Cloning of sequences expressed specifically in tumors of rat

The sequences specifically transcribed in tumor cells are believed to be closely related to transformed phenotypes. For the isolation of such sequences, a cDNA clone library was constructed by using poly(A)⁺ RNAs from azo-dye-induced rat ascites hepatomas. Thirty-one tumor RNA-responsive clones were isolated by screening 4,000 clones of this library with with conventional techniques, differential colony hybridization, and RNA blot hybridization. These clones were categorized into two groups with respect to their size distribution of mRNAs from which clones were derived. The first group was complementary to a single distinct species, either about 1.5 or 0.6 kilobases in length, of poly(A)⁺ RNA, and the second showed no distinct bands but a smear on a RNA blot. Semiquantitative RNA dot blot assays revealed that the sequences of these clones were expressed very little, if at all, in normal and regenerating livers, while generally high in ascites hepatomas. This specificity was also true for other solid lines of tumors, such as Morris hepatoma 5123D of Buffalo rat and Walker 256 carcinosarcoma of Wistar rat. The smear class sequences were transcribed from middle-repetitive sequences of DNA, indicating that a class of middle-repetitive sequences is specifically transcribed in tumor cells.