Primate-specific adaptation of Ku protects transcriptomic integrity by suppressing Alu-mediated alternative splicing.

Accurate pre-mRNA splicing is essential for the transfer of genetic information but faces unique challenge in higher primates due to the massive expansion of intronic Alu elements1-3. While studying Ku, the Ku70/Ku80 heterodimer best known for initiating non-homologous end-joining (NHEJ) by encircling DNA ends4, we discovered that Ku expression increased markedly during primate evolution in parallel with Alu expansion5. Ku binds double-stranded RNA (dsRNA) stem-loops, including those at the antisense Alu (asAlu) elements within introns5. Here, we show that Ku-depletion in human cells has a broad impact on splicing largely independent of cell-cycle states, NHEJ, or innate immune signaling, significantly affecting ~8-10% of quantifiable alternative splicing events. Mechanistically, Ku directly binds exonic asAlu to prevent their aberrant inclusion and binds asAlu within inverted-repeat Alu (irAlu) pairs flanking canonical exons to prevent exon skipping6. Among human tissues, Ku expression in the brain is consistently ~50% lower, correlating with more permissive expression of Alu-derived splice variants, particularly those encoding mitochondrial proteins and RNA-binding factors. Correspondingly, heterozygous Ku loss in patient causes developmental delay, neurological dysfunction, and acidosis. Together our findings identified Ku as a critical suppressor of Alu-associated alternative splicing co-opted during evolution with implications for primate brain function and human disease.