Developmental Skills and Adaptive Functioning in MECP2 Duplication Syndrome

MECP2 Duplication Syndrome (MDS) is a rare, X-linked neurodevelopmental disorder characterized by severe-to-profound intellectual disability, epilepsy, limited-to-absent speech and global functional dependence. Caregivers consistently identify developmental and adaptive functioning as a chief concern. To support investigational therapies entering the clinic, the developmental and behavioral outcomes in MDS, changes over time, and clinical modifiers of functional progression must be defined using standardized, trial-ready instruments. Data were aggregated from prospective studies at a single center. The analysis included 62 males with genetically confirmed MDS who were < 18 years old. Assessments included the Vineland Adaptive Behavior Scales, Second Edition (VABS-II), Bayley Scales of Infant and Toddler Development, Third Edition (BSID-III), and Aberrant Behavior Checklist, Community version (ABC-C). Analyses examined developmental profiles, trajectories across age bins, longitudinal changes, and impacts of epilepsy on functioning and behavior. Cross-sectional analysis demonstrated significant developmental delays across all domains, with median age-equivalent scores not exceeding 20 months despite chronological age up to 15 years. Receptive communication emerged as a relative strength, whereas expressive language, interpersonal relationships, and gross motor skills were weaknesses. No child achieved gross motor function beyond a 24-month level. Longitudinal data showed modest gains in adaptive skills in the pre-epileptic population and mostly during early childhood. Epilepsy was associated with markedly lower functioning (e.g., 9 months reduced gross motor ability) and increased lethargy. VABS-II scores were well-correlated with BSID-III performance measures, suggesting caregiver report is a reliable proxy. ABC-C scores revealed higher levels of social withdrawal and modest repetitive behaviors. Relatively higher functioning levels may be correlated with higher levels of externalizing behaviors. This study provides one of the largest standardized characterizations of adaptive and developmental functioning in MDS. Measurements quantify an eventual plateau in skill acquisition and substantial impact of epilepsy. In clinical trials, a group-mean change of 3 or more raw score points on any VABS-II subdomain would be largely unexpected over 12-to-18 months of follow-up. In individuals who have MDS and epilepsy, even halting progression or stabilization of VABS-II scores could reflect clinical benefit within the same period. Implications for supportive and adjunctive therapies are discussed.