Dibenz[a,h]anthracene (DB[a,h]A) and 7-nitrodibenz[a,h]anthracene (7-NDB[a,h]A) induced liver tumors when administered to neonatal B6C3F(1) mice. For protooncogene analysis, RNA was isolated from each of the liver tumors from treated mice and reverse-transcribed into cDNA. Portions of the K- and H-ras protein coding sequences were then amplified and analyzed for DNA sequence alterations. DB[a,h]A-induced liver tumors had a 100% (23/23)frequency of ras-protooncogene mutation, with 83% (19/23) occurring at the first base of K-ras codon 13 and resulting in GGC --> CGC transversion; the remaining 17% (4/23) of the mutations were located at the second base of H-ras codon 61. In contrast, only four of nine (44%) of 7-NDB[a,h]A-induced liver tumors had ras-protooncogene mutations, with two each at K-ras codon 13 and H-ras codon 61. Combined with previous observations, the results indicate that the nitro substituent perpendicular to the aromatic moiety alters the chemical-induced protooncogene activation frequency and mutational pattern in liver tumors of B6C3F(1) mice.
