MDS1 and EVI1 complex locus protein (MECOM) also known as ecotropic virus integration site 1 protein homolog (EVI-1) or positive regulatory domain zinc finger protein 3 (PRDM3) is a protein that in humans is encoded by the MECOM gene. MECOM was first identified as a common retroviral integration site in AKXD murine myeloid tumors. It has since been identified in a plethora of other organisms, and seems to play a relatively conserved developmental role in embryogenesis. MECOM is a transcription factor involved in many signaling pathways for both coexpression and coactivation of cell cycle genes.
Gene structure The MECOM gene is located in the human genome on chromosome 3 (3q26.2). The gene spans 60 kilobases and encodes 16 exons, 10 of which are protein-coding. The first in-frame ATG start codon is in exon 3.
mRNA
A large number of transcript variations exist, encoding different isoforms or chimeric proteins. Some of the most common ones are: MECOM_1a, MECOM_1b, MECOM_1c, MECOM_1d, and MECOM_3L are all variants in the 5' untranslated region, and all except MECOM_1a are specific to human cells. −Rp9 variant is quite common in human and mouse cells, lacks 9 amino acids in the repression domain. Δ324 found at low levels in human and mouse cells - an alternative splice variant encoding an 88kDa protein lacking zinc fingers 6 and 7 Δ105 variant is unique to mice, and results in a protein truncated by 105 amino acids at the acidic C-terminus. Fusion transcripts with upstream genes such as MDS1/MECOM (ME), AML1/MDS1/MECOM (AME), ETV6/MDS1/MECOM have all been identified
Protein
The MECOM is primarily found in the nucleus, either soluble or bound to DNA. The 145kDa isoform is the most-studied, encoding 1051 amino acids, although there are many MECOM fusion products detectable in cells expressing MECOM.
The MECOM protein contains 2 domains characterized by 7 zinc finger motifs followed by a proline-rich transcription repression domain, 3 more zinc finger motifs and an acidic C-terminus.
Biological role MECOM is a proto-oncogene conserved across humans, mice, and rats, sharing 91% homology in nucleotide sequence and 94% homology in amino acid sequence between humans and mice. It is a transcription factor localized to the nucleus and binds DNA through specific conserved sequences of GACAAGATA with the potential to interact with both corepressors and coactivators.
Embryogenesis The role of MECOM in embryogenesis and development is not completely understood, but it has been shown that MECOM deficiency in mice is an embryonic lethal mutation, characterized primarily by widespread hypocellularity and poor/disrupted development of the cardiovascular and neural system. MECOM is highly expressed in the murine embryo, found in the urinary system, lungs, and heart, but is only minutely detectable in most adult tissues, indicating a likely role in tissue development. MECOM and the fusion transcript MDS1-MECOM are both expressed in the adult human kidney, lung, pancreas, brain, and ovaries.
Cell cycle and differentiation In vitro experiments using both human and mouse cell lines have shown that MECOM prevents the terminal differentiation of bone marrow progenitor cells to granulocytes and erythroid cells, however it favors the differentiation of hematopoietic cell to megakaryocytes. The chimeric gene of AML1-MDS1-MECOM (AME) formed by the chromosomal translocation (3;21)(q26;q22) has also been shown in vitro to upregulate the cell cycle and block granulocytic differentiation of murine hematopoietic cells, as well as to delay the myeloid differentiation of bone marrow progenitors.
Association with cancer MECOM has been described as a proto-oncogene since its first discovery in 1988. Overexpression and aberrant expression of MECOM has been associated with human acute myelogenous leukemia (AML), myelodysplastic syndrome (MDS) and chronic myelogenous leukemia (CML), and more recently has been shown to be an indicator of poor prognosis. Its function in these cells may be regulated by phosphorylation of serine196, in its N-terminal DNA binding domain. All of these involve erratic cellular development and differentiation in the bone marrow leading to dramatic alterations in the normal population of blood cells. MECOM has also been found to play a role in solid ovarian and colon tumors, although it is not yet well characterized in this context. It has been hypothesized that it acts as a survival factor in tumor cell lines, preventing therapeutic-induced apoptosis and rendering the tumor cells more resistant to current treatments.
Role in tumor suppressor signaling and prevention of apoptosis
TGF-β and cell cycle progression
MECOM has been shown to be involved in the downstream signaling pathway of transforming growth factor beta (TGF-β). TGF-β, along with other TGF-β family ligands such as bone morphogenic protein (BMP) and activin are involved in regulating important cellular functions such as proliferation, differentiation, apoptosis, and matrix production. These biological roles are not only important for cellular development, but also in understanding oncogenesis.
