Premed · Premed · Genetics
Lecture 18: CRISPR-Cas9 and Genome Editing
Genetics
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the natural CRISPR-Cas9 system in bacterial adaptive immunity
- Explain how CRISPR-Cas9 has been engineered for genome editing
- Describe the mechanisms of DNA repair following Cas9 cleavage (NHEJ and HDR)
- Compare CRISPR-Cas9 with earlier genome editing tools (ZFNs, TALENs)
- Discuss advanced CRISPR applications (base editing, prime editing, CRISPRi/a, epigenome editing)
- Evaluate the clinical applications and ethical considerations of genome editing
Lecture Content
I. CRISPR-Cas as Bacterial Adaptive Immunity
CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats. First observed in E. coli in 1987 by Yoshizumi Ishino, the system's function as an adaptive immune mechanism was elucidated in 2007. It is found in approximately 40% of bacteria and 90% of archaea.
The natural CRISPR system operates in three stages. During adaptation (spacer acquisition), when a bacterium encounters foreign DNA from a phage, a short segment of approximately 20-40 base pairs (called a spacer) is integrated into the CRISPR array between repeat sequences. During expression (crRNA biogenesis), the CRISPR array is transcribed as a long pre-crRNA and processed into individual crRNAs, each containing one spacer flanked by repeat sequences. During interference, the crRNA guides Cas protein(s) to complementary foreign DNA, which is then cleaved and destroyed. The PAM (Protospacer Adjacent Motif), a short sequence such as 5'-NGG-3' for Streptococcus pyogenes Cas9, must be adjacent to the target in the foreign DNA but is absent from the CRISPR array, allowing the bacterium to distinguish self from non-self.
II. Engineering CRISPR-Cas9 for Genome Editing
The key contribution by Jennifer Doudna and Emmanuelle Charpentier (Nobel Prize in Chemistry, 2020) was simplifying the Type II CRISPR system from S. pyogenes for use as a genome editing tool. The natural system uses a crRNA, a tracrRNA (trans-activating crRNA), and the Cas9 protein. The engineered version fuses the crRNA and tracrRNA into a single guide RNA (sgRNA), reducing the system to just two components: the programmable sgRNA and the Cas9 protein.
The mechanism proceeds as follows. The sgRNA, with its 20-nucleotide guide sequence, directs Cas9 to the complementary target DNA. Cas9 first recognizes the PAM sequence (NGG) on the non-target strand, then the sgRNA base-pairs with the target strand to form an R-loop. Cas9 cleaves both DNA strands, producing a blunt-ended double-strand break 3 base pairs upstream of the PAM. The RuvC domain cleaves the non-target strand while the HNH domain cleaves the target strand. The resulting DSB is then repaired by the cell's endogenous repair machinery.
III. DNA Repair Outcomes After Cas9 Cleavage
The cell can repair the Cas9-induced DSB through two pathways with very different outcomes. Non-Homologous End Joining (NHEJ) is error-prone and introduces small insertions or deletions (indels) at the cut site. Indels within a coding sequence cause frameshifts that lead to gene knockout (loss of function). NHEJ requires no repair template, works in all cell types and cell cycle phases, and is the primary tool for gene disruption/knockout experiments.
Homology-Directed Repair (HDR) provides precise repair using a supplied DNA template with homology arms flanking the desired change. The template can be a plasmid with homology arms or a single-stranded oligonucleotide (ssODN). HDR enables precise gene editing, including point mutations, insertion of tags or reporters, and gene correction. However, HDR is less efficient than NHEJ and is primarily active during S and G2 phases when a sister chromatid is available. Improving HDR efficiency through cell cycle synchronization and NHEJ inhibitors remains a major area of research.
<image>Panel A: Diagram of the CRISPR-Cas9 genome editing mechanism showing: the sgRNA (with 20-nt guide sequence fused to scaffold) complexed with Cas9 protein, binding to the target DNA at the PAM (NGG) site, R-loop formation, and the blunt-ended double-strand break made by the RuvC and HNH domains (cut sites marked). Panel B: Two DNA repair pathways after Cas9 cleavage — left: NHEJ pathway leading to indels (insertions/deletions) and gene knockout (with example frameshift); right: HDR pathway with a donor template leading to precise gene editing (point mutation correction or sequence insertion), with homology arms labeled. Panel C: Comparison of the three generations of programmable nucleases — ZFNs (zinc finger nucleases), TALENs (transcription activator-like effector nucleases), and CRISPR-Cas9 — showing their protein-DNA recognition mechanisms, ease of design, and relative advantages/limitations.</image>
IV. Earlier Genome Editing Technologies
Zinc Finger Nucleases (ZFNs) consist of zinc finger DNA-binding domains fused to the FokI nuclease. Each zinc finger recognizes approximately 3 base pairs, and 3-6 fingers per side provide 9-18 base pair recognition specificity. ZFNs require dimerization (two ZFNs flanking the target site) and are difficult and expensive to engineer with limited target site selection.
TALENs (Transcription Activator-Like Effector Nucleases) use TALE repeats, each recognizing a single base pair, fused to FokI nuclease. The simpler protein-DNA recognition code makes TALENs more flexible than ZFNs, but they still require dimerization and protein engineering for each new target.
