Search

Supplementary Figure Legends

Figure S1. Expression of forkhead family transcription factors in GIST; S2. Motif analysis of the FOXF1 cistrome and validation of FOXF1 binding sites in GIST 48 cells; S3. FOXF1 regulates ETV1, KIT, and ETV1- and KIT-dependent transcriptome as well as cell cycle regulated genes; S4. Representative ChIP-seq and ATAC-seq profiles with siRNAmediated downregulation of ETV1 and FOXF1 in GIST48 cells; S5. FOXF1 regulates KIT expression and signaling; S6. FOXF1 regulates cell cycle progression and is required for growth and proliferation of GIST cells in vitro; S7. Generation of the Foxf1f-HA-Foxf1 allele; S8. Foxf1 expression in mouse GI tract and Foxf1 ablation reduces mutant Kit-mediated hyperplasia of mouse large intestine.

S1. De novo motifs in GIST-Specific ETV1 enhancers; S2. De novo motifs in Prostate Specific ETV1 enhancers; S3. De novo motifs in Shared ETV1 enhancers; S4. De novo motifs in Shared ETV1 promoters; S5: Expression of Forkhead Transcription Factors in GIST and non-GIST tumors from GENT dataset; S6: Custom Gene Sets Used for GSEA; S7: GSEA results of gene sets enriched among downregulated genes in GIST48 transfected with siFOXF1 compared with siScrambled; S8: GSEA results of gene sets enriched among downregulated genes in GIST48 treated with siETV1

Supplementary Figure 1: Etv1 is required for intramuscular and myenteric ICC development. Supplementary Figure 2: Etv1 ablation with tamoxifen decreases Etv1 and Kit expression in Kit^delta558/+;Etv1^flox/flox;Rosa26^CreERT2/CreERT2 mice. Supplementary Figure 3: Etv1 positively regulates Kit expression in murine stomach. Supplementary Figure 4: ETV1 regulates KIT expression in GIST48 cells. Supplementary Figure 5: MAP kinase pathway inhibition decreases ETV1 target gene expression. Supplementary Figure 6: Dual lineage inhibition inhibits tumor initiation in vivo.

Supplementary Table 2: Gene sets enriched among genes DOWNregulated by tamoxifen treatment (Etv1 ablation) in murine GIST-like tumors.

Supplementary Table 3: Gene sets enriched among genes UPregulated by tamoxifen treatment (Etv1 ablation) in murine GIST-like tumors.

Supplementary Table 1: Differentially expressed genes (p1.5) between oil and tamoxifen treated tumors to conditionally ablate Etv1.

Combination of homozygous BrafV600E activation and Trp53 loss induced dramatic ICC hyperplasia in stomach, cecum and large intestine, but not in small intestine.

Gastrointestinal stromal tumor (GIST) is the most common subtype of sarcoma. Despite clinical advances in the treatment of KIT/PDGFRA–mutant GIST, similar progress against KIT/PDGFRA wild-type GIST, including mutant BRAF-driven tumors, has been limited by a lack of model systems. ETV1 is a master regulator in the intestinal cells of Cajal (ICC), thought to be the cells of origin of GIST. Here, we present a model in which the ETV1 promoter is used to specifically and inducibly drive Cre recombinase in ICC as a strategy to study GIST pathogenesis. Using a conditional allele for BrafV600E, a mutation observed in clinical cases of GIST, we observed that BrafV600E activation was sufficient to drive ICC hyperplasia but not GIST tumorigenesis. In contrast, combining BrafV600E activation with Trp53 loss was sufficient to drive both ICC hyperplasia and formation of multifocal GIST-like tumors in the mouse gastrointestinal tract with 100% penetrance. This mouse model of sporadic GIST model was amenable to therapeutic intervention, and it recapitulated clinical responses to RAF inhibition seen in human GIST. Our work offers a useful in vivo model of human sporadic forms of BRAF-mutant GIST to help unravel its pathogenesis and therapeutic response to novel experimental agents. Cancer Res; 77(14); 3758–65. ©2017 AACR.

