Production of high titre helper-free recombinant retroviral vectors by lipofection

Lentiviral Vector Bioprocessing

Lentiviral vectors (LVs) are potent tools for the delivery of genes of interest into mammalian cells and are now commonly utilised within the growing field of cell and gene therapy for the treatment of monogenic diseases and adoptive therapies such as chimeric antigen T-cell (CAR-T) therapy. This is a comprehensive review of the individual bioprocess operations employed in LV production. We highlight the role of envelope proteins in vector design as well as their impact on the bioprocessing of lentiviral vectors. An overview of the current state of these operations provides opportunities for bioprocess discovery and improvement with emphasis on the considerations for optimal and scalable processing of LV during development and clinical production. Upstream culture for LV generation is described with comparisons on the different transfection methods and various bioreactors for suspension and adherent producer cell cultivation. The purification of LV is examined, evaluating different sequences of downstream process operations for both small- and large-scale production requirements. For scalable operations, a key focus is the development in chromatographic purification in addition to an in-depth examination of the application of tangential flow filtration. A summary of vector quantification and characterisation assays is also presented. Finally, the assessment of the whole bioprocess for LV production is discussed to benefit from the broader understanding of potential interactions of the different process options. This review is aimed to assist in the achievement of high quality, high concentration lentiviral vectors from robust and scalable processes.

Biotechnology in the treatment of sensorineural hearing loss: foundations and future of hair cell regeneration

PURPOSE

To provide an overview of the methodologies involved in the field of hair cell regeneration. First, the author provides a tutorial on the biotechnological foundations of this field to assist the reader in the comprehension and interpretation of the research involved in hair cell regeneration. Next, the author presents a review of stem cell and gene therapy and provides a critical appraisal of their application to hair cell regeneration. The methodologies used in these approaches are highlighted.

METHOD

The author conducted a narrative review of the fields of cellular, molecular, and developmental biology, tissue engineering, and stem cell and gene therapy using the PubMed database.

RESULTS

The use of biotechnological approaches to the treatment of hearing loss--approaches such as stem cell and gene therapy-has led to new methods of regenerating cochlear hair cells in mammals.

CONCLUSIONS

Incredible strides have been made in assembling important pieces of the puzzle that comprise hair cell regeneration. However, mammalian hair cell regeneration using stem cell and gene therapy are years--if not decades--away from being clinically feasible. If the goals of the biological approaches are met, these therapies may represent future treatments for hearing loss.

DTL/CDT2 is essential for both CDT1 regulation and the early G2/M checkpoint

Checkpoint genes maintain genomic stability by arresting cells after DNA damage. Many of these genes also control cell cycle events in unperturbed cells. By conducting a screen for checkpoint genes in zebrafish, we found that dtl/cdt2 is an essential component of the early, radiation-induced G2/M checkpoint. We subsequently found that dtl/cdt2 is required for normal cell cycle control, primarily to prevent rereplication. Both the checkpoint and replication roles are conserved in human DTL. Our data indicate that the rereplication reflects a requirement for DTL in regulating CDT1, a protein required for prereplication complex formation. CDT1 is degraded in S phase to prevent rereplication, and following DNA damage to prevent origin firing. We show that DTL associates with the CUL4-DDB1 E3 ubiquitin ligase and is required for CDT1 down-regulation in unperturbed cells and following DNA damage. The cell cycle defects of Dtl-deficient zebrafish are suppressed by reducing Cdt1 levels. In contrast, the early G2/M checkpoint defect appears to be Cdt1-independent. Thus, DTL promotes genomic stability through two distinct mechanisms. First, it is an essential component of the CUL4-DDB1 complex that controls CDT1 levels, thereby preventing rereplication. Second, it is required for the early G2/M checkpoint.

Abnormal intracellular trafficking of high affinity nerve growth factor receptor, Trk, in stable transfectants expressing presenilin 1 protein

The pathogenesis of Alzheimer's disease (AD) is now thought to be tightly linked to Abeta deposition and oxidative stress, but it is still unknown how these factors result in neuronal dysfunction and cell death. Mutations of presenilin 1 (PS1) gene are the causative gene for early onset familial AD (FAD) due to the overproduction and deposition of pathogenic Abeta1-42 peptides. We report here the molecular influences of the overexpression of PS1 protein by stable transfection of PS1 cDNA into SH-SY5Y neuroblastoma cells on the function of high affinity nerve growth factor receptor, Trk, that is essential for neuronal survival and differentiation. We examined the sensitivity of these transfectants to oxidative stress and found that mutant (I143T) PS1-expressing clones showed the highest vulnerability to an oxidative stress inducer, hydrogen peroxide treatment compared with that of mock-transfected clones, whereas wild PS1-expressing cells were less vulnerable to the treatment than mutant PS1 transfectants. Because nerve growth factor (NGF) is known to protect neuronal cells from oxidative stress-induced cell death, we examined the NGF-Trk-mediated intracellular signaling pathway in these transfectants. In the wild and mutant PS1 cDNA-transfected cells, NGF did not elicit the autophosphorylation response of Trk, although their basal levels of tyrosine phosphorylation were higher than those of mock-transfected cells. Immunocytochemical and subcellular fractionation studies revealed that most of Trk proteins are abnormally located in the cytoplasm as well as in the nucleus in PS1-overexpressing clones irrespective of wild and mutant forms. These results strongly indicate that the expression level of PS1 protein has a cross talk with the Trk-dependent neuroprotective intracellular signaling pathway.

