Chromosomal instability can lead to loss of heterozygosity (LOH). In early stages of cancer development, the instability occurs preferentially in genomic regions that are sensitive to replication stress, defined as replication stress. Later in cancer development, the instability is found along the entire genome.
Bester, et. al (2011)
Show oncogene-induced replication stress (HPV E6/E7) in primary keratinocytes leads to increased genomic instability at the most sensitive fragile sites during early cancer development.
HPV E6/E7 expression in primary keratinocytes for 2-4 weeks leads to 2-5 fold reduction in levels of all four dNTPs, can be rescued via exogenous introduction of nucleosides. Also increased gamma H2AX staining (increased DSBs)
Cyclin E overexpression via retrovirus in BJ fibroblasts for 2-4 weeks decreases dNTP pools, can be rescued via exogenous introduction of nucleosides. Also increased gamma H2AX staining (increased DSBs)
Activation of the Rb-E2F pathway by cellular or viral oncogenes results in an insufficient nucleotide pool, leading to replication stress and DNA damage. This replication induced DNA damage can be rescued by exogenous supply of nucleosides.
Cell proliferation depends on coordinated activation of the different nucleotide metabolic genes, which are tightly regulated by the transcription factors and master regulators of cell proliferation.
Unbiased whole transcriptome analysis reveals cyclin E expressing BJ cells have significantly upregulated pathways involved in cell cycle and DNA replication, with no upregulation of nucleotide biosynthesis pathways, leading to insufficient pools of nucleotides required for normal replication
Nucleotide biosynthesis is upregulated by c-Myc, which is not altered following E6/E7 or cyclin E expression
Conclusion
The basis for genome instability by oncogenes activating the Rb-E2F pathway is uncoordinated S phase entry, leading to insufficient factors required for normal DNA replication. Cells are forced to proliferate with an insufficient pool of nucleotides to support normal DNA replication. Under these conditions, the replication machinery fails to achieve regular rate and processivity, resulting in DNA damage and genome instability.
Show oncoproteins leads to perturbed progression of the replication forks and a poor symmetry between forks emerging from the same origin. Indicates a poor processivity which leads to replication fork collapse and DSBs.