Background

Human pluripotent stem cells (hPSCs) possess the potential to differentiate into all cell types in the human body. In particular, patient-specific induced pluripotent stem cells (hiPSCs) are considered key materials for creating organoids, such as brain and kidney organoids. However, in actual research settings, the differentiation efficiency varies significantly between cell lines, which has often posed a problem that reduces the reliability of research results. Even under the same conditions, some hPSC cell lines differentiate well into brain organoids, while others differentiate into other tissues instead of the brain or stop developing. To elucidate the cause, the academic community has tracked genetic factors such as DNA sequence changes and DNA methylation, but these alone could not fully explain the differentiation differences. It has been revealed that specific cell lines lose their differentiation ability due to epigenetic modifications that occur naturally during the culture process. In particular, the phenomenon in which the pathway leading to nerve cells is blocked has been identified as a critical weakness in disease modeling and therapeutic development. There was a need for a technology that could erase the unique memory of stem cells and restore their differentiation ability.

Key Findings

The research team led by Dr. Madeline Lancaster at the University of Cambridge, UK, has discovered that the limit of stem cell differentiation ability is determined at the chromatin level. The research team found that cell lines that fail to form brain organoids are fixed in a posterior epiblast-like state, which forms the posterior tissue during embryonic development. These cells tend to express genes that form the spinal cord or tail early, before differentiating into the anterior tissue, the brain. The researchers confirmed that this abnormal state results from the loss of bivalent chromatin, which regulates gene expression. The research team devised a chemical chromatin restoration (CHR) protocol to normalize chromatin. This three-step technique works by blocking H3K9 methylase, a chromatin repressor marker, and adjusting the concentration of cell growth factors. As a result of applying this treatment, hiPSCs successfully recovered their original undifferentiated state, the competent anterior epiblast-like state. These cell lines began to form cortical organoids. Specific experimental data also confirms this. When the expression rate of the SOX2 marker, an indicator of brain organoid development, was measured, the abnormal cell line sojd3 showed a sharp increase from the previous 4.44% to 60.72% after CHR. The burb1 cell line also increased from 2.75% to 66.71%. The expression rate of PAX8, a kidney organoid development marker, increased from 0.74% to 42.68% for sojd3, and the organoid marker CDX2 also increased significantly from 0.41% to 14.21%.

Significance and Prospects

This discovery shows that the fate of stem cells is not fixed but is a flexible system that can be reset by chemical manipulation. In particular, the fact that the differentiation potential of cells was restored by regulating only histone modifications without touching DNA methylation is considered positive in terms of safety. The CHR technique proposed by the researchers can be performed by treating only with small molecule compounds without gene editing, which has very high industrial application value. This opens the way to bring many patient-derived stem cell lines, which have been put on hold due to differentiation bias, back into the research field. However, there are still many challenges to be solved before this technology can be directly introduced into a standard cell therapy production process. It is necessary to thoroughly verify whether long-term genomic stability is maintained when the CHR protocol is consistently applied to various stem cell lines. Subsequent research should follow to refine the culture medium composition so that bivalent chromatin does not erode again in large-scale culture environments. It is also an important research task to establish criteria for epigenetic fine-tuning to increase the maturity of organoids to an adult level.