The genome editing using engineered nuclease has strategically transformed the idea of gene therapy for monogenic diseases including in hematopoietic stem and progenitor cells (HSPCs) (Osborn et al., 2016; Yu et al., 2016). The genome editing technology enables to create a site specific double-strand break (DSB) by the engineered nucleases that programmable triggering the cell’s endogenous repair machinery to edit the genome in a site-specific manner via the non-homology end joining repair (NHEJ) and the homology directed repair (HDR) mechanisms(Branzei and Foiani, 2008). The approach allows the precise alteration of the disease-causing alleles at the specific locus making it a permanent event that maintains the phenotypical gene expression under the control of endogenous regulatory elements..
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Over the past decade, three major classes of engineered nucleases have been used for genome editing, including zinc-finger nucleases (ZFNs) (Kim et al., 1996; Urnov et al., 2010), transcription activator-like effector nucleases (TALENs) (Li et al., 2011; Miller et al., 2011) and CRISPR–Cas9 (clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) protein 9) (Hsu et al., 2014; Sander and Joung, 2014; Tsai and Joung, 2016; Wiedenheft et al., 2012). ZFNs and TALENs are fusions between arrays of ZF or TALE DNA-binding domains and the dimerization-dependent FokI nuclease domain. The both of ZFN and TELENs nucleases exclusively rely on protein–DNA interactions to mediate site-specific recognition of genomic DNA sequences which requires complex protein engineering for each new targets (Kim and Kim, 2014). By contrast, CRISPR–Cas9 nuclease is a RNA-guided endonuclease. Through the guidance of a 23 nucleotides linked to CRISPR-domain RNA (gRNA), CRISPR-Cas9 finds the complementary protospacer DNA target in a genome where it cuts the double stranded DNA precisely 3 base pairs upstream of a PAM (Protospacer Adjacent Motif). The broken DNA ends generated by those nucleases are repaired either by NHEJ resulting in small insertion/deletions (indels) to disrupt target allele, or by HDR to precisely replace desired nucleotides required with delivery homologous DNA template. Compared to ZFNs and TALEN, the CRISPR/Cas9 system has rapidly become the most promising genome editing tool with demonstrated advantages including simplicity, easy programming, low cost and potential multiplexed editing (Bannikov and Lavrov, 2017; Brunetti et al., 2018; Salsman and Dellaire, 2017; Tsai and Joung, 2016) (Minkenberg et al., 2017). Despite of the genome editing holds tremendous promise for the developing novel gene therapy, the technique has been shown to be more refractory in HSPCs than any other cell types due to their quiescent status associated with low activity of the HDR machinery, and prone to DSB induced toxicity. However since first publication of using the ZFNs editing on human CD34+ cell (Genovese et al., 2014), the substantial developments have been made in last few years to circumvent the problems.
Since all nucleases targeted gene editing occurs through cell cycle progress,
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