Subtelomeric rearrangements in idiopathic mental retardation

To estimate the frequency of subtelomeric rearrangements in patients with sporadic and non-syndromic idiopathic mental retardation (IMR).

A total of 18 IMR patients were taken for the study. Selection criteria included no known syndromes or chromosomes abnormalities and known causes of IMR. All patients signed an informed consent to participate. Chromosome analysis was carried out on all patients to rule out gross chromosome abnormalities. Lymphocyte cultures were initiated and harvested using standard protocols. For fluorescence in situ hybridization (FISH), Chromoprobe Multiprobe-T system was used. This system consists of 24 embossed areas with each area having one reversibly bound subtelomere probe for a specific chromosome. The subtelomere probes were differentially labeled with green fluorescence for short arm and orange for the long arm. Hybridization, washing and staining are done using standard protocols. A minimum of 5 metaphases were analyzed per chromosome per patient.

A total of 2 subtelomeric rearrangements were detected (11.1%). Case 1 involved a 17-year-old with severe MR, profound deafness and dysmorphic features with reciprocal translocation t(3;7)(q26.2; p15.1). The second case involved a 4.6-year-old with mild developmental delay and a terminal deletion of the long arm of chromosome 2, del(2) (q37.3). The frequency of abnormalities detected in our study is in agreement with published reports.

Subtelomeric screening with FISH is a useful tool for investigation of IMR, however, it is not cost effective in all cases. Conventional chromosome analysis coupled with targeted FISH testing might be the optimal strategy for investigation of IMR.