Partial phenotypic correction of human Lesch-Nyhan (hypoxanthine-guanine phosphoribosyltransferase-deficient) lymphoblasts with a transmissible retroviral vector

Erratic journey of CRISPR/Cas9 in oncology from bench-work to successful-clinical therapy

CRISPR is a customized molecular scissor, comprising genetic guide made of RNA and an enzyme, Cas9 which snips DNA in simpler, cheaper and more precise way than any other gene editing tools. In recent years CRISPR/Cas has taken the research world by storm being go-to genome editor for potential gene therapy to fix cancer as well as several hereditary disorders. This review explores the literature around the mechanism of Nobel winning CRISPR/Cas9 and its journey from its discovery to various pre-clinical and clinical trials in oncology, focusing mostly on PD-1 knockout CAR-T cell therapy. It also discusses the hurdles and ethical dispute associated with CRISPR, such as unintended on-target and off-target cuts, embryonic germ-line editing. Despite the controversies regarding the safety of this technique, many studies reported promising results on targeting cancer and other diseases using CRISPR/Cas9. Outcomes from the first successful clinical trial showed the beneficial long term effect on genetically modified T-cells in targeting cancer cells which opens the door for CRISPR to be the most preferred technique to help treating cancer and other diseases in future. As far as germ-line editing is concerned, further studies are needed to support the safety of this technique in humans fixing genetic disorders and mutations. Therefore till date only somatic cell editing is ethically approved.

Human Cardiac Gene Therapy

In the past 10 years, there has been tremendous progress made in the field of gene therapy. Effective treatments of Leber congenital amaurosis, hemophilia, and spinal muscular atrophy have been largely based on the efficiency and safety of adeno-associated vectors. Myocardial gene therapy has been tested in patients with heart failure using adeno-associated vectors with no safety concerns but lacking clinical improvements. Cardiac gene therapy is adapting to the new developments in vectors, delivery systems, targets, and clinical end points and is poised for success in the near future.

The Impact of CRISPR/Cas9 Technology on Cardiac Research: From Disease Modelling to Therapeutic Approaches

Genome-editing technology has emerged as a powerful method that enables the generation of genetically modified cells and organisms necessary to elucidate gene function and mechanisms of human diseases. The clustered regularly interspaced short palindromic repeats- (CRISPR-) associated 9 (Cas9) system has rapidly become one of the most popular approaches for genome editing in basic biomedical research over recent years because of its simplicity and adaptability. CRISPR/Cas9 genome editing has been used to correct DNA mutations ranging from a single base pair to large deletions in both in vitro and in vivo model systems. CRISPR/Cas9 has been used to increase the understanding of many aspects of cardiovascular disorders, including lipid metabolism, electrophysiology and genetic inheritance. The CRISPR/Cas9 technology has been proven to be effective in creating gene knockout (KO) or knockin in human cells and is particularly useful for editing induced pluripotent stem cells (iPSCs). Despite these progresses, some biological, technical, and ethical issues are limiting the therapeutic potential of genome editing in cardiovascular diseases. This review will focus on various applications of CRISPR/Cas9 genome editing in the cardiovascular field, for both disease research and the prospect of in vivo genome-editing therapies in the future.

Advances of gene therapy for primary immunodeficiencies

In the recent past, the gene therapy field has witnessed a remarkable series of successes, many of which have involved primary immunodeficiency diseases, such as X-linked severe combined immunodeficiency, adenosine deaminase deficiency, chronic granulomatous disease, and Wiskott-Aldrich syndrome. While such progress has widened the choice of therapeutic options in some specific cases of primary immunodeficiency, much remains to be done to extend the geographical availability of such an advanced approach and to increase the number of diseases that can be targeted. At the same time, emerging technologies are stimulating intensive investigations that may lead to the application of precise genetic editing as the next form of gene therapy for these and other human genetic diseases.

