"RCL-Pooling Assay": A Simplified Method for the Detection of Replication-Competent Lentiviruses in Vector Batches Using Sequential Pooling.

Nonreplicative recombinant HIV-1-derived lentiviral vectors (LV) are increasingly used in gene therapy of various genetic diseases, infectious diseases, and cancer. Before they are used in humans, preparations of LV must undergo extensive quality control testing. In particular, testing of LV must demonstrate the absence of replication-competent lentiviruses (RCL) with suitable methods, on representative fractions of vector batches. Current methods based on cell culture are challenging because high titers of vector batches translate into high volumes of cell culture to be tested in RCL assays. As vector batch size and titers are continuously increasing because of the improvement of production and purification methods, it became necessary for us to modify the current RCL assay based on the detection of p24 in cultures of indicator cells. Here, we propose a practical optimization of this method using a pairwise pooling strategy enabling easier testing of higher vector inoculum volumes. These modifications significantly decrease material handling and operator time, leading to a cost-effective method, while maintaining optimal sensibility of the RCL testing. This optimized "RCL-pooling assay" ameliorates the feasibility of the quality control of large-scale batches of clinical-grade LV while maintaining the same sensitivity.

Transfusion independence and HMGA2 activation after gene therapy of human ß-thalassaemia.

The ß-haemoglobinopathies are the most prevalent inherited disorders worldwide. Gene therapy of ß-thalassaemia is particularly challenging given the requirement for massive haemoglobin production in a lineage-specific manner and the lack of selective advantage for corrected haematopoietic stem cells. Compound ß(E)/ß(0)-thalassaemia is the most common form of severe thalassaemia in southeast Asian countries and their diasporas. The ß(E)-globin allele bears a point mutation that causes alternative splicing. The abnormally spliced form is non-coding, whereas the correctly spliced messenger RNA expresses a mutated ß(E)-globin with partial instability. When this is compounded with a non-functional ß(0) allele, a profound decrease in ß-globin synthesis results, and approximately half of ß(E)/ß(0)-thalassaemia patients are transfusion-dependent. The only available curative therapy is allogeneic haematopoietic stem cell transplantation, although most patients do not have a human-leukocyte-antigen-matched, geno-identical donor, and those who do still risk rejection or graft-versus-host disease. Here we show that, 33 months after lentiviral ß-globin gene transfer, an adult patient with severe ß(E)/ß(0)-thalassaemia dependent on monthly transfusions since early childhood has become transfusion independent for the past 21 months. Blood haemoglobin is maintained between 9 and 10 g dl(-1), of which one-third contains vector-encoded ß-globin. Most of the therapeutic benefit results from a dominant, myeloid-biased cell clone, in which the integrated vector causes transcriptional activation of HMGA2 in erythroid cells with further increased expression of a truncated HMGA2 mRNA insensitive to degradation by let-7 microRNAs. The clonal dominance that accompanies therapeutic efficacy may be coincidental and stochastic or result from a hitherto benign cell expansion caused by dysregulation of the HMGA2 gene in stem/progenitor cells.

eGFP reporter genes silence LCRbeta-globin transgene expression via CpG dinucleotides.

beta-Globin transgenes regulated by the locus control region (LCR) are dominantly silenced by linked bacterial reporter genes in transgenic mice. Enhanced green fluorescent protein (eGFP) from jellyfish is an alternative reporter used in retrovirus vectors to transfer LCRbeta-globin genes into bone marrow. We show here that the eGFP coding sequence silences LCRbeta-globin in transgenic mice, but the PGK promoter did not provoke such silencing. As eGFP contains 60 CpG dinucleotides, which are targets of DNA methylation, we synthesized a novel CpG-free variant called dmGFP. Its utility was demonstrated in MSCV retrovirus vectors transcriptionally controlled by the viral 5'LTR or internal PGK or EF1alpha promoter. Specific fluorescence was detected from eGFP, and at lower levels from dmGFP, in transduced mouse CFU-S and embryonic stem cells. While eGFP was rarely silenced in CFU-S, dmGFP was not silenced in these progenitors. Moreover, the dmGFP coding sequence did not silence LCRbeta-globin in transgenic mice, showing that the eGFP silencing mechanism acts primarily via CpG dinucleotides. However, LCRbeta-globin expression remained suboptimal, indicating that other silencing pathways recognize dmGFP in the absence of CpG dinucleotides. We conclude that dmGFP ameliorates silencing, but optimal LCRbeta-globin expression is obtained in the absence of nonmammalian reporters.

Long-term doxycycline-regulated secretion of erythropoietin by encapsulated myoblasts.

We developed an ex vivo gene therapy approach for the regulated delivery of therapeutic proteins based on the implantation of encapsulated, genetically engineered C(2)C(12) myoblasts. We investigated doxycycline-based regulation of gene expression to modulate the secretion of erythropoietin (EPO) from encapsulated myoblasts in a mouse model. An autoregulatory tet-off system provided high induction levels with low basal expression in the noninduced state. Stable C(2)C(12) clones constitutively secreted between 25 and 50 IU mouse EPO/10(6)cells/24 hours in the on-state. The clone C15, selected for its in vivo survival characteristics, displayed a desirable secretion profile when encapsulated. Devices released 5 IU EPO per capsule in the on-state, with EPO levels being undetectable upon the addition of doxycycline (dox). Capsules subcutaneously implanted in DBA/2J mice demonstrated a tightly regulated secretion of EPO through up to four on-off cycles during a period lasting 40 weeks. Hematocrits could be modulated between basal levels (40-50%) and elevated levels (70-90%) through the presence or absence of dox in the drinking water. Hematocrit returned to normal levels, paralleling the kinetics observed following capsule explantation, 6 to 8 weeks following dox administration to polycythemic mice. The results of this study suggest that encapsulation and implantation of a tet-off regulated C(2)C(12) cell clone represents a safe method for the controlled long-term delivery of proteins in vivo.