Differentiation of Brain Tumor Initiating Cells

Differentiation is a central key capability of stem cells. Their ability to be multipotent and undergo self-renewal are key identifying features of stem cells. A differentiation assay allows for study of one of the essential features of stem cells, the ability to differentiate into all of the cell types of its lineage, in order to ensure that the cells cultured and utilized in key experiments indeed have stem cell properties. Neural stem cells when plated in differentiation media, differentiate into all three neural lineages: Neurons, Astrocytes, and Oligodendrocytes. Brain tumor initiating cells (BTICs) are cells present in brain tumors that possess stem cell properties and are able to self-renew and differentiate into neural lineages. In the current chapter, we discuss protocols involved in immunofluorescence staining and identification of differentiated cells from BTIC populations.

In Vitro Self-Renewal Assays for Brain Tumor Stem Cells

Early development of human organisms relies on stem cells, a population of non-specialized cells that can divide symmetrically to give rise to two identical daughter cells, or divide asymmetrically to produce one identical daughter cell and another more specialized cell. The capacity to undergo cellular divisions while maintaining an undifferentiated state is termed self-renewal and is responsible for the maintenance of stem cell populations during development. In addition, self-renewal plays a crucial role in the homeostasis of developed organism through replacement of defective cells.Similar to their non-malignant counterparts, it has been postulated that tumor cells follow a differentiation hierarchy, with the least differentiated cells termed cancer stem cells (CSCs) at the apex. These tumor stem cells possess the ability to self-renew, have a higher capacity to initiate tumor growth when xenografted into an animal model, and can recapitulate the cell heterogeneity of the tumor they originate from. Hence, further investigation of mechanisms governing the self-renewal in cancer can lead to development of novel therapies targeting CSCs.In this chapter, we described the soft agar assay and the limiting dilution assay (LDA) as two easy-to-implement and inexpensive assays to measure the stemness properties of brain tumor stem cells (BTSCs). These techniques constitute useful tools for the preclinical evaluation of therapeutic strategies targeting BTSCs clonogenicity.

TAp73 Modifies Metabolism and Positively Regulates Growth of Cancer Stem-Like Cells in a Redox-Sensitive Manner

PURPOSE

Stem-like cancer cells, with characteristic self-renewal abilities, remain highly refractory to various clinical interventions. As such, stemness-inhibiting entities, such as tumor suppressor p53, are therapeutically pursued for their anticancer activities. Interestingly, similar implications for tumor suppressor TAp73 in regulating stemness features within stem-like cancer cells remain unknown.Experimental Design: This study utilizes various in vitro molecular biology techniques, including immunoblotting, qRT-PCR, and mass spectrometry-based proteomics, and metabolomics approaches to study the role of TAp73 in human and murine embryonal carcinoma stem-like cells (ECSLC) as well as human breast cancer stem-like cells (BCSLC). These findings were confirmed using patient-derived brain tumor-initiating cells (BTIC) and in vivo xenograft models.

RESULTS

TAp73 inhibition decreases the expression of stem cell transcription factors Oct4, Nanog, and Sox-2, as well as tumorsphere formation capacity in ECSLCs. In vivo, TAp73-deficient ECSLCs and BCSLCs demonstrate decreased tumorigenic potential when xenografted in mice. Mechanistically, TAp73 modifies the proline regulatory axis through regulation of enzymes GLS, OAT, and PYCR1 involved in the interconversion of proline-glutamine-ornithine. Further, TAp73 deficiency exacerbates glutamine dependency, enhances accumulation of reactive oxygen species through reduced superoxide dismutase 1 (SOD1) expression, and promotes differentiation by arresting cell cycle and elevating autophagy. Most importantly, the knockdown of TAp73 in CD133^HI BTICs, separated from three different glioblastoma patients, strongly decreases the expression of prosurvival factors Sox-2, BMI-1, and SOD1, and profoundly decreases their self-renewal capacity as evidenced through their reduced tumorsphere formation ability.

CONCLUSIONS

Collectively, we reveal a clinically relevant aspect of cancer cell growth and stemness regulation through TAp73-mediated redox-sensitive metabolic reprogramming.

