Oncolytic virus as a cancer stem cell killer: progress and challenges

Introduction to cancer stem cells (CSCs)

CSCs were not isolated until the mid-1990s, but the concept was proposed by two German pathologists, Rudolph Virchow and Julius Cohnheim, more than one and a half centuries ago. The two scientists both noted histological similarities between developing fetuses and certain cancers (1), and then propose that cancer cells might be originated from embryonic tissue. Due to the limited development of technology back then, a formal proof of this conceptualization had not arisen until 1963 when Bruce et al. observed that only a very small portion of lymphoma cells could form in vitro colonies and initiate tumorigenesis in a xenograft transplant (2). A few decades later, in 1994, Dick and colleagues demonstrated that human acute myeloid leukemia (AML) could be regenerated after transplantation of leukemic stem cells possessing CD34⁺CD38^– cell surface markers in non-obese diabetic mice (3). Although the occurrence of CSCs was rare (~1/10⁶), a single leukemic stem cell could give rise to the repertoire of populations of leukemia cells (4). Since then, CSCs have been isolated from a number of tumor types, including brain, breast, colon, ovarian, head and neck, pancreas and liver (5-13). According to the CSC hypothesis, only a small subpopulation of cells is tumorigenic: that is, only CSCs with the ability of self-renewal and self-differentiation can give rise to the variety of rapidly proliferating and differentiated cells that make up the bulk of a tumor.

Till now, the most common way to identify and isolate CSCs is to use the phenotypic markers, based on the expression of which, flow cytometry and magnetic-activated cell separation are widely used to enrich CSCs (14,15). Additionally, functional assays, such as serial transplantation in animal models, have also been applied complementarily to define CSCs. Typically, cells are isolated from existing tumors by using surface markers or enzymatic activity, and then tested for their ability to form tumor spheroids in vitro or tumors in secondary recipients in vivo when orthotopically xenografted into immunodeficiency mice (14).

Despite the progress of CSC research, CSC hypothesis is still under debates due to the following two major concerns. First, many CSC markers identified are demonstrated not exclusively limited to CSC populations, but also to healthy tissue (16), which casts doubt on the reliability of using surface biomarkers or enzymatic activity to identify CSCs. Besides, recent discovery also proposed a dynamic state of marker expression on CSCs, that is, CSC populations may only transiently express the CSC markers (17). Therefore, in order to further elucidate the cellular biology of CSCs, better understanding is required of both the specificity and stability of the CSC markers.

The second concern was raised by a study by Kelly et al. group. They demonstrated that when lymphomas and leukemias of mouse origin are transplanted into histocompatible mice, a very high frequency of tumor cells (1 in 10) could seed new tumor growth (18). Obviously, their findings challenge the idea that the tumor growth is driven by a rare subpopulation of tumor cells, indicating the limitations of human tumor cells’ ability to grow in a foreign (mouse) microenvironment.

Regardless of the uncertainty of the specificity and stability of CSC markers, as well as the debates on the frequency of CSC populations, CSC hypothesis is under dynamic refined as research progresses. With growing insight into the mechanism of CSC development, function, and interplay with the tumor microenvironment, CSCs hold great promise as a therapeutic target for reducing relapse rates and improving long-term outcome for patients with many types of cancers.

CSCs vs. normal stem cells

The term “CSCs” was originally coined according to its similarity to bona fide normal stem cells (19). Like normal stem cells, CSCs also have the ability to self-renew and self-differentiate, by which, they are able to produce daughter cells that constitute the bulk of a tumor similar to the way normal stem cells generate an organ. In particular, CSCs express similar markers to normal stem cells in the same tissues (19-21), such as CD34 in both leukemia CSCs and normal hematopoietic stem cells (3,22), as well as CD133 in brain cancer and normal neural stem cells (NSC) (23-26). Besides, both CSCs and normal stem cells share similar pathways for their proliferation and self-renewal, such as Hedgehog, Wnt and Notch pathways (27-30).

Another feature CSCs share with normal stem cells is a specialized microenvironment, named niche. In 1978, Schofield first proposed the existence of a niche for hematopoietic stem cells that serves as the key regulator of the balance between self-renewal and differentiation (31). The regulation of the niche occurs through cell-to-cell interactions, the extracellular matrix, and the balance of signals received from embryonic signaling pathways (32-34). For example, the NSCs lie within a vascular niche, in which endothelial cells secrete factors that promote stem cell survival and self-renewal and are thought to be a key component of the NSC niche (35-38). Similarly, endothelial cells interact selectively with Nestin⁺/CD133⁺ brain cancer cells that include the CSC fraction. Increasing the number of endothelial cells expanded the numbers of self-renewing Nestin⁺/CD133⁺ cancer cells and accelerated the initiation and growth of tumors, and vice versa (39).

