Lenti in red: progress in gene therapy for human hemoglobinopathies

Reprogramming erythroid cells for lysosomal enzyme production leads to visceral and CNS cross-correction in mice with Hurler syndrome

Restricting transgene expression to maturing erythroid cells can reduce the risk for activating oncogenes in hematopoietic stem cells (HSCs) and their progeny, yet take advantage of their robust protein synthesis machinery for high-level protein production. This study sought to evaluate the feasibility and efficacy of reprogramming erythroid cells for production of a lysosomal enzyme, alpha-L-iduronidase (IDUA). An erythroid-specific hybrid promoter provided inducible IDUA expression and release during in vitro erythroid differentiation in murine erythroleukemia cells, resulting in phenotypical cross-correction in an enzyme-deficient lymphoblastoid cell line derived from patients with mucopolysaccharidosis type I (MPS I). Stable and higher than normal plasma IDUA levels were achieved in vivo in primary and secondary MPS I chimeras for at least 9 months after transplantation of HSCs transduced with the erythroid-specific IDUA-containing lentiviral vector (LV). Moreover, long-term metabolic correction was demonstrated by normalized urinary glycosaminoglycan accumulation in all treated MPS I mice. Complete normalization of tissue pathology was observed in heart, liver, and spleen. Notably, neurological function and brain pathology were significantly improved in MPS I mice by erythroid-derived, higher than normal peripheral IDUA protein. These data demonstrate that late-stage erythroid cells, transduced with a tissue-specific LV, can deliver a lysosomal enzyme continuously at supraphysiological levels to the bloodstream and can correct the disease phenotype in both viscera and CNS of MPS I mice. This approach provides a paradigm for the utilization of RBC precursors as a depot for efficient and potentially safer systemic delivery of nonsecreted proteins by ex vivo HSC gene transfer.

Genotoxic potential of lineage-specific lentivirus vectors carrying the beta-globin locus control region

Insertional mutagenesis by long terminal repeat (LTR) enhancers in gamma-retrovirus-based vectors (GVs) in clinical trials has prompted deeper investigations into vector genotoxicity. Experimentally, self-inactivating (SIN) lentivirus vectors (LVs) and GV containing internal promoters/enhancers show reduced genotoxicity, although strong ubiquitously-active enhancers dysregulate genes independent of vector type/design. Herein, we explored the genotoxicity of beta-globin (BG) locus control region (LCR), a strong long-range lineage-specific-enhancer, with/without insulator (Ins) elements in LV using primary hematopoietic progenitors to generate in vitro immortalization (IVIM) assay mutants. LCR-containing LV had approximately 200-fold lower transforming potential, compared to the conventional GV. The LCR perturbed expression of few genes in a 300 kilobase (kb) proviral vicinity but no upregulation of genes associated with cancer, including an erythroid-specific transcription factor occurred. A further twofold reduction in transforming activity was observed with insulated LCR-containing LV. Our data indicate that toxicology studies of LCR-containing LV in mice will likely not yield any insertional oncogenesis with the numbers of animals that can be practically studied.

Supplying clotting factors from hematopoietic stem cell-derived erythroid and megakaryocytic lineage cells

Systemically distributed proteins such as clotting factors have been traditionally expressed from muscle or liver to achieve therapeutic, long-term expression. As long-lived cell capable of generating an abundant progeny, hematopoietic stem cells (HSCs) also merit consideration for this purpose. To be clinically relevant, this approach would require that hematopoietic cells be capable of expressing high levels of functional, secreted proteins, that the risk of insertional oncogenesis be minimized, and that sufficient stem cell engraftment be achieved following minimally invasive conditioning. Recent reports demonstrate the feasibility of expressing either factor IX (FIX) or factor VIII (FVIII) in erythroblasts and platelets using lineage-restricted vectors, resulting in effective treatments in mouse models of hemophilia. The erythroid system is especially powerful in providing high protein output, yielding FIX levels approaching 1 micro g/ml per vector copy in the plasma of long-term hematopoietic chimeras, a secretion level that vastly outperforms any other current mammalian constitutive or long-terminal repeat (LTR)-driven vector system. These early but promising studies raise the prospect of further developing these strategies for clinical investigation.

