The third installment of our blog series about gene editing focuses on gene edits and editing for research purposes. Hope you that you find it informative – please Contact Us with any comments! View the other posts in this series!
Gene edit: A modification of a specific sequence of A, C, G, T units that instruct the sequence of amino acids that comprise a specific protein. The edit may or may not alter the amino acid sequence and the protein.
Early gene editing experiments were accomplished by mating individuals with different traits. Two well known examples are Mendel’s famous red peas crossed to white peas to yield pink peas (Mendel experiments summarized in this short animation: https://youtu.be/Mehz7tCxjSE), and Mr. Little’s Fancy Mice, popular in the early 1900’s, bred for coat color, formed the basis of the Jackson Laboratory’s inbred mice to study genetic diseases.
Nobel Laureate Mario Capecchi systematically studied the function of mouse genes by mutating them into silence, so called “knock-out” mice (he also spoke at the Foundation’s annual Activated Egg Symposium, in a talk titled “Gene Targeting Into the 21st Century: Mouse Models of Human Disease From Cancer to Neuropsychiatric Disorders”). This was accomplished by flooding cultures of mouse embryonic stem cells with strands of synthetic DNA that could replace the normal gene with an edited copy during DNA replication. The edited gene sequence was designed to not guide the synthesis of the normal protein. Such gene edited cells were combined with early mouse embryos, ultimately becoming part of the tissues of the mouse, including occasionally sperm and eggs. Males with gene edited sperm were mated to females with gene edited eggs to produce offspring containing two copies of the edited, non-functioning genes. Although laborious and time-consuming, this approach has yielded highly valuable information about the normal functions of thousands of genes.
In the past 20 years, other less time consuming methods of silencing genes, or increasing their expression, have been developed, all with the goal of understanding their function in health and disease.
In 2013, the most recent method for gene editing was popularized by scientists at Stanford and MIT. It is an adaptation of a naturally occurring defense mechanism that bacteria have against the viruses that invade them. Termed CRISPR/Cas, it is a complex between a protein that can cut DNA strands and a synthetic single-stranded RNA with a sequence of A, C, G, U that matches the gene being targeted (short video explanation of CRISPR here: https://youtu.be/duKV1lNiqQw). The simplicity and specificity of the system have rapidly led to a wide variety of applications among scientists world-wide.
CRISPR/Cas: “Clustered Regularly Interspaced Short Palindromic Repeats” is a term that describes DNA sequences in the viruses that infect bacteria. The immune system of bacteria includes a family of proteins (CRISPR-associated, Cas) that recognize CRISPR sequences and degrades them. The enzyme, Cas, needs to bind to a specific RNA sequence of 120 units, which can be synthesized synthetically, in order to degrade the DNA. These two components also function well in cell types other than bacteria, and so have become a useful tool for cutting DNA, resulting in either small deletions, or successful insertions of new synthetic DNAs. Both outcomes create an edited (mutated) gene.
Such targeted DNA cuts can edit the gene sequences so they no longer code for a functioning protein, analogous to the natural CCR5 mutation, or opening the DNA strands can allow the incorporation of synthetic DNA sequences into the cut site. This raises the exciting possibility of being able to repair defective human genes. We’ll see you next month, when we’ll discuss how these research gene editing techniques may be used in the potential treatment for diseases.