New tricks from an old oncogene

Abstract

MYB is a leucine zipper transcription factor that is essential for hematopoesis and for renewal of colonic crypts. There is also ample evidence showing that MYB is leukemogenic in several animal species. However, it was not until recently that clear evidence was presented showing that MYB actually is an oncogene rearranged in human cancer. In a recent study, a novel mechanism of activation of MYB involving gene fusion was identified in carcinomas of the breast and head and neck. A t(6;9) translocation was shown to generate fusions between MYB and the transcription factor gene NFIB. The fusions consistently result in loss of the 3´-end of MYB, including several highly conserved target sites for microRNAs that negatively regulate MYB expression. Deletion of these target sites may disrupt the repression of MYB, leading to overexpression of MYB-NFIB transcripts and protein and to transcriptional activation of critical MYB target genes associated with apoptosis, cell cycle control, cell growth/angiogenesis, and cell adhesion. This study, together with previous and recent data showing rearrangements and copy number alterations of the MYB locus in T-cell leukemia and certain solid tumors, will be the main focus of this review.

Introduction: Fusion Oncogenes in Solid Tumors

Tumor-type specific chromosome rearrangements often result in gene fusions encoding potent oncoproteins. To date, close to 400 gene fusions have been identified in human cancer and an increasing number of these fusions are being recognized as important diagnostic and prognostic markers and as targets for new cancer therapies.1,2 Recent estimates have indicated that gene fusions account for 20% of human cancer morbidity. Most gene fusions have thus far been identified in leukemias and sarcomas and only a few in carcinomas.1 This paucity is likely due to an inability to discover these rearrangements rather than a true lack of gene fusions in carcinomas. Recent data suggest that the mechanisms by which they are generated may be partly different from that in leukemias and sarcomas. Thus, previous cytogenetic studies have clearly demonstrated that recurrent balanced chromosome translocations are rare in carcinomas whereas they are common in leukemias and sarcomas.2 Therefore, it is not surprising that several new gene fusions in carcinomas have been shown to result from intrachromosomal rearrangements or cryptic translocations/insertions that are not visible at the cytogenetic level.3–7 In line with this reasoning, it was recently shown that more than 50% of prostate cancers harbor androgen regulated gene fusions in which an ETS (E26 transformation specific) transcription factor gene (ERG or ETV1) is fused to the TMPRSS2 promoter region,4 and subgroups of non-small-cell lung cancers have EML4-ALK, SLC34A2-ROS or CD74-ROS gene fusions.5,8 In addition, fusion oncogenes have been found in subsets of thyroid carcinomas, renal cell carcinomas, breast cancer and mucoepidermoid carcinomas of the salivary and bronchial glands.1,9,10 Continued studies of cancer genomes using massively parallel sequencing and paired-end sequencing strategies will most likely lead to the identification of new gene fusions also in other common epithelial cancers.

Fusion genes are potent oncogenes as demonstrated by their ability to induce tumors in various transgenic mouse models. For example, the RET-PTC1 fusion induces tumors that are morphologically similar to human papillary thyroid carcinomas when specifically overexpressed in the thyroid.11,12 Similarly, the FUS-DDIT3 fusion causes myxoid liposarcomas when expressed under control of a ubiquitously expressed promoter,13 the SYT-SSX2 fusion induces synovial sarcoma-like tumors when expressed in immature myoblasts,14 whereas the PAX3-FKHR fusion induces alveolar rhabdomyosarcomas when targeted to Myf6-expressing skeletal muscle cells.15 In the latter mice, the incidence of tumors is significantly increased by concurrent loss of p53 or Ink4a/Arf function.

