The LIN28B-let-7-PBK pathway is essential for group 3 medulloblastoma tumor growth and survival

Children with Group 3 medulloblastoma (G3 MB) have a very poor prognosis, and many do not survive beyond 5 years after diagnosis. A factor that may contribute to this is the lack of available targeted therapy. Expression of protein lin-28 homolog B (LIN28B), a regulator of developmental timing, is upregulated in several cancers, including G3 MB, and is associated with worse survival in this disease. Here, we investigate the role of the LIN28B pathway in G3 MB and demonstrate that the LIN28B-lethal-7 (let-7; a microRNA that is a tumor suppressor)-lymphokine-activated killer T-cell-originated protein kinase (PBK; also known as PDZ-binding kinase) axis promotes G3 MB proliferation. LIN28B knockdown in G3-MB-patient-derived cell lines leads to a significant reduction in cell viability and proliferation in vitro and in prolonged survival of mice with orthotopic tumors. The LIN28 inhibitor N-methyl-N-[3-(3-methyl-1,2,4-triazolo[4,3-b]pyridazin-6-yl)phenyl]acetamide (1632) significantly reduces G3 MB cell growth and demonstrates efficacy in reducing tumor growth in mouse xenograft models. Inhibiting PBK using HI-TOPK-032 also results in a significant reduction in G3 MB cell viability and proliferation. Together, these results highlight a critical role for the LIN28B-let-7-PBK pathway in G3 MB and provide preliminary preclinical results for drugs targeting this pathway.

Identification and targeting of protein tyrosine kinase 7 (PTK7) as an immunotherapy candidate for neuroblastoma

GD2-targeting immunotherapies have improved survival in children with neuroblastoma, yet on-target, off-tumor toxicities can occur and a subset of patients cease to respond. The majority of neuroblastoma patients who receive immunotherapy have been previously treated with cytotoxic chemotherapy, making it paramount to identify neuroblastoma-specific antigens that remain stable throughout standard treatment. Cell surface glycoproteomics performed on human-derived neuroblastoma tumors in mice following chemotherapy treatment identified protein tyrosine kinase 7 (PTK7) to be abundantly expressed. Furthermore, PTK7 shows minimal expression on pediatric-specific normal tissues. We developed an anti-PTK7 chimeric antigen receptor (CAR) and find PTK7 CAR T cells specifically target and kill PTK7-expressing neuroblastoma in vitro. In vivo, human/murine binding PTK7 CAR T cells regress aggressive neuroblastoma metastatic mouse models and prolong survival with no toxicity. Together, these data demonstrate preclinical efficacy and tolerability for targeting PTK7 and support ongoing investigations to optimize PTK7-targeting CAR T cells for neuroblastoma.

A phase 1b/2, open-label study of amivantamab monotherapy or in combination with standard-of-care chemotherapy in participants with advanced or metastatic colorectal cancer

TPS279

Background: Amivantamab, a fully human EGFR and MET bispecific antibody, has shown clinical activity against tumors with primary activating EGFR mutations, EGFR resistance mutations, or MET pathway activation. Amivantamab has demonstrated activity in both EGFR- and MET-driven non-small cell lung cancer, with preclinical evidence demonstrating its ability to recruit immune effector cells. While two anti-EGFR antibodies are incorporated as part of the standard of care (SoC) for metastatic colorectal cancer (mCRC) patients, MET is highly expressed or amplified in subsets of mCRC and additionally plays a role in mediating resistance to anti-EGFR therapies; therefore, amivantamab may provide benefit in this setting. Methods: This open-label, multicenter, global Ph1b/2 study will assess the safety and anti-tumor activity of amivantamab as a monotherapy and characterize the safety and tolerability of amivantamab in addition to SoC chemotherapy in KRAS, NRAS, BRAF, and EGFR ectodomain wild type participants with advanced or metastatic CRC. The Ph2 amivantamab monotherapy Cohorts A and B will assess the anti-tumor activity in participants with left-sided CRC who have progressed on or after SoC fluoropyrimidine-, oxaliplatin-, and irinotecan-based chemotherapy and an anti-VEGF treatment, without (Cohort A) or with (Cohort B) prior exposure to anti-EGFR treatment. The Ph2 amivantamab monotherapy Cohort C will assess the antitumor activity in participants with right-sided CRC who have progressed on or after SoC fluoropyrimidine-, oxaliplatin-, and irinotecan-based chemotherapy and an anti-VEGF treatment, with or without an anti-EGFR treatment. The Ph1b dose confirmation cohorts (Ph1b-D and Ph1b-E) will assess the safety and confirm the recommended Ph2 combination dose (RP2CD) of amivantamab in addition to SoC chemotherapy regimens (mFOLFOX6 or FOLFIRI). Upon confirmation of the RP2CD, the Ph2 Cohorts D and E, which are distinct cohorts from Ph1b-D or Ph1b-E, will further characterize the safety, tolerability, and preliminary anti-tumor activity of amivantamab in addition to SoC mFOLFOX6 or FOLFIRI in mCRC patients who have progressed after front-line therapy. The primary objectives are to assess the anti-tumor activity of amivantamab as a monotherapy and characterize the safety of amivantamab when added to SoC chemotherapy in participants with mCRC (Ph2 cohorts), as well as to assess the RP2CD of amivantamab when added to SoC chemotherapy (Ph1b). The key secondary objectives are to characterize the safety of amivantamab as a monotherapy and to assess the anti-tumor activity of amivantamab when added to SoC chemotherapy in participants with mCRC. This study is currently enrolling (NCT05379595) as of August 2022 in 12 countries, with goal enrollment of 225 participants. Clinical trial information: NCT05379595 .

Amivantamab in patients with NSCLC with MET exon 14 skipping mutation: Updated results from the CHRYSALIS study

9008

Background: Amivantamab, a fully human bispecific antibody targeting epidermal growth factor receptor (EGFR) and MET, is approved for the treatment of non-small cell lung cancer (NSCLC) with EGFR exon 20 insertion after prior platinum-based chemotherapy. Given its bispecific nature, amivantamab is being explored in patients (pts) with primary MET exon 14 skipping mutation (METex14) in the MET-2 cohort of the CHRYSALIS study. Methods: CHRYSALIS (NCT02609776) is an ongoing phase 1 dose escalation/dose expansion study of amivantamab in pts with advanced NSCLC. Pts with primary METex14 whose disease progressed on or who declined current standard of care therapy were treated with amivantamab 1050 mg (pts <80 kg) or 1400 mg (pts ≥80 kg) weekly in cycle 1 and biweekly thereafter. Response was assessed by investigators using RECIST v1.1. Results: As of 2 Dec 2021, 43 pts with METex14 had received amivantamab. Median age was 70 y (range, 43-88), 58% were women, median prior lines of therapy was 2 (range, 0-10) [eg, crizotinib (n=13), capmatinib (n=11), tepotinib (n=5), anti-MET antibody (n=1)], and 23% had history of brain metastases at baseline. In 36 pts with ≥1 postbaseline disease assessment, median duration of follow-up was 5.8 months (range, 0.3-15.8); 6 pts had no prior treatment, 11 had no prior MET inhibitor, and 19 had a prior MET inhibitor. Overall response rate was 33% (50% [3/6] in treatment-naïve pts, 46% [5/11] in pts with no prior MET inhibitor, and 21% [4/19] in pts with prior MET inhibitor therapy). Clinical benefit rate was >54% regardless of prior treatment (Table). Median duration of response (DOR) was not reached (range, 2.1-12.2 months); 67% (8/13) had DOR ≥6 months. Ten of the 12 responders remain on treatment (6.0-14.4 months) with ongoing responses; 2 discontinued after 2 and 12 months, respectively. Safety profile was consistent with previously reported experience of amivantamab (Sabari 2021 JTO 16(3):S108-109). Treatment-related adverse events leading to dose reduction or discontinuation occurred in 3 pts, each. Conclusions: Amivantamab demonstrates anti-tumor activity in primary METex14 NSCLC including after prior MET inhibitor treatment. Enrollment is ongoing and updated data will be shown. Clinical trial information: NCT02609776. [Table: see text]

