Chromosomal damage studies

At GSI experiments are performed to examine the induction of chromosome damages after exposure of cells to ionizing radiation.
Mammalian cells like Chinese hamster cells and human fibroblasts are irradiated with heavy ions of varying LET (Linear Energy Transfer) or with X rays, and chromosomal lesions are investigated using three different experimental approaches.

To measure the initial number of radiation induced chromatin breaks and their rejoining in interphase cells without the perturbing influence of cell cycle progression delays the PCC technique (Premature Chromosome Condensation) is used. By cell fusion the chromosomes of non-cycling cells are forced to condense the chromatin into chromosome like structures, then chromosome spreads are prepared and stained. The amount of chromatin fragments in excess to the normal chromosome number represents the number of chromatid breaks.

Another approach is to examine chromosome aberrations, that are expressed in the first post-irradiation mitosis. Chromosome preparations are differentially stained with Fluorescense-plus-Giemsa technique to discriminate between first and later cell generations. Additionally, FISH (Fluorescence in-situ hybridization) is performed to stain particular chromosomes and examine their interaction with other chromosomes after radiation-induced breakage. These investigations cover a certain period of time because recent experiments have shown, that the cell cycle transition is delayed according to the number of aberrations carried by the cells.

As a long-term follow-up study cell clones arising from a single irradiated cell are grown for several generations and delayed chromosomal instability is investigated. Delayed chromosomal instability describes the phenomenom, that some cells, that survive the radiation exposure transmit a latent chromosomal instability to their progeny, that leads to a de novo occurrence of aberrations many cell generations later. Identification of the underlying cause and the mechanisms by which ionizing radiation induces a delayed destabilization of the genome are important challenges for future studies.