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The Role of DNA Repair and Damage in Cancer

July 12, 2021
  • 00:00Funding for Yale Cancer Answers
  • 00:02is provided by Smilow Cancer
  • 00:04Hospital and AstraZeneca.
  • 00:08Welcome to Yale Cancer Answers with
  • 00:10your host doctor Anees Chagpar.
  • 00:12Yale Cancer Answers features the
  • 00:14latest information on cancer care by
  • 00:16welcoming oncologists and specialists
  • 00:18who are on the forefront of the
  • 00:20battle to fight cancer. This week,
  • 00:22it's a conversation about DNA
  • 00:24repair with Doctor Megan King.
  • 00:26Doctor King is an associate professor
  • 00:28of cell biology and of molecular,
  • 00:31cellular, and developmental biology
  • 00:32at the Yale School of Medicine,
  • 00:34where Doctor Chagpar is a
  • 00:37professor of surgical oncology.
  • 00:39Megan, maybe we can start off with you
  • 00:42telling us a little bit about yourself
  • 00:45and about your research and how you got
  • 00:47involved in this research project to
  • 00:50begin with.
  • 00:50Yeah, so it's very interesting thinking
  • 00:52back to what drew me towards science.
  • 00:55I'm from a family of engineers,
  • 00:57actually including both of my parents,
  • 00:59but I always gravitated towards science,
  • 01:02and in particular as a high school student,
  • 01:04I took anatomy and Physiology,
  • 01:06and it was actually the
  • 01:08section of my textbook
  • 01:09on cancer that really provided for me,
  • 01:12I think the first kind of window into
  • 01:14how a scientist could have a positive
  • 01:17impact on human health in a way that was
  • 01:20different from becoming a medical doctor,
  • 01:22which I think all of us are a
  • 01:25little bit more familiar with,
  • 01:27certainly as children.
  • 01:28And so I've been reflecting on
  • 01:30that recently because it's been a
  • 01:32bit of a circuitous route that's
  • 01:34brought me back to Cancer Research.
  • 01:36I really trended towards very
  • 01:38fundamental kind of basic science.
  • 01:40Questions for my initial training
  • 01:42as an undergraduate and graduate
  • 01:44student and even into my
  • 01:47postdoc period where one typically
  • 01:49is defining the kind of areas of
  • 01:51research that they will pursue,
  • 01:53and in their independent laboratory.
  • 01:55But I discovered a connection between
  • 01:57the cell biology of the nucleus,
  • 02:00which is something that I had
  • 02:02been training with
  • 02:04Gunter Blobel at Rockefeller
  • 02:06University in Genome Integrity,
  • 02:07so that is the mechanisms that
  • 02:10maintain the DNA blueprint
  • 02:12as it should be and that was
  • 02:14really just something that I had
  • 02:16not focused on before
  • 02:18but it really changed the direction of
  • 02:21my research and I became very interested
  • 02:24in how aspects of how a cell works,
  • 02:26are able to contribute to the mechanisms
  • 02:29that maintain that genetic code.
  • 02:31So tell us more about
  • 02:33that. I think some of us can
  • 02:35remember back to junior high biology
  • 02:38where we kind of know what a cell is.
  • 02:41And we know what a nucleus is and
  • 02:44housed within that nucleus is the
  • 02:46DNA which is responsible for that
  • 02:48genetic blueprint as you say.
  • 02:50So tell us more about the connection
  • 02:52that you found between how a cell
  • 02:55functions and genomic integrity.
  • 02:56Yeah, so I was
  • 02:58also fascinated with this
  • 03:00idea of the the nucleus which
  • 03:02is the organelle that houses the DNA,
  • 03:05being kind of the brain.
  • 03:07Having all of the kind of
  • 03:09control and that plan
  • 03:11for the cell, but I think one of the
  • 03:14things that maybe isn't always captured
  • 03:16when we kind of make that diorama during
  • 03:19you know grade school is that actually
  • 03:22it's not just a big ball of yarn,
  • 03:25but actually the DNA has lots of different
  • 03:28regions and these regions are important
  • 03:30for different aspects of that blueprint.
