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The successful use of gene therapy to correct rare immune system disorders has highlighted the enormous potential of such therapies. We review the current state of gene therapy for childhood immune system disorders, and consider why these conditions have been particularly amenable to genetic correction. As with all emerging therapies, there have been unexpected side effects and their underlying mechanisms are the subject of intense research. Minimising such risks through improved vector design will play an important role in developing the next generation of gene based therapies and extending their applicability.
Of all single gene disorders, inherited immune deficiencies have emerged as the first conditions to be effectively treated by the stable and durable transfer of genetic material. Severe combined immune deficiency (SCID) is an umbrella term for a heterogeneous group of rare genetic disorders characterised by a primary failure of T cell immunity.1 The molecular basis of SCID can now be determined for more than 80% of cases (table 11)) and all these disorders have notable clinical features in common, presenting in the first few months of life with failure to thrive, chronic diarrhoea and serious recurrent infections.
Conventional therapy for SCID relies on haematopoietic stem cell transplantation (HSCT) and survival rates of ~90% for children receiving grafts from HLA matched donors can now be expected.2 However, mis‐matched procedures are less successful and often require continuation of immunoglobulin replacement therapy and antibiotic prophylaxis.2,3 Furthermore, pre‐conditioning for HSCT using chemotherapy carries significant risks and is associated with long term complications, including infertility and growth retardation. Gene therapy offers alternative therapeutic possibilities and there are several important reasons why SCID disorders have been particularly attractive target diseases in this field (table 22)
To date, successful gene therapy has relied on a gene addition strategy, whereby a copy of cDNA encoding a target protein is added to the genome of target cells. Promoter or regulatory sequences are also introduced to ensure sustained expression of the introduced gene, and in the case of retroviral systems these elements are derived from the viral vector. The retrovirus genome comprises single stranded RNA and includes long terminal repeat (LTR) sequences with inherent promoter activity that can be used to drive expression of therapeutic genes. Following entry into a target cell, reverse transcriptase carried by the virus converts the RNA to DNA, and this crosses the nuclear membrane during mitosis and integrates into the host cell genome. Although it had been assumed that integration occurs randomly throughout the genome, it is now apparent that there is a bias towards sites close to the transcriptional start sites of active genes and the relevance of this is being elucidated.6,7
Two conditions, SCID‐X1 due to defects in the cytokine receptor common gamma chain (γc), and adenosine deaminse deficiency (ADA‐SCID) have been the most widely studied to date (table 11).). Successful treatment of patients with SCID‐X1 using retroviral mediated gene transfer of γc was first demonstrated in French trials published in 2000,8 and was subsequently confirmed in studies at our institution.9 The procedure involves the selection of autologous bone marrow HSC expressing the stem cell marker CD34, followed by activation in a cytokine cocktail before exposure to the retroviral vector (fig 11).). The cells are then collected, washed and infused back into the patient without any prior chemotherapeutic pre‐conditioning. Twenty infants with SCID‐X1 have now been treated in the two studies and the majority have had sustained immunological reconstitution with kinetics of recovery similar to that seen after allogeneic HSCT. Natural killer cells were detectable at 2–4 weeks after the procedure, and T cells were present within 12 weeks. Both cellular and antibody responses have been demonstrated in those re‐immunised to date with childhood vaccines.9,10,11 Although adverse events were subsequently reported by the French investigators (discussed below), SCID‐X1 became the first genetic disorder to be corrected by gene therapy. Interestingly, two older patients with SCID‐X1 (one who had undergone HSCT in infancy but in adulthood had poor immune function, and another who had an atypical SCID variant) proved more difficult to treat.12 Despite successful stem cell harvest and transduction, these patients have had poor T cell recovery; this is probably a reflection of age‐related reduction in thymic function in these individuals.
