Background

Hypertensive disorders of pregnancy (Hypertension identified at least twice after the 20th week of gestation with proteinuria present at least twice after the 20th week of gestation (pre-eclampsia) or without proteinuria (gestational hypertension)) are by far the commonest complication of pregnancy and are associated with increased risk of maternal and perinatal mortality, small for gestational age and preterm birth in the short time and with increased risk of cardiovascular disease in mother and offspring in the long term. Hypertensive disorders of pregnancy (HDP) affect 10-20% of pregnancies in developed countries.

Despite the common occurrence of HDP, their important impact on maternal and child health in the short and long-term and the many decades of research into this condition, two recent reviews noted that the pathophysiology of these disorders, how to predict women who are at high risk and how to prevent HDP remained unclear. They note that in part this reflects the failure to appreciated the complex nature of the syndrome which is likely to be affected by a number of pathophysiological pathways; the limited ability in studies to date to explore possible differences in pathophysiology and consequences for potentially different sub-types and the lack of large studies with adequate maternal and fetal/offspring data. With respect to exploring pathophysiology and genetic pathways it has been noted that too few studies have examined maternal and fetal genotype together when taking a candidate gene approach and that no studies to date have examined within one study population a wide array of variants that relate to all of the plausible pathophysiolgogical pathways that are likely to be involved in these conditions. It was noted that studies that restrict samples to first pregnancies are problematic, because although pre-eclampsia is more common in nulliparous women, different subtypes may be more common in women who have experienced previous pregnancies and it is important to try to understand the whole spectrum of this condition.

Classical twin studies have reported heritabilities of around 20% for HDP and have lead some to conclude that the genetic contribution to HDP was smaller than hitherto believed. However, examining levels of association in twin mothers fails to recognise the potential importance of paternal genes expressed in the fetus. Two large studies using record linkage between Norwegian population registers and containing over 1 million participants examined a wide range of familial relationships and concluded that heritability was 30% with fetal paternal genotype. The plausible pathophysiological mechanisms for HDP, including maternal-fetal immunological maladaptation, maternal endothelial dysfunction, oxidative stress, inflammatory response and glucose-insulin metabolism mechanisms, have high heritabilities (greater than 50%).

A number of genome wide association studies (GWAS) of pre-eclampsia or HDP are currently underway. Only a minority of these include genetic information on fetus/offspring as well as mother. Other studies with genome wide data are potentially able to complete GWAS for pre-eclampsia with some having fetal genome wide data only and others maternal data only.

The 50K Cardiochip has densely typed loci for all pathways that are plausibly linked to HDP and cardiovascular disease and therefore provides an efficient way of establishing the many genetic variants that are likely to be associated with HDP. This includes loci that have to date been identified as relating to HDP. Brendan Keating (co-applicant) produced this chip and is responsible for updates/revisions to later versions.

Our aim is to submit a grant proposal to BHF in March 2009 that will have the following objectives:

1. Identify and collate all GWAS of HDP and also potential additional studies with genome wide data and the possibility to define cases of HDP. To combine data from all such studies in order to identify key loci in mothers and fetus associated with HDP.

2. Add any new loci from (1) to an update version of the cardiochip

3. Use the newly updated cardiochip on all ALSPAC mothers and offspring in order to determine the association of genetic variants (in mothers or fetus) associated with HDP, cardiovascular disease and plausible mechanisms for HDP, with patterns of change in blood pressure during pregnancy in ALSPAC mothers and with later blood pressure in ALSPAC offspring ages 7-17 and mothers at the 17 year postnatal clinic.

Methods

PILOT

Before submitting the grant application we would like to complete a small pilot study in ALSPAC. This would involve using the current version of the cardiochip on 1000 ALSPAC samples (500 mothers and their 500 offspring). Debbie Lawlor is able to identify 500 mothers who have had obstetric data abstracted and for whom there is extracted DNA on both mother and offspring. From this 'eligible' sample she will identify 300 mothers-offspring pairs where mother had severe HDP (pre-eclampsia and/or systolic BPgreater than 200 on at least 2 occasions after 20 weeks gestation) and a random sample of 200 mother-offspring pairs where there is no evidence of HDP or existing hypertension in mother. We would like permission to ship DNA from these mother-offspring pairs (1000 samples in total: 500 mothers/500 offspring) to the University of Pennsylvania ASAP (DAL can provide IDs immediately) where Brendan Keating will provide genotyping with the cardiochip. The cardiochip will be provided for free for this pilot and MRC CAiTE will cover shiping costs. Analyses will be completed by DAL, Brendan Keating & Nic Timpson.

Brendan Keating requires 1ug total of DNA (50 or 100ng/ul) not from cell-lines for this work.

FULL GRANT PROPOSAL

The aims of the full grant proposal are detailed above.

With respect to ALSPAC we would request funds to complete genotyping using the cardiochip on all ALSPAC mothers and offspring with DNA. Our plan would be for this genotyping on the complete sample to be completed at University of Pennsylvania with a newly updated cardiochip. Funds would be requested for shipping samples; completion of the genotyping; merging of large amount of new genetic data with the existing ALSPAC data; a statistician to complete analyses under supervision of applicants. If funded for this complete project we would again require 1ug of DNA (50 or 100ng/ul) from whole blood samples and not cell lines for each sample from mothers and offspring.

This proposal has been discussed with Sue Ring who feels that is is feasible.