Functional Correction of Fanconi Anemia Group A Hematopoietic Cells by Retroviral Gene Transfer

Sleeping Beauty-mediated correction of Fanconi anemia type C

BACKGROUND

The Sleeping Beauty (SB) transposon system can insert defined sequences into chromosomes to direct the extended expression of therapeutic genes. Our goal is to develop the SB system for nonviral complementation of Fanconi anemia (FA), a rare autosomal recessive disorder accompanied by progressive bone marrow failure.

METHODS

We used a CytoPulse electroporation system (CytoPulse, Glen Burnie, MD, USA) to introduce SB transposons into human lymphoblastoid cells (LCL) derived from both Fanconi anemia type C (FA-C) defective and normal patients. Correction of the FA-C defect was assessed by resistance to mitomycin C, a DNA-crosslinking agent.

RESULTS

Culture of both cell types with the antioxidant N-acetyl- l-cysteine improved cell viability after electroporation. Co-delivery of enhanced green fluorescent protein (GFP) transposon with SB100X transposase-encoding plasmid supported a 50- to 90-fold increase in stable GFP expression compared to that observed in the absence of SB100X for normal LCL, but in FA-C defective LCL SB100X enhancement of stable GFP-expression was a more moderate five- to 13-fold. SB-mediated integration and expression of the FA-C gene was demonstrated by the emergence of a mitomycin C-resistant population bearing characteristic transposon-chromosome junction sequences and exhibiting a mitomycin dose response identical to that of normal LCL.

CONCLUSIONS

The SB transposon system achieved stable expression of therapeutic FA-C genes, complementing the genetic defect in patient-derived cells by nonviral gene transfer.

Stem cell clonality and genotoxicity in hematopoietic cells: Gene activation side effects should be avoidable

Two serious adverse events involving activation of the LMO2 oncogene through retrovirus vector insertion in the otherwise extremely successful first gene therapy trial for X-linked severe combined immunodeficieny type 1 (SCID-X1) had initially caused widespread concern in the patient and research communities. Careful consideration 1 year after diagnosis of the second case still finds 12 of the treated patients clearly benefiting from gene therapy (freedom from treatment failure, 80%; survival 100%), a situation that should not portend the end of gene therapy for this disease, and is, in fact encouraging. While current approaches are justified to treat patients with otherwise life-threatening disorders, a broad consensus has developed that systematic basic research is required to further understand the pathophysiology of these serious adverse events and to provide new insights, enabling safer and more effective gene therapy strategies. With the continued success of SCID-X1 gene therapy in the majority of patients treated, it is of even greater importance to understand exactly which vector element or combination of elements predispose to toxicity. An in-depth study of the mechanisms behind the activation of the LMO2 and gammac genes will be highly instructive for the development of safer procedures and vectors. We summarize the central observations, ongoing experimental approaches, new concepts, and developments relevant to understanding, interpreting, and eventually overcoming the real and perceived obstacles posed by insertional mutagenesis due to gene transfer vectors.

Update on gene therapy for hereditary hematological disorders

The past 3 years have been characterized by a number of impressive advances as well as setbacks in gene therapy for genetic disease. Children with X-linked severe combined immunodeficiency disorder (SCID-X1) have shown almost complete reconstitution of their immune system after receiving retrovirally transduced autologous CD34+ hematopoietic stem cells (HSCs). However, two of 11 treated patients subsequently developed a leukemia-like disease probablydue to the undesired activation of an oncogene. Gene transfer to HSCs resulted in substantial correction of immune function and multi-lineage engraftment in two patients with adenosine deaminase (ADA)-SCID. Several Phase I clinical trials for treatment of hemophilia A and B have been initiated or completed. Partial correction of hemophilia A, albeit transient, has been reported by ex vivo gene transfer to autologous fibroblasts. Intramuscular injection of adeno-associated viral (AAV) vector to patients with severe hemophilia B resulted in evidence of Factor IX gene transfer to skeletal muscle and a separate trial based on hepatic infusion of AAV vector is ongoing. Sustained therapeutic levels of coagulation factor expression have been achieved in preclinical models using retroviral, lentiviral, AAV and high capacity adenoviral vectors. Efficient lentiviral gene transfer to HSC in murine models of beta-thalassemia and sickle cell disease demonstrated sustained phenotypic correction.

