The MuvB complex sequentially recruits B-Myb and FoxM1 to promote mitotic gene expression.

Cell cycle progression is dependent on two major waves of gene expression. Early cell cycle gene expression occurs during G1/S to generate factors required for DNA replication, while late cell cycle gene expression begins during G2 to prepare for mitosis. Here we demonstrate that the MuvB complex-comprised of LIN9, LIN37, LIN52, LIN54, and RBBP4-serves an essential role in three distinct transcription complexes to regulate cell cycle gene expression. The MuvB complex, together with the Rb-like protein p130, E2F4, and DP1, forms the DREAM complex during quiescence and represses expression of both early and late genes. Upon cell cycle entry, the MuvB complex dissociates from p130/DREAM, binds to B-Myb, and reassociates with the promoters of late genes during S phase. MuvB and B-Myb are required for the subsequent recruitment of FoxM1 to late gene promoters during G2. The MuvB complex remains bound to FoxM1 during peak late cell cycle gene expression, while B-Myb binding is lost when it undergoes phosphorylation-dependent, proteasome-mediated degradation during late S phase. Our results reveal a novel role for the MuvB complex in recruiting B-Myb and FoxM1 to promote late cell cycle gene expression and in regulating cell cycle gene expression from quiescence through mitosis.

MicroRNA-16 Regulates Myeloblastosis Oncogene Expression to Affect Differentiation of Acute Leukemia Cells.

BACKGROUND: This study was designed to evaluate the effects of micro-RNA-16 (miR-16)-regulated expression of myeloblastosis oncogene (MYB) on the differentiation of acute leukemia cells, the expressions of miR-16 and MYB mRNA, and protein in differently differentiated leukemia cells were detected by real-time PCR and western blot. METHODS: 1,25-Dihydroxyvitamin D3 (1,25 D3) induced monocytic differentiation of HL60 cells, and the resulting changes in miR-16 and MYB expressions were detected. Morphology of the cells induced by 1,25 D3, after being transfection with miR-16 mimics, was observed by Wright-Giemsa staining. The expression of mononuclear cell surface marker CD14 was detected by flow cytometry. RESULTS: Minimum miR-16 was expressed in early-differentiation KG-1a cells, while late-differentiation U937 and THP-1 cells had higher expressions (p < 0.01). The expressions of MYB changed oppositely. During the monocytic differentiation of HL60 cells, miR-16 expression showed a time-dependent increase, but MYB expression gradually decreased. Overexpression of miR-16 in HL60 cells promoted 1,25 D3-induced morphological changes and CD14 expression (p < 0.05). CONCLUSIONS: MR-16 facilitated the monocytic differentiation of leukemia HL60 cells by negatively regulating MYB expression.

MAZ induces MYB expression during the exit from quiescence via the E2F site in the MYB promoter.

Most E2F-binding sites repress transcription through the recruitment of Retinoblastoma (RB) family members until the end of the G1 cell-cycle phase. Although the MYB promoter contains an E2F-binding site, its transcription is activated shortly after the exit from quiescence, before RB family members inactivation, by unknown mechanisms. We had previously uncovered a nuclear factor distinct from E2F, Myb-sp, whose DNA-binding site overlapped the E2F element and had hypothesized that this factor might overcome the transcriptional repression of MYB by E2F-RB family members. We have purified Myb-sp and discovered that Myc-associated zinc finger proteins (MAZ) are major components. We show that various MAZ isoforms are present in Myb-sp and activate transcription via the MYB-E2F element. Moreover, while forced RB or p130 expression repressed the activity of a luciferase reporter driven by the MYB-E2F element, co-expression of MAZ proteins not only reverted repression, but also activated transcription. Finally, we show that MAZ binds the MYB promoter in vivo, that its binding site is critical for MYB transactivation, and that MAZ knockdown inhibits MYB expression during the exit from quiescence. Together, these data indicate that MAZ is essential to bypass MYB promoter repression by RB family members and to induce MYB expression.

Recent progress in understanding and manipulating haemoglobin switching for the haemoglobinopathies.

