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Homocysteine and the very elderly: is there a message for cardiovascular risk?

Delles, Christian

doi: 10.1097/HJH.0b013e328360aa8b
Editorial Commentaries

Institute of Cardiovascular & Medical Sciences, University of Glasgow, Glasgow, UK

Correspondence to Dr Christian Delles, BHF Glasgow Cardiovascular Research Centre Institute of Cardiovascular & Medical Sciences, University of Glasgow, 126 University Place, Glasgow, G12 8TA Scotland, UK. Tel: +44 141 330 2749; fax: +44 141 330 7394; e-mail: Christian.Delles@glasgow.ac.uk

Cardiovascular risk prediction has seen important changes in recent years. First, the concept of overall cardiovascular risk has revolutionized our approach to the patient. Research in the larger part of the last century was driven by discovery of the ‘traditional’ risk factors, including hypertension, dyslipidaemia, smoking and diabetes, and a unimodal strategy to target these factors individually. The more recent years, however, have taught us that it not a single risk factor but a holistic view of the combination and interaction of risk factors that provides better estimates and has the potential to drive individualized treatment decisions [1]. Second, markers of subclinical cardiovascular damage have been established that modify an individual's cardiovascular risk. These include markers of renal dysfunction such as estimated glomerular filtration rate and microalbuminuria and markers of cardiac damage such as left ventricular hypertrophy. Presence of subclinical organ damage, and even more so, presence of diabetes or overt cardiovascular disease such as myocardial infarction or stroke, require more stringent control of risk factors [1]. Third, our understanding of the development of cardiovascular disease has improved dramatically. We now recognize the eminent role of inflammation, oxidative stress, apoptosis and extracellular matrix remodelling in the pathophysiology of vascular disorders, but we also understand that traditional risk factors such as hypercholesterolaemia need to be dissected in more detail in order to fully acknowledge their involvement in the disease processes [2].

Our better insights into the pathophysiology, however, have not yet really translated into risk prediction. It is for example well established that vascular diseases are characterized by inflammation of the vessel wall [3], and data from the last few years have revolutionized our understanding of the links between the immune system and the development of hypertension [4]. There is a huge body of evidence suggesting greater cardiovascular risk even if levels of C-reactive protein, a marker of inflammation, are only mildly elevated [5], and robust data also exist for other inflammatory markers. Except for a few examples in which C-reactive protein has been used to reclassify cardiovascular risk [6], current guidelines do, however, not include an assessment of markers of inflammation for cardiovascular risk prediction. This is because the contribution of these ‘emerging’ factors to overall risk is small compared with the eminent contribution of the ‘traditional’ risk factors.

Homocysteine is another emerging risk factor that has been subject to numerous studies in recent years. Homocysteine is a homologue of the amino acid cysteine but is not a building block of proteins. It is synthesized from methionine and can be recycled to methionine with the help of the B-vitamins pyridoxine (B6), folic acid (B9) and cobalamine (B12). Vitamin B deficiency is probably the most common reason for hyperhomocysteinaemia in the population. Homocysteine has the potential to degrade cysteine disulphide bridges and thereby to change protein structure and function. In particular in proteins with slow turnover, such as vascular collagens and elastin, homocysteine can cause a gradual degradation and hence change the architecture of the vessel wall. Homocysteine has been found to be associated with an increased risk of ischaemic heart disease and stroke [7].

In the current issue of Journal of Hypertension, van Dijk et al.[8] report the cross-sectional results of the B-vitamins for the Prevention of Osteoporotic Fractures (B-PROOF) study. This multicentre, randomized, double-blind trial of a combination of folic acid and cobalamin vs. placebo is currently being conducted in a total of 2919 elderly people with hyperhomocysteinaemia. The primary outcome will be osteoporotic fractures [9]; osteoporotic fracture is another condition related to high homocysteine levels. In the present article, the authors studied the association between homocysteine levels and a range of markers of arterial function and structure including aortic pulse wave velocity (aPWV), carotid distensibility and compliance, and central augmentation index at the baseline study visit in a subgroup of 560 individuals. aPWV is a direct marker of arterial stiffness and an independent predictor of cardiovascular morbidity and mortality [10]. van Dijk et al.[8] demonstrate a direct correlation between homocysteine levels and aPWV in their study cohort. On further analysis, they found that the overall correlation was driven by a statistically significant correlation in men, whereas in women this relationship was not statistically significant. Another modifier of the correlation was age wherein the strongest relationship between homocysteine and aPWV was found in the oldest participants. No statistically significant correlations were observed between homocysteine levels and carotid compliance or distensibility and central augmentation index.

