**1. Introduction**

Pulmonary arterial hypertension (PAH), although rare, is a progressive disease with a high morbidity and mortality rate. In 1981, Ernst von Romberg, a German physician described pulmonary vascular lesions as "pulmonary vascular sclerosis", the first description of histological changes in PAH [Fishman 2004]. The average survival time for untreated patient is around 2.8 yrs [D'Alonzo 1991]. Despite remarkable progress made since then, the patho‐ genesis of PAH, however, is not yet well understood; because a large number of cardiopul‐ monary and systemic diseases can lead to PAH, and in addition, multiple signaling pathways have been implicated. Current advances in therapy, have improved the quality of life and delayed the progression of the disease, but have not provided a cure. Lack of cure in PAH is further underscored by a recent study showing persistent large plexiform lesions and inflam‐ matory infiltrates in patients despite having been on a long term prostacyclin therapy [Pogoriler 2012]. One of the main reasons for the failure of therapy is that the diagnosis is often made late because of vague symptoms; and by the time the diagnosis is made extensive pathologic changes have already taken place in pulmonary vasculature. From experimental studies, it is clear that pathological changes in the vasculature occur before the onset of PAH [Huang 2010]. Another problem is that a large number of signaling molecules implicated in PAH may not be relevant in all patients; and the activation of some of these molecules may depend on the stage of the disease.

The current clinical classification updated in 2008 maintains five major groups [Simonneau 2009]. *Group 1*: Pulmonary arterial hypertension (PAH): Included in this group are idiopathic (IPAH) and heritable PAH (HPAH), PAH associated with congenital heart defects (CHD), connective tissue diseases, portal hypertension, infection, chronic hemolytic anemia, drug toxicity and persistent pulmonary hypertension of the newborn (PPHN). Pulmonary venoocclusive disease and pulmonary capillary hemangiomatosis are included in this group as a subcategory. Approximately 70% of HPAH and 26% IPAH exhibit heterozygous germline

© 2013 Mathew; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Mathew; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

mutations in BMPRII, a member of TGFβ superfamily; however, only about 20% of people with BMPRII mutation develop PAH. The incidence of HPAH is reported to be 3.9% of IPAH. [Thompson 2000, Machado 2006, Cogan 2006, Sztrymf 2007, Humbert 2006]. It has recently been shown that in BMPRII+/- mice, a "second hit" such as inflammation or serotonin increases the susceptibility to develop PAH [Song 2008, Long 2006]. In addition, altered metabolism of estrogen resulting in low production of 2 methylestradiol is also thought to be a "second hit" for the development of PAH in females with BMPRII mutation [Austin 2009]. Interestingly, a short exposure to fenfluramine, a diet suppressant, is enough to induce PAH in patients with a BMPRII mutation [Humbert 2002]. Thus, the "second hit" is almost a requirement for the development of PAH in patients with BMPRII mutations. Approximately 6% of adults and children with congenital heart defect CHD and PAH also exhibit BMPRII mutations [Roberts 2004]. In addition, mutations of activin-like receptor kinase 1 (ALK1) and endoglin, both belonging to the TGFβ superfamily have been reported in patients with hereditary hemorrha‐ gic telangiectasia, and some of these patients develop PAH [Trembath 2001]. Recently mutation of SMAD 8, belonging to another member of the TGF-β family, and thrombospon‐ din-1 (TSP-1) were found in patients with PAH and HPAH respectively. Interestingly, TSP-1 is known to regulate the activation of TGF-β [Shintani 2009, Maloney 2012]. Thus, the TGF-β/ BMP signaling pathway has an important role in pulmonary vascular health and disease. Polymorphisms of other genes have been described in PH such as serotonin (LL allele), TRPC6 gene promoter, and Norrie disease with deficiency of monoamine oxidases, which degrades serotonin have been reported in patients with PH [*reviewed in* Mathew 2011]. In addition, polymorphism of the *KCNA5* gene with altered expression and function of Kv1.5 channels has been observed in pulmonary vascular smooth muscle cells (SMC) from IPAH patients [Remillard 2007].

