A homozygous MED11 C-terminal variant causes a lethal neurodegenerative disease

**Background:** The Mediator (MED) complex is a multisubunit protein complex that regulates transcription by bridging transcription factors and RNA polymerase II. Pathogenic variants in MED subunits (e.g., MED17, MED20, MED27) have been linked to rare neurologic disorders with severe neurodevelopmental impairment, microcephaly, and progressive neurodegeneration. MED11 is a component of the head module, but its role in human disease was unknown. This study aimed to characterize the genetic and clinical features of a recurrent homozygous MED11 variant.

**Methods:** Seven affected individuals from 5 unrelated families (South Italian, Saudi/Yemeni, Turkish, Tunisian) were identified through international research networks and Matchmaker Exchange. Exome or genome sequencing was performed, and the candidate variant (MED11 c.325C>T; p.Arg109Ter) was confirmed by Sanger sequencing. Clinical data and brain MRI were reviewed by pediatric neurologists and neuroradiologists. Functional studies included RT-PCR and western blotting on fibroblasts from one affected individual and controls, computational modeling of the MED complex structure, and generation of med11 knockout zebrafish using CRISPR/Cas9. Zebrafish phenotypes were assessed at 5 days post-fertilization (dpf) for head/eye size, survival, and startle responses.

**Key Results:** All 7 individuals carried the same homozygous MED11 c.325C>T variant, leading to a premature stop at amino acid 109 and loss of the last 9 conserved residues. The variant was ultrarare (14 heterozygotes in >500,000 exomes/genomes). Haplotype analysis suggested independent mutational events. Clinical features included congenital microcephaly (head circumference 29-32 cm at birth), profound global developmental delay, refractory myoclonic seizures starting in the first month of life, exaggerated startle responses, axial hypotonia with limb hypertonia, bilateral hearing loss, and vision abnormalities (nystagmus, strabismus, optic atrophy). Brain MRI showed progressive global atrophy (cerebral and cerebellar), cortical dysgyria, white matter immaturity, and basal ganglia degeneration. Four individuals died in early infancy (10 days to 6 years) due to cardiorespiratory failure. Functional studies showed that the truncated MED11 protein escaped nonsense-mediated decay, with similar expression levels in patient fibroblasts vs. controls. Computational modeling indicated that the C-terminal truncation disrupts interactions between MED11, MED28, and MED30C, destabilizing the 4-helix bundle. Zebrafish med11 knockouts exhibited microcephaly, small eyes, heart edema, reduced survival (50% died by 10 dpf, all by 14 dpf), and abnormal startle responses (reduced auditory response, increased visual response).

**Clinical Implications:** This study establishes MED11 as a novel gene for a lethal neurodegenerative disorder with prenatal onset. The recurrent p.Arg109Ter variant likely impairs MED complex stability, leading to severe neurodevelopmental and degenerative phenotypes. The findings expand the spectrum of MED subunit-related disorders and highlight the importance of the mediator complex in brain development. Genetic testing for MED11 should be considered in cases of congenital microcephaly, early-onset refractory seizures, and progressive brain atrophy. The zebrafish model provides a tool for further pathophysiological studies and potential therapeutic screening.