A major breakthrough in classification of soft tissue tumors has been the recent identification of NTRK-fusion related neoplasms which are amenable to highly effective targeted therapies. Despite these therapeutic oportunities, diagnostic challenges have emerged in recognizing tumors characterized by protein kinase fusions, as they are associated with a wide morphologic spectrum, variable risk of malignancy and a rather non-specific immunoprofile. As such, NTRK-related fusions may occur in infantile fibrosarcoma, lipofibromatosis-like neural tumors, tumors resembling malignant peripheral nerve sheath tumors, etc. Triggered by an index case resembling lipofibromatosis-like neural tumor but harboring RET gene rearrangement, we investigated our files for cases showing RET gene abnormalities in order to establish their clinicopathologic features. Tumors were tested with a combination of targeted RNA sequencing and FISH methods. Six cases with RET gene rearrrangements were identified, all except one occured in children, including 4 infants. Their morphologic spectrum was quite diverse, but closely reproduced the phenotype of NTRK-fusion positive tumors, including lipofibromatosis-like neural tumors (n=3), infantile fibrosarcoma-like tumor (n=2) and malignant peripheral nerve sheath tumor-like (n=1). Three cases showed co-expression of S100 and CD34, while the remaining 3 had a non-specific immunoprofile. The tumors ranged morphologically and clinically from benign to highly malignant. None of the lipofibromatosis-like neural tumor cases recurred, while 2 patients with a malignant histology had a highly aggressive course with distant metastases to lung and other viscera. By targeted RNA sequencing these tumors harbored RET fusions with an identical break in exon 12, which retains the tyrosine kinase domain in the fusion oncoprotein, and involving various gene partners (CLIP2, CCDC6, SPECC1L, MYH10 and NCOA4). Our results suggest that RET fusion-positive neoplasms share a similar phenotypic spectrum with the NTRK-positive tumors, displaying either fibroblastic or neural-like differentiation, and spanning a wide spectrum of clinical behavior. These findings open new avenues for targeted therapy with RET inhibitors currently available in clinical trials.

Gastrointestinal stromal tumor (GIST) is the most common subtype of sarcoma. Despite clinical advances in the treatment of KIT/PDGFRA–mutant GIST, similar progress against KIT/PDGFRA wild-type GIST, including mutant BRAF-driven tumors, has been limited by a lack of model systems. ETV1 is a master regulator in the intestinal cells of Cajal (ICC), thought to be the cells of origin of GIST. Here, we present a model in which the ETV1 promoter is used to specifically and inducibly drive Cre recombinase in ICC as a strategy to study GIST pathogenesis. Using a conditional allele for BrafV600E, a mutation observed in clinical cases of GIST, we observed that BrafV600E activation was sufficient to drive ICC hyperplasia but not GIST tumorigenesis. In contrast, combining BrafV600E activation with Trp53 loss was sufficient to drive both ICC hyperplasia and formation of multifocal GIST-like tumors in the mouse gastrointestinal tract with 100% penetrance. This mouse model of sporadic GIST model was amenable to therapeutic intervention, and it recapitulated clinical responses to RAF inhibition seen in human GIST. Our work offers a useful in vivo model of human sporadic forms of BRAF-mutant GIST to help unravel its pathogenesis and therapeutic response to novel experimental agents. Cancer Res; 77(14); 3758–65. ©2017 AACR.

