A homozygous NOBOX truncating variant causes defective transcriptional activation and leads to primary ovarian insufficiency

STUDY QUESTION: Does a novel homozygous NOBOX truncating variant, identified in whole exome sequencing (WES) of patients with primary ovarian insufficiency (POI), cause defective transcriptional activation of multiple oocyte-related genes? SUMMARY ANSWER: A novel homozygous truncating mutation of NOBOX was confirmed to exhibit a loss-of-function effect using welldefined molecular and functional analyses. WHAT IS KNOWN ALREADY: Several NOBOX mutations have been reported to be associated with POI but all of them are heterozygous mutations. STUDY DESIGN, SIZE, DURATION: This is a cross sectional study in 96 patients diagnosed with POI and 211 women not diagnosed with POI in China. PARTICIPANTS/MATERIALS, SETTING, METHODS: Blood samples collected from the participants were subjected to whole exome sequencing. Full-length transcript of NOBOX was cloned directly from human fetal ovary (FO). Functional analysis was performed for a NOBOX sequence variant associated with POI. MAIN RESULTS AND THE ROLE OF CHANCE: One novel homozygous truncating variant, chr7: 144098161delC, in the NOBOX gene was found in a POI patient. The truncating variant showed a severe defect in transcriptional activation of GDF9 a well-known target NOBOX. Furthermore, using real-time quantitative PCR analysis, we found many oocyte-related genes were expressed at lower level in truncating variant cells than in control cells. In addition, we found that the truncated NOBOX lost its ability to induce the G2/M arrest. Notably, our results confirmed that the 1725 bp NOBOX transcript is expressed in human FO and is the only functional isoform in transcriptional activation assays. LIMITATIONS REASONS FOR CAUTION: Although the in vitro assays demonstrated the loss-of-function effect of truncating mutation on NOBOX transcriptional activation, further studies are needed to validate its long-term effects on folliculogenesis and POI. WIDER IMPLICATIONS OF THE FINDINGS: This is the first homozygous mutation of NOBOX associated with POI showing a loss-of-function effect using well-defined molecular and functional analyses. These results will aid both researchers and clinicians in understanding the molecular pathology of NOBOX and POI to develop diagnostic assays or therapeutic approaches.