Hyperglycosylated HCG expression in pregnancy: cellular origin and clinical applications

Employing a monoclonal antibody (B152) specific for a carbohydrate epitope found on a choriocarcinoma derived hCG, it was discovered that a similar hCG isoform is expressed during early pregnancy. This form differs from later pregnancy hCG in carbohydrate moieties. Profiling of these two hCG isoforms throughout pregnancy utilized two IRMA's: B152-B207 ("hyperglycosylated hCG"-specific assay) and B109-B108 (an IRMA for standard intact hCG isoforms in the WHO hCG reference preparation). The WHO hCG standard was used in both assays. Values were presented as a ratio of hCG isoform concentrations (B152/B109 ratio). In early pregnancy urine concentrations of B152 hCG were significantly higher in normal pregnancy (NP) compared to early pregnancy loss (EPL). Matched serum-urine samples from the first and third trimesters revealed that the B152 hCG form is predominant in both serum and urine in the first trimester compared with the third trimester. The proportion of the B152 hCG (HhCG) form is higher in urine than in matched serum. There was a significant difference in the B152/B109 ratio between days 5 and 20 from time of embryo transfer in normally developing pregnancy versus EPL in the urine of IVF patients. In spontaneous abortion (SA) the level of B109 hCG remained higher in NP compared with SA. However, the B152/B109 ratio declined with gestational age faster in SA than in NP suggesting perhaps a different loss mechanism in SA versus EPL. The cellular origin of the different hCG glycoforms was identified by assay of cell media from cytotrophoblasts (CTBs) and syncytiotrophoblasts (STBs). Isolated CTBs expressed predominantly HhCG. The level of expression was the highest in the first trimester. STBs were the source of the less glycosylated B109 hCG isoform. Analysis of hCG glycoforms during early pregnancy can distinguish pregnancies that will fail from those that will proceed normally. Since the B152 assay does not effectively discriminate between intact HhCG and free beta HhCG (HhCGbeta), a new HhCGbeta assay was developed. This assay recognizes the HhCGbeta which is produced by CTBs. We hypothesize that the measurement of HhCGbeta may have a potential use in screening for Down syndrome and perhaps other pregnancy disorders and certain types of cancer.