Combined preconditioning and in vivo chemoselection with 6-thioguanine alone achieves highly efficient reconstitution of normal hematopoiesis with HPRT-deficient bone marrow

In vivo selection of anti-HIV-1 gene-modified human hematopoietic stem/progenitor cells to enhance engraftment and HIV-1 inhibition

Hematopoietic stem/progenitor cell (HSPC)-based anti-HIV-1 gene therapy holds great promise to eradicate HIV-1 or to provide long-term remission through a continuous supply of anti-HIV-1 gene-modified cells without ongoing antiretroviral therapy. However, achieving sufficient engraftment levels of anti-HIV gene-modified HSPC to provide therapeutic efficacy has been a major limitation. Here, we report an in vivo selection strategy for anti-HIV-1 gene-modified HSPC by introducing 6-thioguanine (6TG) chemoresistance through knocking down hypoxanthine-guanine phosphoribosyl transferase (HPRT) expression using RNA interference (RNAi). We developed a lentiviral vector capable of co-expressing short hairpin RNA (shRNA) against HPRT alongside two anti-HIV-1 genes: shRNA targeting HIV-1 co-receptor CCR5 and a membrane-anchored HIV-1 fusion inhibitor, C46, for efficient in vivo selection of anti-HIV-1 gene-modified human HSPC. 6TG-mediated preconditioning and in vivo selection significantly enhanced engraftment of HPRT-knockdown anti-HIV-1 gene-modified cells (>2-fold, p < 0.0001) in humanized bone marrow/liver/thymus (huBLT) mice. Viral load was significantly reduced (>1 log fold, p < 0.001) in 6TG-treated HIV-1-infected huBLT mice compared to 6TG-untreated mice. We demonstrated that 6TG-mediated preconditioning and in vivo selection considerably improved engraftment of HPRT-knockdown anti-HIV-1 gene-modified HSPC and repopulation of anti-HIV-1 gene-modified hematopoietic cells in huBLT mice, allowing for efficient HIV-1 inhibition.

Rare Opportunities: CRISPR/Cas-Based Therapy Development for Rare Genetic Diseases

Rare diseases pose a global challenge, in that their collective impact on health systems is considerable, whereas their individually rare occurrence impedes research and development of efficient therapies. In consequence, patients and their families are often unable to find an expert for their affliction, let alone a cure. The tide is turning as pharmaceutical companies embrace gene therapy development and as serviceable tools for the repair of primary mutations separate the ability to create cures from underlying disease expertise. Whereas gene therapy by gene addition took decades to reach the clinic by incremental disease-specific refinements of vectors and methods, gene therapy by genome editing in its basic form merely requires certainty about the causative mutation. Suddenly we move from concept to trial in 3 years instead of 30: therapy development in the fast lane, with all the positive and negative implications of the phrase. Since their first application to eukaryotic cells in 2013, the proliferation and refinement in particular of tools based on clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) prokaryotic RNA-guided nucleases has prompted a landslide of therapy-development studies for rare diseases. An estimated thousands of orphan diseases are up for adoption, and legislative, entrepreneurial, and research initiatives may finally conspire to find many of them a good home. Here we summarize the most significant recent achievements and remaining hurdles in the application of CRISPR/Cas technology to rare diseases and take a glimpse at the exciting road ahead.

Towards in vivo amplification: Overcoming hurdles in the use of hematopoietic stem cells in transplantation and gene therapy

With the advent of safer and more efficient gene transfer methods, gene therapy has become a viable solution for many inherited and acquired disorders. Hematopoietic stem cells (HSCs) are a prime cell compartment for gene therapy aimed at correcting blood-based disorders, as well as those amenable to metabolic outcomes that can effect cross-correction. While some resounding clinical successes have recently been demonstrated, ample room remains to increase the therapeutic output from HSC-directed gene therapy. In vivo amplification of therapeutic cells is one avenue to achieve enhanced gene product delivery. To date, attempts have been made to provide HSCs with resistance to cytotoxic drugs, to include drug-inducible growth modules specific to HSCs, and to increase the engraftment potential of transduced HSCs. This review aims to summarize amplification strategies that have been developed and tested and to discuss their advantages along with barriers faced towards their clinical adaptation. In addition, next-generation strategies to circumvent current limitations of specific amplification schemas are discussed.