Recent studies have also shown that the ETV6-NTRK3 fusion, typical of human secretory breast carcinoma, can induce breast tumors in mice through transformation of committed alveolar bipotent or CD61(+) luminal progenitor cells,16 and that the EML4-ALK fusion induces lung adenocarcinomas in mice.17 Similarly, studies of the oncogenic role of the TMPRSS2-ERG fusion have shown that ERG has an important role in mouse prostate cancer progression and cooperates with Pten haploinsufficiency to promote progression of high-grade prostatic intraepithelial neoplasia (HGPIN) to invasive prostatic adenocarcinoma and that transgenic TMPRSS2-ERG mice develop PIN, but only in the context of PI3-kinase pathway activation.18,19 Taken together, these studies clearly demonstrate that fusion oncogenes are pathogenetically critical genes in solid tumors and as such also key targets for therapy.

The majority of fusion oncogenes in solid tumors encode aberrant transcription factors while a minority express chimeric proteins that deregulate growth factor signaling.20 Many fusion oncoproteins exert their effects through deregulation of genes encoding cell cycle proteins or by affecting the stability of these proteins. For example, the Ewing sarcoma associated EWSR1-FLI1 fusion protein has been shown to induce expression of MYC, CCND1 and CCNE1 both at the mRNA and protein levels21–23 and also to downregulate CDKN1A (p21), CDKN1B (p27) and CDKN1C (p57), leading to a reduction of the corresponding CDKI (cyclin dependent kinase inhibitor) proteins.21,22,24 Recently, Sanchez et al. also demonstrated that EWS-FLI1 upregulates a more oncogenic isoform of cyclin D1, termed D1b, by promoting the expression of an alternative splice form of the CCND1 transcript.25 Cyclin D1 is also induced by several other fusion oncoproteins such as ETV6-NTRK3, SYT-SSX2 and RET-PTC1.26–28 The PAX3-FKHR fusion protein has been shown to increase proteasome-dependent degradation of p27,29 and the FUS-DDIT3 fusion protein was recently shown to directly bind CDK2 through its DDIT3 part.30 However, the biological consequence of this interaction is not yet fully understood.

Gene Fusions Involving Transcription Factors and Transcriptional Coactivators in Salivary Gland Tumors

Tumors of the salivary glands constitute up to 10% of the tumors of the head and neck region.31 There are more than 30 histopathological subtypes, of which pleomorphic adenomas and Warthin tumors are the most common benign tumors and mucoepidermoid carcinomas and adenoid cystic carcinomas (ACC) are the most common malignant tumors.32 Recent studies have shown that these tumor types are characterized by a highly specific pattern of chromosome translocations resulting in fusion oncogenes.3,6,7,9,33,34 The major targets of the translocations are transcription factors involved in growth factor signaling and cell cycle regulation, transcriptional coactivators and tyrosine kinase receptors.

Pleomorphic adenomas are characterized by translocations resulting in gene fusions involving the transcription factor genes PLAG1 and HMGA2. PLAG1 is a developmentally regulated DNA-binding zinc finger protein that belongs to a family of cell cycle progression-related proteins (reviewed in ref. 9 and 35). Ectopic overexpression of PLAG1 due to promoter swapping with at least five other genes (Fig. 1A), cause deregulation of PLAG1 target genes and activation of the IGF-II mitogenic signaling pathway in salivary gland pleomorphic adenomas.36 PLAG1 is also involved in gene fusions with two other genes (HAS2 and COL1A2) in lipoblastomas (reviewed in ref. 9) and has been shown to induce acute myeloid leukemia in cooperation with the CBFB-MYH11 fusion protein in mice.37

HMGA2 belongs to a family of non-histone nuclear proteins that orchestrate the assembly of nucleoprotein complexes (reviewed in ref. 38). They play important roles in gene transcription, recombination and chromatin structure. Genomic rearrangements of the 3′-part of HMGA2 due to gene fusions (Fig. 1A) or truncations in pleomorphic adenomas and benign mesenchymal tumors result in activation of the expression of HMGA2 and its cellular targets, including the cell cycle regulators CCNA1 and CCNB2.39,40 In a subset of tumors with HMGA2 fusions the gene fusion is amplified together with other closely linked genes such as MDM2.7 The molecular mechanism by which HMGA2 is activated by chromosomal rearrangements is still unclear. Previous studies have suggested that separation of the DNA-binding domains from the acidic domain and the mRNA-destabilizing AUUUA motifs in the 3′-UTR (untranslated region) is the critical event leading to stabilization and overexpression of HMGA2 transcripts (reviewed in ref. 9). However, recent studies have indicated that HMGA2 may also be deregulated by loss of negatively regulating Let-7 microRNA (miRNA) targetsites in the 3′-UTR as a consequence of gene fusion/truncation.41–43

