What
Did Grandi Set
Out To Target, With What, and Why:
Grandi and her fellow researchers focused on the biomarker EGFR vIII,
a mutant form of the tyrosine kinase receptor EGFR. But why use EGFR vIII as a target for this
viral vector therapeutic treatment for glioblastoma
multiform (GMB)? Well, the wild type EGFR is a type of
proto-oncogen, meaning that if mutated in a certain way it could lead to the
cell acquiring cancer like properties. The wild type EGFR elicits a signal transduction cascade that lead to cell
proliferation, DNA synthesis, and acquisition of new phenotypes like cell
migration. All these properties, if left unregulated can lead to cancer. As we have learned in class one of the reasons for signal
transduction pathways being permanently activated is through the truncation of
the extracellular portion of the receptor. This phenomna is seen in EGFR vIII. Because
of an in-frame deletion in the mRNA, the extracellular receptor portion is truncated
causing the kinase to be continually activated due to the ligand independent
binding. This has been shown to enhance the
tumorigenicity and apoptosis resistance of cells containing the EGFR vIII mutant. (2) So, why is this important? EGFR vIII is associated with increased invasiveness and
growth rate of tumors. Mutations in the EGFR, especially the EGFR vIII mutant, are some of the most common mutations found in GMB cells, leading
to higher concentrations of the EGFR vIII mutant on the cell surface compared to
normal cells, which have virtually none. Also,
it has been shown that “several antibodies have been described that are
specific to EGFR vIII and do not cross-react with wild-type EGFR”(1).
To target EGFRvIII, Grandi and her fellow researchers, used a modified version of the oncolytic
virus HSV. An oncolytic virus is a type of virus that causes the infected cell
to lyses. As the virus replicates it not only destroys the host cell but also
keeps this destructive cycle going since it infects adjacent cells causing
subsequent destruction. Though
there are multiple types of viral vector therapies in clinical trials,
oncolytic viruses like HSV show the most promise. The key to Grandi
research was finding a way to engineer the HSV virus to effectively target only
the cell specific surface
receptors EGFR VIII, unique to
cancerous GMB cells, leaving the normal cells unharmed. To generate
this tumor selectivity Grandi implemented the single strand monoclonal antibody
MR1-1. In previous studies
MR1-1 has been shown to have high binding affinity accompanied with long
lasting infectivity with EGFR vIII causing it to be extremely potent to cells
with this mutation. Instead of
having a heparan sulfate binding domain, which allows for the viruses initial binding
with the cell surface, the virus was modified so it would only bind to the EGFR vIII receptor. Because
Grandi created a specific immunotherapeutic target, EGFR vIII, it not only concentrated the
viral vectors specifically to the EGFR vIII containing cells but it also
increases the safety of the treatment and lessens the needed dose.
Testing
Method and Results:
What Grandi proposes would increase the potency of these vectors, by enhancing targeting and
increasing vector infectivity to tumor cells. To test that attaching MR1-1 to
HSV actually does this, Grandi first did a comparative study to see different
monoclonal antibodies: MR1-1 (high affinity expected), MRB (low affinity
expected), and the normal gC binding domain without the HS-binding domain
relative binding affinity. Each of
these “were inserted in the pCONG amplicon which also contains an expression
cassette for green fluorescent protein (GFP) to monitor amplicon vector
infection”(1). Grandi found that the MR1-1 attachment increased the infectivity
rate of the human glioblastomas cells with EGFR vIII 5x. I should note that
this was preformed at low M.O.I (multiplicity of infection) to imitate the
expected in vivo ratio of vectors to tumor cells. To test infectivity of the of MR1-1 modified HSV and make made sure the MR1-1 modified HSV would
only bind to glioma cells with the
EGFR vIII receptors Grandi implanted two sets of U87 cells, a human primary glioblastoma cell line, with and without the EGFR vIII in nude mice
and let them grow. She then infected the modified virus in each set of mice and
monitored the progression over a week as seen in the picture. The infectivity
was monitored by using fluorescent or lacZ reporter genes in vivo that were incorporated into a
virus. A set of infected mice were injected with the normal gC binding domain
lacking the Herparan sufalte binding domain as a control. Grandi observed that
not only did the tumor shrink but also the modified virus had longer
infectivity. This shows incredible promise for future use in cancer
therapies.
Critique
and Concerns: This research certainly provides a creative approach to
target cancer cells for virus-mediated destruction. GMB is such a fast paced
disease, having a median
survival rate of 12-18 months after diagnosis, making research into creating effective treatments like this one
necessary. Though this
article did provide me with a way that viruses could actually destroy cancerous
cells it still left me with many concerns and
causes for future research opportunities.
