Targeting miRNA in hematologic malignancies

Abstract

Purpose of review:

MiRNA are critical regulators for gene expression. Numerous studies have revealed how miRNAs contribute to the pathogenesis of hematologic malignancies.

Recent findings:

The identification of novel miRNA regulatory factors and pathways crucial for miRNA dysregulation has been linked to hematologic malignancies. miRNA expression profiling has shown their potential to predict outcomes and treatment responses. Recently, targeting miRNA biogenesis or pathways has become a promising therapeutic strategy with recent miRNA-therapeutics being developed.

Summary:

We provide a comprehensive overview of the role of miRNAs for diagnosis, prognosis and therapeutic potential in hematologic malignancies.

Introduction

MicroRNAs (miRNAs) are small non-coding RNA with an average 22 nucleotides in length were first identified in 1990s [1, 2]. Most miRNAs are transcribed by RNA polymerase II from the intergenic regions of DNA sequences into primary miRNAs (pri-miRNAs), which are further processed into precursor miRNAs (pre-miRNAs) and mature miRNAs [3, 4]. MiRNAs regulate a broad range of biological processes, and concordantly have been found to be severely dysregulated in disease [5, 6]. An increasing number of studies demonstrated aberrant miRNA expression in a variety of tumor cells and tissues that have established the role of miRNAs in carcinogenesis [7–9]. Different classes of miRNAs may either exhibit oncogenic or tumor-suppressing properties, but can display opposite functions depending on their targets and tissue[10]. Moreover, tissue gene expression studies of tumor versus normal pairs have revealed distinct miRNA expression profiles in tumors that can be used as diagnostic classifiers [11–13]*.

Multiple factors and molecular mechanisms that alter the expression levels of miRNAs have been identified [14]. Generally, miRNAs interact with the 3′ untranslated region (3′ UTR), 5′ UTR coding sequence or gene promoter of target mRNAs to suppress their expression directly; however, they have also been shown to activate gene expression and regulate transcription under certain conditions [15–18]. The impact of the interaction between miRNAs and their target genes is dynamic and is dependent on many other factors such as subcellular location of miRNAs, the abundance of miRNAs and target mRNAs, and the affinity of miRNA-target interactions [19, 20]. miRNAs can be secreted into extracellular fluids and transported to target cells through exosomes, or by binding to proteins [21]. Extracellular miRNAs can act as chemical messengers to regulate cell-cell communications during various physiological and pathological processes [22–24].

The hematologic malignancies are a diverse group of neoplastic disorders that vary in clinical severity. The 3 main types of hematological cancers include the leukemias, lymphomas and myeloma that make up about 10% of new cancer cases in the USA each year with incidence rates still rising [25]. Increasing evidence illustrates the potential role of miRNAs in shaping the tumor microenvironment (TME) [26, 27]. Paggetti et al. have shown that chronic lymphocytic leukemia (CLL)-derived exosomes carry miRNAs to the surrounding microenvironment leading to reprogramming of stromal cells to secrete proangiogenic and immunosuppressive cytokines that support the growth of CLL cells [28]. Moreover, the pivotal role of miRNAs in blood diseases is well established [29–31]. MiRNA biogenesis pathways have been linked to cancer with compelling evidence that disruption in posttranscriptional pathway regulation of miRNAs is crucial for hematologic malignancies [32, 33]. In addition, alterations in miRNA expression may also result from changes in the regulation of transcription [34].

In this review, we summarize the miRNA biogenesis and regulatory mechanisms of miRNA expression and assess the potential clinical implications of miRNAs in blood cancers.

1. Biogenesis of miRNAs

In mammalian cells, miRNAs biogenesis is a complex multi-step process, which starts with the processing of RNA polymerase II (RNAP II) transcripts co- or post transcriptionally [17, 35–37]**. The majority of the currently identified miRNAs are intragenic and processed mostly from introns with relatively few exons of protein coding genes, while the remaining are intergenic, transcribed independently of a host gene and regulated by their own promoters [38, 39]. Usually, miRNAs are transcribed as one long transcript with a seed region that is the predicted sequence for target specificity. MiRNAs with similar seed regions are grouped into a family [40]. Understanding miRNAs biogenesis provides the necessary background to interpret the dysregulated expression described in the malignant phenotype.

