TRA1, a novel mRNA highly expressed in leukemogenic mouse monocytic sublines but not in nonleukemogenic sublines

Mouse monocytic Mm-A, Mm-P, Mm-S1, and Mm-S2 cells are sublines of mouse monocytic and immortalized Mm-1 cells derived from spontaneously differentiated, mouse myeloblastic M1 cells. Although these subline cells retain their monocytic characteristics in vitro, Mm-A and Mm-P cells are highly leukemogenic to syngeneic SL mice and athymic nude mice, whereas Mm-S1 and Mm-S2 cells are not or are only slightly leukemogenic. To better understand the molecular mechanisms of these levels of leukemogenicity, we investigated putative leukemogenesis-associated genes or oncogenes involved in the maintenance of growth, especially in vivo, by means of differential mRNA display. We isolated a fragment clone (15T01) from Mm-P cells. The mRNA probed with 15T01 was expressed at high levels in leukemogenic Mm-P and Mm-A cells but not in nonleukemogenic Mm-S1 and Mm-S2 cells. The gene corresponding to 15T01, named TRA1, was isolated from an Mm-P cDNA library. The longest open reading frame of the TRA1 clone predicts a peptide containing 204 amino acids with a calculated molecular weight of 23,049 D. The predicted TRA1 protein is cysteine-rich and contains multiple cysteine doublets. A putative normal counterpart gene, named NOR1, was also isolated from a normal mouse kidney cDNA library and sequenced. NOR1 cDNA predicts a peptide containing 234 amino acids. The sequence of 201 amino acids from the C-terminal NOR1 was completely identical to that of TRA1, whereas the remaining N-terminal amino acids (33 amino acids) were longer than that (3 amino acids) of TRA1 and the N-terminus of NOR1 protein contained proline-rich sequence. A similarity search against current nucleotide and protein sequence databases indicated that the NOR1/TRA1 gene(s) is conserved in a wide range of eukaryotes, because apparently homologous genes were identified in Caenorhabditis elegans and Saccharomyces cerevisiae genomes. Northern blotting using TRA1-specific and NOR1-specific probes indicated that TRA1 mRNA is exclusively expressed in leukemogenic but not in nonleukemogenic Mm sublines and normal tissues and also indicated that NOR1 mRNA is expressed in normal tissues, especially in kidney, lung, liver, and bone marrow cells but not in any Mm sublines. After leukemogenic Mm-P cells were induced to differentiate into normal macrophages by sodium butyrate, the normal counterpart, NOR1, was expressed, whereas the TRA1 level decreased. Furthermore, transfection of TRA1 converted nonleukemogenic Mm-S1 cells into leukemogenic cells. These results indicate that the TRA1 gene is associated at least in part with the leukemogenesis of monocytic Mm sublines.