An Overview of CRISPR/Cas9 and IPSCs and the Possibility of Creating a Super Force to Treat Human Genetic Disorders
- Owen Conneran
The 14th of April 2003, the US Human Genome Research Institute completed its 15 year multibillion dollar project referred to as the Human Genome Project (HGP) and published the final sequencing map of the human genome. The ability to determine the sequencer of approximately 3 billion nucleotide base pairs that make up DNA and all the genes in DNA provided scientist with the framework to understand genetic diseases and disorders, characterized by an abnormality in one’s DNA.
Research on the human genome has shown that genetic disorders can be caused by a number of factors including a mutation to one gene termed monogenic disorder; mutations to multiple genes (multifactorial inheritance disorder); or by damage to chromosome structure and/or function in part or full. These mutations can be either pass on from parent to offspring (inherited) or developed during the life of an individual.
Further advancements in genomics have enabled the editing and modification of the genome by technologies which can insert, delete and modify targeted regions on a DNA sequence allowing for the control of activation or inactivation of a particular gene (Hsu et al, 2014). However, due to the colossal size of the human genome, it is extremely difficult to manipulate it. Due to the low efficiency of many techniques, a lot of scientific interest has shifted to nuclease-based genome editing techniques and in particular to the CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats) system. This review will discuss this platform for genome editing along with human induced pluripotent stem cells (iPSCs) and the possible combined application of both in gene therapy.
CRISPR/CAS9 System
The CRISPR/Cas9 systems are loci encoded on bacterial genomes that consist of short identical, direct repeats that ranges from 21-47 nucleotide base pairs in length, interspaced with short intervening spacers derived from exogenous DNA targets called protospacers which make up the CRISPR RNA (crRNA) array (Manjunath et al, 2013). These protospacers, in each DNA target are always associated with a sequence motif termed protospacer motif (PAM) which generally vary with respect to the type of CRISPR system in question (Ran et al,2013). The CRISPR loci are surrounded by a group of CRISPR-associated (Cas) genes. The transcribed crRNA product forms a complex with a Cas protein which directs the complex to the sequence of DNA complementary to the spacer sequence. Once here, the Cas protein cuts the DNA to create a double stranded break which is then rebuilt by the cells own repair mechanisms, nonhomologous end-joining (NHEJ) or homology directed repair (HDR) (Figure 1). NHEJ is an error prone method which leaves a mark in the form of insertion or deletion (indel) mutations. Alternatively, the HDR method can be used to repair the cleaved DNA strand(s). The HDR pathway, used at lower and more variable frequencies than its counterpart NHEJ, can be used to create precise, defined modifications at a specific locus with an exogenous repair template added to the mechanism (Ran et al, 2013). This repair template can be in two forms: double-stranded DNA that targets regions with homology arms flanking the sequence to be inserted; or single-stranded DNA oligonucleotides donor sequence. Many groups have opted for this choice of repair template as it a relatively more simple method of making edits in the genome. Ran et al, also states that HDR usually only occurs in dividing cells and that the efficiency of the technique can vary greatly depending on the cell type, genomic locus of interest and the repair template used.