Clinical, cellular, and genomic consequences of a population-enriched SETD1A missense variant

Rare variants in SETD1A , encoding a histone H3K4 methyltransferase, are among the strongest genetic risk factors for schizophrenia. Exome sequencing (n = 3,736) revealed a population-enriched SETD1A missense variant (P596L) in the Lancaster Old Order Amish founder population, presenting a unique opportunity to elucidate variant-specific, multi-scale mechanisms. Psychiatric and cognitive phenotyping revealed nearly two-fold increased risk for bipolar disorder, accompanied by allele dose-dependent cognitive deficits in adulthood. Induced pluripotent stem cells (iPSCs) from homozygous carriers exhibited signatures of SETD1A hypofunction, including reduced proliferation and heightened susceptibility to replication stress and DNA double-strand breaks. During forebrain-directed differentiation, homozygous mutant cells displayed premature activation of neurodevelopmental transcriptional programs but impaired neural rosette formation, reduced neurite complexity, and early progenitor senescence. Multi-omic profiling revealed dysregulation of gene modules converging on replication stress pathways and neuronal regulatory networks enriched for autism and psychiatric risk genes. Pharmacologic inhibition of the H3K4 demethylase KDM5 partially rescued replication stress and neurite deficits, supporting an epigenetic mechanism and suggesting therapeutic tractability. Together, these findings link a population-enriched missense variant to disrupted chromatin regulation, genome stability, and neurodevelopmental timing, bridging human genetic risk with cellular pathophysiology.