This study aims to comprehensively delineate the phenotypic spectrum of ACTL6B-related disorders, previously associated with both autosomal recessive and autosomal dominant neurodevelopmental disorders. Molecularly, the role of the nucleolar protein ACTL6B in contributing to the disease has remained unclear. We identified 105 affected individuals, including 39 previously reported cases, and systematically analyzed detailed clinical and genetic data for all individuals. Additionally, we conducted knockdown experiments in neuronal cells to investigate the role of ACTL6B in ribosome biogenesis. Biallelic variants in ACTL6B are associated with severe-to-profound global developmental delay/intellectual disability, infantile intractable seizures, absent speech, autistic features, dystonia, and increased lethality. De novo monoallelic variants result in moderate-to-severe global developmental delay/intellectual disability, absent speech, and autistic features, whereas seizures and dystonia were less frequently observed. Dysmorphic facial features and brain abnormalities, including hypoplastic corpus callosum, and parenchymal volume loss/atrophy, are common findings in both groups. We reveal that in the nucleolus, ACTL6B plays a crucial role in ribosome biogenesis, particularly in pre-rRNA processing. This study provides a comprehensive characterization of the clinical spectrum of both autosomal recessive and dominant forms of ACTL6B-associated disorders. It offers a comparative analysis of their respective phenotypes provides a plausible molecular explanation and suggests their inclusion within the expanding category of "ribosomopathies."

Background/Objectives: The Mediator (MED) complex is an essential regulator of RNA polymerase II transcription. There is increasing evidence that pathogenic variants in several MED subunits are the cause of neurodegenerative and neurodevelopmental phenotypes, collectively referred to as "MEDopathies". This review aims to summarize current knowledge on the genetic basis, clinical manifestations, and neuroradiological features of MED-related disorders. Methods: We undertook a narrative synthesis of the literature focusing on the MED subunits most commonly associated with neurological disorders, including MED1, MED8, MED11, MED12/MED12L, MED13/MED13L, MED14, MED17, MED20, MED23, MED25, MED27, and CDK8. Sources included peer-reviewed genetic, clinical, and imaging studies, supplemented by relevant case reports and cohort analyses. In addition, representative facial phenotypes associated with selected MED variants (MED11, MED12, MED13, MED13L, MED25) were visualized for educational purposes using artificial intelligence-based image generation derived from standardized clinical descriptors. Results: All MEDopathies show converging clinical patterns: global developmental delay/intellectual disability, hypotonia, epilepsy, speech disorders, and behavioral comorbidity. Non-neurological involvement, such as craniofacial or cardiac anomalies, is subunit-specific. Neuroradiological features include callosal abnormalities (agenesis, thinning, dysmorphia), delayed or hypomyelination, progressive cerebral and cerebellar atrophy, basal ganglia signaling changes, pontine hypoplasia, and, in MED27 deficiency, a "hot cross bun" sign. Gene-specific constellations emphasize catastrophic infantile progression (MED11), X-linked syndromes with callosal defects (MED12/MED12L), language-dominant phenotypes (MED13), and syndromic intellectual disability with systemic features (MED13L). Conclusions: The growing spectrum of MEDopathies argues for their recognition as a unified nosological group with overlapping clinical and radiological signatures. Characteristic MRI constellations may serve as diagnostic clues and guide targeted molecular testing. Future directions include longitudinal imaging to describe disease progression and the integration of genomic data with curated clinical radiological datasets to refine genotype-phenotype correlations.

