Understanding DNA Repair

Dr. Peter Glazer, Understanding DNA Repair
March 20, 2010Welcome to Yale Cancer Center Answers with doctors Francine
Foss and Lynn Wilson.  I am Bruce Barber.  Dr. Foss is a
Professor of Medical Oncology and Dermatology, specializing in the
treatment of lymphomas.  Dr. Wilson is a Professor of
Therapeutic Radiology and he is an expert in the use of radiation
to treat lung cancers and cutaneous lymphomas.  If you would
like to join the conversation, you can contact the doctors
directly.  The address is canceranswers@yale.edu and
the phone number is 1-888-234-4YCC.  This evening, we are
pleased to welcome Dr. Peter Glazer.  Dr. Glazer is Robert E.
Hunter Professor of Therapeutic Radiology, Professor of Genetics,
and Chair of the Department of Therapeutic Radiology at Yale School
of Medicine. He joins us this evening to talk about DNA
repair.  Here is Lynn Wilson.Wilson
 Let's start off by having you tell us a little bit about what DNA
repair is?Glazer
DNA repair is the process in a cell by which the cell tries to fix
damage that occurs to its genes and chromosomes.  DNA repair
actually occurs through several different pathways in response to
various different kinds of DNA damage.  One of the most common
types of DNA damage is what people experience when they go to the
beach and get sunlight damage to their DNA, and that's why
sunscreens are recommended, but the fact of the matter is that the
DNA does get damaged by ultraviolet light in the sun and our cells,
DNA repair enzymes, need to come in and fix that.  You can
have DNA damage from chemicals such as from smoking, or from diesel
fumes, even some of the compounds that we eat lead to reactive
chemicals in the body that cause damage to DNA and then other
sources such as radiation can lead to breaks in the DNA that need
to be repaired.Wilson
 What sorts of effects can chemotherapy have on cells, does that
interact with DNA as well?Glazer
Many of the chemotherapy agents that are used, and that are
effective, have as their targets the DNA in the cancer cells. 
One of the most powerful chemotherapy agents is something called
cisplatin and this agent acts by creating platinum adducts on the
basis in the DNA and blocking the function of the genes.Wilson
 Tell us a little bit about what it is that you do within the
Department of Therapeutic Radiology.Glazer
As you mention, I am chairman of the department and so I have a
fair amount of administrative duties which mostly consist of trying
to keep everybody else happy so they can do their jobs, but I also
do a fair amount of basic science, laboratory research, focused on
cancer biology as well as some clinical care of patients receiving
radiation treatments.Wilson
 What first got you interested in studying genes and cancer?Glazer
Well, I was in college in the mid 70s, which really was the
beginning of the molecular biology revolution, and I took a course
in genetics that opened my eyes to the possibilities of the power
of genetics and how that could be used to solve or address problems
like cancer.3:35 into mp3 file 
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 Talk to our listeners about how DNA works and tell us more details
about DNA repair.Glazer
As you know, DNA is the material that makes up our genes and
chromosomes.  It consists of what our called bases, adenine,
guanine, cytosine, and thymine (AGCT) which form the genetic code,
and DNA actually is a double helix, and that's the famous discovery
of Watson and Crick, and so DNA forms a double helix, but is
subject to different kinds of stresses in the cell and these can
lead to DNA damage either from environmental stressors or
endogenous factors.  I mentioned some of the environmental
stressors earlier, but DNA damage can occur in the cell also when
there is a problem during replication, such as stalling during the
period of time when the DNA is being replicated, so a cell can
divide that can lead to breaks in the DNA strand.  There can
also be errors when the DNA is being copied, and if one chromosome
is being copied into two, those errors are similar to spelling
errors on the computer and you need a spell check, a program to
come in and find the mistakes and then correct them.Wilson
 What's the difference between DNA and RNA?Glazer
DNA has the genetic blueprint that codes for RNA, which is
expressed or transcribed from the DNA, and RNA can take many
forms.  The classical form is called messenger RNA which
conveys the message of the DNA into a protein, so the messenger RNA
becomes translated from an RNA sequence into a polypeptide protein
sequence, and it is the proteins that form all the enzymes and
structural components of our cells.  It turns out there are
many other types of RNAs that have been newly discovered, one of
them is something called microRNAs, which were subjective of a
Nobel Prize awarded recently and these seem to have a very
fascinating regulatory role in the cells.Wilson
 What role does DNA repair play in cancer therapy?Glazer
 Many of the agents that we use to treat cancer, such as radiation
therapy, and many chemotherapies we have talked about such as
cisplatin, cyclophosphamide and others, interact directly with the
DNA and either form chemical changes in the DNA or actually break
the DNA strands, which is what radiation can do, and that's where
DNA repair comes in.  One of the reasons that we are able to
use these sorts of agents to treat cancer effectively is that
cancer cells tend to be dividing more rapidly than our normal
healthy cells, and as a result, they have less time to fix their
DNA.  They also tend to be a little sloppier in terms of how
they repair their DNA.  Some of these pathways are abnormal,
in fact that's some of the area that we study in our own lab but
this sloppiness leads to some increased susceptibility to agents
that target the DNA.  Also, cancer cells, in addition to
dividing more rapidly, have some abnormalities in some of the
processes in the cell that slow down the cell division when DNA
