A naturally occurring genetic variant in the human chorionic gonadotropin-β gene 5 is assembly inefficient

The hCGβ gene family is composed of six homologous genes linked in tandem repeat on chromosome 19; the order of the genes is 7, 8, 5, 1, 2, and 3. Previous studies have shown that hCGβ gene 5 is highly expressed during the first trimester of pregnancy. The purpose of our study was to identify naturally occurring polymorphisms in hCGβ gene 5 and determine whether these alterations affected hCG function. The data presented here show that hCGβ gene 5 was highly conserved in the 334 asymptomatic individuals and 41 infertile patients examined for polymorphisms using PCR followed by single stranded conformational polymorphism analysis. Most of the polymorphisms detected were either silent or located in intron regions. However, one genetic variant identified in β gene 5 exon 3 was a G to A transition that changed the naturally occurring valine residue to methionine in codon 79 (V79M) in 4.2% of the random population studied. The V79M polymorphism was always linked to a silent C to T transition in codon 82 (tyrosine). To determine whether βV79M hCG had biological properties that differed from those of wild-type hCG, a β-subunit containing the V79M substitution was created by site-directed mutagenesis and was coexpressed with the glycoprotein hormone α-subunit in Chinese hamster ovary cells and 293T cells. When we examined βV79M hCG biosynthesis, we detected atypical βV79M hCG folding intermediates, including a βV79M conformational variant that resulted in a β-subunit with impaired ability to assemble with the α-subunit. The inefficient assembly of βV79M hCG appeared to be independent of β-subunit glycosylation or of the cell type studied, but, rather, was due to the inability of the βV79M subunit to fold correctly. The majority of the V79M β-subunit synthesized was secreted as unassembled free β. Although the amount of αβ hCG heterodimer formed and secreted by βV79M-producing cells was less than that by wild-type β-producing cells, the hCG that was secreted as αβ V79M heterodimer exhibited biological activity indistinguishable from that of wild-type hCG.