CRISPR Mediated ELANE Single-Allele Knock-out Restores Proliferation and Myeloid Differentiation of Neutropenia Patient Derived BM HSCs

Background: Mutations in ELANE are a cause of cyclic and severe congenital neutropenia (SCN), predominantly autosomal dominant disorders. ELANE encodes neutrophil elastase (NE), a tissue specific serine protease. Neutropenia occurs because mutant NE impairs survival and maturation of myeloid cells. More than 130 disease causing mutations in ELANE have been identified and cell permeable inhibitors of NE can correct the defect in cell survival and maturation in cellular models. (Makaryan et al. J Leukoc Biol. 2017;102:1143). CRISPR/Cas9 knock-out (KO) of ELANE in patients' hematopoietic stem cells (HSCs) corrects myeloid cell differentiation, (Nasri et al Haematologica 2020:105:598), but this approach targets both alleles, eliminates normal NE and may impair innate immunity.

Hypothesis: Selective, single allele editing is a preferred strategy for correcting ELANE associated neutropenia and other autosomal dominant disorders.

Methods: Emendo Biotherapeutics developed CRISPR-associated allele specific KO based on single nucleotide polymorphisms (SNPs) located in the vicinity of ELANE. Three SNPs cover ~80% of the population. In the current study, editing of the mutated allele was achieved by targeting SNP rs1683564 located downstream to the 3'UTR of ELANE and mediating a biallelic break in intron 4, with excision of the 3'UTR, destabilized the mutated allele transcript. For editing we utilized RNPs assembled from Emendo's proprietary nuclease, OMNI-50, a guide targeting the SNP and a guide targeting a region in intron 4. OMNI-50 has low off-target activity and high allele specific editing when targeting either the reference or the alternative form of the SNPs. The efficacy of this composition was tested on normal and SCN patient-derived HSCs harboring the relevant SNP. (Figure 1)

A patient's bone marrow CD34+ cells with the S126L mutation and rs1683564 were enriched using RosetteSep™ Progenitor Cell Enrichment Cocktail (Stemcell Technologies), purified by density gradient centrifugation using Lymphoprep (Stemcell Technologies). The purified CD34+ cells were expanded 4 days in StemSpan™ SFEM II media, supplemented with StemSpan™ CD34+ Expansion Supplement. Expanded CD34+ cells were further purified using StemSep™ Human CD34 Positive Cocktail and purity verified by FACS analysis using CD34 and CD45 antibodies. Purified CD34+ cells were cryopreserved using serum free CryoStor® CS10 media (Stemcell Technologies) and stored in Liquid Nitrogen Vapor Phase. Normal human blood CD34+ progenitor cells, harboring rs1683564 were similarly treated as the control. Allele specific excision was determined by ddPCR. To determine excision efficiency for the patient and control, we amplified two regions in ELANE, Exon 1 and Exon 5 (excised region), using two different probes labeled with FAM and HEX, respectively.

We used OMNI-50 nuclease (Emendo Bio, Ness Ziona, Israel), electroporation (Lonza 4D nucleofector) and StemCell media (Stemcell Technologies, Vancouver, BC) to modify, grow and differentiate the CD34+ cells. We assessed myeloid cell proliferation and differentiation using daily cell counts, cytospins stained with Diff-Quik, and CD14, CD66b, CD15, CD16, CD11b labeling and flow cytometry.

Results: Unedited patient cells demonstrated significant abnormalities in proliferation and differentiation. Cytospins from unedited patient CD34+ cells at 14 days showed block of myeloid differentiation and 4-fold greater monocytes compared to control, consistent with the hematopoietic defect in SCN patients. Single allele ELANE KO significantly improved these cellular abnormalities: total cell proliferation increased 41% and CD14/CD66b and CD15/CD11b positive cells increased by 107% and 65.5%, respectively. Studies are in progress for additional patients harboring different ELANE mutations.

Conclusions: Single allele KO may resolve concerns about unwanted effects of total elimination of the normal gene products with gene editing for autosomal dominant diseases and be an effective strategy for treating ELANE associate neutropenia.

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