  • 03:33And they're not all created equal.
  • 03:36There are specific regions of the DNA
  • 03:39that are far more prone to damage.
  • 03:42And there are also mechanisms to repair
  • 03:44that damage that may be quite specific,
  • 03:46so if you have a leak
  • 03:49in a pipe you may need a plumber, right?
  • 03:52But if you're siding
  • 03:54has gone downhill,
  • 03:55maybe you need someone who is
  • 03:57more like a Carpenter.
  • 03:59Or for any
  • 04:00new paint you're going
  • 04:02to have a different kind of approach
  • 04:04depending on what the issue is.
  • 04:06And it turns out for cells,
  • 04:08that's similar.
  • 04:09There are actually different
  • 04:10DNA repair mechanisms and you
  • 04:12really need to use the right mechanism
  • 04:14for the right kind of damage,
  • 04:17and it turns out that much of that is
  • 04:19actually dictated by these different
  • 04:21flavors of the regions of DNA and
  • 04:24also physically where those different
  • 04:26regions of the DNA blueprint are
  • 04:29organized inside the nucleus,
  • 04:30because it's a much more
  • 04:33compartmentalized kind of network
  • 04:37than when we just again think of
  • 04:40this string that has all of that
  • 04:43coding material,
  • 04:43so it's not
  • 04:44just where the break occurs in the
  • 04:47DNA or what kind of a break it is,
  • 04:50whether it's a single strand
  • 04:52break or a double strand break,
  • 04:54but where exactly it's
  • 04:56located within the nucleus.
  • 04:58We think about two components.
  • 05:00One exactly as you say physically,
  • 05:03where is that DNA break in the nucleus?
  • 05:06And then there's also the
  • 05:07other attributes of the DNA.
  • 05:09So DNA doesn't live on its own.
  • 05:13It's actually wrapped up and packaged
  • 05:15around proteins that are called
  • 05:17histones and this packaging is
  • 05:19really important for whether a
  • 05:22particular gene may be expressed or not.
  • 05:24It turns out that a DNA break
  • 05:27in a region of the genome
  • 05:29that is coding for a protein,
  • 05:31so it's going to be transcribed
  • 05:33into the messenger RNA and then
  • 05:36translated into a protein.
  • 05:37Those regions of the genome are
  • 05:39a bit different than regions of
  • 05:41the genome that may be silent,
  • 05:43and so that also just leads to both some
  • 05:46challenges for DNA repair mechanisms
  • 05:48and also some activities that
  • 05:51may actually make it more prone to
  • 05:53the accumulation of DNA damage.
  • 05:55And so we think of both
  • 05:57where the break is
  • 05:59physically,
  • 06:00and also where it is in context
  • 06:02of what else is happening in
  • 06:04that region of the DNA.
  • 06:06So we know that DNA can incur various
  • 06:09forms of damage that can be in coding
  • 06:12regions or in non coding regions.
  • 06:15How does that then evolve into
  • 06:17your research with cancer?
  • 06:19So initially as I mentioned our
  • 06:21interest was the idea that these
  • 06:24different locations in the nucleus
  • 06:26might be important for making sure that
  • 06:29those breaks are repaired by the right
  • 06:31process and in order to study that we
  • 06:35really need to be able to watch DNA
  • 06:38repair in a cell that's living while
  • 06:40it's happening and that as it turns out
  • 06:42is actually quite a difficult problem,
  • 06:45and so over the past ten years or so,
  • 06:48one of the things that my group has
  • 06:50invested in, is building so called
  • 06:52assays where we can actually watch
  • 06:55a single DNA break,
  • 06:56which we actually control.
  • 06:58So we induce the break to occur in
  • 07:00exactly the place where we want it to,
  • 07:03and then we actually follow the
  • 07:05repair of that break in real time and
  • 07:08once we built this system,
  • 07:10we became very interested in how we might
  • 07:13leverage it to answer some important
  • 07:15questions that were really arising
  • 07:17in the field of cancer treatments.
