Many human diseases are the result of genetic errors, commonly called mutations. You can think of the four nucleotides (A, C, T, and G) that comprise our DNA as the letters of our genetic alphabet with these nucleotide letters making up our genes which are the “words” of our genome. Mutations result from changes in the “spelling” of a gene, just as misspelling a word can disrupt its meaning. These errors can be as simple as a single nucleotide change in our DNA. For example, changing the letter A to the letter C changes the word “arm” to “crm”, a non-word with no meaning. Likewise, a single nucleotide change in a gene can render that gene defective. Other common mutations include spurious additions or deletions of nucleotides that again disrupt the function of the gene just like adding or removing letters from a word will change or destroy its original meaning. Such inherited mutations account for a variety of genetic diseases including sickle-cell anemia, cystic fibrosis, and hemophilia; individuals with these mutations suffer permanent, lifelong illnesses. In addition to these inborn genetic errors, mutations can occur in any of our cells as we grow and age. These so-called somatic mutations that accumulate in our cells can damage or alter critical genes which frequently leads to cancer development. An important goal of 21st-century science is to find ways to correct genetic errors, restore gene function, and eliminate diseases caused by faulty genes.
A major advance for gene therapy was the discovery of the CRISPR-Cas9 system in bacteria. This system is a defense mechanism against bacterial viruses (bacteriophages) and is used to chop up invading bacteriophage DNA. Scientists soon demonstrated that this system could be harnessed to modify human genes by making precise cuts at specific locations in our genomes. The approach, called CRISPR (pronounced “crisper”) gene editing, has two components, a small RNA that serves as the guide and the Cas9 protein which cuts DNA. Cas9 binds the guide RNA, uses the RNA to find a DNA sequence that matches the RNA, and then cuts the DNA at this matching sequence. In order to cut anywhere in the human genome you synthesize a guide RNA (easy to do in the lab) with a sequence matching the site you want to cut. The guide RNA and Cas9 are introduced into cells where they combine, locate the matching DNA sequence, and cleave the DNA. Using two different guide RNAs that cut on opposite sides of a mutation will result in the removal of that damaged DNA. Simultaneously introducing a piece of DNA containing the correct sequence causes the host cell repair mechanisms to insert the replacement DNA at the cut site, thus swapping the defective DNA with the correct version. This approach has been successful many times in cultured cells in the lab, leading to several human trials in the last four years.
Some of the earliest uses of CRISPR in humans involved cancer patients where gene editing was used to try to thwart tumor growth. Though not very successful at stopping the cancers, the technology showed no adverse effects or unexpected consequences which paved the way for further applications. The most promising results have been with sickle-cell anemia and β-thalassemia, both of these diseases involving defects in hemoglobin which is critical for carrying oxygen in our bodies. Starting in 2019, several patients with these diseases have been treated by CRISPR and are showing impressive benefits with greatly reduced clinical symptoms. While it’s only been a year since the first of these patients was treated, the effects seem to be long-lasting and even stronger than expected. Additional trials are beginning for several other genetic diseases and hopes are high that some previously untreatable illnesses may someday become curable with gene editing. Even though there are still many technical hurdles, the future is bright for curing genetic diseases as CRSIPR technology becomes more refined and even more precise.