The major ß-haemoglobinopathies, sickle cell disease and ß-thalassaemia, represent the most common monogenic disorders worldwide and a steadily increasing global disease burden. Allogeneic haematopoietic stem cell transplantation, the only curative therapy, is only applied to a small minority of patients. Common clinical management strategies act mainly downstream of the root causes of disease. The observation that elevated fetal haemoglobin expression ameliorates these disorders has motivated longstanding investigations into the mechanisms of haemoglobin switching. Landmark studies over the last decade have led to the identification of two potent transcriptional repressors of γ-globin, BCL11A and ZBTB7A. These regulators act with additional trans-acting epigenetic repressive complexes, lineage-defining factors and developmental programs to silence fetal haemoglobin by working on cis-acting sequences at the globin gene loci. Rapidly advancing genetic technology is enabling researchers to probe deeply the interplay between the molecular players required for γ-globin (HBG1/HBG2) silencing. Gene therapies may enable permanent cures with autologous modified haematopoietic stem cells that generate persistent fetal haemoglobin expression. Ultimately rational small molecule pharmacotherapies to reactivate HbF could extend benefits widely to patients.

MYB-GATA1 fusion promotes basophilic leukaemia: involvement of interleukin-33 and nerve growth factor receptors.

Acute basophilic leukaemia (ABL) is a rare subtype of acute myeloblastic leukaemia. We previously described a recurrent t(X;6)(p11;q23) translocation generating an MYB-GATA1 fusion gene in male infants with ABL. To better understand its role, the chimeric MYB-GATA1 transcription factor was expressed in CD34-positive haematopoietic progenitors, which were transplanted into immunodeficient mice. Cells expressing MYB-GATA1 showed increased expression of markers of immaturity (CD34), of granulocytic lineage (CD33 and CD117), and of basophilic differentiation (CD203c and FcϵRI). UT-7 cells also showed basophilic differentiation after MYB-GATA1 transfection. A transcriptomic study identified nine genes deregulated by both MYB-GATA1 and basophilic differentiation. Induction of three of these genes (CCL23, IL1RL1, and NTRK1) was confirmed in MYB-GATA1-expressing CD34-positive cells by reverse transcription quantitative polymerase chain reaction. Interleukin (IL)-33 and nerve growth factor (NGF), the ligands of IL-1 receptor-like 1 (IL1RL1) and neurotrophic receptor tyrosine kinase 1 (NTRK1), respectively, enhanced the basophilic differentiation of MYB-GATA1-expressing UT-7 cells, thus demonstrating the importance of this pathway in the basophilic differentiation of leukaemic cells and CD34-positive primary cells. Finally, gene reporter assays confirmed that MYB and MYB-GATA1 directly activated NTRK1 and IL1RL1 transcription, leading to basophilic skewing of the blasts. MYB-GATA1 is more efficient than MYB, because of better stability. Our results highlight the role of IL-33 and NGF receptors in the basophilic differentiation of normal and leukaemic cells. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

MYB Promotes Desmoplasia in Pancreatic Cancer through Direct Transcriptional Up-regulation and Cooperative Action of Sonic Hedgehog and Adrenomedullin.

Extensive desmoplasia is a prominent pathological characteristic of pancreatic cancer (PC) that not only impacts tumor development, but therapeutic outcome as well. Recently, we demonstrated a novel role of MYB, an oncogenic transcription factor, in PC growth and metastasis. Here we studied its effect on pancreatic tumor histopathology and associated molecular and biological mechanisms. Tumor-xenografts derived from orthotopic-inoculation of MYB-overexpressing PC cells exhibited far-greater desmoplasia in histological analyses compared with those derived from MYB-silenced PC cells. These findings were further confirmed by immunostaining of tumor-xenograft sections with collagen-I, fibronectin (major extracellular-matrix proteins), and α-SMA (well-characterized marker of myofibroblasts or activated pancreatic stellate cells (PSCs)). Likewise, MYB-overexpressing PC cells provided significantly greater growth benefit to PSCs in a co-culture system as compared with the MYB-silenced cells. Interrogation of deep-sequencing data from MYB-overexpressing versus -silenced PC cells identified Sonic-hedgehog (SHH) and Adrenomedullin (ADM) as two differentially-expressed genes among others, which encode for secretory ligands involved in tumor-stromal cross-talk. In-silico analyses predicted putative MYB-binding sites in SHH and ADM promoters, which was later confirmed by chromatin-immunoprecipitation. A cooperative role of SHH and ADM in growth promotion of PSCs was confirmed in co-culture by using their specific-inhibitors and exogenous recombinant-proteins. Importantly, while SHH acted exclusively in a paracrine fashion on PSCs and influenced the growth of PC cells only indirectly, ADM could directly impact the growth of both PC cells and PSCs. In summary, we identified MYB as novel regulator of pancreatic tumor desmoplasia, which is suggestive of its diverse roles in PC pathobiology.