Gene therapy for hereditary hematological disorders

The year 2000 saw the first successful treatment of a genetic disorder by gene therapy. Pediatric patients with X-linked severe combined immunodeficiency disorder (SCID-X1) received autologous CD34+ hematopoietic cells following ex vivo gene transfer using a retroviral vector, with subsequent demonstration of improved immune responses. A number of preclinical and clinical studies have been conducted with the aim of developing gene therapy for hemophilia, Fanconi anemia, sickle cell disease, beta-thalassemia, chronic granulomatous disease, and other inherited hematological disorders. The greatest advances in novel approaches toward treatment of hematological disorders have been made in hemophilia, with 3 current phase I clinical trials ongoing. Two trials are investigating the safety and feasibility of utilizing either an ex vivo, non-viral gene transfer technique or an intravenous infusion of a retroviral vector to treat adults with severe hemophilia A (factor VIII deficiency). The third study involves intramuscular administration of an adeno-associated viral (AAV) vector for expression of factor IX in adult patients with hemophilia B. Results from this study and from preclinical studies preceding the trial demonstrate that it is possible to safely administer high doses of a viral vector in vivo.

Phenotypic correction of primary Fanconi anemia T cells with retroviral vectors as a diagnostic tool

OBJECTIVE

The aim of this study was to develop a rapid laboratory procedure that is capable of subtyping Fanconi anemia (FA) complementation groups FA-A, FA-C, FA-G, and FA-nonACG patients from a small amount of peripheral blood.

MATERIALS AND METHODS

For this test, primary peripheral blood-derived FA T cells were transduced with oncoretroviral vectors that expressed FANCA, FANCC, or FANCG cDNA. We achieved a high efficiency of gene transfer into primary FA T cells by using the fibronectin fragment CH296 during transduction. Transduced cells were analyzed for correction of the characteristic DNA cross-linker hypersensitivity by cell survival or by metaphase analyses.

RESULTS

Retroviral vectors containing the cDNA for FA-A, FA-C, and FA-G, the most frequent complementation groups in North America, allowed rapid identification of the defective gene by complementation of primary T cells from 12 FA patients.

CONCLUSION

Phenotypic correction of FA T cells using retroviral vectors can be used successfully to determine the FA complementation group immediately after diagnosis of the disease.

[Fanconi's anemia and molecular biology research]

Fanconi's anemia is a rare autosomal recessive disease characterized by congenital abnormalities, a progressive pancytopenia and a predisposition to cancer. The diagnosis is based on an abnormal increase of spontaneous chromosome breakage, more specifically on a clear-cut increase of chromosome breakage in the presence of bifunctional alkylating agents. Eight complementation groups (A to H) have been defined, and the genes corresponding to four of these groups have been cloned (FANCA, FANCC, FANCF and FANCG). The function of the proteins encoded by the genes of Fanconi's anemia remains unknown. Numerous studies indicate that different cellular processes are probably involved, including DNA repair pathways, apoptosis, cell cycle regulation and oxygen metabolism. Nevertheless, the exact cellular and molecular mechanisms implicated in Fanconi's anemia remain a challenge for fundamental research. The treatment of Fanconi's anemia is also the subject of intense research, bearing principally upon bone marrow transplantation, which is successful in the case of HLA-identical sibling donors, and gene therapy, which is still at a preliminary stage on the clinical level.

Functional analysis of patient-derived mutations in the Fanconi anemia gene, FANCG/XRCC9

OBJECTIVE

Fanconi anemia (FA) is an autosomal-recessive cancer susceptibility syndrome with seven complementation groups. Six of the FA genes have been cloned (corresponding to subtypes A, C, D2, E, F, and G) and the encoded proteins interact in a common pathway. Patient-derived mutations in FA genes have been helpful in delineating functional domains of FA proteins. The purpose of this work was to subtype FA patient-derived cell lines in our repository and to identify FA gene mutations.

