Corrigendum: Optimized riboswitch-regulated AAV vector for VEGF-B gene therapy.

[This corrects the article DOI: 10.3389/fmed.2022.1052318.].

Development of Large-Scale Downstream Processing for Lentiviral Vectors.

The interest in lentiviral vectors (LVs) has increased prominently for gene therapy applications, but few have reached the later stages of clinical trials. The main challenge has remained in scaling up the manufacturing process for the fragile vector to obtain high titers for in vivo usage. We have previously scaled up the LV production to iCELLis 500, being able to produce up to 180 L of harvest material in one run with perfusion. The following challenge considers the purification and concentration of the product to meet titer and purity requirements for clinical use. We have developed a downstream process, beginning with clarification, buffer exchange, and concentration, by tangential flow filtration. This is followed by a purification step using single membrane-based anion exchange chromatography and final formulation with tangential flow filtration. Different materials and conditions were compared to optimize the process, especially for the chromatography step that has been the bottleneck in lentiviral vector purification scale-up. The final infectious titer of the lentiviral vector product manufactured using the optimized scale-up process was determined to be 1.97 × 109 transducing units (TU)/mL, which can be considered as a high titer for lentiviral vectors.

Preclinical Proof-of-Concept, Analytical Development, and Commercial Scale Production of Lentiviral Vector in Adherent Cells.

The therapeutic efficacy of a lentiviral vector (LV) expressing the herpes simplex virus thymidine kinase (HSV-TK) was studied in an immunocompetent rat glioblastoma model. Intraperitoneal ganciclovir injections (50 mg/kg/day) were administered for 14 consecutive days, resulting in reduced tumor volumes as monitored by MRI. Survival analyses revealed a significant improvement among the LV-expressing HSV-TK (LV-TK)/ganciclovir-treated animals when compared to non-treated control rats. However, a limiting factor in the use of LV has been the suboptimal small-scale production in flasks. Our aim during the translation phase, prior to entering the final pre-clinical and early clinical phases, was to develop a scalable, robust, and disposable manufacturing process for LV-TKs. We also aimed to minimize future process changes and enable production upscaling to make the process suitable for larger patient populations. The upstream process relies on fixed-bed iCELLis technology and transient plasmid transfection. This is the first time iCELLis 500 commercial-scale bioreactor was used for LV production. A testing strategy to determine the pharmacological activity of LV-TK drug product by measuring cell viability was developed, and the specificity of the potency assay was also proven. In this paper we focus on upstream process development while showing analytical development and the proof-of-concept of LV-TK functionality.

Cancer-associated mutations in chromatin remodeler hSNF5 promote chromosomal instability by compromising the mitotic checkpoint.

The hSNF5 subunit of human SWI/SNF ATP-dependent chromatin remodeling complexes is a tumor suppressor that is inactivated in malignant rhabdoid tumors (MRTs). Here, we report that loss of hSNF5 function in MRT-derived cells leads to polyploidization and chromosomal instability. Re-expression of hSNF5 restored the coupling between cell cycle progression and ploidy checkpoints. In contrast, cancer-associated hSNF5 mutants harboring specific single amino acid substitutions exacerbated poly- and aneuploidization, due to abrogated chromosome segregation. We found that hSNF5 activates the mitotic checkpoint through the p16INK4a-cyclinD/CDK4-pRb-E2F pathway. These results establish that poly- and aneuploidy of tumor cells can result from mutations in a chromatin remodeler.

P16INK4a is required for hSNF5 chromatin remodeler-induced cellular senescence in malignant rhabdoid tumor cells.

The hSNF5 chromatin-remodeling factor is a tumor suppressor that is inactivated in malignant rhabdoid tumors (MRTs). A number of studies have shown that hSNF5 re-expression blocks MRT cell proliferation. However, the pathway through which hSNF5 acts remains unknown. To address this question, we generated MRT-derived cell lines in which restoration of hSNF5 expression leads to an accumulation in G(0)/G(1), induces cellular senescence and increased apoptosis. Following hSNF5 expression, we observed transcriptional activation of the tumor suppressor p16(INK4a) but not of p14(ARF), repression of several cyclins and CD44, a cell surface glycoprotein implicated in metastasis. Chromatin immunoprecipitations indicated that hSNF5 activates p16(INK4a) transcription and CD44 down-regulation by mediating recruitment of the SWI/SNF complex. Thus, hSNF5 acts as a dualistic co-regulator that, depending on the promoter context, can either mediate activation or repression. Three lines of evidence established that p16(INK4a) is an essential effector of hSNF5-induced cell cycle arrest. 1) Overexpression of p16(INK4a) mimics the effect of hSNF5 induction and leads to cellular senescence. 2) Expression of a p16(INK4a)-insensitive form of CDK4 obstructs hSNF5-induced cell cycle arrest. 3) Inhibition of p16(INK4a) activation by siRNA blocks hSNF5-mediated cellular senescence. Collectively, these results indicate that in human MRT cells, the p16(INK4a)/pRb, rather than the p14(ARF)/p53 pathway, mediates hSNF5-induced cellular senescence.

Analysis of putative interactions between potyviral replication proteins and plant retinoblastoma proteins.

Sequence comparisons suggest that the RNA-dependent RNA polymerase (NIb) of potyviruses and bymoviruses, as well as the viral polymerase of potexviruses may contain a putative retinoblastoma protein (pRb) binding motif. The possibility that the potyviral NIb may function in the nucleus through interactions with plant pRb-related (RBR) proteins, and the modifications of the cell cycle was investigated by a combination of mutagenesis of the NIb and yeast two-hybrid system (YTHS). Mutation of a highly conserved glutamic acid residue in the putative pRb-binding motif of the NIb had no detectable phenotypic effect on replication of Potato virus A (PVA). Furthermore, the NIb proteins from Potato virus V and PVA failed to interact with maize or tobacco RBR proteins in yeast. Although the conservation of the motif for pRb interaction in plant RNA viruses is intriguing, these proteins from plant RNA viruses appear not to interact with plant RBR proteins.