Absence of tRNA-guanine transglycosylase in a human colon adenocarcinoma cell line

Queuosine (Q), found exclusively in th first position of the anticodons of tRNA^Asp, tRNA^Asn, tRNA^His and tRNA^Tyr, is synthesized in eucaryotes by a base-for-base exchange of queuine, the base of Q, for guanine at tRNA position 34. This reaction is catalyzed by the enzyme tRNA-guaning transglucosylate (EC 2.4.2.29). We measured the specific release of queuine from Q-5′-phsphate (queuine salvage) and the extent of tRNA Q modification in 6 human tumors carried as xenografts in immune-deprived mice. Q-deficient tRNA was found in 3 of the tumors but it did not correlate with diminished queuine salvage. The low tRNA Q content of one tumor, the HxGC₃ colo adenocarcinoma, prompted us to examine a HxGC₃-derived cell line, GC₃/M. GC₃/M completely lacks Q in its tRNA and measurable tRNA-guanine transglycosylase activity; the first example of a higher eucaryotic cell which lacks this enzyme. Exposure of GC₃/M cells to 5-azacytidine induces the transient appearance of Q-positive tRNA. This result suggests that at least one allele of the transglycosylase gene in GC₃/M cells may have been inactivated by DNA methylation. In clinical samples, we found Q-deficient tRNA in 10 of 46 solid tumors, including 2 of 13 colonic carcinomas.