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

Genetics of preeclampsia: what are the challenges?

Despite recent efforts to identify susceptibility genes of preeclampsia, the genetic determinants of the condition remain ill-defined, as is the situation for most disorders of complex inheritance patterns. The angiotensinogen, factor V, and methylenetetrahydrofolate reductase genes have been investigated in different populations, as have other genes involved in blood pressure, vascular volume control, thrombophilia, lipid metabolism, oxidative stress, and endothelial dysfunction. The study of the genetics of complex traits is faced with both methodological and genetic issues; these include adequate sample size to allow for the identification of modest genetic effects, of gene-gene and gene-environment interactions, the study of adequate quantitative traits and extreme phenotypes, haplotype analyses, statistical genetics, genome-wide (hypothesis-free) versus candidate-gene (hypothesis-driven) approaches, and the validation of positive associations. The use of genetically well-characterized populations showing a founder effect, such as the French-Canadian population of Quebec, in genetic association studies, may help to unravel the susceptibility genes of disorders showing complex inheritance, such as preeclampsia. It is necessary to better evaluate the role of the fetal genome in the resulting predisposition to preeclampsia and its complications. Eventually, we may be able to integrate genetic information to better identify the women at risk of developing preeclampsia, and to improve the management of those suffering from this condition.

Pre-eclampsia: maternal risk factors and perinatal outcome

OBJECTIVE

The aim of this study was to throw light on the incidence of pre-eclampsia (PE) in women attending for care and delivery at a hospital in Saudi Arabia, and analyze the maternal risk factors and outcome of mothers and neonates in pregnancies complicated by PE.

METHODS

This retrospective study involved almost all women (n = 27,787) who delivered at King Fahad Hospital of the University in a 10-year period (1992-2001). The maternal records were reviewed for age, parity, gestational age, mode of delivery, antenatal care, onset of PE, severity of proteinuria, and the frequency of antenatal and intrapartum complications. The neonatal records were reviewed for perinatal outcome including birth weight, frequency of stillbirths, and neonatal deaths.

RESULTS

Among the study cohort of pregnancies, 685 women, i.e. 2.47%, were diagnosed as having PE among whom a high proportion (42.0%) were nulliparous women. Similarly, PE was encountered at a high percentage (40.0%) in women at the extreme of their reproductive age (< 20 and >40 years), and more women with PE delivered prematurely (30.2%) as compared to healthy controls (13.5%). Spontaneous vaginal deliveries were less frequent in women with PE (69.2%) as compared with healthy controls (86.2%). Instrumental deliveries, with spontaneous labor, amounted to 15.9% in women with PE, but they comprised only 2.9% in healthy women. The deliveries were more likely to be induced (22.8%) or be performed by cesarean section (14.9%) in women with PE than in healthy controls (6.8% and 9.6%). Placental abruption was the most common maternal complication (12.6%) in women with PE, followed by oligouria (7.9%), coagulopathy (6.0%), and renal failure (4.1%). The perinatal outcome of pregnancies with PE shows that stillbirths (2.34%) and early neonatal deaths (1.02%) comprised an overall mortality rate of 33.6 per 1,000. More stillbirths and neonatal deaths showed a tendency to be associated with the severe form of PE (diastolic BP > or =120), as compared with the mild form (diastolic BP 90-110). Stillbirths and neonatal deaths appear to be associated with women who had no or irregular antenatal care and whose proteinuria amounted to or exceeded 3 g per 24 h, when delivery occurred at 28th gestational week or less, and when the birth-weight of the neonates was between 500 and 1,000 g.

CONCLUSION

We document a hospital-based incidence rate of PE of 2.47%, with a high proportion of PE cases occurring among nulliparous women and those at the extreme ends of the reproductive age. More maternal and neonatal complications were encountered in women with PE when the PE was severe, when the pregnancy had to be terminated early, when there was no regular antenatal care, the birth-weight was low, or the proteinuria was severe.

Trophoblastic remodeling in normal and preeclamptic pregnancies: implication of cytokines

OBJECTIVE

To summarize the recent knowledge on the implications of placenta and cytokines in normal and preeclamptic pregnancies.

