A KDM5 variant outside the catalytic Jumonji C domain disrupts demethylase activity and alters the proximity interactome.

Transcriptional programs regulated by the KDM5 family of chromatin-modifying proteins are dysregulated in cancer and intellectual disability (ID) disorders. To define the fundamental mechanisms by which KDM5 regulates disease-relevant gene expression, we use missense variants in the X-linked KDM5C gene associated with the ID disorder Claes-Jensen syndrome (also known as KDM5C-NDD). Here, we use Drosophila melanogaster to investigate the effects of KDM5A224T, equivalent to human KDM5CA77T, which affects a conserved residue outside the catalytic histone demethylase JmjC domain and alters both enzymatic and non-enzymatic activities. Quantifying levels of H3K4me3, the demethylase substrate of KDM5, in adult brains revealed that Kdm5A224T induced changes indistinguishable from those observed with a catalytically inactive allele. This effect was not due to reduced promoter recruitment of the variant KDM5A224T protein. Instead, TurboID studies demonstrate that KDM5A224T exhibits reduced proximity with proteins involved in promoter activity and chromatin remodeling. Together, these findings show that KDM5-dependent transcriptional regulation cannot be explained by demethylase activity alone and support altered chromatin regulatory interactions as a key mechanism underlying pathogenic KDM5 variants.