CHARACTERIZATION OF 2 ALLELIC VARIANTS OF A HUMAN PREGNANCY-SPECIFIC GLYCOPROTEIN GENE

The pregnancy-specific glycoproteins (PSGs) of the human placenta are a group of proteins that together with the carcinoembryonic antigens c omprise a subfamily within the immunoglobulin superfamily. To study th e control of PSG expression, we isolated and characterized PSG genes a nd identified cis-acting DNA elements in the 5'-flanking gene regions essential for PSG expression. Two overlapping PSG cosmid clones, which contain two allelic variants of a PSG gene (PSG12 and PSG12psi), were isolated from an unamplified library made from a single individual. C osmid 1 contains exons 1 (5'/L) and 2 (L/N) of the PSG12 gene located downstream of a previously identified PSG1-I gene. Cosmid 6 contains a portion of the PSG1-I gene lacking exons 1 and 2 upstream of a comple te PSG12psi transcription unit. Sequence comparison indicates that exo ns 5'/L and L/N in PSG12 and PSG12psi are 99% identical, except that t he L/N exon in the PSG12psi gene contains a stop codon. Both PSG12 and PSG12psi transcripts were detected in the human placenta, indicating that both genes are actively transcribed. However, the PSG12psi gene m ay represent an allelic pseudogene variant of the PSG12 gene, because all identified PSGs contain a functional N-domain. Primer extension an alysis showed that the PSG12 gene starts at a cluster of sites located at -106 to -104 base pairs with respect to the translation start site . In transient transfection assays using a chloramphenicol acetyltrans ferase reporter gene, we demonstrated that the -835 to -34 DNA region upstream of the translation start site of PSG12 or PSG12psi contained both positive and negative elements that control PSG expression. Delet ion analysis showed that nucleotides -172 to -34 in the PSG12 gene cou ld function as a promoter. Gel retardation analysis showed that protei n factors in human placental cell extract formed four complexes (I, II , IIa, and III) with the PSG12(-172/-34) DNA. Site-directed mutagenesi s that prevents protein factor binding to the PSG12 promoter resulted in a marked reduction in transcription activation, locating the core e nhancers at nucleotides-148 to -141 and -60 to -55. Mutagenesis studie s also showed that the ACAGC repeats at nucleotides -84 to -68 in the PSG12 5'-flanking are essential for expression of the PSG12 gene in hu man placental cells.