A plasmid carrying antisense human MYC DNA and the gene encoding Escherichia coli xanthine/guanine phosphoribosyltransferase (Ecogpt) was introduced into human promyelocytic leukemia cell line HL-60 by protoplast fusion. High-level expression of antisense MYC RNA was obtained by selecting cells resistant to progressively higher levels of mycophenolic acid over a period of greater than 6 months. The constitutive production of MYC protein in clones producing high levels of antisense MYC RNA was reduced by 70% compared to parental HL-60 cells. Inhibition of MYC expression was observed not only at the translational but also at the transcriptional level, implying that antisense RNA can regulate transcription of the MYC gene. The Pst I-Pvu II fragment (920 base pairs) of the MYC leader sequence is the primary transcriptional target of the antisense RNA. The suppression of endogenous MYC gene expression by antisense RNA decreases cell proliferation and triggers monocytic differentiation.
Comparative tryptic peptide analysis was used to probe the structure of the TL products coded for by normally expressed TL alleles (Tlaa and Tlac haplotypes) and the structure of the TL product on ERLD, a TL+ leukemia occurring in a strain that does not normally express TL antigens (Tlab haplotype). In mice that have the Tlaa haplotype, the peptide maps of TL glycoproteins from normal thymocytes and from TL+ leukemias were identical, a finding that is consistent with the indistinguishable serologically defined TL phenotype of these cells. Analysis of the TL product on ERLD leukemia cells (TL phenotype, TL.1,2,4) indicates a single TL glycoprotein species with the TL.4 determinant (restricted to leukemias of Tlab and Tlac haplotypes) coexisting on the same glycoprotein as the Tl.1 determinant (normally expressed by thymocytes of Tlaa haplotype). Comparison of the peptides of the TL product of ERLD and the products of the normally expressed Tlaa and Tlac loci showed a high degree of structural similarity (i.e., 70-80%) of shared peptides. In contrast, the products of the Tlaa, Tlab, and Tlac loci shared relatively few peptides with the products of the K, D, and L loci of the closely linked major histocompatibility complex (15-35%). The TL family, therefore, consists of alleles that are highly homologous. This contrasts with the marked diversity and polymorphism among the alleles of the K, D, and L loci.