Abstract 3024: Targeting the RNA binding protein LIN28B in Group 3 medulloblastoma decreases proliferation and promotes apoptosis

Medulloblastoma (MB) is the most common pediatric malignant brain tumor and is currently divided into WNT, SHH, Group 3 and Group 4 subtypes. Even with multimodal chemotherapy, radiotherapy and surgery, many children with Group 3 MBs do not survive. While the molecular aberrations underlying WNT- and SHH-driven MBs are relatively well understood, the oncogenic drivers that lead to Group 3/4 MBs are poorly defined, limiting therapeutic progress. In addition to genetic mutations and alterations, cancers display dysregulated transcription and translation. RNA-binding proteins (RBPs) play key roles in both transcription and translation, and a subset of RBPs are differentially expressed in many different cancers. Indeed, we have previously demonstrated an oncogenic role for the RBP LIN28B in neuroblastoma and it is known to be upregulated in Wilms tumor, hepatoblastoma, germ cell tumors, leukemia among others. LIN28B is a key regulator of let-7 family miRNAs, which in turn inhibit LIN28B and other oncogenes. We hypothesize that LIN28B plays an important role in Group 3 MB and that a better understanding of LIN28B and LIN28B-driven networks will reveal novel therapeutic vulnerabilities. In support of our hypothesis we find that among the four subtypes, LIN28B levels are highest in Group 3 MB, and that overexpression is associated with significantly worse survival. Down-regulation of LIN28B results in significant reduction in cell proliferation by CellTiter-Glo and increased apoptosis by Caspase-Glo (as well as induction of cleaved PARP on immunoblots). In contrast overexpression of LIN28B increases Group 3 cell proliferation and tumor sphere formation. In addition we find that PDZ-binding kinase (PBK) a downstream target of LIN28B is downregulated when LIN28B is depleted. PBK knock down also leads to decreased proliferation of Group 3 MB cells. Finally, in order to robustly define the signaling networks downstream from LIN28B that are involved in Group 3 MB metastasis, we have performed who transcriptome RNA-seq profiling of two group 3 cell lines following LIN28B depletion and plan to interrogate a subset of these based on expression change and functional relevance to LIN28B-mediated Group 3 MB metastasis. This work will help define the role for LIN28B in Group 3 MB aggressiveness and pave the way for similar studies in other cancers.

Citation Format: Shubin W. Shahab, Jo Lynne Rokita, Kyle Juraschka, Sachin Kumar, Michael Taylor, Robert W. Schnepp, Tobey J. MacDonald, Anna M. Kenney. Targeting the RNA binding protein LIN28B in Group 3 medulloblastoma decreases proliferation and promotes apoptosis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3024.

Epigenetic regulator BMI1 promotes alveolar rhabdomyosarcoma proliferation and constitutes a novel therapeutic target

Rhabdomyosarcoma (RMS) is an aggressive pediatric soft tissue sarcoma. There are two main subtypes of RMS, alveolar rhabdomyosarcoma (ARMS) and embryonal rhabdomyosarcoma. ARMS typically encompasses fusion‐positive rhabdomyosarcoma, which expresses either PAX3‐FOXO1 or PAX7‐FOXO1 fusion proteins. There are no targeted therapies for ARMS; however, recent studies have begun to illustrate the cooperation between epigenetic proteins and the PAX3‐FOXO1 fusion, indicating that epigenetic proteins may serve as targets in ARMS. Here, we investigate the contribution of BMI1, given the established role of this epigenetic regulator in sustaining aggression in cancer. We determined that BMI1 is expressed across ARMS tumors, patient‐derived xenografts, and cell lines. We depleted BMI1 using RNAi and inhibitors (PTC‐209 and PTC‐028) and found that this leads to a decrease in cell growth/increase in apoptosis in vitro, and delays tumor growth in vivo. Our data suggest that BMI1 inhibition activates the Hippo pathway via phosphorylation of LATS1/2 and subsequent reduction in YAP levels and YAP/TAZ target genes. These results identify BMI1 as a potential therapeutic vulnerability in ARMS and warrant further investigation of BMI1 in ARMS and other sarcomas.

YAP-Mediated Repression of HRK Regulates Tumor Growth, Therapy Response, and Survival Under Tumor Environmental Stress in Neuroblastoma

Following chemotherapy and relapse, high-risk neuroblastoma tumors harbor more genomic alterations than at diagnosis, including increased transcriptional activity of the Yes-associated protein (YAP), a key downstream component of the Hippo signaling network. Although YAP has been implicated in many cancer types, its functional role in the aggressive pediatric cancer neuroblastoma is not well-characterized. In this study, we performed genetic manipulation of YAP in human-derived neuroblastoma cell lines to investigate YAP function in key aspects of the malignant phenotype, including mesenchymal properties, tumor growth, chemotherapy response, and MEK inhibitor response. Standard cytotoxic therapy induced YAP expression and transcriptional activity in patient-derived xenografts treated in vivo, which may contribute to neuroblastoma recurrence. Moreover, YAP promoted a mesenchymal phenotype in high-risk neuroblastoma that modulated tumor growth and therapy resistance in vivo. Finally, the BH3-only protein, Harakiri (HRK), was identified as a novel target inhibited by YAP, which, when suppressed, prevented apoptosis in response to nutrient deprivation in vitro and promoted tumor aggression, chemotherapy resistance, and MEK inhibitor resistance in vivo. Collectively, these findings suggest that YAP inhibition may improve chemotherapy response in patients with neuroblastoma via its regulation of HRK, thus providing a critical strategic complement to MEK inhibitor therapy. SIGNIFICANCE: This study identifies HRK as a novel tumor suppressor in neuroblastoma and suggests dual MEK and YAP inhibition as a potential therapeutic strategy in RAS-hyperactivated neuroblastomas.

Abstract A47: BMI1 constitutes a novel therapeutic vulnerability in fusion-positive rhabdomyosarcoma

Background: Despite intense efforts within pediatric oncology, novel, effective therapy for alveolar rhabdomyosarcoma, known as fusion-positive rhabdomyosarcoma (FP RMS) given the PAX-FOXO1 fusions characteristic of the disease, remains unrealized. Like many pediatric tumors, FP RMS displays a quiet genomic landscape, when focusing on the coding genome. However, the epigenome plays key roles in shaping tumor aggression, and previous studies have demonstrated that FP RMS is specifically enriched for methylation of Polycomb target genes, suggesting that Polycomb complexes may be deregulated and could constitute novel therapeutic targets. We hypothesized that BMI1, a key member of the Polycomb family and a tractable therapeutic target, represents a novel therapeutic vulnerability in FP RMS.

