Frequent activation of the lck gene by promoter insertion and aberrant splicing in murine leukemia virus-induced rat lymphomas.

We have analysed DNA and RNA from 36 T-cell lymphomas induced in Fischer rats by Moloney murine leukemia virus for alterations affecting the structure or expression of the lck gene. At least five primary tumors (14%) have a proviral insertion upstream of lck. In at least four of the tumors, proviral insertion increases lck mRNA levels an average of eight-fold. Overexpression of lck results from transcription initiating in the viral promoter and extending into lck sequences. Three different structures of hybrid transcript were detected. In all three, the hybrid RNAs are spliced to a normal lck splice acceptor in the first exon of lck, resulting in removal of three out of frame ATG codons which would be expected to increase the translation efficiency of the hybrid message. In one tumor, the viral splice donor is used, in one tumor, proviral insertion generates a splice donor sequence one base pair downstream of the long terminal repeat boundary, and in two tumors, a cryptic splice donor in the upstream lck sequences is used. The significance of these unusual splicing patterns and of the higher frequency of proviral insertions adjacent to lck in rats relative to mice is discussed.