The development of gene therapy: from monogenic recessive disorders to complex diseases such as cancer

During the last 4 decades, gene therapy has moved from preclinical to clinical studies for many diseases ranging from monogenic recessive disorders such as hemophilia to more complex diseases such as cancer, cardiovascular disorders, and human immunodeficiency virus (HIV). To date, more than 1,340 gene therapy clinical trials have been completed, are ongoing, or have been approved in 28 countries, using more than 100 genes. Most of those clinical trials (66.5%) were aimed at the treatment of cancer. Early hype, failures, and tragic events have now largely been replaced by the necessary stepwise progress needed to realize clinical benefits. We now understand better the strengths and weaknesses of various gene transfer vectors; this facilitates the choice of appropriate vectors for individual diseases. Continuous advances in our understanding of tumor biology have allowed the development of elegant, more efficient, and less toxic treatment strategies. In this introductory chapter, we review the history of gene therapy since the early 1960s and present in detail two major recurring themes in gene therapy: (1) the development of vector and delivery systems and (2) the design of strategies to fight or cure particular diseases. The field of cancer gene therapy experienced an "awkward adolescence." Although this field has certainly not yet reached maturity, it still holds the potential of alleviating the suffering of many individuals with cancer.

A roadmap to safe, efficient, and stable lentivirus-mediated gene therapy with hematopoietic cell transplantation

Hematopoietic stem cells comprise a prominent target for gene therapy aimed at treating various genetic and acquired disorders. A number of limitations associated with hematopoietic cell transplantation can be circumvented by the use of cells stably modified by retroviral gene transfer. Oncoretroviral and lentiviral vectors offer means for generating efficient and stable transgene expression. This review summarizes the state of the field today in terms of vector development and clinical experimentation. In particular, concerns with the safety of retroviral vectors intended for clinical gene transfer, applicability of preclinical data in directing clinical trial design, and recent research aimed at resolving some of these issues are addressed. Finally, this review underlines the specific advantages offered by lentiviral gene-transfer vectors for gene therapy in stem cells.

A human neuronal tissue culture model for Lesch-Nyhan disease

Mutations in the gene encoding the purine salvage enzyme, hypoxanthine-guanine phosphoribosyltransferase (HPRT) cause Lesch-Nyhan disease, a neurodevelopmental disorder characterized by cognitive, neurological, and behavioral abnormalities. Despite detailed knowledge of the enzyme's function, the key pathophysiological changes that accompany loss of purine recycling are unclear. To facilitate delineating the consequences of HPRT deficiency, four independent HPRT-deficient sublines of the human dopaminergic neuroblastoma, SK-N-BE(2) M17, were isolated by targeted mutagenesis with triple helix-forming oligonucleotides. As a group, these HPRT-deficient cells showed several significant abnormalities: (i) impaired purine recycling with accumulation of hypoxanthine, guanine, and xanthine, (ii) reduced guanylate energy charge and GTP:GDP ratio, but normal adenylate energy charge and no changes in any adenine nucleotide ratios, (iii) increased levels of UTP and NADP+, (iv) reduced DOPA decarboxylase, but normal monoamines, and (v) reduction in cell soma size. These cells combine the analytical power of multiple lines and a human, neuronal origin to provide an important tool to investigate the pathophysiology of HPRT deficiency.

Post-transduction events in retrovirus-mediated gene therapy involving hematopoietic stem cells: beyond efficiency issues

Numerous incremental technological improvements have occurred recently in the application of therapeutic retrovirus-mediated gene transfer into hematopoietic stem cells (HSCs). Improved transduction efficiencies are now reaching levels that may correct some inherited or acquired disorders. Novel retroviral vector systems likewise offer the possibility for an expanded portfolio of treatment approaches. Most importantly, however, investigators are now also focusing efforts on post-transduction events to fully impact correction. Here we describe recent advances in the field, with a special emphasis on the role of post-transduction processes, for correction of disorders or treatments that involve HSCs or their progeny.

The maturation of human gene therapy

Human gene therapy is two things. It is the concept that human disease might be treated at the level of underlying genetic targets rather than at the level of aberrant metabolism, and it is the implementation of that concept toward a clinical reality. The conceptual aspect is established--gene therapy has become an accepted central driving force in medicine. The second aspect--that of converting the concepts into practical tools for human gene therapy--is maturing rapidly. Over the past several years, the level of expectation had risen to unrealistic proportions and recent initial clinical trials produced disappointment. These early clinical results should, however, be viewed not as failures, but rather as deliberate progress along the learning curve in this new and difficult field of biomedical science.

