DNA: understanding how it is repaired, shed light on the origin of cancer

Sébastien Huet, lecturer at the University of Rennes 1 at IGDR, contributed to a European study of a protein capable of reshaping DNA, called ALC1

The DNA molecule contains the manufacturing plan of our organism. Any modification of this plan following an alteration of the DNA can have severe consequences by inducing the appearance of cancers in particular. It is considered that within each of our cells, DNA undergoes more than 100,000 events of alteration every day. To repair damages and thus preserve the integrity of its genome, the cell utilizes a variety of DNA repair pathways. This is one of the early stages of this process that was studied through a collaboration between scientists from Germany, Denmark, France and the Netherlands.

ALC1, a "remodeling" molecule

When DNA strand is damaged, for example by X-rays, UV or chemicals, various mechanisms are triggered to repair it. The first step is to free access to the damaged area. Indeed, the DNA is not naked in the nucleus but packaged in a very compact structure called chromatin. This is where chromatin "remodelers", such as ALC1 (also known as Amplified in Liver Cancer 1 or CHD1L) come into play by "opening" the chromatin to release access to the DNA for repair.

"We already knew that ALC1 had this ability to open chromatin. Here, we showed how the binding of ALC1 to the damaged DNA makes it possible to activate this function of chromatin remodeling, "explains Sébastien Huet.

"In Cellulo" observation


The activity of ALC1 was observed directly in living cells. To do so, Sébastien Huet used confocal microscopy combined with laser irradiation, which allows to quickly cause damage to DNA at a predefined area of the cell nucleus.

Video
 

For example, by damaging cell nuclei, Professor Gilles Salbert's team members at the IGDR observed the recruitment of ALC1 molecules which gather in a few seconds around the damaged area, as shown in the video below (artificial colors). At 0 seconds, a laser beam damages the DNA. The ALC1 protein, fused to a fluorescent marker visible in yellow, rapidly accumulates around the lesion.

The results
 

These in cellulo observations confirmed the function of the 3 domains of ALC1, studied in vitro by the international partners of the study. ALC1 is composed of: a remodeler domain, a binding domain and a macro domain which binds to a specific mark of the presence of DNA damage: a poly-ADP-ribosylation (PAR). This mark is produced by the PARP1 protein which is able to recognize the presence of breaks along the DNA.

In the absence of damage, the remodeler (R) and macro (M) domains of ALC1 interact with each other, and this inhibits the chromatin remodeling activity of the protein (see diagram below). On the other hand, the binding of the macro domain to the PAR marks, signaling the presence of breaks in the DNA, results in a dissociation of the macro and remodeler domains. The latter can then play its role by “opening” the chromatin. ALC1 is amplified in several types of solid tumors, such as colorectal and ovarian cancers. Other types of tumors can be associated with mutations of ALC1.


        "We have observed that some of these mutations block the interaction between the remodeler and macro domains. ALC1 then becomes able to bind to chromatin, and thus to potentially reshape it even in the absence of damage, "says Sebastien Huet

Figure : S. Huet

Caption: Model of regulation of ALC1 protein activity. In the absence of DNA damage, the ALC1 macro (M) and remodel (R) domains interact, making this protein inactive. Following damage induction, the macro domain binds to these through the PARP1 damage sensor. This binding of the macro domain to the damaged DNA induces a release of the remodeling domain which can then bind to the chromatin to open this structure.

In perspective: a better understanding of the origin of cancers

The study of the functioning of ALC1 thus contributes to a better understanding of the deregulations observed in cancer cells. PARP1, which signals to ALC1 the presence of damage in the DNA, is already a target for cancer treatments. ALC1 also appears as a target with high therapeutic potential. However, before getting there, there is still some way to go! This work published in the journal Molecular Cell represents a further step.

Reference

A Poly-ADP-Ribose Trigger Releases the Auto-Inhibition of a Chromatin Remodeling Oncogene
Hari R. Singh, Aurelio P. Nardozza, Ingvar R. Möller, Gunnar Knobloch, Hans A.V. Kistemaker, Markus Hassler, Nadine Harrer, Charlotte Blessing, Sebastian Eustermann, Christiane Kotthoff, Sébastien Huet, Felix Mueller-Planitz, Dmitri V. Filippov, Gyula Timinszky, Kasper D. Rand, Andreas G. Ladurner
Molecular Cell (2017) Volume 68, Issue 5, 7 December 2017, Pages 860–871.e7  |  doi: 10.1016/j.molcel.2017.11.019

Article made with the contribution of Alice Vettoretti [Plume&Sciences]

Article originally published on the website of the Université of Rennes 1, le 28 décembre 2017.

 

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