The association between four SNPs (rs7482144, rs4671393, rs28384513 and rs4895441) and fetal hemoglobin levels in Chinese Zhuang ß-thalassemia intermedia patients.

Four SNPs (rs7482144, rs4671393, rs28384513 and rs4895441) associated with HbF levels have been identified in different populations worldwide. To explore whether these SNPs modulate HbF expression in Chinese Zhuang population, 436 Chinese Zhuang ß-thalassemia intermedia (ß-TI) patients were divided into high HbF level group (mean HbF=25.5%, n=218) and low group (mean HbF=6.51%, n=218) for genotyping using PCR-HRM method. Results demonstrated that there was a significantly higher minor allele frequency (MAF=34.2%) of rs4895441 (G) in HMIP in high HbF level group than that in low group (MAF=19.8%) (P=0.001, OR=1.73, 95% CI: 1.24-2.57). The cumulative effects of risk genotypes of these loci for patients carrying any combination of 1, 2 or 3 risk genotype had a gradually increased risk of high HbF level phenotype compared to those without the risk genotypes (OR=1.50-9.06, P=0.0008); Gene-gene interaction of rs7842144 and rs4895441 showed the best model with the smallest prediction error (0.4259) and the greatest consistency of coefficient of variation (P=0.01). We concluded that rs4895441, G on HMIP might be a high-risk modifier variant for high HbF level expression, and HBG2, BCL11A and HMIP genes, as HbF quantitative trait loci (QTL) could have a synergistic effect on increasing the HbF level in Chinese Zhuang ß-TI patients.

MYB, MYBL1, MYBL2 and NFIB gene alterations and MYC overexpression in salivary gland adenoid cystic carcinoma.

AIMS: Adenoid cystic carcinoma (AdCC) is one of the most common salivary gland malignancies and the long-term prognosis is poor. In this study, we examined alterations of AdCC-associated genes, MYB, MYBL1, MYBL2 and NFIB, and their target molecules, including MYC. The results were correlated to clinicopathological profile of the patients. METHODS AND RESULTS: Using paraffin tumour sections from 33 cases of salivary gland AdCC, we performed a detailed fluorescence in-situ hybridization (FISH) analysis for gene splits and fusions of MYB, MYBL1, MYBL2 and NFIB. We found that 29 of 33 (88%) AdCC cases showed gene splits in either MYB, MYBL1 or NFIB. None of the cases showed an MYBL2 gene alteration. AdCCs were divided genetically into six gene groups, MYB-NFIB (n = 16), MYB-X (n = 4), MYBL1-NFIB (n = 2), MYBL1-X (n = 1), NFIB-X (n = 6) and gene-split-negative (n = 4). AdCC patients showing the MYB or MYBL1 gene splits were associated with microscopically positive surgical margins (P = 0.0148) and overexpression of MYC (P = 0.0164). MYC expression was detected in both ductal and myoepithelial tumour cells, and MYC overexpression was associated with shorter disease-free survival of the patients (P = 0.0268). CONCLUSIONS: The present study suggests that (1) nearly 90% of AdCCs may have gene alterations of either MYB, MYBL1 or NFIB, suggesting the diagnostic utility of the FISH assay, (2) MYB or MYBL1 gene splits may be associated with local aggressiveness of the tumours and overexpression of MYC, which is one of the oncogenic MYB/MYBL1 targets and (3) MYC overexpression may be a risk factor for disease-free survival in AdCC.

Frequent NFIB-associated Gene Rearrangement in Adenoid Cystic Carcinoma of the Vulva.

