Study identifies a common hidden genetic cause of neurodevelopmental disorder in children

A major genetic study has uncovered a surprisingly common recessive cause of a neurodevelopmental disorder, revealing how hidden changes in a small RNA gene can disrupt brain development and opening new avenues for diagnosis and family counseling.

Study: Biallelic variants in RNU2-2 cause the most common known recessive neurodevelopmental disorder. Image credit: Lightspring / Shutterstock

In a recent study published in Genetics of Naturea group of researchers identified and characterized recessive neurodevelopmental disorders caused by biallelic variants in the RNU2-2 gene and investigated their clinical and molecular features.

Background

Neurodevelopmental disorders affect millions of children worldwide, often leading to lifelong cognitive, behavioral and physical challenges. While many cases remain genetically unexplained, advances in sequencing have revealed that even small non-coding ribonucleic acids (RNA), such as small nuclear RNA (snRNA), play critical roles in brain development.

RNA splicing requires the help of non-coding RNAs. As such, they are extremely important to the overall function of RNA. The possibility that changes in the splicing process could play a role in the development of the disease has not been extensively studied.

Determining how changes in non-coding RNA genes can affect neurological disorders is important to provide clinicians with diagnostic tools and guide future therapeutic development. There is a significant knowledge gap regarding the genetic basis of the disease and further research is needed to fill it.

About the study

The researchers conducted a large-scale genetic association study using data from the 100,000 Genomes Project (100KGP), the Genomic Medicine Service (GMS) and the National Genomic Research Library (NGRL).

The analysis included 14,805 people with neurodevelopmental disorders and 52,861 controls without such conditions. A statistical method, the Bayesian evaluation of variant involvement in Mendelian disease (BeviMed), was applied to identify rare variants associated with the disease.

The researchers analyzed RNU2-2 gene variants using dominant and recessive models. Read-phase techniques were used to determine whether the variants occurred on the same chromosome (cis) or on different chromosomes (trans), allowing the identification of biallelic cases. Variants were ranked according to their disease-causing potential.

Independent datasets of participants of the Undiagnosed Diseases Network (UDN), the Undiagnosed Patients Program (UPP) and the Erasmus Medical Center (Erasmus MC) were also analyzed to confirm the results. Clinical phenotypes were assessed according to Human Phenotype Ontology (HPO) terms. In addition, whole blood RNA sequencing (RNA-seq) data from selected participants was analyzed to determine gene expression levels and to investigate the molecular mechanisms underlying the disorder.

Study results

The study found strong statistical evidence for an association between biallelic variants in the RNU2-2 gene and a residual neurodevelopmental disorder. A high log Bayes factor of 18.2 supported the residual heritability model, indicating a strong association. The researchers identified 18 high-confidence candidates (tier 1), with 5 additional affected siblings and 13 lower-confidence candidates (tier 2).

Clinical examinations showed a common basic pattern of symptoms rather than a completely uniform presentation, as the majority of affected individuals had intellectual disability, global developmental delay, and seizures. Seizure disorders were present in over 90% of grade 1 cases in the 100 KGP cohort. Developmental delay and related neurological features were also common among affected individuals. Additional diagnostic findings include electroencephalographic abnormalities, motor impairments, and skeletal abnormalities, including microcephaly.

Replication in independent cohorts further confirmed the findings. Nine additional cases from international datasets showed similar genetic and clinical profiles, strengthening the reliability of the association. It was also observed that individuals carrying only one variant did not show reported symptoms associated with a neurodevelopmental disorder, which supports that the disease is recessive.

At the molecular level, RNA sequencing revealed a dramatic reduction of over 90% in the expression of RNU2-2 mutant alleles in affected individuals. However, carriers showed partial compensation through increased expression of the normal allele. This suggests that the disease results from a loss-of-function mechanism rather than a toxic gain-of-function.

Structural modeling predicted that pathogenic variants compromise critical stem-loop structures of U2-2 snRNA, suggesting an essential role for this sequence in splicing function. These perturbations are predicted to impair aspects of RNA splicing, a key process essential for proper gene expression. However, splicing defects were not consistently detected and may have been missed due to tissue specificity or the limited number of samples available.

This disorder is probably one of the most common identified recessive neurodevelopmental disorders currently diagnosed by sequencing, accounting for approximately 10% of all families with a currently sequenceable recessive neurodevelopmental disorder, and is approximately 60% as common as the dominant RNU4-U syndrome. Furthermore, it has been shown to be prevalent in both inbred and outbred populations, thus suggesting that it may be an important and under-recognized cause of disease among populations.

conclusions

Biallelic mutations in the RNU2-2 gene represent an important and previously underestimated cause of recessive neurodevelopmental disorders. This finding emphasizes that non-coding RNA plays a vital role in human disease and that disruption of the RNA splicing machinery can cause significant neurological damage.

The identification of a relatively common genotypically diagnosable condition has important implications for the clinical setting, including genetic counseling, early diagnosis, and possible preconception or prenatal genetic counseling in affected families.