  • 07:19And really,
  • 07:20I was driven towards those questions
  • 07:22through my interactions with my
  • 07:24fantastic colleagues here
  • 07:26at the School of Medicine and at
  • 07:29Yale Cancer Center who really brought
  • 07:33a way of connecting the kind of questions
  • 07:36that I had become interested in,
  • 07:38again as a postdoc and kind of just
  • 07:41looking through the microscope
  • 07:42to where we had a real need to
  • 07:45understand specific questions in
  • 07:46the field of DNA repair,
  • 07:49and particularly those that were
  • 07:50relevant to the kind of therapies
  • 07:53that might be used in the context
  • 07:55where patients have
  • 07:56defects in DNA repair within their tumors.
  • 07:59So first question,
  • 08:00how exactly do you watch DNA being
  • 08:03repaired in real time?
  • 08:05I'm kind of blown away by that concept.
  • 08:09I remember back in junior high
  • 08:11biology looking down a microscope at
  • 08:13a cell and looking at the nucleus.
  • 08:16And sometimes you could even see DNA
  • 08:19separating into mitotic figures and so on.
  • 08:22But to actually see DNA being repaired?
  • 08:24I mean presumably that occurs at a base pair
  • 08:28level and that's just fascinating to me.
  • 08:31So how exactly do you do that
  • 08:33and what kind of magnification
  • 08:35would you need even to see that?
  • 08:38Yeah, that's a great question and honestly,
  • 08:40this is why I'm a cell biologist at the end
  • 08:43of the day because we love to just look.
  • 08:46If we have a way we can look at
  • 08:48something happening in real time that
  • 08:50is always the best thing in the world.
  • 08:53However, as you say,
  • 08:54it's not easy and so our work is built
  • 08:56on really critical discoveries that
  • 08:58have driven cell biology, in particular,
  • 09:00and I'll just tell you about two
  • 09:02of those that are critical for
  • 09:04the assays that we've built.
  • 09:05The first is the advent of
  • 09:07these fluorescent proteins.
  • 09:08Green fluorescent proteins
  • 09:09and red fluorescent proteins.
  • 09:11Now we have an entire rainbow
  • 09:13of these fluorescent proteins,
  • 09:14and so these are proteins that
  • 09:16fold up and they're able to make
  • 09:19what's called a chromophore
  • 09:20and we can actually follow that
  • 09:22specific molecule in a microscope.
  • 09:24And what we do is we basically stitch
  • 09:27that fluorescent protein onto a
  • 09:29protein that we're interested in,
  • 09:30and now we can follow our favorite
  • 09:33protein of interest in a live
  • 09:35cell on a fluorescence microscope
  • 09:36that can specifically detect
  • 09:38that fluorescent protein,
  • 09:39and so that's one technology
  • 09:41that's absolutely critical.
  • 09:43The other,
  • 09:43and I think this really speaks to
  • 09:46the importance of kind of basic
  • 09:48science discoveries and what
  • 09:50really has impacts on human
  • 09:52health these days is that we use
  • 09:55tricks to insert a region that's
  • 09:57actually taken from a bacteria,
  • 09:59so it's not native to the cells
  • 10:01that we are modifying,
  • 10:03and we essentially take that little sequence,
  • 10:05and we put it into the place in the genome
  • 10:09we're interested in and then we
  • 10:11have a protein that can bind to
  • 10:13that very specific DNA sequence,
  • 10:15and so we can monitor any kind of
  • 10:18region of the genome that we want
  • 10:20just by doing a little bit of editing
  • 10:23to that genome and putting these
  • 10:25in bacterial gene sequences
  • 10:27into our eukaryotic cell,
  • 10:28because that's what we want to be studying.
  • 10:31In terms of the magnification,
  • 10:33you're absolutely right.
  • 10:34We are able to do a pretty good
  • 10:37job following these events
  • 10:39even with a magnification,
  • 10:41usually between 100 and 1000 fold over
  • 10:44what you could see with the naked eye.
  • 10:47Wow, so essentially you can clip the
  • 10:49DNA where you want to make a break.
