Evaluation of genotoxicity and mutagenic effects of vector/DNA nanocomplexes in transfected mesenchymal stem cells by flow cytometry

In recent years, there has been a great deal of interest in ex-vivo genetic modification of mesenchymal stem cells (MSCs) to meet various biomedical needs. Considering the self-renewal potential of MSCs, it is critically important to ensure that transfection vectors (gene carriers) do not induce genotoxicity because they could theoretically turn a single stem cell into a cancer-initiating cell. Unfortunately, there is currently no reliable, unbiased, and quantitative method to measure genotoxicity (micronuclei formation) of gene carriers directly in transfected MSCs. Consequently, it has not been possible to study the correlation of vectors' physicochemical characteristics with their impact on stem cell genome stability. To address this deficiency, a flow cytometry-based method with a specialized gating protocol was developed that not only measures micronuclei formation, but also determines the mechanism of mutagenesis (i.e., clastogenic vs. aneugenic) of each vector in transfected MSCs. This gating protocol effectively eliminates all interfering signals associated with aggregated nanoparticles (viral and non-viral), exogenous DNA, and apoptotic/necrotic bodies from the micronuclei measurement process. The presented gating protocol for flow cytometry, which is provided as a template, enables investigators in academia, industry and regulatory bodies to rapidly and reliably evaluate the genosafety profiles of gene carriers. The findings of this study also indicate that highly positively charged lipid- and polymeric-based vectors can induce genotoxicity even without manifesting substantial somatic toxicity. Thus, extreme care must be taken before implanting ex-vivo-modified MSCs back into a patient's body. Statement of significance There is a great interest in genetic modification of stem cells (SCs) by using vectors for various biomedical needs. Considering the self-renewal potential of SCs, it is essential to ensure that such vectors do not induce genetic aberrations (genotoxicity) because they could theoretically turn a single stem cell into a cancer-initiating cell. Unfortunately, there is currently no reliable method to measure genotoxicity of vectors directly in transfected SCs. To address this deficiency, a specialized flow cytometry-based method was developed that quantitatively analyzed genotoxicity and determined the mechanism of mutagenesis that occurred in transfected SCs during the transfection process. The developed technique will enable scientists to design safer vectors for genetic modification of stem cells.