Thesis defense Melanie Laarman: gene regulation in the circle of Willis

Intracranial aneurysms
Around 3% of the general population has an intracranial aneurysm, a balloon-shaped dilation on a bloodvessel in the brain. Most people who have an intracranial aneurysm do not notice it, but sometimes an aneurysm can burst. If an aneurysm bursts this results in a so-called subarachnoid hemorrhage, a subtype of stroke that predominantly affects young persons (mean age of 50 years) and has a bad prognosis. Intracranial aneurysms develop on the circle of Willis, a circle of blood vessels at the base of the brain, and the blood vessels that branch off from the circle of Willis (Figure).

DNA and intracranial aneurysms
Developing an intracranial aneurysm is partly due to genetics: first-degree relatives of a person with an aneurysm or subarachnoid hemorrhage have a higher chance to develop an aneurysm themselves. Previous research has found genetic variants that are linked to the presence of intracranial aneurysms. Therefore, these genetic variants are likely to increase the chance of intracranial aneurysm development in some way.

Gene regulation
Such genetic variants that are linked to a disease are often located in non-coding regions, either in between genes or in introns, as was the case for intracranial aneurysm. In addition, such genetic variants often co-localize with regulatory regions of DNA, such as enhancers and promoters, that can affect the gene activity. These regions can determine whether a gene is turned ‘on’ or ‘of’, or something in between. Therefore, Laarman and her colleagues therefore set out to identify the regulatory DNA regions that are active in the circle of Willis. They found that indeed also for intracranial aneurysm, these regulatory DNA regions often overlap with the genetic variants that are linked to intracranial aneurysm. Therefore, it may well be that the genetic variants affect the activity of certain genes through these regulatory regions, and thereby increases the risk of developing an intracranial aneurysm.

Candidate genes
After this, Laarman and her colleagues searched for the genes that are affected by the enhancers, a subset of the regulatory regions, because such enhancers can affect the nearest gene, but also a gene that is much further away (Figure). They identified six genes that interact with the enhancers. These genes are all expressed in the circle of Willis and a change in gene expression may increase the chance of developing an intracranial aneurysm. In addition, Laarman studied whether the enhancers are really able to affect gene expression by using a reporter assay in zebrafish embryos, and she found that most of the enhancers could indeed affect gene expression.

Future perspectives
The relationship between the identified enhancers and the genes that may be affected by them has to be studied in more detail in the future, to find out if the genetic variants linked to intracranial aneurysm can really affect the expression of those genes. In addition, studying the function of these genes in the circle of Willis will ultimately lead to more insight into the pathogenesis of intracranial aneurysms. Such future studies will hopefully lead to successful identification of persons with an increased risk of intracranial aneurysm development in the future.

During her PhD, Laarman worked on a project that was a collaboration between the group of Jeroen Bakkers at the Hubrecht Institute and the Department of Neurology at the University Medical Center Utrecht, where Ynte Ruigrok and Gabriël Rinkel were here supervisors.