Renin-angiotensin-aldosterone system genes and the complex hypertrophic phenotype of hypertrophic cardiomyopathy

Carstens, Nadia (2012-12)

Thesis (PhD)--Stellenbosch University, 2012.

Thesis

ENGLISH ABSTRACT: Left ventricular hypertrophy (LVH) is a strong independent predictor of cardiovascular morbidity and mortality, while its regression is associated with an improved clinical prognosis. It is, therefore, vital to elucidate and fully comprehend the mechanisms that contribute to LVH development and to identify markers that indicate a strong predisposition to the development of severe cardiac hypertrophy, before its occurrence. Hypertrophic cardiomyopathy (HCM) serves as a model to investigate LVH development. This primary cardiac disease is characterised by LVH in the absence of increased external loading conditions and is caused by defective sarcomeric proteins, as a result of mutations within the genes encoding these proteins. However, the hypertrophic phenotype of HCM is largely complex, as we see strong variability in the extent and distribution of LVH in HCM, even in individuals with the same disease-causing mutation from the same family; this points toward the involvement of additional genetic and environmental modifiers. Components of the renin-angiotensin-aldosterone system (RAAS) influence LVH indirectly, through their key role in blood pressure regulation, but also directly, due to the direct cellular hypertrophic effects of some RAAS components. Previous genetic association studies aimed at investigating the contribution of RAAS variants to LVH were largely centred on a subset of polymorphisms within the genes encoding the angiotensin converting enzyme (ACE) and angiotensin II type 1 receptor genes, while the renin section and RAAS components downstream from ACE remained largely neglected. In addition, most previous studies have reported relatively small individual effects for a small subset of RAAS variants on LVH. In the present study we, therefore, employ a family-based genetic association analysis approach to investigate the contribution of the entire RAAS to this complex hypertrophic phenotype by exploring both the individual as well as the compound effects of 84 variants within 22 RAAS genes, in a cohort of 388 individuals from 27 HCM families, in which either of three HCM-founder mutations segregate. During the course of this explorative study, we identified a number of RAAS variants that had significant effects on hypertrophy in HCM, whether alone or within the context of a multi-variant haplotype. Through single variant association analyses, we identified variants within the genes encoding angiotensinogen, renin-binding protein, the mannose-6-phosphate receptor, ACE, ACE2, angiotensin receptors 1 and 2, the mineralocorticoid receptor, as well as the epithelial sodium channel and the Na+/K+-ATPase β-subunits, that contribute to hypertrophy in HCM. Using haplotype-based association analyses, we were able to identify haplotypes within the genes encoding for renin, the mannose-6-phosphate receptor, angiotensin receptor 1, the mineralocorticoid receptor, epithelial sodium channel and Na+/K+-ATPase α- and β subunits, as well as the CYP11B1/B2 locus, that contribute significantly to LVH. In addition, we found that some RAAS variants and haplotypes had statistically significantly different effects in the three HCM founder mutation groups. Finally, we used stepwise selection to identify a set of nine risk-alleles that together predicted a 127.80 g increase in left ventricular mass, as well as a 13.97 mm increase in maximum interventricular septal thickness and a 14.67 mm increase in maximum left ventricular wall thickness in the present cohort. In contrast, we show that a set of previously identified “pro-LVH” polymorphisms rather poorly predicted LVH in the present South African cohort. This is the first RAAS investigation, to our knowledge, to provide clear quantitative effects for a subset of RAAS variants indicative of a risk for LVH development that are representative of the entire pathway. Our findings suggest that the eventual hypertrophic phenotype of HCM is modulated by the compound effect of a number of RAAS modifier loci, where each polymorphism makes a modest contribution towards the eventual phenotype. Research such as that presented here provides a basis on which future studies can build improved risk profiles for LVH development within the context of HCM, and ultimately in all patients with a risk of cardiac hypertrophy.