SUMMARY Uveal melanoma (UM) is characterized by mutually exclusive activating mutations in GNAQ, GNA11, CYSLTR2, and PLCB4, four genes in a linear pathway to activation of PLCβ in almost all tumors and loss of BAP1 in the aggressive subset. We generated mice with melanocyte-specific expression of GNA11Q209L with and without homozygous Bap1 loss. The GNA11Q209L mice recapitulated human Gq-associated melanomas, and they developed pigmented neoplastic lesions from melanocytes of the skin and non-cutaneous organs, including the eye and leptomeninges, as well as at atypical sites, including the lymph nodes and lungs. The addition of Bap1 loss increased tumor proliferation and cutaneous melanoma size. Integrative transcriptome analysis of human and murine melanomas identified RasGRP3 to be specifically expressed in GNAQ/GNA11-driven melanomas. In human UM cell lines and murine models, RasGRP3 is specifically required for GNAQ/GNA11-driven Ras activation and tumorigenesis. This implicates RasGRP3 as a critical node and a potential target in UM., In Brief Moore et al. generate a preclinical mouse model of melanoma that recapitulates features of aggressive uveal melanoma. By comparing murine and human melanomas, they identify a dependency on RasGRP3 in uveal melanoma.

Aberrant activation of MAPK signaling leads to the activation of oncogenic transcriptomes. How MAPK signaling is coupled with the transcriptional response in cancer is not fully understood. In 2 MAPK-activated tumor types, gastrointestinal stromal tumor and melanoma, we found that ETV1 and other Pea3-ETS transcription factors are critical nuclear effectors of MAPK signaling that are regulated through protein stability. Expression of stabilized Pea3-ETS factors can partially rescue the MAPK transcriptome and cell viability after MAPK inhibition. To identify the players involved in this process, we performed a pooled genome-wide RNAi screen using a fluorescence-based ETV1 protein stability sensor and identified COP1, DET1, DDB1, UBE3C, PSMD4, and COP9 signalosome members. COP1 or DET1 loss led to decoupling between MAPK signaling and the downstream transcriptional response, where MAPK inhibition failed to destabilize Pea3 factors and fully inhibit the MAPK transcriptome, thus resulting in decreased sensitivity to MAPK pathway inhibitors. We identified multiple COP1 and DET1 mutations in human tumors that were defective in the degradation of Pea3-ETS factors. Two melanoma patients had de novo DET1 mutations arising after vemurafenib treatment. These observations indicate that MAPK signaling-dependent regulation of Pea3-ETS protein stability is a key signaling node in oncogenesis and therapeutic resistance to MAPK pathway inhibition.

Prostate cancer exhibits a lineage-specific dependence on androgen signaling. Castration resistance involves reactivation of androgen signaling or activation of alternative lineage programs to bypass androgen requirement. We describe an aberrant gastrointestinal-lineage transcriptome expressed in ∼5% of primary prostate cancer that is characterized by abbreviated response to androgen-deprivation therapy and in ∼30% of castration-resistant prostate cancer. This program is governed by a transcriptional circuit consisting of HNF4G and HNF1A. Cistrome and chromatin analyses revealed that HNF4G is a pioneer factor that generates and maintains enhancer landscape at gastrointestinal-lineage genes, independent of androgen-receptor signaling. In HNF4G/HNF1A-double-negative prostate cancer, exogenous expression of HNF4G at physiologic levels recapitulates the gastrointestinal transcriptome, chromatin landscape, and leads to relative castration resistance.

Malignant peripheral nerve sheath tumors (MPNSTs) represent a group of highly aggressive soft-tissue sarcomas that may occur sporadically, in association with neurofibromatosis type I (NF1 associated) or after radiotherapy. Using comprehensive genomic approaches, we identified loss-of-function somatic alterations of the Polycomb repressive complex 2 (PRC2) components (EED or SUZ12) in 92% of sporadic, 70% of NF1-associated and 90% of radiotherapy-associated MPNSTs. MPNSTs with PRC2 loss showed complete loss of trimethylation at lysine 27 of histone H3 (H3K27me3) and aberrant transcriptional activation of multiple PRC2-repressed homeobox master regulators and their regulated developmental pathways. Introduction of the lost PRC2 component in a PRC2-deficient MPNST cell line restored H3K27me3 levels and decreased cell growth. Additionally, we identified frequent somatic alterations of CDKN2A (81% of all MPNSTs) and NF1 (72% of non-NF1-associated MPNSTs), both of which significantly co-occur with PRC2 alterations. The highly recurrent and specific inactivation of PRC2 components, NF1 and CDKN2A highlights their critical and potentially cooperative roles in MPNST pathogenesis.