1.1. The canonical pathway of miRNA biogenesis

The canonical biogenesis pathway is the dominant pathway by which miRNAs are processed, which can be divided into 5 steps. The first step is transcription: pri-miRNAs are transcribed from their genes and then processed into pre-miRNAs [41]. In step 2, the primary transcript must undergo processing to produce a shorter stem loop structure referred to as precursor-miRNA (pre-miRNA) before it reaches the cytoplasm [37]**. This occurs via a microprocessor, which is able to recognize the U6 motif at basal segment and UGU/GUG motif at apical loop (frequently occurring motifs of pri-miRNA), and to excise the upper part of the RNA hairpin [4, 42, 43]. Step 3, the key step is nuclear export of pre-miRNA to the cytoplasm by exportin-5 complex bound to GTP and cofactor Ran [44, 45]. The recognition of pre-miRNA by exportin-5 requires a 22 base pair (bp) long stem and a short 3’ bp overhang [46] and to translocate to the cytoplasm across nuclear pore components embedded in the nuclear plasma membrane [47]. Step 4 is the double-strand miRNA (miRNA duplex) generation. The pre-miRNA encounters Dicer, a RNAse III enzyme that processes the pre-miRNA into a 22 bp double-stranded RNA to interact with the 3’ end of pre-miRNA for loading onto the Argonaute protein that functions as guiding molecule to deliver the complex to target mRNA [48]^,[49]. The final Step 5 is miRNA maturation and is coupled with the formation of the ribonucleoprotein complex known as RISC (miRNA-Induced Silencing Complex) with cleavage of the so-called passenger strand of the miRNA duplex. The guide strand engages with the target mRNA to mediate gene silencing or mRNA degradation while the passenger strand is usually deleted [50].

1.2. Non-canonical miRNA biogenesis pathways

In contrast to the canonical miRNA biogenesis described above, miRNAs can be generated in a Drosha- and/or Dicer-independent (key players in miRNA biogenesis) manner classified as non-canonical biogenesis [51]. Up to date, multiple non-canonical miRNA biogenesis pathways have been elucidated. An example is the Drosophila short RNA duplexes derived from short intronic hairpins termed “mirtrons” that mimic the structural features of pre-miRNAs [52]. The Mirtrons then bypass Drosha process and merge with the canonical miRNA pathway during hairpin export by exportin-5 [53]. After that, they are cleaved by Dicer and incorporated into silencing complexes similar to the canonical pathway [54].

2. The function of miRNAs in hematologic malignancies

MiRNAs are important regulators of various developmental processes [37]**. Dysregulation of miRNA biogenesis and gene silencing are associated with the entire spectrum of hematologic malignancies [55–58].

2.1. miRNAs as oncogenes in hematological malignancies

A screening of deregulated (up- and downregulated) miRNAs in acute lymphoblastic leukemia (ALL) and chronic lymphocytic leukemia (CLL) has recently been published, providing a list of miRNAs involved in leukemogenesis and of candidates for further studies aimed at determining their possible implications either as oncogenes (OGs) or as tumor-suppressor genes (TSGs) or both [59, 60]. MiR-155 overexpression has been described in ALL, CLL, cutaneous T cell lymphoma (CTCL), and diffuse large B cell lymphoma (DLBCL) as well as in other hematologic disorders [61–64]. Fulci et al. have shown that miR-150 was significantly upregulated in CD19+ B cells from CLL patients compared to healthy donors [65]. In contrast to its upregulation in CLL, miR-150 was observed to be significantly downregulated in CD34+ blasts or mononuclear cells isolated from chronic myeloid leukemia (CML) patients than in healthy donors [66]. The levels of miR-221, miR-155 and miR-21 miRNAs have been shown to be upregulated in DLBCL patients, where miR-21 expression also correlated with prognosis [67]. MiRNA profiling in DLBCL patients compared to healthy controls identified a significant upregulation of miR-155 in DLBCL, particularly of the ABC phenotype. Diagnostic profiling indicated that miRNAs have a high diagnostic potential in CTCL of mycosis fungoides (MF) subtype, with upregulated miR-155 and downregulation of miR-203 and miR-205 discriminating from benign dermatoses [68]. Tumor lesions and folliculotropic type of MF demonstrated increased expression of miR-155, indicating that heterogeneity and progression of the disease are related to miR-155 dysregulation [69]. The main oncogenic miRNAs (oncomiRs) with identified target mRNAs are listed in Table 1.

