Detection of subtelomeric rearrangements in 1180 patients: FISH and MLPA contribution

Mental retardation (MR) is a major social, educational, and health problem affecting 3% of the population. Subtelomeric chromosome aberrations are one of the major causes of MR with or without multiple anomalies; previous studies have shown that these rearrangements are responsible for 3-6% of unexplained mental retardation. Between 2000-2010 in the Cytogenetics Unit, Centro de Genética Médica Jacinto de Magalhães, INSA (Portugal), the subtelomeric regions of all the chromosomes were analysed in 1180 individuals, whose karyotype had been considered normal. The reasons for referral included (i) psychomotor development delay or (ii) mental retardation with or without dysmorphisms. Until 2007 the analysis of metaphases, obtained from cultured lymphocytes following standard protocols, were performed by "Fluorescence in situ hybridization” (FISH): the first kit to be used was the Chromoprobe Multiprobe-TM (Cytocell) kit (until 2005), which was followed by the TotelVysion Multi-Color FISH Probe (Vysis). In 2007 the "Multiplex Ligation dependent Probe Amplification” (MLPA) was implemented in the laboratory, using kits P036 and P070 (MRC-Holland). All the unbalanced cases detected by MLPA were confirmed by FISH. Of a total of 1180 individuals, 62 (5.3%) showed chromosomal alterations: 60 in the subtelomeric regions and 2 in the control regions. It was not possible to perform any familial studies in 12 of the 62 cases (1.0%) and therefore the results were considered inconclusive. In the other 50 abnormal cases, the parental investigation allowed us to conclude that 30 (2.5%) of these patients had chromosomal abnormalities “de novo” that might be responsible for the clinical phenotype; the remaining 20 possibly abnormal cases (1.7%) were considered polymorphisms without pathological significance, since the apparent deletion or duplication had been inherited from phenotypically normal parents. The authors compare the results obtained in the individuals in the present study with literature reports and highlight the advantages/disadvantages of each technique