TRUNCATED C-MYB EXPRESSION IN THE HUMAN LEUKEMIA-CELL LINE TK-6

The c-MYB proto-oncogene encodes a transcription factor which plays an important role in hematopoiesis. We detected truncated c-MYB mRNA (2. 0 kb) and c-Myb protein (55 kDa) in the TK-6 cell line, which was esta blished from a patient with chronic myelogenous leukemia in T cell bla st crisis. Mutated c-MYB cDNA clone (WTK-1) was isolated from a TK-6 c ell cDNA library and sequenced in its entirety. Compared with the wild type human c-MYB sequence, the WTK-1 sequence diverged at the 3' ends of exons 9. A termination codon was present as the second codon downst ream from the point of divergence and was followed by a previously unk nown rearranged sequence. The conceptual protein encoded by WTK-1 (Myb (TK-6)) comprises 402 amino acids and lacks the negative regulatory do main of the normal c-Myb, reminiscent of the activated form of Myb pro tein. Luciferase reporter assay in NIH3T3 cells showed that the expres sion vector encoding Myb(TK-6) stimulated Myb-regulated mim-1 promoter more effectively than that encoding wild-type human c-Myb, suggesting that Myb(TK-6) is functional as a transcription factor, and thus as a potential transforming protein. Southern blot and mutant allele-speci fic polymerase chain reaction analyses showed that the same rearrangem ent of the c-MYB gene in TK-6 was present in late, but not in early, s pecimens obtained from the patient, indicating that this mutation had been acquired during disease progression.