A Lawyer’s Guide to CRISPR Gene Editing

In October 2018, I chaired a North Carolina Law Review Symposium on the ethical, legal and policy implications of gene editing. The collected papers from the Symposium are about to appear in a special issue of the Review (v. 97, 2019). In that issue, authors from diverse disciplines explore gene editing’s significant implications for law, ethics, regulation and health policy from their varied perspectives. In my introduction to the issue, I give a brief “CRISPR for Lawyers” overview of the technology. This post is adapted from that introduction, with the permission of the North Carolina Law Review.(Online versions of the final articles will appear here.)

For years, genomic medicine has been hailed as the future of clinical treatment. The general premise is that doctors will use detailed information about a particular patient’s DNA (and other “biomarkers”) to custom-tailor diagnoses, advice, drug choices and doses, and other specifics of treatment. President Obama’s highly publicized Precision Medicine Initiative (now rebranded—cryptically—as the “All of Us” Research Program; https://allofus.nih.gov/) illustrates both the hope and the hype.

Despite this hope and hype, genomic medicine has thus far had limited effect on the day-to-day practice of medicine, and that effect has been most notable in cancer treatment (see, for example, the use of BRCA gene testing in treating breast cancer made famous by Angelina Jolie). The limiting factors have included the facts that: genes tend to influence the probability of getting a disease, but rarely “cause” a disease in a deterministic sense; the relative influences of environment, lifestyle and epigenetic factors (changes in DNA’s immediate chemical environment in the body) on the ways genes are expressed are only beginning to be understood; and, for the rare cases of clear genetic causation, treatment of the resulting diseases has been only symptomatic, since we have had no “cures” at the genetic level.

In fact, the Holy Grail of genomic medicine has always been the ability not just to identify dangerous gene mutations but to fix them: to go into a patient’s (or an embryo’s) cells and change a dangerous DNA sequence to a healthy one. There have been efforts to do “gene therapy” by using viruses and other vectors to add desired DNA into the patient’s cells. There have been some limited successes but also some catastrophic failures, most infamously the death of a teenage boy in Pennsylvania and cases of leukemia-like side effects in France. In hindsight, the problems were probably due to insufficient knowledge about the mechanisms (or vectors) used to deliver the healthy DNA into the patient’s cells.

Now a new “gene-editing” technology, called CRISPR (or CRISPR-Cas9), may have the potential to provide a safe and effective way to cut out mutated sequences of DNA and paste in normal variants. (As is so often the case in science, it is actually a new application of old knowledge—in this case, about the immune systems of bacteria.) There is a long way to go before CRISPR becomes part of patient care, but, for the first time, there seems to be a way to leapfrog the use of potentially risky vectors to deliver DNA into a patient’s cells. The promise and potential value of the technology is reflected in the epic struggle underway over the foundational patent rights, featuring MIT and the Broad Institute on one side and the University of California-Berkeley and several European luminaries on the other—a biomedical Clash of Titans. Meanwhile, in 2015 a Chinese research team reported the first successful gene editing intervention in non-viable human embryos, followed last year by a Chinese scientist’s claim to have edited the genome of twin baby girls.

How CRISPR Works

CRISPR (pronounced crisper, like the lettuce drawer in the refrigerator) stands for Clustered Regularly Interspaced Palindromic Repeats. These are short repeating sequences in the DNA of E. coli and other bacteria that were discovered by Japanese researchers in the 1980s. DNA is made up of long, two-stranded chains of four chemical building blocks, or bases: A, T, C and G. The specific arrangement, or sequence, of these bases determines the nature of the organism—in simplest terms, whether it’s a bacteria or me.

