An in vitro system for efficiently evaluating gene therapy approaches to hemoglobinopathies

Human telomerase reverse transcriptase protects hematopoietic progenitor TF-1 cells from death and quiescence induced by cytokine withdrawal

Telomerase is a complex ribonucleoprotein enzyme that exhibits elevated activity in the majority of cases of human leukemia. We have previously shown that retroviral expression of the catalytic subunit of telomerase, human telomerase reverse transcriptase (hTERT), in human cord blood CD34+ cells leads to an enhanced survival of mature hematopoietic cells. The mechanism for this pro-survival effect is not known. Here, we show that telomerase may play a role in leukemogenesis as a survival factor, independent of its role in maintaining telomere length. Retroviral expression of hTERT in the cytokine-dependent, human hematopoietic progenitor cell line, TF-1, resulted in the survival of cells following the withdrawal of cytokine, with protection from apoptosis, but did not promote unlimited replicative potential. This hTERT-mediated effect on cell survival does not involve Bcl-2 family members, results in accumulation of cells in G1 and appears to operate via autocrine expression of IL-3 and activation of the p53/p21 pathway. Survival in the absence of cytokine stimulation was also observed following retroviral expression of hTERT in normal cord blood CD34+ cells. This study demonstrates a novel pro-survival role for hTERT and may have important implications for the role of hTERT in the pathogenesis of leukemia and drug resistance.

Anabolic effects of a G protein-coupled receptor kinase inhibitor expressed in osteoblasts

G protein-coupled receptors (GPCRs) play a key role in regulating bone remodeling. Whether GPCRs exert anabolic or catabolic osseous effects may be determined by the rate of receptor desensitization in osteoblasts. Receptor desensitization is largely mediated by direct phosphorylation of GPCR proteins by a family of enzymes termed GPCR kinases (GRKs). We have selectively manipulated GRK activity in osteoblasts in vitro and in vivo by overexpressing a GRK inhibitor. We found that expression of a GRK inhibitor enhanced parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor-stimulated cAMP generation and inhibited agonist-induced phosphorylation of this receptor in cell culture systems, consistent with attenuation of receptor desensitization. To determine the effect of GRK inhibition on bone formation in vivo, we targeted the expression of a GRK inhibitor to mature osteoblasts using the mouse osteocalcin gene 2 (OG2) promoter. Transgenic mice demonstrated enhanced bone remodeling as well as enhanced urinary excretion of the osteoclastic activity marker dexoypyridinoline. Both osteoprotegrin and OPG ligand mRNA levels were altered in calvaria of transgenic mice in a pattern that would promote osteoclast activation. The predominant effect of the transgene, however, was anabolic, as evidenced by an increase in bone density and trabecular bone volume in the transgenic mice compared with nontransgenic littermate controls.

Overexpression of Phex in osteoblasts fails to rescue the Hyp mouse phenotype

Inactivating mutations of Phex, a phosphate-regulating endopeptidase, cause hypophosphatemia and impaired mineralization in X-linked hypophosphatemia (XLH) and its mouse homologue, Hyp. Because Phex is predominantly expressed in bone and cultured osteoblasts from Hyp mice display an apparent intrinsic mineralization defect, it is thought that reduced expression of Phex in mature osteoblasts is the primary cause of XLH. To test this hypothesis, we studied both targeted expression of Phex to osteoblasts in vivo under the control of the mouse osteocalcin (OG2) promoter and retroviral mediated overexpression of Phex in Hyp-derived osteoblasts (TMOb-Hyp) in vitro. Targeted overexpression of Phex to osteoblasts of OG2 Phex transgenic Hyp mice normalized Phex endopeptidase activity in bone but failed to correct the hypophosphatemia, rickets, or osteomalacia. OG2 Phex transgenic Hyp mice did exhibit a small, but significant, increase in bone mineral density and dry ashed weight, suggesting a partial mineralization effect from restoration of Phex function in mature osteoblasts. Similarly, retroviral mediated overexpression of Phex in TMOb-Hyp osteoblasts restored Phex mRNA levels, protein expression, and endopeptidase activity but failed to correct their intrinsic mineralization defect. In addition, we failed to detect the Phex substrate FGF-23 in osteoblasts. Taken together, these in vivo and in vitro data indicate that expression of Phex in osteoblasts is not sufficient to rescue the Hyp phenotype and that other sites of Phex expression and/or additional factors are likely to be important in the pathogenesis of XLH.

