The ability to precisely modify human genes has been made possible by the development of tools such as meganucleases, zinc finger nucleases, TALENs, and CRISPR/Cas. These now make it possible to generate targeted deletions, insertions, gene knock outs, and point variants; to modulate gene expression by targeting transcription factors or epigenetic machineries to DNA; or to target and modify RNA. Endogenous repair mechanisms are used to make the modifications required in DNA; they include non-homologous end joining, homology-directed repair, homology-independent targeted integration, microhomology-mediated end joining, base-excision repair, and mismatch repair. Off-target effects can be monitored using in silico prediction and sequencing and minimized using Cas proteins with higher accuracy, such as high-fidelity Cas9, enhanced-specificity Cas9, and hyperaccurate Cas9. Alternatives to Cas9 have been identified, including Cpf1, Cas12a, Cas12b, and smaller Cas9 orthologs such as CjCas9. Delivery of gene-editing components is performed ex vivo using standard techniques or in vivo using AAV, lipid nanoparticles, or cell-penetrating peptides. Clinical development of gene-editing technology is progressing in several fields, including immunotherapy in cancer treatment, antiviral therapy for HIV infection, and treatment of genetic disorders such as β-thalassemia, sickle cell disease, lysosomal storage disorders, and retinal dystrophy. Here we review these technological advances and the challenges to their clinical implementation.

We present an overview of clinical trials involving gene editing using clustered interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 (Cas9), transcription activator-like effector nucleases (TALENs), or zinc finger nucleases (ZFNs) and discuss the underlying mechanisms. In cancer immunotherapy, gene editing is applied ex vivo in T cells, transgenic T cell receptor (tTCR)-T cells, or chimeric antigen receptor (CAR)-T cells to improve adoptive cell therapy for multiple cancer types. This involves knockouts of immune checkpoint regulators such as PD-1, components of the endogenous TCR and histocompatibility leukocyte antigen (HLA) complex to generate universal allogeneic CAR-T cells, and CD7 to prevent self-destruction in adoptive cell therapy. In cervix carcinoma caused by human papillomavirus (HPV), E6 and E7 genes are disrupted using topically applied gene editing machinery. In HIV infection, the CCR5 co-receptor is disrupted ex vivo to generate HIV-resistant T cells, CAR-T cells, or hematopoietic stem cells. In β-thalassemia and sickle cell disease, hematopoietic stem cells are engineered ex vivo to induce the production of fetal hemoglobin. AAV-mediated in vivo gene editing is applied to exploit the liver for systemic production of therapeutic proteins in hemophilia and mucopolysaccharidoses, and in the eye to restore splicing of the CEP920 gene in Leber’s congenital amaurosis. Close consideration of safety aspects and education of stakeholders will be essential for a successful implementation of gene editing technology in the clinic.

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RUNX1 mutations are recurrent aberrations in acute myeloid leukemia (AML) that are either somatically acquired or originate in the germline. Monoallelic pathogenic germline RUNX1 variants cause familial platelet disorder with propensity to AML (FPD/AML), featuring predisposition to develop myeloid malignancies. Recently, Simon et al 1 found that 30% of RUNX1mutated AML patients carried a (nonpolymorphic) RUNX1 germline variant. This high frequency contrasted previously reported frequencies in AML cohort studies. [2][3][4][5] To obtain further insight into the frequency of germline variants within RUNX1-mutated AML patients, we retrieved data from a large independent cohort of AML patients that was previously interrogated by paired genetic sequencing at diagnosis and in complete remission (CR). 6 Targeted next-generation sequencing (NGS) at diagnosis was performed in 763 AML patients, enrolled in the Dutch-Belgian Hemato-Oncology Cooperative Group (HOVON)102 clinical trial (2010-2013), 7 using the TruSight Myeloid Sequencing Panel (Illumina). The HOVON102 trial was designed to investigate the added value of clofarabine in combination with standard remission-induction chemotherapy in adult (18-65 years) AML or myelodysplastic syndrome patients. NGS data were analyzed as previously described, 6 and nonpolymorphic RUNX1 variants were annotated to RefSeq ID NM_001754.5.At diagnosis, at least 1 RUNX1 mutation was detected in 115 of 763 AML patients (15.1%) for a total of 142 RUNX1 mutations (data not shown), corresponding to frequencies of 10% to 15% reported in literature. 3,4,8,9 We detected 2 or more mutations in 23 of 115 patients.

Familial platelet disorder with associated myeloid malignancies (RUNX1-FPD) is caused by heterozygous pathogenic germline variants of RUNX1. In the present study, we evaluate the applicability of transactivation assays to investigate RUNX1 variants in different regions of the protein. We studied 11 variants to independently validate transactivation assays supporting variant classification following the ClinGen Myeloid Malignancies variant curation expert panel guidelines. Variant classification is key for the translation of genetic findings. We showed that new assays need to be developed to assess C-terminal RUNX1 variants. Two variants of uncertain significance (VUS) were reclassified to likely pathogenic. Additionally, our analyses supported the (likely) pathogenic classification of two other variants. We demonstrated functionality of four VUS, but reclassification to (likely) benign was challenging and suggested the need to reevaluate current classification guidelines. Finally, clinical utility of our assays was illustrated in the context of seven families. Our data confirmed RUNX1-FPD suspicion in three families with RUNX1-FPD-specific family history. Whereas for three variants identified in non RUNX1-FPD-typical families, no functional defect was detected. Applying functional assays to support RUNX1 variant classification can be essential for adequate care of index patients and their relatives at risk. It facilitates translation of genetic data into personalized medicine.