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 overexpression of inactive forms of the protein that bind to the wild-type protein to create a dominant negative phenotype. An ATDC cDNA containing a 37 amino acid deletion in the zinc finger region (ATDC delta) was therefore transfected into colorectal carcinoma human tumour cells (RKO) to determine whether its expression would produce a response to radiation similar to that seen in A-T cells. RKO cells have been shown to have normal radiosensitivity and cell cycle regulation and, therefore, 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 ATDC 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 support the conclusion that ATDC is an A-T gene, and suggest that the ATDC protein acts indirectly to suppress radiosensitivity in A-T cells.