EXPRESSION OF THE CANDIDATE A-T GENE ATDC IS NOT DETECTABLE IN A HUMAN CELL-LINE WITH A NORMAL RESPONSE TO IONIZING-RADIATION

Nucleotide sequence analysis of a candidate gene for A-T group D (ATDC ) demonstrated that it is related to a group of proteins that contain both zinc finger and leucine zipper motifs. The presence of a leucine zipper suggested that this protein might form homodimers, and this was confirmed by means of the two-hybrid system in yeast. The activity of some proteins that form homodimers can be effectively eliminated by o verexpression of inactive forms of the protein that bind to the wild-t ype protein to create a dominant negative phenotype. An ATDC cDNA cont aining a 37 amino acid deletion in the zinc finger region (ATDC Delta) was therefore transfected into colorectal carcinoma human tumour cell s (RKO) to determine whether its expression would produce a response t o radiation similar to that seen in A-T cells. RKO cells have been sho wn to have normal radiosensitivity and cell cycle regulation and, ther efore, seemed ideal for this study. Despite the fact that the A-T gene has been found to be important in the radiation damage response, no A TDC mRNA transcripts were detectable in the RKO cell line. In addition , the RKO subclones expressing the ATDC Delta mRNA showed no change in radiosensitivity or cell cycle regulation. These results do not suppo rt the conclusion that ATDC is an A-T gene, and suggest that the ATDC protein acts indirectly to suppress radiosensitivity in A-T cells.