parenchymal lung diseases associated with hypoxia. Major components in this group are vasoconstriction and vascular remodeling. Inflammation plays a significant role in the pathobiology of lung diseases. Recent studies suggest that the pathological changes seen in COPD and idiopathic pulmonary fibrosis are related to oxidative stress and aging as evidenced by increased expression of senescence markers in lungs and enhanced tissue destruction [MacNee 2009, Faner 2012]. Furthermore, senescent pulmonary artery SMC exhibit telomere shortening and increased production of cytokines, thus, contributing to the progression of the disease [Noureddine 2011]. *Group 4*: Included in this group is PH resulting from an incomplete resolution of chronic pulmonary thromboembolism. The incidence of PH in this group is reported to be approximately 4% at 2 yrs. About 10% of patients develop PH even after satisfactory thrombo-endarterectomy. Worsening of the disease is thought to occur because of recurrent thromboembolism, or in situ thrombosis and pulmonary vascular remodeling. Reduction in the expression of eNOS and impaired endothelium-dependent, NO-mediated relaxation response in pulmonary arteries distal to ligation was recently reported in a porcine pulmonary artery ligation model. Importantly, histological features include pulmonary vascular remodeling and plexiform lesions indistinguishable from PAH [Moser 1993, Pengo 2004, Dartevelle 2004, Fadel 2000]. *Group 5*: This group includes a large number of miscella‐ neous diseases such as PH secondary to other systemic diseases such as sarcoidosis, myelo‐ proliferative diseases, metabolic and hematological disorders, Thyroid diseases, Gaucher's

Pathogenesis of Pulmonary Hypertension http://dx.doi.org/10.5772/56179 49

PH in pediatric age group has several different features compared with the adult patients. In children, medial hypertrophy is the main feature; with increasing age other pathological features such as intimal proliferation, concentric fibrosis and subsequently dilatation and plexiform lesions appear [Wagenvoort 1970]. A recent study revealed that females comprised 46% of all PH and 51% of all PAH group patients [van Loon 2011]. The major causes of PH in children are CHD, PPHN, lung diseases such as respiratory distress syndrome (RDS), bron‐ chopulmonary dysplasia (BPD), and congenital defects associated with hypoplasia of the lungs [Mathew 2000]. Antenatal and perinatal problems have adverse effects on vascular and alveolar development. Preterm delivery disrupts normal pulmonary vascular and bronchoalveolar development which leads to reduced cross sectional area of the pulmonary vascula‐ ture resulting in increased pulmonary vascular resistance and PH [Farquhar 2010]. Another interesting difference from the adult group is that >80% of pediatric patients have transient PH. These include resolution of PPHN and in the majority of the cases after surgical correction of CHD [van Loon 2012]. However, poor outcome has been reported in children with IPAH or HPAH associated with BMPRII mutation [Chida 2012]. A new classification for pediatric PH has been proposed that is comprised of 10 major groups and includes prenatal and developmental anomalies. The main categories are: 1. Prenatal or developmental pulmonary hypertensive vascular disease, 2. Prenatal pulmonary vascular maladaptation, 3. Pediatric cardiovascular disease, 4. Bronchopulmonary dysplasia, 5. Isolated pediatric PAH, 6. Pulmo‐ nary hypertensive vascular disease in congenital malformation syndrome, 7. Pediatric lung

disease and chronic renal failure requiring dialysis.