Fusion of the transcriptional coactivators MAML2 and CRTC1 (a.k.a. MECT1, TORC1 or WAMTP1) is a characteristic feature of low-grade mucoepidermoid carcinomas of the salivary, bronchial and thyroid glands (Fig. 1B).44–46 MAML2 belongs to a family of Mastermind-like, nuclear proteins that functions as coactivators for Notch receptors47,48 whereas CRTC1 belongs to a family of highly conserved CREB (cAMP response element-binding protein) coactivators.49,50 The CRTC1-MAML2 fusion encodes a chimeric protein consisting of the CREB-binding domain of CRTC1 fused to the transactivation domain of MAML2.44,45 Functional studies have shown that the N-terminal CREB-binding domain of the fusion is crucial for its transforming activity,51,52 and that CRTC1-MAML2 can activate transcription of cAMP/CREB target genes (Enlund et al. unpublished data and ref. 51 and 52). An identical CRTC1-MAML2 fusion has also been found in Warthin's tumors (Fig. 1B) and in clear cell hidradenomas of the skin,45,53,54 indicating that the fusion is etiologically linked to benign and low-grade malignant, histogenetically related tumor types originating from diverse exocrine glands.

Much to our surprise we recently identified a second gene fusion in cutaneous hidradenomas and MEC. The fusion consists of the amino-terminal domain of EWSR1 and the DNA-binding, C-terminal domain of the transcription factor POU5F1 (Fig. 1B).33 POU5F1 is expressed during early development to maintain the pluripotent status of embryonic stem and germ cells. Interestingly, the morphology of the EWSR1-POU5F1 positive tumors were more immature compared to the CRTC1-MAML2 positive tumors, raising the question of whether gene fusion could be a mechanism of POU5F1 reactivation leading to a more undifferentiated, stem cell-like phenotype in these tumors.

Recently, a hitherto unknown type of salivary gland tumor, mammary analogue secretory carcinoma of salivary glands, was described.55 These tumors show strong histomorphologic and immunohistochemical resemblance to secretory carcinoma of the breast. In addition, both tumor types show an identical ETV6-NTRK3 gene fusion (Fig. 1C), thus further emphasizing the histogenetic similarities between breast and salivary glands.

Finally, a recent study by Persson and co-workers identified a novel mechanism of activation of the MYB oncogene involving gene fusion in adenoid cystic carcinomas of the breast and head and neck.34 In this fusion, a major part of MYB is linked to the last coding exon(s) of the transcription factor gene NFIB (Fig. 1D). The fusion results in overexpression of MYB-NFIB transcripts and protein as well as to transcriptional activation of critical MYB target genes. This study, together with previous and recent data showing rearrangements or copy number alterations of MYB in leukemias and certain solid tumors, will be the main focus of this review.

