Aortic aneurysm pathophysiology

Overview

It is thought that aortic aneurysm is the result of either genetic disturbances and environmental risks such as smoking.

Pathophysiology

The aortic aneurysms are a multifactorial disease associated with genetic and environmental risk factors. Marfan's syndrome and Ehlers-Danlos syndrome are associated with the disease, but there are also rarer syndromes like the Loeys-Dietz syndrome that are associated as well. Even in patients that do not have genetic syndromes, it has been observed that genetics can also play a role on aortic aneurysms' development. There has been evidence of genetic heterogeneity as there has already been documented in intracranial aneurysms. The genetic alterations associated with these genetic syndromes are the following:

Genetic diseases associated with aortic aneurysms

Disease	Involved Cellular Pathway	Mutated Gene(s)	Affected Protein(s)
Ehlers-Danlos syndrome type IV	Extracellular Matrix Proteins	COL3A1	Collagen type III
Marfan's syndrome	Extracellular Matrix Proteins	FBN1	Fibrillin-1
Loeys-Dietz syndrome	TGF-β Pathway	TGFBR1/TGFBR2
Aneurysm-Osteoarthritis Syndrome		SMAD3	SMAD3
Autosomal Dominant Polycystic Kidney Disease	Ciliopathy	PKD1PKD2	Polycystin 1 Polycystin 2
Turner Syndrome	Meiotic Error with Monosomy, Mosaicism, or De Novo Germ Cell Mutation	45X 45XO	Partial or Complete Absence of X Chromosome
Bicuspid Aortic Valve with TAA	Neural Crest Migration	NOTCH1	Notch 1
Familial TAA	Smooth Muscle Contraction Proteins	ACTA2	α-Smooth Muscle Actin
Familial TAA with Patent Ductus Arteriosus	Smooth Muscle Contraction Proteins	MYH11	Smooth Muscle Myosin
Familial TAA	Smooth Muscle Contraction Proteins	MYLK	Myosin Light Chain Kinase
Familial TAA	Smooth Muscle Contraction Proteins	PRKG1	Protein Kinase c-GMP Dependent, type I
Loeys-Dietz Syndrome variants	TGF-β Pathway	TGF-βR1TGF-βR2SMAD3TGF-β2TGF-β3

These genetic diseases mostly affect either the synthesis of extracellular matrix protein or damage the smooth muscle cells both important component's of the aortic wall. Injury to any of these components lead to weakening of the aortic wall and dilation - resulting in aneurysm formation.

The aorta is the largest vessel of the body, but it is not homogenous. Its upper segment is composed by a larger proportion of elastin in comparison to collagen, therefore being more distensible. The lower segment has a larger proportion of collagen, therefore it is less distensible. It is also where most of the atherosclerotic plaques of the aorta are located. Historically it was thought that abdominal and thoracic aortic aneurysms were caused by the same etiology: atherosclerotic degeneration of the aortic wall, but recently it has been theorized that they are indeed different diseases.

The aortic arch mostly derives from the neural crest cell which differentiate into smooth muscle cells. These smooth muscle cells are probably more adapted to remodel the thoracic aorta and manage the higher pulse pressure and ejection volume due to increased production of elastic lamellae during development and growth. The abdominal aorta remains with cells of mesodermal origin, which are more similar to that of the original primitive arterial. That difference results in the neural crest cell precursors of the thoracic aorta being able to respond differently to various cytokines and growth factors than the mesodermal precursors of the abdominal aorta, such as homocysteine and angiotensin II.

When neural crest vascular smooth muscle cells are treated with TGF-β they demonstrate increased collagen production, while mesodermal vascular smooth muscle cell did not. Not coincidently, mutations of the TGF-β receptor can cause thoracic aortic aneurysm but do not cause abdominal aortic ones.

The thoracic and abdominal aorta are very structurally different. While they both have three layers: intimal, medial and adventitia, the media of the thoracic aorta is comprised of approximately 60 units divided into vascular and avascular regions. The abdominal aorta consists of about 30 units and is entirely avascular - being dependent on trans-intimal diffusion of nutrients for its smooth muscle cells to survive. It is believed that both differences explain why the abdominal aorta is more likely to form aneurysms.

The development of aortic aneurysms is defined by: inflammation: infiltration of the vessel wall by lymphocytes and macrophage; extracellular matrix damage: destruction of elastin and collagen by proteases (also metalloproteinases) in the media and adventitia; cellular damage: loss of smooth muscle cells with thinning of the media; and insufficient repair: neovascularization.

In summary:

• The pathogenesis of aortic aneurysm is characterized by progressive dilation, rupture, and may present with dissection;
• The pathological processes that lead to abdominal and thoracic aortic aneurysms may be very different from one another;
• The fibrillin-1 gene mutation has been associated with the development of thoracic aortic aneurysms in Marfan's syndrome;
• Other gene mutations coding for collagen, elastin and other elements of the extracellular matrix have been associated with the development of aneurysms in some genetic disorders, including Ehlers-Danlos syndrome and others;
• Mutations in the TGF-β pathway have also been described in the pathogenesis of aortic aneurysms in multiple genetic disorders, including Loeys-Dietz syndrome.