Methods: We analyzed RNA and protein expression in FP RMS cell lines, patient-derived xenografts (PDXs), and human tumor specimens. We used genetic and pharmacologic approaches to manipulate BMI1 in FP RMS cells and measured effects on proliferation, cell cycle, apoptosis, and signal transduction. To examine the effect of in vivo inhibition of BMI1, we utilized xenograft models of FP RMS.

Results: We examined RNA-Seq tumor datasets and tumor microarrays and demonstrated that BMI1 is robustly expressed in FP RMS tumors, PDXs, and cell line models. Next, in 2 cell line models, we depleted BMI-1 using shRNAs and siRNAs, and found that this led to striking (~70%) decreases in cell growth secondary to both G1/S phase arrest and apoptosis. Given these findings, we asked whether small-molecule inhibitors of BMI-1 mediated similar effects. We treated 4 independent FP RMS cell line models with both PTC-209 (first-generation inhibitor) and PTC-028 (orally available second-generation inhibitor with higher potency). Both compounds inhibited BMI-1 function and greatly reduced cell proliferation in FP rhabdomyosarcoma cell line models. Similar to genetically mediated depletion, pharmacologic inhibition of BMI1 led to G1/S phase arrest and apoptosis, as demonstrated by Annexin V staining and PARP cleavage. Finally, in a xenograft model of an aggressive FP RMS, PTC-028 treatment decreased tumor growth (p=0.0005) and significantly prolonged survival by 17 days (p=0.0002). Importantly, treatment was well tolerated without evidence of toxicity or weight loss.

Conclusions: BMI1 is robustly expressed in FP RMS and both genetic and pharmacologic inhibition of BMI1 lead to striking decreases in cell proliferation, with concomitant cell cycle arrest and apoptosis. BMI1 inhibition significantly decreases tumor growth, prolongs survival, and is well tolerated. Currently, we are further investigating combining BMI1 inhibition with standard chemotherapy and novel agents, as well as defining molecular mechanisms by which BMI1 exerts molecular functions in FP RMS. Targeting BMI-1 could provide a novel therapeutic option for patients with FP RMS, with potential broader implications for additional aggressive sarcomas.

Citation Format: Cara E. Shields, Selma M. Cuya, Sarah Chappell, Komal Rathi, Shiv Patel, Sindhu Potlapalli, Robert W. Schnepp. BMI1 constitutes a novel therapeutic vulnerability in fusion-positive rhabdomyosarcoma [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr A47.

Abstract B48: A LIN28B-PDZ kinase axis promotes neuroblastoma metastasis

Background: Neuroblastoma, an aggressive cancer of the developing sympathetic nervous system, continues to cause significant morbidity and mortality, highlighting the need to identify novel therapeutic vulnerabilities. We previously demonstrated that the RNA binding protein (RBP) LIN28B is an oncogenic driver and induces neuroblastoma proliferation, in part by regulating a RAN GTPase-Aurora kinase A axis. LIN28B blocks the processing of the let-7 family of tumor suppressors and binds mRNAs directly. In addition to alterations in cell cycle/apoptosis, metastatic dissemination is a hallmark of cancer that is incompletely understood, and neuroblastoma exhibits a striking proclivity for widespread metastases. In these studies, we investigated how LIN28B shapes neuroblastoma metastasis.

Methods: We generated GFP-luciferase expressing neuroblastoma cell line models in which LIN28B levels were manipulated, injected these models into the tail veins of NSG mice, and tracked dissemination using an IVIS Spectrum system. We used gain- and loss-of-function approaches (siRNAs, shRNAs, microRNA mimetics) to manipulate transcripts of interest in neuroblastoma cells and measured effects on self-renewal, invasion, and downstream signaling. To discover LIN28B-associated pathways, we assessed clinically annotated mRNA expression datasets.

Results: Mice injected with LIN28B-depleted neuroblastoma cells exhibit a delayed onset of tumor metastasis, reduced tumor burden, and extended survival (103 days vs. 50 days, p&lt;0.0001), compared to mice bearing neuroblastoma cells expressing control scrambled shRNA. We next demonstrated that LIN28B promotes, and let-7 opposes, self-renewal and migration, two hallmarks of metastasis. As we discovered that Aurora kinase A is a novel LIN28B target, we speculated that LIN28B might positively regulate diverse oncogenic kinases to promote metastasis. We evaluated the TARGET dataset of neuroblastoma tumors and found LIN28B mRNA expression to be robustly correlated with PDZ-binding kinase mRNA expression (PBK; r=0.67; p=3.2 × 10-33), a kinase with roles in cell proliferation/survival, self-renewal, and metastasis, and is overexpressed in multiple malignancies. We demonstrated that LIN28B directly promotes, and let-7 opposes, the expression of PBK protein, and, indeed, that PBK is a novel and direct let-7 target. Moreover, we reveal that MYCN binds to the promoter of PBK and positively regulates PBK RNA and protein expression. Finally, PBK depletion mimics the effects of LIN28B depletion, with respect to self-renewal and invasion.

Conclusions: Taken together, our findings suggest that LIN28B/let-7 shapes neuroblastoma metastasis, in part through influencing PBK, a kinase not previously implicated in the pathogenesis of aggressive pediatric solid tumors. Current studies are defining whether PBK, a therapeutically tractable target for which clinically relevant inhibitors exist, represents a novel therapeutic vulnerability in metastatic neuroblastoma.

Citation Format: Dongdong Chen, Julie Cox, Jayabhargav Annam, Melanie Weingart, Grace Essien, Komal Rathi, Priya Khurana, Selma Cuya, Jo Lynne Harenza, Kristopher Bosse, Adeiye Pilgrim, John M. Maris, Robert W. Schnepp. A LIN28B-PDZ kinase axis promotes neuroblastoma metastasis [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr B48.

Limited antitumor activity of combined BET and MEK inhibition in neuroblastoma

BACKGROUND

The treatment of high-risk neuroblastoma continues to present a formidable challenge to pediatric oncology. Previous studies have shown that Bromodomain and extraterminal (BET) inhibitors can inhibit MYCN expression and suppress MYCN-amplified neuroblastoma in vivo. Furthermore, alterations within RAS-MAPK (mitogen-activated protein kinase) signaling play significant roles in neuroblastoma initiation, maintenance, and relapse, and mitogen-activated extracellular signal-regulated kinase (MEK) inhibitors demonstrate efficacy in subsets of neuroblastoma preclinical models. Finally, hyperactivation of RAS-MAPK signaling has been shown to promote resistance to BET inhibitors. Therefore, we examined the antitumor efficacy of combined BET/MEK inhibition utilizing I-BET726 or I-BET762 and trametinib in high-risk neuroblastoma.

PROCEDURE

Utilizing a panel of genomically annotated neuroblastoma cell line models, we investigated the in vitro effects of combined BET/MEK inhibition on cell proliferation and apoptosis. Furthermore, we evaluated the effects of combined inhibition in neuroblastoma xenograft models.

