Thomas R. Bauer

Gains of glycosylation comprise an unexpectedly large group of pathogenic mutations

Mutations involving gains of glycosylation have been considered rare, and the pathogenic role of the new carbohydrate chains has never been formally established. We identified three children with mendelian susceptibility to mycobacterial disease who were homozygous with respect to a missense mutation in IFNGR2 creating a new N-glycosylation site in the IFNγR2 chain. The resulting additional carbohydrate moiety was both necessary and sufficient to abolish the cellular response to IFNγ. We then searched the Human Gene Mutation Database for potential gain-of-N-glycosylation missense mutations; of 10,047 mutations in 577 genes encoding proteins trafficked through the secretory pathway, we identified 142 candidate mutations (∼1.4%) in 77 genes (∼13.3%). Six mutant proteins bore new N-linked carbohydrate moieties. Thus, an unexpectedly high proportion of mutations that cause human genetic disease might lead to the creation of new N-glycosylation sites. Their pathogenic effects may be a direct consequence of the addition of N-linked carbohydrate.

Successful treatment of canine leukocyte adhesion deficiency by foamy virus vectors

Recent successes in treating genetic immunodeficiencies have demonstrated the therapeutic potential of stem cell gene therapy. However, the use of gammaretroviral vectors in these trials led to insertional activation of nearby oncogenes and leukemias in some study subjects, prompting studies of modified or alternative vector systems. Here we describe the use of foamy virus vectors to treat canine leukocyte adhesion deficiency (CLAD). Four of five dogs with CLAD that received nonmyeloablative conditioning and infusion of autologous, CD34+ hematopoietic stem cells transduced by a foamy virus vector expressing canine CD18 had complete reversal of the CLAD phenotype, which was sustained more than 2 years after infusion. In vitro assays showed correction of the lymphocyte proliferation and neutrophil adhesion defects that characterize CLAD. There were no genotoxic complications, and integration site analysis showed polyclonality of transduced cells and a decreased risk of integration near oncogenes as compared to gammaretroviral vectors. These results represent the first successful use of a foamy virus vector to treat a genetic disease, to our knowledge, and suggest that foamy virus vectors will be effective in treating human hematopoietic diseases.

Exposure of human megakaryocytes to high shear rates accelerates platelet production

Platelets originate from megakaryocytes (MKs) by cytoplasmic elongation into proplatelets. Direct platelet release is not seen in bone marrow hematopoietic islands. It was suggested that proplatelet fragmentation into platelets can occur intravascularly, yet evidence of its dependence on hydrodynamic forces is missing. Therefore, we investigated whether platelet production from MKs could be up-regulated by circulatory forces. Human mature MKs were perfused at a high shear rate on von Willebrand factor. Cells were observed in real time by videomicroscopy, and by confocal and electron microscopy after fixation. Dramatic cellular modifications followed exposure to high shear rates: 30% to 45% adherent MKs were converted into proplatelets and released platelets within 20 minutes, contrary to static conditions that required several hours, often without platelet release. Tubulin was present in elongated proplatelets and platelets, thus ruling out membrane tethers. By using inhibitors, we demonstrated the fundamental roles of microtubule assembly and MK receptor GPIb. Secretory granules were present along the proplatelet shafts and in shed platelets, as shown by P-selectin labeling. Platelets generated in vitro were functional since they responded to thrombin by P-selectin expression and cytoskeletal reorganization. In conclusion, MK exposure to high shear rates promotes platelet production via GPIb, depending on microtubule assembly and elongation.

