Mutations in the G-quadruplex silencer element and their relationship to c-MYC overexpression, NM23 repression, and therapeutic rescue.
We have demonstrated that a parallel G-quadruplex structure in the c-MYC promoter functions as a transcriptional repressor element. Furthermore, a specific G-to-A mutation in this element results in destabilization of the G-quadruplex repressor element and an increase in basal transcriptional activity. To validate this model in an in vivo context, we have examined the sequence of this region in human colorectal tumors and the surrounding normal tissue. We have found that approximately 30% of tumors contain one of two specific G-to-A mutations, not present in the surrounding normal tissue, that destabilize the parallel G-quadruplex, which would be expected to give rise to abnormally high expression of c-MYC in these cells. In contrast, G-quadruplex-disruptive mutations were absent in 20 colon adenomas, suggesting that these mutations occur late in tumorigenesis. We have also demonstrated that these same mutations are found in established colorectal cell lines. NM23-H2 levels are lower in cancer tissues and cell lines that harbor these mutations. In cells with repressed levels of NM23-H2, the mutated and destabilized G-quadruplex silencer element can be reinstated by the addition of G-quadruplex-stabilizing compounds, providing an opportunity for therapeutic intervention for patients carrying these mutations.
Grand
Cory L
CL
Arizona Cancer Center, 1515 North Campbell Avenue, Tucson, AZ 85724, USA.
Powell
Tiffanie J
TJ
Nagle
Raymond B
RB
Bearss
David J
DJ
Tye
Denise
D
Gleason-Guzman
Mary
M
Hurley
Laurence H
LH
D001483:Base Sequence; D015179:Colorectal Neoplasms / Q000235:genetics; D017931:DNA Primers; D020868:Gene Silencing; D016259:Genes, myc; D006579:Heterozygote; D006801:Humans; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D054778:NM23 Nucleoside Diphosphate Kinases; D009701:Nucleoside-Diphosphate Kinase; D011506:Proteins / Q000235:genetics*; D013347:Subcellular Fractions / Q000378:metabolism
D017931:DNA Primers; D054778:NM23 Nucleoside Diphosphate Kinases; D011506:Proteins; C517938:NME1 protein, human; D009701:Nucleoside-Diphosphate Kinase