CRISPR-Cas9 offers decisive advantages over both earlier technologies. It is RNA-guided, requiring no protein engineering -- simply changing the 20-nucleotide guide sequence redirects it to a new target. It is simple, fast, and inexpensive to design and produce. It can target multiple genes simultaneously through multiplex editing. And vast guide RNA libraries are available for genome-wide screens.
V. Advanced CRISPR Technologies
Base editing, developed by the David Liu laboratory in 2016, converts one base pair to another without creating a double-strand break. Cytosine base editors (CBE) convert C to T (C-G to T-A) using a catalytically impaired Cas9 (nickase, nCas9) fused to a cytidine deaminase and a uracil glycosylase inhibitor. Adenine base editors (ABE) convert A to G (A-T to G-C) using nCas9 fused to an engineered adenosine deaminase (TadA). Base editing avoids the DSB entirely, requires no donor template, and achieves higher efficiency than HDR for point mutations, though it is restricted to certain conversion types and may cause bystander editing within the editing window.
Prime editing, also from the Liu laboratory (2019), is a "search-and-replace" approach capable of making any point mutation, small insertion, or small deletion. It uses nCas9 fused to a reverse transcriptase along with a prime editing guide RNA (pegRNA) that contains a spacer for target recognition, a primer binding site, and a reverse transcriptase template encoding the desired edit. The nCas9 nicks one strand, the reverse transcriptase copies the edit from the pegRNA template, and the cell resolves the resulting mismatch. Prime editing requires no DSB and no donor template DNA, and it produces lower off-target effects than standard CRISPR.
CRISPRi (interference) uses catalytically dead Cas9 (dCas9) fused to a transcriptional repressor such as KRAB to block transcription without cutting DNA. CRISPRa (activation) fuses dCas9 to transcriptional activators (VP64, p65, Rta) to upregulate gene expression. Epigenome editing fuses dCas9 to epigenetic modifiers such as DNMT3A (for targeted methylation), p300 (for acetylation), or TET1 (for demethylation) to make targeted epigenetic changes.
VI. Applications of CRISPR
In basic research, CRISPR enables gene knockout and knock-in in cell lines and model organisms, genome-wide screens (knockout, activation, and interference) to identify gene function, and disease modeling through creation of cells or animals with specific mutations. In agriculture, CRISPR is used to develop disease-resistant crops, improve nutritional content, and enhance drought tolerance, and in some regulatory frameworks these modifications are distinguished from GMOs because no foreign DNA is introduced.
Therapeutic applications represent the most transformative potential. CASGEVY (exagamglogene autotemcel), the first CRISPR therapy approved by the FDA in 2023, edits the BCL11A enhancer in patient hematopoietic stem cells to reactivate fetal hemoglobin for the treatment of sickle cell disease and beta-thalassemia. Ex vivo editing of CAR-T cells and hematopoietic stem cells is advancing rapidly. In vivo editing approaches include lipid nanoparticle delivery of CRISPR to the liver (such as NTLA-2001 for transthyretin amyloidosis) and gene therapy for inherited retinal dystrophies. Diagnostic applications using CRISPR-based detection platforms (SHERLOCK and DETECTR) exploit the collateral cleavage activity of Cas12 and Cas13 for sensitive nucleic acid detection.
VII. Off-Target Effects and Delivery Challenges
Off-target editing occurs when Cas9 cuts at sites with imperfect complementarity to the guide RNA, particularly at sites with 1-3 mismatches in the PAM-distal region. Strategies to minimize off-target effects include careful guide RNA design using computational tools (Benchling, CRISPOR, CHOPCHOP), use of high-fidelity Cas9 variants (eSpCas9, HiFi Cas9), truncated guides (17-18 nucleotides instead of 20), paired nickases (two nCas9 molecules with offset guides), and anti-CRISPR proteins for temporal control.
Delivery methods vary in their properties and applications. Plasmid DNA provides simple delivery but sustained expression increases off-target risk. mRNA delivery offers transient expression with reduced off-target effects. Ribonucleoprotein (RNP) delivery, in which Cas9 protein is complexed directly with sgRNA, is the most transient and carries the lowest off-target risk. Viral vectors, particularly AAV (adeno-associated virus), enable in vivo delivery but have limited cargo capacity. Lipid nanoparticles (LNPs) deliver mRNA or RNP to the liver and other tissues. Electroporation is used for ex vivo cell editing.
<image>Panel A: Comparison diagram of base editing and prime editing mechanisms. Base editing: nCas9-cytidine deaminase converts C to U (then to T after replication) within an editing window, without a DSB. Prime editing: nCas9-reverse transcriptase with pegRNA nicks one strand, extends with the RT template encoding the desired edit, and the cell resolves the heteroduplex. Panel B: Clinical application flowchart for CRISPR therapy of sickle cell disease: patient HSCs collected → CRISPR editing of BCL11A enhancer ex vivo → expansion of edited cells → myeloablative conditioning → infusion of edited HSCs → production of fetal hemoglobin (HbF) → clinical improvement. Panel C: Delivery methods for CRISPR components illustrated: plasmid DNA (long expression, higher off-target), mRNA (transient), RNP (Cas9 protein + sgRNA, most transient), viral vector (AAV), and lipid nanoparticle, ranked by duration of expression and off-target risk.</image>
