|Tumor Tropism of Intravenously Injected Human iPSC-derived NSCs and their Gene Therapy Application in a Metastatic Breast Cancer Model|
Neural stem cell (NSC)-based cancer therapeutics show great potential as while they display an inherent tropism for sites of brain injury they also effectively target sites of tumourigenesis through as yet unappreciated mechanisms (Aboody et al 2000 and Aboody et al 2008). For this reason NSCs could be utilised in gene therapy strategies against various types of tumours. To avoid an immune-response from the patient it is also desirable for NSCs to be matched to each patient, a strategy which may be completed through the use of induced pluripotent stem cells (iPSCs) and their subsequent differentiation to NSCs. These ideas prompted the study by researchers from the laboratory of Shu Wang at the University of Singapore in which they demonstrate that tumor-tropic iPSC-derived NSCs can be used to attenuate tumor growth in both immunodeficient and immunocompetent mice (Yang and Lam et al).
Coupling the use of a non-toxic fluorescent dye (DiR) to label cells and non-invasive in vivo imaging (as described in a previous study by Zhao et al), iPS-NSCs could be tracked post-injection in both immunodeficient (NSG) and immunocompetent mice (BALB/c) mice. This analysis demonstrated that tail vein-injected iPS-NSCs initially located to the lung region but after 24 hours they gradually moved towards other organs including the spleen and femur with the signal being stable for 14 days in the NSG mice and 7 days in the BALB/c mice. However, no DiR signals were found in the kidney, heart and brain of NSG mice or the heart in the BALB/c mice. These findings where further confirmed through a human-specific DNA PCR probe, which also demonstrated that the iPS-NSCs mainly accumulated in the liver 28 days following injection.
These findings were next compared to iPS-NSC injections into 4T1 breast cancer-bearing mice to assess the impact of tumourigenesis in the localisation of iPSC-NSCs. Continuous growth of the tumour was noted for both mouse strains over 3 weeks, after which time metastasis was obvious in NSG mice. Injected iPS-NSCs accumulated rapidly at the tumour site in NSG mice and remained at a level of 23% of total signal detected across all organs for 14 days. However, in the BALB/c mice, signals from the iPS-NSCs increased over a 14 day period from 1% to 32% with an accompanying strong signal present in the abdomen suggesting that tumor growth and tumor homing of the stem cells counteract immune rejection of human cell xenografts. Ex vivo analysis also found iPS-NSC signals in the lung and femur in the NSG mice, but not in the BALB/c mice, and confirmed the high DiR signal concentration at the tumour site and the spleen as compared to non-tumour controls of immunodeficient and immunocompetent mice.
The apparent tumour-targeting nature of the injected iPS-NSC was then analysed as a potential anti-tumour strategy in 4T1 tumor-bearing athymic BALB/c nude mice, through the baculoviral transduction of the HSVtk (thymidine kinase) suicide gene into iPS-NSCs prior to their injection alongside intraperitoneal Ganciclovir (GCV) treatment. The implementation of this strategy led to the significant prolongation in the lifespan of the tumour bearing-mice when compared to controls (GCV treatment alone, iPS-NSCs alone, or baculovirus-transduced, eGFP expressing iPS-NSCs). Ex vivo bioluminescence imaging found that the signal decreased in all organs as well as in orthotopic 4T1 breast tumors suggesting that this strategy inhibits tumour growth and attenuates metastasis. The role of the immune system was investigated through the repetition of the above experiment in immunocompetent BALB/c mice bearing syngeneic 4T1 tumors, which also demonstrated an increase in the survival time of the NSCtk/GCV group over controls.
Overall, this study shows that iPSC-derived NSC are capable of homing to sites of tumourigenesis so to function properly as part of a cancer gene therapy application, taking another step towards a potentially non-toxic effective and personalised treatment for a range of cancers. At the time of writing a phase 1 clinical trial of gene therapy for recurrent glioblastoma multiforme using a human NSC cell line to deliver a suicide gene is currently ongoing at the City of Hope Medical Center in Duarte, California, and if this is successful, the application of iPSC-derived NSCs in the same context may be an excellent prospect.