Is Angiotensinogen a Good Candidate Gene for Preeclampsia?

The genetic aspects of pre-eclampsia: achievements and limitations

Pre-eclampsia, a life-threatening disease during pregnancy, is a leading cause of global maternal mortality. Although there is substantial evidence of a genetic background, the complexity of the processes involved and nature of the maternal-fetal phenomenon do not make the search for the causative genes easy. Recent retrospective studies on the subject suggest the heritable allelic variations, particularly the utero-placental renin-angiotensin system with defective placental vascular development, could become the cornerstone for the genetics of pre-eclampsia and hence might well be associated with such defective development. Moreover, the role of immune mechanisms (immune maladaptation) deserves not to be ignored. Large-scale studies entailing genomewide scanning, sib-pair linkage analysis, and family-based association studies with appropriate power to detect genes with a lower relative risk are necessary to understand the puzzle of the disease. Moreover, recently, the importance of epigenetic features and the effect of imprinted genes related to trophoblast growth as well as fetal development on hypertension in pregnancy have been highlighted. All these possibilities are intuitively attractive and are supported by some circumstantial evidence. Although the consistent tenor of a series of papers instill some confidence, we need meticulously designed larger-scale investigations including large numbers of affected women and their babies to provide the analytic stringency essential to study the polygenic multifactorial basis of pre-eclampsia.

Molecular epidemiology of preeclampsia

Numerous articles have been published that address the possible genetic influences on the development of preeclampsia (PE). However, to our knowledge, a complete review of the results has not yet been completed. We undertook a MEDLINE search to identify English-language articles published after January 1, 1990 that examined the possible role of specific genes in the etiology of PE. After a brief introduction and a concise review of the prevailing etiologic hypotheses, we have categorized the candidate genes into six categories, based on their hypothesized role in PE etiology. The purpose of this paper is to review the literature, comment on its quality, and provide a reference for researchers interested in the molecular epidemiology of preeclampsia.

TARGET AUDIENCE

Obstetricians & Gynecologists, Family Physicians.

LEARNING OBJECTIVES

After completion of this article, the reader will be able to list the prevailing etiologic hypotheses of preeclampsia, to outline the published data on possible genetic influence on the development of preeclampsia, and to clearly state the definition of preeclampsia.

Searching for preeclampsia genes: the current position

Although there is substantial evidence that preeclampsia has a genetic background, the complexity of the processes involved and the fact that preeclampsia is a maternal-fetal phenomenon does not make the search for the molecular basis of preeclampsia genes easy. It is possible that the single phenotype 'preeclampsia' in fact should be divided into different sub-groups on genetic or biochemical level. In the present review, the preeclampsia phenotype and its pathophysiologic features are discussed. Family studies and postulated inheritance models are summarized. A systematic overview is given on the numerous candidate gene studies and gene-expression studies performed so far and on the currently available genome-wide scan data. Despite extensive research the molecular genetic basis of preeclampsia remains unclear. Future studies will hopefully enhance our insights in the molecular pathogenesis of preeclampsia.

A genome scan in families from Australia and New Zealand confirms the presence of a maternal susceptibility locus for pre-eclampsia, on chromosome 2

Epidemiological studies have shown that genetic factors contribute to the etiology of the common and serious pregnancy-specific disorder pre-eclampsia (PE)/eclampsia (E). Candidate-gene studies have provided evidence (albeit controversial) of linkage to several genes, including angiotensinogen on 1q42-43 and eNOS on 7q36. A recent medium-density genome scan in Icelandic families identified significant linkage to D2S286 (at 94.05 cM) on chromosome 2p12 and suggestive linkage to D2S321 (at 157.5 cM) on chromosome 2q23. In the present article, the authors report the results of a medium-density genome scan in 34 families, representing 121 affected women, from Australia and New Zealand. Multipoint nonparametric linkage analysis, using the GENEHUNTER-PLUS program, showed suggestive evidence of linkage to chromosome 2 (LOD=2.58), at 144.7 cM, between D2S112 and D2S151, and to chromosome 11q23-24, between D11S925 and D11S4151 (LOD=2.02 at 121.3 cM). Given the limited precision of estimates of the map location of disease-predisposing loci for complex traits, the present finding on chromosome 2 is consistent with the finding from the Icelandic study, and it may represent evidence of the same locus segregating in the population from Australia and New Zealand. The authors propose that the PE/E-linked locus on chromosome 2p should be designated the "PREG1" (pre-eclampsia, eclampsia gene 1) locus.