**1.1. PH in pediatric age group**

Among adults, PAH occurs more frequently in women than men. The French national registry revealed female to male ratio to be 1.9:1 and in a recent report from the US registry the ratio was reported to be around 4:1 with better survival rate among the females. The higher incidence of PAH in females in the US was thought to be related to the higher incidence of obesity [Humbert 2006, Shapiro 2012]. In HIV-PAH, however, there is higher incidence in males (M:F 1.5:1), because more male patients have HIV infection [Cicalini 2011]. PAH is the leading cause of death in patients with scleroderma, and the estimated prevalence of PAH in this group is 8-12% [Mathai 2011]. *Group 2*: PH due to left heart diseases such as mitral valve disease, systemic hypertension, ischemic heart disease and cardiomyopathy are included in this group. These diseases lead to LV diastolic overload, impaired function and passive congestion in capillaries. Sustained elevated pressure in pulmonary venous circulation results in structural and functional damage of pulmonary arteries, and endothelial dysfunction leading to PH. Heart failure with preserved ejection fraction (HFpEF) is recognized as the major cause of PH associated with left heart disease. In one study, female preponderance (58%) was observed in HFpEF + PH group. These patients have higher LV end-diastolic pressure. It is important to distinguish this group from PAH (group 1), because the therapy used in PAH is not effective in patients in this group [Guazzi 2010, Thenappan 2011, Hill 2011]. *Group 3*: This group encompasses PH due to chronic obstructive pulmonary disease (COPD) and other parenchymal lung diseases associated with hypoxia. Major components in this group are vasoconstriction and vascular remodeling. Inflammation plays a significant role in the pathobiology of lung diseases. Recent studies suggest that the pathological changes seen in COPD and idiopathic pulmonary fibrosis are related to oxidative stress and aging as evidenced by increased expression of senescence markers in lungs and enhanced tissue destruction [MacNee 2009, Faner 2012]. Furthermore, senescent pulmonary artery SMC exhibit telomere shortening and increased production of cytokines, thus, contributing to the progression of the disease [Noureddine 2011]. *Group 4*: Included in this group is PH resulting from an incomplete resolution of chronic pulmonary thromboembolism. The incidence of PH in this group is reported to be approximately 4% at 2 yrs. About 10% of patients develop PH even after satisfactory thrombo-endarterectomy. Worsening of the disease is thought to occur because of recurrent thromboembolism, or in situ thrombosis and pulmonary vascular remodeling. Reduction in the expression of eNOS and impaired endothelium-dependent, NO-mediated relaxation response in pulmonary arteries distal to ligation was recently reported in a porcine pulmonary artery ligation model. Importantly, histological features include pulmonary vascular remodeling and plexiform lesions indistinguishable from PAH [Moser 1993, Pengo 2004, Dartevelle 2004, Fadel 2000]. *Group 5*: This group includes a large number of miscella‐ neous diseases such as PH secondary to other systemic diseases such as sarcoidosis, myelo‐ proliferative diseases, metabolic and hematological disorders, Thyroid diseases, Gaucher's disease and chronic renal failure requiring dialysis.

#### **1.1. PH in pediatric age group**

mutations in BMPRII, a member of TGFβ superfamily; however, only about 20% of people with BMPRII mutation develop PAH. The incidence of HPAH is reported to be 3.9% of IPAH. [Thompson 2000, Machado 2006, Cogan 2006, Sztrymf 2007, Humbert 2006]. It has recently been shown that in BMPRII+/- mice, a "second hit" such as inflammation or serotonin increases the susceptibility to develop PAH [Song 2008, Long 2006]. In addition, altered metabolism of estrogen resulting in low production of 2 methylestradiol is also thought to be a "second hit" for the development of PAH in females with BMPRII mutation [Austin 2009]. Interestingly, a short exposure to fenfluramine, a diet suppressant, is enough to induce PAH in patients with a BMPRII mutation [Humbert 2002]. Thus, the "second hit" is almost a requirement for the development of PAH in patients with BMPRII mutations. Approximately 6% of adults and children with congenital heart defect CHD and PAH also exhibit BMPRII mutations [Roberts 2004]. In addition, mutations of activin-like receptor kinase 1 (ALK1) and endoglin, both belonging to the TGFβ superfamily have been reported in patients with hereditary hemorrha‐ gic telangiectasia, and some of these patients develop PAH [Trembath 2001]. Recently mutation of SMAD 8, belonging to another member of the TGF-β family, and thrombospon‐ din-1 (TSP-1) were found in patients with PAH and HPAH respectively. Interestingly, TSP-1 is known to regulate the activation of TGF-β [Shintani 2009, Maloney 2012]. Thus, the TGF-β/ BMP signaling pathway has an important role in pulmonary vascular health and disease. Polymorphisms of other genes have been described in PH such as serotonin (LL allele), TRPC6 gene promoter, and Norrie disease with deficiency of monoamine oxidases, which degrades serotonin have been reported in patients with PH [*reviewed in* Mathew 2011]. In addition, polymorphism of the *KCNA5* gene with altered expression and function of Kv1.5 channels has been observed in pulmonary vascular smooth muscle cells (SMC) from IPAH patients

