Publication

Key Clinical Features

Core Manifestations (>99% of cases):

• Mild to profound developmental delay
• Intellectual disability (100% – ranging from mild to severe, most commonly moderate)
• Hypotonia (63%)
• Neurobehavioral issues (~60%) – autistic features, agitation/aggression, restlessness, self-harm, tantrums, overfriendliness, hyperactivity
• Characteristic facial features (>99%) – depressed nasal bridge, bulbous nose, hypotonic open mouth

Other Common Features:

• Musculoskeletal abnormalities (51%) – affecting feet/hands
• Ocular manifestations/vision issues (31%) – primarily strabismus
• Congenital heart defects (23%)
• Seizures (22%)
• Brain imaging abnormalities – ventriculomegaly, myelination defects, thin/absent corpus callosum
• Hearing impairment (6%)

Speech and Motor Development

• Speech: 99% have delayed or completely absent speech; many lack expressive language
• Motor milestones: Sitting age 8-17 months; walking age 20 months to 3.5 years (when achieved)
• Most individuals become ambulatory, though some require assistance

Diagnosis

• Confirmed by molecular genetic testing showing heterozygous pathogenic variant in MED13L
• Typically identified through exome/genome sequencing or intellectual disability gene panels
• ~90-95% of variants detected by sequence analysis
• ~5-10% are deletions/duplications

Genetics

• Inheritance: Autosomal dominant
• Most cases: De novo (new mutation, not inherited)
• Rare cases: Inherited from mosaic, apparently unaffected parent
• Penetrance: Complete (100% of people with variant show symptoms)

Genotype-Phenotype Correlations

• Missense variants appear associated with more severe manifestations including:
  • Severe motor delay
  • Higher rates of seizures
  • More frequent absence of speech and ambulation
  • Increased autistic features

Management Approach

Treatment of Manifestations:

• Standardized developmental, intellectual, and behavioral interventions
• Anti-seizure medications as needed
• Orthopedic treatment for skeletal issues
• Cardiac management for congenital heart defects
• Ophthalmologic care for vision issues
• Hearing aids when appropriate
• Feeding therapy; gastrostomy tube if needed
• Social work and family support

Surveillance Recommendations:

• Monitor developmental progress, educational needs, and behavior at each visit
• Assess mobility and self-help skills regularly
• Clinical scoliosis screening with radiographs as needed
• Annual audiology evaluation
• Regular ophthalmologic assessment
• Monitor nutritional status and feeding safety
• Screen for aspiration/respiratory insufficiency

Educational/Developmental Support

• Early intervention programs (ages 0-3)
• Developmental preschool (ages 3-5)
• Individualized Education Plans (IEP)
• Physical, occupational, and speech therapy
• Alternative communication methods (AAC devices) when needed
• Applied behavior analysis (ABA) for autism-related behaviors

Molecular Mechanism

• MED13L protein is part of the Mediator complex controlling RNA polymerase II transcription
• Pathogenic variants cause transcriptional defects
• Disease likely results from haploinsufficiency (insufficient protein from one working copy)
• May also cause mislocalization of cyclin C, affecting mitochondrial function

Prognosis

• Life span appears normal – several adults have been reported
• Over 100 published cases to date
• Most individuals require ongoing support and specialized education
• Some achieve relative independence with educational support

Prevalence

• Exact prevalence unknown
• More than 100 published cases
• Likely underdiagnosed due to variable presentation