Abstract
Evaluation of: Kanasaki K, Palmsten K, Sugimoto H et al. Deficiency in catechol-O-methyltransferase and 2-methoxyoestradiol is associated with pre-eclampsia. Nature 453 (7198), 1117–1121 (2008).
Despite recent advances in the understanding of dysregulation of placenta-derived anti-angiogenic factors in preeclampsia, the etiology and pathogenesis of this disorder remain elusive. Catechol-O-methyltransferase (COMT) generates 2-methoxyestradiol (2-ME) in the human placenta and has been shown to have decreased activity in the placenta in severe preeclampsia. Kanasaki and colleagues performed a series of experiments in a Comt-/- mouse model, showing that decreased COMT activity and subsequent decreased 2-ME levels resulted in the clinical, histologic and molecular changes characteristic of preeclampsia. Furthermore, both the preeclampsia-like clinical features and characteristic angiogenic abnormalities completely resolved in rescue experiments with 2-ME. In addition, circulating levels of 2-ME were lower in eight women with preeclampsia compared with 13 normotensive pregnant controls. These preliminary data suggest that 2-ME may serve both as a marker and as a therapeutic target in preeclampsia; thus, setting the stage for future comprehensive validation studies in larger patient cohorts.
Summary of methods & results
Kanasaki and colleagues performed a series of experiments on pregnant mice to illuminate the role of catechol-O-methyltransferase (COMT) activity and subsequent 2-methoxyestradiol (2-ME) production in the development of preeclampsia Citation[1]. 2-ME is a natural estradiol metabolite, which is elevated in the third trimester of normotensive pregnancies and inhibits HIF-1α. The Comt-/- pregnant mice exhibited decreased levels of 2-ME in comparison to the wild-type mice, and developed several characteristics that mimic human disease.
Most notably, the pregnant Comt-/- mice developed clinical signs that closely resemble human preeclampsia, including increased maternal blood pressure, fetal wastage, preterm delivery and proteinuria. The combined placenta/decidua and embryo weights were lower in the Comt-/- mice than in the wild types. Furthermore, the placentas of the Comt-/- mice had the typical pathologic lesion of preeclampsia: eosinophil-positive deposition in the deciduae, an increased number of thrombotic lesions, suggestive of endothelial damage, and increased markers of hypoxia.
Electron microscopy analysis of the kidneys of the Comt-/- mice showed lesions that are considered typical of preeclampsia: glomerular endothelial cell detachment, swelling and vacuolization that were not present in the wild-type mice or the 2-ME-treated Comt-/- mice.
Administration of 2-ME to the Comt-/-mice resulted in resolution of the clinical, histologic and molecular changes of preeclampsia, suggesting that the specific mechanism through which Comt-/-mice develop preeclampsia is through the failure to generate adequate levels of 2-ME.
Recent studies have provided evidence that preeclampsia is associated with elevated levels of the soluble fms-like tyrosine kinase receptor (sFlt)-1 for VEGF of placental origin. sFlt-1 may bind and neutralize VEGF and, thus decrease free VEGF and PlGF levels that are required for active fetal and placental angiogenesis Citation[2]. The Comt-/- mice had significantly elevated levels of sFlt-1, which was not present in the controls or in the 2 ME-treated Comt-/- mice. Likewise, decreased levels of endothelial nitric oxide synthase have been shown to be present in preeclampsia. Nitric oxide inhibition may not only induce hypertension and proteinuria during pregnancy Citation[3], but may also result in a reversal of angiotensin II resistance, which is characteristic of normotensive pregnancy Citation[4]. The Comt-/- mice had significantly decreased endothelial nitric oxide synthase expression, which was present in normal levels in controls and 2-ME-treated Comt-/- mice. Inflammatory mediators, such as IFN-γ and natural killer cells, were also found to be elevated only in the Comt-/- mice, as previously demonstrated in preeclampsia.
Levels of HIF-1α were also elevated in the placentas of the Comt-/- mice. This was an important finding, as it elucidates a possible mechanism through which inadequate levels of 2-ME may lead to preeclampsia. HIF-1α is a transcription factor that increases the expression of hypoxia-induced genes when exposed to decreased oxygen tension. Kanasaki and colleagues demonstrated that administration of 2-ME to pregnant Comt-/- mice suppressed placental accumulation of HIF-1α, and prevented an increase in sFlt-1 level, thus restoring the angiogenic factor balance. These observations led to the hypothesis that decreased 2-ME levels in preeclampsia may lead to upregulation of HIF-1α, with the consequent increased expression of hypoxia-induced genes in the placenta. The elevation of anti-angiogenic factors ensues, leading to angiogenic dysfunction and placental insufficiency. A pathologic vicious cycle is set in motion: altered COMT function decreases 2-ME levels, causing upregulation of HIF-1α, increasing the expression of hypoxia-induced genes and anti-angiogenic factors, resulting in placental insufficiency. Placental insufficiency results in decreased production of placenta-derived estrogens and hydroxyestradiols (precursors of 2-ME), which, in turn, leads to a further decrease in 2-ME.
To test this hypothesis in human disease, Kanasaki and colleagues determined that circulating levels of 2-ME in women with preeclampsia were lower than in matched normotensive pregnant controls. They also found that the expressions of both the membrane-bound and soluble forms of the COMT protein in placentas of preeclamptic pregnancies were lower than in the controls.
