Munis DÜNDAR, Yasin ADA, Hakan GÜMÜŞ, Yagut AKBAROVA

FRAJİL X SENDROMU ÖN TANILI HASTALARDA FMR1 GENİNDEKİ 3'LÜ TEKRAR SAYI MUTASYONLARIN BELİRLENMESİ

Frajil X Sendromu (FXS) (OMIM 300624); mental retardasyonların etmenleri arasında Down sendromu'ndan sonra ikinci sırada, ailesel mental retardasyonlar içinde ise birinci sırada yer almaktadır. Toplumdaki sıklığı erkeklerde yaklaşık 4000'de 1 ve kadınlarda yaklaşık 8000'de 1 olarak bildirilmektedir. FXS'nda klinik bulguları mental retardasyon, belirgin kulaklar, uzun ince bir yüz, davranış bozuklukları, pubertal dönemden itibaren makroorşitizmdir. FXS'dan sorumlu gen, Xq27.3'e lokalize, 17 ekzon içeren, 38 kb büyüklüğünde, 4308 bp transkripti (NM_002024) olan FMR1 genidir. Gen, 5' ucunda CpG metilasyon bölgesi ve birinci ekzonun translasyona uğramayan (UTR) bölgesinde yer alan CGG (sitozin, guanin, guanin) üçlü tekrarları içerir. Gen, 632 amino asitlik (NP_002015). FMR1 proteini (FMRP) kodlar. Çeşitli dokularda ifade edilen bu protein özellikle nörönal sinapslardaki fonksiyonu ile zihinsel gelişim için gereklidir. Hastaların % 98-99' unda FMR1 geninin 5' ucundaki CGG tekrarında genişleme ve promotör bölgesinde metillenme, %12'sinde ise gen içi delesyon ve nokta mutasyonları görülür. Normal bireylerde CGG tekrar sayısı ~5-44 arasındadır ve ~45-54 arasında "gri bölge" olarak adlandırılır, ~55-200 arasındaki bireyler ise "premutasyon" yani FXS taşıyıcısıdır. Premutasyon taşıyıcı kadınlarda mayoz bölünmede CGG tekrarında artış gözlenir ve bu olay "antisipasyon" olarak tanımlanır ve bu özellik sendromun kalıtımını diğer X'e bağlı hastalıklardan farklı kılar. CGG tekrar sayısının >200 olması "full mutasyon" olarak tanımlanır ve FXS fenotipine yol açar. Bu çalışmada, kırkbir erkek ve dokuz kız olgu (toplam 50 olgu) FMR1 Sizing PCR (ABBOTT) ve SNP Detective Fragile X (GML) kitleri kullanılarak FXS'na neden olan FMR1 genindeki CGG trinükleotit sayısı ve metilasyon durumu açısından incelendi. Kırkbir erkek olgunun %14.63 'ünde full mutasyon, %2.43' ünde premutasyon ve bir olgu da ise boyut mozaisizmi bulunmuştur. Toplam dokuz kız olgunun %11.11'inde full mutasyon görülmüştür. Full mutasyonların tümü metillenmiş olarak bulundu.

IDENTIFICATION Of TRIPLET REPEAT MUTATIONS IN THE FMR1 GENE IN PATIENTS WITH CLINICAL DIAGNOSIS FOR FRAGILE X SYNDROME

Fragile X Syndrome ( FXS) (OMIM 300624) is most frequently observed familial mental retardation syndrome in males, and second frequent following Down syndrome in general population. The frequency rate is one in every 4000 men as compared to one in every 8000 women. In 98-99% of the cases, the disease is caused by full repeat expansion of CGG (> 200) at 5'UTR of FMR1 gene and methylation of CpG islands in the promoter region. Deletions and point mutations in the gene is responsible from the remaining 1-2% of the cases. Clinical indications of FXS are mental retardation, large ears, narrow face, behavioral disorder, macroorchidism after puberty. The gene is called as FMR1, which is located in Xq27.3, and 38 kb in size containing 17 exons. mRNA of an approximate size of 4308 bp (NM_002024) coding 632 aa (NP_002015) protein called as FMR1 protein (FMRP), expressed in several tissues and function especially in synaptic regions of neurons. CGG repeat lengths are ~5-44 in normal population. Repeats between ~45 and ~54 is called as 'grey zone'. Individuals with ~55-200 repeat lengths are called premutation or FXS carriers. In female premutation carriers, CGG repeats expands during oogenesis, which is called as anticipation and this phenomenon differentiates FXS inheritance from the classical X-linked ressesive inheritance. Individuals with more than 200 CGG repeats are classified as full mutation and associated with FXS. In this study, CGG trinucleotide count in FMR1 gene and the methylation status were investigated in 50 (41 male and 9 female patients) by using FMR1 Sizing PCR (ABBOTT) and SNP Detective Fragile X (GML) kits. Among the 41 male patients, 14.63% had full mutation, 2.43% had permutation and size mosaicism was found in one male patients. Full mutation was found in 11.11% of female patients. All full mutations were methylated

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