Cystatin C and the relation to cardiovascular disease : studies on the relative importance of genetic and environmental factors

Abstract

Various manifestations of atherosclerotic cardiovascular disease are a common clinical problem affecting millions of people each year and the prevalence increases globally. This strengthens the incentives to improve the tools for primary and secondary prevention. Cystatin C is, besides its role as a marker of renal function, a promising biomarker of vascular damage and vascular disease, which might add value to risk prediction in cardiovascular burdened individuals as well as in persons free of cardiovascular disease. Atherosclerotic cardiovascular diseases are polygenic disorders, and disease development is a function of complex relationships between several environmental factors and multiple genetic variations. The relative impact of genes and environment on the variations in, biomarkers such as cystatin C and creatinine and on their association to cardiovascular disease (CVD), is not well-known and requires further studies. The overall aim of this thesis has been to study the relation between cystatin C and cardiovascular disease, to investigate if it may be used as a biomarker for atherosclerotic disease and if it could lead to earlier identification of patients at particularly high risk. In addition, we investigated the heritability of cystatin C and its relation to heritability of cardiovascular disease. Finally we studied the predictive value of cystatin C for incident CVD when controlled for genetic confounding in twin studies.

Material and methods. This thesis is based on studies in two different study groups. The first consists of elderly men with peripheral arterial disease (PAD) (n103=) and matched controls (n=96) and the second is a population based cohort of elderly Swedish twins of both sexes (n=12313). Initially we performed a cross-sectional study in which we investigated differences in cystatin C-levels between PAD-patients and matched controls. We further studied the predictive value of cystatin C with regards to secondary cardiovascular events in the same group. In the twin study group we investigated the heritability of cystatin C and prevalent CVD using a structured equation model (SEM) followed by a genome-wide complex trait analysis (GCTA). Finally the predictive ability of cystatin C for incident atherosclerotic cardiovascular disease (ASCVD) during a follow-up of 71 month was studied in an adjusted Cox-regression model in twins free from CVD at baseline (n= 11402). Twin pairs discordant for incident ASCVD during follow up were identified and within-pair comparisons regarding cystatin C and creatinine levels were performed.

Results. We observed that cystatin C-levels were higher in PAD patients compared to healthy controls even when corrected for differences in eGFR, IL-6 and CRP. In follow-up we could not establish cystatin C as a predictive marker of incident cardiovascular events in patients with manifest PAD, however we unexpectedly found a U-shaped relation between tertiles of cystatin C-concentration and outcome. Further we observed a higher heritability of cystatin C compared with previous studies, for which the GCTA analysis provided independent evidence. We also observed a significant genetic correlation between levels of cystatin C and CVD. Lastly we showed that cystatin C was a predictor for incident myocardial infarction (MI) and stroke. The association for stroke was higher than for MI, and that cystatin C remained a predictor for incident stroke after adjustment for genetic confounding.

Conclusion. Cystatin C is associated to atherosclerotic disease. The covariation between cystatin C and CVD in males indicates that cystatin C and CVD share genetic influences. Variation in cystatin C is associated with incident myocardial infarction and stroke independent of traditional risk factors, with a stronger association to stroke. The finding that cystatin C is related to incident stroke in disease-discordant identical twins indicates that individual specific environmental factors are important. One possible explanation is that cystatin C may be a sensitive marker of early hypertensive end organ damage. It could be of value to expand the usage of cystatin C beyond renal medicine and include it as a tool in the arsenal for cardiovascular risk stratification. However, further research is needed.