Sarcomeric modifiers of hypertrophy in hypertrophic cardiomyopathy (HCM)

Bloem, Liezl Margaretha (2013-03)

Thesis (PhD)--Stellenbosch University, 2013.

Thesis

ENGLISH ABSTRACT: Left ventricular hypertrophy (LVH) is an independent predictor of cardiovascular morbidity and allcause mortality. Significantly, it is considered a modifiable cardiovascular risk factor as its regression increases overall survival and reduces the frequency of adverse cardiac events. A clear understanding of LVH pathogenesis is thus imperative to facilitate improved risk stratification and therapeutic intervention. Hypertrophic cardiomyopathy (HCM), an inherited cardiac disorder, is a model disease for elucidating the molecular mechanisms underlying LVH development. LVH, in the absence of increased external loading conditions, is its quintessential clinical feature, resulting from mutations in genes encoding sarcomeric proteins. The LVH phenotype in HCM exhibits marked variability even amongst family members who carry the same disease-causing mutation. Phenotypic expression is thus determined by the causal mutation and additional determinants including the environment, epigenetics and modifier genes. Thus far, factors investigated as potential hypertrophy modifiers in HCM have been relatively removed from the primary stimulus for LVH; and the few studies that have been replicated yielded inconsistent results. We hypothesized that the factors that closely interact with the primary stimulus of faulty sarcomeric functioning, have a greater capacity to modulate it, and ultimately the LVH phenotype in HCM. Plausible candidate modifiers would include factors relating to the structure or function of the sarcomere, including known HCM-causal genes; and the enzymes that function in sarcomere-based energetics. Indeed, the literature highlights the relevance of sarcomeric proteins, Ca2+-handling and myocardial energetics in the development of LVH in HCM. This study, therefore, set out to evaluate the hypertrophy-modifying capacity of such factors by means of family-based genetic association testing in 27 South African HCM families in which one of three unique HCM-causing founder mutations segregates. Moreover, the single and combined effects of 76 variants within 26 candidate genes encoding sarcomeric or sarcomere-associated proteins were investigated. The study identified a modifying role in the development of hypertrophy in HCM for each of the candidate genes investigated with the exception of the metabolic protein-encoding gene, PRKAG1. More specifically, single variant association analyses identified a modifying role for variants within the genes MYH7, TPM1 and MYL2, which encode proteins of the sarcomere, as well as the genes CPT1B, CKM, ALDOA and PRKAB2, which encode metabolic proteins. Haplotype-based association analyses identified combined modifying effects for variants within the genes ACTC, TPM1, MYL2, MYL3 and MYBPC3, which encode proteins of the sarcomere, as well as the genes CD36, PDK4, CKM, PFKM, PPARA, PPARG, PGC1A, PRKAA2, PRKAG2 and PRKAG3, which encode metabolic proteins. Moreover, a number of variants and haplotypes showed statistically significant differences in effect amongst the three HCM founder mutation groups. The HCM-modifier genes identified were prioritised for future studies according to the number of significant results obtained for the four tests of association performed. The genes TPM1 and MYBPC3, which encode sarcomeric proteins, as well as the genes PFKM and PRKAG2, which encode metabolic proteins, were identified as stronger candidates for future studies as they delivered multiple significant results for various statistical tests. This study makes a novel contribution to the field of hypertrophy research as it tested the hypothesis that structural or energy-related factors located within the sarcomere may act as modifiers of cardiac hypertrophy in HCM, and succeeded in identifying a modifying role for many of the candidate genes selected. The significant results include substantial single and within-genecontext variant effects; and identified sizeable variation in the risk of developing LVH owing to the compound effect of the modifier and the individual founder mutations. Collectively, these findings enhance the current understanding of genotype/phenotype correlations and may, as consequence, improve patient risk stratification and choice of treatment. Moreover, these findings emphasize the potential for modulation of disease by further elucidation of some of the avenues identified.

