Sex matters to the arteries: studies into the (epi)genetic background and clinical outcome of atherosclerotic disease in women and men

The higher incidence of atherosclerotic disease at a younger age in men has directed most cardiovascular research since the early 1980s towards men. Yet, if studied, sex-differences are found in etiology, diagnostics, therapy and prognosis of CVD. It has been suggested that sex hormone status explains the differences between men and women and their progression to CVD. Moreover, sex chromosomes are beginning to be recognized as important players in sex differences in disease development, independent of sex hormones.

This thesis describes studies into genetics and epigenetics of the atherosclerotic plaque, its histological characteristics and clinical outcome of atherosclerotic disease, mainly focusing on sex-differences. In most studies, atherosclerotic plaques from the Athero-Express Biobank, a biobank consisting of atherosclerotic plaque specimens of around 2300 carotid and 1000 (ilio)femoral arteries were used.

A positive association was found between estradiol, the most abundant female sex-hormone, and the amount of intraplaque microvessels and macrophages. Female plaques generally display features that are associated with stable plaque characteristics, such as a large amount of collagen and smooth muscle cells. Although preliminary, these findings could thus point towards a more complex pathophysiological mechanism of the plaque erosion that is often observed in young women.

Genetic variation on the Y chromosome (haplogroups) and its association with characteristics of the atherosclerotic plaque and aneurysmal artery wall was studied. No association was found, nor a different distribution of the haplogroups in these cohorts when compared with a Dutch cohort from the general population. The results make (causal) involvement of the Y chromosome in atherosclerotic disease less likely. However, an association was found between Y chromosomal loss and secondary cardiovascular events during 3-year follow-up. How Y chromosomal loss affects the development of cardiovascular events remains elusive.

Across the genome, many sex-differences were identified in DNA methylation of the atherosclerotic plaque and it was observed that most of them were not associated with atherosclerotic disease risk factors or characteristics of the atherosclerotic plaque. Not many DNA methylation studies investigated diseased tissue. These findings emphasize the need for stratification by sex in epigenetic studies.

Mortality rates after coronary artery bypass grafting surgery between women and men were studied in the IMAGINE trial. A poorer survival for women in univariable analysis, but not in multivariable analysis, was observed. This effect has been observed also in other coronary artery bypass grafting cohorts and warrants further study, preferably in a much larger number of women. When studying long-term follow-up of both carotid and femoral endarterectomy in the Athero-Express Biobank, sex-differences were observed in survival after carotid endarterectomy surgery, where women displayed an advantage over men and followed the survival curve of the cohort of age-matched women from the general population. However, such a sex-difference was not observed after femoral endarterectomy surgery.

Other studies within this thesis include the identification of novel candidate genes using 4C-sequencing of cardiovascular disease associated genetic loci, a review on the poor translatability of murine atherosclerosis genes to humans and time-dependent changes and associations with diabetes mellitus in ilio-femoral atherosclerotic plaque composition.