Therapeutic Targeting of the Premetastatic Stage in Human Lung-to-Brain Metastasis

Brain metastases (BM) result from the spread of primary tumors to the brain and are a leading cause of cancer mortality in adults. Secondary tissue colonization remains the main bottleneck in metastatic development, yet this "premetastatic" stage of the metastatic cascade, when primary tumor cells cross the blood-brain barrier and seed the brain before initiating a secondary tumor, remains poorly characterized. Current studies rely on specimens from fully developed macrometastases to identify therapeutic options in cancer treatment, overlooking the potentially more treatable "premetastatic" phase when colonizing cancer cells could be targeted before they initiate the secondary brain tumor. Here we use our established brain metastasis initiating cell (BMIC) models and gene expression analyses to characterize premetastasis in human lung-to-BM. Premetastatic BMIC engaged invasive and epithelial developmental mechanisms while simultaneously impeding proliferation and apoptosis. We identified the dopamine agonist apomorphine to be a potential premetastasis-targeting drug. In vivo treatment with apomorphine prevented BM formation, potentially by targeting premetastasis-associated genes KIF16B, SEPW1, and TESK2 Low expression of these genes was associated with poor survival of patients with lung adenocarcinoma. These results illuminate the cellular and molecular dynamics of premetastasis, which is subclinical and currently impossible to identify or interrogate in human patients with BM. These data present several novel therapeutic targets and associated pathways to prevent BM initiation.Significance: These findings unveil molecular features of the premetastatic stage of lung-to-brain metastases and offer a potential therapeutic strategy to prevent brain metastases. Cancer Res; 78(17); 5124-34. ©2018 AACR.

Abstract 390: Genome-wide CRISPR screens in brain tumor initiating cells (BTICs) identify potent sensitizers and resistors of conventional chemoradiotherapy

Glioblastoma (GBM) is a highly aggressive and most common form of malignant primary brain tumors in adults (WHO grade IV). Despite surgical and therapeutic interventions, including chemotherapy with the alkylating agent Temozolomide (TMZ) and cranial irradiation, GBM relapse is inevitable with a median survival of <15 months. Extensive intratumoral heterogeneity in GBM is believed to be the leading cause of therapy resistance and disease relapse, suggesting that therapy acts as a bottleneck for tumor evolution. Recently, the advent of CRISPR-Cas9 technology has led to the development of genome-wide libraries of sgRNAs capable of introducing insertion-deletion (indels) within exonic regions of genes, leading to a frameshift mutation two-thirds of the time. Here, we present the first genome-wide CRISPR-Cas9 knockout screen in patient-derived GBM BTICs aimed to discover synthetic lethal sensitizers of conventional chemoradiotherapy. Briefly, we performed genome-wide CRISPR-Cas9 screens in treatment-naïve GBM BTICs subjected to in vitro chemotherapy with TMZ and irradiation. By comparing sgRNA dynamics at each doubling period, we were able to identify potent sensitizer genes exclusive to combined chemoradiotherapy, and not TMZ or irradiation alone. Candidate sensitizer genes were validated in an arrayed format to evaluate impact on GBM BTIC self renewal, proliferation, and sensitivity to TMZ and radiation. We aim to further validate these sensitizers of conventional chemoradiotherapy by performing a focused CRISPR-Cas9 genetic screen in our patient-derived xenograft model of treatment-refractory GBM. Ultimately, adjuvants targeting sensitizer genes could greatly enhance the impact of conventional chemoradiotherapy in GBM patients, leading to an increase in patient survival.

Citation Format: Chirayu Chokshi, David Tieu, Kevin Brown, Chitra Venugopal, Parvez Vora, Katherine Chan, Amy Tong, Maleeha Qazi, Mohini Singh, Neil Savage, Andrea Habsid, Jason Moffat, Sheila Singh. Genome-wide CRISPR screens in brain tumor initiating cells (BTICs) identify potent sensitizers and resistors of conventional chemoradiotherapy [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 390.

Abstract 44: Characterization and targeting of a temporal micro-metastatic signature in human brain metastases