Among adults, PAH occurs more frequently in women than men. The French national registry revealed female to male ratio to be 1.9:1 and in a recent report from the US registry the ratio was reported to be around 4:1 with better survival rate among the females. The higher incidence of PAH in females in the US was thought to be related to the higher incidence of obesity [Humbert 2006, Shapiro 2012]. In HIV-PAH, however, there is higher incidence in males (M:F 1.5:1), because more male patients have HIV infection [Cicalini 2011]. PAH is the leading cause of death in patients with scleroderma, and the estimated prevalence of PAH in this group is 8-12% [Mathai 2011]. *Group 2*: PH due to left heart diseases such as mitral valve disease, systemic hypertension, ischemic heart disease and cardiomyopathy are included in this group. These diseases lead to LV diastolic overload, impaired function and passive congestion in capillaries. Sustained elevated pressure in pulmonary venous circulation results in structural and functional damage of pulmonary arteries, and endothelial dysfunction leading to PH. Heart failure with preserved ejection fraction (HFpEF) is recognized as the major cause of PH associated with left heart disease. In one study, female preponderance (58%) was observed in HFpEF + PH group. These patients have higher LV end-diastolic pressure. It is important to distinguish this group from PAH (group 1), because the therapy used in PAH is not effective in patients in this group [Guazzi 2010, Thenappan 2011, Hill 2011]. *Group 3*: This group encompasses PH due to chronic obstructive pulmonary disease (COPD) and other

[Remillard 2007].

48 Pulmonary Hypertension

PH in pediatric age group has several different features compared with the adult patients. In children, medial hypertrophy is the main feature; with increasing age other pathological features such as intimal proliferation, concentric fibrosis and subsequently dilatation and plexiform lesions appear [Wagenvoort 1970]. A recent study revealed that females comprised 46% of all PH and 51% of all PAH group patients [van Loon 2011]. The major causes of PH in children are CHD, PPHN, lung diseases such as respiratory distress syndrome (RDS), bron‐ chopulmonary dysplasia (BPD), and congenital defects associated with hypoplasia of the lungs [Mathew 2000]. Antenatal and perinatal problems have adverse effects on vascular and alveolar development. Preterm delivery disrupts normal pulmonary vascular and bronchoalveolar development which leads to reduced cross sectional area of the pulmonary vascula‐ ture resulting in increased pulmonary vascular resistance and PH [Farquhar 2010]. Another interesting difference from the adult group is that >80% of pediatric patients have transient PH. These include resolution of PPHN and in the majority of the cases after surgical correction of CHD [van Loon 2012]. However, poor outcome has been reported in children with IPAH or HPAH associated with BMPRII mutation [Chida 2012]. A new classification for pediatric PH has been proposed that is comprised of 10 major groups and includes prenatal and developmental anomalies. The main categories are: 1. Prenatal or developmental pulmonary hypertensive vascular disease, 2. Prenatal pulmonary vascular maladaptation, 3. Pediatric cardiovascular disease, 4. Bronchopulmonary dysplasia, 5. Isolated pediatric PAH, 6. Pulmo‐ nary hypertensive vascular disease in congenital malformation syndrome, 7. Pediatric lung disease, 8. Pediatric thromboembolic disease, pediatric hypobaric hypoxic exposure, 10. Pediatric pulmonary vascular disease associated with other systemic disorders [del Cerro 2011]. Irrespective of the underlying pathology, patients usually present with similar changes in the lungs including endothelial dysfunction, impaired vascular reactivity, activation of inflammatory processes, vascular remodeling, with subsequent neointima formation and eventually right heart failure.

