Introduction

The hunt for genetic variants which confer susceptibility to pre-eclampsia has proved to be challenging, in spite of convincing evidence that the disorder runs in families[i]. Until recently research has focused on candidate genes in the maternal genome – genes with physiological plausibility for association with pre-eclampsia. In line with experience in other genetically complex disorders, studies of candidate genes have provided inconsistent results. Genome-wide association screening (GWAS) provides the opportunity to search through the entire genome for evidence of regions associated with pre-eclampsia without any prior bias towards an underlying pathophysiology.  An important consideration in disorders of pregnancy is that two genomes are involved – maternal and fetal – and there is good epidemiological evidence that both contribute to pre-eclampsia susceptibilityⁱ.

InterPregGen is a consortium of research groups from the UK, Nordic countries, and Central Asia which is using the GWAS approach to identify DNA sequence variants which predispose to pre-eclampsia[ii]. The group has recently published the results of the first GWAS of the fetal genome, in which they compared the genotypes of 2,658 offspring of pre-eclamptic pregnancies from Iceland and the UK with those of over 300,000 controls from the general Icelandic and UK populations[iii].  One DNA variant in the fetal genome near the FLT1 gene, which encodes Fms-like tyrosine kinase 1 (Flt-1), was significantly associated with pre-eclampsia (P=3.2×10^-8). This finding was confirmed in an independent sample of 1722 offspring of pre-eclamptic pregnancies and 1946 offspring of healthy pregnancies from Norway and Finland (P=3.6×10^-4).  Further genotyping showed that at least 2 nearby variants are independently associated with pre-eclampsia, reinforcing the importance of this region.

How do these results affect our understanding of the pathogenesis of pre-eclampsia? Flt-1 and its ligands – vascular endothelial growth factor (VEGF) and placental growth factor (PlGF) – are vital players in the process of angiogenesis, and in maintenance of a healthy vascular endothelium. Alternative splicing of the FLT1 transcript produces a soluble isoform, sFlt-1, which also binds VEGF and PlGF, but lacks the intracellular effector region of the full length Flt-1 protein. Levels of sFlt-1, predominantly of placental origin, rise in maternal plasma in healthy pregnancies, but they are markedly increased in women with pre-eclampsia[iv]. Binding of PlGF in maternal plasma by excess sFlt-1 appears to cause vascular endothelial activation and damage, leading to the characteristic clinical features of pre-eclampsia. The DNA variants associated with pre-eclampsia lie within enhancer regions which could potentially affect the regulation of FLT1 in the placenta. It is not clear at present whether they could affect production of full length Flt-1, the soluble variant sFlt-1, or both, nor do we know the stage of pregnancy at which these enhancers are active.

DNA variants at the FLT1 locus may increase the risk of pre-eclampsia by promoting sFlt-1 production by the placenta, but the InterPregGen study found only modest evidence for this: maternal plasma sFlt-1 in the third trimester was nominally associated with fetal genotype at the FLT1 locus in control pregnancies (P=0.04), but no such association was detected in pre-eclamptic pregnancies. It is possible that subtle changes in gene expression are masked by other pathophysiological changes in established pre-eclampsia. An alternative hypothesis is that the association with genotype at FLT1 may be related to disordered angiogenesis much earlier in pregnancy in the course of placental growth.

InterPregGen found a significant difference in the strength of the association between fetal genotype at FLT1 and pre-eclampsia in subgroups defined by gestation at onset (≤34 weeks vv. >34 weeks) and offspring birthweight (≤10^th centile vv.  >10^th centile). The strongest genetic association was in late onset disease associated with birthweight >10^th centile. By contrast, pre-eclampsia of early onset leading to the birth of a small-for-gestational age baby was weakly associated with fetal FLT1 genotype. This was unexpected, as sFlt-1 is a marker of placental malfunction, predominantly in early onset pre-eclampsia associated with fetal growth restriction[v]. Late onset disease is often regarded as resulting primarily from maternal maladaptation to pregnancy, due to pre-pregnancy hypertensive disease, diabetes or obesity for example. The observation that late onset disease without evidence of impaired fetal growth is strongly associated with the gene encoding Flt-1 suggests that placental pathology is also important in this subgroup.

Translating Findings

Establishing the functional basis of the association between DNA sequence variants in the fetal genome and pre-eclampsia is essential if the findings are to be translated into health care benefits. Genetic information offers researchers a powerful tool for further investigation of the pathogenesis of pre-eclampsia, not least because genotype is unchanged by time. Frozen placental tissue can be genotyped to throw new light on earlier physiological investigations. Adults born of pre-eclamptic pregnancies can provide DNA for retrospective review of clinical data. New approaches will require DNA sequencing, mRNA and proteomic investigations, coupled with sophisticated data analysis. Assembling the necessary tissue samples and expertise for high quality studies is a priority for researchers. Genetics is now opening a door for unique insights into the mechanisms of pre-eclampsia – a further step towards the prevention and treatment of this potentially devastating condition.

References:

[i] Cnattingius, S. Reilly, M. Pawitan, Y. & Lichtenstein, P. Maternal and fetal genetic factors account for most of familial aggregation of preeclampsia: a population-based Swedish cohort study. Am. J. Med. Genet. A. 130A, 365-371 (2004).

[ii] Morgan, L. et al. InterPregGen: genetic studies of pre-eclampsia in three continents. Nor. Epidemiol. 24, 141-146 (2014)

[iii] McGinnis, R. Steinthorsdottir, V. Williams, N.O et al. Variants in the fetal genome near FLT1 are associated with risk of preeclampsia. Nat. Genet.(2017). doi:10.1038/ng.3895

[iv]Maynard, S. E. et al. Excess placental soluble fms-like tyrosine kinase 1 (sFlt1) may contribute to endothelial dysfunction, hypertension, and proteinuria in preeclampsia. J. Clin. Invest. 111, 649-658 (2003).

[v] Staff A. C. et al. Redefining preeclampsia using placenta-derived biomarkers.  Hypertension 61, 932-942 (2013).