damage is sensed.  These processes are sometimes referred to
as checkpoints, where the cell has a process of checking the DNA to
see if it is intact before the cell divides.  Normal cells
will halt cell division to give time for repair, but cancer cells
often lack that checkpoint and as a result would go on to divide
with broken DNA, and again, the cancer therapy that targets the DNA
takes advantage of that.7:59 into mp3 file 
http://yalecancercenter.org/podcast/mar2011-cancer-answers-glazer.mp3Wilson
 It sounds like typically, normal tissues, skin, and other organs,
are better at repairing this damage than a tumor is, for example,
and that gives us a therapeutic advantage in some cases.Glazer
Right, in some cases there are subtle differences between the DNA
repair capacity of cancer cells and normal healthy tissue that
comes from a combination of the factors I talked about before, the
lack of the checkpoints, the more rapid cell division, as well as
some abnormalities in the repair pathways themselves.Wilson
 Tell us a little bit about genetic instability, what is that in
cancer?Glazer
Cancer is not only more rapidly dividing, but as I said, they are a
little bit sloppier in terms of how they manage their DNA, they
lack those checkpoints for regulating the speed of cell division
and as a result they tend to incur many more mistakes and
abnormalities in their DNA as they divide and the genetic
instability can be at the level of broken or missing chromosomes,
or too many chromosomes because the cell division does not occur
properly, so the chromosomes do not segregate to the daughter cells
properly.  There can also be so called point mutations where
single base pairs, those A, G, C, and T letters, are incorrect, but
the cancer cell does not fix that properly.  So there are many
levels of genetic instability and this is a key feature of cancer
cells.  It is obviously not a good thing for normal cells to
have genetic instability, but cancer cells actually can benefit
from the genetic instability because it creates genetic diversity
in the cancer and it sometimes allows the cancer cells to develop
resistance to cancer therapy more quickly.Wilson
 Explain for our listeners, Peter, what a mutation is?  What
does that mean?Glazer
A mutation is a change in the sequence of the DNA.  I
mentioned earlier that the DNA is made of beads on a string with
the letter A, G, C, and T and the Watson and Crick discovery in
part showed that A binds to T and G binds to C and so the double
helix has the sequence code and that sequence code is read during
the transcription of DNA into RNA.  A mutation is an
alteration in that sequence code so it could be a G in place of
what should be an A, it could be a rearrangement where a piece of
the DNA is flipped around backwards, it could be an insertion of a
string of letters from one up to hundreds.  Any change in the
sequence is a form of mutation.Wilson
 And have some of these have been well identified in cancer?Glazer
It turns out that there are a number of key genes that researchers
have found are mutated in cancer cells, and many people feel a link
to the causation of the cancer.  One of the most famous is a
gene called RAS, and another one is called RAF.  Mutations in
RAF are linked to melanoma.  Mutations in RAS are linked to
many types of cancers including cancers in the pancreas, lung, and
other sites.  More recent discoveries have identified
mutations in genes linked to familial breast and ovarian cancers
and those genes go by the name BRCA1 or BRCA2 and BRCA is lingo for
BRCA1 or BRCA2 for a breast cancer associated gene, and individuals
can inherit one of these genes with a preexisting mutation that
inactivates the functions of the gene and what people think happens
is12:29 into mp3 file 
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that the good copy of the gene is lost in a particular cell and
that creates an increased risk of that cell becoming malignant.Wilson
 In certain mutations, do we have some medicines or drugs that
might be more effective for the patient with that problem?  I
have heard some things regarding lung cancer mutations.  A
patient might have a certain mutation and they might be more
sensitive to a particular drug then a patient without that
mutation.Glazer
That's right, so part of the elucidation of these mutations in
these key genes has been understanding their function, how these
mutations impact the function of the protein as well as the
structure of the protein and how it behaves, and that has allowed
people to design agents, small molecule drugs usually, or sometimes
antibodies, the target those specific proteins or mutated proteins
and take advantage of that.  In fact, there is a gene called
Epidermal Growth Factor Receptor, which is a protein that is on the
surface of many cells including cancer cells in the lung, and it
turns out that if an individual's cancer has a mutation in the EGF
receptor gene that cancer will be susceptible to a certain class of
drug. Another example that is somewhat related to research in our
own lab is as I mentioned, the BRCA 1 or BRCA 2 genes linked to
breast cancer. Mutations in those genes actually impact the ability
of the cancer cell to conduct DNA repair by a particular
pathway.  Those mutations give the cancer cell increased
genetic instability and help turn it more malignant, but also
create a little bit of an Achilles heel for the cancer with agents
that exploit that deficiency in DNA repair and there are a number
of agents that are now in clinical trials to take advantage of that
deficiency.  There are some trials going on at Yale-New Haven
Hospital with those agents and in our own lab we are actually not
only testing those agents themselves, but in combinations with
other potential chemotherapy agents as well as radiation, and we
have also been working to develop newer agents that may be even
more selective or more potent.Wilson
 We are going to take a short break for a medical minute. 