Adenoid cystic carcinoma is a rare malignant tumor that usually arises in the major and minor salivary glands and other locations containing secretory glands, including the lower female genital tract. Lower female genital tract carcinomas with adenoid cystic differentiation can be subclassified into 2 distinct groups based on the presence or absence of high-risk HPV. Cervical mixed carcinomas with some adenoid cystic differentiation are high-risk HPV-related but pure adenoid cystic carcinomas of vulvar and cervical origin appear to be unrelated to high-risk HPV. Mechanisms by which normal cells give rise to an HPV-unrelated adenoid cystic carcinoma remain largely unknown. Studies demonstrate that chromosomal translocation involving the genes encoding the transcription factors MYB and NFIB functions as a driving force of adenoid cystic carcinomas development regardless of anatomic site. The current study used fluorescence in situ hybridization with 3 different probes including MYB break-apart probe, NFIB break-apart probe, and MYB-NFIB fusion probe to assess for the presence of gene rearrangements in adenoid cystic carcinomas of the vulva. Six (66.7%) of 9 vulvar adenoid cystic carcinomas demonstrated NFIB rearrangement. Of these 6 cases with a disturbed NFIB, only 2 cases (33.3%) were positive for a MYB rearrangement that was also confirmed by a positive MYB-NFIB fusion pattern. NFIB-associated gene rearrangement is a frequent genetic event in vulvar adenoid cystic carcinomas. Chromosome translocations involving NFIB but with an intact MYB indicate the presence of novel oncogenic mechanisms for the development of adenoid cystic carcinomas of the vulva.

Immunohistochemical expression of MYB in salivary gland basal cell adenocarcinoma and basal cell adenoma.

BACKGROUND: Basal cell predominant salivary gland neoplasms can be difficult to separate histologically. One of the most aggressive of basaloid salivary gland neoplasms is adenoid cystic carcinoma. MYB expression by immunohistochemistry has been documented in adenoid cystic carcinoma. Some investigators have suggested that using this expression can help in establishing the diagnosis of adenoid cystic carcinoma. Utilizing tissue microarrays, we studied a group of basal cell adenocarcinomas and basal cell adenomas to determine: (i) whether either tumor expressed MYB and (ii) the frequency of any expression in either tumors. METHODS: Seventeen salivary gland basal cell adenocarcinomas and 30 salivary gland basal cell adenomas were used to construct microarrays. These tissue microarrays were used to assess for immunohistochemical MYB expression. RESULTS: Fifty-three percent (nine of 17) of salivary gland basal cell adenocarcinomas and 57% (17 of 30) of salivary gland basal cell adenomas showed MYB overexpression. For comparison, we studied 11 adenoid cystic carcinomas for MYB expression and found that 64% (seven of 11) overexpressed MYB. We found no relation to clinical course for basal adenomas or basal cell adenocarcinomas that overexpressed MYB vs those that did not. CONCLUSIONS: MYB expression does not help separate basal cell adenocarcinomas from basal cell adenomas, and our data suggest it does not differentiate between either of these neoplasms and adenoid cystic carcinoma.

Analysis of a Genetic Polymorphism in the Costimulatory Molecule TNFSF4 with Hematopoietic Stem Cell Transplant Outcomes.

Despite stringent procedures to secure the best HLA matching between donors and recipients, life-threatening complications continue to occur after hematopoietic stem cell transplantation (HSCT). Studying single nucleotide polymorphism (SNP) in genes encoding costimulatory molecules could help identify patients at risk for post-HSCT complications. In a stepwise approach we selected SNPs in key costimulatory molecules including CD274, CD40, CD154, CD28, and TNFSF4 and systematically analyzed their association with post-HSCT outcomes. Our discovery cohort analysis of 1157 HLA-A, -B, -C, -DRB1, and -DQB1 matched cases found that patients with donors homozygous for the C variant of rs10912564 in TNFSF4 (48%) had better disease-free survival (P = .029) and overall survival (P = .009) with less treatment-related mortality (P = .006). Our data demonstrate the TNFSF4C variant had a higher affinity for the nuclear transcription factor Myb and increased percentage of TNFSF4-positive B cells after stimulation compared with CT or TT genotypes. However, these associations were not validated in a more recent cohort, potentially because of changes in standard of practice or absence of a true association. Given the discovery cohort, functional data, and importance of TNFSF4 in infection clearance, TNFSF4C may associate with outcomes and warrants future studies.

