Absence of the genetic variant Val79Met in human chorionic gonadotropin-beta gene 5 in five European populations

Structural and functional analysis of rare missense mutations in human chorionic gonadotrophin β-subunit

Heterodimeric hCG is one of the key hormones determining early pregnancy success. We have previously identified rare missense mutations in hCGβ genes with potential pathophysiological importance. The present study assessed the impact of these mutations on the structure and function of hCG by applying a combination of in silico (sequence and structure analysis, molecular dynamics) and in vitro (co-immunoprecipitation, immuno- and bioassays) approaches. The carrier status of each mutation was determined for 1086 North-Europeans [655 patients with recurrent miscarriage (RM)/431 healthy controls from Estonia, Finland and Denmark] using PCR-restriction fragment length polymorphism. The mutation CGB5 p.Val56Leu (rs72556325) was identified in a single heterozygous RM patient and caused a structural hindrance in the formation of the hCGα/β dimer. Although the amount of the mutant hCGβ assembled into secreted intact hCG was only 10% compared with the wild-type, a stronger signaling response was triggered upon binding to its receptor, thus compensating the effect of poor dimerization. The mutation CGB8 p.Pro73Arg (rs72556345) was found in five heterozygotes (three RM cases and two control individuals) and was inherited by two of seven studied live born children. The mutation caused ∼50% of secreted β-subunits to acquire an alternative conformation, but did not affect its biological activity. For the CGB8 p.Arg8Trp (rs72556341) substitution, the applied in vitro methods revealed no alterations in the assembly of intact hCG as also supported by an in silico analysis. In summary, the accumulated data indicate that only mutations with neutral or mild functional consequences might be tolerated in the major hCGβ genes CGB5 and CGB8.

Chorionic gonadotropin beta-gene variants are associated with recurrent miscarriage in two European populations

CONTEXT

The incidence of recurrent miscarriage (RM) (>or=3 consecutive pregnancy losses) is estimated as 1-2% in fertile couples. Familial clustering of RM has suggested the contribution of a genetic component.

OBJECTIVE

A low level of human chorionic gonadotropin (HCG) in maternal serum during the first trimester of the pregnancy is a clinically accepted risk factor for miscarriage. We sought to study whether variation in chorionic gonadotropin beta-subunit genes (CGBs) expressed in placenta may contribute to the risk of RM.

DESIGN

Resequencing of CGB5 and CGB8, the two most actively transcribed loci of the four HCG beta-duplicate genes, was performed.

SETTING

A case-control study involving two sample sets, from Estonia (n = 194) and Finland (n = 185), was performed.

PATIENTS

RM patients (n = 184) and fertile controls (n = 195) participated in the study.

RESULTS

From 71 identified variants in CGB5 and CGB8, 48 polymorphisms were novel. Significant protective effect was associated with two single nucleotide polymorphisms located at identical positions in intron 2 in both CGB5 [P = 0.007; odds ratio (OR) = 0.53] and CGB8 (P = 0.042; OR = 0.15), and with four CGB5 promoter variants (P < 0.03; OR = 0.54-0.58). The carriers of minor alleles had a reduced risk of RM. The haplotype structure of the CGB8 promoter was consistent with balancing selection; a rare mutation in CGB8 initiator element was detected only among patients (n = 3). In addition, three rare nonsynonymous substitutions were identified among RM cases as possible variants increasing the risk of recurrent pregnancy loss.

CONCLUSION

The findings encourage studying the functional effect of the identified variants on CGB expression and HCG hormone activity to elucidate further the role of CGB variation in RM.

No evidence of the human chorionic gonadotropin-beta gene 5 betaV79M polymorphism in Mexican women

Human chorionic gonadotropin (hCG) is a placental hormone essential for the maintenance of pregnancy. Previous studies have shown a G to A transition in exon 3 of the hCGbeta gene 5, which changes the naturally occurring valine to methionine in codon 79. The frequency of this transition varies among different ethnic groups, being high in USA women, and less common, or absent, in various European populations. The purpose of the present study was to determine the frequency of the betaV79M allelic variant of the beta-subunit of hCG in a Mexican population, and to compare this frequency with those found in other ethnic groups. Placental DNA from 161 pregnant Mexican women was genotyped for the betaV79M by polymerase chain reaction (PCR)-restriction fragments length polymorphism analysis. No polymorphic betaV79M alleles were identified in the population studied. The allele and genotypic frequencies of betaV79M polymorphism in Mexican Mestizo women were significantly different from those reported for the US population, but not from five different European populations. In contrast to what has been found in women from the USA, it seems that the hCGbeta V79M polymorphism is absent or extremely rare in Mexican Mestizo women.

An update of the pathophysiology of human gonadotrophin subunit and receptor gene mutations and polymorphisms

New information about mutations and polymorphisms in the genes for the gonadotrophins and their receptors has become available in the last few years. In this short review mutations and polymorphisms in gonadotrophins, their receptors and their pathophysiological effects and implications are discussed. An increasingly clear picture about the structure–function relationships of gonadotrophin action is emerging from the combining the types and the locations of the mutations with their phenotypic effects and the information about the crystal structure of these molecules.

Human FSH beta subunit gene is highly conserved

FSH is a pituitary gonadotropin that along with LH plays a key role in the regulation of gonadal function. The gonadotropic hormones are composed of two subunits, the common alpha subunit and the hormone-specific beta subunit, which determines the binding to specific receptors and induction of biological response. Unlike the LHbeta gene, shown in earlier studies to harbour several amino acid-altering polymorphisms and mutations, information about the eventual sequence variation of the human FSHbeta subunit is not available. In this study, we made sequence analysis and comparison of polymorphisms found in FSHbeta in two Caucasian populations, the Finns and the Danes. It was found that FSHbeta subunit is highly conserved in these populations. Compared with the published sequences, only three silent polymorphisms were detected in the coding regions of the gene, and the promoter sequence was completely identical with the reported sequence. Two of the polymorphisms found were novel, one in the Finnish and one in the Danish population. The results of the sequence analysis show that the human FSHbeta gene is highly conserved and amino acid changing mutations are apparently extremely rare, at least in the samples collected randomly from control populations. This may be due to the crucial role of normal FSH function in the regulation of fertility.