Bone marrow and clinical gene therapy

Remarkable progress has been made in the last 5 years in the use of gene therapy for the treatment of inherited diseases and acquired disorders. This article reviews these applications with particular emphasis on the use of genetically modified hematopoietic cells.

Gene therapy of cancer

Retroviral-mediated gene transfer has permitted the development of clinical protocols for the study and treatment of cancer. These protocols can be divided into gene-labeling and gene therapy proposals. Labeling studies include the tracking of tumor infiltrating lymphocytes (TIL) following the administration of those cells, and the detection, at the time of relapse, of tumor cells from transplanted autologous bone marrow. Most gene therapy protocols are designed to induce an immune attack against the tumor by inserting genes into tumor cells themselves. Although uncertainty about the safety of the procedure still exists, gene therapy of cancer holds much promise as an effective treatment modality.

A brief history of gene therapy

The concepts of gene therapy arose initially during the 1960s and early 1970s whilst the development of genetically marked cells lines and the clarification of mechanisms of cell transformation by the papaovaviruses polyoma and SV40 was in progress. With the arrival of recombinant DNA techniques, cloned genes became available and were used to demonstrate that foreign genes could indeed correct genetic defects and disease phenotypes in mammalian cells in vitro. Efficient retroviral vectors and other gene transfer methods have permitted convincing demonstrations of efficient phenotype correction in vitro and in vivo, now making gene therapy a broadly accepted approach to therapy and justifying clinically applied studies with human patients.

Enzymes as agents for the treatment of disease

The replacement of genetically deficient enzymes in patients with inherited metabolic disorders by infusion of purified enzymes or by organ transplantation has had very limited success, although good results with bone marrow transplantation have been obtained in some patients with mucopolysaccharidosis, Gaucher disease and inherited immunodeficiency diseases. Genetic engineering of the patient's lymphocytes may ultimately render these approaches redundant, at least for some of these diseases. Treatment of chronic pancreatic insufficiency and of disaccharidase deficiency with oral enzymes can be very effective; therapy can be monitored in the latter by measuring the breath hydrogen excretion and in the former by a range of tests of which stool chymotrypsin assay is the most convenient. Treatment of acute myocardial infarction by intracoronary perfusion of thrombolytic enzymes can improve both cardiac function and long-term survival if given early enough. Successful reperfusion can be identified by changes in the kinetics of serum enzyme release and clearance, especially for the isoenzymes and isoforms of creatine kinase. In cancer chemotherapy, L-asparaginase has long been a useful adjunct in the treatment of acute lymphoblastic leukemia, but recent experience suggests a role in acute nonlymphoblastic leukemia as well.

Employment of fibroblasts for gene transfer: applications for grafting into the central nervous system

Genetic modification of primary skin fibroblasts offers a new approach to the focal delivery of deficient transmitter-specific enzymes (e.g., TH) or trophic substances (e.g., NGF) to the damaged or diseased CNS. Although fibroblasts are unable to provide anatomical corrections to defective neural connectivity, they can serve as biological pumps for the enzymes and growth factors in vivo. The capability of genetically engineered cells to ameliorate disease phenotypes in animal models of CNS disorders may ultimately results in the restoration of function. At this time, primary skin fibroblasts appear to be a convenient cellular population for the application of gene transfer and intracerebral grafting for the animal model of Parkinson's disease. It is now important for future investigations to provide data concerning the long-term stable expression of the transgene product (e.g., TH) following intracerebral implantation, as well as determining optimal conditions for the survival of primary cells grafted into the nervous system.

