The alpha thalassemia, mental retardation, X-linked (ATR-X) syndrome is a difference of sex development affecting XY individuals caused by mutations in the chromatin remodelling gene ATRX. Patients display genital abnormalities varying from hypospadias to ambiguous genitalia to male-to-female sex reversal. Gonadal histology reveals small testes containing only a few seminiferous tubules. To investigate the underlying mechanisms of this phenotype, our lab generated mice with Atrx specifically inactivated in Sertoli cells (ScAtrxKO), a cell lineage crucial for seminiferous tubule formation. ScAtrxKO mice showed small testes with fewer tubules because of G2/M cell cycle arrest and apoptosis of Sertoli cells during fetal life.
We wanted to understand why ATRX-deficient Sertoli cells of ScAtrxKO undergo apoptosis. We identified a single giant speckle in the nuclei of these cells which expressed GATA4. Unique to Sertoli cell nuclei, GATA4 foci were identified as PML nuclear bodies (PML NBs) in the wildtype and ScAtrxKO gonads. GATA4-PML NBs are composed of a Y chromosome repetitive DNA, ATRX and DAXX which establish a heterochromatic state of the short arm of the Y chromosome. Although formation of PML bodies is independent of ATRX, the loss of ATRX in Sertoli cells affects the size, protein composition and chromatin regulation of GATA4-PML NBs. The loss of ATRX results in the absence of DAXX, HP1α, and PH3 from the GATA4-PML NBs, leading to the loss of the heterochromatic state, chromatin decondensation and vulnerability to DNA damage. This would explain the reported cell cycle arrest in the late G2- mitosis phase, and consequent apoptosis of Sertoli cells.1 In turn loss of Sertoli cells affects testis cord integrity, resulting in the small testes phenotype seen in the murine model and reported in boys with ATR-X syndrome.