The eNOS gene: a candidate for the preeclampsia susceptibility locus?

OBJECTIVE

To investigate the endothelial cell nitric oxide synthase (eNOS) gene as a candidate for susceptibility to preeclampsia.

METHODS

Twenty-six Australian families containing 11 eclamptics, 59 severe preeclamptics, and 27 mild preeclamptics were used to test for linkage between the eNOS gene region and preeclampsia. Two microsatellite markers (D7S483 and D7S505) in the proximity of the eNOS gene were used.

MAIN OUTCOME MEASURES

Logarithm of odds (LOD) scores were used to examine the cosegregation of alleles with the disease under a variety of inheritance models. Model-independent analysis, affected pedigree member method (AFFPED), and pairwise haplotype sharing between affected sibs were also used.

RESULTS

Two-point LOD score analysis gave no evidence of linkage between preeclampsia and two markers in close proximity to the eNOS gene (LOD scores < 1) for any of the inheritance models investigated, with no evidence of heterogeneity between pedigrees. The AFFPED and the pairwise haplotype sharing test on affected sibs also gave no evidence of linkage (p-values > 0.05).

CONCLUSION

This study provides no evidence for linkage between two markers in close proximity to the eNOS gene and preeclampsia in these families. These results do not support the recent suggestion that eNOS could be a familial pregnancy-induced hypertension gene (Arngrimsson R, et al., Am J Hum Genet 1997;61:354-62). Distinguishing preeclampsia from other hypertensive disorders in pregnancy is difficult. Hypertension appears to be a consequence, rather than a primary cause of preeclampsia. Given the vasodilatory role of the eNOS gene product, it is possible that the linkage recently reported for eNOS reflects its relationship with hypertension rather than preeclampsia.

Hypertension in pregnancy: new recommendations for management

Hypertension in pregnancy is a frequent complication that has substantial adverse perinatal outcomes. Hypertension may be preexisting (chronic) essential or secondary hypertension; a second entity is pregnancy induced (gestational hypertension, preeclampsia). Recent advances have identified newer markers for pregnancy hypertension: several potential candidate genes may explain the apparent family inheritance of preeclampsia, and some thrombophilic markers have been associated with the condition. Management options for mild to moderate hypertension include a short hospital stay to exclude ongoing severe hypertension and to ascertain fetal well-being. Outpatient care with appropriate maternal and fetal surveillance, including umbilical artery doppler velocimetry, is recommended for better perinatal outcomes. Acute care for severe hypertension includes the use of magnesium sulfate to prevent eclampsia and antihypertensive medication. Expeditious delivery is recommended when the maternal or fetal states cannot be stabilized. Follow-up after delivery allows the uncovering of any other coexisting hypertensive or cardiovascular disorder.

New insights into the genetics of preeclampsia

Preeclampsia is familial. Pedigree analyses suggest that one or more common alleles may act as "preeclampsia susceptibility genes." The authors speculate that genes involved in blood pressure control, volume regulation, placental health, vascular disease, and vascular remodeling, underlie familial susceptibility to preeclampsia. Several candidate genes have been examined. These data suggest that a common mutation in the angiotensinogen promoter, A(-6), leads to elevated expression of this gene and pleiotropic effects, including abnormal spiral artery remodeling and failed hypervolemia of pregnancy. The factor V Leiden mutation, which predisposes women to thromboembolic disorders during pregnancy, has been implicated as another preeclampsia susceptibility gene. New insights into the genetics of preeclampsia will contribute to the understanding of this disease and should ultimately lead to improved diagnosis and treatment.

The immunology of preeclampsia

The immune maladaptation hypothesis of preeclampsia is concordant with cytokine-mediated oxidative stress, chronology of endothelial activation, lipid changes, adverse effect of changing partners, and the protective effect of sperm exposure. Genetic factors may involve underlying hereditary thrombophilic disorders and hyperhomocysteinemia, essential hypertension and/or obesity, or control of the Th1/Th2 balance and thus affect the maternal response against fetal antigens. Placental ischemia and increased syncytiotrophoblast deportation are probably end-stage disease phenomena.

