Research conducted at the National Cancer Research Centre (CNIO) has revealed an unexpected behaviour observed in a protein involved in several types of cancer: it manages to self-activate, meaning it gives itself the order to start working in the cell.

This finding shows that its activation process is accelerated, making it much faster than in normal proteins.

This opens up new avenues to seek ways to block this protein, called CCDC6-RET, considered an age-old unresolved problem in cancer research.

CCDC6-RET was discovered more than three decades ago and has been extensively studied for its role in thyroid cancer and pulmonary adenocarcinoma, yet “the structural and molecular determinants that control its function and (oncogenic) mechanism of action are paradoxically elusive in most of the cases,” the authors write in Nature Communications, Iván Plaza, head of CNIO’s Kinase, Protein Phosphorylation and Cancer Group, the corresponding author for the article, and Ana Martín-Hurtado, first author.

Genes that fuse to create an oncogene

Sometimes, two genes abnormally fuse together and create a unique entity.

Some genes are more prone to fuse than others, and often when they do, they make it easier for tumours to form.

The proteins expressed by these fusion genes are ‘chimaeras’ that can be much more active than the proteins derived from the isolated genes.

One gene that is more prone to fusing is RET, which in its ‘normal’, non-fused form is important for cellular multiplication and division.

The protein CCDC6-RET, which is the focus of new CNIO research, is created when RET fuses with another gene.

CCDC6-RET is already being studied as a therapeutic target, but little was known about its structure or its detailed mechanisms of action.

The new study characterises the architecture of CCDC6-RET, discovering that it can activate itself—without interference from other proteins.

It also reveals that this self-activation occurs at a much faster rate than the activation of the protein expressed solely by the RET gene.

Being an oncogenic protein, acceleration is particularly relevant.

Maximising fuel reuse

The CNIO group has succeeded in identifying the mechanism of this self-activation.

The normal RET protein activates progressively by adding a phosphate group to each of its components, one after another.

The phosphate groups come from ATP molecules, which act as fuel for cellular processes.

The molecule left over after that process, once the ATP loses the phosphate group, is called ADP.

However, the authors of the study have observed that CCDC6-RET activates all its components simultaneously and that, after taking a phosphate group from the ATP molecule, it is able to re-energise from ADP.

It is a little like being able to refill the tank of your car with exhaust fumes.

This is the first time that feedback has been detected,  opening up a new paradigm that suggests that ADP is an active signalling molecule, not merely a waste product.

“We knew about protein kinases that specifically use ADP, but this is the first time a kinase capable of utilising both ATP and ADP has been described,” Plaza notes.

The researchers believe that this ability may be associated with the fact that tumour metabolism differs from that of normal cells.

The ability to utilise two distinct energy sources—ATP and ADP—might provide the oncogenic protein with greater flexibility, aiding the tumour cells in adapting to adverse conditions, such as nutrient scarcity or the effects of targeted drugs.

For Plaza, “it is important to meticulously dissect the activation mechanism of CCDC6-RET”.

These results suggest that the current treatments targeting RET fusions might not be wholly effective if they do not consider this dual mechanism of activation.

Consequently, the study opens the door to the development of new, more precise therapeutic strategies.

3D model of its structure

The new study reconstructs a three-dimensional model of the fusion protein CCDC6-RET.

By combining various structural biology techniques and artificial intelligence, they have visualised the architecture of the inactive protein (before binding to ATP) and also of the active form, as well as the changes associated with this activation, and they describe the structure of these models.

“This is the first oncogenic fusion of the RET gene from which detailed information has been obtained at the structural and molecular mechanism level. It is fundamental, because we can apply our methodology and approach to other RET fusions, of which about 20 have been recently described, involved in other types of cancer.”

Source: Centro Nacional de Investigaciones Oncológicas (CNIO)