The term cancer “stem cell” harbors the hypothesis that cells in the tumorigenic fraction harbor all properties of normal stem cells. However, whether this is true is the subject of much debate. One difference between normal stem cells and CSCs might be their relationship with niches. Stem cells exist within protective niches composed of a number of differentiated cell types that provide direct cell contacts and secreted factors to maintain stem cells primarily in a quiescent state (40,41). However, it was proposed that CSCs might escape from the niche control with either intrinsic mutations or extrinsic aberrant niche microenvironment (42,43). If it is true that CSCs depend upon aberrant niches, then these niches might represent targets for treatments of cancer.

In 2006, Yilmaz et al. group reported a solid piece of evidence showing the difference between CSCs and normal stem cells. They find that Pten deletion causes the generation of transplantable leukemia CSCs, but results in the depletion of normal hematopoietic stem cells (44). Their findings provide a potential therapeutic target pathway that has distinct effects on normal stem cells and CSCs within the same tissue.

CSCs are resistant to conventional cancer therapies

The mechanisms by which CSCs are resistant to conventional therapies are very diverse, including slow cell cycle kinetics, efficient DNA repair, upregulation of anti-apoptotic proteins and multidrug resistance type membrane transporters (13,16,45-47). All these evasion mechanisms enable CSCs to repopulate the tumor mass, leading to recurrence of a cancer.

Interestingly, most of the known resistance mechanisms of CSCs to conventional therapies are involved in the stem-like properties embodied by CSCs, such as relative quiescence and high-level of drug efflux transporters. The quiescence allows CSCs escape drugs that are designed only to highly proliferative cells (other cancer cells), such as DNA synthesis inhibitors (16,48); and the upregulation of membrane transporters, such as ATP-binding cassette protein efflux pump ABCG2, makes them drug resistant (20,21). Therefore, the similarity between CSCs and normal stem cells confound an already difficult situation of cancer therapies, that is, besides the concern with the differentiation of CSCs from other tumor cells and normal cells, what is in also great need is a deeper delving into the unique properties of CSCs different from normal stem cells.

Introduction to oncolytic viruses (OVs)

The historical record of OV therapy began in 1912, when DePace reported tumor regression after inoculation of an attenuated rabies vaccine (49), inspired by which, Levaditi and Nicolau, reported viral oncolysis in the mouse in 1922 (50). A human trial was not documented until 1950, when Pack used an attenuated rabies virus against melanoma and yielded a remarkable partial remission (51). After that, numerous emerging OVs were used to treat patients suffering from various types of cancer (52-55). Such viruses fall into three broad categories: (I) wild-type viruses that do not typically infect human cells but are cytotoxic to human cancer cells; (II) attenuated viruses, either by mutations or serial passage, specifically targeting cancer cells; (III) larger viruses that can be armed to express transgenes (16,56).

OVs target cancer cells in four ways: (I) OVs kill tumor cells that contain altered signaling pathways different from normal cells; (II) OVs attack tumor cells due to either genetically engineered mutations or harnessed foreign genes that directly or indirectly result in cancer cell death; (III) OVs infection of cancer cells induces antitumor immunity (57-59); (IV) In relatively rare cases, viruses produce cytotoxic protein during replication in cells, with the protein itself then causing damage to the cell. For example, adenoviruses express the E3 11.6-kDa death protein that is cytotoxic to cells (60).

Targeting CSCs with OVs

OVs kill cancer cells in a way that differs from traditional therapies and thus are able to elude the typical mechanisms in which CSCs are used to resist current chemotherapy and radiotherapy. So far, various OVs and mutant strains have been reported to successfully target different types of CSCs. The list could be found in two review articles (16,57). Here, I will focus on the application of harnessed OVs, and the combination use of OVs and conventional cancer therapies.

Besides initial screening of available OVs for their potentials of killing CSCs, genetic manipulation of OVs has been used to enhance either the efficiency or the specificity. Basically, there are four strategies to be considered. First, engineered OVs are designed to have increased tumor penetration. For example, Zhu et al. used a herpes simplex virus (HSV) carrying endostatin-angiostatin (VAE) to target glioblastoma-derived CSCs (61). VAE specifically inhibits the secretion of vascular endothelial cells and fibroblasts growth factors and consequently disrupts the vascular niche, therefore exposing GSCs for the virus attack. Similarly, a chondroitinase adenosine triphosphate-binding cassette removes the chondroitin sulfate from the tumor extracellular matrix proteoglycans, which also helps HSV spread throughout the glioma spheroids more efficiently and thus enhances anti-tumor activity in vivo (62).