RESULTS

Combined BET and MEK inhibition demonstrated synergistic effects on the growth and survival of a large panel of neuroblastoma cell lines through augmentation of apoptosis. A combination therapy slowed tumor growth in a non-MYCN-amplified, NRAS-mutated neuroblastoma xenograft model, but had no efficacy in an MYCN-amplified model harboring a loss-of-function mutation in NF1.

CONCLUSIONS

Combinatorial BET and MEK inhibition was synergistic in the vast majority of neuroblastoma cell lines in the in vitro setting but showed limited antitumor activity in vivo. Collectively, these data do not support clinical development of this combination in high-risk neuroblastoma.

LIN28B promotes neuroblastoma metastasis and regulates PDZ binding kinase

Neuroblastoma is an aggressive pediatric malignancy of the neural crest with suboptimal cure rates and a striking predilection for widespread metastases, underscoring the need to identify novel therapeutic vulnerabilities. We recently identified the RNA binding protein LIN28B as a driver in high-risk neuroblastoma and demonstrated it promotes oncogenic cell proliferation by coordinating a RAN-Aurora kinase A network. Here, we demonstrate that LIN28B influences another key hallmark of cancer, metastatic dissemination. Using a murine xenograft model of neuroblastoma dissemination, we show that LIN28B promotes metastasis. We demonstrate that this is in part due to the effects of LIN28B on self-renewal and migration, providing an understanding of how LIN28B shapes the metastatic phenotype. Our studies reveal that the let-7 family, which LIN28B inhibits, decreases self-renewal and migration. Next, we identify PDZ Binding Kinase (PBK) as a novel LIN28B target. PBK is a serine/threonine kinase that promotes the proliferation and self-renewal of neural stem cells and serves as an oncogenic driver in multiple aggressive malignancies. We demonstrate that PBK is both a novel direct target of let-7i and that MYCN regulates PBK expression, thus elucidating two oncogenic drivers that converge on PBK. Functionally, PBK promotes self-renewal and migration, phenocopying LIN28B. Taken together, our findings define a role for LIN28B in neuroblastoma metastasis and define the targetable kinase PBK as a potential novel vulnerability in metastatic neuroblastoma.

Abstract 3838: Targeting epigenetic regulator BMI-1 in alveolar rhabdomyosarcoma

Background: Rhabdomyosarcoma (RMS) is an extremely aggressive soft tissue sarcoma which affects mainly children. There are two subtypes: alveolar rhabdomyosarcoma (ARMS) and embryonal rhabdomyosarcoma (ERMS). ARMS is characterized by PAX-FOXO1 fusion proteins, whereas subsets of ERMS harbor alterations within RAS and TP53 pathways. Currently, the outcomes for ARMS (especially when metastatic) remain dismal, thus underscoring the urgent need to identify novel targets for this cancer. The genomic landscape of many pediatric cancers, including ARMS, is relatively sparse. This led us to ask whether key epigenetic factors are driving tumor aggressiveness and could constitute novel approaches for treating ARMS. Notably, the epigenetic complexes PRC1 and PRC2 are overexpressed in a variety of sarcomas and are associated with worse overall survival. We took a hypothesis-based approach and focused on PRC1. We discovered that B lymphoma Mo-MLV insertion region 1 (BMI-1), a protein member of PRC1, is overexpressed in ARMS cells. BMI-1 is a known oncogene in other cancers, but its potential oncogenic role in ARMS and other pediatric malignancies has not yet been interrogated; thus we aim to study it within this context.

Methods: To analyze the function of BMI-1 in ARMS, we depleted the protein in ARMS cell line models by both shRNA/siRNA knockdown and measured expression, cell proliferation and apoptosis. We utilized two small molecule inhibitors, PTC-209 and PTC-028, to obtain IC50s in these cell lines, then determined effects on cell proliferation and apoptosis.

Results: We examined RNA-Seq tumor datasets and determined that BMI1 is robustly expressed in ARMS tumors. Additionally, we confirmed that BMI-1 is also overexpressed in ARMS cell lines at the levels of RNA and protein. Next, we depleted BMI-1 using multiple shRNAs and siRNAS and found that this led to striking (~70%) decreases in cell growth. We also observed increased levels of apoptosis within knockdown cells. Given these results, we asked whether small molecule inhibitors of BMI-1 mediated similar phenotypes, and so we used the inhibitors PTC-209 and PTC-028. PTC-209 is a first-generation BMI-1 inhibitor, while PTC-028 is a second-generation orally available inhibitor with higher potency. Both compounds inhibited BMI-1 function and greatly reduced cell proliferation in ARMS cell lines within the nanomolar range; however, as expected, PTC-028 showed a more pronounced effect compared to PTC-209.

Conclusions: BMI1 supports proliferation and survival in cell line models of ARMS. Both chemical and pharmacologic inhibition of BMI1 led to striking decreases in cell proliferation. Currently, we are further investigating the molecular impact of BMI1 inhibition, with plans to investigate its effectiveness within an in vivo ARMS model. Targeting BMI-1 pharmacologically could provide a novel therapeutic option for patients with ARMS and may apply more broadly to other sarcomas.

Citation Format: Cara E. Shields, Selma M. Cuya, Sarah K. Chappell, Komal Rathi, Shiv Patel, Sindhu Potlapalli, Robert W. Schnepp. Targeting epigenetic regulator BMI-1 in alveolar rhabdomyosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3838.

Abstract 3668: A LIN28B-PBK Axis promotes neuroblastoma dissemination and aggression

LIN28B is an RNA binding protein that plays key roles in normal development and, when deregulated, oncogenesis; mechanistically, it blocks the processing of the let-7 family of tumor suppressors and binds mRNAs directly. We previously demonstrated that LIN28B induces neuroblastoma proliferation, in part by regulating the expression of RAN GTPase and Aurora kinase A (AURKA). However, given the widespread metastases seen within neuroblastoma, we speculated that LIN28B might also influence neuroblastoma dissemination. We used gain and loss of function approaches to genetically manipulate transcripts of interest in neuroblastoma cells and measured effects on self-renewal, invasion, and downstream signaling. To examine the impact of LIN28B on dissemination, we generated GFP-luciferase expressing neuroblastoma cell line models in which LIN28B levels were manipulated, injected these lines into the tail veins of NSG mice, and tracked dissemination using an IVIS Spectrum system. Results show that depletion of LIN28B significantly delayed the onset of tumor metastasis, reduced tumor burden, and extended mouse survival (104 days versus 50 days, p&lt;0.0001) compared to control cells. While LIN28B did not impact anoikis resistance, it did increase both tumorsphere number and size, linking self-renewal to metastatic dissemination. Additionally, LIN28B promoted cellular invasion. These effects were largely opposed by let-7. We next sought to understand how LIN28B promotes aggression and metastasis, specifically focusing on novel networks that are currently therapeutically targetable. Given our discovery of AURKA as a novel LIN28B target, we speculated that LIN28B might promote the expression of additional oncogenic kinases, perhaps revealing novel therapeutic possibilities to target the LIN28B network. We evaluated the TARGET dataset of neuroblastoma tumors and, focusing on the top 10 kinases most significantly and positively correlated with high LIN28B expression, nominated PBK for further study (4/10 of top correlated kinases). PBK (PDZ-binding kinase) is a Ser/Thr protein kinase expressed in normal embryonic tissues and various tumor types that plays a role in both mitosis and metastasis. Depletion of PBK mimicked the effects of LIN28B depletion, with respect to self-renewal and invasion. Depletion of LIN28B and overexpression of let-7 both reduced PBK protein expression, suggesting that PBK is a direct or indirect let-7 target. Taken together, our findings suggest that LIN28B/let-7 shapes neuroblastoma aggression, in part through influencing PBK, a kinase not previously implicated in the pathogenesis of neuroblastoma or other aggressive pediatric solid tumors. Current studies are further dissecting the functional and molecular relationships among LIN28B, let-7, and PBK in neuroblastoma.