downstream effector of NO, sGC has been shown to compartmentalize in caveolae to facilitate its activation. In caveolin-1 knockout mice, the loss of caveolin-1 is associated with the hyperactivation of eNOS, and increased cGMP production. The hyper-activation of eNOS subse‐ quently leading to PKG nitration-induced stress is considered responsible for PH in these mice; and re-expression of endothelial caveolin-1 restores vascular and cardiac abnormalities [*reviewed in* Mathew 2011b]. Caveolin-1 functions as an antiproliferative molecule; it negatively regulates proliferative pathways such as mitogen-activated protein kinase/extracellular signalregulated kinase (MAPK/ERK), tyrosine- phosphorylated signal transducer and activator of transcription (PY-STAT) 3, EGF and platelet-derived growth factor (PDGF). Caveolin-1 also regulates cell cycle and apoptosis. In addition, caveolin-1 interacts with major ion channels

number of molecules responsible for Ca2+ handling such as inositol triphosphate receptor (IP3R), heterodimeric GTP binding protein, Ca2+ ATPase and several transient receptor potential channels in caveolae. Through these interactions, caveolin-1 modulates cell prolif‐ eration and cell cycle progression. In SMC, caveolin-1 regulates Ca2+ entry and enables vasoconstriction. The localization of Ca2+ regulating proteins in caveolae and the proximity to the sarcoplasmic reticulum suggests an important role for caveolae/caveolin-1 for Ca2+ homeostasis [*reviewed in* Mathew 2011b]. RhoA interacts directly with caveolin-1, and the translocation of RhoA to caveolae is essential for myogenic tone. The CSD peptide of caveolin-1 has been shown to inhibit the agonist-induced redistribution of RhoA and PKC-α. Caveolin-1 blockage results in impaired formation of capillary tubes, and the overexpression of caveolin-1 accelerates EC differentiation and tube formation [Santibanz 2008, Liu 2002]. Furthermore, caveolin-1 modulates inflammation. It has recently been shown that caveolin-1 inhibits HIV

BMPRII is predominantly expressed in EC, and a part of BMPRII colocalizes with caveolin-1 in caveolae and also in golgi bodies. BMPRII signaling, essential for BMP-mediated regulation of vascular SMC growth and differentiation also protects EC from apoptosis [Yu 2008, Teichert-Kuliszewska 2006]. BMPRII directly modulates proteins involved in cytoskeletal organization, possibly through Mas1 (G-protein-coupled receptor) interaction with Rho GTPase. Recently discovered angiotensin converting enzyme (ACE) 2, an endogenous inhibitor of ACE, is endothelium-bound. ACE2 cleaves angiotensin (Ang) I to Ang 1-9 which is an inactive compound. ACE2 metabolizes Ang I to produce Ang 1-7 which is a physiological antagonist of Ang II. ACE2/Ang (1-7) pathway antagonizes Ang II acting through Mas1, increases NO production via the Akt-dependent pathway, releases PGI2 and it inhibits Ang II-induced reactive oxygen species (ROS) formation within the cell nucleus. Loss of ACE2 causes increased vascular permeability, pulmonary edema and worsening lung function. The over-expression of Ang-(1-7) has a protective effect on MCT-induced PH and bleomycin-induced lung fibrosis. Interestingly, inhibition of Rho kinase has been shown to activate the ACE2/Ang-(1-9) pathway resulting in increased eNOS expression and amelioration of hypertension [Johnson 2012, Burton 2011, Lovern 2008, Mathew 2011, Ocaranza 2011, Shenoy 2010]. Thus, under normal conditions EC maintain homeostasis by producing cell protective factors and inhibiting

channels (Kv1.5), and a

Pathogenesis of Pulmonary Hypertension http://dx.doi.org/10.5772/56179 51

such as Ca2+ -dependent potassium channels, voltage-dependent K+

replication through NF-κB [Wang 2011].

inflammation and cell proliferation.
