Pattern of secretion of pregnancy-associated plasma protein-A (PAPP-A) during pregnancies complicated by fetal aneuploidy, in vivo and in vitro

Abstract

Background: Pregnancy-associated placental protein-A (PAPP-A) is a metalloprotease which circulates as an hetero-tetramer in maternal blood. Its maternal serum concentration in fetal trisomy 21 is decreased during the first trimester, so that PAPP-A is a useful screening biomarker. However, the regulation of PAPP-A placental secretion is unclear. We therefore investigated the secretion of PAPP-A in pregnancies complicated by fetal aneuploidies, both in vivo and in vitro.

Methods: Maternal serum collected between 10 WG and 33 WG during 7014 normal pregnancies and 96 pregnancies complicated by fetal trisomy 21, 18, and 13 were assayed for PAPP-A using the Immulite 2000xpi system®. The pregnancies were monitored using ultrasound scanning, fetal karyotyping and placental analysis. Villous cytotrophoblasts were isolated from normal and trisomic placenta and cultured to investigate PAPP-A secretion in vitro (n=6).

Results: An increased nuchal translucency during the first trimester is a common feature of many chromosomal defect but each aneuploidy has its own syndromic pattern of abnormalities detectable at the prenatal ultrasound scanning and confirmed at the fetal examination thereafter. PAPP-A levels rise throughout normal pregnancy whereas in trisomy 21, PAPP-A levels were significantly decreased, but only during the first trimester. PAPP-A levels were decreased in trisomy 13 and sharply in trisomy 18, whatever the gestational age. In vitro, PAPP-A secretion was decreased in aneuploidy, and associated with decreased hCG secretion in Trisomy 21 and 18. These biochemical profiles did not appear to be linked to any specific histological lesions affecting the placenta.

Conclusions: These profiles may reflect different quantitative and qualitative placental dysfunctions in the context of these aneuploidies.

Figure 1

Maternal serum levels of PAPP-A throughout normal gestation and in gestations complicated by fetal aneuploidy. PAPP-A was measured using the assay developed on the Immulite 2000 analyser (Siemens, Germany) specific for the heterotetrameric complex (htPAPP-A) in the maternal serum of 7014 normal pregnancies (controls) and 96 pregnancies affected by fetal trisomy (68 trisomy 21, 21 trisomy 18, 7 trisomy 13) from 10WG to 33WG. Results are expressed in median values, and 10-90ciles for the controls.

Figure 2

In vitro secretion of PAPP-A and hCG by villous trophoblast tissues isolated from normal placentas and placentas complicated by aneuploidy. Villous cytototrophoblasts were cultured after being isolated from aneuploid placentas (one trisomy 21 at 20 WG, one trisomy 18 at 19WG, one trisomy 13 at 18WG) and from three age matched normal placentas. PAPP-A was measured in the culture supernatant from 24 hours to 72 hours of culture using the assay developed on the Immulite 2000 analyser® (Siemens, Germany) specific for the heterotetrameric complex (htPAPP-A). hCG was measured with the assay developed on Advia Centaur XP analyser® (Siemens, Germany) to assess the formation of the syncytiotrophoblast.

Figure 3

Microscopic analysis of placental villi in normal pregnancy and pregnancy complicated by aneuploidy. After termination of pregnancy, 55 aneuploid placentae were analysed by experimented pathologists at the macroscopic level, and the histological level after being fixed in formalin, included in paraffin and stained with hematein-eosin-saffron. Mature villi i.e. small diameter, strong staining, presence of fetal vessels, decreased connective tissue, covered by trophoblastic tissue. Immature hydropic villi i.e. large diameter, pale staining, few fetal vessels, connective tissue, decreased trophoblastic tissue.