  • 10:52Insert a bacterial strand of genetic
  • 10:55material, flag it with a particular
  • 10:57flag so you know where the break is and
  • 11:00then have these chromophores which can
  • 11:03light up when they approach that break.
  • 11:09That's right, so another critical aspect is
  • 11:11we have to know a lot about DNA repair,
  • 11:13and fortunately,
  • 11:15DNA repair has been a really rich
  • 11:17area of research for many decades,
  • 11:19and so building again on the knowledge
  • 11:21of many others we know pretty well
  • 11:23about the kind of timing and the events
  • 11:25that are taking place and repair.
  • 11:27So protein X shows up,
  • 11:29and it always shows up before protein Y.
  • 11:32And as you said,
  • 11:33we want to know what's happening
  • 11:35at the base pair level,
  • 11:37like the smallest unit of DNA.
  • 11:39We can't really see something
  • 11:41that small in this assay,
  • 11:42so we're using proxies of factors
  • 11:44that we know will show up at different
  • 11:47points and that allows us to
  • 11:49essentially monitor distinct events,
  • 11:51because if we build up our
  • 11:54library of these different flags that
  • 11:56indicate different times and repair
  • 11:58them more able to monitor those events,
  • 12:01and we're also able to
  • 12:03monitor them in single,
  • 12:05individual cells,
  • 12:06and it's turned out that that's
  • 12:08really important.
  • 12:09Because if we look at a million
  • 12:12cells doing something they all kind
  • 12:13of do it on a little bit different
  • 12:16time over a little bit different time,
  • 12:18then the cell next
  • 12:20door and so by actually watching
  • 12:22these events in single cells,
  • 12:24that really gives us a resolution
  • 12:26that's really important for
  • 12:27being able to make very
  • 12:29mechanistic conclusions from the data.
  • 12:31So we understand that you've got
  • 12:33DNA that can get injured and it can
  • 12:36get injured in a variety of ways
  • 12:39at a variety of places,
  • 12:41each of which requires a
  • 12:43specific mechanism to repair it.
  • 12:45And we now understand that you've
  • 12:47built this model to kind of see
  • 12:49how DNA repairs itself overtime,
  • 12:51so tell us more about how this gets
  • 12:53into cancer and into therapeutics
  • 12:56And we'll have to do that as soon as
  • 12:59we take a break for a medical minute.
  • 13:02So please stay tuned to learn more about
  • 13:05DNA repair and cancer with my guest
  • 13:08Doctor Megan King.
  • 13:09Funding for Yale Cancer Answers
  • 13:12comes from AstraZeneca, working
  • 13:14to eliminate cancer as a cause of death.
  • 13:18Learn more at astrazeneca-us.com.
  • 13:21Breast cancer is one of the most common
  • 13:24cancers in women. In Connecticut alone,
  • 13:27approximately 3500 women will be
  • 13:29diagnosed with breast cancer this year,
  • 13:31but there is hope,
  • 13:32thanks to earlier detection,
  • 13:34noninvasive treatments and the development
  • 13:36of novel therapies to fight breast cancer.
  • 13:38Women should schedule a baseline
  • 13:40mammogram beginning at age 40 or
  • 13:43earlier if they have risk factors
  • 13:45associated with the disease.
  • 13:46With screening, early detection,
  • 13:48and a healthy lifestyle,
  • 13:49breast cancer can be defeated.
  • 13:52Clinical trials are currently
  • 13:54underway at federally designated
  • 13:56Comprehensive cancer centers such
  • 13:58as Yale Cancer Center and Smilow
  • 14:00Cancer Hospital to make innovative
  • 14:03new treatments available to patients.
  • 14:05Digital breast tomosynthesis, or 3D
  • 14:07mammography is also transforming breast
  • 14:10cancer screening by significantly
  • 14:12reducing unnecessary procedures
  • 14:13while picking up more cancers.
  • 14:15More information is available at
  • 14:18yalecancercenter.org. You're listening
  • 14:20to Connecticut Public Radio.
  • 14:22Welcome
  • 14:22back to Yale Cancer Answers.
