Ionizing Irradiation Induced Genomic Instability In The Repairing Intestinal Epithelium Nnd Chromolomnl Aberrations In Bone Mnrrow Cells

PART.1:It has been reported that cultured mammalian cells cannot sustain cell cycle arrest in a long run after the number of DNA double strand breaks drop to a relative low level and enter mitosis with DNA breaks. Here we aimed at assess the checkpoint activation and cell cycle restarting in process of intestinal crypt regeneration.Crypt regeneration of intestine was induced by a single dose of12Gy abdominal irradiation. γ-H2AX,53BP1were used as DNA repair surrogates to investigate the inherent ability of crypt stem cells to recognize and repair double-strand breaks. The Ki67staining and5-bromo-2’-deoxyuridine incorporation assay were used to study patterns of cell proliferation in regenerating crypts and ATM, P53and Check2staining to study checkpoint activation and release. Apoptosis was evaluated by H&E staining and Terminal deoxynucleotidyl transferase dUTP nick end labeling.After reaching to very low levels after irradiation, the DSBs in crypt stem cells rose again in crypts underwent regeneration. A sudden rose of chromosomal bridges was also observed in this process. ATM-Chk2-P53pathway was activated immediately after irradiation. Nevertheless, to our surprise, this genomic surveillance pathway was depressed during the regeneration phase despite the presence of a second wave of DNA damage, including DSBs and chromosomal bridges, in the cells in the regenerating crypts.The y-H2AX is present in early and late apoptotic cells whereras ATM, Chk2are present in early apoptotic cells not in apoptotic cells in regeneration phase. Intestinal stem cells can adapt to IR-induced checkpoint arrest before regeneration. ATM-Chk2-P53pathway was activated immediately after irradiation. Nevertheless, to our surprise, this genomic surveillance pathway was depressed during the regeneration phase despite the presence of a second wave of DNA damage.This process in characterized by chromosomal instability. It was switch to reliance on mitotic cell death rather than on cell cycle delay or apoptosis to eliminate the cells with severe DNA damage or CIN that would prevent cell division.The DNA damage response proteins such as ATM, Chk2,53BP1,γ-H2AX are present in apoptotic cells or apoptototic bodies. PART.2:For radiation induced bone marrow chromosomal aberration studies, the whole bodies of6-week-old male Kun-Ming mice were exposed to different doses of12C6+ion or x-rays. Chromosomal aberrations of bone marrow (gaps, terminal deletions and breaks, fragments, inter-hromosomal fusions and sister-chromatid union) were scored in metaphase at9hour after exposure corresponded to cells exposed in the G2-phase of first mitosis cycle. Dose-response relationships for frequency of chromosomal aberrations were plotted both by linear and linear-quadratic equations. The data showed that there was a dose-related increase in frequency of chromosomal aberrations in all treated groups compared to controls. Linear-quadratic equation was well fitted by both radiation qualities. The compound theory of dual radiation action was applied to decipher the bigger curvature (D2) of x-rays dose-response curves compared to12C6+ion. Different distribution of the five types of aberrations and different degree of homogeneity have been found between12C6+ion and x-rays irradiations and the possible underlying mechanism for these phenomena had been analysed according to the differences in the spatial energy deposition of both radiation qualities.