2012 OCRP Investigator Vignette
Title: Targeting Cell Surface Proteins in Molecular Photoacoustic Imaging to Detect
Ovarian Cancer Early
Investigator: Charles Drescher, MD; Fred Hutchinson Cancer Research Center
I have a grant currently through the DoD OCRP looking at targeting cell surface proteins
for photoacoustics to detect ovarian cancer early.
And this is motivated by the understanding that the conventional imaging tests that we
have are really not sufficient to detect ovarian cancer in an early curable phase.
Perhaps one of the most promising imaging strategies for ovarian cancer early detection
is photoacoustic imaging.
And this approach combines a molecular contrast agent with a hybrid imaging probe that uses
nanosecond pulses of laser light to excite tissue based on different physical characteristics.
And those excited tissues then generate sound waves of particular ranges which a traditional
ultrasound probe can then use to kind of reconstruct into a 3D image of the tissue.
And the current research funding that I have focuses on only one step in that process and
that’s basically to try to identify the proteins that are expressed on a surface of
an ovarian cancer cell so that we can then target them with a molecular imaging probe.
We have adapted a sort of a new type of chemistry to be able to very selectively and specifically
bind what are known as N-linked glycoproteins.
And glycoproteins are proteins that have sugar attached to them and they’re very enriched
on the surface of cells and particularly on the surface of tumor cells.
So we adapted a particular chemical strategy that allows us to essentially bind these glycoproteins
on intact viable ovarian cancer cells.
And once we bind them, then we can use different affinity capture methods to basically strip
them off of the cells.
And then we subject them to mass spectrometry so that we can then go ahead and identify
them.
This is just a summary of where we are with these
analyses.
And so what you can see is an OVCAR3, which is a cell line, we essentially identified
a total of 275 proteins that contain the motif.
And then we went on using bioinformatic methods to try to determine whether in fact these
proteins are cell surface proteins.
And we used either gene ontology annotation and a number of prediction algorithms and
then, in addition, literature searches based on experimental data.
And we were able to confirm that of these 275 proteins that were identified on OVCAR3
cell lines that contain the motif, 235 of them are in fact cell surface proteins.
If you contrast that to the peptides that don’t contain this motif, you can see where
there’s 196 and of those really only 14 were ultimately confirmed to be cell surface
proteins.
So you can see that our method is very highly selective for identifying the proteins of
interest.
And we subsequently prioritized these proteins for those that we think might be excellent
targets for molecular imaging probes and then gone on to validate them with more kind of
standard immunohistochemical means just looking to see are these proteins expressed in ovarian
cancer cells from patients’ tissue samples that are collected in surgery.
And in fact, so far eight out of the nine that we’ve selected have actually validated.
So this is milk fat globulin 8 and what you can see here is very specific staining in
ovarian cancer cells.
And if you look carefully you can see that there are some cytoplasts that are staining
but this sort of deep line in these pictures demonstrate that there’s abundant membrane
expression.
This is another one of our candidates, TAC-STD2.
This is a protein that’s involved in cell signaling, may play a role in invasion and
proliferation of ovary cancer, and again we see abundant staining on the membrane and
cytoplasmically in serious ovary cancer.
So we feel like we’re off to a good start and we’re optimistic that we, you know,
will ultimately find some markers that can be translated really hopefully after some
animal studies pretty rapidly into the clinic. �