Our research focuses on cellular responses to DNA damage.
Preservation of the integrity of the genome is essential to survival
of individual cells and the whole organism. Genetic mutations can
lead to disruption of the normal pattern of development and to cancer.
A wide variety of environmental agents (e.g., UV radiation, toxic
chemicals, etc.) as well as intrinsic metabolic processes (e.g., oxidative
stress) can damage DNA leading to mutations. We are interested in
understanding the mechanisms by which organisms respond to DNA damage
to prevent mutations and cell death. We are using tools of molecular
biology and biochemistry combined with genetic approaches to address
these questions. Although much of our work is carried out in cultured
mammalian cells and cell-free systems, we also make use of other experimental
systems, such as yeast and transgenic mice.
Currently, we are investigating the nature of the DNA damage signal
that activates protein kinases (especially ATM and ATR) that phosphorylate
proteins involved in DNA replication and DNA repair, including the
single-stranded DNA binding protein RPA, the MRE11/RAD50/NBS1 complex,
and the BLM protein. We showed that the signal from UV damage is
the blockage of the DNA replication complex at sites of damage.
It is thought that similar blockage occurs occasionally during normal
DNA replication and a failure to properly overcome the blockage
is a basis for certain human genomic instability syndromes. DNA
damage-induced phosphorylation of RPA causes changes in RPA function
which may lead to inhibition of its role in DNA replication and
enhancement of its role in DNA repair. Further work in this area
will involve proteomic approaches to investigating alterations in
protein partnering associated with phosphorylation of RPA.
We are also interested in understanding more about the mechanisms
of action of two known human carcinogens, chromium and arsenic.
We have used mutagenesis assay systems from yeast, mammalian cells
and transgenic mice to investigate the molecular mechanisms of chromate
mutagenesis. These studies have shown that chromate is highly mutagenic
and causes oxidative-type DNA damage. Chromate induces mutations
in all three assay systems and the mutagenic activity depends on
the redox state of the cells and the ability of the cells to repair
oxidative DNA damage. In contrast, arsenite does not appear to be
mutagenic by itself, but it amplifies mutagenesis by other agents
that damage DNA (e.g., UV radiation, methylmethane sulfonate, etc.).
Our studies show that arsenite interferes with the removal of UV-induced
DNA damage causing enhanced DNA damage signaling and prolonged cell
cycle checkpoints. In addition, arsenite alone causes an accumulation
of cells in mitosis. Future work will focus on understanding the
molecular mechanism by which arsenite interferes with removal of
DNA damage.
Any link on the below references will take you off
of the BMCB site and to an abstract of that particular paper.
Robison, J.G., J. Elliott, K. Dixon, and G.G. Oakley.
2004. Replication protein A and the Mre11-Rad50-Nbs1 complex co-localize
and interact at sites of stalled replication forks. Journal
of Biological Chemistry 279: 34802-34810.
Oakley, G.G., S.M. Patrick, J. Yao, M.P. Carty, J.J.
Turchi, and K. Dixon. 2003. RPA phosphorylation at mitosis alters
DNA binding and protein/protein interactions. Biochemistry 42: 3255-3264.
Langland, G., J. Elliott, Y. Li, J. Creaney, K. Dixon,
and J. Groden. 2002. The BLM helicase is necessary for normal DNA
double-strand break repair. Cancer
Research 62: 2766-2770.
Yao, J., K. Dixon, and M.P. Carty. 2001. A single (6-4)
photoproduct inhibits plasmid DNA replication in xeroderma pigmentosum
variant cell extracts. Environmental
and Molecular Mutagenesis 38: 19-29.
King, N, G.G. Oakley, M. Medvedovic, and K. Dixon. 2001.
The XPA protein alters the specificity of ultraviolet light-induced
mutagenesis in vitro. Environmental
and Molecular Mutagenesis 37: 329-339.
King, N., M. Carty, and K. Dixon. 2001. In vitro replication
and mutagenesis of a novel reversion vector with selective damage
in the SupF gene. Mutation
Research 476: 21-28.
Li, Y., M.P. Carty, G.G. Oakley, M.M. Seidman, M. Medvedovic,
and K. Dixon. 2001. Expression of ATM in ataxia telangiectasia fibroblasts
rescues defects in DNA double strand break repair in nuclear extracts. Environmental and Molecular Mutagenesis 37: 128-140.
Oakley, G.G., L.I. Loberg, J. Yao, M.A. Reisinger, R.I.
Yunker, M. Zernik-Kobak, K.K. Khanna, M.F. Lavin, M.P. Carty, and
K. Dixon. 2001. UV-induced hyperphosphorylation of replication protein
A depends on DNA replication and expression of ATM protein. Molecular
Biology of the Cell 12: 1199-1213.
Medvedovic, M., P. Succop, R. Shukla, and K. Dixon.
2001. Clustering mutational spectra via classification likelihood
and Markov Chain Monte Carlo algorithms. Journal of Agricultural, Biological and Environmental Statistics 6:19-37.
