Innovations in Down syndrome screening

Abstract

Down syndrome (DS) is the most common chromosomal abnormality, with an incidence of approximately 1 per 500 to 800 live births. The first-trimester combined test is mostly used for the prenatal prediction of carrying a child with DS. The test is composed of the maternal serum parameters pregnancy-associated plasma protein A (PAPP-A) and the free beta subunit of human chorionic gonadotropin (f?-hCG), and an ultrasound measurement of the foetal nuchal translucency (NT), combined with maternal age. The detection rate (DR) of DS screening in the Netherlands is currently 70-75%, which is rather low as compared to other countries. The aim of this research was to investigate ways to improve the performance of the current DS screening programme. The extra chromosome in DS not only leads to anomalies of the foetus, but also of the placenta. The inability of placental cells to develop properly is associated with a decrease of trophoblastic products. If this differential expression is traceable in maternal blood these products could have potential as new screening markers. Several markers have been investigated and found to be potentially useful as predictors of DS. One of those markers is placental protein 13 (PP13) which was found to be decreased in DS pregnancies. Moreover, serum concentrations of a disintegrin and metalloprotease 12 (ADAM12) and placental growth factor (PlGF) are decreased, while total hCG (thCG) is increased in first-trimester DS pregnancies. However, the addition of these markers to the current first-trimester combined test only slightly increases the DR. Therefore, a dedicated search for more markers was set up. An extensive review of the literature was carried out to study normal placental development and function during early pregnancy. Furthermore, a bioinformatics approach was developed using data from the literature on genes and protein expression. This way, a list of potential DS screening markers was generated. The list included three biomarkers that are already used for DS screening and several others, among which PP13 and PlGF. A more experimental approach was carried out by analyzing 90 different proteins from a pre-existing immunoassay. By comparing the protein concentrations in a small cohort of DS and control sera, seven potential screening markers were identified. To confirm the predictive value of these seven markers a subsequent validation study was carried out. Epidermal growth factor (EGF) and EN-RAGE were confirmed to be potential screening markers for DS and improved the DR of the current first-trimester combined test with approximately 6%. In addition, Cancer Antigen 19-9 (CA19-9), was found to be strongly predictive for DS and even further increased the DR. It turned out that the predictive power of serum markers differs within the first trimester. Therefore, it would be useful to draw two separate blood samples and analyze several markers to increase the DR of first-trimester screening to almost 90%. If such a screening test is to be developed, simultaneous assessment of markers is crucial and demands innovation of the test, i.e. by using Antibody microarrays.