Loss of Uhrf1 in neural stem cells leads to activation of retroviral elements and delayed neurodegeneration

In order to understand whether early epigenetic mechanisms instruct the long-term behavior of neural stem cells (NSCs) and their progeny, we examined Uhrf1 (ubiquitin-like PHD ring finger-1; also known as Np95), as it is highly expressed in NSCs of the developing brain and rapidly down-regulated upon differentiation. Conditional deletion of Uhrfl in the developing cerebral cortex resulted in rather normal proliferation and neurogenesis but severe postnatal neurodegeneration. During development, deletion of Uhrfl lead to global DNA hypomethylation with a strong activation of the intracisternal A particle (IAP) family of endogenous retroviral elements, accompanied by an increase in 5-hydroxymethylcytosine. Down-regulation of Tet enzymes rescued the IAP activation in Uhrfl conditional knockout (cKO) cells, suggesting an antagonistic interplay between Uhrfl and Tet on IAP regulation. As IAP up-regulation persists into postnatal stages in the Uhrfl cKO mice, our data show the lack of means to repress IAPs in differentiating neurons that normally never express Uhrfl. The high load of viral proteins and other transcriptional deregulation ultimately led to postnatal neurodegeneration. Taken together, these data show that early developmental NSC factors can have long-term effects in neuronal differentiation and survival. Moreover, they highlight how specific the consequences of widespread changes in DNA methylation are for certain classes of retroviral elements.