The p53 Gene

            The p53 tumor suppressor gene is responsible for producing the p53 protein, which works to repair the cell when DNA damage is detected. Some of its roles include arresting the cell cycle before replication, inducing apoptosis, or binding to promoter regions to prevent transcription of certain genes. There are about 100 proteins that p53 regulates, and the article I am going to discuss focuses on its relation to skin cancer and regulating UV-induced DNA damage. The article also looks at how p53 can serve as a biological endpoint to evaluate the efficacy of sunscreens in preventing UV-induced skin cancer.

            A signature mutation of UV-induced DNA damage is the C–>T and CC–>TT mutation, which are found in the p53 gene. The researchers in this article conducted an experiment in which mice heterozygous (+/-) and homozygous (-/-) for the p53 gene, were exposed to UV in order to observe tumor development in relation to presence of absence of the p53 gene. The study concluded that the order of probability for developing tumors went from wildtype (+/+) mice being the least at risk, to the heterozygous, and finally to the homozygous (-/-), which had the highest risk. What I’m curious to know more about is how the researchers ranked these “at risk” levels, and what measurement they used to quantify likelihood of tumor development. The results imply that having the p53 gene is better than not having it, because although p53 mutations can occur from the UV, it’s better to have the gene so that non-mutants can continue to repair damage.

Another experiment carried out studied the effectiveness of sunscreen in inhibiting p53 mutations. The SPF of sunscreen is determined by its ability to prevent sunburn, but is not indicative of how well you are protected from other damages caused by UV exposure. Because p53 mutations correlate with tumor development as studied in the previous experiment I discussed, researchers looked at sunscreen’s ability to inhibit these mutations from occurring. In one of the experiments, mice were exposed with UVB from a sunlamp 5 times per week for 12 weeks. One group of mice received UVB irradiation and sunscreen vehicle control, while another had UVB absorbing sunscreen applied 30 minutes prior to exposure, and the last group had UVB+UVA absorbing sunscreen applied 30 minutes prior. The results showed that in comparison to the control, there was an 88% reduction in mutation frequency for the UVB absorbing sunscreen, and a 92% reduction when UVA+UVB sunscreen was used. But, as the study indicated, using sunlamps weren’t as realistic as if the mice were to be exposed to actual sunlight, and so they redid the experiment using solar simulator radiation (SSR). The results were claimed to be extremely similar, although the exact numbers were not indicated. I question whether it was necessary for them to initially use sunlamps if they knew it wouldn’t be an accurate representation of real sunlight, and I also would like to know more about what makes SSR better (i.e. how are the UV waves emitted differently, how is its intensity different, etc.). The number of mice studied was not indicated, but I find the results reliable because the experiment was controlled in terms of type of mice, exposure amount, type of exposure, and treatment of each group of mice. If all else besides type of product applied is the same, I think it’s plausible to claim the mutation frequencies were due to the type of sunscreen used. What I am curious to know more about is what the actual mutation rates were like, and whether an 88% or 92% reduction meant that mutations were still occurring, and tumors were still likely to develop, or if a reduction of those amounts were enough for the cell to later repair and avoid tumor development.

The article also discusses an experiment done to examine the effect of sunscreen on human skin, by using skin grafts of human skin used for breast reconstruction, which tested negative for presence of p53. I think it was important for the researchers to make sure the skin was not predisposed to having the gene expressed because that would have indicated the skin was responding to DNA damage, and would have factored into whether developing tumors were a result of the UVB exposure or to other elements already in the skin. In the study, the skin was grafted onto mice, and after the mice healed from the surgery, they were exposed to solar-simulated UVB radiation 5 times per week, and p53 mutations were measured periodically by AS-PCR, although the time points at which these measurements were taken are not specified. The following table depicts the number of mutations occurring in skin grafts out of the total number of mice irradiated.

 
As you can see, the number of mice tested was very small, and it is also questionable whether some mice had multiple mutations, and what combinations of mutations occurred. It is also strange that there are more mice irradiated by 12 weeks. But the overall trend shows that by week 12, all skin grafts had some sort of mutation when sunscreen was not used, and that in mice that were treated with sunscreen, there was 1 C-->T mutation, which leads me to question why it’s noted that for total skin grafts with mutations a zero is indicated. Although the sample size is too small to give an in depth analysis of the complete effect of sunscreen, it is apparent that there is a benefit of using sunscreen in regards to mutations arising based on the 10 fold difference in number of mutations by week 12.

            Overall, I though the article did a good job of analyzing its own experimental data, and noting what they could do to improve upon, as well as the credibility of their numbers. Several times, the article would discuss the results, and then indicate the implications, and the error in their analysis. I am interested to learn more about how sunscreen actually inhibits p53 gene mutations, and to what extent sunscreen must be used in order to reduce mutation frequency and likelihood of tumor development in the long run.    

Sources:
1. Benjamin, C. L., Ullrich, S. E., Kripke, M. L. and Ananthaswamy, H. N. (2008), p53 Tumor Suppressor Gene: A Critical Molecular Target for UV Induction and Prevention of Skin Cancer. Photochemistry and Photobiology, 84: 55–62. doi: 10.1111/j.1751-1097.2007.00213.x