Adverse health outcomes in relation to blood pressure and genetic, epigenetic and environmental risk factors

Identification of risk factors is pivotal in disease prevention.  The objectives of this doctoral thesis were to address the contribution of common risk factors to the incidence of adverse health outcomes.  In line with the title of my thesis, I structured my research around three themes: hypertension, genetic and epigenetic variants that potentially carry risk and exposure to environmental pollutants.  The focus on hypertension was obvious in line with the research track of the group that hosted me.  Moreover, hypertension is the world’s number one killer and the leading modifiable cardiovascular risk factor.  I engaged in the research of genetic and epigenetic variation of potentially risk carrying genes in an attempt to translate findings of my basic-science promoters into tangible health outcomes in the population at large.  Over the past decades, the health effects of exposure to environmental pollutants raised high on the research agenda and were a self-evident part of the research theme addressed by my dissertation.  

Resources

Access to the Flemish Study on Environment, Genes and Health Outcomes (FLEMENGHO), a family-based population study running in North Limburg, Belgium, allowed me to assess the incidence of adverse health outcomes in relation to genetic and epigenetic variation in MEOX2/TCF15 and PEAR1 (Chapters 3.1, 3.2 and 3.3) and left ventricular structure and function in relation to environmental pollutants (Chapters 4.1 and 4.2). The International Database of Ambulatory Blood Pressure in Relation to Cardiovascular Outcome (IDACO) includes 13,111 individuals enrolled in 12 countries.  This international resource allowed me to estimate the number of ambulatory blood pressure readings required to assess a person’s blood pressure in a reproducible manner (Chapter 2.1) and to build a hierarchy of information that can be extracted from office and ambulatory blood pressure measurements to refine risk stratification based on blood pressure (Chapter 2.2).  The Study for Promotion of Health in Recycling Lead (SPHERL; NCT02243904) enrolled newly hired workers at lead recycling plants in the United States, who were studied prior to occupational lead exposure.  SPHERL data allowed me exploring the association of ECG voltages with central and peripheral blood pressure (Chapter 2.3) and the correlation of office and ambulatory blood pressure with blood lead at low-level environmental exposure. 

Blood pressure as risk factor

Hypertension guidelines unanimously recommend that ambulatory monitoring is the method of choice to measure a person’s blood pressure.  In Chapter 2.1, using IDACO data, we established that in large clinical and epidemiological studies 8 readings during wakefulness and 4 during sleep are sufficient to estimate an individual’s blood pressure level reliably.  For this conclusion to be reached, we checked three criteria, including a correct diagnosis of the blood pressure level, a correct cross-classification between office and ambulatory hypertension and preservation of accurate risk prediction.  The results of Chapter 2.1 will help researchers to find a balance between the larger number of required ambulatory readings recommended in guidelines and the preservation of valuable information contained in a smaller number of readings.  Chapter 2.2 demonstrated that for total and cardiovascular mortality and for most composite cause-specific cardiovascular endpoints made up of fatal and non-fatal events, nighttime blood pressure as prognosticator outperforms the nocturnal dipping ratio (the night-to-day blood pressure ratio), dipping status, daytime and 24‑h ambulatory pressure and office blood pressure, irrespective of whether the latter is measured conventionally or automatically.  These findings were consistent for systolic and diastolic pressure and for pulse pressure.  In Chapter 3.2, among workers enrolled in SPHERL, ECG voltages were not closer related to central than peripheral ambulatory blood pressure, confirming that emphasis in clinical practice should remain on the accurate assessment of brachial blood pressure rather than the more complex approach required to measure central blood pressure. 

Genetic and epigenetic risk factors

In Chapter 3.1, we related in a general population the incidence of coronary heart disease to genetic variation in MEOX2 and TCF15, two transcription factors that are highly expressed by cardiac endothelium and that in a heterodimeric fashion interfere with cardiac energy metabolism by driving endothelial CD36 and LPL expression, thereby facilitating fatty acid transport across the cardiac endothelium.  In multivariable-adjusted analyses of 2027 Flemish participants with correction for multiple testing, coronary risk was associated with MEOX2 SNPs (P≤0.049) and the MEOX2 GTCCGC haplotype (P=0.0054), but not with TCF15 SNPs (P≥0.29).  The MEOX2 GTCCGC haplotype significantly improved the prediction of coronary heart disease over and beyond traditional risk factors and was associated with a similar population-attributable risk as smoking (18.7% vs. 16.2%).  In the same cohort, the incidence of colorectal cancer (Chapter 3.3), but not cardiovascular disease (Chapter 3.2), was associated with genetic variation in PEAR1.  The encoded protein modulates endothelial proliferation and platelet contact-induced activation and sustains platelet aggregation.  Among 2532 Flemish over 18.1 years of follow-up, the risk of colorectal cancer was associated with genetic variation in PEAR1, as captured by rs12566888, which is in complete linkage disequilibrium (r2=0.99; D’=1.0) with rs12041331, a SNP related to PEAR1 expressions.  Moreover, we identified two CpG sites in the PEAR1 promoter associated with lower risk of colorectal cancer and two with higher risk.  The in silico analysis showed that colorectal-cancer related transcription factors including p53, PAX5 and E2F-1 are potentially involved in the regulation of PEAR1 expression. 

