A mutated version of Myc is found in many cancers, which causes Myc to be constitutively (persistently) expressed. This leads to the unregulated expression of many genes, some of which involved in cell proliferation, and results in the formation of cancer.
A common human translocation involving Myc is t(8;14), which is critical to the development of most cases of Burkitt's Lymphoma. Malfunctions in Myc have also been found in carcinoma of the cervix, colon, breast, lung and stomach. Nobel laureate James D. Watson and others believe that Myc is a promising target for anti-cancer drugs.
In the human genome, Myc is located on chromosome 8 and is believed to regulate expression of 15% of all genes through binding on Enhancer Box sequences (E-boxes) and recruiting histone acetyltransferases (HATs). This means that in addition to its role as a classical transcription factor, Myc also functions to regulate global chromatin structure by regulating histone acetylation both in gene-rich regions and at sites far from any known gene.
Myc gene was first discovered in Burkitt's lymphoma patients. In Burkitt's lymphoma, cancer cells show chromosomal translocations, in which Chromosome 8 is frequently involved. Cloning the break-point of the fusion chromosomes revealed a gene that was similar to myelocytomatosis viral oncogene (v-Myc). Thus, the newfound cellular gene was named c-Myc.
Myc protein belongs to Myc family of transcription factors, which also includes N-Myc and L-Myc genes. Myc family of transcription factors contain bHLH/LZ (basic Helix-Loop-Helix Leucine Zipper) domain. Myc protein, through its bHLH domain can bind to DNA, while the leucine zipper domain allows the dimerization with its partner Max, another bHLH transcription factor.
Myc mRNA contains an IRES (internal ribosome entry site) that allows the RNA to be translated into protein when 5' cap-dependent translation is inhibited, such as during viral infection.
Myc protein is a transcription factor that activates expression of many genes through binding on consensus sequences (Enhancer Box sequences (E-boxes)) and recruiting histone acetyltransferases (HATs). It can also act as a transcriptional repressor. By binding Miz-1 transcription factor and displacing the p300co-activator, it inhibits expression of Miz-1 target genes. In addition, myc has a direct role in the control of DNA replication.
Myc is activated upon various mitogenic signals such as Wnt, Shh and EGF (via the MAPK/ERK pathway). By modifying the expression of its target genes, Myc activation results in numerous biological effects. The first to be discovered was its capability to drive cell proliferation (upregulates cyclins, downregulates p21), but it also plays a very important role in regulating cell growth (upregulates ribosomal RNA and proteins), apoptosis (downregulates Bcl-2), differentiation, and stem cell self-renewal. Myc is a very strong proto-oncogene and it is very often found to be upregulated in many types of cancers. Myc overexpression stimulates gene amplification, presumably through DNA over-replication.
There have been several studies that have clearly indicated Myc's role in cell competition.
The cleavage of Myc by calpains is a constitutive process but is enhanced under conditions that require rapid downregulation of Myc levels, such as during terminal differentiation. Upon cleavage, the C-terminus of Myc (containing the DNA binding domain) is degraded, while Myc-nick, the N-terminal segment 298-residue segment remains in the cytoplasm. Myc-nick contains binding domains for histone acetyltransferases and for ubiquitin ligases.
The functions of Myc-nick are currently under investigation, but this new Myc family member was found to regulate cell morphology, at least in part, by interacting with acetyl transferases to promote the acetylation of α-tubulin. Ectopic expression of Myc-nick accelerates the differentiation of committed myoblasts into muscle cells.
During the discovery of Myc gene, it was realized that chromosomes that translocate to Chromosome 8 contained immunoglobulin genes at the break-point. Enhancers that normally drive expression of immunoglobin genes now lead to overexpression of Myc proto-oncogene in lymphoma cells. To study the mechanism of tumorigenesis in Burkitt's lymphoma by mimicking expression pattern of Myc in these cancer cells, transgenic mouse models were developed. Myc gene placed under the control of IgM heavy chain enhancer in transgenic mice gives rise to mainly lymphomas. Later on, in order to study effects of Myc in other types of cancer, transgenic mice that overexpress Myc in different tissues (liver, breast) were also made. In all these mouse models overexpression of Myc causes tumorigenesis, illustrating the potency of Myc oncogene.
Overview of signal transduction pathways involved in apoptosis.
A major effect of Myc is B cell proliferation.
c-Myc induces AEG-1 or MTDH gene expression and in turn itself requires AEG-1 oncogene for its expression.
It is associated with carcinoma of the cervix, colon, breast, lung, and stomach.
Except for early response genes, Myc universally upregulates gene expression. Furthermore the upregulation is nonlinear. Genes whose expression is already significantly upregulated in the absence of Myc are strongly boosted in the presence of Myc, whereas genes whose expression is low in the absence Myc get only a small boost when Myc is present.
Inactivation of SUMO-activating enzyme (SAE1 / SAE2) in the presence of Myc hyperactivation results in mitotic catastrophe and cell death in cancer cells. Hence inhibitors of SUMOylation may be a possible treatment for cancer.
Amplification of the MYC gene was found in a significant number of epithelial ovarian cancer cases.
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