Identification of a Novel Mechanism of Activation of MYB Involving Gene Fusion

The c-Myb oncogene was first identified almost 30 years ago as the cellular homologue of the transforming v-Myb gene of two avian retroviruses that induce leukemia, avian myeloblastosis virus (AMV) and E26 leukemia virus.56–59 The v-myb oncogenes of AMV and E26 encode N- and C-terminally truncated myb proteins. The murine Myb locus is also a common site of retroviral insertional mutagenesis.60 MYB belongs to a family of transcription factors that include the closely related family members MYBL1 (a.k.a. AMYB) and MYBL2 (a.k.a. BMYB). MYB proteins function as transcriptional regulators and contain three functional key domains, an N-terminal DNA-binding domain comprised of three tandem 50 amino acid myb repeats that specifically bind to the sequence PyAACG/TG, a centrally located transcription activation domain and a C-terminal negative regulatory domain (NRD) involved in transcriptional repression (reviewed in refs. 61 and 62). The latter contains a leucine zipper-like motif and an EVES-motif that can modulate the activity of MYB. Disruption of the leucine zipper-like motif enhances the transforming activity of Myb and the EVES-motif has been shown to regulate the activity of Myb by inter- and intramolecular interactions.63 Myb may also be sumoylated at two sites within the NRD leading to a reduction in Myb activity.64

The wild-type MYB-protein has a half-life of about 30 minutes and is post-translationally modified by phosphorylation, acetylation, sumoylation and ubiquitylation. These modifications may affect protein levels, DNA-binding and/or the transactivation capacity of MYB. MYB plays an important role in the control of cell proliferation, apoptosis and differentiation of in particular hematopoietic progenitor cells (reviewed in refs. 61 and 62). Loss of Myb function in mice results in embryonic lethality due to failure of fetal hepatic hematopoiesis and conditional Myb knockout in adult hematopoietic stem cells was recently shown to result in loss of self-renewal due to impaired proliferation and accelerated differentiation.65,66 More recent work has also demonstrated that MYB is important during colon development (reviewed in ref. 62). MYB is highly expressed in immature, proliferating epithelial, endothelial and hematopoietic cells and is downregulated as cells become more differentiated. High expression levels of MYB have also been found in pancreatic, colon and breast tumors as well as in most leukemias and lymphomas (reviewed in ref. 62). Overexpression of MYB in colorectal cancer has been attributed to frequent mutations in attenuation sequences in the first intron that are known to regulate the transcription of MYB. In breast cancer, ERα (estrogen receptor-α) relieves the attenuation leading to increased expression of MYB, a characteristic feature of most ERα⁺ breast cancers.

However, it was not until recently that clear evidence was presented showing that MYB actually is an oncogene rearranged in human cancer.34,60,67 The most compelling evidence in support of this observation derives from studies of a recurrent t(6;9)(q22–23;p23–24) translocation in ACC.34 This translocation was originally described in 1986 and was subsequently shown to be a primary and tumor-type specific aberration in ACC.68,69 The t(6;9) translocation is found in approximately one third of karyotypically abnormal ACCs. Positional cloning of the 6q22–23 and 9p23–24 breakpoint regions revealed a fusion between the two transcription factor genes MYB and NFIB.34 Due to alternative splicing and variable breakpoints in MYB and NFIB, at least 11 fusion transcript variants have been identified. Fusions consisting of MYB exon 14 linked to NFIB exons 8c and/or 9 predominate (Fig. 2), suggesting that most breakpoints occur in MYB intron 14 and in NFIB intron 8. The majority of transcripts contained one or more of the alternatively spliced NFIB exons 8a, 8a alternative, 8b, and/or 8c and MYB exon 9a. Preliminary studies of these transcript variants, although admittedly still limited, do not indicate that they have prognostic significance (unpublished data). However, since the fusion was found in a high frequency of ACCs, irrespective of whether they were derived from the breast, salivary glands, lacrimal glands or ceruminal glands of the ear, but not in any of 25 non-ACC tumor samples, we believe that the MYB-NFIB fusion is an important new diagnostic biomarker for this tumor entity. These findings unequivocally identify gene fusion as a novel mechanism for activation of MYB in human cancer and add to the evidence that MYB is an important human oncogene.