Environmental pollutants as risk factors

In view of the increasing heart failure epidemic and the growing awareness of the adverse health effects of environmental pollution, we searched for association of left ventricular structure and function with fine particulate matter suspended in the air (Chapter 4.1) and with the body burden of lead and cadmium (Chapter 4.2).  In 671 randomly recruited Flemish, annual exposures to black carbon (BC), particulate matter with a diameter of 2.5 and 10 mm (PM2.5, PM10) and NO2 averaged 1.19, 13.0, 17.7, and 16.8 µg/m3.  Systolic left ventricular function was worse (P≤0.027) with higher BC, PM2.5, PM10 and NO2 with association sizes per interquartile interval increment ranging from ‑0.339 to ‑0.458% for longitudinal strain and from ‑0.033 to ‑0.049 s‑1 for longitudinal strain rate.  Mitral E and a’ peak velocities were lower (P≤0.021) with higher BC, PM2.5 and PM10 with association sizes ranging from ‑1.727 to ‑1.947 cm/s and from ‑0.175 to ‑0.235 cm/s, respectively.  In the geographic analysis, the systolic longitudinal strain followed gradients in air pollution.  The path analysis identified systemic inflammation as a possible mediator of associations with BC.  To study lead and cadmium in Chapter 4.2, we included 179 participants randomly recruited FLEMENGHO participants.  The key findings were that higher blood lead concentrations at baseline predicted impaired systolic left ventricular function a decade later and that regional longitudinal strain rate and regional radial strain at follow-up were lower if the baseline 24-h urinary cadmium excretion was higher, whereas diastolic left ventricular function was not associated with exposure to lead or cadmium. 

In view of decreasing lead exposure in the United States and guidelines endorsing ambulatory above office blood pressure measurement, we reassessed in the framework of SPHERL the association of blood pressure with blood lead in 236 newly employed men (mean age, 28.6 years) without previous lead exposure and not treated for hypertension.  
The geometric mean blood lead concentration was 4.5 mg/dL.  On office measurement, blood pressure tended to be higher with elevated blood lead concentration with effect sizes for a doubling of the exposure of around 0.8 mm Hg systolic (P≥0.11) and 0.9 mm Hg diastolic (P≤0.045).  The 24‑h, awake and asleep systolic and diastolic blood pressures in unadjusted and multivariable-adjusted analyses were unrelated to the blood lead concentration.  The risk of office or ambulatory hypertension was not correlated with blood lead.  The white-coat effect explained the significant association of office diastolic blood pressure with blood lead.  Our findings do not support the hypothesis that at current environmental exposure levels in developed countries, lead is a major cause of hypertension. 

Conclusions and perspectives

With its focus on risk factors, including blood pressure, genetic and epigenetic variation and environmental pollutants, this dissertation introduced novel concepts.  Our results could potentially contribute to the refinement of risk stratification and if implemented in guidelines or policy plans might benefit many people.  We determined the minimum number of ambulatory blood pressure readings required to characterise an individual’s blood pressure in a reliable manner.  We showed that among all measures derived from the office and ambulatory blood pressure that the nocturnal blood pressure is the best predictor of cardiovascular complications.  Central compared with brachial blood pressure is not closer related to electrocardiographic left ventricular mass.  Variation in the MEOX2 gene predicts cardiovascular complications.  Genetic and epigenetic variation in PEAR1 predicts colorectal cancer, but not cardiovascular complications.  A subclinical decrease in left ventricular function occurs in response to air pollutant and environmental exposure to lead and cadmium, whereas the blood pressure level is unrelated to blood lead.  Our current findings support hypertension current guidelines positioning ambulatory monitoring of brachial blood pressure as the method of choice for the diagnosis and management of hypertension and endorse the worldwide drive aiming at reducing the exposure of populations to environmental pollutants.