Citation Format: Dongdong Chen, Julie Cox, Jayabhargav Annam, Melanie Weingart, Grace Essien, Komal Rathi, Priya Khurana, Selma M. Cuya, Robert W. Schnepp. A LIN28B-PBK Axis promotes neuroblastoma dissemination and aggression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3668.

A LIN28B-RAN-AURKA Signaling Network Promotes Neuroblastoma Tumorigenesis

A more complete understanding of aberrant oncogenic signaling in neuroblastoma, a malignancy of the developing sympathetic nervous system, is paramount to improving patient outcomes. Recently, we identified LIN28B as an oncogenic driver in high-risk neuroblastoma. Here, we identify the oncogene RAN as a LIN28B target and show regional gain of chromosome 12q24 as an additional somatic alteration resulting in increased RAN expression. We show that LIN28B influences RAN expression by promoting RAN Binding Protein 2 expression and by directly binding RAN mRNA. Further, we demonstrate a convergence of LIN28B and RAN signaling on Aurora kinase A activity. Collectively, these findings demonstrate that LIN28B-RAN-AURKA signaling drives neuroblastoma oncogenesis, suggesting that this pathway may be amenable to therapeutic targeting.

Abstract B34: Antitumor activity and sensitivity evaluation of novel BET inhibitors in neuroblastoma

Children with neuroblastoma exhibit marked variability in outcome based on age at diagnosis, disease stage and tumor biologic characteristics. Genomic amplification of MYCN is a potent oncogenic driver and negative prognostic marker in neuroblastoma patients. Therefore, there is a pressing need to identify therapeutic agents that target MYCN gene expression. The bromodomain and extra-terminal (BET) family of proteins are epigenetic regulators known to control expression of genes involved in cell growth and oncogenesis. Previous reports demonstrate that BET inhibition (BETi) significantly attenuates cellular proliferation in numerous cancer models, perhaps via modulation of the MYC and/or MYCN oncogenes. Here, we show that potent BET inhibition induces anti-tumor effects in preclinical neuroblastoma models. Specifically, GSK726 and GSK762 (GlaxoSmithKline) were used for in vitro cytotoxicity and in vivo therapeutic studies, respectively. Neuroblastoma cell lines (n=22) were treated with GSK726 to calculate IC50 values in a luminescence-based cell viability assay and differential sensitivity was observed with an IC50 range of 27 nM to 9.5 µM. In sensitive cell lines, cell cycle distribution and induction of apoptosis were also measured. In these cell lines, GSK726 treatment resulted in MYCN depletion, G1 arrest within 24 hours, and apoptosis as measured by cleaved-PARP. To assess in vivo efficacy, GSK762 was subcutaneously administered in xenograft models and in genetically engineered neuroblastoma mouse models overexpressing MYCN and MYCN/ALK F1174L in the neural crest. In both models, GSK762 treatment resulted in tumor growth delay. Further assessment of results seen in vitro and in vivo indicated that MYCN amplification status did not fully predict sensitivity to GSK726 or GSK762. Thus, to determine additional biomarkers of sensitivity, we examined baseline gene expression data at the extremes of IC50 values by comparing sensitive (n=6; IC50&lt;128 nM) and resistant (n=4; IC50&gt;940 nM) neuroblastoma cell lines. Gene expression data from the neuroblastoma cell lines generated on HuGene1.0ST expression microarrays (Affymetrix) were utilized, and data were analyzed using the Limma package in R/Bioconductor. Univariate analysis, with a false discovery rate less than 0.25, revealed 6 genes (PTER, PCDHB14, RFTN1, JAK2, MYCBP, and DACH1) that were differentially expressed between sensitive and resistant cell lines in the MYCN amplified setting. While all 6 genes predicted BETi sensitivity in the MYCN-amplified subset of cell lines, only DACH1 expression predicted sensitivity to BET inhibition irrespective of MYCN amplification status (p=7.92x10-7). In addition, high DACH1 expression was correlated with poor patient outcome (p=1.74x10-5). As a result, high DACH1 expression serves as a candidate biomarker for future studies with respect to sensitivity to BETi. Ultimately, these studies will help to optimize the clinical utility of BETi in neuroblastoma and perhaps other MYC-driven malignancies.

Citation Format: Jason R. Healy, Robert W. Schnepp, Lori S. Hart, Pichai Raman, Laura Danielson, Michael Russell, Priya Khurana, Maria Gagliardi, Ryan M. Kinsey, Anastasia Wyce, Olena Barbash, Peter J. Tummino, Louis Chesler, John M. Maris. Antitumor activity and sensitivity evaluation of novel BET inhibitors in neuroblastoma. [abstract]. In: Proceedings of the AACR Special Conference on Myc: From Biology to Therapy; Jan 7-10, 2015; La Jolla, CA. Philadelphia (PA): AACR; Mol Cancer Res 2015;13(10 Suppl):Abstract nr B34.

Abstract 4734: A LIN28B/RAN/AURKA signaling network promotes neuroblastoma tumorigenesis

Background: Neuroblastoma, a childhood cancer of the sympathetic nervous system, accounts for approximately 10-15% of pediatric oncology deaths. The genetic basis of neuroblastoma has grown clearer, with genome-wide association studies performed by our laboratory uncovering CASC15, BARD1, NBPF23, LMO1, HACE1, TP53 and LIN28B as susceptibility genes, with many (if not all) playing a major role in tumorigenesis. Here we focus on LIN28B, which binds mRNAs directly and is a master regulator of the let-7 family of tumor suppressor microRNAs, as we previously showed that high LIN28B expression is associated with advanced stage disease and worse patient outcome.

Methods: To discover LIN28B-associated pathways in neuroblastoma, we performed gene set enrichment analysis (GSEA) on mRNA expression datasets and analyzed SNP-array based DNA copy number datasets. We used siRNAs, shRNAs, and microRNA mimetics to perturb transcripts of interest in neuroblastoma cells and measured effects on downstream signaling, protein-protein interactions, and proliferation.