Prostate cancer exhibits a remarkable lineage-specific dependence on androgen signaling. Lineage-directed therapy using androgen deprivation has been the mainstay of prostate cancer treatment for 70 years. Castration resistance involves reactivation of androgen signaling or activation of alternative lineage programs to bypass androgen requirement. Our studies found that an aberrant gastrointestinal lineage transcriptome is expressed in ~5% of primary prostate cancer that is characterized by abbreviated response to androgen deprivation therapy and in ~30% of castration-resistant prostate cancer. This program is governed by a transcriptional circuit consisting of HNF4G and HNF1A. Cistrome and chromatin analyses revealed that HNF4G is a pioneer factor that generates and maintains enhancer landscape at gastrointestinal lineage genes, independent of AR signaling. In HNF4G/1A-negative prostate cancer, exogenous expression of HNF4G at physiologic levels recapitulates the GI transcriptome, chromatin landscape and leads to relative castration resistance. Citation Format: Shipra Shukla, Joanna Cyrta, Devan Murphy, Edward Walczak, Leili Ran, Praveen Agrawal, Yuanyuan Xie, Yuedan Chen, Shangqian Wang, Yu Zhan, Wai Pung E. Wong, Andrea Sboner, Himisha Beltran, Juan-Miguel Mosquera, Jessica Sher, Zhen Cao, John Wongvipat, Richard P. Koche, Anuradha Gopalan, Deyou Zheng, Mark Rubin, Howard I. Scher, Ping Chi, Yu Chen. Aberrant activation of a gastrointestinal transcriptional circuit in prostate cancer mediates castration resistance [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr A074.

The cellular context that integrates upstream signaling and downstream nuclear response dictates the oncogenic behaviour and shapes treatment responses in distinct cancer types. Here, we uncover that in GIST, the forkhead family member, FOXF1, directly controls the transcription of two master regulators, KIT and ETV1, both required for GIST precursor-interstitial cells of Cajal (ICC) lineage-specification and GIST tumorigenesis. Further, FOXF1 co-localizes with ETV1 at enhancers and functions as a pioneer factor that regulates the ETV1-dependent GIST-lineage specific transcriptome through modulation of the local chromatin context, including chromatin accessibility, enhancer maintenance and ETV1 binding. Functionally, FOXF1 is required for human GIST cell growth in vitro and murine GIST tumor growth and maintenance in vivo. The simultaneous control of the upstream signaling and nuclear response sets up a unique regulatory paradigm and highlights the critical role of FOXF1 in enforcing the GIST cellular context for highly lineage-restricted clinical behaviour and treatment response. Citation Format: Leili Ran, Yuedan Chen, Wai Pung E. Wong, Dan Li, Zhen Cao, Shipra Shukla, Yuanyuan Xie, Deyou Zheng, Richard P. Koche, Cristina R. Antonescu, Yu Chen, Ping Chi. FOXF1 defines the core-regulatory circuitry in gastrointestinal stromal tumor (GIST) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3358.