DATA SOURCES

A literature search was conducted of applicable articles related to interactions between trophoblast and cytokines in generating preeclampsia.

CONCLUSIONS

The initiating event in preeclampsia has been postulated to be the reduced uteroplacental perfusion as a result of abnormal extravillous cytotrophoblast invasion and remodeling of the uterine spiral arteries. Focal ischemia and hypoxia, deportation of hypoxemic trophoblast cells and abnormal expression of various placental biologic molecules, particularly the cytokines, are thought to lead to widespread dysfunction of the maternal vascular endothelium resulting in overproduction of endothelin and thromboxane, enhanced vascular sensitivity to angiotensin II, and reduced secretion of vasodilators such as nitric oxide and prostacyclin. These alterations, in turn, cause hypertension, proteinuria and edema, and pathologies in many organ systems (kidney, lung, liver, brain).

Genome-wide mapping of human loci for essential hypertension

BACKGROUND

Blood pressure may contribute to 50% of the global cardiovascular disease epidemic. By understanding the genes predisposing to common disorders such as human essential hypertension we may gain insights into novel pathophysiological mechanisms and potential therapeutic targets. In the Medical Research Council BRItish Genetics of HyperTension (BRIGHT) study, we aim to identify these genetic factors by scanning the human genome for susceptibility genes for essential hypertension. We describe the results of a genome scan for hypertension in a large white European population.

METHODS

We phenotyped 2010 affected sibling pairs drawn from 1599 severely hypertensive families, and completed a 10 centimorgan genome-wide scan. After rigorous quality control, we analysed the genotypic data by non-parametric linkage, which tests whether genes are shared in excess among the affected sibling pairs. Lod scores, calculated at regular points along each chromosome, were used to assess the support for linkage.

FINDINGS

Linkage analysis identified a principle locus on chromosome 6q, with a lod score of 3.21 that attained genome-wide significance (p=0.042). The inclusion of three further loci with lod scores higher than 1.57 (2q, 5q, and 9q) also show genome-wide significance (p=0.017) when assessed under a locus-counting analysis.

INTERPRETATION

These findings imply that human essential hypertension has an oligogenic element (a few genes may be involved in determination of the trait) possibly superimposed on more minor genetic effects, and that several genes may be tractable to a positional cloning strategy.

Microsatellite marker association at chromosome region 2p13 in Finnish patients with preeclampsia and obstetric cholestasis suggests a common risk locus

The pathophysiology of preeclampsia is incompletely understood, but the familial nature of the disease has long been recognized. Recent genome-scan studies have indicated linkage at the p23 region of chromosome 2. We have previously reported microsatellite marker association at chromosome region 2p13 in patients with obstetric cholestasis. We conducted population-based association screening with microsatellite markers to find potential preeclampsia-associated loci on chromosome region 2p13-p12 and to test whether preeclampsia and obstetric cholestasis share a single risk locus. The study was carried out among 115 unrelated control women, 133 preeclamptic women and 57 cholestatic women. Screening with microsatellite markers at the 2p13-p12 region revealed that the marker D2S286 was significantly associated with obstetric cholestasis in the overall association analysis (P=0.03), while it revealed only borderline association with preeclampsia (P=0.08). However, single allele association analysis indicated that both preeclampsia and obstetric cholestasis showed a statistically significant association with a common allele (P < 0.05), which was overrepresented in both the obstetric cholestasis (0.42) and preeclamptic (0.37) groups when compared with the control group (0.28). In conclusion, These findings suggest a possible genetic link between chromosome region 2p13-p12, preeclampsia and obstetric cholestasis. More specifically, these data suggest that there may be a common risk locus associated with both obstetric complications located in the vicinity of the 2p13-p12 association region.