Second, given that cancer cell antigen is a weak antigen for host immune sensitization, OVs armed with antitumor immunity factors show promises as well. An HSV armed with talimogene laherparepvec, a cytokine granulocyte macrophage colony stimulating factor, showed promises on metastatic melanoma (63). Another HSV, when armed with murine IL-12, lysed human glioblastoma (CD133⁺) CSC-like cells (GSCs) and targeted mouse GSC in vivo (64).

Third, as virus replication is optimal in highly proliferating cells, the ability to induce programmed cell death is likely to increase the therapeutic potential of the OVs for CSCs, which are largely quiescent cells. For example, a multimutated HSV synergized with the PI3K/Akt pathway inhibitors in killing CSCs through enhanced apoptosis (65).

Lastly, CD133⁺-targeted measles viruses enhanced oncolytic activity of CD133⁺ specific viruses when compared with the unmodified ones (66).

As for the combination of OVs with chemo- or radiation therapy, it will likely to achieve superior outcome of eliminating CSCs. OVs may sensitize cells to radiation, and radiation can enhance viral infection, replication, and gene expression, resulting in greater tumor cell death (67). Low-dose chemotherapy with agents like cyclophosphamide can reduce the antiviral immune response and thus enhance oncolysis (68). Besides, virotherapy may complement high-dose chemotherapy that causes toxic side effect. For example, the combination of low-dose etoposide with G47Δ significantly kills GSC-derived tumors (69), and so does the combination of alkylating agent temozolomide (TMZ) and G47Δ in killing GSCs (70).

Challenges and perspectives

The designs of conventional therapies are usually based on tumor subtypes, stage, grade and other molecular files. However, due to the tumor heterogeneity, both within the tumor and between patients, the efficacy of current treatment strategies is limited. In addition, chemo and radio-resistance and subsequent patient relapse are still a big concern. By contrast, OVs kill cancer cells, including CSCs in a way that is irrespective of tumor subtypes, and also with minimal toxic side effects to their normal cell and tissue counterparts. Apparently, OVs provide a lot of excitement about developing therapies that target CSCs, but great amount of efforts still need to be put in toward a more successful clinic translation.

To begin with, a better understanding of CSC biology is required for the development of OV therapy that specifically targets CSCs. It requires a more accurate differentiation of CSCs from other tumor cells, normal cells and particularly normal stem cells so that OV may specifically target CSCs. The identification of CSCs includes unique surface antigens, the niche and signaling pathways. Besides, it is equally important to investigate the origins of CSCs and the occurring frequency of CSCs in a broad range of human cancers. It is possible that the number of CSCs may directly correlate with the stage of tumor development. In that case, the occurrence and abundance of CSCs in primary and metastatic, especially in benign tumors should be different. In addition to the intrinsic driving forces, an extrinsic stimulation, such as chemotherapy, may also regulate the numbers of CSCs, or the biological features of CSCs. Addressing all these questions will present a multifaceted landscape of CSCs.

Another way to improve the efficiency of OV includes enhancing virus delivery, replication and increasing the tumor-directed immune response. In addition, the combination therapy with chemotherapeutics, radiation, monoclonal antibodies, and small-molecule inhibitors may also achieve better outcomes. But currently, except some armed viruses designed specifically for targeting CSCs, most studies use viruses that generally kill both cancer cells and CSCs. Of course, with better understanding of CSCs, OVs could be harnessed to meet the specificity, but discovery of new OVs is also important. Further, although physical and physiological barriers within the tumor microenvironment limit viral spread to CSCs, it is possible to turn these barriers into incubators, that is, by using the unique features of CSC niche, a OV can be designed to replicate only when it reaches the CSC niche.

The last but not the least challenge for the researchers is to discover the unique mechanisms of OVs killing CSCs. Most studies have been focusing on the phenotypic features of OVs on CSCs, and only a few have reported the underlying mechanisms, such as induction of autophagy and robust of DNA damages (70,71). However, these observations on mechanisms are not exclusive to CSCs, which limits the significance.

Acknowledgements

None.

Footnote

Conflicts of Interest: The author has no conflicts of interest to declare.

