Expanding the phenotypic spectrum of Xq28 duplication involving MECP2: a familial case report.

X-linked intellectual disability (XLID) is a well-recognized group of neurodevelopmental disorders, with pathogenic variants in X-chromosomal genes accounting for approximately 16% of intellectual disability cases in males. Clinical expression in females is variable and depends on patterns of X-chromosome inactivation. We describe three affected individuals from a single family with XLID caused by a confirmed duplication of the Xq28 region, including the genes SLC6A8, L1CAM, MECP2, TKTL1, FLNA, and GDI1. Two male siblings presented with severe phenotypes, including profound intellectual disability, severe speech impairment, behavioral issues, facial dysmorphism, spastic cerebral palsy, epilepsy, and cutaneous abnormalities. Their mother showed mild intellectual disability and skin manifestations. Family history suggested additional affected male relatives with a similar or even more severe clinical presentation. The duplication of multiple dosage-sensitive genes within the Xq28 region likely explains the multisystem involvement and the marked phenotypic variability observed between male and female family members. This report highlights the importance of considering Xq28 duplication, the most common X-linked copy number variation associated with intellectual disability, in the differential diagnosis of families with X-linked intellectual disability, especially if it is accompanied by additional neurological impairment.