Aberrant activation of the mitogen activate kinase (MAPK) pathway is highly prevalent in cancer and therapies targeting the pathway are approved or under active investigation in multiple malignancies. MAPK signaling leads to activation of a transcriptional program that includes general growth promoting genes, negative feedback regulators of the MAPK pathway, and lineage-specific genes. While the mechanisms of upstream signal transduction that leads to MAPK activation have been studied in detail, how MAPK activation is dynamically coupled with downstream nuclear transcriptional response is not fully understood. In gastrointestinal stomal tumor (GIST) and melanoma, two malignancies with aberrant MAPK activation, we find that Pea3 family ETS transcription factors ETV1, ETV4, and ETV5 are critical nuclear effectors of MAPK signaling. We find that the primary mechanism linking MAPK and Pea3 activity is through protein stability via the COP1 E3 ligase. The loss of COP1 leads to decoupling between upstream MAPK signaling and downstream transcription with constitutively stabilized Pea3 protein levels, constitutively high MAPK transcriptome, yet decreased upstream signaling due to Pea3-mediated transcription of negative feedback regulators. This leads to decreased therapeutic sensitivity to MAPK pathway inhibition in vitro and in vivo. These observations indicate that MAPK signaling-dependent regulation of Pea3 ETS protein stability is a crucial pathway that couples downstream transcriptional response to MAPK signaling and can shape the therapeutic sensitivity to MAPK pathway inhibition in cancer. Citation Format: Yuanyuan Xie, Zhen Cao, Wai Pung Wong, Youxin Guan, Jenny Zhang, Edward Walczak, Devan Murphy, Leili Ran, Inna Sirota, Shangqian Wang, Shipra Shukla, Dong Gao, John Wongvipat, Simon Knott, Kenneth Chang, Cristina Antonescu, Gregory Hannon, Ping Chi, Yu Chen. COP1-ETS axis regulates ERK transcriptional output and modulates sensitivity to MAPK inhibitors [abstract]. In: Proceedings of the AACR Conference on Advances in Sarcomas: From Basic Science to Clinical Translation; May 16-19, 2017; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(2_Suppl):Abstract nr B13.

Several types of bone/cartilage tumors reportedly contain high-frequency missense mutations in histone H3, yet the underlying oncogenic mechanism remains poorly characterized. We find that the H3 lysine 36 to methionine (H3K36M) mutation, identified in >90% of chondroblastomas, impairs the differentiation of mesenchymal progenitor cells and generates undifferentiated sarcoma in vivo. H3K36M mutant nucleosomes inhibit the enzymatic activities of several H3K36 methyltransferases, resulting in globally diminished H3K36 methylation. Depletion of H3K36 methyltransferases, or expressing an H3K36I mutant that similarly inhibits H3K36 methylation, is sufficient to phenocopy H3K36M mutation. Following the loss of H3K36 methylation, a genome-wide gain in H3K27 methylation leads to a redistribution of polycomb repressive complex 1 and de-repression of polycomb target genes known to block mesenchymal differentiation. Our findings are mirrored in human undifferentiated sarcomas where novel K36M/I mutations in H3.1 are identified in rare pediatric cases. This abstract is also being presented as Poster A12. Citation Format: Chao Lu, Siddhant Jain, Dominik Hoelper, Denise Bechet, Leili Ran, Irene Andrulis, Brendan Dickson, Jacek Majewski, Craig Thompson, Ping Chi, Benjamin Garcia, Nada Jabado, Peter Lewis, C. David Allis. Histone H3K36 mutations promote sarcomagenesis through altered histone methylation landscape [abstract]. In: Proceedings of the AACR Conference on Advances in Sarcomas: From Basic Science to Clinical Translation; May 16-19, 2017; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(2_Suppl):Abstract nr PR12.