Multiplex Genome-Edited T-cell Manufacturing Platform for "Off-the-Shelf" Adoptive T-cell Immunotherapies

Adoptive immunotherapy using autologous T cells endowed with chimeric antigen receptors (CAR) has emerged as a powerful means of treating cancer. However, a limitation of this approach is that autologous CAR T cells must be generated on a custom-made basis. Here we show that electroporation of transcription activator-like effector nuclease (TALEN) mRNA allows highly efficient multiplex gene editing in primary human T cells. We use this TALEN-mediated editing approach to develop a process for the large-scale manufacturing of T cells deficient in expression of both their αβ T-cell receptor (TCR) and CD52, a protein targeted by alemtuzumab, a chemotherapeutic agent. Functionally, T cells manufactured with this process do not mediate graft-versus-host reactions and are rendered resistant to destruction by alemtuzumab. These characteristics enable the administration of alemtuzumab concurrently or prior to engineered T cells, supporting their engraftment. Furthermore, endowing the TALEN-engineered cells with a CD19 CAR led to efficient destruction of CD19(+) tumor targets even in the presence of the chemotherapeutic agent. These results demonstrate the applicability of TALEN-mediated genome editing to a scalable process, which enables the manufacturing of third-party CAR T-cell immunotherapies against arbitrary targets. As such, CAR T-cell immunotherapies can therefore be used in an "off-the-shelf" manner akin to other biologic immunopharmaceuticals

Genetic modification of mouse bone marrow by lentiviral vector-mediated delivery of hypoxanthine-Guanine phosphoribosyltransferase short hairpin RNA confers chemoprotection against 6-thioguanine cytotoxicity

We have recently developed a novel and highly efficient strategy that exclusively uses the purine analog 6-thioguanine (6TG) for both pretransplantation conditioning and post-transplantation chemoselection of hypoxanthine-guanine phosphoribosyltransferase (HPRT)-deficient bone marrow (BM). In a mouse BM transplantation model, combined 6TG preconditioning and in vivo chemoselection consistently achieved >95% engraftment of HPRT-deficient donor BM and long-term reconstitution of histologically and immunophenotypically normal hematopoiesis in both primary and secondary recipients, without significant toxicity and in the absence of any other cytotoxic conditioning regimen. To translate this strategy for combined 6TG conditioning and chemoselection into a clinically feasible approach, it is necessary to develop methods for genetic modification of normal hematopoietic stem cells (HSC) to render them HPRT-deficient and thus 6TG-resistant. Here we investigated a strategy to reduce HPRT expression and thereby confer protection against 6TG myelotoxicity to primary murine BM cells by RNA interference (RNAi). Accordingly, we constructed and validated a lentiviral gene transfer vector expressing short-hairpin RNA (shRNA) that targets the murine HPRT gene. Our results showed that lentiviral vector-mediated delivery of HPRT-targeted shRNA could achieve effective and long-term reduction of HPRT expression. Furthermore, in both an established murine cell line as well as in primary murine BM cells, lentiviral transduction with HPRT-targeted shRNA was associated with enhanced resistance to 6TG cytotoxicity in vitro. Hence this represents a translationally feasible method to genetically engineer HSC for implementation of 6TG-mediated preconditioning and in vivo chemoselection.

Strong inhibition of thioredoxin reductase by highly cytotoxic gold(I) complexes. DNA binding studies

Biological properties of a series of aminophosphine-thiolate gold(I) complexes [Au(SR)(PPh2NHpy)] [Ph2PNHpy=2-(diphenylphosphinoamino)pyridine; HSR=2-mercaptopyridine (2-HSpy) (3), 2-mercaptonicotinic acid (2-H2-mna) (4), 2-thiouracil (2-HTU) (5) or 2-thiocytosine (2-HTC) (6)] and [Au(SR){PPh2NH(Htrz)}] [Ph2PNH(Htrz)=3-(diphenylphosphinoamino)-1,2,4-triazole]; HSR=2-mercaptopyridine (2-HSpy) (7), 2-thiocytosine (2-HTC) (8) or 6-thioguanine (6-HTG) (9) have been studied. Their antitumor properties have been tested in vitro against two tumor human cell lines, HeLa (derived from cervical cancer) and MCF-7 (derived from breast cancer), using a metabolic activity test (3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyl tetrazolium bromide, MTT). Some of them showed excellent cytotoxic activity. With the aim to obtain more information about the mechanisms of action of these derivatives, the interactions of complexes 3, 5, 7 and 9 with thioredoxin reductase in HeLa cells were studied. They showed a potent inhibition of thioredoxin reductase activity. In order to complete this study, interactions of the complexes with calf thymus (CT-) DNA and with different bacterial DNAs, namely the plasmid pEMBL9 and the promoter region of the furA (ferric uptake regulator A) gene from Anabaena sp. PCC 7120 were investigated. Although interactions of complexes with CT-DNA have been verified, none of them cause significant changes in its structure.

