Gene name - diminutive
Synonyms - Myc, dMyc
Cytological map position - 3D5
Function - transcription factor
Symbol - dm
Genetic map position -
Classification - bHLH - leucine zipper
Cellular location - nuclear
myc is a mammalian oncogene involved in a wide variety of tumors. Oncogenic activation depends on elevated expression of the short lived Myc protein. In normal cells, myc expression is dependent on mitogenic stimuli. Myc is required for both cell proliferation and to prevent differentiation. Unless growth factors are provided, activation of myc can concomitantly induce programmed cell death (apoptosis).
Before discussing the avid interest Myc holds for developmental biologists, a brief description is given of the interaction of Myc with three other proteins (MAD, MAX and MXI) and their combined effects on both cell activation and quiescence (Amati,1994 and references).
Myc proteins contain two regions characteristic of transcription factors: an N-terminal transactivation domain, and a C-terminal basic helix-loop-helix (bHLH) leucine zipper motif known to mediate dimerization and sequence specific DNA binding. Myc functions as a heterodimer with other bHLH leucine zipper proteins. Max specifically dimerizerizes with Myc, and Myc-Max heterodimers function as transcriptional activators, binding a hexanucleotide motif called the E-box, which is often the target of other bHLH proteins.
Antagonizing the cell cycle promoting activity of Myc-Max in mammals are two other proteins, Mad and Mxi-1. Mad and Mxi can heterodimerize with Max, depriving Myc of a partner. The Max/Mad or Max/Mxi-1 partners either fail to activate or actively repress transcription, leading to a state of growth inhibition or possibly cell differentiation. Max proteins are metabolically stable and constitutively expressed, while Myc, Mad and Mxi-1 are unstable, responding to the level of mitotic stimulation in the cell. Mitogen stimulation induces a rapid rise in Myc levels and thus a shift in the equilibrium from Max/Max homodimers to Myc/Max heterodimers, the combination that promotes entry into cell cycle. Conversely, mitogen withdrawal or differentiation stimuli suppress myc expression and may result in the induction of either or both mad and mxi, leading to a state of growth inhibition. Thus Myc, Max and Mxi function as a protein network leading to alternative states of cell activation or quiescence (Amati, 1994 and references).
Of particular interest to developmental biologists is the question of whether Myc, Mad and Max play a role in normal development. This question became more complex and interesting with the discovery in vertebrates of multiple Myc genes. There is a neurally expressed myc (N-myc), for example. In mammals N-myc and myc show contrasting expression patterns during gastrulation. myc is most abundant in extraembryonic cells; in contrast, N-myc is found at highest levels in the expanding primitive streak and other portions of the embryonic mesoderm. Differentiation of mesoderm to epithelioid cells is accompanied by diminished expression of N-myc. Expression of myc is not an inevitable correlate of cellular proliferation. Instead, the gene appears to be regulated in concert with changes that affect diverse cellular properties, including proliferation, invasiveness and differentiation. For example, myc is selectively down-regulated in the primitive ectoderm, the most highly proliferative tissue of the embryo. A decline in myc expression correlates with terminal differentiation of brown adipose tissue and liver. In contrast, N-myc continues to be expressed in post-mitotic cells of neuronal origin (Downs, 1989 and Hirning, 1991).
Isolation of Drosophila Myc, properly termed Diminutive but referred to as dMyc, was accomplished by means of its conserved affinity for Max. Human Max was used to screen a two-hybid library prepared from Drosophila cDNAs. A second yeast two-hybrid screen was performed using the bHLH leucine zipper region of dMyc as bait. This experiment resulted in the identification of dMax (Gallant, 1996).
What is the phenotypic effect of loss of dmyc function? The results are inconclusive, but it is believed that a Drosophila mutation (known as diminutive), resulting in an abnormally small body size and female sterility, is due to a hypomorphic mutation in dmyc. Defects are observed in both germ cell nuclei and follicle cells; they fail to migrate and aberrantly undergo the transition to columnar epithelium. This phenotype suggests dysfunction in either follicle or nurse cells, or in communication between these two cell types. Interestingly the degeneration of the egg chamber in dm mutant females occurs at stage 8, at a time when cell division does not occur.
It is thought that a stage-specific downregulation of dmyc expression in diminished mutants results in a loss of the capacity of the follicle cells to grow and migrate. A possibly related effect has been observed in mice: hypomorphic N-myc mutation results in a loss of induction of tissue specific differentiation (Moens, 1992). In both cases diminished Myc expression in progenitor cells may result in their inability to respond to inductive signals. In addition, the smaller size of the diminished mutants may also result from partial loss of dMyc function in other tissues (Gallant, 1996). Recent studies in mice demonstrate that alterations in the cell cycle can significantly influence overall size. Mice lacking p27Kip1, an inhibitor of cyclin dependent kinase, display increased body size without an increase in growth hormone levels (Fero, 1996).
Is the small body size of diminutive mutants due to a hormonal effect or is it due to cell autonomous dmyc deficiency? Mosaic experiments should yield clues as to dMyc's influence on specific organs and developmental timing.
dmax is expressed as a single 1.2 kb transcript at constant levels throughout development. In contrast, a dmyc transcript of 6 kb is expressed at the highest levels during early embryogenesis and in adult females. It is barely detectable during the larval and pupal stages. Early embryos and adult females contain smaller dmyc transcripts that appear to differ primarily in their untranslated regions (Gallant, 1996).
The dMyc protein is only 26% identical to human c-Myc over its entire amino acid sequence. The N-terminal region shows a 57% identity to human c-Myc box II, which contains the conserved sequence DCMW, mutations that abrogate myc activity. The centrally located acidic region shows a 57% identity to vertebrate c-Myc, and the C-terminal bHLH leucine zipper motif retains a 40% identity. The Myc-box I may also be present, but its sequence is less conserved. All but one of the basic region residues predicted to contact DNA are identical to those in human c-Myc. The dmyc gene is comprised of three exons with the protein-coding region spanning exons 2 and 3, as in mammalian myc (Gallant, 1996).
Myc proteins consist of two N-terminal segments (referred to as Myc boxes 1 and 2), a central block of acidic residues, and a highly charged carboxyl-terminal region. Myc box 1 contains two of the four phosphorylation sites identified to date in mammalian myc: one for p34cdc2 (see Drosophila Cyclin B, along with its dimerization partner cdc2) or a related kinase and a second for an unidentified kinase. The C-terminal region contains three structural motifs: the leucine zipper, helix-loop-helix, and basic regions. The HLH and leucine zipper regions have been shown to mediate protein oligomerization, while the basic region, when oligomerized, allows sequence specific DNA binding (Pendergast, 1992).
date revised: 12 Dec 96
Home page: The Interactive Fly © 1995, 1996 Thomas B. Brody, Ph.D.
The Interactive Fly resides on the
Society for Developmental Biology's Web server.