Silencing of optogenetic and chemogenetic transgenes in human iPSCs involves promoter methylation and methylation-independent mechanisms

The transplantation of neural progenitor cells derived from induced pluripotent stem cells (iPSCs) has therapeutic potential for the treatment of neurological diseases. However, the functional integration of transplanted iPSC-derived neurons into host neural networks remains controversial. Optogenetic and chemogenetic tools offer the means to assess such integration. However, constructing modifiable iPSC-derived neurons requires efficient gene editing. Here, we used CRISPR/Cas9 (targeting the <i>AAVS1</i> safe harbor) and PiggyBac transposon systems to insert optogenetic and chemogenetic receptors (ChR2/hM4Di) into human iPSCs. While both systems successfully integrated genes into the genomes of HEK293T cells and iPSCs, receptor expression was detected only in HEK293T cells. Bisulfite sequencing revealed extensive methylation of the TRE3G BI promoter (95.3-98.2%) in iPSCs, in contrast to low methylation (5.9%) in HEK293T cells. For PiggyBac, the methylation of CMV/EF1α promoters in iPSCs exhibited integration site-dependent variability (0-95.2%). Notably, even hypomethylated clones failed to show gene expression, suggesting that additional regulatory mechanisms, such as histone modifications or chromatin remodeling, may contribute to transcriptional silencing. Differentiation into neural stem cells does not reverse methylation nor restore protein expression. Our findings demonstrate that the CRISPR/Cas9 and PiggyBac systems enable the integration of optochemical receptor genes into iPSCs. However, promoter methylation or other epigenetic and non-epigenetic gene-silencing mechanisms could pose barriers to efficient protein expression from the integrated transgene in iPSCs.