Biallelic variants in RNU2-2 cause the most prevalent known recessive neurodevelopmental disorder.

We recently showed that mutations in the snRNA genes RNU4-2 and RNU2-2 are prevalent causes of dominant neurodevelopmental disorders (NDDs). Here, by genetic association, we demonstrate the existence of a recessive form of RNU2-2 syndrome. We inferred a log Bayes factor for a recessive model of association of 18.2. Conditional on that model, 17 rare variants had a posterior probability of pathogenicity >0.8. This conservative threshold identified 18 probands and 5 affected siblings, each carrying two alleles in trans at these variants. A relaxed threshold of >0.6 identified a further 13 candidate probands. We identified nine further cases in replication collections. Affected individuals have intellectual disability, global developmental delay and seizures. Recessive RNU2-2 syndrome accounts for ~10% of families with a recessive NDD presently diagnosable by sequencing and affects ~60% as many families as the dominant RNU4-2-related NDD ReNU syndrome. The variants are predicted to destabilize stem loops and binding domains of U2-2 snRNA. Whole-blood RNA sequencing data showed a >90% reduction in the expression of pathogenic U2-2 alleles in biallelic cases and monoallelic carriers, albeit with wild-type compensation in carriers, pointing to a loss-of-expression mechanism.