A novel human gamma-globin gene vector for genetic correction of sickle cell anemia in a humanized sickle mouse model: critical determinants for successful correction

We show that lentiviral delivery of human gamma-globin gene under beta-globin regulatory control elements in hematopoietic stem cells (HSCs) results in sufficient postnatal fetal hemoglobin (HbF) expression to correct sickle cell anemia (SCA) in the Berkeley "humanized" sickle mouse. Upon de-escalating the amount of transduced HSCs in transplant recipients, using reduced-intensity conditioning and varying gene transfer efficiency and vector copy number, we assessed critical parameters needed for correction. A systematic quantification of functional and hematologic red blood cell (RBC) indices, organ pathology, and life span was used to determine the minimal amount of HbF, F cells, HbF/F-cell, and gene-modified HSCs required for correcting the sickle phenotype. We show that long-term amelioration of disease occurred (1) when HbF exceeded 10%, F cells constituted two-thirds of the circulating RBCs, and HbF/F cell was one-third of the total hemoglobin in sickle RBCs; and (2) when approximately 20% gene-modified HSCs repopulated the marrow. Moreover, we show a novel model using reduced-intensity conditioning to determine genetically corrected HSC threshold that corrects a hematopoietic disease. These studies provide a strong preclinical model for what it would take to genetically correct SCA and are a foundation for the use of this vector in a human clinical trial.

The sea urchin sns5 insulator protects retroviral vectors from chromosomal position effects by maintaining active chromatin structure

Silencing and position-effect (PE) variegation (PEV), which is due to integration of viral vectors in heterochromatin regions, are considered significant obstacles to obtaining a consistent level of transgene expression in gene therapy. The inclusion of chromatin insulators into vectors has been proposed to counteract this position-dependent variegation of transgene expression. Here, we show that the sea urchin chromatin insulator, sns5, protects a recombinant gamma-retroviral vector from the negative influence of chromatin in erythroid milieu. This element increases the probability of vector expression at different chromosomal integration sites, which reduces both silencing and PEV. By chromatin immunoprecipitation (ChIP) analysis, we demonstrated the specific binding of GATA1 and OCT1 transcription factors and the enrichment of hyperacetylated nucleosomes to sns5 sequences. The results suggest that this new insulator is able to maintain a euchromatin state inside the provirus locus with mechanisms that are common to other characterized insulators. On the basis of its ability to function as barrier element in erythroid milieu and to bind the erythroid specific factor GATA1, the inclusion of sns5 insulator in viral vectors may be of practical benefit in gene transfer applications and, in particular, for gene therapy of erythroid disorders.

Harnessing endogenous miR-181a to segregate transgenic antigen receptor expression in developing versus post-thymic T cells in murine hematopoietic chimeras

MicroRNAs (miRNAs) are small, noncoding RNAs that regulate gene expression by targeting complementary sequences, referred to as miRNA recognition elements (MREs), typically located in the 3' untranslated region of mRNAs. miR-181a is highly expressed in developing thymocytes and markedly downregulated in post-thymic T cells. We investigated whether endogenous miR-181a can be harnessed to segregate expression of chimeric antigen receptors (CARs) and TCRs between developing and mature T cells. Lentiviral-encoded antigen receptors were tagged with a miR-181a-specific MRE and transduced into mouse BM cells that were used to generate hematopoietic chimeras. Expression of a CAR specific for human CD19 (hCD19) was selectively suppressed in late double-negative and double-positive thymocytes, coinciding with the peak in endogenous miR-181a expression. Receptor expression was fully restored in post-thymic resting and activated T cells, affording protection against a subsequent challenge with hCD19+ tumors. Hematopoietic mouse chimeras engrafted with a conalbumin-specific TCR prone to thymic clonal deletion acquired peptide-specific T cell responsiveness only when the vector-encoded TCR transcript was similarly engineered to be subject to regulation by miR-181a. These results demonstrate the potential of miRNA-regulated transgene expression in stem cell-based therapies, including cancer immunotherapy.