The genuine empty follicle syndrome: is the king naked?

ZP1 mutations are associated with empty follicle syndrome: evidence for the existence of an intact oocyte and a zona pellucida in follicles up to the early antral stage. A case report

Empty follicle syndrome (EFS) is the complete failure to retrieve oocytes after ovarian stimulation. Although LHCGR and ZP3 were identified as causative genes, it is still unclear what happens to these patients' oocytes, and the pathogenesis of EFS remains obscure. Here, we identified six novel ZP1 mutations associated with EFS and female infertility that was inherited recessively in five unrelated families. Studies in CHO-K1 cells showed that these mutations resulted in either degradation or truncation of ZP1 protein. Immunohistochemistry using ovarian serial sections demonstrated that all preantral follicles had normal architecture, but with a thin ZP, lacking ZP1, surrounding the growing oocytes. The antral follicles were also defective in normal cumulus-oocyte complex organisation, leading us to speculate that the lack of ZP1 might lead to oocyte degeneration or increased fragility of the oocyte during follicular puncture, ultimately resulting in EFS. To our knowledge, this is the first study that presents morphological evidence showing normal preantral folliculogenesis with abnormal ZP assembly in EFS patients. Our data provides a better understanding of the biological functions of ZP1 in human ZP assembly and folliculogenesis and gives new insights into the pathogenesis of EFS and possible therapeutic developments.

Novel biallelic loss-of-function variants in ZP1 identified in an infertile female with empty follicle syndrome

PURPOSE

Empty follicle syndrome (EFS) refers to the inability to obtain mature oocytes after appropriate ovarian stimulation during the process of in vitro fertilization (IVF). However, the specific cause and mechanism of action underlying EFS remain to be further explored. Herein we aimed to investigate the clinical and genetic characteristics of EFS.

METHODS

After data were collected in an infertile family, we performed whole-exome sequencing (WES) on the patient and confirmed the pathogenic mutations through Sanger sequencing. Western immunoblotting, immunofluorescence staining, and minigene assay were further used to investigate the negative effects of these mutations.

RESULTS

Absence of oocytes was observed over 2 cycles of IVF in the patient, and we evaluated the novel compound heterozygous mutations c.2T>A (p. M1K) and c.1112+1G>T of the zona pellucida glycoprotein 1 gene (ZP1, MIM# 195000) by WES. For the family under study, EFS was classified as an autosomal recessive inheritance pattern. The results of western blotting and immunofluorescence staining analyses confirmed that the missense mutation of c.2T>A [p. M1K] resulted in almost missing protein production. Additionally, using a minigene assay, we demonstrated the deleterious effect on the ZP1 gene of the splice site mutation c.1112+1G>T, which caused truncation of ZP1 protein.

CONCLUSIONS

The compound heterozygous mutations of ZP1 gene identified in this study by genetic and functional experiments constituted a novel genetic cause of EFS, and further study will expand its use in the clinical and molecular diagnoses of EFS.

Does the empty follicle syndrome occur in cases of low number of maturing follicles in assisted reproduction?

The pathophysiology of the genuine empty follicle syndrome (EFS) is still debated. Ovarian aging has been contested as a cause of this condition. Our aim was to investigate the occurrence of the genuine EFS in cases of a low number of mature follicles in a prospective manner. Ninety-five infertile women were recruited and evaluated following conventional controlled ovarian stimulation (COS) with ≤ six follicles of ≥14 mm diameter on the day of hCG administration. Enrolled women were 37.5 ± 5.2 years of age with basal FSH level of 9.1 ± 3.7 mIU/L, antral follicle count (AFC) of 6.9 ± 4.6, and number of ≥14 mm follicles (on the day of hCG) of 3.4 ± 1.5. Among the 95 women, four were complicated by the genuine EFS (4.2%) with features of the depleted ovarian reserve. Comparison between these four cases and the 91 controls revealed significant differences between age, AFC, number of ≥14 mm follicles, and serum E₂ level corresponding to 41.8 ± 1.7 versus 37.4 ± 5.2 years, 1.7 ± 0.6 versus 7.1 ± 4.5, 2.0 ± 0.8 versus 3.4 ± 1.5 follicles, and 356 ± 200 versus 975 ± 557 pg/mL, respectively. Post hoc analysis revealed that 56 among the 95 women fulfilled the Bologna criteria for poor ovarian response and all four cases matched the definition of the genuine EFS raising its incidence to 7.1% in this group. A logistic regression analysis showed that AFC was a significant factor in the development of the genuine EFS. We conclude that the genuine EFS complicates infertile women characterized by a low number of mature follicles. Our findings suggest that the mechanism behind this occurrence is associated with a more exhausted ovarian reserve.

Empty follicle syndrome revisited: definition, incidence, aetiology, early diagnosis and treatment

In this review, the definition, incidence and possible causes of empty follicle syndrome (EFS), including molecular mechanisms that may underlie the syndrome, are discussed, along with prevention and treatment options. EFS is the complete failure to retrieve oocytes after ovarian stimulation, despite apparently normal follicle development and adequate follicular steroidogenesis. Two variants of EFS have been described: the 'genuine' form (gEFS), which occurs in the presence of adequate circulating HCG levels at the time of oocyte aspiration, and the 'false' form (f-EFS), which is associated with circulating HCG below a critical threshold. Heterogeneous HCG concentration thresholds, however, have been used to define gEFS, and to date no standardization exist. The situation is unclear when GnRH-analogues are used for ovulation trigger, as the threshold circulating LH and progesterone levels used to define EFS as 'genuine' are not established. The cause of fEFS has been clearly identified as an error in HCG administration at the time of ovulation trigger; in contrast, the cause of gEFS is still unclear, although some pathogenetic hypotheses have been proposed. Optimal treatment and prognosis of these patients are still poorly understood. Large, systematic multi-centre studies are needed to increase the understanding of EFS.