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Editor—Cryptic telomeric rearrangements are a significant cause of idiopathic mental retardation. Knightet al 1 found 7.4% of these rearrangements in children with moderate to severe mental retardation. Clinical selection criteria for testing patients with subtelomeric chromosome specific probes are still not clear cut and the importance of other surveys to define this point has been stressed.2With this aim, we examined 200 patients with idiopathic mental retardation, either isolated or associated with dysmorphism and/or congenital anomalies, using FISH analysis with subtelomeric chromosome specific probes.
Material and methods
The sample was collected in four Italian Genetic Centres. Patients were selected on the basis of the following criteria: (1) the presence of mental retardation that was classified as mild, moderate, or severe according to DSM IV3; (2) patients under 1 year of age, too young for psychological assessment, were selected for the presence of developmental delay; (3) exclusion of pre- or perinatal distress through an accurate evaluation of the pre-, peri-, and postnatal patient history; and (4) exclusion of Mendelian syndromes and of genomic disorders4 ,5 for which a specific diagnostic test is available.
The essential elements of evaluation also included family history, a complete physical and neurological examination of the patients with particular attention to the presence of mental retardation and multiple congenital anomalies, and assessment of the behavioural phenotype. Electroencephalograms, brain CT scan, and MRI were performed in specific situations.
Abnormal methylation and expansion at FRAXA and FRAXE6 were excluded in 52 and 50 males and in 37 and 32 females, respectively.
Routine cytogenetic analysis at the 400-550 band level was performed in all the patients. In those patients in whom a cryptic subtelomeric rearrangement was identified by FISH, prometaphase chromosomes were also analysed to determine if the rearrangement could be detected in retrospect by …