Molecular Consequences of the MYB-NFIB Gene Fusion

The expression of wild-type MYB is low in normal adult salivary gland and breast tissues.34 In contrast, the MYB-NFIB fusion is highly expressed in translocation-positive tumors; the expression of the 5′-part of MYB is significantly higher than that of the 3′-part, consistent with disruption of the 3′-part of the gene as a consequence of fusion to NFIB. This observation indicates that the increased MYB expression is not due to expression of the non-rearranged wild-type MYB allele. The MYB-NFIB protein is also highly expressed in fusion-positive ACCs (Fig. 3A). The immunoreactivity was, however, somewhat variable between tumors expressing similar levels of fusion transcript. The reason for this is unclear but could for example be due to heterogeneous expression of the fusion protein in different tumor cell populations or to fixation artifacts of the tissue specimens. Transient transfection of CHEF/18,70 and NIH-3T3 cells with a GFP-tagged MYB-NFIB construct confirmed the nuclear localization of the fusion oncoprotein. A diffuse nuclear staining was observed in both cell types (Fig. 3B).

The predicted MYB-NFIB fusion protein retains the DNA-binding and transactivation domains of wild-type MYB and is therefore expected to activate MYB target genes. Indeed, 14 of 16 confirmed MYB targets were overexpressed in fusion-positive tumors relative to normal salivary gland tissue (Fig. 4).34 These included genes associated with cell cycle control (CCNB1, CDC2 and MAD1L1), apoptosis (API5, BCL2, BIRC3, HSPA8 and SET), cell growth and angiogenesis (MYC, KIT, VEGFA, FGF2, CD53) and cell adhesion (CD34). Several of these genes have previously also been reported to be overexpressed in ACC (reviewed in ref. 34), thus further emphasizing their potential role in the pathogenesis of ACC. Continued studies of these and other transcriptional targets of the MYB-NFIB fusion will provide important new insights into their potential role as new diagnostic biomarkers and therapeutic targets for ACC.

The molecular mechanism by which MYB is activated by the t(6;9) translocation is still obscure. Since the minimal common part of MYB lost due to gene fusion with NFIB is exon 15, including the 3′-UTR, it has been proposed that MYB may be deregulated as a result of loss of binding sites for negatively regulating miRNAs.34

Is the MYB-fusion Another Example of Disruption of miRNA-Directed Repression of an Oncogene Activated by Chromosomal Translocation?

As previously indicated, MYB expression is tightly regulated by attenuation sequences located in the first intron of the gene and by a leucine-rich NRD in the C-terminus (see above). Mutation of the attenuation sequences or truncation/disruption of the NRD correlate with elevated expression levels of MYB in both human and experimentally induced neoplasms. However, recent studies have also indicated that MYB is negatively regulated by several miRNAs. The perhaps most compelling evidence of the latter mechanism derives from studies of miR-150, a miRNA selectively expressed in mature, resting B and T cells, but not in their progenitors. miR-150 directly regulates MYB expression in vivo through two conserved target sites in the 3′-UTR of MYB mRNA, and recent studies have shown that Myb is highly expressed during B-cell activation in Mir150^-/- knockout mice and conversely that Myb expression is significantly reduced in mice overexpressing miR-150.71 These findings have subsequently been confirmed by others showing that miR-150 indeed is an important regulator of MYB expression during differentiation.72–75 In addition to miR-150, there are at least three other miRNAs, miR-15a, miR-16 and miR-34a, with target sites in the 3′-UTR of MYB that have been shown to negatively regulate MYB expression in vitro.76–78 Thus, several lines of evidence point to an intricate miRNA-mediated regulation of MYB expression during development and differentiation.

