Complex transcriptional regulation of myc family gene expression in the developing mouse brain and liver

Targeting Myc Interacting Proteins as a Winding Path in Cancer Therapy

MYC, as a well-known oncogene, plays essential roles in promoting tumor occurrence, development, invasion and metastasis in many kinds of solid tumors and hematologic neoplasms. In tumors, the low expression and the short half-life of Myc are reversed, cause tumorigenesis. And proteins that directly interact with different Myc domains have exerted a significant impact in the process of Myc-driven carcinogenesis. Apart from affecting the transcription of Myc target genes, Myc interaction proteins also regulate the stability of Myc through acetylation, methylation, phosphorylation and other post-translational modifications, as well as competitive combination with Myc. In this review, we summarize a series of Myc interacting proteins and recent advances in the related inhibitors, hoping that can provide new opportunities for Myc-driven cancer treatment.

Mutations in the MYB intron I regulatory sequence increase transcription in colon cancers

Although MYB overexpression in colorectal cancer (CRC) is known to be a prognostic indicator for poor survival, the basis for this overexpression is unclear. Among multiple levels of MYB regulation, the most dynamic is the control of transcriptional elongation by sequences within intron 1. The authors have proposed that this regulatory sequence is transcribed into an RNA stem-loop and 19-residue polyuridine tract, and is subject to mutation in CRC. When this region was examined in colorectal and breast carcinoma cell lines and tissues, the authors found frequent mutations only in CRC. It was determined that these mutations allowed increased transcription compared with the wild type sequence. These data suggest that this MYB regulatory region within intron 1 is subject to mutations in CRC but not breast cancer, perhaps consistent with the mutagenic insult that occurs within the colon and not mammary tissue. In CRC, these mutations may contribute to MYB overexpression, highlighting the importance of noncoding sequences in the regulation of key cancer genes.

Definition of the human N-myc promoter region during development in a transgenic mouse model

The N-myc oncogene directs organogenesis, and gene amplification is associated with aggressive forms of neuroblastoma, a common malignant tumor in children. N-myc is expressed in fetal epithelium, and expression decreases markedly postnatally. To localize sequences responsible for directing expression, we have analyzed the human N-myc promoter. We noted previously that N-myc promoter regions 5' to exon 1 directed reporter gene expression in all cell lines, including those without detectable N-myc transcripts. However, when promoter constructs included 3' exon 1 and the 5' portion of intron 1, reporter activity was detected only when there was expression of the endogenous gene. To determine the role of this "tissue-specific region" in directing expression during development, we generated transgenic mice carrying N-myc promoter lacZ minigenes that contained 5' N-myc promoter elements alone or the promoter linked to the 3' exon 1/5' intron 1 tissue-specific region. Animals lacking the tissue-specific exon 1/intron 1 region showed beta-galactosidase expression in the CNS, but expression was not observed in other organs in which endogenously derived N-myc transcripts were seen. Within the CNS, transgene expression was seen mainly in the olfactory system and was not observed in other areas in which expression of the murine gene has been noted. In contrast, no transgene expression was observed in any of the animals carrying the tissue-specific exon 1/intron 1 region. Thus, sequences that direct expression within the olfactory system were contained within our 5' promoter transgene, whereas sequences that guide the ubiquitous expression of N-myc during organogenesis lie outside the regions studied here. Finally, the exon 1/intron 1 region seems to act in a dominant fashion to repress expression in the CNS from the immediate 5' N-myc promoter.

MYC oncogenes and human neoplastic disease

c-myc, N-myc and L-myc are the three members of the myc oncoprotein family whose role in the pathogenesis of many human neoplastic diseases has received wide empirical support. In this review, we first summarize data, derived mainly from non-clinical studies, indicating that these oncoproteins actually serve quite different roles in vivo. This concept necessarily lies at the heart of the basis for the observation that the deregulated expression of each MYC gene is reproducibly associated with only certain naturally occurring malignancies in humans and that these genes are not interchangeable with respect to their aberrant functional consequences. We also review evidence implicating each of the above MYC genes in specific neoplastic diseases and have attempted to identify unresolved questions which deserve further basic or clinical investigation. We have made every attempt to review those diseases for which significant and confirmatory evidence, based on studies with primary tumor material, exists to implicate MYC members in their causation and/or progression.

C-myc deregulation during transformation induction: involvement of 7SK RNA

The process of oncogenic transformation has been widely studied but is still poorly understood. We have focused on the mechanism of deregulation of the c-myc gene during transformation of a temperature-sensitive SV40-transformed mouse cell line. Run-on transcription assays showed that the two c-myc minor promoters, P1 and P3, are transiently activated following induction of transformation and that peak activation of both promoters is preceded by a large increase in transcription of a small RNA (7SK). To test the possibility that this RNA might participate in promoter activation, we transfected cells with sense and antisense oligodeoxynucleotides corresponding to different regions of the 7SK RNA predicted to be accessible within the RNP particle. Out of 14 oligos tested, inhibition of activation of P1 and/or P3 was observed with four antisense oligonucleotides corresponding to looped regions in the putative 7SK secondary structure. To identify c-myc promoter sequences which might serve as targets for 7SK activity, we carried out mobility-shift assays with either whole or 7SK-depleted cell extracts. The CT element of the c-myc promoter formed a 7SK-dependent complex which could be competed only with the same antisense 7SK oligo that suppressed P1 and P3 activation in vivo. Taken together these results suggest that 7SK RNP participates in transformation-dependent c-myc deregulation.