Haploidentical In Utero Hematopoietic Cell Transplantation Improves Phenotype and Can Induce Tolerance for Postnatal Same-Donor Transplants in the Canine Leukocyte Adhesion Deficiency Model

In the murine model, in utero hematopoietic cell transplantation (IUHCT) has been shown to achieve low levels of allogeneic chimerism and associated donor-specific tolerance permitting minimal conditioning postnatal hematopoietic stem cell transplantation (HSCT). In this pilot study, we investigated IUHCT in the canine leukocyte adhesion deficiency (CLAD) model. Haploidentical IUHCT resulted in stable low-level donor cell chimerism in all dogs that could be analyzed by sensitive detection methodology (4 of 10) through 18 months of follow-up. In the 2 CLAD recipients, low-level chimerism resulted in amelioration and complete reversal of the CLAD phenotype, respectively. Six recipients of IUHCT (5 carriers and 1 CLAD) subsequently received postnatal HSCT from the same haploidentical prenatal donor after minimal conditioning with busulfan 10 mg/kg. Chimerism in 2 of 5 CLAD carriers that underwent HSCT increased from < 1% pre-HSCT to sustained levels of 35% to 45%. Control animals undergoing postnatal haploidentical HSCT without IUHCT had no detectable donor chimerism. These results demonstrate that haploidentical IUHCT in the CLAD model can result in low-level donor chimerism that can prevent the lethal phenotype in CLAD dogs, and can result in donor-specific tolerance that can facilitate postnatal minimal conditioning HSCT.

Mixed chimeric hematopoietic stem cell transplant reverses the disease phenotype in canine leukocyte adhesion deficiency.

The genetic disease canine leukocyte adhesion deficiency (CLAD) is characterized by recurrent, severe bacterial infections, typically culminating in death by 6 months of age. CLAD is due to a mutation in the leukocyte integrin CD18 subunit, which prevents surface expression of the CD11/CD18 leukocyte integrin complex. We demonstrate that stable mixed donor:host hematopoietic chimerism, achieved by a non-myeloablative bone marrow transplant from a histocompatible littermate, reverses the disease phenotype in CLAD. Donor chimerism following the transplant was demonstrated both by flow cytometric detection of donor-derived CD18-positive leukocytes in the peripheral blood of the recipient, and by the demonstration of donor-derived DNA microsatellite repeats in the peripheral blood leukocytes of the recipient. These results indicate that mixed hematopoietic chimerism reverses the clinical phenotype in CLAD and represents a potential therapeutic approach for the human disease leukocyte adhesion deficiency.

Leukocyte Adhesion Deficiency in Children and Irish Setter Dogs

Children with the genetic immunodeficiency disease leukocyte adhesion deficiency, or LAD, develop life-threatening bacterial infections as a result of the inability of their leukocytes to adhere to the vessel wall and migrate to the sites of infection. Recently, the canine counterpart to LAD, known as canine leukocyte adhesion deficiency, or CLAD, has been described in Irish setter dogs. This review describes how the clinical phenotype of dogs with CLAD closely parallels that of children with the severe deficiency phenotype of LAD, thus enabling the CLAD dog to provide a disease-specific, large-animal model for testing novel hematopoietic stem cell and gene therapy strategies before their translation to children with LAD.

Long-Term Follow-up of Foamy Viral Vector-Mediated Gene Therapy for Canine Leukocyte Adhesion Deficiency

The development of leukemia following gammaretroviral vector-mediated gene therapy for X-linked severe combined immunodeficiency disease and chronic granulomatous disease (CGD) has emphasized the need for long-term follow-up in animals treated with hematopoietic stem cell gene therapy. In this study, we report the long-term follow-up (4-7 years) of four dogs with canine leukocyte adhesion deficiency (CLAD) treated with foamy viral (FV) vector-mediated gene therapy. All four CLAD dogs previously received nonmyeloablative conditioning with 200 cGy total body irradiation followed by infusion of autologous, CD34(+) hematopoietic stem cells transduced by a FV vector expressing canine CD18 from an internal Murine Stem Cell Virus (MSCV) promoter. CD18(+) leukocyte levels were >2% following infusion of vector-transduced cells leading to ongoing reversal of the CLAD phenotype for >4 years. There was no clinical development of lymphoid or myeloid leukemia in any of the four dogs and integration site analysis did not reveal insertional oncogenesis. These results showing disease correction/amelioration of disease in CLAD without significant adverse events provide support for the use of a FV vector to treat children with leukocyte adhesion deficiency type 1 (LAD-1) in a human gene therapy clinical trial.

