Characterization and analysis of oncofetal tRNA and its possible application for cancer diagnosis and therapy.
We determined the primary structures of various tumor-specific tRNAs as well as of their normal counterparts by postlabeling RNA-sequencing procedures. The results clearly indicated that tumor-specific tRNAs are mostly formed by undermodification of hypermodified nucleosides located in the anticodon loop: no new tRNA transcripts have so far been found in tumor cells. Among the modified nucleosides affected by tumorigenesis, queuosine and Y base are the most interesting. In various tumor tRNAPhe species, hydroxy Y base located next to the anticodon is undermodified to form hypomodified hydroxy Y base lacking methyl and carboxymethyl groups. This Y base analog should be a good marker for analyzing the state of cancer cells. Queuosine, located in the first position of the anticodon, is partly or completely replaced by guanosine in all tumor cells tested so far. The amount of G-tRNA decreased markedly when the cells differentiated into mature erythroid cells, with concomitant increase of Q-tRNA. This indicates that the presence of G-tRNA is closely related to the state of the cells, not merely to the fast growth rate of tumor cells. The enzyme tRNA-guanine transglycosylase, which is a key enzyme in biosynthesis of queuosine in tRNA (inserting Q base into tRNA by a transglycosylase reaction), is active in both tumor cells and normal cells. Administration of chemically synthesized Q base to tumor-bearing mice resulted in complete conversion of G-tRNA to Q-tRNA in tumor cells, indicating that exogenously added Q base is effectively incorporated into G-tRNA. Various Q base analogs that can be used as substrates for tRNA-guanine transglycosylase were synthesized chemically. These compounds may be used for clinical cancer diagnosis, since they should be incorporated selectively into tRNA in tumor cells. In addition to use as cancer chemotherapeutic reagents, it should be possible to develop new Q base analogs that induce miscoding or blocking of protein synthesis after insertion into G-tRNA.