Epigenetics: Is mode of conception a marker for future cardiovascular risk?

Assisted reproduction technologies (ART) although available worldwide, have not yet resulted in widely available databases that permit analysis of the health of children resulting from these advances. There is limited statistical evaluation of relative health (other than congenitally evident anomalies) of this population when compared to children not born of ART. In today’s journal, an evaluation of the vascular systems of a small number of patients born as a result of IVF demonstrates that “vascular aging”, first identified at a time approximating onset of puberty may precede elevated blood pressure identified in adolescence. The authors are to be commended for recruiting approximately 75% of the previously described cohort and restudying them 5 years later (1). Variables measured included flow mediated dilation, intimal media thickness, pulse wave velocity, and ambulatory 24-hour blood pressure recordings. The current study’s purpose was to determine if the vascular dysfunction initially reported had downstream consequences on development of subsequent hypertension in a population conceived of ART.

There are ever increasing numbers of children conceived by ART since its development in 1978. We are only now beginning to see the effects of such advances on population statistics. Today, approximately 1.7% of all infants born in the United States every year are conceived using ART (1). Older maternity may alter maternal morbidity and mortality, but also effect childhood health and development. Based on the Center for Disease Control’s Fertility Clinic Success Rates Report, among 463 reporting clinics in the United States, there were 76,930 live born infants conceived with the help of ART during 2016. Indeed, worldwide there are currently over 6 million born of assisted reproduction worldwide. And the use of ART has doubled over the past decade.

It is estimated from limited data that rates of hypertension in US adolescents are approximately 3.5% (2). The major finding of the current study was that prevalence of criteria for hypertension, based on a single 24-hour ambulatory blood pressure monitor (ABPM), were met in 1/43 (2.3%) adolescent participants in the control group, and 8/52 (15.4%) participants born of ART (Fischer’s Exact test, p-value =0.04). The mean 24-hour systolic and diastolic blood pressure on ABPM was approximately 4 mmHg and 2 mmHg higher, respectively, among adolescents born of ART as compared to healthy controls. This observation, derived from a relatively small cohort, may actually understate the importance of this problem for ART populations because higher risk populations for development of hypertension such as multiple birth pregnancies, and those resulting from maternal factors of excess (eclampsia, chronic hypertension, diabetes or obesity) risk were excluded from the study.

The population studied represents descendants of a relatively youthful cohort (mean maternal age <30 years, BMI<23 kg/m2, with a low prevalence of maternal hypertension, smoking and diabetes). This population may not represent populations undergoing ART today (in some societies child rearing is voluntarily delayed, cryopreservation is more likely to be used, obesity more likely, or ART commenced after immunosuppressive or antineoplastic therapies responsible for infertility). These expanded indications for ART may be associated with even greater risk for vascular aging in subsequent generations. If adolescent hypertension risk is really six-fold higher in ART patients (and potentially subsequent generations) consequences for longevity will be vast given the millions of patients whose births were permitted by ART methods. Early study, detection, and treatment of ART conceived individuals may be the appropriate ounce of prevention.

In vitro or extra-uterine fertilization exposes gamete and embryo to variable physical and hormonal environments before implantation. Exposure is brief in fresh fertilizations and more prolonged in gamete or embryo cryopreservation, and such epigenetic modifications of the environment may have downstream effects on the vascular system. Estradiol alone has a dose related effect on vascular function and hormonal support of pregnancy may have a long-term impact on vascular structure and function. Laboratory animal research and human study evaluations have demonstrated disorders of DNA methylation levels and imprinting associated with extra-uterine fertilization (3–6). For example, epigenome-wide scans of methylation of DNA derived from cord blood demonstrate hypomethylation globally (7) and relative hypermethylation at CpG islands (8), which are often found in promoters and other regulatory regions of the genome. The impact of the epigenome on later-life blood pressure was recently assessed in a meta-analysis of over 17,000 adult participants of community-based cohorts. (9) Thirteen replicated differentially methylated loci were identified in DNA derived from whole blood leukocytes in relation to blood pressure. Computational approaches to identify causal contributors to elevated blood pressure implicated methylation and expression of TSPAN2, a protein involved in signal transduction, linked to the contractility and differentiation of vascular smooth muscle cells. These results support the premise that epigenetic changes can influence later life blood pressure regulation.

If alteration of temperature of gametes is associated with more long-term vascular stress, it is possible that procedure modification could reduce epigenetic changes without lowering success rates. This reported cohort is too small to test hypotheses that altering the physical environment of ART (fresh vs. frozen embryo transfer) might possibly prevent later vascular dysfunction. 80% of the ART births in this study resulted from fresh embryo transfer rather than freezing, and no comparison has been made as to whether embryo or gamete freezing might create a milieu that leads to endothelial disruption. One could hypothesize that normal conception occurring within a relatively uniform intrauterine environment may be disturbed by temperature variations leading to endothelial dysfunction of the conceptus. These questions cannot be answered without larger cohorts and require large-scale registries or controlled ART trials. A study of this size will not lead to changes in ART.

Genetic studies at some centers and enrollment in procedural studies in France [Study of the Impact of Freezing-thawing Procedures and the Prolonged Culture of Embryos on Epigenetic Regulation in Humans ClinicalTrials.gov Identifier: NCT02171884] or cross sectional follow ups in Australia may help answer questions raised by the current study. (10) This study requires confirmation and should stimulate additional research to determine whether growing populations, born of advances in technology is subject to previously unacknowledged requiring early detection.

While six-fold higher hypertension prevalence in the ART-conceived group as compared to the control group is striking, it is defined only by a single 24-hour ABPM without office blood pressure or repeat measurement. Whether these elevated ABPM values represent masked hypertension or would regress to the mean with a repeat assessment is unknown. In addition, no evidence of target end-organ damage, such as left-ventricular hypertrophy was demonstrated. The pediatric hypertension clinical practice guidelines, (11) recommend annual office-based hypertension screening for all children after 3 years of age. For certain high-risk subgroups (e.g. those with chronic kidney disease or repaired aortic coarctation), screening is recommended at every health encounter. Despite limitations in the current study, it appears reasonable to be more vigilant for development of elevated blood pressure among children and adolescents conceived with ART in order to implement early lifestyle-based modifications and if necessary, pharmacotherapy. In addition, a suspicion of long-standing changes should be maintained for adults conceived with ART and found to be hypertensive to lead to assessment for left ventricular hypertrophy or dysfunction.

This study demonstrates that children conceived by ART are more likely to develop vascular structure and function changes with a higher prevalence of hypertension on a single measure in adolescence than a matched cohort who are products of natural conception. Further studies are needed to determine the potential epigenetic-mediated basis for this phenomenon as it may reveal novel mechanisms and therapeutic targets. Clinicians should consider adding information about mode of conception to the electronic health record in order to track patients at potential risk and implement appropriate monitoring. The future of precision medicine guided approaches should incorporate emerging clinical factors, such as history of ART conception in a growing proportion of the population, in addition to an array of novel molecular markers.