Haptoglobin and its association with the HELLP syndrome.

Haptoglobin (Hp) is an acute phase α2-sialoglycoprotein, which is characterised by molecular heterogeneity. Owing to a genetic polymorphism, different Hp phenotypes exist of which Hp1-1, Hp1-2, and Hp2-2 are the three major isoforms in humans. Hp consists of two different polypeptide chains, the heavy β chain, which is identical in all haptoglobins, and the light α chain, consisting of two α chains and a α chain, modifications of which result in the different Hp phenotypes. The most important function of Hp is capturing haemoglobin, thereby preventing iron loss and subsequent oxidative damage generated by free iron in the vascular system of the kidneys. Binding of haemoglobin to Hp is beneficial for the human body in several other ways. Hp is protective against cell damage by scavenging free radicals, such as the hydroxyl radical, the formation of which is promoted by the presence of free haemoglobin. Furthermore, the Hp-haemoglobin complex inhibits the vasodilatory effect of nitric oxide and provides a non-specific defence against bacterial invasion, since free haem iron is necessary for bacterial growth. Furthermore, Hp itself was identified as a serum angiogenic factor and plays a role in proliferation and differentiation of vascular endothelium. Hp2-2 has stronger angiogenic functionality than Hp1-1, whereas Hp1-1 has the highest affinity for haemoglobin and is therefore associated with the antioxidant capacity of Hp. Pre-eclampsia, which is characterised by pregnancy induced hypertension and concurrent proteinuria, can be complicated by the haemolysis, elevated liver enzymes, and low platelets (HELLP) syndrome, which may also occur alone. The pathogenesis of pre-eclampsia and HELLP is largely unknown, although it is postulated that maladaptation of trophoblast invasion may result in poor placental perfusion and local oxidative stress, which could subsequently affect maternal circulation. Systemic maternal oxidative stress may result in the clinical manifestations seen in women with preeclampsia, including dysfunction of the vascular endothelium. A previous study associated a higher incidence of the Hp2-2 phenotype with the occurrence of pregnancy induced hypertension. Since Hp2-2 has poor affinity for haemoglobin and may therefore be less capable of preventing oxidative damage induced by free haemoglobin present after haemolysis, we hypothesised that occurrence of the Hp2-2 genotype may be associated with the HELLP syndrome. Therefore, we investigated the prevalence of the Hp2-2 genotype in patients with a history of severe pre-eclampsia with or without HELLP syndrome as compared to women with uncomplicated pregnancies only.