  • 14:24This is doctor Anees Chagpar and I'm
  • 14:27joined tonight by my guest doctor Megan King.
  • 14:31We're talking about DNA repair and cancer,
  • 14:33and right before the break we had
  • 14:36gotten to the point in the story
  • 14:39where we were talking about the fact
  • 14:41that DNA gets injured and it can
  • 14:44get damaged in a variety of places.
  • 14:47And each of these breaks may be
  • 14:49specific and may require a specific
  • 14:52mechanism to repair it and we also
  • 14:55talked about the fact that Doctor King's
  • 14:58laboratory had figured out a way to
  • 15:01actually watch how DNA gets repaired.
  • 15:04right under a microscope,
  • 15:06which was just fascinating.
  • 15:08But now Megan,
  • 15:09maybe you can help us to understand
  • 15:11how this really evolves into
  • 15:13understanding a little bit more
  • 15:14about cancer and therapeutics.
  • 15:16We built the capability now of
  • 15:19monitoring DNA repair and these single cells.
  • 15:22And now we get to the point
  • 15:24in a basic scientist life where you
  • 15:27think about, I've built this assay,
  • 15:30it took us many years to do it.
  • 15:33What do we want to study?
  • 15:35And it's about this time that I had
  • 15:38been interacting increasingly
  • 15:40with members of Yale Cancer
  • 15:42Center and hearing about their
  • 15:44work in the clinic and their work
  • 15:47that is more translational.
  • 15:49So that's when we kind of apply basic
  • 15:51science and fundamental principles,
  • 15:53directly to new treatments.
  • 15:56And through these interactions we became
  • 15:58very interested in how we might use this
  • 16:02assay to answer a question that has arisen
  • 16:05that was clearly critical to the treatment
  • 16:07of breast and ovarian cancer that is
  • 16:09tied to this familial cancer susceptibility
  • 16:12genes BRCA one and 2.
  • 16:14I allways have a soft spot in my heart
  • 16:16for BRCA 1 because it
  • 16:19was discovered by Mary Claire King.
  • 16:20No relation but we have the same
  • 16:23initials and last name and in fact
  • 16:25over the years I've gotten emails
  • 16:27intended for Mary Claire King.
  • 16:29So we've struck up already a kind
  • 16:31of back and forth just because
  • 16:33of people getting us mixed up.
  • 16:37And so BRCA one really had
  • 16:40become a success story of an approach
  • 16:43to therapy called synthetic lethality.
  • 16:46And so the idea is that
  • 16:48BRCA one is very important,
  • 16:51particularly in a type of DNA repair
  • 16:54called homologous or combination
  • 16:55and in individuals who have a
  • 16:58loss of function and BRCA one,
  • 17:01this leads to an increased susceptibility
  • 17:03to breast and ovarian cancer in women.
  • 17:07And so you are probably quite familiar
  • 17:09with this because it's become very well known.
  • 17:13And it's also well known
  • 17:15even on the scientific front
  • 17:18because of the advent of a therapy
  • 17:20which is called PARP inhibitor
  • 17:22therapies that specifically kill
  • 17:24tumor cells that are defective in the
  • 17:27functions of BRCA one or two,
  • 17:29and actually more broadly in DNA
  • 17:32repair through this mechanism
  • 17:33called homologous recombination.
  • 17:35And so this is fantastic.
  • 17:37What does that mean for a patient?
  • 17:39It means that all of their normal
  • 17:42tissues can tolerate these drugs.
  • 17:44They really only attack the cells
  • 17:46that don't have functional DNA repair.
  • 17:48So DNA repair is this kind of
  • 17:50double edged sword, on the one hand,
  • 17:53a defect in DNA repair can lead
  • 17:55an individual to be vulnerable
  • 17:57to developing a cancer.
  • 17:58But if the cancer is defective
  • 18:01in DNA repair,
  • 18:02it also opens up a window
  • 18:04for therapies and PARP
  • 18:05Inhibitors were something that
  • 18:07could kind of fit into that window,
  • 18:09so this was really a very exciting
  • 18:12time and continues to be a really new
  • 18:14approach to treating cancers that are
  • 18:16tied to homologous or combination
  • 18:18defects which we now know include a
  • 18:21number of contexts that do not involve
  • 18:24just BRCA 1 and 2.