Etiology and pathogenesis of preeclampsia: current concepts

The etiology of preeclampsia is unknown. At present, 4 hypotheses are the subject of extensive investigation, as follows: (1) Placental ischemia-Increased trophoblast deportation, as a consequence of ischemia, may inflict endothelial cell dysfunction. (2) Very low-density lipoprotein versus toxicity-preventing activity-In compensation for increased energy demand during pregnancy, nonesterified fatty acids are mobilized. In women with low albumin concentrations, transporting extra nonesterified fatty acids from adipose tissues to the liver is likely to reduce albumin's antitoxic activity to a point at which very-low density lipoprotein toxicity is expressed. (3) Immune maladaptation-Interaction between decidual leukocytes and invading cytotrophoblast cells is essential for normal trophoblast invasion and development. Immune maladaptation may cause shallow invasion of spiral arteries by endovascular cytotrophoblast cells and endothelial cell dysfunction mediated by an increased decidual release of cytokines, proteolytic enzymes, and free radical species. (4) Genetic imprinting-Development of preeclampsia-eclampsia may be based on a single recessive gene or a dominant gene with incomplete penetrance. Penetrance may be dependent on fetal genotype. The possibility of genetic imprinting should be considered in future genetic investigations of preeclampsia.

Pre-eclampsia--still a disease of theories?

The pathophysiology of pre-eclampsia remains poorly understood. Moreover, there is no reliable predictive test and no effective prophylactic therapy for this disease. Advances have, however, recently been made in our understanding of the genetics of pre-eclampsia and in the processes which lead to abnormal trophoblastic invasion in pre-eclampsia. Prediction and prevention are intimately linked, and both problems will only be solved by further unravelling of the complex pathophysiology of pre-eclampsia.

Angiotensinogen gene variation in a population case-control study of preeclampsia/eclampsia in Australians and Chinese

Preeclampsia/eclampsia (PE/E) is a common disease of human pregnancy with a strong genetic component. The etiology of PE/E is unknown. Two recent reports indicated that the angiotensinogen gene (AGT) could be involved in susceptibility to PE/E. We performed a population-based case-control study in Australian and Chinese populations to investigate whether AGT is a good candidate gene for PE/E. A microsatellite polymorphism within AGT was typed as well as a molecular variant T235 (Met-->Thr) of AGT using allele-specific PCR and allele-induced restriction site PCR. The allele distributions of the microsatellite and the variant T235 of AGT were significantly different between the two ethnic groups. However, no significant allele associations were found with disease when comparing PE/E patients and controls in Australian or Chinese populations, which is in contrast to the two earlier reports. The results suggest that the contribution of AGT to the occurrence of PE/E is small, if anything, and is not constant across populations.

Evidence for a familial pregnancy-induced hypertension locus in the eNOS-gene region

Pregnancy-induced hypertension may be regarded as a manifestation of endothelial-cell dysfunction. The role of the eNOS gene in the development of a familial pregnancy-induced hypertension was evaluated by analysis of linkage among affected sisters and in multiplex families (n = 50). Markers from a 4-cM region encoding the eNOS gene showed distortion from the expected allele sharing among affected sisters (P = .001-.05), and the statistic obtained from the multilocus application of the affected-pedigree-member method also showed distortion (T[f(P)=sqrt(P)] = 3.53; P < .001). A LOD score of 3.36 was obtained for D7S505 when a best-fitting model derived from genetic epidemiological data was used, and LOD scores of 2.54-4.03 were obtained when various other genetic models were used. Estimates of recombination rate, rather than maximum LOD-score values, were affected by changes in the genetic parameters. The transmission-disequilibrium test, a model-free estimate of linkage, showed strongest association and linkage with a microsatellite within intron 13 of the eNOS gene (P = .005). These results support the localization of a familial pregnancy-induced hypertension-susceptibility locus in the region of chromosome 7q36 encoding the eNOS gene.