| The multistage model of carcinogenesis states that an accumulation of mutations in genes that are important for maintaining cellular homeostasis may lead to cancer. A specific type of mutation observed in particular types of cancer is the CC→TT mutation found in the TP53 gene of patients with basal or squamous cell carcinoma. Although sunlight (or UV radiation) and oxidative stress have been shown to lead to increased frequencies of CC→TT mutations, the mechanism of formation is not yet determined. Other factors that have previously been shown to play a role in tandem mutations are: cytosine deamination, the pol eta translesion bypass polymerase, the nucleotide excision repair pathway, and the presence of functional P53. A mammalian reversion assay was designed to detect single C→T or tandem CC→TT mutations for the purpose of characterizing additional factors that could contribute to the formation of the tandem mutation. The work presented herein describes several novel factors that were discovered, via the use of the reversion assay, to contribute to the formation of tandem mutations. These factors include: the mismatch repair pathway, DNA sequence context, endogenous oxidative stress in the presence of UV radiation, and antioxidant treatment in the presence of UV radiation. Consideration of the novel factors in the context of what is already known about tandem mutations led to the development of a model suggesting that CC→TT tandem mutations are formed as a result of tandem cytosine deamination, followed by a round of replication in which adenines are placed across the deaminated cytosines (uracils), followed by another round in which thymines are placed across the adenines. Future experiments are proposed to test this model. |