Neurodevelopmental disorders (NDDs) exhibit complex genotype-phenotype associations that frequently result in inconclusive variant interpretations, contributing to suboptimal diagnostic yields (~ 40%). Koolen-de Vries syndrome (KdVS), an autosomal dominant NDD caused by KANSL1 haploinsufficiency, exemplifies this diagnostic challenge with its multisystem manifestations and lack of systematic genotype-phenotype associations. To address this gap, we constructed a comprehensive KdVS genotype-phenotype repository by systematically integrating all molecularly confirmed cases from global literature. Comprehensive phenotypic analysis revealed that core KdVS features include developmental delay/intellectual disability, characteristic craniofacial dysmorphism, hypotonia, and multisystem abnormalities. Phenotypic association analysis identified 249 significant correlations, demonstrating that KdVS clinical manifestations are highly interconnected rather than representing isolated features, such as the association between strabismus and hydrocephalus (OR = 14.26). Application of this repository to screen a Chinese rare disease cohort identified 53 KANSL1 variants. Among these, one de novo nonsense variant (NM_001193466.2: c.902T > G, p.Leu301Ter) was classified as pathogenic in a Chinese boy with classic KdVS features. The remaining 52 variants were categorized as variants of uncertain significance (VUS), approximately half of which were absent from gnomAD databases. Each VUS was comprehensively annotated with detailed clinical profiles to facilitate phenotype-driven reinterpretation. In conclusion, this study establishes KdVS as a highly interconnected multisystem disorder and demonstrates that deep phenotypic association analysis enhanced genetic diagnosis. This disease-specific repository approach provides a scalable framework for improving molecular diagnostics across rare NDDs.

TNRC6B encodes a protein crucial for RNA silencing, and heterozygous variants of TNRC6B have been associated with developmental delay/intellectual disability, speech and language delay, fine and motor delay, and a range of neurobehavioral phenotypes, including autism and attention deficit and hyperactivity disorder (ADHD). Rett syndrome (RTT) is a neurodevelopmental disorder primarily affecting girls, characterized by loss of acquired speech and motor skills, repetitive hand movements, breathing irregularities, seizures, and is a prevalent cause of intellectual disability in females. Most RTT cases are due to pathogenic variants in the MECP2 gene located at Xq28, encoding methyl-CpG binding protein 2 (MeCP2). The phenotypic spectrum of heterozygous TNRC6B variants combined with MECP2 gene deletion has not been well described. A 17-month-old Chinese female patient with severe malnutrition and global developmental delay (GDD) was enrolled in this study. Whole-exome sequencing was conducted, and clinical data were obtained retrospectively from medical history and formal neuropsychological evaluation. The heterozygous TNRC6B variants (c.1409A > G; p.Asp470Ser) and a 4.066 Kb intragenic deletion of Xq28 encompassing the MECP2 gene were found. This expands the genetic spectrum of TNRC6B variants. The patient exhibited GDD, behavioral abnormalities, stunting, underweight, microcephaly and facial dysmorphism, including low-set ears, wide-set eyes, upslanting lateral canthi, underbite and hypertonia. The patient has received feeding guidance and rehabilitation training, and is currently under regular follow-up. This case broadens the phenotypic spectrum associated with TNRC6B variants and MECP2 gene deletion. This is the first report of a Xq28 microdeletion encompassing the MECP2 gene combined with heterozygous variants in TNRC6B. Our study expands the genotypic and phenotypic spectrum of TNRC6B deficiency syndrome and Rett syndrome. Our findings suggest that patients with TNRC6B and MECP2 gene deficiencies may experience more severe developmental delay and malnutrition.

O'Donnell-Luria-Rodan (ODLURO) syndrome is an autosomal dominant neurodevelopmental disorder mainly characterized by global development delay/intellectual disability, white matter abnormalities, and behavioral manifestations. It is caused by pathogenic variants in the KMT2E gene. Here we report seven new patients with loss-of-function KMT2E variants, six harboring frameshift/nonsense changes, and one with a 7q22.3 microdeletion encompassing the entire gene-locus. We further characterize both the clinical phenotype as well as its associated pathogenic variants' spectrum providing new information on sex-related phenotype distribution, according to the variant groups. We also highlight different epilepsy phenotype-genotype correlation with preferential association of generalized epilepsy and/or developmental and epileptic encephalopathy with missense pathogenic variants and focal epilepsy, childhood absence epilepsy and/or febrile seizures with pathogenic truncating variants and structural rearrangements. By a systematic review of the previously reported series, we also discuss previously unappreciated findings, including progressive macrocephaly, apraxia, and higher risk of bone fractures.