Intracranial aneurysms, outwards bulges ballooning out of an artery, are surprisingly common in middle age, with an estimated prevalence of 2% in the general population.

While only a small fraction of these common aneurysms actually go on to rupture, one-fourth of these ruptured aneurysms will lead to sudden death before hospitalization. There is almost never a warning sign of the event, as it is typically preceded by a headache or dizziness followed by collapse.

Intracranial aneurysms can be effectively treated with modern endovascular techniques if caught early, but the ability to identify these patients has evaded detectability to date.

Until this past week, the only genes identified as contributing to aneurysm risk were known to impact aneurysm risk by merely 2-10%. These were mostly genome-wide association studies (GWAS) involving analysis of noncoding region single-nucleotide polymorphism (SNPs).

Investigators at Yale University have reported on the discovery of the first identified familial gene in PPIL4 as published in the December 9, 2021, issue of Nature Medicine. PPIL4 encodes for the peptidyl-prolyl cis-trans isomerase-like 4 protein, a member of the cyclophilin family.

Principal investigator Murat Gunel, professor and chief of neurovascular surgery at Yale, New Haven, Connecticut, told BioWorld Science, "In genetics there are two ways you can prove function. One way involves statistical analysis in which you need large numbers of samples for that to reach significance, but in collaboration with investigators like Stefania Nicoli who use zebrafish models, we could show that the PPIL4 gene serves a fundamental function in the normal formation of blood vessels and in humans too most likely. We don't know for sure yet, but it is expected to be embryonic lethal if there is no PPIL4."

Stefania Nicoli, associate professor in genetics and medicine (cardiology) and director of the Yale Zebrafish Phenotyping Core that brings Yale as premier center of disease phenotyping using zebrafish, explained "The Ppil4 variant, while itself rare, points to the importance of vascular density during development as a key underlying cause of aneurysms and strokes. That new insight might be used as a marker for future disease risk even in the very young and the approach could be applicable to many other cardiovascular diseases with a genetic signature."

Previously, Gunel's group published several other papers focused on the genetics and risk of developing aneurysms. Comorbidities like high blood pressure, asthma and diabetes have been related to aneurysm susceptibility thus indicating that it is an exceptionally complex issue to determine disease risk.

GWAS SNP analysis was mostly in the noncoding region epigenome where the contribution to risk was merely 5-10%. One of the strongest areas in the genome increasing the risk for brain aneurysms was near a gene called SOX17, a transcription factor which was recently shown to play a major role in arterial development as well as other vital developmental processes, while its role in aneurysm formation in adulthood was demonstrated by other groups using endothelial specific depletion of Sox17 in a mouse model.

Given the relatively high frequency of the polymorphisms identified via GWAS, they confer small risk in IA formation. Thus, these studies cannot explain the genetic risk in familial cases. Thus, the researchers have been searching for rare coding mutations (seen in < 1% of the population) in the genome that can lead to the formation of aneurysms by using whole exome sequencing and have now successfully identified a gene that can increase disease risk by over 10-fold.

Gunel explained the process, "The important thing about genetics is that it is hypothesis forming. You take a genome-wide unbiased approach, you identify clues and then you pursue them. This is unlike classic biology, which starts with a test that you hypothesize."

Gunel and colleagues have been collecting samples from families with multiple affected individuals and index intracranial aneurysm patients since 1994. Associate research scientist and first author Tanyeri Barak performed analysis on over 300 patient-derived samples after whole exome sequencing to identify coding mutations in PPIL4 segregating with all five affected individuals of an intracranial aneurysm family.

Furthermore, Barak and the research team including Emma Ristori, who is the other co-first author in the study, Gulhan Sencicek, Andrew Prendergast, Ketu Mishra-Gorur and several other scientists demonstrated the essential role of PPIL4 in brain vascular development and hemodynamics using two vertebrate models, zebrafish and frog.

Barak emphasized, "Elucidating the molecular mechanism of intracranial aneurysms is the beginning of the journey which will eventually lead us to develop diagnostic and therapeutic tools for this deadly disease. This study is just a small brick in the wall; however, a big motivation for our continuous dedication and the upcoming discoveries."

The researchers explained that there does not appear to be anybody walking around with homozygous loss of the PPIL4 gene as this is almost certainly early lethal, but individuals that are heterozygous appear to be highly susceptible to these aneurysms. In certain conditions, environmental factors involving high blood pressure are expected to lead to the rupture event.

Ketu Mishra-Gorur, a research scientist in the Department of Neurosurgery and co-corresponding author in the study, unraveled the mechanistic aspects of PPIL4 signaling by identifying the PPIL4-JMJD6 nexus and helped the group demonstrate a novel PPIL4-JMJD6-Wnt signaling axis that is critical in aneurysm formation in humans.

Mishra-Gorur said "Unruptured cerebral aneurysms are asymptomatic and are therefore unable to be detected based on history and physical exam alone. Studies like ours not only provide a genetic and mechanistic window into disease pathogenesis but present a remarkable potential for new pathways in IA screening, early diagnosis and treatment."

Gunel emphasized, "how powerful this approach is... sometimes it takes three decades to open the door. But I think it's a major door opening here."