Treating achondroplasia: the effects of BMN-111 in mice

Biomarin and the French group headed by Dr. Laurence Legeai-Mallet had their last paper evaluating BMN-111 in mice just published by the American Journal of Human Genetics: http://www.cell.com/AJHG/abstract/S0002-9297(12)00537-X.

What is important about this study?

This new study is important because it brings more consistent results of the tests made with the C-type natriuretic peptide (CNP) analogue in a mice model of achondroplasia.

We have already reviewed CNP in this previous article but I think that some concepts are worth to be seen again. Things may seem difficult to understand given the multitude of acronyms, expressions or jargon used in the science language. In a recent message a parent asked me to explain more the implications of this study in a more palatable way. Well, I will try.

So, let’s talk about some concepts that may help the interested but not specialist reader to understand the implications of the use of a CNP analogue in the treatment of achondroplasia.

What is CNP? What is a peptide?

CNP is a relatively small molecule called peptide and is composed by a chain of amino acids naturally built by cells of our body. It is made in the same way the proteins are: there is a gene in our DNA which encodes (carries the chemical information needed to produce) it. Try to think that CNP is a kind of a multicolored lego block chain. Proteins and peptides are quite the same thing; they will be called one way or another according to their size, peptides being smaller than proteins.

We also have learned that many proteins are called enzymes for their chemical properties. These enzymes can catalyze (trigger) chemical reactions (grossly, small electric shocks caused by the transfer of charged atoms or electrons) which in turn will generate reactions in and/or by our cells. Proteins and peptides are in fact the great masters of life, ruling all aspects of life as we know it.

Nature is smart and during the millions of years of life evolution, it made uncountable chemical experiences creating peptides and proteins with increasingly more specific properties. So, these molecules, although electrically charged, will only react with other specific molecules (or targets) that will fit with them, in the same way we combine certain lego blocks with some others but not with all of them.

This specific nature of proteins and peptides is so important that the main ones ruling the key reactions that allow life have a backbone structure very well conserved across the majority of life forms. In such proteins, depending on the species, we will find a number of switches in amino acids, some lacking, some added, but the main structure will be there, no matter the animal we study. Why are we talking about this? Well, I guess someone would ask why we are testing a mouse to see if a peptide would work in man, isn’t it? The simple reason is that CNP (or a similar molecule) is present in man and in mice and in the majority (all?) of the more advanced life forms in Earth. And more importantly: it does about the same thing in all of them. The same is valid for the fibroblast growth factor receptor 3 (FGFR3).

What does everything of this have with achondroplasia?

Let’s continue our short journey reviewing what these proteins and peptides do.

Proteins can act outside, across and inside the cells. We have learned here that FGFR3, a protein with electrical properties, therefore an enzyme, is placed across the cell membrane of the chondrocyte in an off mode. It works like an antenna, transmitting the outside messages to let the cell react accordingly. It is there just waiting to be turned on when connected by another… protein, which in this case is a fibroblast growth factor (FGF). When a protein or a peptide acts in such a way, starting a reaction by binding to its target, we call it a ligand (coming from Latin, meaning the one which binds to another one). Thus, ligands are no more than postmen delivering chemical messages to specific addresses in cells across the many tissues and organs of the body. CNP is no other thing than a ligand. It has its own receptor in the chondrocyte cell membrane and when it connects to this receptor called natriuretic peptide receptor B (NPR-B), it starts a cascade of reactions inside the cell. The natural function of CNP is to stimulate chondrocytes to grow. But how does it accomplish this task?

Complex chemical reactions

We have learned that proteins and peptides have been created while life was evolving (it still is). As the life forms were becoming more complex, more of these distinct molecules also developed, leading to more complex functions and so on, in a continuous virtuous cycle. However, normally proteins don’t have the capacity to decide if it is time to work or to stop working, they have that designed function and once turned on they will work non-stop. You see, with those so much reactive molecules working without balance, life would not be possible, so control mechanisms, based in other proteins have also evolved. For the majority of the chemical reactions we have in the body there are control mechanisms regulating their intensity, amount, rhythm, frequency, etc. 

