Differential expression of the normal and of the translocated human c-myc oncogenes in B cells.

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Abstract
Whether the translocated and the untranslocated human c-myc oncogenes of Burkitt lymphoma cells are equally or differentially expressed in host mouse B cells was investigated. The human c-myc mRNA levels in somatic cell hybrids between mouse plasmacytoma cells and Burkitt lymphoma cells with either t(8;14) or the t(2;8) chromosome translocation were determined by using the nuclease S1 protection procedure. Although both the human parental lines and the hybrid cells carrying the translocated c-myc oncogene expressed high levels of human specific c-myc transcripts, the hybrid cells carrying the untranslocated c-myc gene on normal chromosome 8 did not contain human specific c-myc mRNA. The translocated human c-myc oncogene has apparently escaped the normal transcriptional control to which the untranslocated c-myc gene remains subjected. This interpretation is also supported by the finding that the expression of the c-myc genes of lymphoblastoid cells and of HL-60 promyelocytic leukemia cells are repressed when they are transferred into a mouse plasmacytoma background. The ability of the translocated c-myc oncogene to escape the normal transcriptional control occurring in B cells may be important for the expression of B cell neoplasia in mouse and man. The Burkitt 14q+ chromosome carrying a translocated c-myc oncogene was transferred into mouse LM-TK- fibroblasts and the levels of the human c-myc transcripts was studied in the hybrids. Because the levels of human c-myc transcripts in the fibroblast hybrids are dramatically decreased in comparison to the plasmacytoma hybrids, the levels of transcripts of the translocated c-myc oncogene depend on the differentiated state of the cells harboring the translocated chromosome.