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Ashis Basu
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(b. 1950)
Postdoctoral Fellow, MIT, 1984
B.S., M.S., Univ. of Calcutta, 1971, 1973
Ph.D., Wayne State University, 1984
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| Chemical carcinogens, radiation,
and many chemical antitumor agents share a common property
in that they exert their biological effects through mechanisms
involving DNA damage. Certain DNA-bound forms of chemotherapeutic
agents are believed to be responsible for arresting tumor
cell growth. On the other hand, some of the damages induced
by carcinogenic chemicals are thought to be the mediators
of mutational changes which, in turn, are likely to be
necessary prerequisites for carcinogenic transformation.
Our research focuses on understanding the mechanism of
these agents at the molecular level. To accomplish this
objective we utilize the tools of synthetic organic chemistry,
physical organic chemistry, and biochemistry/molecular
biology. |
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| We are investigating the mode of action
of antitumor agents such as mitomycin C and its analogs
with DNA. A short fragment of DNA (an oligonucleotide)
with a specific antitumor agent-induced DNA modification
is chemically synthesized. Physico-chemical studies are
carried out on this modified oligonucleotide to understand
the architectural effect of the DNA-bound form of the
antitumor agent. In vitro studies are performed
to find out the kinetics of repair by the excision repair
proteins, such as UvrABC in Escherichia coli. In
vitro studies using DNA polymerases are carried out
to address whether such adducts cause replication blocks
and, in the case of a bypass, if the polymerase bypass
is error-free. Using a set of recombinant DNA techniques,
the lesion-containing oligonucleotide is introduced in
a plasmid or into the genome of a virus at a preselected
site. The modified genome is then introduced in cells,
where the DNA replication and repair systems act upon
the single DNA adduct. Progeny DNA molecules are isolated,
and the mutagenic as well as toxic effect of the single
adduct are studied. The primary goal of this work
is to define the relationship between the structure and
three-dimensional effects of a lesion in DNA and the mutagenicity
and cytotoxicity that it may cause. Another important
aim is to establish which cellular genes protect cells
against the acutely toxic (or mutagenic) properties of
the DNA damaging agent. These studies on the mechanism
of antitumor agents are likely to provide us the insight
necessary to design new analogs of these compounds. |
The other
major area of our interest is the mechanism of mutagenesis
by various DNA damages induced by ionizing radiation,
oxidation, or chemical carcinogens. An approach similar
in design to that described for drug-DNA adducts is
used, although the major focus here is mutagenicity
rather than toxicity. The DNA lesions we investigate
include those formed by ionizing radiation/oxidation
and the nitro-aromatics (e.g.. 1-nitropyrene), which
are common environmental pollutants formed during the
combustion of fossil fuels. A major focus in our
research is investigation of the mechanism of various
biological endpoints such as mutation, toxicity, and
repair.
The model on the right shows how a slippage
during replication may occur causing a two-base deletion,
which occurs frequently with the nitro-aromatic carcinogens. Hypotheses
or working models such as this one are investigated
by a combination of in vitro translesion synthesis by
DNA polymerases, in vivo mutagenesis studies, structural
investigations (such as NMR and thermal DNA melting),
and molecular modeling.
These investigations are aimed at probing
the cause and treatment of cancer and entail working
at the interface between chemistry and biology. One
of the long term goals our research is to elucidate
the biochemical mechanisms by which cells defend against
specific forms of DNA damage. The mechanistic information
gained from this research is likely to be of central
importance in understanding how tumor cells respond
to antitumor drugs and how they acquire resistance.
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