- In our lab, we wanted to interrogate whether HER3 is required in
HER2-amplified breast cancers or if eventually, cancer cells can find
a way to grow and survive independently of HER3.

The current hypothesis in the field is that HER3 is required for the
growth of HER2-amplified cancer cells. This means that HER2-amplified
cancer cells need HER3 expression to grow and survive.

I had some preliminary data that I was generating in the lab, some
small evidence hidden there in the small experiments in-vitro, that
was telling me that cancer cells in the absence of HER3 were still
able to form tumors.

To test this hypothesis, we really had to go in-vivo and go into mice.
Interrogating this HER2-amplified human cells in mice, will directly
give us an idea of the tumorigenicity of the cells. In our case, we
chose immunocompromised mice, known as nude mice, that means that the
immune system of the mouse has been compromised so, it will not react
against the human cancer cells that we are going to inject into the
mouse.

There are different approaches to target HER3 and it's activity.
Targeting the expression of HER3 by using shRNA, short hairpin RNA or
siRNA, small interfering RNA or the CRISPR-Cas9 technology for
targeting the activity of the protein by using an inhibitor. shRNA and
siRNA, both techniques allow you to reduce the expression of a protein
by targeting the messenger RNA of the protein. In both cases, you are
reducing the level of expression, you are not completely eliminating
the expression of the protein. Ideally, we'll have used an inhibitor
for HER3 but currently, there are not inhibitors available. Right now,
HER3 remains undruggable. Therefore, we went with CRISPR and the
reason for that is that this approach allows you to eliminate
completely the expression of the protein.

We wanted to go in-vivo. We wanted to interrogate this question in
mice and having a complete knockout, a genetically altered cell line
not expressing HER3 will allow us to study the absence of HER3 in the
cells in-vivo. The predicted outcome is that those cells were not
going to form tumors and indeed, they didn't. Initially, for one
month, two months, three months, there were no tumors there and we
kept monitoring those mice and waiting and waiting and suddenly, like
four months and a half, we started to see a small bump and that was
the beginning of a tumor. Not only in one mouse, actually two thirds
of the mice developed tumors after five, six months. The scientific
literature indicates that HER3 is essential for the growth of this
cells but what we did in this experiment is to push out the limit and
monitor the tumor growth of this mice for longer than what's done
before and therefore, we managed to see that actually these cells are
eventually able to adapt to the lack of HER3.