Metastases to the brain (BM) are the most common neoplasms to affect the adult central nervous system, arising in 40% of cancer patients and at a rate 10 times greater than primary brain tumors. Despite the prevalence and poor survival rates, therapeutic strategies for BM remain limited, and a substantial cause is the lack of proper preclinical models available to interrogate the intricacies of BM development. Previous work in our lab utilized BM samples from patient-derived lung-to-brain metastases to successfully establish clinically relevant in vivo models of BM. Here we further characterized the cells responsible for BM, termed brain metastasis initiation cells (BMIC). Patient-derived BMICs were injected via intracranial (BT), intracardiac (IC) and intrathoracic (IT) routes into NOD/SCID mice and re-isolated at different stages of metastatic progression. We isolated cells from primary lung tumors (LT), cells injected via intra-thoracic route that crossed the blood brain barrier to seed the brain forming micro-metastases (BMIT), and cells injected via intracardiac route that seeded large macro-metastases (BMIC). Through RNA sequencing we determined cells from BMIT (micro-metastasis stage) to retain a vastly different genetic profile compared to BMICs isolated at other stages of metastasis. Several of these genes belonged to pathways implicated in autonomic central nervous system neoplasms and neural development. Through connectivity mapping of BMIT profiles we discovered drugs that could inhibit the micro-metatsasis signature, and further in vitro validation revealed apomorphine to reduce BMIC sphere formation and proliferation. In vivo treatment with apomorphine blocked both micro- and macro-metastatic stages of our BMIC model. Of 5 BMIT genes identified to be specifically targeted by apomorphine, KIF16B, TESK2 and SEPW1 were shown to have significant value when applied as an independent prognostic signature in a cohort of lung adenocarcinoma patients. Future work will further validate the efficacy of apomorphine in targeting BMICs in primary lung cancer patient samples. With this work we present a possible new avenue for therapeutic targeting toward the prevention of BM development, where it is anticipated to transform a uniformly fatal disease into one that is eminently more treatable.

Citation Format: Mohini Singh, Chitra Venugopal, Tomas Tokar, Nicole McFarlane, Minomi Subhapanditha, David Bakhshinyan, Maleeha Qazi, Parvez Vora, Neil Savage, Naresh K. Murty, Igor Jurisica, Sheila K. Singh. Characterization and targeting of a temporal micro-metastatic signature in human brain metastases [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 44.

Abstract 1763: BiTEs vs CAR-Ts: Preclinical targeting of CD133+ brain tumor initiating cells using immunotherapy-based treatment strategies

Glioblastoma (GBM) is a uniformly fatal primary brain tumor, characterized by extensive cellular heterogeneity. Numerous studies have implicated CD133+ brain tumor initiating cells (BTICs) as drivers of chemo- and radio-resistance in GBM. We have recently demonstrated that a CD133-driven gene signature is predictive of poor overall survival and targeting CD133+ treatment-refractory cells may be an effective strategy to block GBM recurrence.

Bispecific T-Cell engaging antibodies (BiTEs) and Chimeric antigen receptors (CARs) present promising immunotherapeutic approaches that have not yet been validated for recurrent GBM. Using CellectSeq, a novel methodology that combines the use of phage-displayed synthetic antibody libraries and DNA sequencing, we developed the CD133-specific monoclonal antibody ‘RW03'. We constructed CD133-specific BiTEs that consist of two arms; one recognizes the tumor antigen (CD133) and the second is specific to the CD3 antigen that binds to human GBMs and PBMC-derived T cells, respectively. We observed BiTEs redirecting T cells to kill GBMs, with greater efficiency observed in CD133high GBMs, validating BiTE target specificity. Incubating T-cells with BiTEs and the CD133high GBMs resulted in increased expression of T cell activation markers. In parallel, we derived the single chain variable fragment (scFv) from RW03 we engineered a second-generation CAR T cell. CD133-specific CAR-T cells were cytotoxic to CD133+ GBMs. Co-culturing CD133 CAR-T cells with GBMs triggered T cell activation and proliferation. Treatment of GBM tumor-bearing mice with CD133-specific CAR-T cells yielded extended survival in mice and significant reductions in brain tumor burden.

Furthermore, we uniquely adapted the existing chemoradiotherapy protocol for GBM patients for treatment of immunocompromised mice engrafted with human GBMs. Within this model, we have initiated treatment of recurrent GBM directed against CD133+ BTICs, to allow for a direct prospective comparison of toxicity and efficacy of BiTEs and CAR-T cell strategies.

Citation Format: Parvez Vora, Jarrett Adams, Mohini Singh, Chitra Venugopal, Nazanin Tatari, Chirayu Chokshi, Maleeha Qazi, Sabra Salim, Sujeivan Mahendram, David Bakhshinyan, Max London, Neil Savage, Minomi Subapanditha, Nicole McFarlane, James Pan, Jonathan Bramson, Sachdev Sidhu, Jason Moffat, Sheila Singh. BiTEs vs CAR-Ts: Preclinical targeting of CD133+ brain tumor initiating cells using immunotherapy-based treatment strategies [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 1763.

