Reduced RNA expression of the FMR1 gene in women with low (CGGn&lt;26) repeats

Qi Wang,David F Albertini,Norbert Gleicher,David H Barad,Emanuela Lazzaroni-Tealdi,Lin Zhang,Sarah K Darmon,Yan-Guang Wu,Vitaly A Kushnir,Cornelis B Lambalk

doi:10.1371/journal.pone.0209309

Abstract

Low FMR1 variants (CGGn<26) have been associated with premature ovarian aging, female infertility and poor IVF treatment success. Until now, there is little published information concerning possible molecular mechanisms for this effect. We wished to examine whether relative expression of RNA and the FMR1 gene’s fragile X mental retardation protein (FMRP) RNA isoforms differ in women with various FMR1 sub-genotypes (normal, low CGGn<26 and/or high CGGn≥34). This prospective cohort study was conducted between 2014 and 2017 in a clinical research unit of the Center for Human Reproduction in New York City. The study involved a total of 98 study subjects, including 18 young oocyte donors and 80 older infertility patients undergoing routine in vitro fertilization (IVF) cycles. The main outcome measure was RNA expression in human luteinized granulosa cells of 5 groups of FMRP isoforms. The relative expression of FMR1 RNA in human luteinized granulosa cells was measured by real-time PCR and a possible association with CGGn was explored. All 5 groups of FMRP RNA isoforms examined were found to be differentially expressed in human luteinized granulosa cells. The relative expression of four FMR1 RNA isoforms showed significant differences among 6 FMR1 sub-genotypes. Women with at least one low allele expressed significantly lower levels of all 5 sets of FRMP isoforms in comparison to the non-low group. While it would be of interest to see whether FMRP is also decreased in the low-group we recognize that in recent years it has been increasingly documented that information flow of genetics may be regulated by non-coding RNA, that is, without translation to a protein product. We, thus, conclude that various CGG expansions of FMR1 allele may lead to changes of RNA levels and ratios of distinct FMRP RNA isoforms, which could regulate the translation and/or cellular localization of FMRP, affect the expression of steroidogenic enzymes and hormonal receptors, or act in some other epigenetic process and therefore result in the ovarian dysfunction in infertility.

Highlights

The most common cause of the fragile X syndrome (FXS) is the impaired expression of the Fragile X mental retardation 1 (FMR1) gene, resulting from the unstable expansion of a CGG repeat in its 5’ untranslated region
In this study aimed to investigate whether relative expression of FMR1 RNA differ in infertile women with various ranges of CGGn
We found differential quantitative expression of FMR1 RNA isoforms in the granulosa cells of women undergoing oocyte retrieval following ovulation induction for in vitro fertilization (IVF)

Summary

Introduction

The most common cause of the fragile X syndrome (FXS) is the impaired expression of the Fragile X mental retardation 1 (FMR1) gene, resulting from the unstable expansion of a CGG repeat in its 5’ untranslated region. Lack of expression of FMR1 gene, which encodes a RNA-binding protein, fragile X mental retardation protein (FMRP), is responsible for the mental retardation and the associated pleiotropic clinical phenotype. FMRP Isoform 1, the full-length protein, contains two K-homology domains and an arginine–glycine–glycine (RGG) box which are responsible for RNA-binding, a nuclear localization signal at its N-terminus and a nuclear export signal at C-terminus, suggesting shuttling between the nucleus and cytoplasm, and post-translational modification sites through phosphorylation and methylation [5, 7,8,9,10,11,12]. The largest expansions found in patients (>230 repeats) are abnormally methylated, leading to the silencing of FMR1 gene transcription [13, 14]

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