Exploring the use of aptamers to treat achondroplasia

Introduction

For a long time we know that there are a multitude of proteins (molecule chains made of amino acids) which are capable to connect directly with distinct regions of the DNA (molecule chains made of nucleotides), either to stimulate the start of a gene reading process (transcription) or to block it. So, it should be readily conceivable that molecules made of nucleotides could also be capable to connect directly with proteins.

In the nineties, with this concept in mind, scientists started to identify a large number of molecules made of DNA or RNA nucleotide sequences, which they ended to name aptamers (1,2). I don’t know if you have already read the last article of this blog. There, I tried to compare the way researchers find new drugs with some kind of fishing or filtering technique. For aptamers, we can use the same concept, since the current technology used to identify them, known by the acronym SELEX, also uses a filtering method to select the best candidate molecules.

Like those proteins capable to bind directly and specifically some target nucleotide sequences of the DNA in the cell nucleus, aptamers are also very specific for their own targets. You could select any part of the target as bait for aptamers. In the case of a receptor enzyme such as our familiar fibroblast growth factor receptor type 3 (FGFR3), this bait could be the external part of the enzyme, or a transmembrane segment or the ATP pockets (figure) we have already reviewed about in previous articles.

This special property, to have a single, specific target, makes aptamers a potential strategy for the treatment of diseases or conditions where a protein is not working properly. Can you imagine that? Let’s give an example of an aptamer already in use in the clinic.

An aptamer in action

Macular degeneration is a condition that affects the eyes of aged people and can lead to blindness. One of the mechanisms thought to drive the disorder is an excess of vascular proliferation (creation of an excessive number of new blood vessels in the retina). The production of new vessels is managed by the activation of a receptor in the cell membrane of cells present in the retina called vascular endothelial growth factor receptor (VEGFR), and of course, the molecule which binds it is a VEGF (also called ligand).

There are several therapies available for the treatment of this condition, with variable efficiency, but one is unique: pegaptanib, the first aptamer to reach the pharmacy. Sold as Macugen™, pegaptanib is an aptamer specific for VEGF. Its mode of action is simple: binding VEGF, it blocks the ability of this ligand to connect to the receptor (figure). As the receptor cannot be activated, then new vessel production is not stimulated and the condition does not progress in the same pace it would if left untreated.

Since the discovery of the aptamers, many have been developed for therapeutic purposes and several are now being explored in clinical trials, mostly in cancer. Cancer is truly a huge challenge and a lot of effort has been given to find new therapies to beat it. This includes targets such as cell membrane receptors like the epithelial growth factor receptor (EGFR), another enzyme located across the cell membrane in the same way FGFR3 is. EGFR excessive functioning is often found in breast cancer (and in other kinds, too), and today some of the most effective therapies for breast cancer are directed against EGFRs.

Controlling the amount of signals from the antenna

Before we continue our flight over the aptamers, it will be good to understand how the cell manages the many, many active receptors located in the cell membrane. This will be important for us to understand what we could expect about one of the possible ways an aptamer targeting FGFR3 could work.

As I said, there are hundreds of different receptors installed across the cell membrane, all functioning like chemical antennas to help the cell respond to what is going on in the environment. From immune defense to apoptosis, an incredible number of cell interactions have already been described guided by those antennas.

However, these antennas are not permanent. As for any biological process to keep life on balance, there are a number of control systems that regulate the action span of these receptors. When a receptor enzyme such as FGFR3 is activated by a FGF it forms a molecule complex that starts to signal to its specific chemical cascades (reviewed here), and is attracted to the interior of the cell. 

Then, this complex is spotted by proteins such as ubiquitin, which will tag it and drive it to degradation systems, where the proteins are dismantled. By using these control systems, the cell is able to regulate for how long a receptor can conduct signals from abroad to the cell nucleus. Do you see where we are going?

Aptamers designed to bind EGFR2 (HERB2) causes its accelerated degradation and reduce cancer growth.

In a recent study, researchers have identified a couple of aptamers specific for EGFR2. EGFR2 is one of the four receptor enzymes of the EGFR family and is an important driver of some forms of breast and other cancers. Blocking or disabling EGFR2 causes impairment of tumor growth and reduces the recurring rate in appropriately treated cases. In this study, Mahlknecht G et al. (3) performed several tests with the aptamers they identified and found that they worked by forcing the receptor to degradation without being activated. This, in turn, resulted in inhibition of tumor growth in the models the researchers used. The aptamers tested did not show major concerns in terms of toxicity, which is also important information.

In summary, this study gives evidence that an aptamer having a target located outside the cell is able to block its function and causes the planned effect (here, tumor growth inhibition). The same strategy could be applied for FGFRs.

A strategy using aptamers for achondroplasia could have advantages:

• Aptamers are small molecules, probably free to transit in the growth plate and reach the chondrocyte;

• Aptamers are very specific, making low the risk of having off-target effects (when a drug causes effects due to interactions with a non-intended target);

• Aptamers use to have a low toxicity risk profile.

Nevertheless, aptamers are somewhat fragile and natural victims of the body enzymatic system. They need protection to reach their targets. This can be achieved by cloaking them with one of the several transport and delivery systems already available (we reviewed this here). This is nothing that cannot be overcome.

Concluding

In the end of 2012, in a conversation with an expert in drug development and venture capital investment, I mentioned the aptamers as a strategy to be explored as potential therapy for achondroplasia, but this expert was skeptical about them. My opinion is that we don’t know. Since I learned about aptamers I have been making random searches in medical databases such as Pubmed and never found a study testing an aptamer against FGFR3. Thus, here, the challenge is again to find an investigator with open mind, and with appropriate resources, to explore the possibility of aptamers being used to treat achondroplasia.

References

1. Tuerk C, Gold L. Systematic evolution of ligands by exponential enrichment: RNA ligands to bacteriophage T4 DNA polymerase. Science 1990; 249 (4968): 505-10.

2. Ellington AD, Szostak JW. In vitro selection of RNA molecules that bind specific ligands. Nature 1990; 346(6287):818–22.

3. Mahlknecht G et al. Aptamer to ErbB-2/HER2 enhances degradation of the target and inhibits tumorigenic growth. PNAS 2013; 110 (20): 8170-8175.