Activation of HERV-K(HML-2) disrupts cortical patterning and neuronal differentiation by increasing NTRK3

The biological function and disease association of human endogenous retroviral (HERV) elements is largely elusive. HERV-K(HML-2) has been associated with neurotoxicity but there is no clear understanding of its role or mechanistic basis. We addressed the physiological functions of HERV-K(HML-2) in neuronal differentiation by using CRISPR engineering to activate or repress its expression levels in a human pluripotent stem cell-based system. We found that elevated HERV-K(HML-2) transcription is detrimental for the development and function of cortical neurons. These effects are cell type-specific, as dopaminergic neurons are unaffected. Moreover, high HERV-K(HML-2) transcription alters cortical layer formation in forebrain organoids. HERV-K(HML-2) transcriptional activation leads to hyperactivation of NTRK3 expression and other neurodegeneration-related genes. Direct activation of NTRK3 phenotypically resembles HERV-K(HML-2) induction, and reducing NTRK3 levels in context of HERV-K(HML-2) induction restores cortical neuron differentiation. Hence, these findings unravel a cell type-specific role for HERV-K(HML-2) in cortical neuron development.