Abstract 1911: Uncovering novel targets of recurrent glioblastoma using transcriptomic profiling in a patient-derived xenograft model

Glioblastoma (GBM) is the most common and aggressive adult primary brain tumor feared for its near uniformly fatal prognosis despite advances in multimodal therapy including surgical resection, chemotherapy and radiation. Poor patient survival due to tumor relapse is thought to be linked to intratumoral heterogeneity (ITH), driven by various environmental cues including chemotherapy and radiation treatment. ITH can be explained at the cellular level by the existence of multiple populations of cancer cells, including cancer stem cells (CSCs), which have acquired stemness properties like self-renewal, proliferation, and multilineage differentiation. In brain tumors, CSCs or brain tumor initiating cells (BTICs), have been shown to be resistant to both chemotherapy and radiation treatment, allowing them to escape therapy and allowing for tumor recurrence. To profile ITH as it evolves through therapy delivery, we have developed a novel and dynamic BTIC patient-derived xenograft (PDX) model of treatment-refractory human GBM, allowing for multimodal profiling of GBM BTICs through tumor engraftment, remission, and recurrence. In this study, we present the transcriptomic profiling at each stage, and novel target selection and validation through CRISPR/Cas9 knockouts, well-established in vitro stem cell assays, and in vivo characterization of their tumor initiation, development, and maintenance properties. Despite the fact that the BTIC population is responsible for GBM recurrence and thus patient demise, it remains a largely unknown landscape. Consequently, therapies that focus on targeting the BTIC compartment within the bulk tumor would provide better treatment and prognosis for patients with brain tumors. The study we present provides a unique therapeutic window into the recurrence of GBM, which drives patient mortality, yet is profiled far less than primary treatment-naïve GBM.

Citation Format: Nicolas Yelle, Chirayu Chokshi, Parvez Vora, Kevin R. Brown, Maleeha A. Qazi, Mohini Singh, Jarrett J. Adams, Chitra Venugopal, Sachdev Sidhu, Jason Moffat, Sheila K. Singh. Uncovering novel targets of recurrent glioblastoma using transcriptomic profiling in a patient-derived xenograft model [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 1911.

Abstract 1140: Genes preserving stem cell state in group 3 medulloblastoma brain tumor initiating cells contribute to therapy evasion and relapse

Medulloblastoma (MB) is the most common malignant pediatric brain tumor. Of current molecular subgroups, Group 3 patients face the highest incidence of metastatic spread and overall patient survival of less than 50%. Current clinical trials for recurrent MB patients based on genomic profiles of primary, treatment-naïve tumors provide limited clinical benefit, since recurrent metastatic MBs are highly genetically divergent from their primary tumors. By adapting the existing COG (Children's Oncology Group) protocol for children with newly diagnosed high-risk MB to the treatment of immuno-deficient mice intracranially engrafted with human MB brain tumor initiating cells (BTICs), we have characterized the rare treatment-refractory cell population in Group 3 MBs. MB cell populations recovered separately from brains and spines during the course of tumor development and therapy were comprehensively profiled for gene expression analysis, stem cell and molecular features to generate a global, comparative profile of MB cells through therapy to relapse. One of the most intriguing observations from our gene expression data was consistent over-expression in the treatment-refractory cell population of proteins belonging to the Inhibitor of DNA-binding/differentiation (ID) family (transcription factors with a basic helix-loop-helix motif that act as suppressors cellular differentiation), and a longevity-associated protein known as bactericidal/permeability-increasing fold-containing-family-B-member-4 (BPIFB4). This persistent upregulation of genes preserving undifferentiated state and cellular longevity further strengthens the hypothesis of stem-cell like cells driving tumor relapse in MB. Targeting ID1 and BPIFB4 using both knockdown (KD) and knockout (KO) strategies has resulted in decreased self-renewal and tumorigenicity of both primary and recurrent MB cells, further highlighting their potential as novel therapeutic targets in MB. Our differential genomic and gene expression profiles of the “treatment-responsive” tumors against those that fail therapy have successfully contributed to discovery and characterization of novel therapeutic targets for the most aggressive subgroup of MB.

Citation Format: Sheila Kumari Singh, David Bakhshinyan, Chitra Venugopal, Ashley Adile, Mohini Singh, Maleeha Qazi, Branavan Manoranjan, Michelle Kameda-Smith. Genes preserving stem cell state in group 3 medulloblastoma brain tumor initiating cells contribute to therapy evasion and relapse [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 1140.

