ABSTRACT
Development and implementation of high-density DNA arrays demonstrated the important role of copy number changes on the X chromosome in the etiology of developmental delay and mental retardation (MR). We describe two unrelated patients with developmental delay due to similar interstitial duplications at Xp21-p22. The first patient is a 6-month-old male with multiple affected family members including many females. The second patient is a 5-year-old adopted female. In both patients, chromosome analysis and array comparative genomic hybridization (aCGH) showed duplications of overlapping regions at Xp21-p22. The duplicated segments contain numerous genes associated with MR, including AP1S2, NHS, CDKL5, RPS6KA3, SMS, and ARX. Except for developmental delay, there is little phenotypic overlap between the male and the female patient. Additionally, the female patient and affected female relatives of the male patient have variable severities of cognitive impairment, likely due to different X-inactivation patterns and effects of other, nonduplicated genes important for normal development. These cases illustrate that increased gene dosage of X-linked MR genes lead to cognitive impairment. Precise delineation of chromosome rearrangements by aCGH and identification of genes within duplicated segments helped in establishing genotype-phenotype correlations for each of our patients, in comparing them to each other, as well as with previously reported cases of Xp21-p22 duplications. However, we show that even with detailed molecular characterization, phenotype prediction remains challenging in patients with structural abnormalities of the X chromosome.
