Around six years after its first use in animals, CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats)/Cas9 gene editing is now being tested in early clinical trials for several disorders. The Cas9 enzyme can be programmed to target specific stretches of genetic code and edit DNA at precise locations, which make diseases caused by a single gene mutation – such as sickle cell disease (SCD), cystic fibrosis and Huntington’s disease – a good first target for genome editing.
SCD encompasses a group of genetic blood disorders affecting haemoglobin, the molecule in red blood cells that delivers oxygen to cells throughout the body. These disorders, which can cause severe symptoms, including periodic episodes of pain, repeated infections, anaemia and reduced life expectancy, are caused by a single nucleotide change (point mutation) in the Haemoglobin Subunit Beta (HBB) gene that misshapes red blood cells. It is estimated that more than 300,000 children are born every year with SCD, with Nigeria, India and the Democratic Republic of Congo bearing half the global burden of disease. Furthermore, numbers are expected to climb and reach 400,000 babies born annually with SCD by 2050.
Treatment strategies for SCD largely focus on addressing the symptoms of the condition; currently, the only means of cure is a bone marrow transplant for those with a matched donor. Gene-editing approaches, which are currently being tested in clinical trials, are generating excitement and offering new hope for development of a therapy able to address the underlying cause of the disease. News recently broke of the first publicly identified patient who was being treated using CRISPR gene editing for SCD. While it will take months before scientists can determine whether the treatment is working as intended, longer still to determine whether the tool is improving patients’ health outcomes, and many years before there is a clear understanding of whether the benefits last a lifetime, this is a groundbreaking milestone.
Sickle cell anaemia (SCA) is a prime target for gene therapy for several reasons, including its high prevalence, significant morbidity and mortality, and the resulting high cost of medical care associated with treatment. Additionally, single point mutations typically allow for greater gene correction efficiency and offer the potential of cure with a single treatment.
The challenge
CRISPR/Cas9 gene editing is versatile and allows researchers to alter the genetic composition of a cell or entire organism more precisely, cost-effectively and faster than previous gene editing technologies (including gene targeting and Zinc-Finger Nucleases). It targets specific DNA sequences of the genome of any organism, and is comprised of two components: the nuclease (Cas9), responsible for cleavage of the DNA, and an RNA guide, which guides the complex to the target.
However, while CRISPR/Cas9 gene editing offers unparalleled genome editing efficiency in various cell types and species, challenges remain due to low target site specificity. Many researchers have tried modifying the guide RNA and mutant Cas9 proteins to improve target specificity, but these alterations often also reduce on-target editing performance.
Also, the consequences of off-target editing can be potentially harmful, as they may affect genes such as oncogenes or tumour suppressors, which can trigger processes that lead to cancer. Even the effects of on-target editing need to be carefully assessed in the context of introducing corrective changes that are not simply a conversion back to healthy or wild-type sequence as genes often serve multiple purposes, which are not always fully understood or known.
Research at IDT has predominantly focused on improving the properties of CRISPR nucleases, such as developing a new, high-fidelity Cas9 protein that reduces off-target gene editing while maintaining high on-target activity in living cells. The resulting HiFi Cas9 protein is now being used in several research projects and clinical trials targeting SCA.
The public opinion
Current media interest and hype surrounding gene editing and CRISPR/Cas9 has been driven by the significant advances seen in the field in the last decade, but there remain many hurdles to overcome before patients will widely benefit from the fruits of this technology.
Beyond the technical challenges encountered in the laboratory, there is a need to raise the scientific literacy of the general public so that they may engage in discourse regarding the applications of gene editing. In the US, a recent Pew Research Center survey showed that a significant portion of the public – 42% – had heard ‘nothing at all’ about gene editing, while 48% had heard ‘a little’ and only 9% ‘a lot’, underscoring the opportunity to increase awareness and better inform the debate.
It is important to note that publicly available information can be difficult to navigate, sometimes inaccurate and often complicated to understand for the non-expert reader. We have seen before, in the case of genetically modified organisms, for example, that an uninformed or confused public can have negative implications for scientific progress. Therefore, it is paramount that the societal debate keeps up with the rapid pace of research, that the public understands the benefits of this technology, and is confident that it will be used to reduce the global burden of disease and suffering associated with these disorders.
One step closer to personalised medicine
When the Human Genome Project (HGP) – an international research effort that aimed to sequence and map all the genes that make up the human genome – was completed, it opened the door to the idea of personalised medicine, in which disease risk, reaction to certain medications and appropriate treatment can be determined by an individual’s genetic makeup.
While hurdles remain, CRISPR gene editing is one option for developing personalised medicine therapies for a variety of genetic disorders. Since people can have different mutations causing the same disease outcome, this methodology would allow individuals to receive different synthetic nucleic acid constructs to treat their individual mutations, a possibility which still seems far-fetched but could revolutionise modern medicine.
In conclusion
The rapid pace of progress in gene editing is exciting and holds promise for finding cures for some of the most severe disorders, including both recessive genetic disorders, such as cystic fibrosis, SCA, Huntington’s disease, etc, and acquired diseases associated with genetic mechanisms, including cancers and certain viral infections, such as HIV/AIDS.
However, the threat posed by polarized opinions and inaccurate information, as well as a need to assess the risks of procedures and the moral implications involved, remain core challenges that will require the entire scientific community, and beyond, to overcome.
Christopher Vakulskas is senior staff scientist at Integrated DNA Technologies (IDT)