Obstetric prognosis in sisters of preeclamptic women - implications for genetic linkage studies

BACKGROUND: To investigate obstetric prognosis in sisters of preeclamptic women. METHODS: We identified consecutive 635 sib pairs from the Birth Registry data of Kuopio University Hospital who had their first delivery between January 1989 and December 1999 in our institution. Of these, in 530 pairs both sisters had non-preeclamptic pregnancies (the reference group), in 63 pairs one of the sisters had preeclampsia and the unaffected sisters were studied (study group I). In 42 pairs both sister's first delivery was affected (study group II). Pregnancy outcome measures in these groups were compared. RESULTS: Unaffected sisters of the index patients had uncompromised fetal growth in their pregnancies, and overall, as good obstetric outcomes as in the reference group. The data on affected sisters of the index patients showed an increased prematurity rate, and increased incidences of low birth weight and small-for-gestational age infants, as expected. CONCLUSION: Unaffected sisters of the index patients had no signs of utero-placental insufficiency and they were at low risk with regard to adverse obstetric outcome, whereas affected sisters were high-risk. Clinically, affected versus unaffected status appears to be clear-cut in first-degree relatives regardless of their genetic susceptibility and unaffected sisters do not need special antepartum surveillance.

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.

Susceptibility loci for preeclampsia on chromosomes 2p25 and 9p13 in Finnish families

Preeclampsia is a common, pregnancy-specific disorder characterized by reduced placental perfusion, endothelial dysfunction, elevated blood pressure, and proteinuria. The pathogenesis of this heterogeneous disorder is incompletely understood, but it has a familial component, which suggests that one or more common alleles may act as susceptibility genes. We hypothesized that, in a founder population, the genetic background of preeclampsia might also show reduced heterogeneity, and we have performed a genomewide scan in 15 multiplex families recruited predominantly in the Kainuu province in central eastern Finland. We found two loci that exceeded the threshold for significant linkage: chromosome 2p25, near marker D2S168 (nonparametric linkage [NPL] score 3.77; P=.000761) at 21.70 cM, and 9p13, near marker D9S169 (NPL score 3.74; P=.000821) at 38.90 cM. In addition, there was a locus showing suggestive linkage at chromosome 4q32 between D4S413 and D4S3046 (NPL score 3.13; P=.003238) at 163.00 cM. In the present study the susceptibility locus on chromosome 2p25 is clearly different (21.70 cM) from the locus at 2p12 found in an Icelandic study (94.05 cM) and the locus at 2q23 (144.7 cM) found in an Australian/New Zealand study. The locus at 9p13 has been shown to be a candidate region for type 2 diabetes in two recently published genomewide scans from Finland and China. The regions on chromosomes 2p25 and 9p13 may harbor susceptibility genes for 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.

Paternal age and preeclampsia

BACKGROUND

Paternal aging is associated with premeiotic damage to spermatogonia, a mechanism by which new point mutations are introduced into the gene pool. We hypothesized that paternal age might contribute to preeclampsia.

METHODS

We studied the incidence of preeclampsia in 81,213 deliveries surveyed in 1964-1976 in the Jerusalem Perinatal Study. We controlled for maternal age, parity and other risk factors using logistic regression.

RESULTS

Preeclampsia was reported in 1303 deliveries (1.6%). Compared with fathers age 25-34 years, the odds ratios (ORs) for preeclampsia were 1.24 (95% confidence interval = 1.05-1.46) for age 35-44 and 1.80 (1.40-2.31) for age 45+. For fathers age <25, the OR was 1.25 (1.04-1.51). Although weaker than maternal age effects, paternal effects were consistent within subgroups of other variables.

CONCLUSIONS

These findings support the hypothesis that a modest proportion of preeclampsia might be explained by new mutations acquired from fathers and add to a growing body of evidence for paternal age effects in birth defects, neuropsychiatric disease and neoplasia.

Two exonic single nucleotide polymorphisms in the microsomal epoxide hydrolase gene are jointly associated with preeclampsia