Since the minimal common part of MYB lost due to gene fusion with NFIB is exon 15 (encoding the last 38 amino acids of the MYB protein), including the 3′-UTR, it has been proposed that MYB may be deregulated as a result of loss of binding sites for negatively regulating miRNAs in the 3′-UTR.34 This hypothesis is supported by the observation that the most common fusion types do not disrupt the C-terminal NRD of MYB encoded by exons 10–13.61,79–81 Forced overexpression of miR-150, miR-15a and miR-16 in short-term cultured fusion-positive ACC cells as well as in a T-cell acute lymphoblastic leukemia (T-ALL) cell line with MYB activation due to genomic duplication67 resulted in a 30% downregulation of MYB mRNA in T-ALL cells after 45 hrs, whereas transfection of primary ACC cells did not significantly alter the expression of MYB. The reduction in MYB mRNA in the control T-ALL cells is comparable to previously published data.73 The miRNAs miR-150 and miR-15a/16 are also expressed in fusion-positive ACCs as well as in normal salivary gland and breast tissues.34 Taken together, these findings support the hypothesis that deletion of miRNA target sites in the 3′-UTR of MYB through gene fusion may disrupt the repression of MYB by miRNAs. Whether this is the major mechanism for activation of MYB in ACC or if also sequences in the 3′-UTR of NFIB (exon 9 only encodes the last five amino acids of the NFIB protein) and/or infrequent disruption of the MYB NRD may contribute to the deregulation of MYB remains to be elucidated.

In this respect MYB is reminiscent of HMGA2, another oncogene frequently activated by chromosomal translocations. Recent studies have shown that deregulation of HMGA2 by loss of negatively regulating Let-7 miRNA target sites in the 3′-UTR due to gene fusion, is an important pathogenetic mechanism in several tumor types, including pleomorphic salivary gland adenomas.9,41–43 Interestingly, in a subset of the latter cases the activation is due to fusions of the last five amino acids of the NFIB protein to the DNA-binding domains of HMGA2,9,82 that is the same part of NFIB that is fused to MYB in ACC. Taken together available data, although admittedly still limited, suggest that MYB-NFIB may be another example of disruption of miRNA-directed repression of an oncogene activated by chromosomal translocations. However, additional studies are necessary to clarify the molecular mechanism underlying the activation of MYB by the t(6;9) and the potential contribution of NFIB.

Copy Number Alterations and Genomic Rearrangements of MYB in T-ALL and Epithelial Cancers

As predicted from the early studies demonstrating that Myb is leukemogenic in several animal species, it has now become clear that MYB is also rearranged in human leukemias. High-resolution comparative genomic hybridization array studies have revealed that a subset of T-ALL show selective amplification and overexpression of MYB (MYB^dup).60,67 Interestingly, knockdown of MYB expression in T-ALL cell lines induces T-cell differentiation but has only limited effects on viability and cell proliferation.67 However, γ-secretase inhibitors were shown to synergize with MYB knockdown to inhibit the growth of MYB^dup T-ALL cells in cases with concomitant NOTCH1 mutations, suggesting that MYB indeed may be an interesting target for therapy in T-ALL. Studies on the mechanism by which the MYB duplications occur in T-ALL indicate that they are mediated somatically by homologous recombination between Alu-elements flanking the MYB locus.83 In all cases analyzed, MYB was tandemly duplicated on one chromosome 6. The Myb locus is also a frequent insertion site in retrovirally-induced leukemias in mice; there are multiple known integration sites located both up- and downstream of Myb (reviewed in ref. 60). Recent studies have also identified a novel subgroup of T-ALL with t(6;7)(q23;q34) translocations that target MYB.60 In these cases MYB and TCRB (T-cell receptor beta) are juxta-posed on the derivative chromosome 6, leading to activation of MYB expression (Fig. 5). The translocation breakpoints in 6q23 maps to two clusters located up to more than 50 kilobases telomeric of MYB. The TCRB-MYB translocation, which does not result in a true fusion gene, defines a new subgroup of T-ALL associated with very young age and a proliferation/mitosis signature.

There are also examples of solid tumors in which MYB has been identified as a target for gene amplification. Thus, about one third of BRCA1 mutated hereditary breast cancers show amplification of MYB and a corresponding overexpression of MYB mRNA and protein.84 In contrast, MYB amplification is only found in 2% of sporadic breast cancers. Furthermore, selective MYB amplification has been found in approximately 10% of pancreatic cancers85 as well as in two colorectal cancer cell lines86 and two glioblastoma cell lines.87 The above-mentioned studies, together with our recent findings of MYB-NFIB gene fusions in ACC, provide ample evidence that MYB indeed is an important human oncogene that is rearranged in a variety of human neoplasias originating from multiple cell lineages. Large-scale sequencing is currently changing our perceptions of the cancer genome, and by using high-resolution copy number arrays in combination with paired-end transcriptome sequencing strategies,88,89 we expect that additional MYB gene rearrangements, including gene fusions, will be found also in other tumor types.