  • 18:26However,
  • 18:27we also knew quite early on
  • 18:29that these patients
  • 18:30would often have acquired
  • 18:31resistance to the PARP inhibitiors.
  • 18:33They would initially respond very well,
  • 18:35but the response would not
  • 18:37be as durable as they and their
  • 18:40physicians would like it to be,
  • 18:42and investigators had gone in to
  • 18:44try to ask how is it that these
  • 18:47tumors are evolving, essentially,
  • 18:48to become resistant to PARP inhibitors,
  • 18:50and particularly in the case of BRCA 1
  • 18:53they found that there
  • 18:55seemed to be secondary loss
  • 18:57of other repair factors that were
  • 18:59involved and we became excited
  • 19:02about the potential of our assay
  • 19:04to maybe provide some insight
  • 19:07into how is it that these tumors
  • 19:09are getting around this therapy,
  • 19:12and even more importantly,
  • 19:13might there be ways that we could
  • 19:16actually target these cells again?
  • 19:19So kind of re-sensitize them
  • 19:21to PARP inhibitors,
  • 19:23and so we modeled these mutations,
  • 19:29so that cells no longer express a number
  • 19:32of other factors called 53BP1
  • 19:34on a complex called shieldin.
  • 19:37And somehow this allows cells that
  • 19:40don't have functional BRCA one
  • 19:42to still survive in the presence
  • 19:44of PARP inhibitors,
  • 19:46and so we investigated those using
  • 19:48this assay and we discovered that the
  • 19:51loss of these factors that drove
  • 19:54this PARP inhibitor to no longer work were
  • 19:57affecting DNA repair in a very
  • 19:59specific way by unleashing
  • 20:01a DNA repair factor that really
  • 20:03shouldn't be functioning and this is
  • 20:05a protein called the bloom's helicase
  • 20:07and it was able to kind of step in for
  • 20:10BRCA one when these other factors
  • 20:12are silenced and take over and so
  • 20:15in a sense that seems like a bad thing.
  • 20:17Some other protein can come in and
  • 20:19and take the place of BRCA one,
  • 20:22but it turns out one of the things we
  • 20:24learned in our experiments was that
  • 20:26there was kind of a new liability.
  • 20:29That this activation of this
  • 20:31bloom's helicase brought along,
  • 20:33and it's actually now this
  • 20:36angle that we're targeting,
  • 20:37with the idea that there will be
  • 20:41new combination therapies that will
  • 20:43re sensitize these tumors to PARP
  • 20:45inhibitors in combination with either
  • 20:48inhibitors of the bloom helicase itself,
  • 20:51but also some other additional
  • 20:54treatments that have already been being
  • 20:57pushed forward.
  • 21:00Things like the DNA damage checkpoint,
  • 21:02which is something that acts
  • 21:04downstream of unresolved DNA damage,
  • 21:05so we're pretty excited that these
  • 21:08kind of very fundamental insights
  • 21:10from this assay that I've described
  • 21:12are really leading us to consider
  • 21:14new combinations of drugs that
  • 21:16may allow for
  • 21:18not necessarily to make the PARP inhibitor
  • 21:20but be a good therapy on its own for longer,
  • 21:24but how we might use combinations
  • 21:26that will allow for a very
  • 21:28durable response for these patients.
  • 21:30Let me make sure that we've got
  • 21:33that straight for all of our listeners.
  • 21:35So normally everybody has functional
  • 21:38BRCA but when you have a mutation in
  • 21:41that it no longer becomes effective
  • 21:44and the function of that BRCA gene is
  • 21:47really to repair DNA because DNA we
  • 21:50have in all of our cells and sometimes
  • 21:53it can just get damaged and BRCA
  • 21:57actually forms is a very important gene
  • 22:00that can help us to repair that DNA,
  • 22:03but when that's defective we get cancers.
