with cancer (22). In this review, we summarized the association of the miRNA expression with chromosomal translocations in acute leukemia, with a specific focus on acute lymphoblastic leukemia.

2. miRNA expression and oncoproteins in acute lymphoblastic leukemia

TEL/AML1; t(12;21)(p13;q22)

TEL/AML1 t(12;21) (p13;q22) fusion gene, resulting from 12;21 chromosomal translocation, is believed to be the most common molecular genetic abnormality in childhood acute lymphoblastic leukemia (23). The resulting fusion protein has the AML1 DNA binding domain and the TEL protein interaction domain and has been shown to maintain transcription factor properties and bind DNA (24,25). TEL/AML1 act as a transcription factor that reduce the expression of tumor suppressor and increase antiapoptotic genes (26) and also alters the regulation of miRNA expression (27,28) (Table I). For example, the miR-320a and miR-498 tumor suppressor are significantly lower in TEL/AML1-positive acute lymphoblastic leukemias (28). Interestingly, these miRNAs can inhibit the expression of survivin in leukemia cells, inducing apoptosis, suggests that miR-320a and miR-498 are potential tumor suppressors or miRNAs that may play a critical role in the leukemic process (28). The TEL/AML1 protein is generally a transcriptional repressor due to its known ability to recruit chromatin repressors such as histone deacetylases, nuclear receptor corepressors (Fig. 1).

Figure 1 TEL/AML oncoprotein in the regulation of miR-320a and miR-494 and cellular processes affected in ALL. TEL, transcription factor ETV6; AML, acute myeloid leukemia 1 protein; ALL, acute lymphoblastic leukemia.

Table I Expression of miRNAs in rearrangement-positive ALL.

Table I

Expression of miRNAs in rearrangement-positive ALL.

miRNAs	Function in cancera	Expression	Authors/(Ref.)
TEL/AML1-positive rearrangement
miR-26b	OG/TSG	Downregulation	Diakos et al (28)
miR-320a	TSG
miR-494	TSG
miR-213	TSG	Downregulation
miR-221	OG/TSG
miR-99a	OG	Upregulation	Schotte et al (27)
miR-100	OG/TSG
miR-125b	OG
miR-126	OG/TSG
miR-383	OG/TSG
miR-629	OG
miR-125b	OG	Upregulation	Gefen et al (29)
BCR/ABL-positive rearrangement
miR-17	OG	Upregulation	Scherr et al (33)
miR-18	OG
miR-19a	OG
miR-20a	OG
miR-93	OG	Downregulation	Schotte et al (27)
miR-103	OG/TSG
miR-148b	TSG
miR-210	OG/TSG
miR-301	TSG
miR-331	TSG
miR-345	OG/TSG
miR-484	TSG
miR-1226	TSG
MLL/AF4-positive rearrangement
Let-7b	TSG	Downregulation	Mi et al (18)
miR-128a	OG/TSG	Upregulation
miR-128b	OG/TSG
miR-128a	OG/TSG	Upregulation	Oliveira et al (47)
miR-128b	OG/TSG
miR-181b	OG
miR-143	TSG	Downregulation	Dou et al (46)
mir-196b	OG	Upregulation	Popovic et al (50) and Li et al (51)
TCF3/PBX1-positive rearrangement
miR-24	OG/TSG	Downregulation	Schotte et al (27)
miR-126	OG/TSG
miR-146a	TSG
miR-193a	TSG
miR-365	TSG
miR-511	TSG
miR-545	TSG
miR-181	OG	Upregulation	Schotte et al (56)
miR-708	OG
mir-196b	TSG	Downregulation

{ label (or @symbol) needed for fn[@id='tfn1-or-36-03-1226'] } OG, oncogene; TSG, tumor suppressor gene.

a The miRBase website (http://www.mirbase.org/) was used to assign the functions of miRNAs.

Other upregulation miRNAs in TEL/AML1-positive acute lymphoblastic leukemias have also been reported (27–29) and it has been observed that AML1 can associate with coactivators to regulate the promoters of its target genes (30). For example, miR-125b is an oncogenic miRNA by its anti-apoptotic activity which is associated with a marked inhibition of caspase 3 activation and the cleavage of its substrate PARP (29), suggesting an active role for miR-125b in the leukemogenesis (31) and its expression is an independent event from TEL/AML1 protein (Fig. 1).