Myeloprotection by cytidine deaminase gene transfer in antileukemic therapy

Gene transfer of drug resistance (CTX-R) genes can be used to protect the hematopoietic system from the toxicity of anticancer chemotherapy and this concept recently has been proven by overexpression of a mutant O(6)-methylguaninemethyltransferase in the hematopoietic system of glioblastoma patients treated with temozolomide. Given its protection capacity against such relevant drugs as cytosine arabinoside (ara-C), gemcitabine, decitabine, or azacytidine and the highly hematopoiesis-specific toxicity profile of several of these agents, cytidine deaminase (CDD) represents another interesting candidate CTX-R gene and our group recently has established the myeloprotective capacity of CDD gene transfer in a number of murine transplant studies. Clinically, CDD overexpression appears particularly suited to optimize treatment strategies for acute leukemias and myelodysplasias given the efficacy of ara-C (and to a lesser degree decitabine and azacytidine) in these disease entities. This article will review the current state of the art with regard to CDD gene transfer and point out potential scenarios for a clinical application of this strategy. In addition, risks and potential side effects associated with this approach as well as strategies to overcome these problems will be highlighted.

Knockdown of HPRT for selection of genetically modified human hematopoietic progenitor cells

The inability to obtain sufficient numbers of transduced cells remains a limitation in gene therapy. One strategy to address this limitation is in vivo pharmacologic selection of transduced cells. We have previously shown that knockdown of HPRT using lentiviral delivered shRNA facilitates efficient selection of transduced murine hematopoietic progenitor cells (HPC) using 6-thioguanine (6TG). Herein, we now extend these studies to human HPC. We tested multiple shRNA constructs in human derived cell lines and identified the optimal shRNA sequence for knockdown of HPRT and 6TG resistance. We then tested this vector in human umbilical cord blood derived HPC in vitro and in NOD/SCID recipients. Knockdown of HPRT effectively provided resistance to 6TG in vitro. 6TG treatment of mice resulted in increased percentages of transduced human CD45(+) cells in the peripheral blood and in the spleen in particular, in both myeloid and lymphoid compartments. 6TG treatment of secondary recipients resulted in higher percentages of transduced human cells in the bone marrow, confirming selection from the progeny of long-term repopulating HPCs. However, the extent of selection of cells in the bone marrow at the doses of 6TG tested and the toxicity of higher doses, suggest that this strategy may be limited to selection of more committed progenitor cells. Together, these data suggest that human HPC can be programmed to be resistant to purine analogs, but that HPRT knockdown/6TG-based selection may not be robust enough for in vivo selection.

Impairment of adenylyl cyclase 2 function and expression in hypoxanthine phosphoribosyltransferase-deficient rat B103 neuroblastoma cells as model for Lesch-Nyhan disease: BODIPY-forskolin as pharmacological tool

Hypoxanthine phosphoribosyl transferase (HPRT) deficiency results in Lesch-Nyhan disease (LND). The link between the HPRT defect and the self-injurious behavior in LND is still unknown. HPRT-deficient rat B103 neuroblastoma cells serve as a model system for LND. In B103 cell membranes, HPRT deficiency is associated with a decrease of basal and guanosine triphosphate-stimulated adenylyl cyclase (AC) activity (Pinto and Seifert, J Neurochem 96:454-459, 2006). Since recombinant AC2 possesses a high basal activity, we tested the hypothesis that AC2 function and expression is impaired in HPRT deficiency. We examined AC regulation in B103 cell membranes, cAMP accumulation in intact B103 cells, AC isoform expression, and performed morphological studies. As most important pharmacological tool, we used 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene forskolin (BODIPY-FS) that inhibits recombinant AC2 but activates ACs 1 and 5 (Erdorf et al., Biochem Pharmacol 82:1673-1681, 2011). In B103 control membranes, BODIPY-FS reduced catalysis, but in HPRT(-) membranes, BODIPY-FS was rather stimulatory. 2'(3')-O-(N-methylanthraniloyl) (MANT)-nucleoside 5'-[γ-thio]triphosphates inhibit recombinant ACs 1 and 5 more potently than AC2. In B103 control membranes, MANT-guanosine 5'-[γ-thio]triphosphate inhibited catalysis in control membranes less potently than in HPRT(-) membranes. Quantitative real-time PCR revealed that in HPRT deficiency, AC2 was virtually absent. In contrast, AC5 was up-regulated. Forskolin (FS) and BODIPY-FS induced cell clustering and rounding and neurite extension in B103 cells. The effects of FS and BODIPY-FS were much more prominent in control than in HPRT(-) cells, indicative for a differentiation defect in HPRT deficiency. Neither FS nor BODIPY-FS significantly changed cAMP concentrations in intact B103 cells. Collectively, our data show that HPRT deficiency in B103 cells is associated with impaired AC2 function and expression and reduced sensitivity to differentiation induced by FS and BODIPY-FS. We discuss the pathophysiological implications of our data for LND.