Turner Syndrome Complicated by a NONO Gene Variant

Non-POU domain-containing octamer-binding (NONO)-associated X-linked intellectual disability syndrome (NAXIS) is a rare disorder characterized by findings that include cardiomyopathy, feeding difficulties, intellectual disability, and seizures that has previously been reported only in males. Here, we describe a girl with Turner syndrome (TS) due to (45,X/46,X,r(X)) mosaicism. She demonstrates typical features of TS, such as a bicuspid aortic valve and growth failure, but also displays symptoms not typical of TS that are more severe than expected in classic TS. Whole-exome sequencing revealed a likely pathogenic variant in the NONO gene, providing an explanation for her unexpected symptoms. This case highlights the importance of considering an evaluation for X-linked disorders in individuals with TS presenting with atypical symptoms, as this can assist the family and the medical team with expectant management.