Studies of gene correction of ADA deficient SCID date back to 1990 and initially targeted peripheral blood mononuclear cells rather than bone marrow stem cells. A retroviral vector was used to transduce patient T cells and after expansion the T cells were returned to the patients.13 As the patients remained on enzyme (PEG‐ADA) replacement therapy, the effectiveness of the genetically modified T cell populations was not clearly established, although recent follow‐up of those patients revealed clear evidence of long term persistence (>10 years) of the gene modified T cells.14 Other studies have targeted lymphocytes, bone marrow stem cells and cord stem cells with retroviral vectors encoding the ADA gene,15,16,17 but again the continued administration of PEG‐ADA has made it difficult to establish efficacy. The continued use of PEG‐ADA may also have reduced the competitive advantage of the genetically modified cells and discontinuation of enzyme replacement therapy has been associated with improved immunity from gene corrected T cells.18 Subsequent trials have therefore tried to enhance engraftment of gene modified CD34 cells by withholding PEG‐ADA, and have also included a mild pre‐conditioning regimen to “make space” for the corrected cells. The use of chemotherapy, even at reduced intensity, has introduced a possibility of related complications (extended cytopaenia was recently reported in a patient with a pre‐existing abnormality of marrow cytogenetics19), but overall this strategy has now been shown be highly effective in achieving systemic detoxification in ADA‐SCID, and good recovery of cellular immunity has now been demonstrated in a number of infants.20,21
Unexpected lymphoproliferative complications following gene therapy have occurred in one of the gene therapy trials for SCID‐X1.22,23 Four infants developed lymphoproliferative disorders, with rapid expansion of a limited number of T cell clones. Chemotherapy was used to successfully induce remission, but one patient relapsed and subsequently died following allogeneic stem cell transplantation. Researchers have rigorously investigated the molecular mechanisms underlying these events. Possibilities included the mobilisation of replication competent vector leading to multiple integration events, toxic side effects due to unregulated transgene expression, or the unexpected activation/disruption of oncogenes following vector integration. When newly developed molecular techniques identified where in the genome vector integration had occurred in the expanded cells, the site mapped close to a known T cell proto‐oncogene, LMO2 in two of the cases.23 Thus, unanticipated vector mediated oncogene activation seems to have precipitated leukaemogenesis, but a number of other factors may have been relevant in these patients. In the context of lymphopaenia, gene corrected cells have a notable proliferation advantage and this combined with reduced immune surveillance in SCID patients, may have allowed abnormal clones to escape challenge and become leukaemogenic. There is also a possibility that in SCID‐X1 the nature of the γc transgene, with its central role in T cell development and function, is important, although suggestions that vector mediated expression of common γc may directly cause leukaemogenesis have been controversial.24,25
There are a number of generic issues for all gene therapy trials, relating to efficiency, efficacy and safety that are currently being addressed. Strategies are under development to limit the risk of insertional mutagenesis and to provide better regulated gene expression. One safety measure has been to design self‐inactivating vectors, including lentiviral based systems, that lack LTR promoter activity after reverse transcription.26 Additional tissue or gene specific promoter elements can be incorporated to provide better controlled transgene expression. Further modifications might include the use of insulator sequences to minimise transcriptional read‐through, or the targeting of gene insertion to sites that are known to be safe. With improvements in the safety profile of vectors in the pipeline, which other disorders might be corrected by gene therapy in the near future? The issues which favoured SCID as a gene therapy target, such as accessibility to the target tissue, the survival advantage conferred to gene modified cells, and the reduced likelihood of a host immune response against the gene product, will favour other conditions that are currently treated by HSCT. In these conditions it is generally more acceptable to use chemotherapy to promote the engraftment and to reduce the risk of immune mediated clearance of the gene modified stem cells. Patients with chronic granulomatous disease (CGD) have received infusions of autologous HSC modified to express gp91phox, the defective protein in X‐linked CGD. Gene corrected neutrophils facilitated the clearance of long‐standing infections, although longer term efficacy and safety have to be evaluated in more detail.27 Gene therapy trials for Fanconi anaemia (type A) showed transient effects but have been limited by the poor CD34+ stem cell harvests from patients with the disease (depletion of the bone marrow compartment is a characteristic of the condition).28 Gene therapy studies for other inherited conditions including β thalassaemia, adrenoleukodystrophy and Wiskott‐Aldrich syndrome have recently been initiated. Notable trials using similar vector technologies for other (non‐inherited) disorders that have recently reported successes have included experimental anti‐viral treatments in patients with HIV disease29 and novel anti‐tumour strategies to re‐direct T cell specificity towards tumour antigens in patients with malignant melanoma.30
In summary, gene therapy has been shown to be a valuable alternative to mismatched stem cell transplantation for infants with SCID, and with appropriate safeguards, should offer therapeutic benefit in a range of blood and immune system disorders in the next few years.
ADA - adenosine deaminase
CGD - chronic granulomatous disease
HSCT - haematopoietic stem cell transplantation
LTR - long terminal repeat
SCID - severe combined immune deficiency
Waseem Qasim is supported by the Leukaemia Research Fund and Adrian Thrasher is a Senior Wellcome Trust fellow.
Competing interests: None.