Evidence that derivation of the adenocarcinoma LT85 predated establishment of the H-2km2 mutation in a C3Hf colony of mice.

The adenocarcinoma LT85 was chemically induced in a mouse from a C3Hf colony shown subsequently to be inbred for a gene conversion-like mutation at the H-2K locus, characterized by a clustered four nucleotide substitution in exon 3. The H-2K phenotype of LT85, however, more closely resembles that of C3H rather than the mutant strain now designated C3Hf H-2km2. We cloned and sequenced the H-2K gene from this tumor to determine whether (i) LT85 might carry a tumor-associated somatic reversion of the H-2Kkm2 germline mutation of (ii) the tumor was induced in a mouse that was genetically H-2k rather than H-2km2. Our analysis confirmed that the LT85 H-2K allele is identical throughout the entire coding region to C3H H-2Kk. To exclude the possibility of a somatic reversion by recombination, we used an oligonucleotide probe corresponding to the region altered in H-2k to show that the C3Hf genome lacks the necessary coding information to reverse the H-2Kkm2 mutation through a sequence exchange with another class I locus. Since it is unlikely that multiple independent point mutations would account for restoration of this stretch of H-2Kk sequences, the tumor was probably established in a mouse carrying a normal H-2Kk allele, possibly at a point prior to the establishment of the H-2Kkm2 mutation as a homozygous trait within the C3Hf colony.