Results: We applied GSEA to mRNA expression profiles from 250 neuroblastoma tumors and found LIN28B expression to be robustly correlated with several biologically relevant gene sets, including “RAN signaling.” We focused on RAN signaling as RAN is a member of the Ras family of GTPases implicated in the pathogenesis of several malignancies and we demonstrated a strong positive correlation between LIN28B and RAN expression, most strikingly in the MYCN-amplified context (p = 2.2×10−10). We next analyzed 374 high-risk neuroblastoma tumors and found that 28% of them displayed recurrent somatic copy number gain of chromosome 12q24, the genomic location of RAN, which was associated with increased RAN expression (p = 0.0004) and was inversely related to MYCN amplification (p = 0.0021). Increased RAN expression was associated with stage 4 disease (p = 0.0047) and decreased overall survival (p = 0.0002). To further dissect the LIN28B-RAN relationship, we depleted LIN28B using shRNAs, showing that it reduced RAN RNA and protein levels. LIN28B directly bound RAN mRNA, likely enhancing its translation. As RAN promotes the phosphorylation and activation of Aurora kinase A (AURKA), we then demonstrated that LIN28B leads to AURKA activation via RAN. Moreover, we demonstrated that AURKA is a direct let-7 target, defining a separate mechanism by which LIN28B/let-7 influences AURKA expression. Finally, we showed that RAN depletion resulted in decreased neuroblastoma proliferation, phenocopying LIN28B depletion.

Conclusions: These results demonstrate that enhanced LIN28B expression and chromosome 12q24 gain each independently promote RAN expression and that LIN28B and RAN signaling further converge on AURKA. Collectively, our studies support LIN28B as a master regulator of multiple oncogenes implicated in neuroblastoma pathogenesis, nominating it as a candidate for therapeutic targeting.

Citation Format: Robert W. Schnepp, Priya Khurana, Edward F. Attiyeh, Sara Chodosh, Pichai Raman, Derek A. Oldridge, Maria E. Gagliardi, Karina Conkrite, Shahab Asgharzadeh, Robert C. Seeger, Blair Madison, Anil Rustgi, John M. Maris, Sharon J. Diskin. A LIN28B/RAN/AURKA signaling network promotes neuroblastoma tumorigenesis. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4734. doi:10.1158/1538-7445.AM2015-4734

Targeting MYCN: a good BET for improving neuroblastoma therapy?

Direct targeting of oncogenic MYC proteins has been an elusive goal of many cancer drug development efforts. In this issue of Cancer Discovery, Stegmaier and colleagues demonstrate that pharmacologically interfering with the bromodomain and extraterminal (BET) class of proteins potently depletes MYCN in neuroblastoma cells, resulting in cellular cytotoxicity and thus providing a novel approach with a potential impact on a previously undruggable major oncogene.

Dual CDK4/CDK6 inhibition induces cell-cycle arrest and senescence in neuroblastoma

PURPOSE

Neuroblastoma is a pediatric cancer that continues to exact significant morbidity and mortality. Recently, a number of cell-cycle proteins, particularly those within the Cyclin D/CDK4/CDK6/RB network, have been shown to exert oncogenic roles in neuroblastoma, suggesting that their therapeutic exploitation might improve patient outcomes.

EXPERIMENTAL PROCEDURES

We evaluated the effect of dual CDK4/CDK6 inhibition on neuroblastoma viability using LEE011 (Novartis Oncology), a highly specific CDK4/6 inhibitor.

RESULTS

Treatment with LEE011 significantly reduced proliferation in 12 of 17 human neuroblastoma-derived cell lines by inducing cytostasis at nanomolar concentrations (mean IC50 = 307 ± 68 nmol/L in sensitive lines). LEE011 caused cell-cycle arrest and cellular senescence that was attributed to dose-dependent decreases in phosphorylated RB and FOXM1, respectively. In addition, responsiveness of neuroblastoma xenografts to LEE011 translated to the in vivo setting in that there was a direct correlation of in vitro IC50 values with degree of subcutaneous xenograft growth delay. Although our data indicate that neuroblastomas sensitive to LEE011 were more likely to contain genomic amplification of MYCN (P = 0.01), the identification of additional clinically accessible biomarkers is of high importance.

CONCLUSIONS

Taken together, our data show that LEE011 is active in a large subset of neuroblastoma cell line and xenograft models, and supports the clinical development of this CDK4/6 inhibitor as a therapy for patients with this disease. Clin Cancer Res; 19(22); 6173-82. ©2013 AACR.

Abstract 3809: The role of LIN28B and RAN in promoting neuroblastoma tumorigenesis

Background: : Neuroblastoma is a childhood cancer of the sympathetic nervous system that accounts for approximately 10% of pediatric oncology deaths. The genetic basis of neuroblastoma has begun to be elucidated, with genome-wide association studies uncovering BARD1 and LMO1 as both predisposition genes and oncogenic drivers in a large subset of established neuroblastomas. Recently, our laboratory has shown that germline variation in LIN28B, a master regulator of the let-7 family of tumor suppressor microRNAs, promotes susceptibility to neuroblastoma. Neuroblastomas harboring the risk genotype show high LIN28B expression and this is highly associated with increased cellular proliferation, advanced stage disease and worse patient outcome (Nat. Gen., 2012).

Methods and Results: : To investigate the mechanism by which LIN28B influences the malignant phenotype, we first performed in silico analyses of several newly created and publicly available mRNA expression data sets along with SNP-array based DNA copy number data from 648 primary neuroblastomas. We observed a strong positive correlation between LIN28B and RAN expression in multiple data sets (P = 2.9 x 10−8 to 2.7 x 10−3). RAN is a member of the Ras family of GTPases that participates in mitosis and nuclear trafficking and is implicated as an oncogene in multiple malignancies. Recurrent somatic copy number gain of 12q24, where RAN is located, was observed in 15% (97/648) of neuroblastomas (p &lt; 0.0001), and this gain was associated with increased RAN expression (P = 0.0041), stage 4 (P = 0.0047), high-risk (P = 0.0005), and the MYCN non-amplified subset of high-risk in particular (P = 0.0021). Pathway analysis of the MYCN amplified subset of high-risk neuroblastoma showed that high LIN28B expression was strongly associated with increased RAN signaling (P = 7.1 x 10−8), suggesting LIN28B may regulate RAN activity. Western blotting in a panel of six neuroblastoma cell lines confirmed a tight correlation at the protein level. RNA interference targeting LIN28B resulted in depletion of RAN protein and a potent antiproliferative effect that was phenococpied by siRNA-mediated depletion of RAN.

Conclusions and Future Directions: Our data show that LIN28B expression and 12q24 gain promote the expression of RAN, suggesting two mechanisms that help to drive the malignant phenotype of neuroblastoma. We are currently determining whether LIN28B drives expression of the activated GTP-bound form of RAN. In addition, we are studying whether LIN28B regulates RAN in a let-7 dependent manner. Finally, we plan to determine whether targeting exportin1/XPO1/CRM1, which complexes with RAN in the nuclear membrane to export proteins and mRNA to the cytoplasm, will demonstrate therapeutic effect in neuroblastoma in a LIN28B- and/or RAN-dependent manner.

