Genom Mühendisliği: CRISPR/Cas9 Teknolojisi
Özet
Metin, prokaryotlarda faj ve plazmitlere karşı bir adaptif bağışıklık mekanizması olan ve 1987'de Ishino ve arkadaşları tarafından tanımlanıp 2013 yılında hedefli bir genom düzenleme aracına dönüştürülen CRISPR/Cas9 teknolojisini ayrıntılı olarak incelemektedir. Sistem, RNA ve Cas protein bileşenlerinden oluşmakta olup hücresel düzeyde adaptasyon, pre-crRNA ekspresyonu ve işlenmesi ile interferans olmak üzere üç temel aşamada işlev göstermektedir. CRISPR-Cas sistemleri efektör modüllerinin yapısına göre iki ana sınıfa ayrılır: Çoklu alt birim efektör kompleksleri içeren Sınıf 1 (Tip I, Tip III, Tip IV) ve tek bir büyük efektör protein içeren Sınıf 2 (Tip II, Tip V, Tip VI). Yaygın olarak kullanılan Tip II CRISPR/Cas9 sistemi, Streptococcus pyogenes bakterisinden elde edilen Cas9 endonükleazı ile sgRNA (crRNA ve tracrRNA bileşimi) rehberliğinde hedef DNA dizisini hassasiyetle tanır. 5'-NGG-3' PAM motifini tanıyarak çift zincir kırılması (DSB) meydana getiren bu yapı, hücrenin NHEJ veya HDR onarım mekanizmalarını tetikleyerek gen susturma, ekleme ya da değiştirme işlemlerini gerçekleştirir. Ayrıca, Cas9 proteininin nükleaz alanlarındaki mutasyonlarla elde edilen nickaz Cas9 (nCas9) hedef DNA'da yalnızca tek zincir kesimi yaparak hedef dışı etkileri azaltırken, nükleaz aktivitesi tamamen durdurulmuş dCas9 ise geni kalıcı olarak mutasyona uğratmadan transkripsiyonel düzenleme ve genomik görüntüleme olanakları sunmaktadır.
The text thoroughly examines CRISPR/Cas9 technology, an adaptive immune mechanism in prokaryotes against phages and plasmids that was identified in 1987 by Ishino et al. and transformed into a targeted genome editing tool in 2013. Composed of RNA and Cas protein components, the system functions at the cellular level through three main stages: adaptation, pre-crRNA expression and processing, and interference. CRISPR-Cas systems are divided into two main classes based on the architecture of their effector modules: Class 1 (Types I, III, and IV), which contains multi-subunit effector complexes, and Class 2 (Types II, V, and VI), which features a single large effector protein. The widely used Type II CRISPR/Cas9 system precisely recognizes target DNA sequences under the guidance of sgRNA (a combination of crRNA and tracrRNA) along with the Cas9 endonuclease derived from the bacterium Streptococcus pyogenes. By recognizing the 5'-NGG-3' PAM motif and generating a double-strand break (DSB), this complex triggers cellular NHEJ or HDR repair mechanisms to perform gene silencing, insertion, or replacement. Furthermore, nickase Cas9 (nCas9), engineered through mutations in the nuclease domains of the Cas9 protein, generates only single-strand breaks in target DNA to reduce off-target effects, whereas nuclease-deficient dCas9 offers opportunities for transcriptional regulation and genomic imaging without permanently mutating the gene.
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