Single-cell analysis of fetal testis reveals dysfunction of human Leydig cells in Klinefelter syndrome

Klinefelter syndrome (KS), the most common sex chromosome aneuploidy (affecting approximately 1 in 650 live male births), causes severe infertility. The extra X chromosome can impair the development of fetal germ cells, but its effects on somatic cells, especially Leydig cells, are still not well known. We performed single-cell RNA-sequencing analysis of fetal KS and control testicular cells and found 2 clusters of KS Sertoli cells, with the XIST-negative cluster showing distinct gene expression pattern and abnormally increased G2/M ratio. Fetal KS Leydig cells showed increased proliferation and immature differentiation with high level of MAPK signaling pathway and X-linked EIF1AX. Inhibition of MAPK signaling partially rescued overproliferation and defective differentiation and androgen secretion in KS Leydig cells, while overexpression of EIF1AX recapitulated the phenotypes of increased proliferation and decline in testosterone synthesis capacity in the Leydig cell line. These findings reveal the early pathological mechanisms of KS somatic cells and lay the groundwork for developing early intervention strategies.