Molecular Understanding of Non-Transfusion-Dependent Thalassemia Associated with Hemoglobin E-ß-Thalassemia in Northeast Thailand.

Non-transfusion-dependent thalassemia (NTDT) is associated with various forms of thalassemia and genetic modifiers. We report the molecular basis of NTDT in hemoglobin (Hb) E-ß-thalassemia disease. This study was done in 73 adult patients encountered at the prenatal diagnosis center of Khon Kaen University, Northeast Thailand. Hematological parameters and Hb patterns were collected, and α- and ß-globin gene mutations were determined. Multiple single-nucleotide polymorphisms (SNPs) including the rs7482144/Gγ-XmnI polymorphism, rs2297339, rs2838513, rs4895441, and rs9399137 in the HBS1L-MYB gene, rs4671393 and rs11886868 in the BCL11A gene, and G176AfsX179 in the KLF1 gene were examined. Five ß0-thalassemia mutations and a severe ß+-thalassemia mutation in trans to the ßE gene were identified. No significant difference in hematological parameters was observed among ß-thalassemia genotypes. Coinheritance of α-thalassemia was observed in 31 of the 73 subjects (42.5%). Four SNPs including Gγ-XmnI, rs2297339, rs4895441, and rs9399137 of HBS1L-MYB were found to be associated with high Hb F levels in 39 (53.4%) subjects. The molecular basis of NTDT in the remaining 3 (4.1%) cases could not be defined. These results indicate multiple genetic factors in NTDT patients and underline the importance of complete genotyping to provide proper management, make clinical predictions, and improve genetic counseling.

Differences in MYB expression and gene abnormalities further confirm that salivary cribriform basal cell tumors and adenoid cystic carcinoma are two distinct tumor entities.

BACKGROUND: In practices, some cases of salivary basal cell tumors that consist mainly of cribriform growth pattern are difficult to differentiate from adenoid cystic carcinoma (AdCC). Identification of reliable molecular biomarkers for the differential diagnosis between them is required. METHODS: Twenty-two cases of cribriform salivary basal cell tumors (at least 10% cribriform pattern present in each tumor) comprising 18 cases of basal cell adenoma (BCA) and four cases of basal cell adenocarcinoma (BcAC) were collected between 1985 and 2008. Twenty cases of cribriform AdCC were retrieved from our archives. MYB protein expression and gene abnormalities were detected in all cases by immunohistochemistry (IHC) and fluorescent in situ hybridization (FISH) analyses, respectively. RESULTS: Neither MYB protein nor split genes were detected in any of the cases of cribriform basal cell tumors, while 55% (11/20) of cases of cribriform AdCC had MYB protein expression. High MYB expression was detected in 81.8% (9/11) cases, while low expression was found in the remaining cases. FISH analysis indicated that nine AdCC tumors with high MYB protein expression were split gene-positive, while MYB gene splitting was not detected in the 11 cases with low or absent MYB protein expression. CONCLUSION: The molecular changes in AdCC differ from those associated with cribriform basal cell tumors, which further confirms that cribriform basal cell tumors and AdCC are two distinct tumor entities. Simultaneous detection of MYB protein expression and the associated molecular changes could be beneficial in differentiating salivary cribriform basal cell tumors from AdCC.

Genetic variant in the BCL11A (rs1427407), but not HBS1-MYB (rs6934903) loci associate with fetal hemoglobin levels in Indian sickle cell disease patients.

India along with Nigeria and DRC contribute to 57% of the world sickle cell anemia population. The annual number of newborns in India with SCA was estimated at 44,000 in 2010. Even with this high prevalence there is minimal information about genetic factors that influence the disease course in Indian patients. The current study was conducted on 240 patients with SCD and 60 with sickle cell trait, to determine the association of genetic variants at the BCL11A (rs1427407) and HBS1-MYB (rs6934903) loci with fetal hemoglobin levels (HbF). Both these loci have been implicated with influencing HbF levels, a powerful modulator of the clinical and hematologic features of SCD. Our results indicate the BCL11A rs1427407 G>T variant to be significantly associated with HbF levels {19.12±6.61 (GG), 20.27±6.92 (GT) and 24.83±2.92 (TT) respectively} contributing to ~23% of the trait variance. Interestingly no association of the HBS1L-MYB rs6934903 with the HbF levels was seen. The present study indicates the BCL11A (rs1427407) but not HMIP (rs6934903) to be associated with elevated HbF levels in Indian patient. Further interrogation of additional variants at both the loci; as also a GWAS which may help uncover new loci controlling HbF levels.

