ONGOING MUTATIONS IN THE N-TERMINAL DOMAIN OF C-MYC AFFECT TRANSACTIVATION IN BURKITTS-LYMPHOMA CELL-LINES

A panel of 18 Burkitt's lymphoma (BL) and nine other cell Lines was ex amined for mutations in the N-terminal transactivation domain of c-Myc . Mutations leading to exchange of amino acids were detected in 13 BL but in none of the control cell lines. Mutations in c-Mye clustered be tween amino acid positions 57 and 62. Thr-58 and Ser-62 are known phos phorylation sites of c-Myc in vivo. BL cell lines derived from the sam e tumour revealed different mutations. Mutant cDNAs of the BL cell lin e Raji differed at 14 positions indicating ongoing mutation of the tra nslocated c-myc during long-term propagation in cell culture. The effe ct of mutations on transactivation by c-Mye was tested by expression o f GAL4/c-Myc fusion proteins in the BL cell line Raji. Mutants with an amino acid exchange at positions 58 or 60 transactivated a reporter g ene two- to fivefold weaker than wildtype c-Myc, Thr-58 and Ser-62 wer e replaced by aspartic acid to mimic constitutively phosphorylated for ms of c-Myc. These mutants transactivated two- to three-fold weaker th an wildtype c-Mye indicating that a negative charge at positions 58 an d/or 62 per se does not enhance transactivation. We propose that mutat ions in the N-terminal domain of c-Mye correlate with reduced transact ivation and provide a growth advantage for BL cells.