In brief, van Dijk et al.[8] show that aPWV was highest in very old men with the highest homocysteine levels. This is probably not surprising, but the independence of the association between homocysteine and aPWV of other cardiovascular risk factors in this group points towards a direct effect of homocysteine on vascular stiffness that could be responsible for the increased cardiovascular morbidity and mortality associated with hyperhomocysteinaemia. Pathophysiologically, it is plausible that long-term exposure to high homocysteine levels affects arterial elasticity by changing the collagen and elastin content of the arterial wall and thus increases vascular stiffness and thereby aPWV. The authors provide convincing data to support this hypothesis.

There are, however, a few findings in the article by van Dijk et al.[8] that cannot be explained easily, the most striking of which is the absence of a modulating effect of homocysteine on aPWV in women. One could speculate about differences in homocysteine levels and in the vascular phenotypes between men and women in this study cohort. Other factors, during the premenopausal state, may have protected the vasculature for many decades in life so that potential homocysteine-related degradation of the vascular matrix may have started later in women than in men. The authors pick some of these arguments up in their discussion, but clearly in an observational study, such explanations remain entirely speculative. Striking is also the absence of correlations between homocysteine levels and the other markers of arterial function and structure in this study. van Dijk et al.[8] present a number of potential explanations, for example the different architecture of the carotid artery compared with the abdominal aorta, but there may be additional reasons to explain these discrepancies. One reason could be related to technical issues wherein despite the excellent phenotypic quality in the B-PROOF study, there may still be challenges with accurate measurement of carotid phenotypes or with performing arterial applanation tonometry, whereas the assessment of aPWV, although still technically difficult, may be more accurate and reproducible. The other reason could be that it is not the absence of correlations with other vascular phenotypes that is wrong but the presence of an association with aPWV, that is that the observed relationship is a false-positive finding. The scatterplots and the multivariate analyses make this unlikely, but of course, only confirmation in other cohorts will provide complete reassurance.

What makes the study by van Dijk et al.[8] interesting is the possibility to look at an elderly population in which there may have been sufficient time for homocysteinaemia to cause harm to the vessels, and the thorough investigation of the vascular phenotype. Some of the interpretation of the data relies on the assumption that participants had hyperhomocysteinaemia for a longer period of time prior to inclusion in the study, but there are no data available to support this assumption. The critical reader may therefore ask what we can learn from the fact that men at an age of 77–98 years with very high homocysteine levels (median in the whole cohort, 14.2 μmol/l, but the highest homocysteine tertile had 15.7–49.0 μmol/l) have a very high aPWV. One could even argue that these particular participants’ age is greater than the mean age of the male population in most Western societies, despite their high aPWV. This view may be short-sighted. It is often from the extremes of the phenotypic variation that we learn most about the pathophysiology of disease. The observation in the present study shows that it requires time and a risk factor (homocysteine), possibly together with another risk factor (male sex) to lead to specific vascular damage. The message of van Dijk et al. [8] is not that homocysteine is a marker of risk in the very elderly but that damage accumulates over a long period of time until it can be assessed with crude techniques such as aPWV.

This is also an important message for any attempts to interfere with the adverse effect that homocysteine may have on vascular structure. With an effect that can be seen most convincingly in the oldest tertile of the present study cohort, it would be unrealistic to expect effects of short or medium-term vitamin B replacement on cardiovascular events. In fact, studies have shown B vitamins to not reduce cardiovascular events in patients with vascular disease or in patients with renal failure followed-up for 5 or 3.6 years, respectively [11,12]. One would expect from the study by van Dijk et al.[8] that much longer treatment is required aiming at lower homocysteine levels over a lifetime. It has also been proposed that preventive homocysteine-lowering therapy in people without overt cardiovascular disease may be a more promising approach compared to homocysteine-lowering therapy in people in whom vascular damage is already present. One can, however, also conclude that aPWV may be a surrogate parameter for the assessment of treatment benefits in future studies. The same principles may apply to other ‘emerging’ cardiovascular risk factors in which the effect size is weaker than that of ‘traditional’ risk factors and evidence from intermediate phenotypes such as aPWV may be all we can get within a reasonable period of time.

The B-PROOF study is a prospective clinical trial that will also look at markers of vascular structure and function at the 2-year follow-up visit. From the present data, it is unlikely that there will be major changes in aPWV in response to vitamin B treatment in a relatively short period of time, but the scientific community and especially the affected participants would of course hope to be positively surprised.

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ACKNOWLEDGEMENTS

Conflicts of interest

C.D. is supported by the European Commission collaborative project ‘EU-MASCARA’.

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