Transcription factor and microRNA interactions in lung cells: an inhibitory link between NK2 homeobox 1, miR-200c and the developmental and oncogenic factors Nfib and Myb.

BACKGROUND: The transcription factor NK2 homeobox 1 (Nkx2-1) plays essential roles in epithelial cell proliferation and differentiation in mouse and human lung development and tumorigenesis. A better understanding of genes and pathways downstream of Nkx2-1 will clarify the multiple roles of this critical lung factor. Nkx2-1 regulates directly or indirectly numerous protein-coding genes; however, there is a paucity of information about Nkx2-1-regulated microRNAs (miRNAs). METHODS AND RESULTS: By miRNA array analyses of mouse epithelial cell lines in which endogenous Nkx2-1 was knocked-down, we revealed that 29 miRNAs were negatively regulated including miR-200c, and 39 miRNAs were positively regulated by Nkx2-1 including miR-1195. Mouse lungs lacking functional phosphorylated Nkx2-1 showed increased expression of miR-200c and alterations in the expression of other top regulated miRNAs. Moreover, chromatin immunoprecipitation assays showed binding of NKX2-1 protein to regulatory regions of these miRNAs. Promoter reporter assays indicated that 1kb of the miR-200c 5' flanking region was transcriptionally active but did not mediate Nkx2-1- repression of miR-200c expression. 3'UTR reporter assays support a direct regulation of the predicted targets Nfib and Myb by miR-200c. CONCLUSIONS: These studies suggest that Nkx2-1 controls the expression of specific miRNAs in lung epithelial cells. In particular, we identified a regulatory link between Nkx2-1, the known tumor suppressor miR-200c, and the developmental and oncogenic transcription factors Nfib and Myb, adding new players to the regulatory mechanisms driven by Nkx2-1 in lung epithelial cells that may have implications in lung development and tumorigenesis.

GFI1B, EVI5, MYB--additional genes that cooperate with the human BCL6 gene to promote the development of lymphomas.

The BCL6 gene, which is expressed in certain B- and T-cell human lymphomas, is involved with chromosomal rearrangements and mutations in a number of these neoplasms. Lymphomagenesis is believed to evolve through a multi-step accumulation of genetic alterations in these tumors. We used retroviral insertional mutagenesis in transgenic mice expressing the human BCL6 transgene in order to identify genes that cooperate with BCL6 during lymphomatous transformation. We previously reported PIM1 as the most frequently recurring cooperating gene in this model. We now report three newly identified cooperating genes-GFI1B, EVI5, and MYB-that we identified in the lymphomas of retroviral-injected BCL6 transgenic mice (but not in retroviral-injected non-transgenic controls); mRNA and protein expression of GFI1B and EVI5 were decreased in the murine tumors, whereas MYB mRNA and protein expression were increased or decreased. These findings correlated with protein expression in human lymphomas, both B- and T-cell. Improved therapy of lymphomas may necessitate the development of combinations of drugs that target the alterations specific to each neoplasm.

Downregulation of miR-15a due to LMP1 promotes cell proliferation and predicts poor prognosis in nasal NK/T-cell lymphoma.

Nasal NK/T-cell lymphoma (NNKTL) is an Epstein-Barr virus (EBV)-associated malignancy and has distinct clinical and histological features. However, its genetic features are hitherto unclear. MicroRNAs (miRNAs) play a crucial role in the pathogenesis of several malignancies via regulating gene expression. In this study, we investigated whether the specific microRNAs were related to the tumor behaviors in NNKTL. MiRNA array and Quantitative RT-PCR analyses revealed that miR-15a was expressed at a much lower level in NNKTL cells (SNK-1, SNK-6, and SNT-8) than in normal peripheral NK cells and EBV-negative NK cell line KHYG-1. Quantitative PCR and western blot analyses showed that the expression of MYB and cyclin D1, which are validated targets of miR-15a, was higher in NNKTL cells. Transfection of NNKTL cells (SNK-6 and SNT-8) with a miR-15a precursor decreased MYB and cyclin D1 levels, thereby blocking G1/S transition and cell proliferation. Knockdown of EBV-encoded latent membrane protein 1 (LMP1) significantly increased miR-15a expression in SNK-6 cells. In NNKTL tissues, we found that reduced miR-15a expression, which correlated with MYB and cyclin D1 expression, was associated with poor prognosis of NNKTL patients. These data suggest that downregulation of miR-15a, possibly due to LMP1, implicates in the pathogenesis of NNKTL by inducing cell proliferation via MYB and cyclin D1. Thus, miR-15a could be a potential target for antitumor therapy and a prognostic predictor for NNKTL.

