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

The hCG beta gene family is composed of six homologous genes linked in tand em repeat on chromosome 19; the order of the genes is 7, 8, 5, 1, 2, and 3. Previous studies have shown that hCG beta gene 5 is highly expressed durin g the first trimester of pregnancy. The purpose of our study was to identif y naturally occurring polymorphisms in hCG beta gene 5 and determine whethe r these alterations affected hCG function. The data presented here show tha t hCG beta 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 polymorph isms detected were either silent or located in intron regions. However, one genetic variant identified in beta gene 5 exon 3 was a G to A transition t hat changed the naturally occurring valine residue to methionine in codon 7 9 (V79M) in 4.2% of the random population studied. The V79M polymorphism wa s always linked to a silent C to T transition in codon 82 (tyrosine). To de termine whether beta V79M hCG had biological properties that differed from those of wild-type hCG, a beta-subunit containing the V79M substitution was created by site-directed mutagenesis and was coexpressed with the glycopro tein hormone alpha-subunit in Chinese hamster ovary cells and 293T cells. W hen we examined beta V79M hCG biosynthesis, we detected atypical beta V79M hCG folding intermediates, including a beta V79M conformational variant tha t resulted in a beta-subunit with impaired ability to assemble with the alp ha-subunit. The inefficient assembly of beta V79M hCG appeared to be indepe ndent of beta-subunit glycosylation or of the cell type studied, but, rathe r, was due to the inability of the beta V79M subunit to fold correctly. The majority of the V79M P-subunit synthesized was secreted as unassembled fre e beta. Although the amount of crp hCG heterodimer formed and secreted by b eta V79M-producing cells was less than that by wild-type beta-producing cel ls, the hCG that was secreted as alpha beta V79M heterodimer exhibited biol ogical activity indistinguishable from that of wild-type hCG.