First of all this study did not specify the necessary dosage of the virus and
at what concentration of EGFR vIII mutant on the cell surface is necessary for
the modified virus to be able to target and actually have the desired effect on
the cancer cell. Another major aspect that I feel was left out of this study is
the implication of the blood brain barrier. Grandi did not present any data on
how the blood brain barrier would affect the MR1-1 EGFR vIII accessibility to
the GMB tumor. Even though the wild type HSV virus is known to be able to
penetrate the blood brain barrier it was not mentioned if the attachment of the
monoclonal antibody would have any effect on the modified virus passing this
barrier. Because this study has only been done in nude mice, and even in these
studies the tumor was not grafted in the cranial portion of the mouse, the
effect of this modification has not been shown in respect of it passing the
blood brain barrier.
Since this process is dependent on the
presence of the specific mutant EGFR vIII being present on the cell surface, I
am skeptical that cancer cells with prolonged exposure to this treatment will
not mutate and become resistant to this treatment. As discussed in class there
have been multiple other drugs that target oncoproteins that after an extended
length of time are found to be useless because resistance has been confirmed in
the cancerous cells. Two of the most relatable drugs to this are Herceptin and Gleevec.
As we have learned Herceptin uses a Monoclonal antibody to target and block
function of the HER2, a type of Epidermal growth factor receptor, by preventing
dimerization. But upon tumor relapse, Herceptin elicits no response. This is
because the tumor cells that some how survived the first round of Herceptin
treatment managed to mutate and developed alternate means to propel growth
independent of the HER2 oncogene expression. Or, in the case of Gleevec, the
oncoprotein it was designed to bind to mutated causing a change of various
amino acids in the binding domain, resulting in the inability for the treatment
to bind and target the cell. So what is to say the same thing does not happen
to the MR1-1 HSV treatment? If
either of these processes occur, finding another path to cause cell growth
while avoiding the expression of EGFR vIII or modifying the binding domain of
EGFR vIII, this therapy in its current state would be rendered useless.
Therefore there needs to further research into other monoclonal antibodies
attachment to the virus allowing for the continuation of binding to EGFR vIII
and or new cell surface kinase targets for the HSV virus. Also, additional research should explore how
the addition of multiple monoclonal antibodies to the HSV virus would accommodate
for these changes in the cancer cell’s morphology.
Since
this treatment uses a virus, I am left to question how the body’s immune
response will affect the efficacy of this therapy. As with increased exposure
to any virus the body’s natural immune system starts to acquire, for the lack
of a better word, memory of the virus, making the virus a key target for immune
systems attack. If this treatment calls for the virus therapy to be
administered frequently in a short length of time, I am concerned that the bodys’
own immune system will start attacking the virus and gain immunity to it,
greatly diminishing the effects of the treatment. If this happened, it would
not matter that the virus can target the cancer; the virus would be prevented
from propagating leaving the cancer cell unharmed and the patient in no better
condition then before treatment
In
this study Grandi experimented on U87 cells, which were taken from a stage 4
GMB tumor cell lineage, but the article does not specify at what stage the EGFR vIII mutation begins to appears on the cancer cell’s surface. This would definitely
need to be explored because if it is only expressed in certain stages this
would dramatically effect the window of opportunity for the drug. And since the ERGF vIII is seen on
multiple cancer types and is not limited to GMB I feel it would be beneficial
to explore the use of this treatment for metastasized cancer. And lastly, I feel there will be public and ethical barriers to
overcome. Intentionally injecting a virus into someone does not come without hostile
response and apprehension.
Work
Cited:
Grandi, Paola, et al. “Targeting HSV-1 virions for specific binding to epidermal growth
factor receptor-vIII bearing
tumor cells.” Cancer Gene
Ther. 2010 September ; 17(9): 655–663. Web. 20 May, 2012 <https://www.dropbox.com/s/hcydsgsh7bz1gqe/nihms-194496.pdf>
Kuan, C-T. , et al. “EGF
mutant receptor vIII as a molecular target in cancer therapy.” Endocrine-Related
Cancer. 2001 June; 8: 83–96. Web. 20
May, 2012
Introduction picture from:
Simula, Pekka. “Maturing data in 2011 will get us
focused on the oncolytic virus industry.” Developing
oncolytic virus therapies. Web. 22 May, 2012. <http://oncolyticvirus.files.wordpress.com/2010/12/oncolytic-virus-in-action.jpg>
