New AccI polymorphism in the follicle-stimulating hormone beta-subunit gene and its prevalence in three Southeast Asian populations

Identification of common genetic polymorphisms associated with down-regulated gonadotropin levels in an exome-wide association study

OBJECTIVE

To investigate whether common genetic polymorphisms are associated with gonadotropin levels after down-regulation with daily gonadotropin-releasing hormone agonist and whether the polymorphisms of candidate variants influence the ovarian response to exogenous gonadotropins.

DESIGN

Genetic association study.

SETTING

University-affiliated in vitro fertilization center.

PATIENTS

Subjects enrolled in an exploratory exome-wide association study (n = 862), a replication exome-wide association study (n = 86), and a classifier validation study (n = 148) were recruited from September 2016 to October 2018, September 2019 to September 2020, and January 2021 to December 2021, respectively. The included patients were aged ≤40 years and had a basal follicle-stimulating hormone (FSH) ≤12 IU/L.

INTERVENTIONS

All participants received a luteal phase down-regulation long protocol. Genome DNA was extracted from the peripheral blood leukocytes. For the exploratory and replication cohorts, exome sequencing was conducted on a HiSeq 2500 sequencing platform. The multiplex polymerase chain reaction amplification technique and next-generation sequencing also were performed in the exploratory and replication cohorts. For the samples of the validation cohort, Sanger sequencing was performed.

MAIN OUTCOME MEASURES

The primary endpoint was the gonadotropin levels after down-regulation, and the secondary endpoints were hormone levels and follicle diameters during stimulation, the total dose of FSH, duration of FSH stimulation, number of oocytes retrieved, and clinical pregnancy rate.

RESULTS

In the exploratory cohort, we identified that FSHB rs6169 (P=2.71 × 10-24) and its single-nucleotide polymorphisms in high linkage disequilibrium were associated with the down-regulated FSH level. The same locus was confirmed in the replication cohort. Women carrying the C allele of FSHB rs6169 exhibited higher average estradiol level during stimulation (P=6.82 × 10-5), shorter duration of stimulation, and less amount of exogenous FSH (Pduration=0.0002; Pdose=0.0024). In the independent validation set, adding rs6169 genotypes into the prediction model for FSH level after down-regulation enhanced the area under the curve from 0.560 to 0.712 in a logistic regression model, and increased prediction accuracy by 41.05% when a support vector machine classifier was applied.

CONCLUSION

The C allele of FSHB rs6169 is a susceptibility site for the relatively high level of FSH after down-regulation, which may be associated with increased ovarian FSH sensitivity.

Haplotype structure of FSHB, the beta-subunit gene for fertility-associated follicle-stimulating hormone: possible influence of balancing selection

Follicle-stimulating hormone (FSH) is essential for human reproduction. The unique functions of this hormone are provided by the FSH receptor-binding beta-subunit encoded by the FSHB gene. Resequencing and genotyping of FSHB in three European, two Asian and one African population, as well as in the great apes (chimpanzee, gorilla, orangutan), revealed low diversity and significant excess of polymorphisms with intermediate frequency alleles. Statistical tests for FSHB showed deviations from neutrality in all populations suggesting a possible effect of balancing selection. Two core haplotypes were identified (carried by 76-96.6% of each population's sample), the sequences of which are clearly separated from each other. As fertility most directly affects an organism's fitness, the carriers of these haplotypes have apparently had more success in human history to contribute to the next generation. There is a preliminary observation suggesting that the second most frequent FSHB haplotype may be associated with rapid conception success in females. Interestingly, the same haplotype is related to an ancestral FSHB variant shared with the ancestor of the great apes. The determination of the functional consequence of the two core FSHB variants may have implications for understanding and regulating human fertility, as well as in assisting infertility treatments.

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.

The role of mutations affecting gonadotrophin secretion and action in disorders of pubertal development

A number of mutations that disturb the development and function of the hypothalamic-pituitary-gonadal (HPG) axis and cause disturbances in pubertal development are known today. These mutations have effects at all levels of the HPG axis, from the migration of gonadotrophin releasing hormone (GnRH) neurones from the nasal cavity to the hypothalamus, GnRH secretion, GnRH action, pituitary gonadotroph differentiation, gonadotrophin synthesis and secretion, right through to gonadotrophin action. Most of the mutations are inactivating, thus causing hypogonadism and arrest or delay of pubertal development. One exception is the activating mutations of the LH receptor, which causes the male-limited gonadotrophin-independent precocious puberty. The human mutations and animal models with disrupted function of orthologous genes have clarified the molecular pathogenesis of hypogonadism and disturbances of pubertal development. The correct diagnosis of these disorders using molecular biological techniques is now possible. This allows the selection of specific treatments and correct counselling of the patients and their families.

Association of AccI polymorphism in the follicle-stimulating hormone beta gene with polycystic ovary syndrome

OBJECTIVE

To search for FSH beta-subunit gene mutations in patients with polycystic ovary syndrome (PCOS) and determine the association between the mutations and the syndrome.

DESIGN

Clinical and molecular studies.

SETTING

Clinics and laboratories of the National University Hospital Obstetrics and Gynecology Department in Singapore.

PATIENT(S)

One hundred thirty-five patients with PCOS and 105 normal control subjects.

INTERVENTION(S)

Exons two and three were screened for mutations by single-stranded conformational polymorphism and DNA sequencing.

MAIN OUTCOME MEASURE(S)

Polymerase chain reaction followed by restriction enzyme analysis.

RESULT(S)

No missense mutation was found in the functional units of the FSHbeta gene in patients with PCOS, but a thymine-cytosine substitution in exon 3 (codon 76, TAT to TAC) was identified. The nucleotide change led to creation of an AccI digestion site. The distribution pattern of AccI polymorphism in the patients was significantly different from that in the control group, and the occurrence of homozygous carriers was significantly higher in patients (12.6%) than in the control group (3.8%). The frequency of polymorphism and prevalence of homozygosity were significantly higher in patients with PCOS with obesity (0.50% and 31.0%, respectively) than in those with menstrual disorders only (0.366% and 8.5%, respectively), which correlated with significantly higher androgen levels in the obese patients.

CONCLUSION(S)

The AccI polymorphism in FSHbeta gene may be associated with PCOS in some women, especially those with obesity.