Quantification of Gene Expression by Competitive RT-PCR: The hCGβ/LH/β Gene Cluster

hCG is a dimeric hormone synthesized by trophoblast cells, and it promotes steroidogenesis by the luteal body during early pregnancy. This hormone is composed of two noncovalently bound α- and β-subunits (1). The clinical relevance of the measurement of hCG and its free beta subunit is in the diagnosis of early pregnancy, and in prenatal screening for trisomy 21 (2). In addition, hCG and its free β-subunit are used as biological markers, mainly of trophoblast-derived and gonadal malignancies, but also of neoplasms of nongonadal origin, such as bladder carcinoma (3). The hCGβ subunit can be encoded by four distinct genes, namely hCGβ-7, -8, -5, and -3, highly homologous with regard to their nucleotide sequences. These four genes are clustered together with the luteinizing hormone beta subunit gene (hLHβ) and map to 19ql3 (4, 5). The discriminative quantification of the hCGβ genes is achieved using reverse transcription in combination with competitive PCR.