Gastrointestinal stromal tumor (GIST) is the most common subtype of sarcoma, characterized by activating mutations in KIT , PDGFRA , BRAF, N/H/KRA S, and loss of function genetic/epigenetic alterations in NF1 , SDH -complex. Despite clinical advances for KIT/PDGFRA-mutant GIST, the scientific and clinical advances for KIT/PDGFRA-wild type GISTs have remained extremely limited due to the lack of model systems. Due to the lack of ICC-lineage specific cre system, modeling for sporadic GIST has not been feasible. ETV1 has recently been described as an ICC and GIST-lineage specific master regulator. Here, using the cre-activated Rosa26 CAGGS-LSL-EYFP system, we demonstrate that Etv1 CREERT2 preferentially activates cre in the Etv1 -expressing ICC lineage by tamoxifen. Using the Braf CA conditional allele for cre-activated Braf V600E , we observed that Braf V600E alone, while leading to development of ICC hyperplasia, is not sufficient to drive GIST tumorigenesis. Combination of Braf V600E activation and Trp53 loss not only gives rise to ICC hyperplasia, but also leads to multifocal GIST-like tumors in mouse GI tract with 100% penetrance. We further demonstrate that the Braf V600E ; Tp53-/- sporadic GIST mouse models are amenable for therapeutic intervention and recapitulate clinical responses to RAF inhibitors in human GIST. Our observations provide the first in vivo model for understanding the pathogenesis and therapeutic responses in BRAF-mutant GISTs. Importantly, these observations provide the proof of principle for modeling sporadic GIST in adult mice using the Etv1 CREERT2 system and establish the feasibility and utility of establishing model systems for human GISTs that do not have any model systems available for mechanistic studies and therapeutic development. Citation Format: Leili Ran, Devan Murphy, Jessica Sher, Zhen Cao, Shangqian Wang, Edward Walczak, Youxin Guan, Yuanyuan Xie, Shipra Shukla, Yu Zhan, Cristina R. Antonescu, Yu Chen, Ping Chi. Modeling sporadic gastrointestinal stromal tumor with BRAFV600E mutation [abstract]. In: Proceedings of the AACR Conference on Advances in Sarcomas: From Basic Science to Clinical Translation; May 16-19, 2017; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(2_Suppl):Abstract nr A15.

Activation of oncogenes by mechanisms other than genetic aberrations such as mutations, translocations, or amplifications is largely undefined. Here we report a novel isoform of the anaplastic lymphoma kinase (ALK) that is expressed in ∼11% of melanomas and sporadically in other human cancer types, but not in normal tissues. The novel ALK transcript initiates from a de novo alternative transcription initiation (ATI) site in ALK intron 19, and was termed ALK(ATI). In ALK(ATI)-expressing tumours, the ATI site is enriched for H3K4me3 and RNA polymerase II, chromatin marks characteristic of active transcription initiation sites. ALK(ATI) is expressed from both ALK alleles, and no recurrent genetic aberrations are found at the ALK locus, indicating that the transcriptional activation is independent of genetic aberrations at the ALK locus. The ALK(ATI) transcript encodes three proteins with molecular weights of 61.1, 60.8 and 58.7 kilodaltons, consisting primarily of the intracellular tyrosine kinase domain. ALK(ATI) stimulates multiple oncogenic signalling pathways, drives growth-factor-independent cell proliferation in vitro, and promotes tumorigenesis in vivo in mouse models. ALK inhibitors can suppress the kinase activity of ALK(ATI), suggesting that patients with ALK(ATI)-expressing tumours may benefit from ALK inhibitors. Our findings suggest a novel mechanism of oncogene activation in cancer through de novo alternative transcription initiation.

Gastrointestinal stromal tumor (GIST), originating from the interstitial cells of Cajal (ICC), is characterized by frequent activating mutations of the KIT receptor tyrosine kinase. Despite the clinical success of imatinib, which targets KIT, most patients with advanced GIST develop resistance and eventually die of the disease. The ETS family transcription factor ETV1 is a master regulator of the ICC lineage. Using mouse models of Kit activation and Etv1 ablation, we demonstrate that ETV1 is required for GIST initiation and proliferation in vivo, validating it as a therapeutic target. We further uncover a positive feedback circuit where MAP kinase activation downstream of KIT stabilizes the ETV1 protein, and ETV1 positively regulates KIT expression. Combined targeting of ETV1 stability by imatinib and MEK162 resulted in increased growth suppression in vitro and complete tumor regression in vivo. The combination strategy to target ETV1 may provide an effective therapeutic strategy in GIST clinical management. Significance: ETV1 is a lineage-specific oncogenic transcription factor required for the growth and survival of GIST. We describe a novel strategy of targeting ETV1 protein stability by the combination of MEK and KIT inhibitors that synergistically suppress tumor growth. This strategy has the potential to change first-line therapy in GIST clinical management. Cancer Discov; 5(3); 304–15. ©2015 AACR. See related commentary by Duensing, p. 231 This article is highlighted in the In This Issue feature, p. 213