  • 22:06But these PARP inhibitors
  • 22:09are very effective against tumors
  • 22:11that have DNA damage that is not
  • 22:15being repaired by BRCA.
  • 22:16But then you've got this bloom helicase
  • 22:20which can step in for BRCA.
  • 22:24It's almost like a fail
  • 22:27safe kind of belt and suspenders
  • 22:31where if one
  • 22:34repair mechanism doesn't work,
  • 22:36then another repair mechanism can work,
  • 22:38but in cancer cells you really
  • 22:40don't want it to work.
  • 22:42So what you're now doing is trying
  • 22:45to find inhibitors to that secondary
  • 22:47repair mechanism to ensure that the PARP
  • 22:50inhibitors can kill off those cancer cells.
  • 22:54Yes, that's exactly right,
  • 22:55and it had been known for a while that
  • 22:58there might be these two kind of parallel
  • 23:01mechanisms to carry out a specific
  • 23:04step in homologous recombination and
  • 23:06indeed, it was known already that
  • 23:08these two mechanisms existed,
  • 23:09but actually we didn't know very much
  • 23:12about how a cell could decide to use one
  • 23:15mechanism that would be this kind of BRCA
  • 23:17one mechanism which works with
  • 23:21this blooms' helicase pathway,
  • 23:22which as you said is kind
  • 23:24of a backup mechanism.
  • 23:25One of the things we've discovered is that
  • 23:28we think that the bloom's helicase mechanism,
  • 23:30although it's a backup,
  • 23:31it's really not supposed to
  • 23:33be working in normal cells,
  • 23:35and that's why there are a number of factors
  • 23:38that keep it off and that
  • 23:40includes these proteins,
  • 23:41the loss of which can drive
  • 23:43PARP inhibitor resistance.
  • 23:44So we think that actually there's
  • 23:46kind of a gain.
  • 23:47We would call it a gain of function
  • 23:49of the bloom's helicase that underlies
  • 23:52the PARP inhibitor resistance.
  • 23:54Why might cells not want to be using
  • 23:56this bloom's helicase all the time?
  • 23:58We think that it's because actually it's
  • 24:01not a very well controlled enzyme,
  • 24:03so its activity in the repair process
  • 24:06kind of goes wild a bit.
  • 24:09And even though this allows the cells
  • 24:11to get around the PARP inhibitor,
  • 24:13it actually may make them susceptible to
  • 24:17additional targets
  • 24:18that are being developed,
  • 24:19and so we think
  • 24:21just like a DNA repair defect
  • 24:23opens up a therapeutic window,
  • 24:25we think this kind of rewiring from
  • 24:28BRCA one to the bloom's helicase may
  • 24:30also open up new ways that we could
  • 24:33go about treating these tumors.
  • 24:35So then the next question is,
  • 24:37is there a way for us to
  • 24:40figure out either upfront before
  • 24:42we give any therapy whether a
  • 24:44particular patient is going to have
  • 24:47this bloom's helicase turned on or not,
  • 24:49so that upfront we can decide whether
  • 24:51we should just give up our PARP inhibitor,
  • 24:54or whether we need to give dual
  • 24:56therapy or in a productive manner
  • 24:58where we can say, well,
  • 25:01if somebody hasn't responded to the
  • 25:03PARP inhibitor as we would anticipate,
  • 25:05is there a way for us to figure out
  • 25:08if this is the mechanism by which
  • 25:11the cell is getting around that
  • 25:13PARP inhibitor and developing resistance
  • 25:15so that we can add in another drug.
  • 25:17Do we have those kinds of diagnostics?
  • 25:21You're absolutely right,
  • 25:22this is exactly what we would like to have,
  • 25:25but we don't have it yet,
  • 25:27so we would like to be able to take a
  • 25:32tumor sample and ask the question,
  • 25:34what is happening in this tumor?
  • 25:36Is this patient likely to
  • 25:38respond to the PARP inhibitor?
  • 25:40We know that if they have
  • 25:41a defect in DNA repair,
  • 25:43they're likely to respond.
