Nicked free beta-subunit of human chorionic gonadotropin: a potential new marker for Down syndrome screening.

Abstract

Our purpose was to investigate maternal serum levels of nicked free beta-subunit in normal and Down syndrome pregnancies. Serum specimens were obtained from 64 karyotypically normal and 6 Down syndrome pregnancies before amniocentesis. Two immunoenzymometric assays for the beta- subunit of human chorionic gonadotropin were used to determine the level of free beta-subunits with peptide linkage breaks ("nicks") between residues 43 to 48. The mean level of nicked free beta-subunit in Down syndrome was 4.76 multiples of the median, which was significantly elevated compared with normal controls (1.07 multiples of the median, p<0.05). In 5 of 6 cases levels were > or = 2 multiples of the median, with a range of 1.96 to 15.43 multiples of the median, which was 2- to 3-fold higher than the regular free beta-subunit assay (mean level 1.53 multiples of the median, range 0.70 to 3.10 multiples of the median). In one case no nicked free-beta subunits were detectable. Our preliminary data indicate that nicked free beta-subunit of human chorionic gonadotropin might be a sensitive marker for Down syndrome screening.