PerspectiveMedicine

Gene Therapy That Works

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Science  23 Aug 2013:
Vol. 341, Issue 6148, pp. 853-855
DOI: 10.1126/science.1242551

The concept of gene therapy is disarmingly simple: Introduce a healthy gene in a patient and its product should alleviate the defect caused by a faulty gene or slow the progression of disease (1). Why then, over the past three decades, have there been so few clinical successes in treating patients with this approach? A major obstacle has been the delivery of genes to the appropriate cell, tissue, and organ. How does one introduce a gene into the brain with trillions of cells, or the liver with billions of cells, or the rare hematopoietic adult stem cell that has the potential to populate all lineages of lymphoid and myeloid cells? Much effort has been devoted to finding ways to efficiently deliver a therapeutic gene to the desired cell type, resulting in sustained production of the gene product, ideally through the entire life of the recipient, without unwanted side effects like genotoxicity or unsettling the immune balance (2). On pages 1233158 and 1233151 in this issue, Biffi et al. (3) and Aiuti et al. (4) report encouraging results using lentivirus-mediated gene therapy to treat children with rare genetic defects.

For scientists in the field of gene therapy, good news, tinged with occasional set-backs, has been trickling in over the past decade, starting with the successful clinical trials of children with X-linked severe combined immunodeficiency disease (SCID) (5). Currently, more than 1700 clinical trials are under way worldwide, drawing on a wide array of gene therapy approaches for both acquired and inherited diseases (6). The approach involves genetically engineering a virus so that it infects a target cell to deliver a gene, but does not cause disease. Retroviruses (such as lentiviruses) integrate their genetic material, including the new gene, in to the host cell genome. Such transduced host cells are transplanted back into the patient and proliferate with the correct gene, producing healthy cells (see the figure). Biffi et al. and Aiuti et al. provide new hope to children with metachromatic leukodystrophy (MLD) and Wiskott-Aldrich syndrome (WAS), respectively, both genetic defects that result in a deficiency of proteins essential for the early years of life and lack any effective treatments. MLD is an autosomal recessive lysosomal storage disease caused by mutations in the ARSA gene, leading to a deficiency of the enzyme arylsulfatase A, and hence the buildup of toxic sulfatide, causing widespread demyelination and neurodegeneration. Children with this disease appear healthy at birth, but gradually lose their cognitive and motor skills, with no possibility of arresting the neurodegenerative process. Children born with WAS, an X-linked primary immunodeficiency, lack WASP, a protein that regulates the cytoskeleton. Its loss leads to a faulty immune system that makes them vulnerable to the development of infections, autoimmune diseases, and cancer, as well as causing a defect in platelets that results in frequent bleeding. Bone marrow transplantations, when feasible, have proved to be a successful therapeutic approach for these two diseases (7). So, in 2010, clinical trials were initiated using lentiviral vectors to transfer functional ARSA and WASP genes in bone marrow–derived hematopoietic stem cells (expressing the marker CD34+) from 16 patients, 6 of whom suffered from WAS and 10 from MLD. The studies of Biffi et al. and Aiuti et al. report results from three patients from each group, for whom sufficient time has passed since administration of the therapy to allow conclusions to be drawn regarding its safety and efficacy.

Clinical benefit.

Children with the rare genetic diseases shown were treated with gene therapy. The approach delivered a normal gene, whose product halted or slowed gene progression up to 2 years after treatment.

CREDIT: V. ALTOUNIAN/SCIENCE

Biffi et al. found that in three presymptomatic children with late infantile MLD, treatment halted disease manifestation or progression for follow-up times ranging from 18 to 24 months, as compared to predicted disease onset in 7 to 21 months. Similarly, in three children with symptoms of WAS, Aiuti et al. showed that pretreatment eczema (chronic inflammation of the skin) resolved between 6 and 12 months; decreased, progressively, the frequency of infections; and improved platelet counts after gene therapy. In both clinical trials, no clonal domination was observed. Analysis of the vector insertion site in hematopoietic cells showed no preferential integrations in a particular gene locus, thus decreasing the likelihood of generating an abundance of abnormally proliferating cells. Increasing presence over time of CD34+ progenitors and mature cells of myeloid and lymphoid lineages marked by identical integration sites of the delivered gene is strong evidence of self-renewal and multilineage potential of vector-transduced hematopoietic stem cells after engraftment. In this regard, the studies of Biffi et al. and Aiuti et al. are similar to those reported for two boys with X-linked adrenoleukodystrophy, a severe demyelinating disease caused by deficiency of an adenosine 5′-triphosphate transporter, where no clonal dominance was observed in lentiviral vector–transduced hematopoietic stem cells (8). In all three trials, partial bone marrow ablation of the patients was required to achieve maximum transplantation efficiency.

In the last 12 to 13 years, more than 50 patients affected by primary immunodeficiencies have been treated with genetically transduced autologous hematopoietic stem cells, mostly with gamma retroviral vectors (9). Most patients received clinical benefits, but occurrence of leukemia and myelodysplasia in some patients with SCID-X1, chronic granulomatous disease, and WAS have raised questions about their long-term safety. These adverse events have generally been ascribed to vector integration in the vicinity of specific protooncogenes, leading to their aberrant expression and resulting in neoplasias (1012). The integration sites can be identified by deep sequencing of genomes, but because it is not possible to efficiently propagate clonal populations of hematopoietic stem cells, analysis can be performed only after transplantation. Although data from lentiviral-transduced hematopoietic stem cells in patients is limited, it appears not to favor any specific integration sites, though the appearance of a self-limiting dominant clone in the myeloid compartment of an individual with β(E)/β(0)-thalassemia whose hematopoietic stem cells were transduced with lentiviral vectors caused concern. In this clone, activation of the gene HMGA2 (which encodes a transcriptional regulator) was caused by vector-mediated generation of a truncated transcript whose overexpression in mice is associated with the development of proliferative hematopoiesis and clonal expansion (13). Nevertheless, there was no clinical evidence supporting the existence of a preleukemic state or a substantial hematopoietic imbalance.

Why are gamma retroviral vectors prone to increased genotoxicity? Perhaps the simplest explanation is that their genome contains long-terminal repeats, harboring intact promoter and enhancer sequences which, upon integration in the vicinity of a growth-promoting gene of the host cell, can enhance its transcription, leading to abnormal cell growth. Self-inactivating (SIN) gamma retroviral vectors are being tested in clinical trials for comparison with SIN-lentivectors in terms of their genotoxicity, efficiency of transduction, and sustained expression of the transgene. Another unknown is the conditioning regime and the precise state of the hematopoietic stem cells being transduced, which may differ from patient to patient.

With continued progress in gene therapy, scientists will likely take somatic cells from a patient, convert them into induced pluripotent stem cells, replace the defective gene “surgically” with the normal gene by homologous recombination, and differentiate them into hematopoietic stem cells, followed by conventional transplantation back into the patient. Alternatively, ways will be found to grow and proliferate hematopoietic stem cells, and bypass the need for generation of induced pluripotent stem cells. This is all good news for patients suffering from incurable genetic diseases.

References and Notes

  1. Acknowledgments: I.M.V is an inventor of lentiviral vector technology that is owned by the Salk Institute and is entitled to receive royalties (U.S. Patent No. 6,013,516 with the Salk Institute).
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