Canine leukocyte adhesion deficiency colony for investigation of novel hematopoietic therapies

The genetic immunodeficiency disease canine leukocyte adhesion deficiency (CLAD) was originally described in juvenile Irish Setters with severe, recurrent bacterial infections. CLAD was subsequently shown to result from a mutation in the leukocyte integrin CD18 subunit which prevents leukocyte surface expression of the CD11/CD18 complex. We describe the development of a mixed-breed CLAD colony with clinical features that closely parallel those described in Irish Setters. We demonstrate that the early identification of CLAD heterozygotes and CLAD-affected dogs by a combination of flow cytometry and DNA sequencing allows the CLAD-affected animals to receive life-saving antibiotic therapy. The distinct clinical phenotype in CLAD, the ability to detect CD18 on the leukocyte surface by flow cytometry, and the history of the canine model in marrow transplantation, enable CLAD to serve as an attractive large-animal model for the investigation of novel hematopoietic stem cell and gene therapy strategies.

Gene Therapy of Canine Leukocyte Adhesion Deficiency Using Lentiviral Vectors With Human CD11b and CD18 Promoters Driving Canine CD18 Expression

To identify cellular promoters in a self-inactivating (SIN) lentiviral vector that might be beneficial in treating children with leukocyte adhesion deficiency type 1 (LAD-1), we tested lentiviral vectors with human CD11 and CD18 leukocyte integrin proximal promoter elements directing expression of canine CD18 in animals with canine LAD (CLAD). Lentiviral vectors with either the human CD11b (637 bp) proximal promoter or the human CD18 (1,060 bp) proximal promoter resulted in the highest percentages of CD18(+) CLAD CD34(+) cells in vitro. Subsequently, two CLAD dogs were infused with autologous CD34(+) cells transduced with the hCD11b (637 bp)-cCD18 vector, and two CLAD dogs were infused with autologous CD34(+) cells transduced with the hCD18 (1,060 bp)-cCD18 vector. Each dog received a nonmyeloablative dose of 200 cGy total body irradiation (TBI) before the infusion of transduced cells. The two CLAD dogs treated with the hCD18 (1,060 bp)-cCD18 vector, and one of the two dogs treated with the hCD11b (637 bp)-cCD18 vector, had reversal of the CLAD phenotype. These studies using endogenous leukocyte integrin proximal promoters represent an important step in the development of gene therapy for children with LAD-1.

Treatment of canine leukocyte adhesion deficiency by foamy virus vectors expressing CD18 from a PGK promoter

Proto-oncogene activation caused by retroviral vector integration can cause malignancies in gene therapy trials. This has led investigators to search for less genotoxic vectors with minimal enhancer activity and a decreased risk of influencing neighboring chromosomal gene expression after integration. We previously showed that foamy virus (FV) vectors expressing the canine CD18 gene from an internal murine stem cell virus (MSCV) promoter could cure canine leukocyte adhesion deficiency (LAD). Here, we have repeated these studies using a FV vector expressing canine CD18 from a phosphoglycerate kinase (PGK) gene promoter. In vitro analysis showed that this vector did not contain an enhancer that activated neighboring genes, and it expressed CD18 efficiently in canine neutrophils and CD34+ cells. However, dogs that received hematopoietic stem cells transduced with the PGK-CD18 vector continued to suffer from LAD, and sometimes died prematurely of the disease. These studies show that the PGK promoter cannot effectively replace the MSCV promoter in CD18-expressing FV vectors, and they suggest that vectors containing a strong promoter–enhancer may be necessary for the treatment of human LAD.