Induction of HIV-specific CTL and antibody responses in mice using retroviral vector-transduced cells

Recombinant retroviral vectors can efficiently transduce and express foreign genes in mammalian cells. We have examined the utility of retroviral vector-mediated gene transfer to deliver genes which encode human immunodeficiency virus type I (HIV) antigens capable of stimulating specific immune responses. Murine fibroblast cell lines were transduced with a nonreplicating murine retroviral vector carrying the gene encoding the HIV-IIIB envelope protein and were shown to express the gp160/120 protein. Mice immunized with syngeneic vector-transduced cells developed CD8+, class I major histocompatibility complex (MHC)-restricted cytotoxic T lymphocytes (CTL) specific for targets expressing the HIV envelope protein. The CTL also exhibited lytic activity on target cells coated with synthetic peptides derived from the gp120 V3 hypervariable region of both the HIV-IIIB and HIV(MN) isolates. Following adoptive transfer in a murine tumor model, these CTL were shown to be effective in vivo by their ability to eliminate established tumor cells expressing the HIV protein. Vector-transduced syngeneic cells were also capable of eliciting HIV envelope-specific antibody responses in immunized mice. Sera obtained from these mice were found to bind to the HIV-IIIB gp160 protein as well as a peptide-defined neutralizing antibody epitope contained within the V3 domain of gp120. These sera exhibited virus-neutralizing activity in that they markedly reduced the ability of HIV to infect and form syncytia of a human T-cell line. This is the first demonstration that cells transduced with a retroviral vector encoding the HIV-IIIB envelope protein are capable of inducing effective HIV-specific cellular and humoral immune responses in mice.

Retroviral-mediated gene transfer of the leukocyte integrin CD18 subunit

Children with leukocyte adherence deficiency (LAD) exhibit heterogeneous defects in the leukocyte integrin CD18 subunit that prevent surface expression of functional CD11/CD18 leukocyte integrin adherence complexes. We used a retroviral vector, designated LCD18SN, to transfer the CD18 cDNA into K562 human myeloid leukemia cells and into EBV B-cells from a child with LAD. Transfer of the LCD18SN retroviral construct, which expresses the CD18 cDNA from the Moloney Murine leukemia virus (MoMLV) long terminal repeat (LTR), into K562 cells resulted in relatively high levels of CD18 mRNA and intracellular protein. Retroviral-mediated gene transfer of CD18 into LAD EBV B-cells resulted in low, but readily measurable, levels of surface expression of the CD11a/CD18 complex in these previously deficient lymphocytes. The reconstitution of surface expression of the CD11a/CD18 complex by gene transfer of the CD18 cDNA into LAD EBV B-cells indicates that this syndrome represents a candidate disorder for gene therapy.

Factors affecting long-term stability of Moloney murine leukemia virus-based vectors

We have examined the long-term functional and structural stability of retroviral vectors in infected murine cells. We have used Moloney murine leukemia virus-based vectors expressing human HPRT, firefly luciferase (luc), and Escherichia coli beta-galactosidase (lacZ) as reporter genes, and the human HPRT and the transposon Tn5 neomycin resistance (neo) gene as selectable markers. All vectors, whether single or double gene, yielded both stable and unstable clones. Stability of the proviruses was dependent on a number of factors, including the nature of the infected cell, the reporter gene, the integration site of the provirus, the relative positions of the component genes in multigene vectors, and the presence or absence of selection pressure. Selection pressure was helpful, but not universally effective, in maintaining provirus structural and functional integrity. Reporter gene expression from an internal promoter was likely to be unstable with or without selection for an upstream, LTR-driven neo gene. In some clones, loss of proviral gene expression was accompanied by deletions, while other inactive clones retained an apparently intact provirus. In the latter clones, treatment with 5-azacytidine failed to reactivate the reporter genes, but superinfection with helper virus resulted in the reappearance of transmissible vector, indicating a reversible epigenetic mechanism for proviral shutdown. The design of effective retroviral vectors and their possible use in vivo will require further characterization of these determinants of provirus stability.

Progress toward human gene therapy

Current therapies for most human genetic diseases are inadequate. In response to the need for effective treatments, modern molecular genetics is providing tools for an unprecedented new approach to disease treatment through an attack directly on mutant genes. Recent results with several target organs and gene transfer techniques have led to broad medical and scientific acceptance of the feasibility of this "gene therapy" concept for disorders of the bone marrow, liver, and central nervous system; some kinds of cancer; and deficiencies of circulating enzymes, hormones, and coagulation factors. The most well-developed models involve alteration of mutant target genes by gene transfer with recombinant pathogenic viruses in order to express new genetic information and to correct disease phenotypes--the conversion of the swords of pathology into the plowshares of therapy.