METHODS

We subtyped 62 FA patients as type A, G, C, or non-ACG by using a combination of retroviral gene transfer and immunoblot analysis. Among these FA patients, we identified six FA-G patients for further analysis. We used a strategy involving amplification of FANCG/XRCC9 exons and direct sequencing to identify novel FANCG mutations in cell lines derived from these FA-G patients. We functionally analyzed FANCG mutant alleles by transducing the corresponding cDNAs into a known FA-G indicator cell line and scoring correction of MMC sensitivity.

RESULTS

Our results demonstrate a wide range of mutations in the FANCG gene (splice, nonsense, and missense mutations). Based on this mutational screen, a carboxy terminal functional domain of the FANCG protein appears to be required for complementation of FA-G cells and for normal assembly of the FANCA/FANCG/FANCC protein complex.

CONCLUSION

The identification of patient-derived mutant alleles of FA genes can provide important insights to the function of FA proteins. FA subtyping is also a necessary precondition for gene therapy.

Current knowledge on the pathophysiology of Fanconi anemia: from genes to phenotypes

Fanconi anemia (FA) is an autosomal recessive disease characterized by congenital anomalies, bone marrow failure, and leukemia susceptibility. FA cells show chromosome instability and hypersensitivity to DNA cross-linking agents such as mitomycin C. Recent studies indicate that there are at least 8 genetically distinct FA groups (A, B, C, D1, D2, E, F, G). To date, 6 genes (for A, C, D2, E, F, and G) have been cloned. In this review, we describe the structures and functions of FA proteins. Increasing evidence indicates that the multiple FA proteins cooperate in a biochemical pathway and/or a multimer complex. FANCD2, a downstream component of the FA pathway, has recently been shown to be ubiquitinated in response to DNA damage and to translocate to nuclear foci containing BRCA1, a breast cancer susceptibility gene product, suggesting a role for this protein in DNA repair functions. We also describe 2 emerging issues: genotype-phenotype relationships and mosaicism. The FA pathway is likely to play a critical role as a caretaker of genomic integrity in hematopoietic stem cells. Clarifying the molecular basis of this disease may provide new insights into the pathogenesis of bone marrow failure syndromes and myeloid malignancies.

Persistent low-level engraftment of rhesus peripheral blood progenitor cells transduced with the fanconi anemia C gene after conditioning with low-dose irradiation

The hematopoietic stem cell has long been considered an ideal target for the introduction of therapeutic genes to treat human disorders such as Fanconi anemia (FA). Although recent progress in large animal models is encouraging, application to nonmalignant conditions is limited by the perceived necessity of myeloablative conditioning. We and others have shown that very low irradiation doses are sufficient to allow significant hematopoietic engraftment in murine hosts even after the introduction of xenogeneic genes. To determine the degree of engraftment of genetically modified cells attainable with very low irradiation doses in larger animals, we employed the rhesus macaque competitive repopulation model. Four animals underwent mobilization with stem cell factor (SCF) and granulocyte colony-stimulating factor (G-CSF) followed by apheresis. The apheresis product was enriched for the CD34-positive fraction by immunomagnetic selection and split equally for transduction with either G1FC26, a retroviral vector carrying the Fanconi anemia complementation group C gene, or PLII, a nonexpression control retroviral vector carrying both neomycin and beta-galactosidase gene sequences modified to prevent translation. Transductions were performed daily in the presence of fresh IL-3, IL-6, SCF, and Flt-3 ligand on fibronectin-coated plates over 96 h. Animals were conditioned with a single dose of either 100 (n = 2) or 200 (n = 2) cGy and received the combined products of transduction on the following day. None of the animals experienced clinically significant neutropenia nor required the use of central line placement, transfusional support with blood products, or intravenous antibiotics. Using real-time PCR, circulating levels of genetically modified cells as high as 1% were initially detected. Stable, albeit, significantly lower levels from both vector-transduced aliquots (<0.1%) persisted beyond 12 months posttransplant in all four animals. Although not sufficient to correct the phenotype in many human disorders, stable low-level engraftment by genetically modified cells following low-intensity conditioning may prove adequate in disorders such as FA due to the selective advantage conferred upon corrected cells.