The CRISPR regions were an enigma to the scientists who first noticed them. Their function was unknown for about 20 years, when food scientists using bacteria to make yogurt figured out that they are part of the bacteria’s immune system. These scientists realized that the CRISPR sequences resemble the DNA of viruses. In fact, the CRISPR sequences are taken from viral DNA that the bacteria has captured during past viral invasions. When a new viral attack occurs, the bacteria’s immune system compares the virus’s genetic material to the sequences stored in CRISPR; if it detects a match, it launches enzymes (a class of proteins that facilitate chemical reactions) to cut up the incoming viral DNA and repel the invasion.

The details of this recognize-and-destroy process have proved critical to developing CRISPR’s gene-editing potential. But first a bit more terminology: An organism’s genome is the entirety of its DNA; genes are those DNA sequences that function to build, or encode, proteins. Genes account for only a small portion of the DNA in the genome. Other portions of the genome have regulatory functions, controlling when particular genes switch on and off, while other areas have no known current function. RNA is a single-stranded cousin of DNA that performs many functions in the cell.

The bacterial CRISPR sequences are always accompanied by genes that code for enzymes that can cut DNA. The original CRISPR scientists called them Cas (for CRISPR-associated) genes. Later research revealed that when viruses invade a bacterial cell, the CRISPR regions produce RNA versions of the viral DNA sequences that they have captured and stored. These RNA sequences are cradled by the Cas enzymes and carried around the cell. When an RNA sequence encounters its viral DNA counterpart, it latches on and the Cas enzyme cuts the DNA, which stops the virus from replicating.

Current CRISPR gene-editing technology mimics this natural process. Researchers at the University of California-Berkeley chose a pair of Cas enzymes called Cas9. They supplied the enzymes with the RNA counterpart of the genetic sequence they wanted to edit—the target gene. The RNA finds and binds to the target DNA and the Cas9 enzymes cut it at its two ends. With the target gene excised, the cell can be induced to make a new one. In the simplest application, the CRISPR mechanism finds and cuts out a “defective” gene—for example, one that causes a single-gene disease such as cystic fibrosis, hemophilia or sickle cell disease—and the cell replaces it with a normal one. CRISPR technology can also be used to introduce a new gene into the space.

The image below provides a simple visual representation of how CRISPR-Cas9 is used to find and cut a target gene (the g in gRNA stands for guide; PAM is a DNA sequence adjacent to the target sequence that Cas9 recognizes):

(This image, created by Marius Walter, is reproduced from this website, under the Creative Commons Attribution-ShareAlike 4.0 International license, available here.)

CRISPR is not the first gene-editing technology. Other approaches include Zinc-finger nucleases (ZFN) and Transcription activator-like effector nucleases (TALENs). ZFN, which dates to the early 1990s, employs custom-engineered proteins that find, bind to and cut target DNA sequences. ZFN improved on prior technology by significantly improving the accuracy of gene editing, in particular by reducing “off-target” edits that hit the wrong DNA sequences, with unpredictable consequences. However, ZFN’s effective rate is only about 10 percent and the custom-engineering of proteins for each new target gene makes it slow, expensive and inefficient. TALENs, which appeared in 2009, is generally similar to ZFN but simpler and more efficient. CRISPR represents a major advance over both in terms of efficiency and accuracy.

There is a long way to go before CRISPR gene editing becomes part of everyday patient care, but it has the potential both to “fix” the causes of single-gene diseases and to contribute to the prevention or treatment of diseases that are caused by a complex interaction of genes and environmental factors, including cancer and heart disease. Such uses seem—at least at first glance—to be ethically unproblematic, though there are worries about such safety issues as off-target edits. But other possible uses are already engendering profound ethical concerns. Those uses include enhancement, or gene editing to improve on normal human traits; editing human sperm or egg cells, which raises concerns about the intergenerational protection of those who might inherit edited genomes; gene editing of embryos, the subject of the recent Chinese claims; gene editing of animals, for a variety of purposes; and attempting to alter ecology, as in the proposed use of CRISPR to eliminate malarial mosquitoes. Such concerns are the subject of many of the articles in the forthcoming North Carolina Law Review Symposium.