Inhibition of orotidylate decarboxylase by 4(5H)-oxo-1-beta-D-ribofuranosylpyrazolo[3,4-d] pyrimidine-3-thiocarboxamide (APR-TC) in B lymphoblasts. Activation by adenosine kinase

The nucleoside allopurinol riboside-3-thiocarboxamide (APR-TC; 4-(5H)oxo-1-beta-D-ribofuranosylpyrazolo[3,4,d]pyrimidine-3-thioca rboxamide) demonstrates potent in vitro antiviral activity against various DNA and RNA viruses and cytostatic activity against a variety of cell lines in culture. The IC50 for APR-TC in the splenic derived B lymphoblast cell line, WI-L2, was 0.3 microM. Adenosine kinase-deficient WI-L2 cells were resistant to growth inhibition by APR-TC, indicating that adenosine kinase (EC 2.7.1.20) is responsible for phosphorylation of APR-TC to form the monophosphate derivative (APR-TC-5'P). A 4-hr incubation of cells with 50 microM APR-TC resulted in severe depletion of intracellular pyrimidine nucleotide pools and the accumulation of 3 microM APR-TC-5'P. The cytotoxicity of APR-TC was reversed by uridine, indicating that the active form of this compound inhibits the de novo pyrimidine biosynthetic pathway. Further, APR-TC-treated cells could not utilize the pyrimidine nucleotide precursor [6-14C]orotic acid, suggesting that the UMP synthase complex is the major cellular site of inhibition. In studies utilizing cell-free lysates of WI-L2, chemically prepared APR-TC-5'P provided potent inhibition of the orotidylate decarboxylase activity (ODCase, EC 4.1.1.23) of the UMP synthase complex. APR-TC-5'P was competitive with OMP, and a Ki value of 0.35 nM was determined.

Characterization, evolutionary relationships, and chromosome location of processed mouse HPRT pseudogene

Studies on a cell line with amplified copies of the mouse hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene and HPRT gene transfer experiments revealed the existence of a nonfunctional HPRT-related sequence in the mouse genome. This sequence was isolated and found to be a processed HPRT pseudogene. With the exception of a small internal deletion, the pseudogene is believed to comprise a complete reverse transcript of HPRT mRNA, although the 3' end of the pseudogene was lost in the cloning process. A probe from a region flanking the mouse pseudogene was used to investigate the evolutionary relationships of mammalian HPRT pseudogenes. The pseudogenes in mouse and Chinese hamster appear to have a common origin, but no homology to any of the four known human HPRT pseudogenes was detected. A pseudogene-linked restriction fragment length polymorphism was used to map the pseudogene to the distal end of mouse chromosome 17.

Grafting genetically modified cells to the brain: possibilities for the future

Diagnostic and therapeutic approaches to disorders of the central nervous system (CNS) are particularly difficult to develop because of the relative inaccessibility of the mammalian brain to study and chemical treatment, the complexity and interconnectedness of CNS subsystems, and the profound and continued lack of fundamental understanding of the relationship between structure and function in the CNS. Neural grafting in the CNS has recently suggested a potential approach to CNS therapy through the selective replacement of cells lost as a result of disease or damage. Independently, studies aimed at direct genetic therapy in model systems have recently begun to suggest conceptually new approaches to the treatment of several kinds of human genetic disease, especially those caused by single-gene enzyme deficiencies. We suggest that a combination of these two approaches, namely the grafting into the CNS of genetically modified cells, may provide a new approach toward the restoration of some functions in the damaged or diseased CNS. We present evidence for the feasibility of this approach, including a description of some current techniques for mammalian cell gene transfer and CNS grafting, and several possible approaches to clinical applications.