Discussion
There is accumulating evidence suggesting that endothelial dysfunction caused by placental factors that enter the maternal circulation may play a central role in the pathogenesis of preeclampsia. Several studies have provided evidence that an imbalance between pro- (VEGF and PlGF) and anti-angiogenic factors (sFlt-1 and soluble endoglin), in favor of the latter, may lead to endothelial dysfunction in preeclampsia and, ultimately, hypertension and proteinuria Citation[5]. Maynard et al. created a sFlt-1 animal model of preeclampsia, generated by injecting the recombinant adenovirus encoding the murine sFlt-1 gene into pregnant rats, which produced sustained systemic elevations of sFlt-1 Citation[6]. These rats developed hypertension, proteinuria and the renal pathologic changes of preeclampsia (i.e., glomerular endotheliosis). In contrast to previous preeclamptic animal models that showed mainly hypertensive damage Citation[7], the characteristic renal lesion of preeclampsia, namely glomerular endotheliosis, was present in the sFlt-1 model, further supporting the role of sFlt-1 in the pathogenesis of this disorder. Several clinical studies followed, showing that sFlt-1, soluble endoglin and PlGF levels, although particularly high in women with early-onset and severe preeclampsia, displayed significant overlap with the milder forms of preeclampsia closer to full-term and normotensive pregnancies Citation[2,8,9].
The relatively poor sensitivity and specificity of angiogenic markers for the diagnosis of preeclampsia, and the current view that these are released as a result of placental hypoxia, led Kanasaki and colleagues to look for an ‘upstream molecular defect’ that may contribute to the pathophysiology of preeclampsia. They studied 2-ME as a candidate protein, as it has been shown to inhibit HIF-1α, and is generated by COMT in the placenta. Previously, placental COMT activity has been shown to be suppressed in women with severe preeclampsia Citation[10]. By creating the Comt-/- mouse model, Kanasaki and colleagues showed that genetic variants in COMT production and activity may be an upstream molecular defect that leads to decreased 2-ME levels and subsequent preeclampsia.
How do these findings complement or add to our current knowledge of the mechanisms underlying preeclampsia? It appears that COMT and 2-ME deficiencies are indeed upstream events that, through HIF-1α, may regulate angiogenic factors. In human disease, polymorphisms of the COMT gene may have effects on COMT activity and/or protein levels, thus affecting downstream signaling, 2-ME levels and its effects on HIF-1α expression and related pathways. However, is 2-ME deficiency the causative event? The role of inadequate, superficial placentation in the pathogenesis of preeclampsia is widely recognized Citation[11]. This is probably the cause, rather than the effect, of upregulation of HIF-1α and sFlt-1, thus suggesting the presence of additional upstream molecular defects. However, the authors have presented data suggesting that 2-ME may serve both as a marker and as a therapeutic target in preeclampsia. Further studies in a larger sample size are clearly needed. In addition, the mechanisms by which glomerular endotheliosis and proteinuria develop in the Comt-/- mouse model remain unknown. This is a crucial question for studies of preeclampsia, as proteinuria is the feature that distinguishes preeclampsia from other hypertensive disorders, with recent studies suggesting that it may occur in association with podocyturia (i.e., urinary excretion of podocytes and glomerular epithelial cells) Citation[9]. Although endothelial dysfunction is frequently cited as a cause of proteinuria, the signaling mechanisms that underlie this association are poorly understood.
Five-year view
The maternal preeclamptic syndrome is a well-defined phenotype, probably resulting from complex interactions among genetic and environmental factors. Indeed, several epidemiological studies have indicated that genetic factors may contribute to the etiology of preeclampsia. Further research of human Comt genetic polymorphisms could further elucidate the hypothesis that COMT inactivity and subsequent inadequate levels of 2-ME may contribute to preeclampsia. The role of 2-ME as a diagnostic marker of preeclampsia should be evaluated in large clinical studies. From a treatment standpoint, therapeutic trials administering 2-ME to available animal models of preeclampsia may provide an early insight into its role as a possible therapeutic agent.
Key issues
• With a catechol-O-methyltransferase (Comt)-/- mouse model, Kanasaki and colleagues showed that COMT-deficient mice develop the clinical signs, histologic appearance and molecular markers of preeclampsia.
• The clinical signs, histologic appearance and molecular markers of preeclampsia were not present in Comt-/- mice who received exogenous 2-methoxyestradiol (2-ME), specifically illustrating how COMT affects the development of preeclampsia.
• The authors suggest a pathologic spiral of events that leads to the development of preeclampsia: decreased 2-ME leads to less suppression of HIF-1α, which, in turn, increases the expressions of anti-angiogenesis factors. This results in placental pathology and decreased placenta-derived estrogens and hydroxyestradiols, the precursors of 2-ME, thus further decreasing 2-ME production.
• Further research of human Comt genetic polymorphisms may elucidate the hypothesis that COMT inactivity and subsequent inadequate levels of 2-ME may contribute to preeclampsia.
• Future studies should establish the role of 2-ME, both as a marker and a therapeutic target in preeclampsia.
Financial & competing interests disclosure
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
No writing assistance was utilized in the production of this manuscript.
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