ARRAY-BASED COMPARATIVE GENOMIC HYBRIDIZATION ANALYSIS IN CHILDREN WITH DEVELOPMENTAL DELAY/INTELLECTUAL DISABILITY
Türkyılmaz A, Geckinli BB, Tekin E, Ates EA, Yarali O, Cebi AH, Arman A
*Corresponding Author: Ayberk Türkyılmaz, M.D., Assistant Professor, Department of Medical Genetics, Karadeniz Technical University Faculty of Medicine, Farabi Street, 61080 Ortahisar, Trabzon, Turkey. Tel: +90-505-812-03-34. Fax: +90-462-377-51-06. E-mail: ayberkturkyilmaz@gmail.com
page: 15

INTRODUCTION

Developmental delay (DD) is a condition wherein developmental milestones and learning skills do not occur at the expected age range for patients under 5 years of age. Areas used for evaluating developmental stages are gross and fine motor skills, speech and language skills, cognition, and personal-social development. Intellectual disability (ID) is characterized by limited or insufficient development of mental abilities, including intellectual functioning impairments, such as learning and cause-effect relationship [1]. Intellectual disability cases are often diagnosed in the early school-age period. The incidence of DD is 1.0-3.0% in the general population, whereas that of ID is approximately 2.7% among early school-age children [2]. Some cases have DD or ID as the only finding and are called isolated cases. Conversely, cases accompanied by facial dysmorphism, autism spectrum disorder (ASD), epilepsy and congenital anomalies, are called syndromic DD/ID [3]. Recent studies have shown that biological signaling pathways causing DD/ID, ASD, and epilepsy phenotypes are common. Additionally, the relationship between signaling pathways involved in early brain development, synaptic plasticity, and neuronal migration and the formation of these phenotypes has been demonstrated [4]. Isolated and syndromic DD/ID cases show extreme genetic heterogeneity. Genetic etiology can be detected in approximately 40.0% of the cases, whereas chromosomal abnormalities are observed in 25.0% [5,6]. Conventional cytogenetic testing can be used for detecting >5 Mb chromosome abnormalities. Moreover, specific chromosomal abnormalities can be investigated using fluorescence in situ hybridization (FISH) techniques. The diagnostic yield of both techniques for detecting DD/ID cases is approximately 5.0-6.0% [7]. Array-based comparative genomic hybridization (aCGH) can detect copy number variations (CNVs) in the whole genome at higher resolution than conventional cytogenetic methods. Copy number variations are defined as changes >1 kb resulting in an increase and/or decrease in the genomic DNA [8]. The CNVs are divided into two groups: recurrent and non recurrent. Recurrent CNVs often arise during meiosis from non-allelic homologous recombination (NAHR) between low copy repeat elements (LCRs). Non recurrent novel microdele-tion/microduplication syndromes have been identified in recent years owing to the widespread application of aCGH in diagnosis [9,10]. In a review published by Miller et al. [11] in 2010, the diagnostic yield of aCGH was 12.2% in 21,698 DD/ID cases retrieved from 33 different studies. The aCGH is currently recommended as the first-tier genetic test for DD/ID cases worldwide [11]. The aim of this study was to discuss the clinical findings and aCGH analysis results of isolated and syndromic DD/ ID cases in the context of genotype-phenotype correlation.



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