The emerging genetic and molecular basis of Fanconi anaemia

The past few years have witnessed a considerable expansion in our understanding of the pathways that maintain chromosome stability in dividing cells through the identification of genes that are mutated in certain human chromosome instability disorders. Cells that are derived from patients with Fanconi anaemia (FA) show spontaneous chromosomal instability and mutagen hypersensitivity, but FA poses a unique challenge as the nature of the DNA-damage-response pathway thought to be affected by the disease has long been a mystery. However, the recent cloning of most of the FA-associated genes, and the characterization of their protein products, has provided tantalizing clues as to the molecular basis of this disease.

Gene therapy for genetic haematological disorders and immunodeficiencies

Gene transfer and autologous transplantation of haematopoietic stem cells (HSCs) from patients with genetic haematological disorders and immunodeficiencies could provide the same benefits as allogeneic HSC transplantation, without the attendant immunological complications. Inefficient gene delivery to human HSCs has imposed the major limitation to successful application of gene therapy. A recently reported clinical trial of gene transfer into HSCs of infants with X-linked severe combined immunodeficiency (SCID) has achieved immune restoration because of the selective outgrowth of the gene-corrected lymphocytes. Newer methods for manipulating HSCs may lead to efficacy for other disorders. The problems and progress in this area are reviewed herein.

The genetics of Fanconi's anaemia

Fanconi's anaemia (FA) is an inherited bone marrow failure syndrome characterized by considerable clinical and cellular heterogeneity. This has also been recently demonstrated at the genetic and molecular levels following cloning of four out of the seven FA genes. Although this now enables molecular diagnosis in the majority of patients, because of the considerable molecular heterogeneity, the diepoxybutane/mitomycin-C stress test based on the increased chromosomal instability seen in FA cells, compared to normal controls, remains the front-line diagnostic test. This FA cell hallmark has led to the suggestion that FA may represent a defect in DNA repair although the precise function of the cloned FA genes remains unknown. Recent data suggest that they function in a novel cell pathway which has an important role in maintaining chromosome stability. The advances in the genetics of FA have already had some impact on diagnosis--for example, identification of patients with somatic mosaicism who have atypical clinical presentations--but to date they have had little impact on treatment. However, new treatments may now follow; indeed, for a number of reasons, FA may be a good candidate for haemopoietic gene therapy.

Viral vectors for gene transfer: a review of their use in the treatment of human diseases

The efficient delivery of therapeutic genes and appropriate gene expression are the crucial issues for clinically relevant gene therapy. Viruses are naturally evolved vehicles which efficiently transfer their genes into host cells. This ability made them desirable for engineering virus vector systems for the delivery of therapeutic genes. The viral vectors recently in laboratory and clinical use are based on RNA and DNA viruses processing very different genomic structures and host ranges. Particular viruses have been selected as gene delivery vehicles because of their capacities to carry foreign genes and their ability to efficiently deliver these genes associated with efficient gene expression. These are the major reasons why viral vectors derived from retroviruses, adenovirus, adeno-associated virus, herpesvirus and poxvirus are employed in more than 70% of clinical gene therapy trials worldwide. Among these vector systems, retrovirus vectors represent the most prominent delivery system, since these vectors have high gene transfer efficiency and mediate high expression of therapeutic genes. Members of the DNA virus family such as adenovirus-, adeno-associated virus or herpesvirus have also become attractive for efficient gene delivery as reflected by the fast growing number of clinical trials using these vectors. The first clinical trials were designed to test the feasibility and safety of viral vectors. Numerous viral vector systems have been developed for ex vivo and in vivo applications. More recently, increasing efforts have been made to improve infectivity, viral targeting, cell type specific expression and the duration of expression. These features are essential for higher efficacy and safety of RNA- and DNA-virus vectors. From the beginning of development and utilisation of viral vectors it was apparent that they harbour risks such as toxicities, immunoresponses towards viral antigens or potential viral recombination, which limit their clinical use. However, many achievements have been made in vector safety, the retargeting of virus vectors and improving the expression properties by refining vector design and virus production. This review addresses important issues of the current status of viral vector design and discusses their key features as delivery systems in gene therapy of human inherited and acquired diseases at the level of laboratory developments and of clinical applications.