References

1. Pappenheim A. Prinzipen der neuren Morphologischen Haematozytologie nach zytogenetischer Grundlage. Folia Haematol 1917;21:91-101.
2. Bruce WR, van der Gaag H. A quantitative assay for the number of murine lymphoma cells capable of proliferation in vivo. Nature 1963;199:79-80. [PubMed]
3. Lapidot T, Sirard C, Vormoor J, et al. A cell initiating human acute myeloid leukaemia after transplantation into SCID mice. Nature 1994;367:645-8. [PubMed]
4. Hope KJ, Jin L, Dick JE. Acute myeloid leukemia originates from a hierarchy of leukemic stem cell classes that differ in self-renewal capacity. Nat Immunol 2004;5:738-43. [PubMed]
5. Bao S, Wu Q, McLendon RE, et al. Glioma stem cells promote radioresistance by preferential activation of the DNA damage response. Nature 2006;444:756-60. [PubMed]
6. Tan S, Chen JS, Sun LJ, et al. Selective enrichment of hepatocellular cancer stem cells by chemotherapy. J Int Med Res 2009;37:1046-56. [PubMed]
7. Ricci-Vitiani L, Lombardi DG, Pilozzi E, et al. Identification and expansion of human colon-cancer-initiating cells. Nature 2007;445:111-5. [PubMed]
8. Li C, Lee CJ, Simeone DM. Identification of human pancreatic cancer stem cells. Methods Mol Biol 2009;568:161-73. [PubMed]
9. Dalerba P, Dylla SJ, Park IK, et al. Phenotypic characterization of human colorectal cancer stem cells. Proc Natl Acad Sci U S A 2007;104:10158-63. [PubMed]
10. Collins AT, Berry PA, Hyde C, et al. Prospective identification of tumorigenic prostate cancer stem cells. Cancer Res 2005;65:10946-51. [PubMed]
11. Dontu G, El-Ashry D, Wicha MS. Breast cancer, stem/progenitor cells and the estrogen receptor. Trends Endocrinol Metab 2004;15:193-7. [PubMed]
12. Prince ME, Sivanandan R, Kaczorowski A, et al. Identification of a subpopulation of cells with cancer stem cell properties in head and neck squamous cell carcinoma. Proc Natl Acad Sci U S A 2007;104:973-8. [PubMed]
13. Friedman GK, Gillespie GY. Cancer Stem Cells and Pediatric Solid Tumors. Cancers (Basel) 2011;3:298-318. [PubMed]
14. Clarke MF, Dick JE, Dirks PB, et al. Cancer stem cells--perspectives on current status and future directions: AACR Workshop on cancer stem cells. Cancer Res 2006;66:9339-44. [PubMed]
15. Visvader JE, Lindeman GJ. Cancer stem cells in solid tumours: accumulating evidence and unresolved questions. Nat Rev Cancer 2008;8:755-68. [PubMed]
16. Smith TT, Roth JC, Friedman GK, et al. Oncolytic viral therapy: targeting cancer stem cells. Oncolytic Virother 2014;2014:21-33.
17. Meyer MJ, Fleming JM, Ali MA, et al. Dynamic regulation of CD24 and the invasive, CD44posCD24neg phenotype in breast cancer cell lines. Breast Cancer Res 2009;11:R82. [PubMed]
18. Kelly PN, Dakic A, Adams JM, et al. Tumor growth need not be driven by rare cancer stem cells. Science 2007;317:337. [PubMed]
19. Jordan CT. Cancer stem cells: controversial or just misunderstood? Cell Stem Cell 2009;4:203-5. [PubMed]
20. Islam MO, Kanemura Y, Tajria J, et al. Functional expression of ABCG2 transporter in human neural stem/progenitor cells. Neurosci Res 2005;52:75-82. [PubMed]
21. Donnenberg VS, Donnenberg AD. Multiple drug resistance in cancer revisited: the cancer stem cell hypothesis. J Clin Pharmacol 2005;45:872-7. [PubMed]
22. Bonnet D, Dick JE. Human acute myeloid leukemia is organized as a hierarchy that originates from a primitive hematopoietic cell. Nat Med 1997;3:730-7. [PubMed]
23. Hemmati HD, Nakano I, Lazareff JA, et al. Cancerous stem cells can arise from pediatric brain tumors. Proc Natl Acad Sci U S A 2003;100:15178-83. [PubMed]
24. Lessard J, Sauvageau G. Bmi-1 determines the proliferative capacity of normal and leukaemic stem cells. Nature 2003;423:255-60. [PubMed]
