Human chromosome-specific changes in a human-hamster hybrid cell line (AL) assessed by fluorescent in situ...

To quantitatively assess all gamma-ray induced chromosomal changes confined to one human chromosome using fluorescence microscopy and in situ hybridization with a fluorescently labeled human chromosome specific nucleic acid probe.

Synchronized human-hamster hybrid cells containing human chromosome 11 were obtained by a modified mitotic shake-off procedure. G1 phase cells (> 95%) were irradiated with 137Cs gamma rays (0, 0.5, 1.0, 1.5, 2.0, 4.0, 6.0, 8.0, and 10.0 Gy) at a dose rate of 1.1 Gy/min and mitotic cells collected 16-20 h later; chromosomal spreads were prepared, denatured, and hybridized with a fluorescein-tagged nucleic acid probe against total human DNA. Chromosomes were examined by fluorescence microscopy and all categories of change involving the human chromosome 11 as target, recorded.

Overall, of the 3104 human-hamster hybrid cells examined, 82.1% were euploid, of which 88.6% contained one copy of human chromosome 11, 6.2% contained two copies, and 5.2% contained 0 copies. This is compatible with mitotic nondisjunction in a small fraction of cells. Of the remaining 17.9% of cells, 85.2% were tetraploid cells with two copies of human chromosome 11. For all aberrations involving human chromosome 11 there was a linear relationship between yield and absorbed dose of 0.1 aberrations per chromosome per Gy. The yield of dicentrics, translocations, and terminal deletions that involve one lesion on the human chromosome was linear, while the yield of interstitial deletions that arise from two interacting lesions on the human chromosome was curvilinear. The frequencies of dicentrics and translocations were about equal, while there was a high (40-60%) incidence of incomplete exchanges between human and hamster chromosomes.

Fluorescent in situ hybridization (FISH) procedures allow for the efficient detection of a broad range of induced changes in target chromosomes. Symmetrical exchanges induced in G1 (translocations) were readily scored and found to equate with the complementary asymmetrical exchanges (dicentrics). That is, nonlethal stable changes, which might be of concern in carcinogenic processes, complement lethal, unstable changes. Interstitial deletions that may contribute to the loss of antioncogenes as well as to lethality are also readily detected with enhanced levels detected at higher doses. The high level of induced terminal deletions and of incomplete dicentrics and translocations indicate a partial failure of interaction between lesions induced in human and hamster DNA, and suggest that such interspecies interactions lack the fidelity of intraspecies DNA lesion interactions. This suggests caution in the use of such model systems as indicators of human cell responsiveness.