We identified a novel compound heterozygous FSHR mutation in four siblings from an un-consanguineous Han Chinese family, moreover, and firstly identified a mutation located in FSHR IL2 region. The variant c.1090_1091del carried by the father results in a truncated protein with 391 amino acids and chaos of the remaining transmembrane helix and intracellular loop. The variant c.1412T > G carried by the mother leading to a missense substitution (p.Ile471Ser) in the intracellular loop 2 which is predicted to be deleterious in silicon analysis and validated in the cAMP assay. The phenotype separating well with the genotype identified by WES and Sanger sequencing suggests that this compound heterozygous mutation is the disease-associated mutation in this family. Notably, c.1412T > G is the first variant in the FSHR intracellular loop 2 identified in POI patients so far.
During the initiation of follicular growth, FSH-FSHR signaling is not required[35]. FSHR begins to express between the preliminary and antral stage and is essential for later follicular development [36]. FSHR-caused POI is due to the arrest but not depletion of follicles usually with some ovarian reserve as determined by AMH (Anti-Mullerian Hormone) and AFC (antral follicle count). There is a correlation, which could be inferred but not completely reflected by in-vitro functional assay, between the specific mutation with the phenotype[13, 30]. POI patients caused by FSHR mutations identified previously most showed small follicles(3–5 mm) by transvaginal ultrasound or follicular arrest at the small antral stage histologically[18, 19, 29]. In other cases, a block of follicular growth after the primary stage, and no antral follicles was observed in patients with specific mutations[21, 22, 24, 26].
In this study, all the hypergonadotropic patients with primary amenorrhea have partially normal secondary sexual characteristics, after exogenous estrogen and progesterone therapy, and two of them received reproductive treatment and conceived after a donor’s oocyte-based IVF treatment indicating that they are not in complete sexual infantilism. However, the transvaginal ultrasound showed a hypoplastic uterus and small bilateral ovaries with no visible antral follicles of the four patients in this study. The phenotype indicated a more severe defect of in vivo FSHR function than that of cases with ultrasound-visible or small antral follicles. It is not difficult to infer that the 2 bp exon6 deletion (c.1090_1091del) resulting in a truncated protein with no TMD and intracellular structure causes a complete loss of function. As to the first IL2 mutation (c.1412T > G) identified in POI patients so far, the in vitro functional assay showed complete loss of the production of FSH induced cAMP. Together with the genotype-phenotype correlation, it suggested that the novel compound heterozygous mutation would remain with no residual FSHR activity. Our patients did not have ovarian histology and AMH tests performed, but we guess some follicles that arrest at different stages ranging from primordial follicles to preantral follicles in their ovaries. The normal male sibling who is fertile (II-7) was not genotyped and examined. However, we could not exclude the possibility that the male sibling also carries the two mutations in a compound heterozygous, for the FSHR defection may not affect male fertility in most case[13].
Since the first inactivating mutation p.Ala189Val in the FSHR ECD was reported in Finnish women[16], more variants were identified mostly in ECD, TMD, and ELs[30]. It is well established that the ECD is the primary hormone-binding domain, whereas the TMD is the signal-transducing domain. ELs also play an essential role in ligand binding and FSHR trafficking in cell membranes[37]. ILs and ICD mutations in POI were relatively uncommon since the first variant in the C-terminal domain(p.Pro688Thr) in POI was just identified recently and only one mutation (p.Arg573Cys) was reported in IL3 so far[18, 31]. Previous studies have provided evidence showing the key role of IL2 in Gs coupling, cAMP production, and also receptor phosphorylation [38, 39], but there was no IL2 variation identified in POI patients before. The missense mutation in this case locates close to the highly conserved class A GPCR ERW motif, which is pivotal for receptor function[40]. Also, it locates at the boundary of the region associated with 14-3-3 tau protein, which is important for ER localization of membrane proteins, interacting activity [39, 41]. These all, with the cAMP assay result, support the c.1412T > G mutation as a variant of certain significance. However, as we didn’t perform membrane localization assay, there is still a possibility that the c.1412T > G FSHR protein decreased at cell membrane though no structure change by in silicon prediction. The cAMP induction assay was not necessary for the c.1090_1091del mutation, for the 2 bp deletion frameshift mutation could result in a badly deleterious effect on the receptor structure.