Key result
CRISPR-Cas9 targeted transgenes at the AAVS1 locus in hiPSCs exhibited highly variable expression between clones and significant silencing during mesodermal differentiation, demonstrating it is not a true safe harbour.
Why the study?
Human induced pluripotent stem cell reporter lines are useful for disease modelling such as hypertrophic cardiomyopathy, and the AAVS1 safe harbour locus is favoured for creating stable gene targeted reporter lines.
Targeted knock-ins to the AAVS1 locus in hiPSCs exhibit variable expression and significant transgene silencing during cardiomyocyte differentiation, challenging its status as a completely safe harbour for disease modeling.
Answers are grounded in this paper and cite their sources.
AAVS1-targeted hiPSCs risk transgene silencing in cardiomyocytes; leaves open optimal safe-harbour loci for cardiac disease modeling.
Background: Diseases such as hypertrophic cardiomyopathy (HCM) can lead to severe outcomes including sudden death. The generation of human induced pluripotent stem cell (hiPSC) reporter lines can be useful for disease modelling and drug screening by providing physiologically relevant in vitro models of disease. The AAVS1 locus is cited as a safe harbour that is permissive for stable transgene expression, and hence is favoured for creating gene targeted reporter lines. Methods : We generated hiPSC reporters using a plasmid-based CRISPR/Cas9 nickase strategy. The first intron of PPP1R12C , the AAVS1 locus, was targeted with constructs expressing a genetically encoded calcium indicator (R-GECO1.0) or HOXA9-T2A-mScarlet reporter under the control of a pCAG or inducible pTRE promoter, respectively. Transgene expression was compared between clones before, during and/or after directed differentiation to mesodermal lineages. Results : Successful targeting to AAVS1 was confirmed by PCR and sequencing. Of 24 hiPSC clones targeted with pCAG-R-GECO1.0, only 20 expressed the transgene and in these, the percentage of positive cells ranged from 0% to 99.5%. Differentiation of a subset of clones produced cardiomyocytes, wherein the percentage of cells positive for R-GECO1.0 ranged from 2.1% to 93.1%. In the highest expressing R-GECO1.0 clones, transgene silencing occurred during cardiomyocyte differentiation causing a decrease in expression from 98.93% to 1.3%. In HOXA9-T2A-mScarlet hiPSC reporter lines directed towards mesoderm lineages, doxycycline induced a peak in transgene expression after two days but this reduced by up to ten-thousand-fold over the next 8-10 days. Nevertheless, for R-GECO1.0 lines differentiated into cardiomyocytes, transgene expression was rescued by continuous puromycin drug selection, which allowed the Ca 2+ responses associated with HCM to be investigated in vitro using single cell analysis. Conclusions: Targeted knock-ins to AAVS1 can be used to create reporter lines but variability between clones and transgene silencing requires careful attention by researchers seeking robust reporter gene expression.
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Bhagwan et al. (2019) studied Hypertrophic cardiomyopathy (in vitro modeling). CRISPR-Cas9 targeting of AAVS1 locus vs. Untargeted or different clones was evaluated on Transgene expression percentage. CRISPR-Cas9 targeted transgenes at the AAVS1 locus in hiPSCs exhibited highly variable expression between clones and significant silencing during mesodermal differentiation, demonstrating it is not a true safe harbour.
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