Citation Format: Robert W. Schnepp, Edward F. Attiyeh, Maria E. Gagliardi, Sara Chodosh, John M. Maris, Sharon J. Diskin. The role of LIN28B and RAN in promoting neuroblastoma tumorigenesis. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3809. doi:10.1158/1538-7445.AM2013-3809

Common variation at BARD1 results in the expression of an oncogenic isoform that influences neuroblastoma susceptibility and oncogenicity

The mechanisms underlying genetic susceptibility at loci discovered by genome-wide association study (GWAS) approaches in human cancer remain largely undefined. In this study, we characterized the high-risk neuroblastoma association at the BRCA1-related locus, BARD1, showing that disease-associated variations correlate with increased expression of the oncogenically activated isoform, BARD1β. In neuroblastoma cells, silencing of BARD1β showed genotype-specific cytotoxic effects, including decreased substrate-adherence, anchorage-independence, and foci growth. In established murine fibroblasts, overexpression of BARD1β was sufficient for neoplastic transformation. BARD1β stabilized the Aurora family of kinases in neuroblastoma cells, suggesting both a mechanism for the observed effect and a potential therapeutic strategy. Together, our findings identify BARD1β as an oncogenic driver of high-risk neuroblastoma tumorigenesis, and more generally, they illustrate how robust GWAS signals offer genomic landmarks to identify molecular mechanisms involved in both tumor initiation and malignant progression. The interaction of BARD1β with the Aurora family of kinases lends strong support to the ongoing work to develop Aurora kinase inhibitors for clinically aggressive neuroblastoma.

Menin-mediated caspase 8 expression in suppressing multiple endocrine neoplasia type 1

Multiple endocrine neoplasia type 1 (MEN1) is a familial tumor syndrome linked to mutation of the MEN1 gene, which encodes a tumor suppressor, menin. We previously reported that menin up-regulates the caspase 8 expression and promotes TNF-alpha-induced apoptosis. However, it remains unclear how menin up-regulates caspase 8 expression and whether menin-mediated caspase 8 expression plays a role in repressing MEN1 development. Here we show that menin binds the 5'-untranslated region (5'-UTR) of the Caspase 8 locus in vivo and activates transcription of a reporter gene through the 5'-UTR. Menin directly binds the 5'-UTR in a sequence-independent manner in vitro. Moreover, Men1 ablation in cells reduces acetylation of histones H3 and H4 at the 5'-UTR of the caspase 8 locus bound by menin in vivo. Notably, the MEN1-derived menin point mutants lose their ability to bind the caspase 8 locus and fail to induce caspase 8 expression and TNF-alpha-mediated apoptosis. Consistent with these observations, the expression level of caspase 8 is markedly reduced in insulinomas from Men1(+/-) mice. Together, our results indicate that menin enhances the caspase 8 expression by binding the caspase 8 locus, and suggest that menin suppresses MEN1 tumorigenesis, at least in part, by up-regulating caspase 8 expression.

Mutation of tumor suppressor gene Men1 acutely enhances proliferation of pancreatic islet cells

Multiple endocrine neoplasia type 1 (MEN1), an inherited tumor syndrome affecting endocrine organs including pancreatic islets, results from mutation of the tumor suppressor gene Men1 that encodes protein menin. Although menin is known to be involved in regulating cell proliferation in vitro, it is not clear how menin regulates cell cycle and whether mutation of Men1 acutely promotes pancreatic islet cell proliferation in vivo. Here we show that excision of the floxed Men1 in mouse embryonic fibroblasts (MEF) accelerates G(0)/G(1) to S phase entry. This accelerated S-phase entry is accompanied by increased cyclin-dependent kinase 2 (CDK2) activity as well as decreased expression of CDK inhibitors p18(Ink4c) and p27(Kip1). Moreover, Men1 excision results in decreased expression of p18(Ink4c) and p27(Kip1) in the pancreas. Furthermore, complementation of menin-null cells with wild-type menin represses S-phase entry. To extend the role of menin in repressing cell cycle in cultured cells to in vivo pancreatic islets, we generated a system in which floxed Men1 alleles can be excised in a temporally controllable manner. As early as 7 days following Men1 excision, pancreatic islet cells display increased proliferation, leading to detectable enlargement of pancreatic islets 14 days after Men1 excision. These observations are consistent with the notion that an acute effect of Men1 mutation is accelerated S-phase entry and enhanced cell proliferation in pancreatic islets. Together, these results suggest a molecular mechanism whereby menin suppresses MEN1 tumorigenesis at least partly through repression of G(0)/G(1) to S transition.

Cardiorespiratory adjustments of homing pigeons to steady wind tunnel flight

We made detailed cardiorespiratory measurements from homing pigeons during quiet rest and steady wind tunnel flight. Our pigeons satisfied their 17.4-fold increase in oxygen consumption during flight with a 7.4-fold increase in cardiac output (Q) and a 2.4-fold increase in blood oxygen extraction. Q was increased primarily by increasing heart rate sixfold. Comparisons between our study and those from the only other detailed cardiorespiratory study on flying birds reveal a number of similarities and important differences. Although the avian allometric equations from this earlier study accurately predicted the flight Q of our pigeons, this was primarily due to due to compensating discrepancies in their heart rate and stroke volume predictions. Additionally, the measured heart mass (MH)-specific Q (Q/MH) of our pigeons during wind tunnel flight was about 22% lower than the estimated value. Compared to running mammals in previous studies, the 1.65-fold Q of our pigeons is consistent with their larger heart mass.

Tumor suppressor menin: the essential role of nuclear localization signal domains in coordinating gene expression

Menin is encoded by the tumor suppressor gene MEN1 that is mutated in patients with an inherited tumor syndrome, multiple endocrine neoplasia type 1 (MEN1). Although menin is a nuclear protein and directly binds to DNA through its nuclear localization signals (NLSs), the precise role for each of the NLSs in nuclear translocation and gene expression remains to be elucidated. Here, we show that point mutations in three individual NLSs, NLS1, NLS2, and a novel accessory NLS, NLSa, do not block nuclear translocation, but compromise the ability of menin to repress expression of the endogenous insulin-like growth factor binding protein-2 (IGFBP-2) gene. This repression is not released by an inhibitor of histone deacetylases. Although subtle mutations in menin NLSs do not affect menin association with chromatin, they abolish menin binding to the IGFBP-2 promoter in vivo. Furthermore, each of the NLSs is also crucial for menin-mediated induction of caspase 8 expression. Together, these results suggest that menin may act as a scaffold protein in coordinating activation and repression of gene transcription and that its NLSs play a more important role in controlling gene transcription than merely targeting menin into the nucleus.

Menin and MLL cooperatively regulate expression of cyclin-dependent kinase inhibitors

Mutations in the MEN1 gene are associated with the multiple endocrine neoplasia syndrome type 1 (MEN1), which is characterized by parathyroid hyperplasia and tumors of the pituitary and pancreatic islets. The mechanism by which MEN1 acts as a tumor suppressor is unclear. We have recently shown that menin, the MEN1 protein product, interacts with mixed lineage leukemia (MLL) family proteins in a histone methyltransferase complex including Ash2, Rbbp5, and WDR5. Here, we show that menin directly regulates expression of the cyclin-dependent kinase inhibitors p27Kip1 and p18Ink4c. Menin activates transcription by means of a mechanism involving recruitment of MLL to the p27Kip1 and p18Ink4c promoters and coding regions. Loss of function of either MLL or menin results in down-regulation of p27Kip1 and p18Ink4c expression and deregulated cell growth. These findings suggest that regulation of cyclin-dependent kinase inhibitor transcription by cooperative interaction between menin and MLL plays a central role in menin's activity as a tumor suppressor.

