Altered Placental Expression of PAPPA2 Does Not Affect Birth Weight in Mice

Background: Pregnancy-associated plasma protein A2 (PAPPA2) is an insulin-like growth factor binding protein (IGFBP)protease expressed in the placenta and upregulated in pregnancies complicated by pre-eclampsia. The mechanismlinking PAPPA2 expression and pre-eclampsia and the consequences of altered PAPPA2 expression remain unknown.We previously identified PAPPA2 as a candidate gene for a quantitative trait locus (QTL) affecting growth in mice and inthe present study examined whether this QTL affects placental PAPPA2 expression and, in turn, placental or embryonicgrowth.Methods: Using a line of mice that are genetically homogenous apart from a 1 megabase QTL region containing thePAPPA2 gene, we bred mice homozygous for alternate QTL genotypes and collected and weighed placentae andembryos at E12.5. We used quantitative RT-PCR to measure the mRNA levels of PAPPA2, as well as mRNA levels ofIGFBP-5 (PAPPA2's substrate), and PAPPA (a closely related IGFBP protease) to examine potential feedback andcompensation effects. Western blotting was used to quantify PAPPA2 protein. Birth weight was measured inpregnancies allowed to proceed to parturition.Results: PAPPA2 mRNA and protein expression levels in the placenta differed by a factor of 2.5 between genotypes, butwe did not find a significant difference between genotypes in embryonic PAPPA2 mRNA levels. Placental IGFBP-5 andPAPPA mRNA expression levels were not altered in response to PAPPA2 levels, and we could not detect IGFBP-5 proteinin the placenta by Western blotting. The observed difference in placental PAPPA2 expression had no significant effecton placental or embryonic mass at mid-gestation, birth weight or litter size.Conclusions: Despite a significant difference between genotypes in placental PAPPA2 expression similar in magnitudeto the difference between pre-eclamptic and normal placentae previously reported, we observed no difference inembryonic, placental or birth weight. Our results suggest that elevated PAPPA2 levels are a consequence, rather than acause, of pregnancy complications.