Please stay tuned to learn more information about DNA repair with
Dr. Peter Glazer.16:26 into mp3 file 
http://yalecancercenter.org/podcast/mar2011-cancer-answers-glazer.mp3Wilson
Welcome back to Yale Cancer Center Answers.  This is Dr. Lynn
Wilson, today I am joined by Dr. Peter Glazer and we are discussing
DNA repair.  Peter, in the previous part of the show you
mentioned antibodies, monoclonal antibodies, and mall molecules,
define for us what these things are, what they mean, and how they
work on the cell?Glazer
Antibodies are actually large protein molecules that our own bodies
make as part of our immune defense mechanisms against infectious
agents, and research almost 30 years ago found that antibodies
could be raised in mice and the cells making them could be
harvested such that the antibodies could be produced in a test tube
and collected for use as a medicine, and the key there is that the
mice are challenged with different proteins or antigens that might
be related to factors that are important for the cancer cell to
grow.  So, in this way, monoclonal antibodies are developed
and monoclonal simply means it comes from a single clone of an
antibody producing cell, and I will not go into all the mechanics
of doing that, but there are ways of isolating that cell and
collecting the antibody for use as a medicine, and these are
particularly promising types of cancer therapy agents.  Some
of the current ones that are being used in the clinic are
Herceptin, which targets a protein on the surface of breast cancer
cells.  There are several that target the Epidermal Growth
Factor Receptor I mentioned before, which is a protein on the
surface of several types of cancer cells including those in the
lung and the colon, as well as cancers in the head and neck region,
and many more antibodies are being developed as cancer
therapeutics, and how they work is still being studied, but part of
the way they work is interfering with the normal function of
whatever protein they stick to by binding to it and sort of mucking
it up.  They may also help to elicit an immune response to the
cells that they stick to, so a combination of those actions is
important for the effectiveness of an antibody.  Small
molecules are simply a term used to broadly refer to chemicals that
are the more conventional types of medicines that we have and these
can be synthesized in a chemistry laboratory by pharmaceutical
companies and they are similar to many of the medicines that we
take everyday in pill form.Wilson
 Why are they called small molecules, is that because they need to
get themselves inside of another cell?Glazer
This is a little bit of scientific jargon to differentiate the
molecules I just described from larger molecules like antibodies or
things of that nature.Wilson
 Tell us a little bit about what hypoxia is and how that can play a
role in cancer therapy?Glazer
One of the key features of tumors is that their growth outstrips
their blood supply, not so much that they do not grow, but to the
point where many regions of the cancer do not get the kind of blood
flow that healthy parts of the body get, and as a result they do
not live in an area with the level of oxygen that is present in our
healthy tissues. The lower oxygen levels in parts of tumors are
referred to as regions of hypoxia, meaning low oxygen.  What
happens with hypoxia, if it is extreme it will prevent the growth
of the cells, but it usually does not get so extreme that the
growth is completely blocked, it may be slowed, but hypoxia
profoundly changes the biology of21:24 into mp3 file 
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the cancer cell.  One of the main things it does, is it
stimulates the cancer cell to send signals to nearby blood vessels
to grow and expand so that more blood vessels grow into the
tumor.  The process of recruiting blood vessels into a tumor
so that it can grow and expand and get bigger is called
angiogenesis and is one of the fundamental discoveries of the last
20 or so years that has led to a lot of effort to develop medicines
and strategies to interfere with angiogenesis.  However,
another thing that the low oxygen or hypoxia does is it changes the
internal biology of the cancer cells.  One aspect of that is,
it makes the cancer cells more dependent on the sugar called
glucose for their energy source and so that is in part the basis of
why we use PET scanning today to identify tumors that take
advantage of the high utilization of glucose in tumors.  In
addition, hypoxia affects the expression of many genes in the
cancer and so it changes the constellation of activities in the
cancer cell creating a situation where the cancer cell is somewhat
different than healthy tissue and that is an area that our lab, and
several labs at Yale, are interested in exploiting for cancer
