Researchers have identified a genetic mutation in patients with an arteriovenous malformation (AVM) of the brain, a leading cause of hemorrhagic stroke in young adults and children.
Results of their new study suggest that these malformations are a result of KRAS-induced activation of the mitogen-activated protein kinase–extracellular signal-regulated kinase (MAPK-ERK) signaling pathway in brain endothelial cells.
“The study is important because it shows us a very likely cause of brain arteriovenous malformations,” study author Ivan Radovanovic, MD, PhD, Krembil Research Institute, University Health Network, and assistant professor of surgery (neurosurgery), Faculty of Medicine, University of Toronto, Ontario, Canada, told Medscape Medical News.
“I think the value of the study is to show, as a principle, that a genetic mutation in a somatic cell can cause sporadic brain AVMs. We also showed that a significant number of these malformations are likely caused by specific KRAS mutations.”
Vascular neurosurgeons like himself have been “frustrated” with the lack of management options for some patients with these malformations, he said.
“To understand what causes them is a significant advance and it opens some avenues and hope for maybe improving their treatment, adding some options.”
The study was published online January 3 in the New England Journal of Medicine.
Treatment Difficult
In older adults, hemorrhagic strokes are often the result of high blood pressure. In young adults and children, a leading cause of stroke is an underlying vascular malformation, or abnormal connections between arteries and veins in the brain’s vasculature.
About half of these malformations are identified because patients present with a bleeding episode. Other cases are uncovered incidentally when patients get brain imaging for an unrelated condition. Or patients might present with neurologic symptoms, such as headaches or seizures.
These malformations are difficult to treat because they require complex brain surgery, radiation, endovascular embolization, or a combination of these modalities, said Dr Radovanovic.
He stressed that these treatments carry “significant risks” and complications, and some cases are simply untreatable, he said.
Dr Radovanovic and his colleagues suspected that these sporadic vascular malformations had a genetic component. However, because most cases don’t have a familial link, they were convinced that it isn’t “a classical genetic disease with a mutation in the germline,” he said.
“Our hypothesis was that maybe it’s a somatic genetic disease” or due to a mutation in the genes of a single cell.
For the study, he and his team analyzed tissue from 39 adult patients with AVMs of the brain who had undergone surgery in Toronto. They also had matched blood samples from 21 of these patients.
The blood “is a surrogate for germ-line DNA as it’s part of the body that is not affected” by the AVM, explained Dr Radovanovic.
DNA Sequencing
In collaboration with a group at the University of Geneva (Dr S.I. Nikolaev and Dr S.E. Antonarakis) and the group of Dr J.E. Fish from University of Toronto, they performed exome DNA sequencing of the tissue samples of 26 patients and found somatic activating KRAS mutations in some of them. This was accompanied by dysregulation of the MAPK-ERK pathway.
To confirm their findings, they performed droplet digital polymerase chain reaction (PCR) analysis of all 26 tissue samples tested with exome sequencing and 13 additional samples. In this case, “we knew what we were looking for, so we just looked for mutations in KRAS,” said Dr Radovanovic.
This droplet digital PCR analysis revealed KRAS mutations in tissue samples from 29 of the 39 patients (74%).
The study included an independent validation group from Finland. “In this type of study, it’s important to have a validation cohort,” said Dr Radovanovic. “This was an independent cohort analyzed by the group that included Dr J. Frösen from Kuopio University in Finland.”
The team in Finland replicated the tests and found the same mutation in 16 of 33 patients (48%) with AVM.
In total, the researchers detected somatic activating KRAS mutations in tissue samples from 45 of 72 patients and in none of the 21 paired blood samples.
It’s important to determine whether the mutation is specific to one type of vascular cell, for example, endothelial cells that line the lumen of vessels and regulate exchanges between tissue and blood and help prevent clotting, or vascular smooth muscle cells in the vessel wall that regulate the vessel diameter and adjust the blood flow like a faucet would.
“If it was restricted to one type of cell, it would be more logical in terms of our view of how these malformations develop,” noted Dr Radovanovic.
In some patients, the researchers were able to isolate endothelial cells from AVMs and test these as well as endothelial cell–depleted fractions.
“When we did this targeted analysis, and looked for the mutation in this population that we separated, we found the mutation only in the fraction that contained endothelial cells,” said Dr Radovanovic.
This, write the authors, “suggests that KRAS mutations are probably specific to endothelial cells and that dysregulation of the biology of endothelial cells is a key feature of the formation of arteriovenous malformations of the brain.”
This new research “opens a very interesting prospect in terms of understanding the developmental biology of the brain vasculature and the importance of KRAS and its downstream MAPK-ERK signaling pathway in endothelial cell development,” commented Dr Radovanovic.
He explained that the MAPK-ERK signaling network is downstream of KRAS. Other researchers have found a mutation in the same pathway, but downstream of KRAS, in AVMs outside the brain.
“It’s also possible that brain AVMs negative for mutations in KRAS have mutations in other genes of the MAPK-ERK pathway downstream of KRAS or even in KRAS itself, but we just missed it,” he said.
The researchers believe that these malformations probably develop during a time window when the blood vessels form and grow.
“During embryonic development and early childhood, these vessels grow, branch out, and sprout in the brain to form the vascular tree, and this is probably a window of vulnerability where we can imagine that the mutation may occur,” said Dr Radovanovic.
He thinks it’s unlikely that these rare malformations can be prevented or that the findings may be used for screening patients for brain AVMs. However, there is some evidence from mouse models that the malformations can be reversed, he said.
Dr Radovanovic noted that this same mutation is present in certain cancers, although cancer is “a very complex disease with many different mutations.”
In the absence of direct inhibitors of KRAS, what might help is a drug that inhibits the MAPK-ERK signalling pathway downstream of KRAS that could mitigate symptoms and shrink the malformation.
“Mutations in KRAS or genes of the MAPK-ERK pathway are important in many cancers, such as melanoma, colorectal or pancreatic cancers, and drugs that inhibit this pathway are already used in clinical practice,” said Dr Radovanovic.
Dr Radovanovic was supported by the Timothy P. Susco Chair of Research Award from the Brain Aneurysm Foundation and the Toronto General and Western Hospital Foundation, and he received seed support from the Department of Surgery and Division of Neurosurgery at the University Health Network.
N Eng J Med. Published online January 3, 2018. Abstract
For more Medscape Neurology news, join us on Facebook and Twitter
Tidak ada komentar:
Posting Komentar