Phenotypic correction of Fanconi anemia group C knockout mice

Fanconi anemia (FA) is a genetic disorder characterized by bone marrow failure, congenital anomalies, and a predisposition to malignancy. FA cells demonstrate hypersensitivity to DNA cross-linking agents, such as mitomycin C (MMC). Mice with a targeted disruption of the FANCC gene (fancc −/− nullizygous mice) exhibit many of the characteristic features of FA and provide a valuable tool for testing novel therapeutic strategies. We have exploited the inherent hypersensitivity offancc −/− hematopoietic cells to assay for phenotypic correction following transfer of the FANCC complementary DNA (cDNA) into bone marrow cells. Murine fancc −/− bone marrow cells were transduced with the use of retrovirus carrying the humanfancc cDNA and injected into lethally irradiated recipients. Mitomycin C (MMC) dosing, known to induce pancytopenia, was used to challenge the transplanted animals. Phenotypic correction was determined by assessment of peripheral blood counts. Mice that received cells transduced with virus carrying the wild-type gene maintained normal blood counts following MMC administration. All nullizygous control animals receiving MMC exhibited pancytopenia shortly before death. Clonogenic assay and polymerase chain reaction analysis confirmed gene transfer of progenitor cells. These results indicate that selective pressure promotes in vivo enrichment offancc-transduced hematopoietic stem/progenitor cells. In addition, MMC resistance coupled with detection of the transgene in secondary recipients suggests transduction and phenotypic correction of long-term repopulating stem cells.

Engraftment of hematopoietic progenitor cells transduced with the Fanconi anemia group C gene (FANCC)

Fanconi anemia (FA) is an autosomal recessive disorder that leads to aplastic anemia. Mutations in the FANCC gene account for 10-15% of cases. FA cells are abnormally sensitive to DNA-damaging agents such as mitomycin C (MMC). Transfection of normal FANCC into mutant cells corrects this hypersensitivity and improves their viability in vitro. Four FA patients, representing the three major FANCC mutation subgroups, were entered into a clinical trial of gene transduction aimed at correction of the hematopoietic defect. Three patients received three or four cycles of gene transfer, each consisting of one or two infusions of autologous hematopoietic progenitor cells that had been transduced ex vivo with a retroviral vector carrying the normal FANCC gene. Prior to infusion, the FANCC transgene was demonstrated in transduced CD34-enriched progenitor cells. After infusion, FANCC was also present transiently in peripheral blood (PB) and bone marrow (BM) cells. Function of the normal FANCC transgene was suggested by a marked increase in hematopoietic colonies measured by in vitro cultures, including colonies grown in the presence of MMC, after successive gene therapy cycles in all patients. Transient improvement in BM cellularity coincided with this expansion of hematopoietic progenitors. A fourth patient, who received a single infusion of transduced CD34-enriched BM cells, was given radiation therapy for a concurrent gynecologic malignancy. The FANCC transgene was detected in her PB and BM cells only after recovery from radiation-induced aplasia, suggesting that FANCC gene transduction confers a selective engraftment advantage. These experiments highlight both the potential and difficulties in applying gene therapy to FA.

Intracellular localization of the Fanconi anemia complementation group A protein

Mutations in the Fanconi anemia (FA) complementation group A (FANCA) gene leads to bone marrow failure, developmental abnormalities and cancer predisposition. To map the intracellular site of FANCA, we constructed a plasmid vector which linked in-frame the enhanced green fluorescent protein (EGFP cDNA) to the 5' end of the FANCA cDNA (pDAS-3). We studied the expression of pDAS-3 in the FANCA mutant fibroblast cell line (GM6914). MMC sensitivity of pDAS-3 transfected cells was comparable to wild-type fibroblasts. The resulting fluorescence pattern in the stable pDAS-3 cell line expressing the fusion protein was primarily nuclear. EGFP-selected cells (lacking FANCA) remain hypersensitive to MMC and maintained a cytoplasmic fluorescence pattern. Using deletion mutants of pDAS-3, a nuclear localization domain was identified at the amino terminus of the polypeptide. Western blot results of FANCA protein confirmed the presence of FANCA in nuclear fractions and FANCA protein levels did not vary during cell cycling. This nuclear trafficking of FANCA should guide future work in defining the function of this protein.