As
discussed under Research, the root cause of achondroplasia is too
much “anti-growth” activity of FGFR3 in the cells of growing
bones. That is, increase in FGFR3 activity within the cells of growing
bones results in specific signals that reduce or block growth. Several
therapeutic strategies have been proposed to reduce this activity, and most are
targeted to the cellular networks that carry instructions from FGFR3 to nucleus
within the cells. These networks are complex and dispersed throughout the cell.
We have discovered a new pathway that bypasses these established networks and
potentially allows instructions to be transferred directly from FGFR3 residing
on the surface of a cell to its nucleus, where the regulation of genes involved
in controlling growth occurs. We believe this novel pathway mediates many
of the anti-growth signals of FGFR3, including those responsible for
achondroplasia. We also believe that defining this new pathway will lead
to the identification of new and potentially better targets for therapeutic
intervention for achondroplasia.
Indeed,
we have already identified one potential target. A key feature of this
new pathway involves cutting of the intact FGFR3 molecule present at the cell
surface to produce a small fragment that moves to the nucleus to deliver its
anti-growth message, and promotes nuclear signaling of FGFR3. To our
surprise, the machinery that cuts FGFR3 also cleaves a protein implicated in
Alzheimer’s disease. Other than sharing this common protein cutting
machinery, the two proteins and also the two diseases are completely unrelated.
In fact, abnormal cutting of this protein in many patients with
Alzheimer’s disease has identified this cutting machinery as a therapeutic
target for Alzheimer’s disease, which in turn has led to considerable
investment by the pharmaceutical industry in drugs to correct the abnormality.
So
how is this relevant to achondroplasia? As we investigated the cleavage of FGFR3,
we discovered that achondroplastic FGFR3 is cut abnormally creating a scenario
similar to that for Alzheimer’s disease in which abnormal cleavage contributes
to disease. If so, the distinct possibility arises that drugs designed to
favorably alter the cleavage of the Alzheimer protein might also normalize the
cleavage of FGFR3 and at least partially alleviate the manifestations of
achondroplasia. In economic terms, our findings raise the possibility of
leveraging the enormous investment in Alzheimer’s drugs for a novel treatment
for achondroplasia.
Our
goal is to move our project from its current stage of initial discovery to the
point where it merits further development by industry. For this we need
to define the new pathway in more depth. We need to demonstrate that this
pathway is involved and necessary for normal FGFR3 effects on growing bone,
that it is altered in achondroplasia, and that restoration of normal cleavage
corrects the disturbance caused by the achondroplasia mutation. In short,
we need to show that the newly discovered pathway influences bone growth, plays
a role in achondroplasia and is amenable to therapeutic manipulation. We
anticipate reaching these goals over the next 2-3 years.
We
will discuss how we are addressing these points in future blogs. However,
for this blog suffice it to say that many of our experiments are being
performed in cultured cells. We have developed and continue to refine
assays that measure precisely how much of the cleaved FGFR3 fragment reaches
the cell nucleus. By comparing normal FGFR3 to FGFR3 bearing the
achondroplasia and other relevant mutations, we can begin to understand the
impact of the mutations on the fate of the cleaved fragments. We are also
setting up assays to determine the precise location of cleavage within the
FGFR3 protein and how this is affected by the achondroplasia mutation.
Once this information is known, we will determine if Alzheimer drugs can
normalize cleavage and transfer of the fragment to the nucleus.
We
expect there to be direct target genes of FGFR3 in the nucleus. To
identify them, we are using a technique called Chromatin Immunoprecipitation
(ChIP). It will be explained in a future blog. Identifying even a
few FGFR3 target genes will provide valuable insights into how FGFR3 regulates
bone growth. Measuring the expression of these genes will further
validate the effects of the achondroplasia mutation and of candidate drugs on
FGFR3 functions.
To
complement our cell culture studies, we are also genetically engineering mice
to validate the importance of the FGFR3 nuclear signaling pathway in live
animals. FGFR3 in these mice will be tagged to allow microscopic and
biochemical detection. The mice will have normal FGFR3, FGFR3 bearing the
achondroplasia mutation, FGFR3 bearing a mutation that blocks cleavage and
prevents the fragment from being produced and FGFR3 comprised of only the
cleavage fragment destined for the nucleus. By analyzing and comparing
these mice in detail, we will determine the biologic significance of the direct
nuclear signaling by FGFR3. These mice will also be used to test the
Alzheimer’s drugs.
As
noted above, future blogs will focus on progress within the different aspects
of this overall investigation.
