Science: The RNA-binding protein FMRP was found to allow tumors to evade recognition by the immune system and become resistant to immunotherapy

Immunotherapy is a cutting-edge approach to treating cancer that pits a patient's own immune system against their tumor
.
Our growing understanding of the mechanisms by which the body regulates immune responses has been transformative in our fight against cancer
.

But despite its high success rates, immunotherapy has hit a stubborn hurdle time and time again: Tumor cells often evade the "radar" scans
of immune cells trying to destroy them.
This can lead to treatment resistance, which in many cases will benefit from a deeper understanding
of the mechanisms that can help circumvent it.

In a new study, researchers from research institutions such as the Swiss Federal Institute of Technology in Lausanne, the University of Bern and the University Hospital of Lausanne have discovered that a protein plays a key role
in helping tumors evade immune destruction.
The RNA-binding protein, named fragile X mental retardation protein (FMRP), regulates a network of genes and cells in the tumor microenvironment, helping to improve its ability to
"evade" immune cells.
Normally, FMRP is involved in regulating protein translation and mRNA stability
in neurons.
But they found that it was abnormally upregulated
in many types of cancer.
The findings were published in the Nov.
18, 2022 issue of Science in the paper "Aberrant hyperexpression of the RNA binding protein FMRP in tumors mediates immune evasion.
"

But why FMRP? The idea comes from previous studies
that showed that cancer cells that naturally overexpress FMRP are invasive and metastatic.
However, other studies have shown that if FMRP is not expressed in developing neurons, it can lead to cognitive deficits
.

With this evidence in mind, the authors set out to study FMRP expression
in human tumors.
They then evaluated its tumor-promoting function in mouse cancer models and finally investigated its relationship
with the prognosis of human cancer patients.

The new study involved several data collection steps
.
First, these authors performed immunostaining
of tissues derived from human tumors against FMRP.
Most tumors test positive, while corresponding normal tissue does not
.
This means that FMRP is specifically and highly expressed
in cancer cells.
They then moved on to the main part of their study, which was to determine the functional significance of FMRP in these tumors--- they expressed this protein, but what role does it do?

FMRP is associated with the immune system

To explore this, the authors developed so-called "gene knockout" cancer cell lines
.
A knockout cell or organism loses --- "knocks out" a specific gene through genetic modification in ---order to find clues
about its function.
Essentially, any changes that occur in a knockout cell can usually be traced back to this lost gene, compared to cells that still have the gene
(called "wild type").

In the new study, the authors used the well-known CRISPR-Cas9 gene-editing technique to knock out the gene that produces FMRP (called FMR1)
in cancer cells caused by mouse pancreas, colon, breast and skin melanocytes.
They then compared
cancer cells that knocked out FMRP to cancer cells that still had the FMR1 gene and therefore expressed the FMRP protein.

FMRP allows tumors to evade attacks
by the immune system.
Image from Science, 2022, doi:10.
1126/science.
abl7207
.

They assessed the survival of mice containing FMRP knockout cancer cells and mice with FMRP-positive wild-type cancer cells, first in mice with compromised immune systems
.
Their comparison showed that the two groups of mice had similar survival rates
.
Remarkably, when they compared the gene-knockout tumors to wild-type tumors grown in mice with functioning immune systems, they found that tumors without FMRP grew more slowly, while they survived longer
.

What this part of the study shows is that FMRP is not involved in stimulating tumor growth itself, but is related
to the adaptive immune system (the part of the immune system that we "train" with vaccines).

Their observations --- wild-type tumors have few invasive T cells, while FMRP knockout tumors are highly inflamed--- further confirming this
.
T cells in tumors knocked out of the FMRP gene caused them to start growing faster and reduced survival in mice, meaning that FMRP was somehow involved in the tumor's evasion
of the immune system.

How FMRP-containing tumors defend immune cells

These authors went on to perform molecular genetic analysis
of FMRP knockout tumors and wild-type tumors.
This revealed significant differences in gene transcription across the genome, indicating that FMRP interacts
with multiple genes.
In addition, there were significant differences between FMRP knockout tumors and wild-type tumors in the abundance of cancer cells, macrophages, and T cells, further suggesting the role of FMRP in regulating components of
the immune system.

The next phase of the new study explores the production
of specific factors associated with a unique immune response--- evasion or attack ---.
The authors found that tumors expressing FMRP (i.
e.
, wild-type tumors) produce interleukin-33 (IL-33), a protein
that induces regulatory T cells --- a specific subset of T cells that suppress immune responses--- produced.

FMRP-expressing tumors also produce protein S, a glycoprotein
known to promote tumor growth.
Finally, FMRP-expressing tumors produce exosomes--- which have been shown to trigger vesicles
produced by a class of macrophages that normally aid wound healing and tissue repair.
Overall, all three factors have immunosuppressive effects that help protect tumors from attack
by T cells.

Conversely, FMRP knockout tumor cells actually lowered all three factors (IL-33, protein S, and exosomes) while upregulating a different chemokine called C-C motif chemokine ligand 7 (CCL7), which helps recruit and activate T cells
.
Macrophages that induce immunostimulatory rather than immunosuppressive properties further aid this process
.
These immune-stimulating macrophages produce three other pro-inflammatory proteins that, along with CCL7, recruit T cells
.

Predicting immunotherapy outcomes in human patients

One clinical question is whether FMRP levels can help determine the prognosis
for cancer patients undergoing immunotherapy.
However, counterintuitively, neither the mRNA nor FMRP protein levels of the FMR1 gene were sufficient to predict immunotherapy outcomes
in the cancer patient cohort.

To address this, the authors established the fact that in cells, FMRP regulates its stability
up and down by directly binding mRNA.
This means that FMRP may alter the levels of RNA that can be acquired in transcriptome datasets, which can be collected to identify "gene signatures"
that can help track its functional activity.
The approach worked, allowing them to track the genetic signature
of FMRP's cancer-regulatory activity through a network of 156 genes.

FMRP cancer network active gene signatures have been shown to have a prognostic effect on adverse survival in a variety of human cancers, which is consistent with the immunosuppressive effects of FMRP and, in some patients, it is associated with
adverse effects of immunotherapy.

The new study shows that FMRP regulates genes and cellular networks in the tumor microenvironment, all of which help tumors evade immune destruction
.

Douglas Hanahan, corresponding author of the paper, from the Ecole Polytechnique Fédérale de Lausanne, said: "I have been studying the complex cellular composition of solid tumors for decades, and I was personally surprised
by our discovery that ---a co-used neuronal regulatory protein (FMRP) can coordinate to form a multifaceted protective barrier to block the immune system's attack on the tumor, limiting the benefits of immunotherapy, which makes FMRP a new cancer treatment target ---.
(Biovalley Bioon.
com)

Resources:

Qiqun Zeng et al.
 Aberrant hyperexpression of the RNA binding protein FMRP in tumors mediates immune evasion, Science, 2022, doi:10.
1126/science.
abl7207.