AFRIKAANSE OPSOMMING: Linker ventrikulêre hipertrofie (LVH) is 'n sterk onafhanklike voorspeller van kardiovaskulêre morbiditeit en mortaliteit, terwyl LVH regressie verband hou met ‘n verbeterde kliniese voorspelling. Dit is dus noodsaaklik om die meganismes wat bydra to LVH ontwikkeling ten volle te verstaan en merkers wat 'n sterk geneigdheid tot die ontwikkeling van ernstige kardiale hipertrofie te identifiseer, voordat dit voorkom. Hipertrofiese kardiomiopatie (HKM) dien as 'n model om LVH ontwikkeling te ondersoek. Hierdie primêre hartsiekte word gekenmerk deur LVH en word meestal veroorsaak deur foutiewe sarkomeer proteïene as gevolg van mutasies binne die gene wat kodeer vir hierdie proteïene. Die hipertrofiese fenotipe van HKM is egter grootliks kompleks; ons sien, by voorbeeld, sterk veranderlikheid in die omvang en die verspreiding van LVH in HKM, selfs in individue met dieselfde siekte-veroorsakende mutasie binne dieselfde gesin, wat dui op die betrokkenheid van addisionele genetiese en omgewing modifiseerders. Komponente van die renien-angiotensien-aldosteroon sisteem (RAAS) beïnvloed LVH indirek, deur middel van hul belangrike rol in bloeddruk regulasie, maar ook direk, as gevolg van die direkte sellulêre hipertrofiese gevolge van sommige RAAS komponente. Vorige genetiese assosiasie studies wat daarop gemik was om die bydrae van RAAS variante LVH te ondersoek, was hoofsaaklik gesentreer op 'n groepie polimorfismes binne die gene wat kodeer vir die “angiotensin converting enzyme” (ACE) en angiotensien II tipe 1-reseptor gene, terwyl die renien gedeelte en RAAS komponente stroomaf van ACE meestal nie ondersoek was nie. Daarbenewens het die meeste vorige studies relatief klein individuele gevolge gerapporteer vir 'n klein groepie RAAS variante op LVH. In die huidige studie het ons dus 'n familie-gebaseerde genetiese assosiasie-analise benadering gebruik om die bydrae van die hele RAAS tot hierdie komplekse hipertrofiese fenotipe te ondersoek deur 'n studie van die individuele-, sowel as die saamgestelde effekte van 84 variante binne 22 RAAS gene, in 'n groep van 388 individue vanaf 27 HKM families, waarin een van drie HCM-stigter mutasies seggregeer. Gedurende die loop van hierdie studie het ons 'n aantal RAAS variante wat ‘n beduidende uitwerking op HKM hipertrofie geïdentifiseer, hetsy alleen of binne die konteks van' n multi-variant haplotipe. Deur middel van enkele variant assosiasie toetsing het ons variante geïdentifiseer binne die gene wat kodeer vir angiotensinogen, renien-bindende proteïen, die mannose-6-fosfaat reseptor, ACE, ACE2, angiotensien reseptore 1 en 2, die mineralokortikoïd reseptor, sowel as die epiteel natrium kanaal en Na+/ K+-ATPase β-subeenhede, wat bydra tot HKM hipertrofie. Deur die gebruik van haplotipe-gebaseerde assosiasie ontleding was ons in staat om haplotipes te identifiseer binne die gene wat kodeer vir renien, die mannose-6-fosfaat reseptor angiotensien reseptor 1, die mineralokortikoïd reseptor, epiteel natrium kanaal en die Na+/ K+-ATPase α-en β subeenhede, sowel as die CYP11B1/B2 lokus, wat aansienlik bydra tot LVH. Verder het ons bevind dat sommige RAAS variante en haplotipes statisties beduidende verskillende effekte gehad het in die drie HKM stigter mutasie groepe. Laastens, het ons stapsgewyse seleksie gebruik om 'n stel van nege risiko-allele wat saam' n toename van 127.80 g in linker ventrikulêre massa, sowel as 'n 13.97 mm toename in maksimum ventrikulêre septale dikte, en' n 14.67 mm verhoging in maksimum linker ventrikulêre wanddikte voorspel, te identifiseer in die huidige kohort. In teenstelling hiermee wys ons dat 'n stel van voorheen geïdentifiseerde "pro-LVH" polimorfismes swakker gevaar het as LVH-voorspellers in die huidige Suid-Afrikaanse kohort. Hierdie is die eerste RAAS ondersoek, tot ons kennis, wat ‘n duidelike kwantitatiewe gevolge vir 'n stel RAAS variante wat ‘n verhoogde risiko tot LVH ontwikkeling aandui, wat verteenwoordigend is van die hele RAAS. Ons bevindinge dui daarop dat die uiteindelike hipertrofiese fenotipe van HKM gemoduleer word deur die saamgestelde effek van 'n aantal RAAS wysiger loki, waar elke polimorfisme ' n beskeie bydrae maak tot die uiteindelike fenotipe. Navorsing soos dié wat hier aangebied word dien as 'n basis waarop toekomstige studies kan bou vir ‘n verbeterde risiko-profiel vir LVH ontwikkeling binne die konteks van die HKM, en uiteindelik in alle pasiënte met' n verhoogde risiko vir kardiale hipertrofie.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/71949
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