Table 1.

miRNAs act as oncogenes in hematological malignancies

MicroRNA	Expression	Identified targets	Reference
miR-150	MDS, CLL; AML, BL; AML	MYB, FLT3, CBL, EGR2, DKC1, AKT2	References[60, 65, 66, 110, 142–150]
miR-155	DLBCL(ABC), CLL, HL, PMBL, PTLD, pediatric BL; adult BL; CTCL	AGTR1, FADD, IKKɛ, RIPK1, TP53INP1, BACH1, STAT5	References[61, 63, 151–153]
miR-96	CML, ALL	USF2	References[144, 154]
miR-21	CLL, DLBCL(ABC), MM	PTEN, BCL2	References[67, 155, 156]
miR-221	DLBCL(ABC), MM	c-KIT, p27KIP1	References[157–159]
miR-29	CLL, AML	TCL1, DNMT3A, DNMT3B	References[59, 160–162]
miR-143/145	CLL, DLBCL, MALT, BL	ERK5, TGFβ	References[71, 163]*
miR-142/146	Translocated in B-PLL patient, DLBCL, MCL, AML	ASH1L	References[164–166]
miR-125b	Translocated in B-ALL patient, AML	CDX2, CBFβ	References[167, 168]
miR-331	CLL, ALL	JAK/STAT, SOCS1	References[60, 169]
miR-130	Adult AML	SKNO-1	References[170]
miR223	AML, CLL	FBXW7	References[59, 171]

Abbreviations: ABC, activated B-cell phenotype; AML, acute myeloid leukemia; APL, acute promyelocytic leukemia; BL, Burkit’s lymphoma; CLL, chronic lymphocytic leukemia; CTCL, cutaneous T cell lymphoma; DLBCL, diffuse large B-cell lymphoma; HL, Hodgkin lymphoma; MALT, marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue; MCL, Mantle cell lymphoma; MDS, myelodysplastic syndrome; MM, multiple myeloma; OG, oncogene; B-PLL, prolymphocytic leukemia PMBL, primary mediastinal B-cell lymphoma; USF2, upstream stimulatory factor 2.

2.2. miRNAs as tumor suppressor genes in hematological malignancies

In acute promyelocytic leukemia (APL)following treatment with the differentiating agent ATRA (all-trans-retinoic acid), showed upregulation and the potential tumor suppressor function of a group of miRNAs in this disease [70]. The group included the oncosuppressor miRNAs including miR-223, −342 and −107, and members of the let-7 family and miR-15a, −15b and −16–1 with confirmed targets in hematopoiesis [59]. The data demonstrates that ATRA-induced miR-107 to target NFI-A, a gene involved in granulocytic differentiation and the miRNA regulators let-7a- and miR-15a/miR-16-1, to downregulate anti-apoptotic RAS and Bcl-2, respectively. Also, miR-143 and miR-145 have been demonstrated to be underexpressed in a variety of B-cell malignancies including B-cell lymphomas, CLL cell lines and Epstein-Barr virus-transformed B-cell lines [71]. The nature of this silencing is still poorly understood but does not rely on genomic aberrations or epigenetic modifications. Restoration of these two miRNAs in Raji (Burkitt) cells exerted a dose-dependent growth inhibition effect, and was associated with downregulation of Erk5 protein (with normal levels of ERK5 mRNA), a recently characterized MAPK, most similar to the well-studied ERK1/2 subfamily [72]. Interestingly ERK5 is targeted by miR-143 and −145 both in colon cancer DLD-1 cells, and in Burkitt’s lymphoma (BL) Raji cells, therefore suggesting a wider impact of the ERK5 pathway in human carcinogenesis [71, 73]. The miRNAs with a clear oncosuppressor role in hematological malignancies are listed in Table 2.