Modulation of RNA editing by functional nucleolar sequestration of ADAR2

The adenosine deaminases that act on RNA (ADARs) catalyze the site-specific conversion of adenosine to inosine (A to I) in primary mRNA transcripts, thereby affecting the splicing pattern or coding potential of mature mRNAs. Although the subnuclear localization of A-to-I editing has not been precisely defined, ADARs have been shown to act before splicing, suggesting that they function near nucleoplasmic sites of transcription. Here we demonstrate that ADAR2, a member of the vertebrate ADAR family, is concentrated in the nucleolus, a subnuclear domain disparate from the sites of mRNA transcription. Selective inhibition of ribosomal RNA synthesis or the introduction of mutations in the double-stranded RNA-binding domains within ADAR2 results in translocation of the protein to the nucleoplasm, suggesting that nucleolar association of ADAR2 depends on its ability to bind to ribosomal RNA. Fluorescence recovery after photobleaching reveals that ADAR2 can shuttle rapidly between subnuclear compartments. Enhanced translocation of endogenous ADAR2 from the nucleolus to the nucleoplasm results in increased editing of endogenous ADAR2 substrates. These observations indicate that the nucleolar localization of ADAR2 represents an important mechanism by which RNA editing can be modulated by the sequestration of enzymatic activity from potential RNA substrates in the nucleoplasm.

Deletion of a short, untranslated region adjacent to the polypurine tract in Moloney murine leukemia virus leads to formation of aberrant 5' plus-strand DNA ends in vivo

Experiments were performed to determine the function of a 28-nucleotide untranslated sequence lying between the envelope gene and the polypurine tract (PPT) sequence in the Moloney murine leukemia virus (Mo-MuLV) genome. A mutant virus carrying a deletion of this sequence (Mo-MuLVDelta28) replicated more slowly than wild-type (wt) virus and reverted by recombination with endogenous sequences during growth in NIH 3T3 cells. We show that this deletion did not affect the level of viral protein expression or genomic RNA packaging. Mo-MuLVDelta28 served as a helper virus as efficiently as the wt virus; in contrast, a retroviral vector harboring this mutation exhibited reduced transduction efficiency, indicating that the mutation acts not in trans but in cis. Analysis of acutely infected cells revealed that reduced levels of viral DNA were generated by reverse transcription of the Mo-MuLVDelta28 RNA as compared to the wt RNA. Analysis of DNA circle junctions revealed that plus-strand DNA of Mo-MuLVDelta28 but not wt virus often retained the PPT and additional upstream sequences. These structures suggest that aberrant 5' ends of plus-strand DNA were generated by a failure to remove the PPT RNA primer and/or by mispriming at sites upstream of the PPT. These data demonstrate that the major role of the sequences immediately upstream of the PPT is specifying efficient and accurate plus-strand DNA synthesis.

Genetic correction of dystrophin deficiency and skeletal muscle remodeling in adult MDX mouse via transplantation of retroviral producer cells

Duchenne muscular dystrophy (DMD) is an X-linked, lethal disease caused by mutations of the dystrophin gene. No effective therapy is available, but dystrophin gene transfer to skeletal muscle has been proposed as a treatment for DMD. We have developed a strategy for efficient in vivo gene transfer of dystrophin cDNA into regenerating skeletal muscle. Retroviral producer cells, which release a vector carrying the therapeutically active dystrophin minigene, were mitotically inactivated and transplanted in adult nude/mdx mice. Transplantation of 3 x 10(6) producer cells in a single site of the tibialis anterior muscle resulted in the transduction of between 5.5 and 18% total muscle fibers. The same procedure proved also feasible in immunocompetent mdx mice under short-term pharmacological immunosuppression. Minidystrophin expression was stable for up to 6 mo and led to alpha-sarcoglycan reexpression. Muscle stem cells could be transduced in vivo using this procedure. Transduced dystrophic skeletal muscle showed evidence of active remodeling reminiscent of the genetic normalization process which takes place in female DMD carriers. Overall, these results demonstrate that retroviral-mediated dystrophin gene transfer via transplantation of producer cells is a valid approach towards the long-term goal of gene therapy of DMD.

Efficiency of in vivo gene transfer using murine retroviral vectors is strain-dependent in mice

Retroviral vectors can be used to transduce cultured cells at high frequencies, but efficient transduction of target cells in vivo has proved difficult and little is known about the factors that influence the efficiency of retroviral infection. Many commonly used mouse strains harbor endogenous C-type proviruses, some of which are expressed and have circulating antibodies against the viral envelope glycoproteins that cross-react with the Moloney strain of murine leukemia virus (MoMLV), from which most current retroviral vectors are derived. We have investigated the relative efficiency of retroviral-mediated gene transfer into regenerating skeletal muscle of a variety of mouse strains using a MoMLV-based vector. Humoral immune competence and interference between endogenous MLVs and exogenous recombinant MoMLV were observed to affect the efficiency of retroviral-mediated transfection in vivo. Our results indicate that the mouse genetic background and immune status need to be considered when choosing a preclinical model for in vivo retroviral-mediated gene transfer.