This study determined whether genetic variability in exons 3 and 4 of the microsomal epoxide hydrolase gene jointly modifies individual preeclampsia risk. The study also determined whether genetic variability in the gene encoding for microsomal epoxide hydrolase (EPHX) contributes to individual differences in susceptibility to the development of preeclampsia. The study involved 133 preeclamptic and 115 healthy control pregnant women who were genotyped for two single nucleotide polymorphisms (SNPs), T-->C (Tyr113His) in exon 3 and A-->G (His139Arg) in exon 4, in the EPHX gene. Chi-square analysis was used to assess genotype and allele frequency differences between the preeclamptic and control groups. In addition, single-point analysis was expanded to pair of loci haplotype analysis to examine the estimated haplotype frequencies of the two SNPs, of unknown phase, among the preeclamptic and control groups. Estimated haplotype frequencies were assessed using the maximum-likelihood method, employing an expectation-maximization (EM) algorithm. Single-point allele and genotype distributions in exons 3 and 4 of the EPHX gene were not statistically different between the groups. However, according to the haplotype estimation analysis, we observed a significantly elevated frequency of haplotype T-A (Tyr113-His139) among the preeclampsia group vs the control group (P=0.01). The odds ratio for preeclampsia associated with the high-activity haplotype T-A (Tyr113-His139) was 1.61 (95% CI: 1.12-2.32). The use of two intragenic SNPs jointly in haplotype analysis of association demonstrated that the genetically determined high-activity haplotype T-A (Tyr113-His139) was significantly associated with preeclampsia.

Preeclampsia. Part 2: experimental and genetic considerations

Preeclampsia-eclampsia is still one of the leading causes of maternal and fetal morbidity and mortality. Despite active research for many years, the etiology of this disorder exclusive to human pregnancy is an enigma. Recent evidence suggests there may be several underlying causes or predispositions leading to the signs of hypertension, proteinuria, and edema, findings that allow us to make the diagnosis of the "syndrome" of preeclampsia. Despite improved prenatal care, severe preeclampsia and eclampsia still occur. Although understanding of the pathophysiology of these disorders has improved, treatment has not changed significantly in over 50 years. Although postponement of delivery in selected women with severe preeclampsia improves fetal outcome to a degree, this is not done without risk to the mother. In the United States, magnesium sulfate and hydralazine are the most commonly used medications for seizure prophylaxis and hypertension in the intrapartum period. The search for the underlying cause of this disorder and for a clinical marker to predict those women who will develop preeclampsia-eclampsia is ongoing, with its prevention the ultimate goal. This review began with the clinical and pathophysiologic aspects of preeclampsia-eclampsia (Part 1). Now, in Part 2, the experimental observations, the search for predictive factors, and the genetics of this disorder are reviewed.

Angiotensinogen gene M235T variant and pre-eclampsia in Romanian pregnant women

BACKGROUND

Association between the human angiotensinogen gene and essential hypertension has been confirmed in recent studies. Pre-eclampsia is a complication of pregnancy characterised by increased vascular resistance, high blood pressure, proteinuria and oedema, that appears in the second and third trimester of pregnancy. The aim of our study was the analysis of M235T mutation in the gene encoding angiotensinogen in Romanian women with different forms of hypertension during pregnancy.

METHODS

Fourteen women with obstetric complications were tested for M235T angiotensinogen gene mutation. Indications for testing were: severe or mild pre-eclampsia and pre-eclampsia associated with chronic hypertension. We also tested for control 6 healthy women. The M235T angiotensinogen gene mutation was analysed by polymerase chain reaction followed by enzymatic digestion with Tth 111I restriction endonuclease enzyme and agarose gel electrophoresis of the products.

RESULTS

Eleven (78.57%) of the 14 women with complications of pregnancy had M235T mutation: 9 (64.28%) were found to be heterozygous carriers of the M235T variant of the angiotensinogen gene and 2 (14.28%) were found to be homozygous carriers. In the group of women with normal pregnancy, 3 (50%) of the 6 women had M235T mutation: 2 (33.33%) were found to be heterozygous carriers of the M235T variant of the angiotensinogen gene and 1 (16.66%) was found to be homozygous carrier.

CONCLUSIONS

Our study shows that the M235T variant in the gene encoding angiotensinogen could be a risk factor in mild and severe pre-eclampsia.

Linkage and association studies of IL1B and IL1RN gene polymorphisms in preeclampsia

OBJECTIVE

To determine whether preeclampsia is either associated with or linked to two polymorphisms in the IL1B gene (IL1B-TaqI and IL1B-511) and one polymorphism in the IL1RN gene (IL1RN-IVS2).