Perspectives: Towards Development of New Diagnostic and Therapeutic Strategies Based on the MYB-NFIB Fusion Oncogene

Previous studies of ACC has been seriously hampered by a number of factors, including a limited knowledge of the cellular and molecular biology of ACC as well as of the pathogenetic mechanisms leading to tumorigenesis, a lack of relevant in vitro and animal models to study disease pathogenesis and treatment, and a paucity of critical targets for development of new therapy. In particular, the fact that six commonly used ACC cell lines recently were shown to be cross-contaminated or misidentified has prompted a re-evaluation of published data regarding signaling pathways and new targets for therapy in ACC.90 The recent discovery of the MYB-NFIB gene fusion provides important clues to the molecular pathogenesis of this highly malignant tumor and opens up new avenues for improved diagnosis and identification of novel targets for pharmaceutical intervention.

The finding of the MYB-NFIB fusion in a high frequency of ACCs suggests that it may be used as a new diagnostic biomarker. We have recently developed a robust RT-PCR screening method for the fusion in formalin-fixed paraffin embedded tumor material (unpublished data) which should be particularly useful in the differential diagnosis of ACC. FISH using MYB-NFIB specific probes and immunohistochemistry using antibodies to the MYB or MYB-NFIB proteins will also be powerful adjunctive diagnostic tools. Efforts are also underway to develop noninvasive methods for detection of MYB-NFIB fusion transcripts in saliva from patients with primary salivary gland ACC. Similar studies of ERG gene fusions in urine samples from patients with prostate cancer has recently proved to be useful for noninvasive detection of prostate cancer.91,92

ACC is a slow-growing tumor with an often protracted clinical course. The poor long-term prognosis for these patients is mainly due to local recurrences and late onset of distant metastases.93 To date, there is no effective systemic therapy available that inhibits the progression of the disease and improves the survival of ACC patients. Therefore, new therapeutic approaches are needed. As previously indicated, the MYB-NFIB fusion is a promising target for development of new therapeutic strategies for patients with this disease.34 However, targeting transcription factors has proven to be notoriously difficult and accordingly there are few options available to directly target MYB. One such possibility was recently described by Williams and co-workers94 who used a DNA vaccine encoding a fusion protein in which MYB was flanked by two tetanus toxin peptides. This vaccine was shown to significantly suppress tumor growth of an aggressive colon cancer syngenic transplant. It would be interesting to see whether such a vaccine also has similar effects against fusion-positive ACC xenografts in mice. In addition, clinical trials using antisense MYB oligodeoxinucleotides (ODN) have been conducted in patients with chronic myelogenous leukemia and other refractory leukemias.95

An alternative therapeutic approach for fusion-positive ACCs would be to target the genes that are regulated by the MYB-NFIB fusion. Studies are now in progress to identify new transcriptional targets and to verify those that were recently suggested (Fig. 4). Among the latter are several that have recently been subjected to pharmaceutical intervention. For example, VEGFA may be inhibited by bevacizumab, a humanized VEGF-antibody96 as well as by several VEGFR inhibitors,97 and BCL2 is inhibited by the small molecules ABT-737 and the related orally active ABT-263.98 Moreover, the tyrosine kinase inhibitor (TKI) PD173074 inhibits FGF2 mediated proliferation through interaction with FGFR-1 and FGFR-2,99,100 and the pan histone deacetylase inhibitor LBH589 (panobinostat) has been shown to downregulate both MYC and MAD1L1101 whereas BIRC3 (CIAP2) and BCL2 may be downregulated by for example fucoxantin and fucoxantinol.102 KIT, which is a tyrosine kinase receptor for stem cell factor, is effectively inhibited by the TKI imatinib mesylate. However, despite the fact that KIT is highly overexpressed both at the RNA and protein levels in most cases of ACC, clinical studies have shown that imatinib has no major effects on advanced ACC of the head and neck.103,104 With the exception of imatinib, the abovementioned drugs and drugs with similar target specificities are interesting examples of potential MYB target gene inhibitors that deserve to be tested as single agents or in combination in in vitro and/or xenograft models to evaluate their efficacy in the treatment of ACC. Such efforts will hopefully lead to the emergence of potent therapies that will improve the survival of patients affected by this disease.

