Bi-allelic PRMT9 loss-of-function variants cause a syndromic form of intellectual disability.

Protein arginine methyltransferase 9 (PRMT9) is part of the PRMT family, and it is suspected to function in pathways relevant to neurodevelopment. It is thought to participate in alternative splicing through interactions with the splicing factor SF3B2 (SAP145). In this study, we report 26 families (35 individuals) with bi-allelic loss-of-function variants in PRMT9, implicating PRMT9 in an autosomal-recessive human disease. Individuals primarily present with a neurodevelopmental disorder characterized by global developmental delay, learning disabilities, mild to severe intellectual disability, autism spectrum disorder, epilepsy, and hypotonia. The mutation spectrum includes 26 different variants such as frameshifting indels, nonsense variants, missense variants, and two copy-number variants. Mapping of the disease-causing missense variants onto the crystal structure of PRMT9 revealed that several of the variants reside within the catalytically active module of PRMT9, likely impairing its methyltransferase activity and resulting in a loss of function. In skin fibroblasts derived from affected individuals, we observed reduced expression at the RNA and/or protein level and subsequent aberrant methylation activity. Moreover, transcriptomic analysis of fibroblasts from affected individuals indicated differential expression of genes related to intellectual disability, autism, and cilia, suggesting a role of PRMT9 during ciliogenesis. Under ciliogenesis conditions, the skin-derived fibroblasts exhibited anomalies in the length of primary cilia but normal amounts of cilia. In addition, a prmt9 knockout zebrafish model displayed abnormal social preference in adult animals. Altogether, our findings implicate bi-allelic PRMT9 loss-of-function variants as causal for neurodevelopmental disorders.