The Potential of CRISPR/Cas9 in Hematotherapy

CRISPR/Cas9 has been called the biggest biotechnological game changer since the polymerase chain reaction, and particularly in hematotherapy, CRISPR/Cas9 holds a remarkable potential. In my opinion, the two major applications in this area are gene therapy in hematopoietic stem cells (HSCs) and genetic engineering of immune cells for adoptive cell therapies. Development of gene editing therapies for beta-globinopathies is particularly advanced, where clinical trials have recently been initiated in Europe for beta thalassemia. With regard to immunotherapy, the community still awaits results from early clinical trials in China using CRISPR/Cas9-engineered T cells for cancer immunotherapy. Clinical trials with Chimeric Antigen Receptor (CAR) T cells engineered with CRISPR/Cas9 are expected to initiate soon in the United States and Europe, but results with cells engineered with TALE nucleases have already demonstrated a great potential [1].

Unique to the CRISPR/Cas9 system is the possibility of delivering the components as a completely DNA-free system using precomplexed ribonucleoprotein [2]. We previously combined this with the development of synthetic and chemically modified single guide RNAs, which now represents a state-of-the-art tool for high frequency gene editing in primary blood cells such as HSCs and T cells [3]. Combined with delivery of repair templates for homologous recombination (HR) facilitated by viral vectors (AAV6), we have shown efficient correction of the disease-causing mutation in HSCs from sickle cell disease patients [4]. The exact same platform has been used to engineer CAR T cells with higher potency than conventional retrovirally engineered cells [5]. With these optimized techniques, it is now possible, in some cell types, to obtain HR rates that exceed those of gene-disrupting nonhomologous end-joining—a shift in balance between these two competing pathways that is oftentimes mistakenly assumed impossible. Remaining concerns about CRISPR/Cas9 in hematotherapy include (1) achieving high HR frequencies in long-term repopulating HSCs, (2) determining the clinical relevance of off-target activity and potential genomic rearrangements, and (3) establishing cost-efficient manufacturing procedures. With regard to the latter, recent advances in using gene editing to generate allogeneic universal “off-the-shelf” cells for immunotherapy are particularly exciting [6].

I was fortunate to spend my postdoctoral training at Stanford University in the laboratory of one of the early gene editing pioneers, Dr. Matthew Porteus. This was an amazing opportunity to learn from someone who has been in the field from the beginning, and in this CRISPR era, I think we too often undervalue the importance of previous discoveries that have facilitated CRISPR to rocket forward with such great success. This entails prominent discoveries on DNA repair mechanisms, early programmable nucleases, and viral and nonviral delivery platforms. Dr. Porteus fostered an outstanding training and developmental environment, and in addition to highly experienced and knowledgeable project mentoring, I particularly appreciated the responsibility and trust I was given to operate with a high degree of freedom. I also highly valued the direct and indirect mentoring on other aspects such as career planning, navigation in academic politics, and how to lead and manage a laboratory. Giving students and postdoctorals fellows insights into these facets of academic life requires a high level of inclusion, which is a key lesson I will bring to my own mentoring.