TGF-β signaling induces transcription of the cyclin-dependent kinase (CDK) inhibitors p15Ink4B or p21Cip1, which, as a consequence, act to halt the cell cycle and stop proliferation. This inhibition can lead to cellular differentiation or apoptosis, and therefore any resistance to TGF-β is thought to contribute in some way to human leukemogenesis. The downstream effectors of TGF-β are the SMAD proteins. SMAD2 and SMAD3 are phosphorylated in response to TGF-β ligand binding at a TGF-β receptor, and translocate into the nucleus of the cell, where they can then bind to DNA and other transcription factors. Stable binding to promoters occurs through a conserved MH1 domain, and transcription activation occurs through an MH2 domain, and involves accompanying coactivators such as CBP/p300 and Sp1.
The majority of literature discusses the interaction between MECOM and SMAD3, however there have been some experiments done showing that MECOM interacts with all of the SMAD proteins at varying levels, indicating a potential involvement in all of the pathways that include SMADs as downstream effectors. The translocation of phosphorylated SMAD3 into the nucleus allows for direct interaction with MECOM, mediated by the first zinc finger domain on MECOM and the MH2 domain on SMAD3. As the SMAD3 MH2 domain is required for transcription activation, MECOM binding effectively prevents transcription of the TGF-β induced anti-growth genes through structural blocking, and also leads to recruitment of other transcriptional repressors (see Epigenetics). By inhibiting an important checkpoint pathway for tumor suppression and growth control, overexpression or aberrant expression of MECOM has characteristic oncogenic activity.
As an additional confirmation of the role of MECOM expression on cell cycle progression, it has been shown that high MECOM expression is correlated with the well-known tumor suppressor and cell cycle mediator Retinoblastoma, remaining in a hyperphosphorylated state, even in the presence of TGF-β.
JNK and inhibition of apoptosis
c-Jun N-terminal kinase (JNK) is a MAP kinase activated by extracellular stress signals such as gamma-radiation, ultraviolet light, Fas ligand, tumor necrosis factor α (TNF-α), and interleukin-1. Phosphorylation on two separate residues, Thr183 and Tyr185, cause JNK to become activated and translocate to the nucleus to phosphorylate and activate key transcription factors for the apoptotic response.
Experiments co-expressing MECOM and JNK have shown that levels of JNK-phosphorylated transcription factors (such as c-Jun) are drastically decreased in the presence of MECOM. Binding of MECOM and JNK has been shown to occur through the first zinc finger motif on MECOM, and that this interaction does not block JNK phosphorylation and activation, but blocks JNK binding to substrate in the nucleus. Subsequent in vitro assays showed that stress-induced cell death from a variety of stimuli is significantly inhibited by MECOM and JNK binding.
MECOM does not bind other MAP kinases such as p38 or ERK.
Oncogenesis and induced proliferation of HSCs Among the many other observed defects, MECOM−/− mouse embryos have been shown to have defects in both the development and proliferation of hematopoietic stem cells (HSCs). It is presumed that this is due to direct interaction with the transcription factor GATA-2, which is crucial for HSC development. It has subsequently been shown many times in vitro that MECOM upregulation can induce proliferation and differentiation of HSCs and some other cell types such as rat fibroblasts.
However, existing data is inconclusive regarding the absolute role of MECOM in cell cycle progression. It appears to depend on the specific cell type, cell line and growth conditions being used as to whether MECOM expression induces growth arrest or cell differentiation/proliferation, or whether it has any effect at all. The data showing direct interaction of MECOM with the promoters for a diverse array of genes supports the theory that this is a complex transcription factor associated with many different signalling pathways involved in development and growth.
Angiogenesis Although the literature is limited on the subject, the well-documented effects on HSCs imply that there is a potential indirect effect of aberrant MECOM expression on tumoral angiogenesis. HSCs secrete angiopoietin, and its receptor molecule Tie2 has been implicated in angiogenesis of tumors in both humans and mice. Upregulation of Tie2 has been shown to occur under hypoxic conditions, and to increase angiogenesis when coinjected with tumor cells in mice. Observations that MECOM−/− mutants have substantially downregulated Tie2 and Ang-I expression, therefore, hints at an interesting role of high MECOM expression in tumor progression. This is likely, at least in part, a reason for the widespread hemorrhaging and minimal vascular development in MECOM deleted embryos, and has potential to indicate yet another reason for poor prognosis of MECOM positive cancers.
Epigenetics MECOM has also been shown to directly interact with C-terminal-binding protein (CtBP, a known transcriptional repressor) through in vitro techniques such as yeast 2-hybrid screens and immunoprecipitation. This interaction has been specifically shown to rely on amino acids 544-607 on the MECOM protein, a stretch that contains two CtBP-binding consensus motifs. This binding leads to recruitment of histone deacetylases (HDACs) as well as many other corepressor molecules leading to transcription repression via chromatin remodelling.
MECOM interaction with Smad3 followed by recruitment of corepressors can inhibit transcription and de-sensitize a cell to TGF-β signaling without ever displacing Smad3 from a gene's promoter. The epigenetic modification is clearly enough to make the DNA inaccessible to the transcription machinery.