Functional interaction between tumor suppressor menin and activator of S-phase kinase

Multiple endocrine neoplasia type I (MEN1), a hereditary tumor syndrome, is characterized by the development of tumors in multiple endocrine organs. The gene mutated in MEN1 patients, Men1, encodes a tumor suppressor, menin. Overexpression of menin leads to inhibition of Ras-transformed cells. However, it is unclear whether menin is essential for repression of cell proliferation, and if it is, how it inhibits cell proliferation. Here, we show that targeted disruption of the Men1 gene leads to enhanced cell proliferation, whereas complementation of menin-null cells with menin reduces cell proliferation. Moreover, menin interacts with activator of S-phase kinase (ASK), a component of the Cdc7/ASK kinase complex that is crucial for cell proliferation, but does not appear to alter Cdc7 kinase activity in in vitro kinase assays. We identify the COOH terminus of menin as the domain that mediates the specific interaction with ASK. Notably, wild-type menin completely represses ASK-induced cell proliferation, although it does not obviously affect the steady-state cell cycle profile of ASK-infected cells. Interestingly, disease-related COOH-terminal menin mutants that do not interact with ASK completely fail to repress ASK-induced cell proliferation. Together, these findings demonstrate a functional link between menin and ASK in the regulation of cell proliferation.

Direct binding of DNA by tumor suppressor menin

Menin is a tumor suppressor that is mutated in patients with multiple endocrine neoplasia type I (MEN1), an inherited tumor-prone syndrome. Because there is no obvious conserved structural domain in menin that suggests a biochemical function, little is known as to how menin suppresses tumorigenesis. Although menin interacts with a variety of nuclear proteins including transcription factors, it is unknown whether menin itself can directly bind DNA. Here we show that menin directly binds to double-stranded DNA. It also binds a variety of DNA structures, including Y-structures, branched structures, and 4-way junction structures. The COOH terminus of menin mediates binding to DNA, but MEN1 disease-derived mutations in the COOH terminus abolish the ability of menin to bind DNA. Importantly, these MEN1 disease-related menin mutants also fail to repress cell proliferation as well as cell cycle progression at the G2/M phase. Furthermore, detailed mutagenesis studies indicate that positively charged residues in two nuclear localization signals mediate direct DNA binding as well as repression of cell proliferation. Collectively, these results demonstrate, for the first time, a novel biochemical activity of menin, binding to DNA, and link its DNA binding to the regulation of cell proliferation.

Tumor suppressor menin regulates expression of insulin-like growth factor binding protein 2

Multiple endocrine neoplasia type I (MEN1) is an inherited tumor syndrome characterized by development of tumors in multiple endocrine organs. The gene mutated in MEN1 patients, Men1, encodes a nuclear protein, menin. Menin interacts with several transcription factors and inhibits their activities. However, it is unclear whether menin is essential for the repression of the expression of endogenous genes. Here, using menin-null cells, we show that menin is essential for repression of the endogenous IGFBP-2, a gene that can regulate cell proliferation. Additionally, complementation of menin-null cells with wild-type menin, but not with a MEN1 disease-related point mutant, restores the function of menin in repressing IGFBP-2. Consistent with this, the promoter of IGFBP-2 is repressed by wild-type menin, but not by a MEN1-related point mutant. Menin also alters the structure of the chromatin surrounding the promoter of the IGFBP-2 gene, as demonstrated by the deoxyribonuclease I hypersensitivity assay. Furthermore, nuclear localization signals in menin are crucial for repressing the expression of IGFBP-2. Together, these results suggest that menin regulates the expression of the endogenous IGFBP-2 gene at least in part through the promoter of IGFBP-2.

Menin induces apoptosis in murine embryonic fibroblasts

Multiple endocrine neoplasia type I (MEN1) is a hereditary tumor syndrome characterized by multiple endocrine and occasionally non-endocrine tumors. The tumor suppressor gene Men1, which is frequently mutated in MEN1 patients, encodes the nuclear protein menin. Although many tumor suppressor genes are involved in the regulation of apoptosis, it is unclear whether menin facilitates apoptosis. Here we show that ectopic overexpression of menin via adenoviruses induces apoptosis in murine embryonic fibroblasts. The induction of apoptosis depends on Bax and Bak, two proapoptotic proteins. Moreover, loss of menin expression compromises apoptosis induced by UV irradiation and tumor necrosis factor-alpha (TNF-alpha), whereas complementation of menin-null cells with menin restores sensitivity to UV- and TNF-alpha-induced apoptosis. Interestingly, loss of menin reduces the expression of procaspase 8, a critical protease that is essential for apoptosis induced by death-related receptors, whereas complementation of the menin-null cells up-regulates the expression of procaspase 8. Furthermore, complementation of menin-null cells with menin increases the activation of caspase 8 in response to TNF-alpha treatment. These results suggest a proapoptotic function for menin that may be important in suppressing the development of MEN1.

Menin associates with FANCD2, a protein involved in repair of DNA damage

Multiple endocrine neoplasia type I (MEN1) is an inherited tumor syndrome characterized by tumors in multiple endocrine organs including the parathyroids, pancreatic islets, and the pituitary. The gene mutated in MEN1 patients, Men1, encodes a protein of 610 amino acid residues, menin, and mutations in the Men1 gene lead to the MEN1 syndrome. Although the chromosomal instability in the peripheral lymphocytes from the MEN1 patients has been reported previously, it is not clear whether menin is involved in repair of DNA damage. Here we show that menin specifically interacts with FANCD2, a protein encoded by a gene involved in DNA repair and mutated in patients with an inherited cancer-prone syndrome, Fanconi anemia. The interaction between menin and FANCD2 is enhanced by gamma-irradiation. Moreover, loss of menin expression in mouse embryonic fibroblasts leads to increased sensitivity to DNA damage. Furthermore, menin is localized to chromatin and nuclear matrix, and the association with nuclear matrix is enhanced by gamma-irradiation. Together, these results suggest that menin plays a critical role in repair of DNA damage in concert with FANCD2.

Fusion proteins of retinoid receptors antagonize TGF-beta-induced growth inhibition of lung epithelial cells

Transforming growth factor-beta1 (TGF-beta) is a growth factor that has multiple functions including potent inhibition of cell growth. TGF-beta signals by binding to its cell surface serine/threonine kinase receptors, which in turn phosphorylate downstream signal transducers, Smad2 and Smad3. Phosphorylated Smad2 and Smad3, together with Smad4, enter the nucleus and associate with various transcription factors. This complex of transcription factors regulates transcription of a diverse group of genes, leading to growth arrest at G1 phase. Through a functional expression cloning approach, a gag-retinoid X receptor beta (gag-RXRbeta) fusion protein was found to antagonize TGF-beta-induced growth inhibition of mink lung epithelial cells and the fusion between gag and RXRbeta is essential for resistance to the growth inhibition. Like gag-RXRbeta, the oncogenic PLZF-RARalpha fusion protein also antagonizes TGF-beta-induced growth inhibition, and the fusion between PLZF and RARalpha is essential for resistance to TGF-beta. Moreover, TGF-beta and retinoic acid (RA) cooperatively induce growth inhibition as well as transcription of the p15(ink4b) gene, while PLZF-RARalpha represses TGF-beta-induced expression of the p15(ink4b) gene. Together, these results suggest that the TGF-beta and RA pathways cooperate to inhibit cell growth and that PLZF-RARalpha -mediated resistance to TGF-beta may facilitate the development of the PLZF-RARalpha-induced leukemia.