Sialic acid-deficient invasive trophoblast antigen (sd-ITA): a new urinary variant for gestational Down syndrome screening.

BACKGROUND

Invasive trophoblast antigen (ITA), also known as hyperglycosylated hCG, is a glycosylation variant of hCG produced by invasive cytotrophoblast cells. Publications show that ITA is potentially a sensitive test for Down syndrome screening, reportedly detecting 38 and 80% of cases at a 5% false-positive rate in the first and second trimesters of pregnancy respectively. ITA has six oligosaccharides varying greatly in terminal sialic acid content (0-19 residues), leading to wide-charge heterogeneity. With the objective of developing an improved Down syndrome screening test, we examined the charge, or sialic acid variants, of ITA.

METHODS

Twenty urine samples were collected from the 16th to 18th week of gestation from 10 patients with trisomy 21 fetuses, demonstrated by karyotype analysis, and from 10 patients with normal karyotype pregnancies. Proteins in each sample were separated by preparative isoelectric focusing. Fractions were collected, pooled into the pl ranges of 3.0 to 4.0, 4.1 to 5.0, 5.1 to 6.0 and 6.1 to 7.0 and assayed for ITA.

RESULTS

A difference in the distribution of ITA charge variants was demonstrated between cases and controls, most apparent in the less acidic (pl 5.1-7.0) range. The controls and Down syndrome samples contained 26.7 +/- 14% versus 45.6 +/- 20%, respectively, of the less acidic form of ITA (P = 0.02).

CONCLUSIONS

The ITA in Down syndrome cases is composed of a much higher proportion of less acidic ITA. This could be the major source of elevated levels of ITA in Down syndrome pregnancies. It is postulated that the development of a new assay specifically measuring ITA that is deficient in sialic acid might enhance the utility of the already highly sensitive screening tests for Down syndrome.