Effects of heterochromatin in colorectal cancer stem cells on radiosensitivity

Abstract

Radiotherapy (RT) is a major non-surgical modality in the comprehensive treatment for colorectal adenocarcinoma. The radioresistance of cancer stem cells (CSCs) is a key factor that influences therapeutic effectiveness. This study was to investigate the effects of specific chromosome structure and histone modification in CSCs in colorectal adenocarcinoma radioresistance. Methods: Samples were collected from resected human colorectal adenocarcinomas. Subcutaneous colorectal cancer model was established in nude mice. Immunohistochemistry showed that xenografts generated from bulk colorectal cancer cells resembled the original tumor specimen. Flow cytometry was performed to sort CSCs (CD133+) and non-CSCs (CD133-) from both resected samples of colorectal adenocarcinoma and xenograft before and after high single-dose radiation. The markers labeling heterochromatin (H3K9me3, HP1-α and H3K4me1) and euchromatin (H3K4me3) in CD133+ and CDI33- nucleus were detected by immunoflurescence. Results: There was distinct difference in chromatin structure between colorectal CSCs (CD133+) and non-CSCs (CD133). The chromatin displayed compact patches in CD133+ nucleus, but loosely latticed structure in CD133- nucleus; immunoflurescence verified that the compact patches existing in CSCs was generated from heterochromatin construction. In addition, the vacuole-like defect in heterochromatin regions of CSCs was observed within 24 h after exposure to 10 gray (Gy) single-dose RT. Interestingly, this phenomenon was repaired from 96 h, and recovered to dense plaque structure in heterochromatin regions of CSCs after 144 h. However, no significant difference in non-CSCs was observed after RT exception for a loose chromatin structure. Conclusions: CSCs play a role in radiosensitivity in colorectal cancer. The mechanism may be related to heterchromatin formation and histone methylation.