Table 2.

miRNAs function as tumor suppressor genes in hematological malignancies

MicroRNA	Expression	Identified targets	Reference
miR-15a/16–1	CLL, APL, MM	BCL2	References[58, 155, 172–175]
miR-29b	DLBCL, B-cell lymphoma, MCL	E2F1	References61–63
Let-7	CLL, AML	RAS, MYC	References[70, 176]
miR-143	CLL, B-cell malignancies	ERK5	References[71]
miR-151	CLL, DLBCL(ABC)	PTEN, BCL2	References[58, 107]
miR-495	MLL-rearranged leukemia	PBX3, MEIS1	References[177]
miR-34a	MM, CLL	BCL2, CDK6, NOTCH1	References[178–180]

Abbreviations: ERK5, extracellular-signal-regulated kinase 5, PTEN, Pphosphatase and tensin homolog; PBX3, PBX homeobox 3; MEIS1, myeloid ecotropic viral integration site-1; CDK6, cyclin-dependent kinase 6; CLL, chronic lymphocytic leukemia; APL, acute promyelocytic leukemia; DLBCL, diffuse large B cell lymphoma; MCL, mantle cell lymphoma; MM, multiple myeloma.

2.3. The dual regulatory function of miRNAs in hematological subtypes

By analyzing the expression of miRNAs in PBMCs or bone marrow samples of 30 acute myeloid leukemia (AML) patients, high levels of miR-181a have been described in tumors with M1 or M2 morphology, compared with the samples with M4 or M5 morphology [74]. Interestingly miR-181a has been recently found to be consistently downregulated in CLL, revealing a dual oncogenic/suppressor nature depending on the cellular context [75]. In addition, miR-17–92 cluster is oncogenic, but can have tumor suppressor effects by targeting the cell cycle gene E2F1, revealing a dual nature both as OG and TSG, which depends on the conditions as described for many other miRNAs [76]. Studies have shown that miR-204 promotes DLBCL and AML development by targeting c-KIT and p27KIP1 genes [77, 78], but also has an anti-tumor role in AML by targeting apoptosis protein repeat containing 6 (BIRC6), a gene involved in the regulation of apoptosis [79]. Key miRNAs that play a dual role in hematologic diseases are listed in Table 3.

Table 3.

The dual role miRNAs in hematological disorders

MicroRNA	Expression	Identified targets	Reference
MiR-181	CLL, AML (M1, M2); APL, AML (M4, M5)	TCL1, HOXA11	References[58, 74, 160, 181,182]
MiR-17–92	DLBCL, B-cell lymphoma, MCL	E2F1, PTEN, RB2	References[183, 184]
MiR-204	DLBCL(ABC), ALL, AML	c-KIT, p27KIP1, BIRC6	References[60, 77, 79, 102]

Abbreviations: ALL, acute lymphoblastic leukemia; AML, acute myeloid leukemia; APL, acute promyelocytic leukemia; DLBCL, diffuse large B-cell lymphoma; CLL, chronic lymphocytic leukemia; CML, chronic myeloid leukemia; MCL, mantle cell lymphoma; TCL1, T cell leukemia/lymphoma 1.

3. Regulatory mechanism of miRNA expression in hematologic malignancies

Since miRNAs are required to maintain the proper regulation of cellular processes, their deregulation leads to hematologic malignancy development, progression, and metastasis.