BCR/ABL1; t(9;22)(q34,q11)

BCR/ABL1 or Philadelphia (Ph) chromosome is a product of the t(9;22), which fuses the Abelson kinase gene (ABL1) from chromosome 9 with the breakpoint cluster region (BCR) from chromosome 22 that expresses the BCR-ABL1 fusion protein: a constitutively active tyrosine kinase. The BCR/ABL1 fusion protein is a hallmark present in a fraction of B progenitor ALL cases that have a particularly poor prognosis (32).

Several miRNAs are overexpressed in BCR/ABL1-positive ALL (27,33) (Table I). BCR/ABL has been shown to upregulate Pim-1 (34,35), E2F3 (36) and c-Myc (35) who regulate the transcriptional expression of miR-17-92 clusters. Recently, it was reported that BIM, a proapoptotic member, is a direct target of miR-17-92 clusters (37). It was demonstrated that BIM is poorly expressed in ALL (38), while the members of the miR-17-92 clusters that have been reported to be upregulated in BCR/ABL1-positive ALL (33). These data demonstrate that the upregulation of miR-17-92 cluster expression observed in BCR/ABL1-positive ALL could have an important role in survival of cells in leukemia (Fig. 2).

Figure 2 BCR/ABL oncoprotein in the regulation of miR-17-92 cluster and miR-93 and cellular processes affected in ALL. BCR, breakpoint cluster region protein; ABL, abelson murine leukemia viral oncogene homolog; ALL, acute lymphoblastic leukemia.

Interestingly, a direct relationship between BCR/ABL activity and cyclin D2 expression in BCR/ABL-positive cells has been demonstrated (39,40); these reports suggest the importance of cyclin D2 in mediating the proliferative signals from BCR/ABL and show that BCR/ABL regulates cyclin D2 expression at the transcriptional level. A recent study showed that miR-93 is downregulated in BCR/ABL-positive patients (27). Interestingly, this miRNA can inhibit the expression of cyclin D2 which leads to cell cycle progression (41). Overexpression of cyclins D2 in BCR/ABL-positive hematopoietic cell have been reported (39,40). The abnormal expression of cyclin D2 may influence the initiation of tumorigenesis, including leukemogenesis (42), suggesting that downregulation of miR-93 could be involved to the leukemogenesis at least partly via upregulation of cyclin D2 expression in BCR/ABL-positive cells (Fig. 2).

MLL/AF4; t(4;11)(q21;q23)

The t(4;11)(q21;q23) involving the genes MLL and AF4 (alias for AFF1) is detected in 50-70% of infant leukemia. MLL/AF4, an MLL fusion protein that is associated with infant pro-B acute lymphoblastic leukemias (43), and it is usually associated with a poor prognosis (44). Rearrangements of the MLL gene result in a fusion protein that retain the DNA binding capacities, the CXXC domain, which binds to non-methylated CpG DNA sites, as well as their DNA methyltransferase activity (transcriptional activation/repression domain of MLL), it is possible that the MLL-fusion proteins directly regulate a subset of genes (45).

Recent studies indicate that MLL/AF4 regulates the expression of miRNas (Table I), which can play a role in leukemogenesis by stimulating proliferation and inducing a block in differentiation of hematopoietic cells (46,47). Dou et al demonstrated that the expression levels of miR-143 are significantly lower in MLL-AF4-positive cells (46). PI3K/AKT/mTOR is a survival pathway and plays an important role in cell proliferation, differentiation and apoptosis. Its abnormal activation has been found in cases of MLL/AF4-positive ALL (48). miR-143 has been shown to decrease the level of Akt at translational level (49). miR-143 is epigenetically repressed by promoter hypermethylation in MLL/AF4-positive primary blasts and cell lines, which was directly associated with expression of the MLL/AF4 oncogene (46). Additionally, it was shown that MLL/AF4 is a target for miR-143, and that DNA methylation decreases the expression of miR-143 in MLL/AF4-positive ALL (46), it is possible that the MLL/AF4 proteins directly downregulate the miR143 expression by the DNA methyltransferase activity of MLL (Fig. 3).

Figure 3 AF4/MLL oncoprotein in the regulation of miR-143 and miR-125b and cellular processes affected in ALL. AF4, AF4/FMR2 family member 1; MLL, mixed lineage leukemia; ALL, acute lymphoblastic leukemia.

miR-196b is an oncogene and appears to be required for MLL/AF4-mediated cell transformation and it was reported that miR-196b is upregulated in MLL/AF4-positive cell lines (50). MLL/AF4 protein regulate the transcriptional expression of miR-196b by increasing levels of K79me2 on the promoter of miR-196b (50,51). FAS plays a central role in the physiological regulation of apoptosis (52), and it has been implicated in the leukemogenesis (51). FAS is direct target of miR-196b and is negatively regulated at the mRNA and protein levels (51). It was demonstrated that FAS is poorly expressed in MLL/AF4-positive human leukemic cells, compared to the normal cells (Table I) (50,51). Li et al found that the low expression of FAS leads to inhibition of FAS-mediated apoptosis (51). These data demonstrate that the upregulation of miR-196b expression observed in MLL/AF4-positive cell has an important role in cell survival in leukemia by targeting FAS mRNA in MLL/AF4-positive cells (Fig. 3).