Abstract 148: Canonical Wnt activation as a therapeutic strategy in pediatric medulloblastoma

Brain tumors represent the leading cause of childhood cancer mortality, of which medulloblastoma (MB) is the most frequent malignant pediatric brain tumor. Current molecular subgroups of MB recognize distinct disease entities of which activated Wnt signaling (monosomy 6, exon 3 mutations in CTNNB1, and Wnt gene signature) is associated with a distinct subgroup and the best overall outcome. In contrast, only non-Wnt MBs are characterized by metastatic disease, increased rate of recurrence, and poor overall survivorship. Given the excellent clinical outcome in patients with Wnt-driven MB, we aimed to convert treatment-resistant MB subgroups into an ostensibly benign tumor through selective activation of the canonical Wnt pathway. Initial characterization of patient-derived Wnt and non-Wnt MB lines demonstrated a significant reduction in in vitro self-renewal and proliferative capacity of Wnt MBs. This was further validated by RNA-seq, which identified a marked reduction in the expression of stem cell self-renewal genes Bmi1 and Sox2 in Wnt MBs compared to non-Wnt MBs. Further, Wnt MB-derived xenografts maintained a significant increase in overall survival compared to non-Wnt MB xengrafts, further highlighting the protective nature of activated Wnt signaling in MB. Activated Wnt signaling by way of small molecule Wnt agonists in treatment-refractory MBs resulted in decreased in vitro self-renewal and expression of self-renewal genes, Bmi1 and Sox2. In order to validate the therapy-sensitive nature of Wnt-activated cells, we developed stable patient-derived lines containing a 7XTOPFlash reporter for endogenous Wnt signaling. Rare subclonal Wnt-active cells demonstrated a reduced self-renewal and tumor-initiating capacity through in vivo limiting dilution assays when compared to bulk Wnt-inactive cells. The therapeutic relevance of these findings were demonstrated with an in vivo survival advantage in those mice with orthotopic injections of cells with endogenous Wnt activity when compared to xenografts generated from Wnt-inactive cells. To develop a rationale clinical therapeutic, we used a novel substrate-competitive peptide inhibitor for GSK. Treatment with our peptide inhibitor showed a significant reduction in tumor burden with a corresponding increase in survival of patient-derived tumors that were otherwise treatment-resistant. The clinical utility of our findings is further supported by our analysis of integrated genomics data from 763 primary MBs, in which a validated Wnt gene signature was found to predict improved survivorship among children with poor-outcome and metastatic MBs. Our work establishes activated Wnt signaling as a novel treatment paradigm in childhood MB, identifies a rationale therapeutic approach for recurrent MB, and provides evidence for the context-specific tumor suppressive function of the canonical Wnt pathway.

Citation Format: Sheila Kumari Singh, Branavan Manoranjan, Anna Dvorkin-Gheva, Chitra Venugopal, Steven Moreira, Michelle Kameda-Smith, Minomi Subapanditha, Ashley Adile, David Bakhshinyan, Neil Savage, Blake Yarascavitch, Olufemi Ajani, Adam Fleming, Bradley Doble. Canonical Wnt activation as a therapeutic strategy in pediatric medulloblastoma [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 148.

Cotargeting Ephrin Receptor Tyrosine Kinases A2 and A3 in Cancer Stem Cells Reduces Growth of Recurrent Glioblastoma

Glioblastoma (GBM) carries a dismal prognosis and inevitably relapses despite aggressive therapy. Many members of the Eph receptor tyrosine kinase (EphR) family are expressed by GBM stem cells (GSC), which have been implicated in resistance to GBM therapy. In this study, we identify several EphRs that mark a therapeutically targetable GSC population in treatment-refractory, recurrent GBM (rGBM). Using a highly specific EphR antibody panel and CyTOF (cytometry by time-of-flight), we characterized the expression of all 14 EphR in primary and recurrent patient-derived GSCs to identify putative rGBM-specific EphR. EPHA2 and EPHA3 coexpression marked a highly tumorigenic cell population in rGBM that was enriched in GSC marker expression. Knockdown of EPHA2 and EPHA3 together led to increased expression of differentiation marker GFAP and blocked clonogenic and tumorigenic potential, promoting significantly higher survival in vivo Treatment of rGBM with a bispecific antibody against EPHA2/A3 reduced clonogenicity in vitro and tumorigenic potential of xenografted recurrent GBM in vivo via downregulation of AKT and ERK and increased cellular differentiation. In conclusion, we show that EPHA2 and EPHA3 together mark a GSC population in rGBM and that strategic cotargeting of EPHA2 and EPHA3 presents a novel and rational therapeutic approach for rGBM.Significance: Treatment of rGBM with a novel bispecific antibody against EPHA2 and EPHA3 reduces tumor burden, paving the way for the development of therapeutic approaches against biologically relevant targets in rGBM. Cancer Res; 78(17); 5023-37. ©2018 AACR.