3.1. Genetic alterations affect miRNA expression

Abnormal miRNA expression in hematologic malignancies can arise from genomic variations in miRNA genomic loci. For example, the genomic locus of the miR-15/miR-16 cluster is deleted at high frequency in B-CLL [80, 81]. Reduced miR-146a is also repressed as a consequence of the deletion of chromosome 5q in AML [82]. In addition, pre‐mir142 is mutated in about 20% of DLBCL [83]. In the following sections, the effect of altered methylation patterns and transcriptional regulation on miRNAs expression will be further discussed.

3.2. Epigenetic modification plays a crucial role in the regulation of miRNA expression

Mounting evidence suggests epigenetic interaction between DNA methylation modification and miRNA expression contributes to the malignant phenotype in hematologic malignancies. Notably, the transcription of pri-miRNA is affected by so-called epigenetic factors, particularly the methylation of the promoter-associated CpG island [84]. The silencing of miRNAs by global methylation has been extensively studied using genome-wide methylation array and targeted methylation assays in CLL [85, 86]. In addition to DNA methylation, histone modification is known to have profound effects on controlling miRNA gene expression by tying DNA methylation with chromatin remodeling [87]. Thus, it is necessary to understand the mechanism of the regulation of miRNA in hematologic malignancy.

3.3. Transcription factors also regulate miRNA expression

miRNA expression is also controlled at the transcriptional level via transcription factors. For instance, the transcriptional co-factor meningioma 1 (MN1) gene is highly expressed in AML patients, and its upregulation is correlated inversely with miR-20a and miR-181b transcripts [88]**. Another example is the c-Myc oncogenic transcription factor (MYC)-activated miR-17–92 that promotes cancer progression by controlling expressions of E2F1, connective tissue growth factor (CTGF), thrombospondin 1 (THBS1), phosphatase and tensin homolog (PTEN) and the MYC transactivation of expression of the miR-17–92 cluster in B cell lymphoma [89]. In contrast, MYC can suppress the expression of genes of oncosuppressor miRNAs in lymphoma, such as miR-30, miR-26, miR-29, and let-7 family members [90, 91]. In addition, activation protein 1 (AP1), Ets family transcription factor C/EBPα, PU.1, nuclear factor I (NFI), and signal transducer and activator of transcription 3 (STAT3) activate miR-21 transcription by binding to the miR-21 promoter [92].

Nuclear receptors (NRs) are also ligand-activated transcription factors regulating miRNA expression by binding to the regulatory regions or specific DNA sequences of target genes. Since its initial discovery the NR superfamily consists now of 48 members including the hormone receptors [93]. Rainer et al. have shown that glucocorticoids upregulated miR-223, miR-15, and miR-16 through activation of both MR and GR in leukemia cell lines [94]. Therefore, increased understanding of the molecular mechanism of the regulation of miRNA expression by NRs may enable new therapeutic interventions and preventions for hematologic malignancy patients.

4. Clinical application of miRNAs in hematologic malignancies

Significant efforts have been made to discovering miRNA biomarkers in blood and/or plasma samples for improving the diagnosis, progression and treatment outcomes of hematologic diseases.

4.1. miRNA as biomarker for diagnostics and prognostics of hematologic malignancies

Multiple studies have shown that the impairment of the pathological regulatory mechanisms caused by microRNAs is connected to hematological malignancy development [95–100]. Aberrant expression of miRNAs has also been observed in these malignancies [101–103], which may be used as a biomarker for cancer classification, grading, and patient’s outcome prediction in addition to providing the clinical rationale for individual therapy [98, 104–106]. In addition, miRNAs are highly stable within exosomes and micro-vesicles with potential for cell-cell communication. Based on the miRNA transported they have become important tumor diagnostic and prognostic biomarkers, in addition to regulators for disease progression [107–110].

