Gene therapy: WAS (not) just for kids

In this issue of Blood, Morris et al describe application of autologous transplant/gene therapy for Wiskott-Aldrich syndrome (WAS), first demonstrated to be efficacious and safe in children, to bring relief to a severely affected adult.¹  In recent years, gene therapy for blood cell diseases in which autologous hematopoietic stem cells (HSCs) are corrected using a lentiviral vector has led to clinical benefits similar to those from allogeneic transplant. Much of the focus has been on severe diseases affecting infants and young children (severe combined immunodeficiency, X-linked adrenoleukodystrophy, metachromatic leukodystrophy) in which early intervention has the potential to prevent the development of major cumulative disease-related complications. More recently, adult patients with β-thalassemia, chronic granulomatous disease, and even X-linked severe combined immunodeficiency, previously having suboptimal responses to nonconditioned allogeneic hematopoietic stem cell transplantation, have shown good response to autologous gene therapy.^2,3  These results have now been extended to WAS.

Several related clinical trials have been performed using a lentiviral vector in which WAS complementary DNA is under transcriptional control of a WAS gene promoter and expresses in all hematopoietic lineages.⁴  Results of clinical trials using this lentiviral vector to add the normal WAS gene to autologous CD34⁺ cells in 10 WAS subjects with severe disease manifestations have been reported (ages at transplant, 1 to 15 years).^5,6  Treated subjects have shown good immune recovery, decreases in inflammatory problems, and absence of new autoimmune problems; effects on elevating platelet counts have been more modest, but nonetheless they generally did increase from pretransplant levels. The reduced intensity conditioning transplants were well tolerated and there were no vector-related complications.

The patient treated by Morris et al was significantly older, age 30 years at transplant, with multiple serious lifelong complications from WAS.¹  He had recurrent bleeding and thrombocytopenia in early childhood (leading to diagnosis) that resolved with splenectomy; multiple autoimmune and inflammatory problems, including recurrent uveitis and arthritis, a characteristic leukocytoclastic vasculitis, with markedly elevated indices of inflammation (cytokines, C-reactive protein); and nonmalignant lymphoproliferation. He also had severely decreased renal function (glomerular filtration rate ∼24-34 mL/min), presumably from multiple prior courses of antibiotics and immunosuppressants, giving him a high pretransplant comorbidity index. He did not have matched family or unrelated donors identified and therefore was treated with gene therapy using autologous stem cells. Besides the absence of risk for graft-versus-host disease, another potential advantage of autologous transplant is that it allows avoidance of immunosuppressive agents that could further impair renal function.

The treatment approach, which is now a relatively standard process for gene therapy using HSC, involved mobilization of autologous HSC with granulocyte colony-stimulating factor and plerixafor for leukapheresis, isolation of CD34⁺ cells, and culture for 2 to 3 days for lentiviral-mediated gene transfer. During that time, he received moderate-intensity conditioning (busulfan 3.2 mg/kg daily × 3 days; fludarabine 40 mg/m² daily × 3) and then the gene-modified HSCs were reinfused. Frequent autoimmune and inflammatory complications in WAS patients could persist or recur after transplant; therefore, some immunosuppressive conditioning (such as fludarabine or serotherapy) is thought to be needed in addition to the myeloid ablative conditioning component (such as busulfan) needed for engraftment of the gene-modified autologous HSC.

He had a very satisfactory outcome with excellent improvement in quality of life, based on this report through 20 months after transplant. From the immune function perspective, there were clinically meaningful improvements in T- and B-cell function by multiple analyses (absolute numbers, responses to mitogens and antigens, T-cell receptor spectratyping, thymic recombinant excision circle quantification) as well as increases of immunoglobulins A and M into the normal ranges. He was able to stop immunoglobulin replacement therapy and then responded to vaccinations (albeit, partially). The frequency of gene-marked blood cells was determined using lineage-sorted cells and quantitative polymerase chain reaction. This analysis demonstrated stable persistence of gene-containing cells of each lineage from 3 months after transplant, with lineage-specific levels of “gene chimerism” ranging from ∼10% in granulocytes and monocytes (0.1 vector copy number on average), reflecting the frequency of gene-corrected stem cells; to ∼100% in T and B lymphocytes (1.0 vector copy number), demonstrating selective amplification of these cells because of their dependence on WAS protein function. As in the other trials reported using lentiviral vectors in HSC, the vector integration site analysis demonstrated high polyclonality, meaning many distinctly gene-marked stem cells have engrafted; most important, there was no evidence of clonal expansion from vector integrations.⁷  Over the time of follow-up, there have been decreases in the levels of inflammatory cytokines and other markers, although they have not fully normalized. He has been off immunosuppression without further vasculitis or arthritis and has resumed normal activities, which he was too ill to do before.

Gene therapy for WAS, which has already been shown to be effective and safe in children, may now be considered for older WAS patients surviving with severe disease manifestations and may provide a safer treatment than allogeneic transplant. There are large prevalent populations of adult patients affected with hemoglobinopathies and other immunodeficiencies who did not receive prior allogeneic transplants because of an absence of a suitable donor or limiting disease-related morbidities. Autologous transplant with gene therapy may be able to provide benefits to them as it becomes available. Of course, advances in graft engineering and the use of posttransplant cyclophosphamide are also bringing improved allogeneic transplant options and outcomes for patients of all ages with genetic blood cell diseases, and may the best therapy win for them!