25. Park IK, Qian D, Kiel M, et al. Bmi-1 is required for maintenance of adult self-renewing haematopoietic stem cells. Nature 2003;423:302-5. [PubMed]
26. Singh SK, Clarke ID, Terasaki M, et al. Identification of a cancer stem cell in human brain tumors. Cancer Res 2003;63:5821-8. [PubMed]
27. Taipale J, Beachy PA. The Hedgehog and Wnt signalling pathways in cancer. Nature 2001;411:349-54. [PubMed]
28. Reya T, Morrison SJ, Clarke MF, et al. Stem cells, cancer, and cancer stem cells. Nature 2001;414:105-11. [PubMed]
29. Pardal R, Clarke MF, Morrison SJ. Applying the principles of stem-cell biology to cancer. Nat Rev Cancer 2003;3:895-902. [PubMed]
30. Molofsky AV, Pardal R, Morrison SJ. Diverse mechanisms regulate stem cell self-renewal. Curr Opin Cell Biol 2004;16:700-7. [PubMed]
31. Schofield R. The relationship between the spleen colony-forming cell and the haemopoietic stem cell. Blood Cells 1978;4:7-25. [PubMed]
32. Moore SW. Developmental genes and cancer in children. Pediatr Blood Cancer 2009;52:755-60. [PubMed]
33. Das B, Tsuchida R, Malkin D, et al. Hypoxia enhances tumor stemness by increasing the invasive and tumorigenic side population fraction. Stem Cells 2008;26:1818-30. [PubMed]
34. Sneddon JB, Werb Z. Location, location, location: the cancer stem cell niche. Cell Stem Cell 2007;1:607-11. [PubMed]
35. Louissaint A Jr, Rao S, Leventhal C, et al. Coordinated interaction of neurogenesis and angiogenesis in the adult songbird brain. Neuron 2002;34:945-60. [PubMed]
36. Shen Q, Goderie SK, Jin L, et al. Endothelial cells stimulate self-renewal and expand neurogenesis of neural stem cells. Science 2004;304:1338-40. [PubMed]
37. Ramírez-Castillejo C, Sánchez-Sánchez F, Andreu-Agulló C, et al. Pigment epithelium-derived factor is a niche signal for neural stem cell renewal. Nat Neurosci 2006;9:331-9. [PubMed]
38. Palmer TD, Willhoite AR, Gage FH. Vascular niche for adult hippocampal neurogenesis. J Comp Neurol 2000;425:479-94. [PubMed]
39. Calabrese C, Poppleton H, Kocak M, et al. A perivascular niche for brain tumor stem cells. Cancer Cell 2007;11:69-82. [PubMed]
40. Fuchs E, Tumbar T, Guasch G. Socializing with the neighbors: stem cells and their niche. Cell 2004;116:769-78. [PubMed]
41. Moore KA, Lemischka IR. Stem cells and their niches. Science 2006;311:1880-5. [PubMed]
42. Wodarz A, Gonzalez C. Connecting cancer to the asymmetric division of stem cells. Cell 2006;124:1121-3. [PubMed]
43. Clarke MF, Fuller M. Stem cells and cancer: two faces of eve. Cell 2006;124:1111-5. [PubMed]
44. Yilmaz OH, Valdez R, Theisen BK, et al. Pten dependence distinguishes haematopoietic stem cells from leukaemia-initiating cells. Nature 2006;441:475-82. [PubMed]
45. Chuthapisith S, Eremin J, El-Sheemey M, et al. Breast cancer chemoresistance: emerging importance of cancer stem cells. Surg Oncol 2010;19:27-32. [PubMed]
46. Eramo A, Ricci-Vitiani L, Zeuner A, et al. Chemotherapy resistance of glioblastoma stem cells. Cell Death Differ 2006;13:1238-41. [PubMed]
47. Labialle S, Gayet L, Marthinet E, et al. Transcriptional regulators of the human multidrug resistance 1 gene: recent views. Biochem Pharmacol 2002;64:943-8. [PubMed]
48. Raguz S, Yagüe E. Resistance to chemotherapy: new treatments and novel insights into an old problem. Br J Cancer 2008;99:387-91. [PubMed]
49. DePace N. Sulla scomparsa di un enorme cancro vegetante del collo dell’utero senza cura chirurgica. Ginecologia 1912;9:82-9.
50. Levaditi C, Nicolau S. Sur le culture du virus vaccinal dans les neoplasmes epithelieux. CR Soc Biol 1922;86:928.