Basic mechanisms of transcript elongation and its regulation

Ternary complexes of DNA-dependent RNA polymerase with its DNA template and nascent transcript are central intermediates in transcription. In recent years, several unusual biochemical reactions have been discovered that affect the progression of RNA polymerase in ternary complexes through various transcription units. These reactions can be signaled intrinsically, by nucleic acid sequences and the RNA polymerase, or extrinsically, by protein or other regulatory factors. These factors can affect any of these processes, including promoter proximal and promoter distal pausing in both prokaryotes and eukaryotes, and therefore play a central role in regulation of gene expression. In eukaryotic systems, at least two of these factors appear to be related to cellular transformation and human cancers. New models for the structure of ternary complexes, and for the mechanism by which they move along DNA, provide plausible explanations for novel biochemical reactions that have been observed. These models predict that RNA polymerase moves along DNA without the constant possibility of dissociation and consequent termination. A further prediction of these models is that the polymerase can move in a discontinuous or inchworm-like manner. Many direct predictions of these models have been confirmed. However, one feature of RNA chain elongation not predicted by the model is that the DNA sequence can determine whether the enzyme moves discontinuously or monotonically. In at least two cases, the encounter between the RNA polymerase and a DNA block to elongation appears to specifically induce a discontinuous mode of synthesis. These findings provide important new insights into the RNA chain elongation process and offer the prospect of understanding many significant biological regulatory systems at the molecular level.

Bovine opsin gene expression exhibits a late fetal to adult regulatory switch

Rates of bovine photoreceptor gene transcription, as measured by nuclear run-on assays, exhibit gene-specific patterns of regulation. Here we investigate initiation and elongation in nuclear run-on assays with the use of sarkosyl to further understand the nature of these gene-specific elements. Opsin transcription, alone among several genes tested, proved sarkosyl-sensitive. This sensitivity is maximal in adult retinas, with inhibition first detected in mid-third trimester fetal retinas. Therefore, opsin transcription appears to involve different regulatory elements in adult and fetal retinas, implying a fetal to adult switch in the control of opsin gene expression. Although this regulatory switch is initially activated at a time when the fetal outer nuclear layer of the retina first achieves adult-like morphology, further maturation of opsin regulation takes place postpartum since levels of sarkosyl sensitivity are almost 5-fold greater in adult retinas compared to the 7.5 month fetus. We also show that the sarkosyl-induced reduction of opsin transcription is not due to prevention of de novo RNA polymerase II initiation in the run-on reaction, suggesting the detergent alters a positive-acting, postinitiation component of the transcriptional apparatus. Since levels of opsin transcription with sarkosyl are similar to those of the other visual transduction genes with or without sarkosyl, this detergent-sensitive transcriptional component appears to account for the singularly high, gene-specific rate of opsin transcription in retinal photoreceptor cells.

Course of c-myc mRNA expression in the regenerating mouse testis determined by competitive reverse transcriptase polymerase chain reaction

The c-myc proto-oncogene is a reliable marker of the "G0-early G1" transition, and its down-regulation is believed to be necessary to obtain cellular differentiation. In murine spermatogenesis, the level of c-myc transcripts does not correlate with the rate of cellular division. Proliferation of supposed staminal spermatogonia to reproduce themselves is induced with a local 5 Gy X-ray dose in 90-day-old C57Bl/6 mice. c-myc quantification by a newly developed competitive reverse transcriptase polymerase chain reaction (RT-PCR) was carried out to follow the expression course of this proto-oncogene. Damage and restoration of spermatogenesis were analyzed at days 3, 6, 9, 10, 13, 30, and 60 after injury by relative testes/body weight determination and histological examination. Proliferative status was determined by histone H3 Northern blot analysis. c-myc mRNA level was 10 times higher after 3 days in the irradiated animals compared to the controls. An increasing number of copies were noted up to 10 days, but promptly decreased to the base level found for irradiated mice from 13 to 60 days. Interestingly, the expression of histone H3 detected S phase only in testes at 60 days from damage.

Common mechanisms for the control of eukaryotic transcriptional elongation

Regulation of transcriptional elongation is emerging as an important control mechanism for eukaryotic gene expression. In this essay, we review the basis of the current view of the regulation of elongation in the human c-myc gene and discuss similarities in elongation control among the c-myc, Drosophila hsp70 and the HIV-1 genes. Based upon these similarities, we propose a model for control of expression of these genes at the elongation phase of transcription. This model suggests that distinct promoter elements direct the assembly of RNA polymerase II transcription complexes which differ in their elongation efficiency.

Cell-specific expression of preproenkephalin intronic heteronuclear RNA in the rat forebrain

Using in situ hybridization with multiple probes to the rat preproenkephalin gene, we have identified a novel population of cells in the reticular thalamic nucleus and basal forebrain which express RNA derived from the preproenkephalin gene. These cells contain nuclear RNA from downstream of an alternate transcription start site in intron A of the preproenkephalin gene (Kilpatrick et al., Mol. Cell Biol., 10 (1990) 3717-3726), while in the same cells preproenkephalin exon 2 RNA is undetectable. The results suggest that in this population of cells, preproenkephalin gene transcription initiates from the intron A initiation site, and is regulated by an additional mechanism which results in the accumulation of nuclear preproenkephalin intron A-derived heteronuclear RNA. The anatomical distribution of these cells indicates that they may be involved in the control of cerebral cortical function.