SummaryThe lack of in vitro prostate cancer models that recapitulate the diversity of human prostate cancer has hampered progress in understanding disease pathogenesis and therapy response. Using a 3D organoid system, we report success in long-term culture of prostate cancer from biopsy specimens and circulating tumor cells. The first seven fully characterized organoid lines recapitulate the molecular diversity of prostate cancer subtypes, including TMPRSS2-ERG fusion, SPOP mutation, SPINK1 overexpression, and CHD1 loss. Whole-exome sequencing shows a low mutational burden, consistent with genomics studies, but with mutations in FOXA1 and PIK3R1, as well as in DNA repair and chromatin modifier pathways that have been reported in advanced disease. Loss of p53 and RB tumor suppressor pathway function are the most common feature shared across the organoid lines. The methodology described here should enable the generation of a large repertoire of patient-derived prostate cancer lines amenable to genetic and pharmacologic studies.

Inhibition of human mitochondrial peptide deformylase (HsPDF) depolarizes the mitochondrial membrane, reduces mitochondrial protein translation and causes apoptosis in Burkitt's lymphoma. We showed that HsPDF mRNA and protein levels were overexpressed in cancer cells and primary acute myeloid leukemia samples. Myc regulates mitochondria and metabolism; we also demonstrated c-myc regulated the expression of HsPDF, likely indirectly. Inhibition of HsPDF by actinonin blocked mitochondrial protein translation and caused apoptotic death of myc-positive Burkitt's lymphoma, but not myc-negative B cells. Inhibition of mitochondrial translation by chloramphenicol or tetracycline, structurally different inhibitors of the mitochondrial ribosome, which is upstream of deformylase activity, followed by treatment with actinonin, resulted in reversal of the biochemical events and abrogation of the apoptosis induced by actinonin. This reversal was specific to inhibitors of HsPDF. Inhibition of HsPDF resulted in a mitochondrial unfolded protein response (increased transcription factors CHOP and CEB/P and the mitochondrial protease Lon), which may be a mechanism mediating cell death. Therefore, HsPDF may be a therapeutic target for these hematopoietic cancers, acting via a new mechanism.

Acetylation and Autophagy Autophagy allows cells to digest their own components when necessary to survive stressful conditions. Lin et al. (p. 477) and Yi et al. (p. 474) describe signaling mechanisms in mammalian cells and yeast, respectively, by which autophagy is regulated by protein acetylation. In mammalian cells deprived of serum, the acetyltransferase TIP60 was activated by phosphorylation by the protein kinase GSK3 (glycogen synthase kinase 3). TIP60's target appeared to be a protein kinase central to autophagy regulation, ULK1. This activating pathway was required for autophagy in the absence of serum, but was not needed for autophagy in cells deprived of glucose. In the yeast Saccharomyces cerevisiae starved of nitrogen, another acetylation mechanism was uncovered. Starvation led to activation of the histone acetyltransferase Esa1, which acetylated the protein Atg3, a key component of the autophagy machinery, thus increasing its interaction with another autophagy protein, Atg8.

