Sex chromosome overdosage in Klinefelter Syndrome (KS; 47,XXY) and high-grade sex-chromosome aneuploidies leads to a shared broad spectrum of clinical features, including motor, speech, and language delays. The severity of intellectual disabilities directly correlates with the number of extra X chromosomes. However, the in vitro modelling of these conditions is, to date, largely unexplored. We investigated the impact of X-chromosome aneuploidies during early neurodevelopment using an induced pluripotent stem cells-based disease-modelling approach.

We postulated that the neurodevelopmental deficits in KS and high-grade SCAs subjects lie in the overdosage of escape genes located within the pseudoautosomal region (PAR) of the X chromosome. We used a paradigmatic cohort of KS, high-grade SCAs, and 46,XY iPSCs to obtain 2D neuronal cultures and 3D cortical brain organoids. Combining a transcriptomic and functional approach, we demonstrated that X chromosome overdosage negatively impacts the electrophysiological properties of iPSC-derived neurons in a dosage-sensitive manner.