TCF3/PBX1; t(1;19)(q23;p13)

The t(1;19)(q23;p13) is one of the most frequent translocations in B-acute lymphoblastic leukemia (B-ALL), and is observed in both adult and pediatric populations at an overall frequency of 6% (53). This translocation is the result from the fusion of TCF3 (transcription factor 3) found at 19p13 and PBX1 (pre-B cell leukemia homebox 1) found at 1q23 forming a chimeric gene whose protein product alters cell differentiation arrest, among other cellular processes (54). TCF3/PBX1 encodes a transcription factor bearing the transactivation domain of TCF3 and the DNA-binding domain of PBX1, which facilitates the activation or repression of genes (55).

Several miRNAs are downregulated or upregulated in TCF3/PBX1-positive ALL (27,56) (Table I). For example, miR-126 plays a pivotal role in restraining cell cycle progression of hematopoietic stem cell in vitro and in vivo, it was observed that the downregulation of this miRNA resulted in cell cycle progression and hematopoietic stem cell expansion (57). miR-126 is significantly lower expressed in TCF3/PBX1-ALL and was shown to correlate with in vitro resistance to vincristine and daunorubicin in childhood ALL (27). This suggests that miR-126 may have potential in prognosis prediction and therapeutic application in ALL patients.

3. miRNAs in diagnosis and prognosis of ALL

MicroRNAs are associated with the regulation of normal hematopoiesis and their disruption has been related to many types of cancer, including hematological malignancies. Pediatric ALL samples showed lower expression levels of miR-100, miR-196b and let-7e, while miR-128a and miR-181b were overexpressed compared to normal pediatric samples (47). miR-451 and miR-373 were downregulated, while an increase in miR-222, mIR-339 and miR-142-3p was identified in pre-B-ALL cells compared to control CD19+ (58). It is also possible to identify between different ALL subtypes (B-ALL and T-ALL) using miRNA signatures: miR-151 (downregulated in T-ALL), miR-148a and miR-424 (both highly expressed in T-ALL) could all be used to discriminate between these two leukemic subtypes (59,60). Furthermore, it is possible to distinguish between B-ALL subtypes using miRNA expression, as exemplified by miR-629 having high expression in MLL-AF4, miR-425-5p miR-191 and miR-128 being highly expressed in E2A-PBX1. BCR-Abl also generates higher expression of miR-146b and miR-126 (59).

Prognosis can be tested by miRNA expression. miR-708 was upregulated in relapse, whereas both miR-223 and miR-27a were highly expressed in patients during remission, suggesting that these dysregulated miRNAs play important roles in controlling disease (61). It also was reported that vincristine-resistant ALL patients expressed higher levels of miR-125b (27). The miR-10a, miR-134 and miR-214 expression was linked to a favorable outcome in pediatric ALL, while the expression of miR-33 was associated with an unfavorable prognosis in T-ALL compared to in normal thymocytes (27).

4. Conclusions

The discovery of miRNAs and their association with TEL/AML1, BCR/ABL, MLL/AF4, E2A/PBX1 oncoproteins in acute lymphoblastic leukemia have provided valuable information on potential diagnostic and/or prognostic biomarkers, as well as monitoring the disease progression. Moreover, a potential role of the microRNAs has been suggested in specific subtypes of acute lymphoblastic leukemia and in the development of different phenotypes. Also it is noted that the changes of miRNA expression levels may play an important role in the genesis and evolution of acute lymphoblastic leukemia. Thus, the mechanism of miRNA regulation by TEL/AML1, BCR/ABL, MLL/AF4, E2A/PBX1 oncoproteins is very complex. Studies are needed to clearly demonstrate the effect of miRNAs in leukemogenesis and its practical implications.

Acknowledgments

J.O.N and Y.G.G. were recipients of fellowships from the Programa de Apoyo a los Estudios de Posgrado, Universidad Nacional Autónoma de Mexico (PAEP-UNAM).

References