This includes, for instance, the way FGFR3 works. In normal conditions, soon after FGFR3 is activated by a FGF it is captured by an intracellular system made of other proteins and is directed to degradation. In other words, there are proteins to provoke reactions and there are proteins to stop the formers to provoke reactions. Interestingly, this is just one way control, or balance, of the myriad of chemical reactions is obtained. Sometimes, for a reaction chain working for a given purpose, there is another chemical cascade counteracting it. We could say that the CNP cascade is one example of these other ways of control.

Connecting to achondroplasia

Let’s see this figure from Dr William Horton’s review published in the GGH Journal, in 2006. It shows both the FGFR3 and CNP cascades in the chondrocyte and the way they interact.

FGFR3 is designed to cause a series of reactions inside the cells this enzyme is produced. For the great majority of these cells, when FGFR3 is activated, they will react increasing their proliferation (multiplication) and maturation rates. The remarkable exception is exactly the chondrocyte: the same cascade of enzymes activated by FGFR3 will force the cell to stop proliferating and reduce its capacity to maturate. That’s why we say that FGFR3 is a negative controller of bone growth. In normal conditions, the FGFR3 will have a limited time to exert its effects, both because of the above mentioned cell control systems and also for there is at least one other system directly reducing the activation of the FGFR3 cascade, the CNP system. How do we know all of this?

Mutations can occur in any protein, with the more diverse consequences. In the case of FGFR3, if the mutation makes it works more or better, it will impair growth. Studies in mice showed that when you abolish FGFR3 production, the consequence in bones is overgrowth. In the case of the CNP system, if CNP or its receptor is abolished the affected individual will have dwarfism. On the contrary, a mutation in NPR-B turning it permanently activated causes overgrowth.

If you saw the figure mentioned above, you learned that the intracellular cascade of reactions triggered by CNP intercepts one of the most important cascades of the FGFR3 pathway, the so called mitogen-activated protein kinase (MAPK). This previous article reviews the FGFR3 cascade. The MAPK enzymes are responsible mainly for the reactions inside the cell nucleus which regulates the pace of chondrocyte maturation, a stage we call hypertrophy. In this stage chondrocytes are stopping the proliferation mode and start enlarging (hypertrophying) while producing large amounts of new components for the cartilage matrix. If FGFR3 is working too much as in achondroplasia, few chondrocytes reach the mature stage and less bone is formed. This is indeed one of the hallmarks of achondroplasia.

The CNP cascade blocks the activity of MAPK enzymes in the level of one called Raf, therefore reducing the FGFR3 negative influence on the chondrocyte transition to the hypertrophy stage. With more chondrocytes proliferating and reaching the hypertrophic stage, more bone is produced.

In summary

The study by Biomarin shows that with the CNP analogue BMN-111 treatment, mice bearing a mutation in FGFR3 that causes similar characteristics in the affected animals we see in achondroplasia, had their bone growth significantly rescued, although the rescue was not complete. The treated mice had longer and straighter limb bones, enlarged spinal bones and better development of the cranial and midface bones. The microscopic studies showed that the growth plate in those treated animals were larger than in the non-treated ones and that they closely resemble the growth plate of normal animals (but were not completely alike). These results can be seen as a proof-of-concept that CNP is a valid approach to rescue bone growth in individuals bearing activating mutations in the FGFR3 gene, such as in achondroplasia and hypocondroplasia.

One could think that a treatment directed to the most common of dwarfism would look like an esthetic one. However, we can anticipate that, even though an improved final height is a desired outcome, the other many improvements any therapy directed against FGFR3 in achondroplasia will bring are as much as desired, too. By reducing the common orthopedic, otolaryngological, neurological and sociopsychological complications, an effective therapy for achondroplasia could reduce the need for medical care or exposure to hard surgeries, and will help those affected beloved kids to have a healthier childhood.