Neurodevelopmental disorders (NDDs) affect approximately 2–4% of the population, and nearly half of cases remain genetically unresolved (1). Variants in the spliceosomal small nuclear RNA (snRNA) genes RNU4-2 and RNU2-2 have recently emerged as important causes of monogenic NDDs (2). In 2024, de novo variants in RNU4-2 were shown to cause ReNU syndrome, one of the most common known monogenic NDDs, accounting for approximately 0.5% of all NDD cases (2).
Recently, three independent groups demonstrated that biallelic variants in RNU2-2 cause a recessive neurodevelopmental disorder and characterized the syndrome at the phenotypic, genetic, and mechanistic levels (1–3). The recessive disorder is distinct from the previously described dominant RNU2-2 syndrome, although the phenotypes partially overlap (1).
Clinically, both dominant and recessive RNU2-2 disorders are characterized by severe-to-profound global developmental delay, intellectual disability, and epileptic seizures (3). Additional features of the recessive syndrome may include movement disorders, gastrostomy dependence, abnormal electroencephalographic findings, and cerebral atrophy on neuroimaging (1). Current data suggest that the phenotypic spectrum of the recessive disorder may be broader than that of the dominant form.
At the genetic level, the dominant RNU2-2 disorder is caused by a limited spectrum of recurrent variants clustered at the 5′ end of the gene, with two major recurrent hotspots. In contrast, biallelic variants associated with the recessive disorder are distributed more broadly across RNU2-2 (1). Mechanistically, individuals with dominant RNU2-2 syndrome show measurable aberrant splicing signatures in blood, whereas such abnormalities have not been consistently detected in the recessive disorder (1).
Despite its unexpectedly high prevalence, several factors hindered the identification of recessive RNU2-2 syndrome. Variant detection in snRNA genes is technically challenging because of sequence redundancy, incomplete genome annotations, and difficulties distinguishing functional genes from pseudogenes. In addition, the absence of the RNU2 locus from the GRCh37 assembly and the lack of strong founder variants further complicated discovery efforts (1).
Current evidence suggests that recessive RNU2-2 syndrome is among the most common currently identifiable recessive NDDs, accounting for approximately 10% of genetically solved recessive NDD families in some cohorts and potentially occurring more frequently than the dominant RNU2-2 disorder (1–3).
Key Points:
- Biallelic variants in RNU2-2, encoding small nuclear RNAs, are associated with recessive RNU2-2 syndrome characterized by neurodevelopmental delay and seizures. The condition is genetically and mechanistically distinct from the already reported dominant RNU2-2 syndrome.
- Recessive RNU2-2 syndrome is the most frequent NDD and may be approximately twofold more frequent than dominant RNU2-2 syndrome.
- At the genetic levels, candidate biallelic variants associated with the recessive disorder are more widely distributed throughout RNU2-2 gene. Aberrant splicing events observed in blood in the dominant RNU2-2 disorder are not detected in the recessive disorder.
References:
- Jackson, A., Blakes, A.J.M., Alhaddad, B. et al. Biallelic variants in RNU2-2 cause a remarkably frequent developmental and epileptic encephalopathy. Nat Genet (2026). doi.org/10.1038/s41588-026-02551-9
- Greene, D., Mendez, R., Lees, J. et al. Biallelic variants in RNU2-2 cause the most prevalent known recessive neurodevelopmental disorder. Nat Genet (2026). doi.org/10.1038/s41588-026-02539-5
- Leitão, E., Santini, A., Cogne, B. et al. Systematic analysis of snRNA genes reveals frequent RNU2-2 variants in dominant and recessive developmental and epileptic encephalopathies. Nat Genet (2026). doi.org/10.1038/s41588-026-02547-5
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