Animal-specific C-terminal domain links myeloblastosis oncoprotein (Myb) to an ancient repressor complex.

Members of the Myb oncoprotein and E2F-Rb tumor suppressor protein families are present within the same highly conserved multiprotein transcriptional repressor complex, named either as Myb and synthetic multivuval class B (Myb-MuvB) or as Drosophila Rb E2F and Myb-interacting proteins (dREAM). We now report that the animal-specific C terminus of Drosophila Myb but not the more highly conserved N-terminal DNA-binding domain is necessary and sufficient for (i) adult viability, (ii) proper localization to chromosomes in vivo, (iii) regulation of gene expression in vivo, and (iv) interaction with the highly conserved core of the MuvB/dREAM transcriptional repressor complex. In addition, we have identified a conserved peptide motif that is required for this interaction. Our results imply that an ancient function of Myb in regulating G2/M genes in both plants and animals appears to have been transferred from the DNA-binding domain to the animal-specific C-terminal domain. Increased expression of B-MYB/MYBL2, the human ortholog of Drosophila Myb, correlates with poor prognosis in human patients with breast cancer. Therefore, our results imply that the specific interaction of the C terminus of Myb with the MuvB/dREAM core complex may provide an attractive target for the development of cancer therapeutics.

Regulation of a Myb transcription factor by cyclin-dependent kinase 2 in Giardia lamblia.

The protozoan Giardia lamblia parasitizes the human small intestine to cause diseases. It undergoes differentiation into infectious cysts by responding to intestinal stimulation. How the activated signal transduction pathways relate to encystation stimulation remain largely unknown. During encystation, genes encoding cyst wall proteins (CWPs) are coordinately up-regulated by a Myb2 transcription factor. Because cell differentiation is linked to cell cycle regulation, we tried to understand the role of cell cycle regulators, cyclin-dependent kinases (Cdks), in encystation. We found that the recombinant Myb2 was phosphorylated by Cdk-associated complexes and the levels of phosphorylation increased significantly during encystation. We have identified a putative cdk gene (cdk2) by searching the Giardia genome database. Cdk2 was found to localize in the cytoplasm with higher expression during encystation. Interestingly, overexpression of Cdk2 resulted in a significant increase of the levels of cwp gene expression and cyst formation. In addition, the Cdk2-associated complexes can phosphorylate Myb2 and the levels of phosphorylation increased significantly during encystation. Mutations of important catalytic residues of Cdk2 resulted in a significant decrease of kinase activity and ability of inducing cyst formation. Addition of a Cdk inhibitor, purvalanol A, significantly decreased the Cdk2 kinase activity and the levels of cwp gene expression and cyst formation. Our results suggest that the Cdk2 pathway may be involved in phosphorylation of Myb2, leading to activation of the Myb2 function and up-regulation of cwp genes during encystation. The results provide insights into the use of Cdk inhibitory drugs in disruption of Giardia differentiation into cysts.

The interaction between MYB proteins and their target DNA binding sites.

Members of the MYB family of transcription factors are found in all eukaryotic lineages, where they function to regulate either fundamental cellular processes, or specific facets of metabolism or cellular differentiation. MYB transcription factors regulate these processes through modulation of transcription at target genes, to which they bind in a sequence-specific manner. Over the past decades, insights have been gained into the molecular interactions between MYB proteins and their cognate DNA targets. This review focuses on those insights, the emergence of common themes in DNA binding by diverse MYB family members. The review also considers gaps in the current knowledge of MYB-DNA interactions, particularly for plant MYB proteins, and how emerging techniques that examine protein-DNA interactions can fill these gaps.