therapy.  One particular way that hypoxia leads to a
difference in cancer cells is by altering the DNA repair capability
of the cell.  It turns out hypoxic stress causes the cancer
cells to decrease their capacity to carry out DNA repair in certain
ways and our own research has discovered some of these steps and
now we are designing new medicines that might be used to exploit
that.Wilson
 How does hypoxia impact the efficacy of radiation treatment, for
example?Glazer
It turns out that radiation causes damage to tissue when the x-rays
or photons pass through the tissue and knock electrons off of atoms
in the tissue, and that is a process called ionization.  That
process creates radicals, so called ion radicals, which can then
interact with other molecules in the cell including DNA.  In
the presence of oxygen, there are additional radicals and other
species are produced that are longer lived and can cause more
damage throughout the cell.  When there is no oxygen present,
there is less damage that is produced to the DNA.Wilson
 I see.  So, preferentially, we would not want to have hypoxic
tumors if we want radiation to work as well as it can?Glazer
That is right.  Hypoxia is a double edge sword, or maybe a
triple or quadruple edge sword really, because it affects the
cancer cell in many ways, some to make them more susceptible to
therapy, but some to make them less susceptible.Wilson
 Let's shift gears for a minute Peter, the NIH, the National
Institutes of Health, is our governmental funding agency and for
our listeners, that process by which we can obtain grants for
research funding is extremely competitive. There are lots of
different kinds of grants and recently you were awarded what is
called a Program Project Grant, and you are the principal
investigator of that program.  Can you tell us the details
about that and how important that is?Glazer
The Program Project Grant is a type of funding mechanism from the
National Cancer Institute, of the National Institutes of Health,
that is designed to support cancer-related research of a team at
a26:01 into mp3 file 
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particular institution, whereby investigators doing related work
can get together and develop an integrated interactive research
program that is highly focused on some aspect of cancer
research.  It is a little bit different than a research grant
awarded to an individual investigator, which is a very important
and highly valuable thing.  This is a way to bring together
investigators in a focused way.Wilson
 Tell us a bit of the details.  Are there multiple projects
within this Program Project Grant?Glazer
There are four projects, and each is led by a project leader, not
only myself, but three other faculty at Yale, Alan Sartorelli,
Joann Sweasy and Patrick Sung, and these projects are focused in
the area of DNA repair, with the ultimate goal of developing new
cancer therapies.  My particular project is looking at how DNA
repair is different in cancer cells and how that can be exploited
for therapy.  Joann Sweasy is looking at the fact that some
DNA repair genes are mutated in cancers and whether cancers
with specific mutations and specific DNA repair genes may be more
or less susceptible to existing therapies.  Patrick Sung is
looking at some novel interactions among DNA repair factors in a
cell and has discovered that there are some key interactions that
could be the targets for novel therapies, and Alan Sartorelli
continues a long track record of developing novel agents that can
be used specifically to target cancer cells.  He is a
pharmacologist and he has already synthesized a number of molecules
that are in clinical trials and this Program Project is supporting
his work in that area.Wilson
 Obviously important discoveries cannot be made without that
support.  Peter, what do you think have been some of the most
important DNA repair discoveries say over the last ten years?Glazer
It is interesting, when I started in this field 20 years or so ago,
DNA repair was thought to be just a house keeping function of the
cell and was not thought to be that important.  Well, it turns
out that as I said, many DNA repair genes are associated with
inherited cancer such as the BRCA1 or BRCA2 genes and so our
understanding of that has now opened up the possibility of
exploiting these DNA repair deficiencies in cancer.Dr. Peter Glazer is Robert E. Hunter Professor of
Therapeutic Radiology, Professor of Genetics and Chair of the
Department of Therapeutic Radiology at Yale School of
Medicine.  If you have questions or would like to share your
comments, visit yalecancercenter.org where you can also subscribe
to our podcast and find written transcripts of past programs. 
I am Bruce Barber and you are listening to the WNPR Health Forum on
the Connecticut Public Broadcasting Network.