Analysis of recombinant Phex: an endopeptidase in search of a substrate

X-linked hypophosphatemia (XLH) is caused by inactivating mutations of Phex, a phosphate-regulating endopeptidase. Further advances in our knowledge of the pathogenesis of XLH require identification of the biological function of Phex and its physiologically relevant substrates. We evaluated several potential substrates using mouse recombinant wild-type Phex proteins (rPhex-WT) and inactive mutant Phex proteins (rPhex-3'M) lacking the COOH-terminal catalytic domain as controls. By Western blot analysis, we demonstrated that Phex is a membrane-bound 100-kDa glycosylated monomer. Neither casein, a substrate for the related endopeptidase thermolysin, human stanniocalcin 1 (hSTC-1), an osteoblast-derived phosphate-regulating factor, nor FGF-23 peptide (amino acid 172-186), comprising the region mutated in autosomal dominant hypophosphatemia, was cleaved by rPhex-WT. In addition, membranes expressing rPhex-WT, rPhex-3'M, and the empty vector hydrolyzed parathyroid hormone-(1-34), indicating the lack of Phex-specific cleavage of parathyroid hormone. In contrast, rPhex-WT did display an EDTA-dependent cleavage of the neutral endopeptidase substrate [Leu]enkephalin. Further studies with wild-type and mutant rPhex proteins should permit the identification of physiologically relevant substrates involved in the pathogenesis of XLH.

Efficient retrovirus-mediated PIG-A gene transfer and stable restoration of GPI-anchored protein expression in cells with the PNH phenotype

Paroxysmal nocturnal hemoglobinuria (PNH) is a clonal hematopoietic stem cell disorder characterized by complement-mediated hemolysis due to deficiencies of glycosylphosphatidylinositol-anchored proteins (GPI-APs) in subpopulations of blood cells. Acquired mutations in the X-linked phosphatidylinositol glycan-class A (PIG-A) gene appear to be the characteristic and pathogenetic cause of PNH. To develop a gene therapy approach for PNH, a retroviral vector construct, termed MPIN, was made containing the PIG-A complementary DNA along with an internal ribosome entry site and the nerve growth factor receptor (NGFR) as a selectable marker. MPIN transduction led to efficient and stable PIG-A and NGFR gene expression in a PIG-A-deficient B-cell line (JY5), a PIG-A-deficient K562 cell line, an Epstein-Barr virus-transformed B-cell line (TK-14(-)) established from a patient with PNH, as well as peripheral blood (PB) mononuclear cells from a patient with PNH. PIG-A expression in these cell lines stably restored GPI-AP expression. MPIN was transduced into bone marrow mononuclear cells from a patient with PNH, and myeloid/erythroid colonies and erythroid cells were derived. These transduced erythroid cells restored surface expression of GPI-APs and resistance to hemolysis. These results indicate that MPIN is capable of efficient and stable functional restoration of GPI-APs in a variety of PIG-A-deficient hematopoietic cell types. Furthermore, MPIN also transduced into PB CD34(+) cells from a normal donor, indicating that MPIN can transduce primitive human progenitors. These findings set the stage for determining whether MPIN can restore PIG-A function in multipotential stem cells, thereby providing a potential new therapeutic option in PNH.

Current status of retroviral vector mediated gene transfer into human hematopoietic stem cells

Genetic modification of hematopoietic stem cells (HSCs) has been proposed as a treatment strategy for a variety of hematologic diseases, tracking marked cells or conferring resistance to chemotherapeutic agents. Despite early enthusiasm, the results of clinical studies involving gene transfer into HSCs has not resulted in therapeutic benefits for the vast majority of treated patients. This review describes the limitations and advances that have been made in the areas of gene transfer vectors, identification of the appropriate HSCs to target for genetic modifications and the methods used to perform gene transfer.

Current status of retroviral vector mediated gene transfer into human hematopoietic stem cells

Genetic modification of hematopoietic stem cells (HSCs) has been proposed as a treatment strategy for a variety of hematologic diseases, tracking marked cells or conferring resistance to chemotherapeutic agents. Despite early enthusiasm, the results of clinical studies involving gene transfer into HSCs have not resulted in therapeutic benefits for the vast majority of treated patients. This review describes the limitations and advances that have been made in the areas of gene transfer vectors, identification of the appropriate HSCs to target for genetic modifications and the methods used to perform gene transfer.