  • 25:45We know, as I told you, this bloom's
  • 25:47helicase tends to go kind of overboard,
  • 25:49and we think that we can design
  • 25:52what we would call a
  • 25:53biomarker of that activity,
  • 25:55because it generates far too much of
  • 25:57this single stranded DNA generating
  • 25:59single strand of DNA is a critical
  • 26:01part of homologous or combination,
  • 26:03but again,
  • 26:04bloom's helicase seems to do too much of this,
  • 26:07and we think that we might be
  • 26:09able to use proteins that bind
  • 26:11to that single stranded DNA,
  • 26:13kind of quantitatively,
  • 26:14and that may be an indication
  • 26:16that this is the mechanism by which
  • 26:19these cells elevated PARP inhibitors.
  • 26:21Another major mechanism
  • 26:22of PARP inhibitor resistance
  • 26:23are so called reversion mutations.
  • 26:25This is where there's actually a
  • 26:27second mutation in the BRCA gene,
  • 26:29which essentially can reconstitute
  • 26:30its normal function.
  • 26:31In this case,
  • 26:32the tumor no longer has
  • 26:34a DNA repair defect,
  • 26:35and so we'd really like to
  • 26:37be able to tell is there a
  • 26:39reconstitution of normal repair.
  • 26:41But maybe that repair still has
  • 26:43some defects that we can target,
  • 26:45or is repair kind of totally normal,
  • 26:47in which case we know we're going
  • 26:49to have to think about another
  • 26:51type of therapy to treat that patient.
  • 26:53So these are in development and
  • 26:55this is something we're really
  • 26:57interested in,
  • 26:59particularly again with our
  • 27:00colleagues here and at Yale Cancer Center.
  • 27:02To continue to push forward by
  • 27:04partnering with those clinicians who
  • 27:06are running clinical trials in this space.
  • 27:09In patients with BRCA or NOTE Confidence: 0.98452777
  • 27:11other homologous recombination
  • 27:12defects who have been enrolled
  • 27:13on PARP inhibitors and looking at
  • 27:15those resistance mechanisms.
  • 27:17And if we can develop these
  • 27:19types of biomarkers.
  • 27:21I mean it's so fascinating
  • 27:23thinking about the fact that
  • 27:26when we started this conversation,
  • 27:28we started by saying that you know DNA
  • 27:30can be damaged in different ways and each
  • 27:33requires a specific repair mechanism.
  • 27:35But now thinking about how you're
  • 27:38actually taking your science
  • 27:40and in a way kind of again,
  • 27:42moving towards personalized medicine,
  • 27:43figuring out, well,
  • 27:44if somebody develops resistance,
  • 27:47how exactly is that resistance
  • 27:49mechanism functioning?
  • 27:49And how can we get around it?
  • 27:54Absolutely, and I want to highlight
  • 27:56we can do this really efficiently
  • 27:58in cells in a laboratory that's
  • 28:00never going to tell us about what
  • 28:02is happening in individual patients.
  • 28:05So really, this discovery requires the
  • 28:07commitment of patients who've been
  • 28:09enrolled on these clinical trials.
  • 28:11That's not an easy thing to
  • 28:13ask of patients in this case.
  • 28:15For example, they've signed up for
  • 28:17serial biopsies of their tumor,
  • 28:19but that is absolutely essential
  • 28:21for us to continue to discover
  • 28:24the mechanisms that are at play and for
  • 28:26us to come up with better treatments.
  • 28:29Doctor Megan King is an associate
  • 28:31professor of cell biology and of molecular,
  • 28:33cellular, and developmental biology
  • 28:35at the Yale School of Medicine.
  • 28:37If you have questions,
  • 28:39the address is cancer answers at
  • 28:41yale.edu and past editions of the
  • 28:43program are available in audio and
  • 28:45written form at yalecancercenter.org.
  • 28:47We hope you'll join us next week to
  • 28:49learn more about the fight against
  • 28:52cancer here on Connecticut Public Radio.
  • 28:54Funding for Yale Cancer
  • 28:56Answers is provided by Smilow
  • 28:58Cancer Hospital and AstraZeneca.