METHODS

Genotyping was performed in 150 affected sib-pair families and 104 healthy Dutch blood donors. Genotype and allele frequencies as well as allelic associations were assessed in three groups of unrelated women from these 150 families; 133 with either eclampsia, preeclampsia or the haemolysis, elevated liver enzymes, low platelets (HELLP) syndrome, 101 with preeclampsia only, and 63 with HELLP syndrome only. These frequencies were compared to those in controls. Frequencies of transmitted and nontransmitted haplotypes, inferred from the three polymorphisms, were compared. Allele sharing between affected siblings from all 150 families was assessed by means of multipoint nonparametric affected sib-pair analyses.

RESULTS

No significant differences in genotype and allele frequencies were found between the unrelated study groups and controls. No allelic associations were apparent, nor were there differences in frequencies of transmitted and nontransmitted haplotypes within affected families. Excess allele sharing for any of the three polymorphic markers was absent in affected sib-pairs.

CONCLUSIONS

None of the IL1B and IL1RN polymorphisms provided evidence for either association or linkage with the risk for (pre)eclampsia/HELLP syndrome, preeclampsia only or HELLP syndrome only.

Polymorphism in the tumor necrosis factor-alpha gene in women with preeclampsia

PURPOSE

We determined whether genetic variability in the gene encoding for tumor necrosis factor-alpha (TNF-alpha) contributes to individual differences in susceptibility to the development of preeclampsia.

METHODS

The study involved 133 preeclamptic and 115 healthy control pregnant women who were genotyped for C-850T polymorphism in the TNF-alpha gene promoter. Chi-square analysis was used to assess genotype and allele frequency differences between preeclamptic women and controls.

RESULTS

A significantly different genotype distribution of C-850T polymorphism was observed between the two groups, with the frequency of the variant T allele being significantly reduced in the preeclamptic group (4.5%) when compared with the control group (9.6%) (P = 0.03; OR = 0.45, 95% CI = 0.22-0.92). Accordingly, the odds ratio for preeclampsia associated with the pooled TT and CT genotypes was 0.367 (P = 0.02; 95% CI = 0.159-0.847).

CONCLUSIONS

The T allele of the TNF-alpha gene may modify individual preeclampsia risk, being protective against the development of the complication.

A genome-wide scan for preeclampsia in the Netherlands

Preeclampsia, hallmarked by de novo hypertension and proteinuria in pregnancy, has a familial tendency. Recently, a large Icelandic genome-wide scan provided evidence for a maternal susceptibility locus for preeclampsia on chromosome 2p13 which was confirmed by a genome scan from Australia and New Zealand (NZ). The current study reports on a genome-wide scan of Dutch affected sib-pair families. In total 67 Dutch affected sib-pair families, comprising at least two siblings with proteinuric preeclampsia, eclampsia or HELLP-syndrome, were typed for 293 polymorphic markers throughout the genome and linkage analysis was performed. The highest allele sharing lod score of 1.99 was seen on chromosome 12q at 109.5 cM. Two peaks overlapped in the same regions between the Dutch and Icelandic genome-wide scan at chromosome 3p and chromosome 15q. No overlap was seen on 2p. Re-analysis in 38 families without HELLP-syndrome (preeclampsia families) and 34 families with at least one sibling with HELLP syndrome (HELLP families), revealed two peaks with suggestive evidence for linkage in the non-HELLP families on chromosome 10q (lod score 2.38, D10S1432, 93.9 cM) and 22q (lod score 2.41, D22S685, 32.4 cM). The peak on 12q appeared to be associated with HELLP syndrome; it increased to a lod score of 2.1 in the HELLP families and almost disappeared in the preeclampsia families. A nominal peak on chromosome 11 in the preeclampsia families showed overlap with the second highest peak in the Australian/NZ study. Results from our Dutch genome-wide scan indicate that HELLP syndrome might have a different genetic background than preeclampsia.

Pathogenesis and genetics of pre-eclampsia

After more than a century of intensive research, pre-eclampsia and eclampsia remain an enigmatic set of conditions. Aberration of the interaction between placental and maternal tissue is probably the primary cause, but the exact nature of the differences from normal pregnancy remain elusive. In this review attempts to understand the sequence of physiological changes have concentrated on vascular endothelium and oxidative stress issues. There are genetic components to susceptibility, but the relative contributions of maternal and fetal genotypes are still unclear. Whole-genome mapping could ultimately define the causative genes.