Abbreviations

ETS	=	E26 transformation specific
HGPIN	=	high-grade prostatic intraepithelial neoplasia
CDKI	=	cyclin dependent kinase inhibitor
ACC	=	adenoid cystic carcinoma
UTR	=	untranslated region
miRNA	=	microRNA
CREB	=	cAMP response element-binding protein
AMV	=	avian myeloblastosis virus
NRD	=	negative regulatory domain
DBD	=	DNA binding domain
TAD	=	transactivation domain
ERα	=	estrogen receptor-α
T-ALL	=	T-cell acute lymphoblastic leukemia
TCRB	=	T-cell receptor beta
ODN	=	oligodeoxinucleotides
TKI	=	tyrosine kinase inhibitor
HDAC	=	histone deacetylase
DAPI	=	4′,6′-diamidino-2′-phenylindole dihydrochloride
cen	=	centromere
tel	=	telomere

Figures and Tables

Figure 1 Gene fusions in salivary gland tumors. (A) PLAG1- and HMGA2-fusions in pleomorphic adenomas. (B) MAML2- and POU5F1-fusions in mucoepidermoid carcinomas and Warthin's tumors (metaplastic variant). (C) ETV6-NTRK3 fusions in mammary analogue secretory carcinoma of salivary glands and (D) the recently identified novel MYB-NFIB fusion in adenoid cystic carcinoma.

Figure 2 Schematic illustration of the MYB and NFIB genes and of two MYB-NFIB fusion variants (coding exons are shown i darker red and blue colors) and the resulting fusion proteins. Translocation breakpoints are shown by vertical arrows and miRNA binding sites for miR-15a/16 and miR-150 in the 3′-UTR of MYB are indicated by asteriks. The alternatively spliced MYB exon 9a and NFIB exons 8a, 8b and 8c are indicated. DBD, DNA binding domain; TAD, transactivation domain; NRD, negative regulatory domain.

Figure 3 MYB-NFIB is a nuclear protein. (A) Immunostaining of the MYB-NFIB fusion protein in a translocation-positive ACC. Note the predominant nuclear staining of tumor cells whereas stromal cells are negative. (B) Chinese hamster embryo fibroblasts (CHEF/18 cells) and NIH-3T3 cells (inset) transfected with a MYB-NFIB-GFP expression construct. Nuclei are counterstained in blue with 4′,6′-diamidino-2′-phenylindole dihydrochloride (DAPI). Fluorescent cells were fixed on glass slides and imaged using a Zeiss LSM510 META confocal microscope system.

Figure 4 Confirmed MYB downstream target genes that are overexpressed in fusion-positive ACCs relative to normal salivary gland tissue.

Figure 5 Schematic illustration of the translocation breakpoint region on the der(6) marker chromosome generated by the t(6;7)(q23;q34) translocation in T-ALL. Note that this translocation, unlike the t(6;9) in ACC, does not result in a MYB gene fusion. cen, centromere; tel, telomere. Adapted from reference 60.

Acknowledgements

This work was supported by the Swedish Cancer Society and the IngaBritt and Arne Lundberg Research Foundation. We thank Ulric Pedersen for help preparing the illustrations. The authors have declared that no competing interests exist.