4.1.1. miRNA as biomarkers for diagnosis and prognosis in leukemia

Increased levels of circulating miR-181b-5p and miR-155–3p were demonstrated in the blood of AML patients and miR-181–5p was associated with shorter overall survival [109]. Moreover, miR-10–5p expression was significantly higher in relapsed patients compared to patients, who achieved complete remission [111]. In addition, elevated levels of miR-222, miR-511, and miR-34a were observed in the plasma of B-cell acute lymphoblastic leukemia (B-ALL) patients, while reduced levels of miR-223, miR-221, miR-199a-3p, and miR-26a were seen [112]. Importantly, miR-155 level in the plasma of CLL patients was helpful to predict overall survival; its plasma level was lower in patients who achieved complete remission [113]. Of note, miR-150 was highly expressed in the serum of CLL patients and was associated with poor prognosis [114]. miRNA profiling has also been used to distinguish clinical subtypes of CLL. In one study, the authors investigated miRNAs that distinguished aggressive from indolent disease in 17p deletion cases [115]. In another study, miR-29c and miR-223 downregulation were associated with poor treatment-free survival and overall survival in CLL patients with the CD19+CD5+CD23+ phenotype [116].

4.1.2. miRNA as biomarkers for diagnosis and prognosis in lymphoma

The most common subtypes of NHL are DLBCL and Follicular Lymphoma (FL). To date, few studies have attempted to identify circulating miRNA profiles in NHL patients. Increased miR-210, miR-155, and miR-21 levels were present in the serum of DLBCL patients and higher expression of miR-21 was correlated positively with relapse-free survival [117]. Iqbal et al. identified predictive miRNA biomarker signatures in DLBCL, including high expression of miR-155 [118]. In another study, serum levels of DLCBL patients identified a panel of 4 significantly elevated miRNAs (miR-15a, miR-16, miR-29c, miR-155) together with low level of miR-34a expression as potential biomarkers for early detection in this disease [119]. Moreover, miR-125b expression was associated with poorer prognosis in DLCBL [120]. There is no study that has adequately investigated the role of miRNAs as biomarkers in FL. Association between miRNA expression and patient prognosis was studied in CTCL patients with miR-155 and miR-200b significantly associated with overall survival [121]**. For adult T-cell leukemia/lymphoma (ATLL), high expression of miR-155 and low expression of miR-126 in the plasma of T-ALL patients correlated with longer overall survival [122]. Querfeld C et al. have shown the role of microRNAs in the molecular diagnosis of mycosis fungoides (MF) [123]**.

4.1.3. miRNA as biomarkers for diagnosis and prognosis in multiple myeloma

Multiple myeloma (MM), characterized by the malignant proliferation of monoclonal plasma cells in the bone marrow and diagnosed by a bone marrow biopsy, which is a painful and invasive procedure for patients. Therefore, circulating mRNA profiles have been suggested as a more sensitive, convenient, and noninvasive method for clinical diagnosis, prognosis and/or relapse of MM. One study has identified that miR-720, miR-1246, miR-451, miR-1915, miR-1308, and miR-638 are differentially expressed in MM patients compared to healthy individuals [124]. Another study has shown that miR-92a expression changed based on the stage of the disease and the response to therapy [125]. In contrast, reduced levels of miR-744 and let-7e were associated with disease progression and/or shorter survival of patients with MM [98]. Of note the combination of high serum levels of miR-25 and miR-16 in MM patients correlated with better overall survival compared with increased levels of miR-25 alone [126]. Low expression levels of miR-19a in MM patients were associated with a better response to bortezomib treatment and prolonged survival [127]. Circulating microRNA expressions predicted the outcome of lenalidomide plus low-dose dexamethasone treatment in patients with refractory/relapsed multiple myeloma [128]. Downregulated miR-331–3p, miR-29c-3p, miR-30b-5p, miR-30c-5p, and miR-26a-5p were associated with shorter time to progression and overall survival. In addition, miR-331 and miR-19b were found to correlate with longer progression-free survival of autologous transplanted MM patients [129].

4.2. miRNA-targeted therapy

The recent progress of miRNA studies are paving the way for the discovery of safer, targeted and more effective treatments for hematologic malignancy. Therefore, the most encouraging clinical aspect of miRNAs maybe their application for targeted therapy.