AFRIKAANSE OPSOMMING: Linker ventrikulêre hipertrofie (LVH) is ‘n onafhanklike voorspeller van kardiovaskulêre morbiditeit en van mortaliteit weens alle oorsake. Van belang is dat dit ‘n wysigbare kardiovaskulêre risiko faktor is, aangesien die afname daarvan algehele oorlewing verhoog en die frekwensie van nadelige kardiale voorvalle verlaag. ‘n Duidelike begrip van LVH patogenese is dus noodsaaklik om verbeterde risiko stratifikasie en terapeutiese intervensie te fasiliteer. Hipertrofiese kardiomiopatie (HKM), ‘n oorerflike hart-siekte, is ‘n model-siekte vir die uitpluis van die molekulêre meganismes onderliggend aan die ontwikkeling van LVH. LVH, in die afwesigheid van verhoogde eksterne lading, is die kern kliniese simptoom van HKM en die gevolg van mutasies in die gene wat kodeer vir sarkomeriese proteïene. Die LVH fenotiepe in HKM toon merkbare veranderlikheid selfs in familie-lede wat dieselfde siekte-veroorsakende mutasie dra. Die fenotiepe word dus bepaal deur die siekte-veroorsakende mutasie asook addisionele determinante insluitend die omgewing, epigenetika en modifiserende gene. Potensiële hipertrofie-modifiseerders wat tot dusver bestudeer is, is betreklik verwyder van die primêre stimulus vir LVH en die paar studies wat gerepliseer is, het teenstrydige resultate gelewer. Ons hipoteseer dat die faktore wat in noue interaksie met die primêre stimulus van foutiewe sarkomeriese funksionering is, ‘n groter kapasitieit het om dit en uiteindelik die LVH fenotiepe in HKM, te moduleer. Aanneemlike kandidaat-modifiseerders sou insluit faktore wat betrekking het tot die struktuur en funksie van die sarkomeer insluitend HKM-oorsaaklike gene en die ensieme wat funksioneer in sarkomeer-gebaseerde energetika. Die literatuur beklemtoon inderdaad die relevansie van sarkomeriese proteïene, Ca2+-hantering en miokardiese energetika in die ontwikkeling van LVM in HKM. Hierdie studie het beoog om die hipertrofie-modifiserende kapasiteit van sulke faktore te evalueer deur middel van familie-gebaseerde genetiese assosiasie toetse in 27 Suid-Afrikaanse HKM families waarin een van drie unieke HKM-stigter mutasies segregeer. Verder was die enkel en gekombineerde effekte van 76 variante binne 26 kandidaat gene wat kodeer vir sarkomeer en sarkomeer-geassosieerde proteïene, ondersoek. Hierdie studie het ‘n modifiserende rol in die ontwikkeling van hipertrofie in HKM geïdentifiseer vir elk van die kandidaat gene wat ondersoek is, met uitsluiting van die PRKAG1, wat kodeer vir ‘n metaboliese proteïen. Meer spesifiek, enkel variant assosiasie analises het ‘n modifiserende rol geïdentifiseer vir variante in die gene MYH7, TPM1 en MYL2, wat kodeer vir sarkomeriese proteïene, asook die gene CPT1B, CKM, ALDOA en PRKAB2, wat kodeer vir metabolise proteïene. Haplotipe-gebaseerde assosiasie-analises het gekombineerde modifiserende effekte geïdentifiseer vir variante in die gene ACTC, TPM1, MYL2, MYL3 en MYBPC3, wat kodeer vir strukturele proteïene van die sarkomeer asook die gene CD36, PDK4, CKM, PFKM, PPARA, PPARG, PGC1A, PRKAA2, PRKAG2 en PRKAG3, wat kodeer vir metabolise proteïene. Verder het ‘n aantal variante en haplotipes statisties betekenisvolle verskille in effek tussen die drie HKM-stigter mutasie groepe getoon. Die HKM-modifiserende gene wat geïdentifiseer is, is verder geprioritiseer vir toekomstige studies volgens die aantal beduidende resultate wat vir die vier assosiasie toetse verkry is. Die gene TPM1 and MYBPC3, wat kodeer vir sarkomeriese proteïene, asook die gene PFKM and PRKAG2, wat kodeer vir metaboliese proteïene, is geïdentifiseer as sterker kandidate vir verdere studies omdat veelvuldige beduidende resultate vir die verskeie statistiese toetse deur hulle gelewer is. Hierdie studie maak ‘n nuwe bydrae tot die veld van hipertrofie navorsing omdat dit die hipotese dat strukturele en energie-verwante faktore, wat binne die sarkomeer geposisioneer is, potensieel as modifiseerders van kardiale hipertropfie in HKM kan optree, ondersoek het. Dit slaag ook daarin om ‘n modifiserende rol vir baie van die geselekteerde kandidaatgene te identifiseer. Die beduidende resultate sluit in aansienlike enkel en binne-geen-konteks variant-effekte en aansienlike variasie in die risiko vir LVH ontwikkeling verskuldig aan die gekombineerde effek van modifiseerder en individuele stigter mutasies. Gesamentlik verbeter hierdie bevindinge die huidige begrip van genotipe/fenotipe korrelasies en dit mag tot gevolg hê verbeterde pasiënt risiko stratifikasie en keuse van behandeling. Verder beklemtoon hierdie bevindinge die potensiaal vir siekte modulering deur verdere uitpluis van sekere van hierdie geïdentifiseerde navorsingsrigtings.

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