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Cancer ?Associated Mutants of DNA Polymerase Beta
Joann B. Sweasy, Yale University School of Medicine
We currently have an incomplete understanding of the events that lead to cancer. Tumors result
from the uncontrolled growth of cells. Therefore, cancer researchers would like to know what
types of events in cells result in their uncontrolled growth. One approach that has been taken by
cancer researchers is to determine whether the genes that act to control cell growth are mutated in
tumors. Mutation of these growth control genes in tumors would imply that these genes are not
able to function to control cell growth. This avenue of research has uncovered some genes that
are mutated in tumors, and further research has provided important insights into how mutations of
specific genes result in uncontrolled growth. In an attempt to understand how growth control
genes are mutated, scientists discovered that there are large numbers of mutations in tumors and
much smaller numbers of mutations in normal cells. This suggests that tumors, perhaps at very
early stages, have acquired a property that does not exist in normal cells and that makes it more
likely for genes in tumors to be mutated. The net result would be a large and aggressive tumor.
What is this property? Before addressing this question, we need to describe some basic biology.
Genes are made of DNA. Mutations are changes in one of the four bases of DNA. DNA has to
be copied by enzymes before cells divide, in order for each cell to have a complete copy of the all
of the genes comprising an individual’s genome. Many times during the course of a day, the
bases in the DNA are damaged and the cell has enzymes that act to repair that damage. Enzymes
are proteins that are made from the information contained within the DNA. Enzymes can make
mistakes while they copy or act to repair the DNA. For example, during the copying of the DNA,
an enzyme called a DNA polymerase might insert a T opposite a G, instead of inserting a C
opposite the G. This action by the polymerase can result in a mutation. Many researchers think
that the property that tumors have acquired is an altered DNA polymerase or repair enzyme that
makes mistakes. This would increase the chances for mutations to occur in growth control genes,
ultimately resulting in cancer.
We began this project by surveying the literature and found that 35% of 90 tumors studied have
mutations within the DNA polymerase beta (Pol beta) gene. None of the mutations that were
found in tumors are present within the normal population. The role of Pol ß in the cell is to copy
DNA during the process of DNA repair. Therefore, we are interested in testing the hypothesis
that the altered forms of Pol beta found in the tumors have a direct link to cancer. We have
initiated our work on the I260M and K289M mutants of Pol beta. I260M was found to be present
in a prostate carcinoma, and K289M was identified in a colon carcinoma. First, we expressed
these two different tumor-associated forms of Pol beta in normal mouse cells. The mouse cells
usually grow as a single layer in flasks. However, when we expressed the tumor-associated
forms of Pol beta in the mouse cells, they no longer grew as a single layer, but instead, grew on
top of each other. This is a characteristic of uncontrolled cell growth. Cells containing the
tumor-associated Pol beta also grow in low serum, which is another characteristic of uncontrolled
cell growth. These studies suggest that the mutant forms of Pol beta that are found in tumors
might be directly related to cancer.
Next, we characterized the I260M and K289M proteins to determine if their properties were
consistent with the ability to induce uncontrolled growth in normal cells. We found that both of
these proteins are able to introduce errors into DNA that are quite different than the errors
induced by the normal (non-mutated) Pol beta. This supports the idea that mistakes introduced by
these Pol beta variants could result in uncontrolled cell growth.
In summary, our data are consistent with the interpretation that the variant forms of Pol beta
found in tumors have a direct link to cancer. When the DNA of a cell is damaged, the damage is
removed, leaving a single-stranded gap in the double helix. One function of Pol beta is to
fill that gap by copying the DNA on the other strand. We have shown that the I260M or K289M
variant proteins are able to fill the gap. However, when these tumor-associated proteins fill the
gap, they are likely to make a mistake. If this mistake occurs within a key growth control gene it
could lead to expansive growth of tumor cells and to cancer. An important implication of our
studies concerns the treatment of cancer with certain types of drugs called alkylating agents. One
way these drugs kill cancer cells is by damaging their DNA. However, the cells try to repair that
damage. We suggest that if variant forms, like I260M or K289M of Pol beta, are present within
the cells, mutations may arise during the repair of the drug-induced damage. These mutations
may result in the acquisition of drug resistance and a more aggressive cancer.