Sssh, we are about to begin

signaling update home

SOURCE:

Overexpression of one component of the pre-replication complex represses the expression of the INK4a, INK4b and ARF tumor suppressors and promotes the oncogenic transformation of cells.

The replication of DNA is initiated at specific sites (origins of replication), which are bound by a specific protein complex (the pre-replication complex). Manuel Serrano and colleagues have found that the overexpression of one component of this complex is able to repress the expression of the INK4a, INK4b and ARF tumour suppressors and promote the oncogenic transformation of cells.

Some transcriptional regulatory sequences are found at, or close to, origins of DNA replication. Serrano and colleagues identified a putative origin of DNA replication 1.5 kb upstream of the ATG start codon of the INK4b tumour-suppressor gene, which is the first of the 3 genes in the CDKN2B–CDKN2A locus. A core sequence of 350 bp within this region was found to be highly conserved between mammalian species. The sequences of DNA replication origins are known to vary, but transcriptional initiation sites are generally conserved, prompting the authors to investigate whether this site, which they termed regulation domain (RD)^INK4/ARF, might regulate the transcription of the CDKN2B–CDKN2A locus.

RD^INK4/ARF reporter-gene experiments and RD^INK4/ARF RNA-interference assays — which affect the conformation of the chromatin (heterochromatinization) at complementary genomic DNA sites — indicated that RD^INK4/ARF could control the transcription of both CDKN2B and CDKN2A. As RD^INK4/ARF is thought to be a replication origin it seemed plausible that the proteins of the pre-replication complex might be involved. CDC6 is one such protein that is also overexpressed in several human cancers. The authors found that both endogenous CDC6 and an epitope-tagged CDC6 protein bound to RD^INK4/ARF in several human cell lines, but not to neighbouring regions of the DNA. The fusion protein was also shown to bind the well-characterized lamin B2 DNA replication origin. Other replication origin proteins were also bound to RD^INK4/ARF, indicating that a full pre-replication complex can form at this site.

High expression levels of CDC6 repressed the expression of all three tumour-suppressor proteins that are encoded by the CDKN2B–CDKN2A locus, but did not have an effect on other genes, such as MYC and DNMT1 that also reside close to origins of replication. Moreover, mouse-embryo fibroblasts (MEFs) expressing high levels of CDC6 were able to form colonies in culture, unlike wild-type MEFs, but did not show increased levels of proliferation, indicating that CDC6 overexpression is not oncogenic per se. But, CDC6-expressing MEFs were transformed by the expression of a Ras oncogene, indicating that the repression of the INK4 and ARF tumour-suppressor pathways by CDC6 increases the likelihood of oncogenic transformation.

Does the expression of CDC6 in human tumours correlate with a loss of INK4a expression? The authors examined this in 162 non-small-cell lung carcinomas. Tumours with no expression of INK4a were not included, as loss of INK4a could be caused by mechanisms other than CDC6 overexpression. However, tumours that expressed low levels of INK4a were found to express high levels of CDC6, indicating that this mechanism of INK4 and ARF repression might be relevant to human cancer.

Nicola McCarthy

References

Gonzalez, S. et al. Oncogenic activity of Cdc6 through repression of the INK4/ARF locus. Nature 440, 702–706 (2006). | Article | PubMed |