51. Pack GT. Note on the experimental use of rabies vaccine for melanomatosis. AMA Arch Derm Syphilol 1950;62:694-5. [PubMed]
52. Newman W, Southam CM. Virus treatment in advanced cancer; a pathological study of fifty-seven cases. Cancer 1954;7:106-18. [PubMed]
53. Southam CM, Noyes WF, Mellors R. Virus in human cancer cells in vivo. Virology 1958;5:395-8. [PubMed]
54. Southam CM, Hilleman MR, Werner JH. Pathogenicity and oncolytic capacity of RI virus strain RI-67 in man. J Lab Clin Med 1956;47:573-82. [PubMed]
55. Southam CM, Moore AE. Induced virus infections in man by the Egypt isolates of West Nile virus. Am J Trop Med Hyg 1954;3:19-50. [PubMed]
56. Kirn DH, Thorne SH. Targeted and armed oncolytic poxviruses: a novel multi-mechanistic therapeutic class for cancer. Nat Rev Cancer 2009;9:64-71. [PubMed]
57. Cripe TP, Wang PY, Marcato P, et al. Targeting cancer-initiating cells with oncolytic viruses. Mol Ther 2009;17:1677-82. [PubMed]
58. Ikeda K, Ichikawa T, Wakimoto H, et al. Oncolytic virus therapy of multiple tumors in the brain requires suppression of innate and elicited antiviral responses. Nat Med 1999;5:881-7. [PubMed]
59. Wakimoto H, Johnson PR, Knipe DM, et al. Effects of innate immunity on herpes simplex virus and its ability to kill tumor cells. Gene Ther 2003;10:983-90. [PubMed]
60. Zou A, Atencio I, Huang WM, et al. Overexpression of adenovirus E3-11.6K protein induces cell killing by both caspase-dependent and caspase-independent mechanisms. Virology 2004;326:240-9. [PubMed]
61. Zhu G, Su W, Jin G, et al. Glioma stem cells targeted by oncolytic virus carrying endostatin-angiostatin fusion gene and the expression of its exogenous gene in vitro. Brain Res 2011;1390:59-69. [PubMed]
62. Dmitrieva N, Yu L, Viapiano M, et al. Chondroitinase ABC I-mediated enhancement of oncolytic virus spread and antitumor efficacy. Clin Cancer Res 2011;17:1362-72. [PubMed]
63. Senzer NN, Kaufman HL, Amatruda T, et al. Phase II clinical trial of a granulocyte-macrophage colony-stimulating factor-encoding, second-generation oncolytic herpesvirus in patients with unresectable metastatic melanoma. J Clin Oncol 2009;27:5763-71. [PubMed]
64. Cheema TA, Wakimoto H, Fecci PE, et al. Multifaceted oncolytic virus therapy for glioblastoma in an immunocompetent cancer stem cell model. Proc Natl Acad Sci U S A 2013;110:12006-11. [PubMed]
65. Kanai R, Wakimoto H, Martuza RL, et al. A novel oncolytic herpes simplex virus that synergizes with phosphoinositide 3-kinase/Akt pathway inhibitors to target glioblastoma stem cells. Clin Cancer Res 2011;17:3686-96. [PubMed]
66. Bach P, Abel T, Hoffmann C, et al. Specific elimination of CD133+ tumor cells with targeted oncolytic measles virus. Cancer Res 2013;73:865-74. [PubMed]
67. Touchefeu Y, Vassaux G, Harrington KJ. Oncolytic viruses in radiation oncology. Radiother Oncol 2011;99:262-70. [PubMed]
68. Kumar S, Gao L, Yeagy B, et al. Virus combinations and chemotherapy for the treatment of human cancers. Curr Opin Mol Ther 2008;10:371-9. [PubMed]
69. Cheema TA, Kanai R, Kim GW, et al. Enhanced antitumor efficacy of low-dose Etoposide with oncolytic herpes simplex virus in human glioblastoma stem cell xenografts. Clin Cancer Res 2011;17:7383-93. [PubMed]
70. Kanai R, Rabkin SD, Yip S, et al. Oncolytic virus-mediated manipulation of DNA damage responses: synergy with chemotherapy in killing glioblastoma stem cells. J Natl Cancer Inst 2012;104:42-55. [PubMed]
71. Colunga A, Bollino D, Schech A, et al. Calpain-dependent clearance of the autophagy protein p62/SQSTM1 is a contributor to ΔPK oncolytic activity in melanoma. Gene Ther 2014;21:371-8. [PubMed]