Epidemiology and genetics of intracranial aneurysms

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Abstract

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.

Introduction

Intracranial (saccular or berry) aneurysms (IAs) are acquired lesions, accounting for about 80% of all nontraumatic subarachnoid hemorrhages (SAH) [1].

IAs affects 5–10% of the general population [2], a fraction of which will rupture and lead to devastating consequences.

Unruptured IAs are rarely noted in children (0,5–4,6% of all aneurysms) and appear to develop with increasing age [3], [4]: the prevalence of harboring an IA within the population aged over 30 years is between 3.6 and 6.5% [5], [6], [7], [8], [9].

Women may be more likely to have an aneurysm than men (3:1 ratio of women compared with men in unruptured – 8, 9). IA may occur alone (70–75%) or as multiple aneurysms (25–30%) [10].

The incidence of unruptured IAs seems to be increasing with the continuous evolution of Magnetic Resonance angiography (MRA) and Computed Tomography angiography (CTA) imaging techniques [11], [12].

SAH due to IA rupture occurs around 1.24 times more often in women than in men [13] and 2.1 times more often in blacks than whites [14].

The annual incidence of SAH has wide variations between geographical regions. A World Health Organization (WHO) study found a 10-fold variation in the age-adjusted annual incidence in Europe and Asia, from 2.0 per 100,000 population in China to 22.5 cases per 100,000 in Finland [15]. The risk of rupture depends on the size and location of the aneurysms and has been reported to be 2.7% per year in a Japanese population [11] and 1.9% in a white population [16].

Most patients have no symptoms or complaints until the aneurysm ruptures. In some cases, there are some warning signs that an aneurysm is present, such as pain above and behind the eye, cranial nerve paralysis, or headache and neck pain secondary to a leakage of blood from the aneurysm, so-called “sentinel bleed” [17].

Section snippets

Risk factors for intracranial aneurysms

The exact etiology of IA formation remains unclear; they are usually acquired lesions because of congenital defects in the wall of a blood vessel, atherosclerotic changes, trauma, or infectious emboli [17].

The progress in understanding the pathogenesis of IAs has been hampered by its multifactorial nature.

High-resolution modern imaging technique, often performed for the evaluation of vague and nonspecific symptoms and concerns, have increased the detection of asymptomatic sporadic IA. When an

Association with heritable conditions

Other than familiar occurrence, there are several heritable conditions associated with IA formation [38], the most common of whom is autosomal dominant polycystic kidney disease (ADPKD), with a prevalence of 10%.

ADPKD is the most common of the inherited renal cystic disease (Fig. 5), a group of disorders characterized by the development of renal cysts and a variety of extrarenal manifestation.

The disease occurs worldwide and in all races with a prevalence estimated to be between 1:400 and

Genetic studies

The familial occurrence and the association with heritable conditions indicate that genetic factors may play a role in the development of IAs; therefore, many studies have focused on genetic determinants for IAs the last decade.

Obviously, if a genetic marker associated with increased risk of formation and rupture of an IA could be identified, the necessity for screening and urgency of treatment could be determined more easily [12].

However the work is not easy, because the mode of transmission

Conclusions

The presence of an IA is a serious medical condition that can lead to permanent neurological deficit or death if the aneurysm ruptures. However, the low incidence of SAH in comparison with the prevalence of unruptured IAs suggests that the vast majority of IAs do not rupture and that identifying those at highest risk is important in defining the optimal management [18].

The most important factors predicting IA rupture are IA size and site; moreover, familial predisposition is a well recognized

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      Moreover, IAs are associated with heritable diseases, including autosomal dominant polycystic kidney disease, Ehlers-Danlos syndrome type IV, neurofibromatosis type I, and Marfan syndrome.17 Taken collectively, familiar and hereditable conditions associated with IAs underline a genetic potential role in aSAH, even although a specific gene-disease association has not been established.16 The pathophysiology underlying IA rupture is not entirely understood, although the main actors seem to be endothelial cells (ECs) and the inflammatory process in response to the impaired wall shear stress.18

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    1

    Ferdinando Caranci, Unit of Neuroradiology, Department of Diagnostic Radiology and Radiotherapy, Federico II University, Naples via S. Pansini 5, 80131 Naples (Italy). Tel.: +39 817464646; fax: +39 817462597.

    2

    Francesco Briganti, via S. Pansini 5, 80131 Naples, Italy. Tel.: +39 817464251; fax: +39 817462597.

    3

    Neuroradiology service Bellaria Hospital, Bologna, Italy.

    4

    Neuroradiology service, Bellaria Hospital, Bologna, Italy.

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