4.2.1. miRNA mimics to re-express down-regulated miRNAs in hematologic malignancies

Many miRNAs can function as tumor suppressors, and their downregulation is inevitably correlated with tumorigenesis. For example, miR-9 has been demonstrated to act as a growth suppressor and differentiation inducer in t (8;21) AML [130]. The miR-29 family functions as tumor suppressors in leukemogenesis. It has been demonstrated that c-Myc induces the down-regulation of miR-29 family members that led to AML development [131]. One study has shown that miR-30e acted as a tumor suppressor via down-regulating BCR-ABL expression in chronic myelogenous leukemia (CML) [132]. Thus, chemically synthesized double-stranded RNA molecules to re-express the down-regulated miRNAs (miRNA mimics) have been described as a therapeutic approach for hematologic malignancies treatment [133, 134].

4.2.2. Inhibition of the up-regulated miRNAs in hematologic malignancies

Multiple miRNAs that function as oncogenes can be down-regulated by applying interference-type strategies. The down-regulation of miR-126 expression induced apoptosis and decreased the clonogenic capacity of AML leukemia stem cells (LSCs), this finding suggests that miR-126 may be a therapeutic target to specifically eradicate LSCs and improve outcomes in patients with AML [135]. Strategies have been developed to address miRNA over-expression, including the use of anti-miRNA oligonucleotides (antagomiRs), miRNA masking, and miRNA sponges. Currently, antagomirs are the most promising approach for miRNA intervention. MiR-214 levels are significantly higher in malignant T cells from CTCL patients than from healthy donors. Targeting miR-214 in transgenic IL-15 mice using an antagomir-214 has shown to decrease disease severity and longer survival compared to untreated mice [136]*. An antagomir of miR-155, cobomarsen, has already demonstrated clinical activity against CTCL in phase 1/2 clinical trials [137]*. Genes associated with pathways implicated in CTCL pathogenesis such as PI3K/AKT, JAK/STAT and NFκB pathways were decreased compared to their baseline biopsies. Velu et al. have used the antagomiRs-21 and −196b to deplete leukemia-initiating (transformation) cell activity by decreasing homeobox (HOX) transcription factors, which led to leukemia-free survival in a murine AML model and delayed disease onset in xenograft models [138]. Anti-miRNA oligonucleotides are limited by their inability to bind more than one miRNA [139]. MicroRNA sponges are novel constructs that can compete with a family of miRNAs of interest and as a result silence the expression of several genes simultaneously [140]. In contrast, microRNA masking the miRNA 3’-untranslated regions of its mRNA target binding site, thus preventing miRNAs from binding to it and as a result inhibiting its function [141].

Conclusions and perspective

Since the first miRNA discovered, tremendous progress has been made on miRNA synthesis, regulation of expression, and their potential clinical role relevant to hematologic malignancies. Emerging technological advances in miRNA research has provided detailed knowledge of miRNA biogenesis. The expression of miRNAs can be regulated on multiple levels and the mechanisms of miRNA expression regulation is dependent on cell type, physiological conditions and external factors. Multiple miRNAs have been proposed as promising biomarkers for the diagnosis and prognostication for a wide variety of hematologic malignancies. While several clinical miRNA-therapeutics have been developed in clinical trials such as for CTCL, to date none has entered routine clinical practice. Thus, it is anticipated that miRNAs will in the future have an important role in the treatment of hematologic malignancies.

Key points.

• miRNAs generally control the expression of genes by post-transcriptional modulation of the expression of key targets

• Dysregulation of miRNA biogenesis pathways has been linked to hematologic malignancies

• miRNA display disease-specific pattern of expression in hematologic malignancies and distinct miRNA expression profiles can be used as diagnostic classifiers and predictive biomarkers

• miRNAs are promising therapeutic targets and novel miRNA therapeutics are currently in clinical trials for various hematologic diseases