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Abstract Intellectual disability (ID) is characterized by significant limitations in intellectual functioning and adaptive behavior and is often associated with developmental delay and multiple congenital anomalies. It has a major effect on the life of the affected person his family and the society and it is a frequently occurring disorder. The study of the intellectual disability is complicated because of the high clinical and genetic heterogeneity. Intellectual disability can be caused by genetic defects as well as environmental causes affecting the development and function of the nervous system. Although Intellectual disability can be caused by environmental insults but a large proportion is caused by genetic abnormalities. Still, around 60% of cases of Intellectual disability do not have a known etiology. Systematic assessment of a child with intellectual disability has many steps starting with detailed history, full physical examination, and screening laboratory testing. Karyotype considered the traditional diagnostic test for genetic evaluation in patients with idiopathic non- syndromic intellectual disability. Other tests include screening tests, metabolic tests, TORCH screens, and neuroimaging, all of which have limited diagnostic value. Chromosomal analysis using the classical cytogenetic studies is considered the standard method for investigating syndromes suspected to have a chromosomal etiology but it cannot detect chromosomal imbalances smaller than 5-10 Mb. So, many subtelomeric copy number variance especially those close to the telomeres cannot be detected by conventional karyotype. Subtelomeric regions are gene-rich parts of the chromosome. Microdeletions and subtelomeric rearrangements that disrupt genes in the telomeric region can cause intellectual disability. The identification of subtelomeric copy number variance as the genetic cause of intellectual disability is of high value because it allows proper medical care, genetic assistance and prognosis. The majority of cases with subtelomeric copy number variance had no characteristic phenotype, so a general subtelomeric screen is required to reach a diagnosis. Molecular studies focusing on subtelomeric copy number variance in patients with idiopathic intllecual disability have been performed mainly by FISH using subtelomere specific probes. Commercial subtelomeric FISH is expensive, time consuming and laborious so different approaches had been tried. from these methods MLPA technique, it is a very promising technique. It appears to represent an improvement compared to the laborious and costly FISH. MLPA is a sensitive and reliable technique for the detection of subtelomeric copy number variant. It is molecular genetic technique that identifies subtelomeric copy number variance, such as duplications or deletions. MLPA consists of comparative quantification of specifically bound probes that are amplified by PCR with universal primers. MLPA is an easy-to-perform reaction requiring only 20 ng of human DNA. We recommend to start with cytogenetics study in patient with intellectual disability if no abnormalities detected proceed to MLPA which is a targete test used for screening of large number of intellectual disability patients in short time with low cost, positive cases with MLPA should be confirmed by FISH. If no abnormality detected by MLPA proceed to array CGH. The aim of our study was to determine the prevalence, and characterization of copy number variance of subtelomeric regions through the multiplex ligation dependent probe amplification (MLPA) method in pediatric patients with idiopathic intellectual disability. This study was conducted on 30 selected patients who had intellectual disability attending Genetic Clinic, Pediatric Hospital, Ain Shams University and Outpatient clinics of the Clinical Genetic Department, National Research Centre. The study group included 30 patients with idiopathic intellectual disability and normal karyotype as major criteria. Minor criteria included dysmorphic features, nonfacial dysmorphism or congenital anomalies abnormal growth, behavioral disorder, family history of intellectual disability and/or family history of miscarriages or perinatal death as the patients group. Patients were subjected to full medical history, pedigree analysis and physical examination. 3 patients with intellectual disability and chromosomal aberrations as detected by karyotype were included as positive controls with provisional diagnosis by karyotype for the technique and 6 normal persons as a negative control group All patients were subjected to the following: a) Clinical Evaluation: 1. Medical history: included age, sex, and history of the present illness. 2. Pedigree Analysis: this was performed for the patients taking in consideration consanguinity, similarly affected siblings and other affected family members. 3. Physical examination: General physical examination was done for all patients. Anthropometric measurements at the time of clinical assessment as regard to weight, length/height and head circumference were assessed. b) Laboratory method: 1. DNA extraction: 8ml of venous blood on PAXgene tube for DNA extraction by PAXgene DNA purification kit and measurement of quality and concentration of DNA (PreAnalytix, Hiden, Germany).2. Multiplex Ligation-dependent Probe Amplification (MLPA) (MRC-Holland 2002): The MLPA assay was performed using the SALSA MLPA probemix P070 Subtelomeres Mix 2B. MLPA analysis was carried out as recommended by the manufacturer. 3. Fluorescence in situ hybridization (FISH) to confirm positive results according to (Pinkel et al., 1986). The results showed that: All 30 selected patients subjected to subtelomeric screening by multiplex ligation dependent probe amplification using P070 kit revealed no Subtlomeric copy number variance in any of the patients included. On the other hand; the 3 positive control patients showed abnormalties by MLPA confirmed by FISH analysis. Positive control patient number one showed by karyotype abnormality in q arm of chromosome 1. When included as positive case in MLPA analysis both 1q subtelomere deletion and 4q subtelomere duplications were detected. FISH was done using subtelomere 4q and 1q pobes and revealed deletion of 1q subtelomere and duplications 4q subtelomere as detected by MLPA. Positive control patient number two showed by karyotype abnormality in q arm of chromosome 22 when included as positive case in MLPA analysis 9p subtelomere duplication was revealed. FISH was done using 9p subtelomere probe and showed duplication in 9p. Positive control patient number three showed by karyotype abnormality in q arm of chromosome 14. When included as positive case in MLPA analysis 9p subtelomere duplication was revealed. FISH was done using total subtelomere 9p probe and showed duplication in 9p subtelomere. The length of deletion and duplication needs further investigation including microarray for detection of the exact length or whether it is limited to subtelomere or other areas are included |