Gastrointestinal stromal tumour (GIST) is one of the most common types of human sarcoma and is primarily defined by activating mutations in the KIT or PDGFRA receptor tyrosine kinases. Despite the initial clinical success of imatinib that specifically target mutant KIT and PDGFRA, imatinib resistance has become the biggest challenge in the management of advanced GIST patient. Novel therapeutics that can improve the efficacy of first line imatinib therapy and/or prevent and overcome imatinib resistance is imperative. The ETS family member, ETV1, has been previously identified as a lineage-specific survival factor that cooperates with mutant KIT by ETV1 protein stabilization by active MAP kinase signaling downstream of KIT signaling in GIST oncogenesis. Here we demonstrate that ETV1 is required for GIST initiation and maintenance in vivo using compound genetically engineered mouse models (GEMM). We further identified that ETV1 forms a feed forward circuit in GIST oncogenesis where the ETV1 protein is stabilized by active MAP kinase signaling downstream of KIT and stabilized ETV1enhances KIT expression through direct binding to the KIT enhancer regions in both GIST mouse models and human GIST cell lines. The dual lineage targeting of KIT by imatinib and ETV1 by MEK162 (an MEK inhibitor) disrupts the ETV1-KIT feed forward circuit and induces more apoptosis than single agent imatinib or MEK162 in human GIST cells. The combination therapy resulted in complete tumor regression whereas single agent imatinib or MEK162 resulted in stabilization of disease in human GIST xenograft studies. Moreover, the combination therapy also induced more tumor fibrosis than single agent imatinib or MEK162 in GIST GEMM. These observations demonstrate the in vivo role of ETV1 in GIST oncogenesis and the feasibility of targeting ETV1 protein stability by inhibiting MAP kinase signaling. They also suggest that the dual lineage targeting of ETV1 and KIT by the combination therapy may provide a more effective therapeutic strategy than imatinib alone in GIST management. Citation Format: Leili Ran, Inna Sirota, Zhen Cao, Devan Murphy, Shipra Shukla, Ferdinando Rossi, John Wongvipat, William D. Tap, Peter Besmer, Cristina R. Antonescu, Yu Chen, Ping Chi. Dual lineage inhibition of ETV1 and KIT disrupts the ETV1-KIT feed forward circuit and potentiates imatinib antitumor effect in GIST oncogenesis. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3396. doi:10.1158/1538-7445.AM2014-3396

In prokaryotes, the removal of an N-terminal formyl group by the peptide deformylase is required for post-translational protein processing. Our identification of human mitochondrial PDF (HsPDF) and deformylated peptides in the mitochondria shows that a similar post-translational processing of the 13 human mitochondrial DNA-encoded, and formylated, proteins can occur. Inhibition of HsPDF reduced proliferation of many human cancer lines. We developed a monoclonal antibody to hsPDF; profiling of cancer and normal cell lines by three immunoassays, flow cytometry, immunoprecipitation and in-cell western, showed over-expression of HsPDF in cancer cell lines. This data was confirmed at the mRNA level by RT-PCR. As the c-myc oncogene regulates mitochondria and metabolism in cancer, we investigated its role in regulating HsPDF. In a tet-off c-myc cell line, the expression of HsPDF was regulated by c-myc; inhibiting this protein resulted in the selective inhibition of mitochondrial protein translation, and eventual death of myc-positive, but not myc-negative cells. Furthmore, knocking down c-myc in a Burkitt's Lymphoma cell line significantly reduced the expression of both the mRNA and the protein levels of HsPDF relative to beta-Actin. The mechanism of death by actinonin was apoptosis in two c-myc over-expressing cell lines. Death was initiated with mitochondrial membrane depolarization, followed by inhibition of mitochondrial translation, and eventually caspase-3 activation and apoptosis. To confirm the specificity of PDF in cell death, we used two other hydroxamic acid peptidomimetics that do not target HsPDF. These compounds did not result in apoptotic death. Furthermore, inhibition of translation of the mitochondrial DNA by chloramphenicol and tetracycline, two structurally different inhibitors of the mitochondrial ribosome, which is upstream of deformylation, followed by treatment with actinonin resulted in a delay and marked reduction in apoptosis by actinonin, but not by staurosporine and the HDAC inhibitors SAHA and Trichostatin A. Thus, death was not a result of ATP depletion, but a unique PDF-mediated mechanism. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1128. doi:1538-7445.AM2012-1128