A potential animal model for Lesch-Nyhan syndrome through introduction of HPRT mutations into mice

The human Lesch-Nyhan syndrome is a rare neurological and behavioural disorder, affecting only males, which is caused by an inherited deficiency in the level of activity of the purine salvage enzyme hypoxanthine-guanosine phosphoribosyl transferase (HPRT). How the resulting alterations in purine metabolism lead to the severe symptoms characteristic of Lesch-Nyhan patients is still not understood. No mutations at the Hprt locus leading to loss of activity have been described in laboratory animals. To derive an animal model for the Lesch-Nyhan syndrome, we have used cultured mouse embryonic stem cells, mutagenized by retroviral insertion and selected for loss of HPRT activity, to construct chimaeric mice. Two clonal lines carrying different mutant Hprt alleles have given rise to germ cells in chimaeras, allowing the derivation of strains of mutant mice having the same biochemical defect as Lesch-Nyhan patients. Male mice carrying the mutant alleles are viable and analysis of their cells shows a total lack of HPRT activity.

The application of bone marrow transplantation to the treatment of genetic diseases

Genetic diseases can be treated by transplantation of either normal allogeneic bone marrow or, potentially, autologous bone marrow into which the normal gene has been inserted in vitro (gene therapy). Histocompatible allogeneic bone marrow transplantation is used for the treatment of genetic diseases whose clinical expression is restricted to lymphoid or hematopoietic cells. The therapeutic role of bone marrow transplantation in the treatment of generalized genetic diseases, especially those affecting the central nervous system, is under investigation. The response of a generalized genetic disease to allogeneic bone marrow transplantation may be predicted by experiments in vitro. Gene therapy can be used only when the gene responsible for the disease has been characterized. Success of gene therapy for a specific genetic disease may be predicted by its clinical response to allogeneic bone marrow transplantation.

Retroviral vector-mediated gene transfer into human hematopoietic progenitor cells

The transfer of the human gene for hypoxanthine phosphoribosyltransferase (HPRT) into human bone marrow cells was accomplished by use of a retroviral vector. The cells were infected in vitro with a replication-incompetent murine retroviral vector that carried and expressed a mutant HPRT complementary DNA. The infected cells were superinfected with a helper virus and maintained in long-term culture. The production of progeny HPRT virus by the bone marrow cells was demonstrated with a colony formation assay on cultured HPRT-deficient, ouabain-resistant murine fibroblasts. Hematopoietic progenitor cells able to form colonies of granulocytes or macrophages (or both) in semisolid medium in the presence of colony stimulating factor were present in the nonadherent cell population. Colony forming units cloned in agar and subsequently cultured in liquid medium produced progeny HPRT virus, indicating infection of this class of hematopoietic progenitor cell.

Alterations of inosinate branchpoint enzymes in cultured human lymphoblasts

The specific activities of the three enzymes of the inosinate branchpoint are independently regulated when lymphoblasts are grown under various tissue culture conditions. In comparison to rapidly dividing cells, lymphoblasts at high cell density with no cellular division have decreased activity of the enzymes which commit inosinate to adenylate or guanylate, while cytoplasmic 5'-nucleotidase is relatively preserved. A linear relationship between inosinate dehydrogenase activity and growth rate (r = 0.92) exists in lymphoblasts with slowed growth rates. In contrast, in dividing cells adenylosuccinate synthetase and 5'-nucleotidase do not vary with growth rate. Adenylosuccinate synthetase and inosinate dehydrogenase activities appear to be related to the presence or rate of cellular division, as opposed to the presence or degree of neoplastic transformation. Lymphoblast lines with alterations of specific purine metabolic enzymes have characteristic alteration of the inosinate utilizing enzymes. Deficiencies of purine nucleoside phosphorylase or hypoxanthine phosphoribosyltransferase, abnormalities which render the cell unable to salvage purine effectively, are associated with depressed inosinate dehydrogenase activity. Insertion of the hypoxanthine phosphoribosyltransferase gene into hypoxanthine phosphoribosyltransferase-deficient cells normalizes inosinate dehydrogenase activity, while a hypoxanthine phosphoribosyltransferase-deficient mutant selected from a hypoxanthine phosphoribosyltransferase-containing line has depressed inosinate dehydrogenase activity. In contrast, overactivity of phosphoribosylpyrophosphate synthetase, with enhanced excretion of purines due to excessive production, is associated with elevated inosinate dehydrogenase activity. Inosinate dehydrogenase appears to be regulated according to the availability of purine nucleotides. Patients who overproduce uric acid and potentially have undescribed purine metabolic defects are now being screened for abnormalities in the inosinate branchpoint enzymes.