Epidemiology and genetics of intracranial aneurysms

Intracranial aneurysms are acquired lesions (5-10% of the population), a fraction of which rupture leading to subarachnoid hemorrhage with devastating consequences. Until now, the exact etiology of intracranial aneurysms formation remains unclear. The low incidence of subarachnoid hemorrhage in comparison with the prevalence of unruptured IAs suggests that the vast majority of intracranial aneurysms do not rupture and that identifying those at highest risk is important in defining the optimal management. The most important factors predicting rupture are aneurysm size and site. In addition to ambiental factors (smoking, excessive alcohol consumption and hypertension), epidemiological studies have demonstrated a familiar influence contributing to the pathogenesis of intracranial aneurysms, with increased frequency in first-and second-degree relatives of people with subarachnoid hemorrhage. In comparison to sporadic aneurysms, familial aneurysms tend to be larger, more often located at the middle cerebral artery, and more likely to be multiple. Other than familiar occurrence, there are several heritable conditions associated with intracranial aneurysm formation, including autosomal dominant polycystic kidney disease, neurofibromatosis type I, Marfan syndrome, multiple endocrine neoplasia type I, pseudoxanthoma elasticum, hereditary hemorrhagic telangiectasia, and Ehlers-Danlos syndrome type II and IV. The familial occurrence and the association with heritable conditions indicate that genetic factors may play a role in the development of intracranial aneurysms. Genome-wide linkage studies in families and sib pairs with intracranial aneurysms have identified several loci on chromosomes showing suggestive evidence of linkage, particularly on chromosomes 1p34.3-p36.13, 7q11, 19q13.3, and Xp22. For the loci on 1p34.3-p36.13 and 7q11, a moderate positive association with positional candidate genes has been demonstrated (perlecan gene, elastin gene, collagen type 1 A2 gene). Moreover, 3 of the polymorphisms analyzed in 2 genes (endothelial nitric oxide synthase T786C, interleukin-6 G572C, and interleukin-6 G174C) were found to be significantly associated with ruptured/unruptured aneurysms: the endothelial nitric oxide synthase gene single-nucleotide polymorphisms increased the risk, while IL-6 G174C seemed protective. More recently, two genomic loci (endothelin receptor A and cyclin-dependent kinase inhibitor 2BAS) have been found to be significantly associated with intracranial aneurysms in the Japanese population; endothelin-1 is a potent vasoconstrictor produced by the endothelial cells. Until now, there are no diagnostic tests for specific genetic risk factors to identify patients who are at a high risk of developing intracranial aneurysms. Knowledge of the genetic determinants may be useful in order to allow clues on stopping aneurysm formation and obtain diagnostic tools for identifying individuals at increased risk. Further multicenter studies have to be carried out.