ESPE Abstracts

Background: Turner syndrome (TS) arises from a complete or partial loss of one X chromosome (45,X) and is the most common genetic cause of primary ovarian insufficiency (POI) in women. Surprisingly little is understood about the pathogenesis of POI in TS beyond an acknowledged germ cell loss throughout the second trimester. Although X chromosome haploinsufficiency likely contributes, the variability in reproductive phenotype in 45,X TS suggests it is not the only explanation. We aimed to identify novel potential mechanisms of ovarian dysfunction in TS.

Methods: We performed focused single-nuclei RNA sequencing (snRNAseq) of 46,XX (n=2) and 45,X (n=2) fetal ovaries at 12-13 weeks post conception (peri-meiosis)(10x Genomics). Samples were obtained in collaboration with the Human Developmental Resource Bank (HDBR). Tissue karyotype was confirmed on skin biopsy and array.

Results: The 45,X ovary transcriptome is globally abnormal at single nuclei resolution. Within all oogonia snRNAseq cell subpopulations, the 45,X ovary had fewer germ cells than the 46,XX ovary. X chromosome inactivation dynamics were clearly disrupted in 45,X ovaries: 45,X germ cells expressed less XIST and JPX than 46,XX germ cells, although did express some XACT until the oogonia stage. A cluster of “synaptic oogonia” was absent from 45,X ovaries; genes differentially expressed in this cluster compared to shared clusters related to the synaptonemal complex, molecular chaperones (e.g., heat shock, CCT complex), and protein synthesis regulation. Key genes that were lower in the 45,X ovary include: 1) genes involved in intracellular protein regulation (“proteostasis”), including X-linked HUWE1 and RPS4X, an X chromosome gene that escapes X-inactivation; 2) heat shock protein genes (HIST1H2AA, HSP90AA1) and histone methylases (KDM1A, KDM3A) in fetal germ cells; 3) genes involved in meiotic progression (BUB1B); and 4) genes related to mitochondrial energy production (ATP11C; COX6C). One key gene higher in 45,X ovary somatic cells was NR4A1, encoding an orphan nuclear receptor (NUR77) involved in development, inflammation, and steroidogenesis.

Conclusions: While failure of sex chromosome synapsis at meiotic recombination may be the primary defect in 45,X germ cells, these data suggest that the beginnings of meiotic failure in 45,X germ cells precede this. Aberrant ribosomal biosynthesis and proteostasis, possibly due to X chromosome haploinsufficiency or even a primary energy deficiency, may result in 45,X germ cell incompetence evident from the very earliest stages of germ cell development. Taken together, these data provide novel insights into potential mechanisms of ovarian insufficiency in Turner Syndrome and identify new therapeutic targets.