Intratumoral heterogeneity: pathways to treatment resistance and relapse in human glioblastoma

Intratumoral heterogeneity (ITH) has increasingly being described for multiple cancers as the root cause of therapy resistance. Recent studies have started to explore the scope of ITH in glioblastoma (GBM), a highly aggressive and fatal form of brain tumor, to explain its inevitable therapy resistance and disease relapse. In this review, we detail the emerging data that explores the extensive genetic, cellular and functional ITH present in GBM. We discuss current experimental models of human GBM recurrence and suggest harnessing new technologies (CRISPR-Cas9 screening, CyTOF, cellular barcoding, single cell analysis) to delineate GBM ITH and identify treatment-refractory cell populations, thus opening new therapeutic windows. We will also explore why current therapeutics have failed in clinical trials and how ITH can inform us on developing empiric therapies for the treatment of recurrent GBM.

Introduction to Cancer Stem Cells: Past, Present, and Future

The Cancer Stem Cell (CSC) hypothesis postulates the existence of a small population of cancer cells with intrinsic properties allowing for resistance to conventional radiochemotherapy regiments and increased metastatic potential. Clinically, the aggressive nature of CSCs has been shown to correlate with increased tumor recurrence, metastatic spread, and overall poor patient outcome across multiple cancer subtypes. Traditionally, isolation of CSCs has been achieved through utilization of cell surface markers, while the functional differences between CSCs and remaining tumor cells have been described through proliferation, differentiation, and limiting dilution assays. The generated insights into CSC biology have further highlighted the importance of studying intratumoral heterogeneity through advanced functional assays, including CRISPR-Cas9 screens in the search of novel targeted therapies. In this chapter, we review the discovery and characterization of cancer stem cells populations within several major cancer subtypes, recent developments of novel assays used in studying therapy resistant tumor cells, as well as recent developments in therapies targeted at cancer stem cells.

RNAi screen identifies essential regulators of human brain metastasis-initiating cells

Brain metastases (BM) are the most common brain tumor in adults and are a leading cause of cancer mortality. Metastatic lesions contain subclones derived from their primary lesion, yet their functional characterization is limited by a paucity of preclinical models accurately recapitulating the metastatic cascade, emphasizing the need for a novel approach to BM and their treatment. We identified a unique subset of stem-like cells from primary human patient brain metastases, termed brain metastasis-initiating cells (BMICs). We now establish a BMIC patient-derived xenotransplantation (PDXT) model as an investigative tool to comprehensively interrogate human BM. Using both in vitro and in vivo RNA interference screens of these BMIC models, we identified SPOCK1 and TWIST2 as essential BMIC regulators. SPOCK1 in particular is a novel regulator of BMIC self-renewal, modulating tumor initiation and metastasis from the lung to the brain. A prospective cohort of primary lung cancer specimens showed that SPOCK1 was overexpressed only in patients who ultimately developed BM. Protein-protein interaction network mapping between SPOCK1 and TWIST2 identified novel pathway interactors with significant prognostic value in lung cancer patients. Of these genes, INHBA, a TGF-β ligand found mutated in lung adenocarcinoma, showed reduced expression in BMICs with knockdown of SPOCK1. In conclusion, we have developed a useful preclinical model of BM, which has served to identify novel putative BMIC regulators, presenting potential therapeutic targets that block the metastatic process, and transform a uniformly fatal systemic disease into a locally controlled and eminently more treatable one.

Brain tumor initiating cells: with great technology will come greater understanding

The discovery of the brain tumor initiating cells resulted in a paradigm shift within the cancer research community to consider brain tumors as an outcome of developmental mechanisms gone awry. This review will guide the reader through the technological advances that hold the powerful potential to allow brain cancer researchers to develop an intimate understanding of the dynamic and complex mechanism governing brain tumor behavior.