VAV1-GFP Fusion Protein Generated By Intron-Based Genome Editing through CRISPR/Cas9 Leads to Spontaneous Activation in TCR Signaling

Recurrent VAV1 mutations and gene fusions (VAV1-THAP4, VAV1-MYO1F, and VAV1-S100A7) have been identified in peripheral T-cell lymphoma (PTCL) including angioimmunoblastic T-cell lymphoma (AITL) patients. A common theme of these genetic aberrations is the loss of the auto-inhibitory C-terminal SH3 domain of VAV1 resulting in aberrant activation of VAV1 independent of normal activation events. Although mouse models support VAV1 mutation/fusion as having a driver oncogenic role in the pathogenesis of PTCL, investigations on VAV1 activity in human cells were performed mainly on the Jurkat cell line with exogenous expression of VAV1 fusion proteins. This approach has un-physiological expression of VAV1 and the functions of VAV1 fusion/mutation under normal endogenous regulation need to be explored.

In this study, we introduced a fusion gene, similar to what has been observed in PTCL, into the endogenous VAV1 locus. The fusion gene was under normal regulatory controls instead of being over-expressed by a viral vector, thus providing a more accurate assessment of its function in vivo. To simulate VAV1 fusion, we knocked in a green fluorescence protein (GFP) sequence followed by a simian virus 40 (SV40) poly(A) signal into intron 25 of VAV1 locus by clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9 (Cas9) technology. A homologous DNA repair (HDR) template with tandem homologous sequences of VAV1 gene, GFP gene, and a SV40 transcription poly(A) signal was electroporated into Jurkat cells together with the Cas9/sgRNA ribonucleoprotein (RNP) complex. This knock-in disrupted the transcription of exon 26 and exon 27, resulting in an in-frame fusion protein with GFP fused to the C-terminal of SH2 of VAV1 (VAV1_SH2-GFP)(Figure A). Because our guide RNA targeted the intron 25 sequence by CRISPR/Cas9 system, any possible indels caused by non-homologous end joining will occur within the intron and will not change the protein sequence of the wild type VAV1. The GFP expressing cells were isolated from the edited cell population by FACS. The fusion of GFP with VAV1 in the sorted cells was confirmed by western blot (Figure B) and these cells displayed a heterozygous VAV1_SH2-GFP fusion/wild type (WT) phenotype that mimicked the VAV1 translocations observed in PTCL patients.

Jurkat cells with VAV1_SH2-GFP showed spontaneous activation of the T-cell receptor (TCR) signaling pathway. Analysis of signaling events downstream of VAV1 demonstrated increased phosphorylation in ITK, LCK, and subsequent ERK in Jurkat cells with VAV1_SH2-GFP compared with WT Jurkat cells by western blot (Figure B). We also observed consistently elevated pERK in Jurkat cells with VAV1_SH2-GFP by flow cytometry (Figure C). Notably, this elevation in pERK was spontaneous and independent of TCR stimulation with anti-CD3 antibody.

VAV1_SH2-GFP fusion protein also led to marked activation of downstream NFAT and NF-κB pathways as shown by Luciferase reporter assays (Figure D). Similarly, the enhanced NFAT and NF-κB pathway activation in Jurkat with the fusion protein was independent of TCR stimulation. Interestingly, with antiCD3 stimulation, Jurkat cells with VAV1_SH2-GFP showed significantly lower pERK, NFAT and NF-κB activity compared to WT Jurkat cells with anti-CD3 stimulation.

In conclusion, in Jurkat cells genetically edited with VAV1_SH2-GFP, spontaneous activation of TCR signaling and subsequently increased NFAT and NF-κB activity were observed. Our findings further support that VAV1 C-terminal SH3 domain plays an important regulatory role in blocking VAV1 activity in the absence of proper activation. Removing C-terminal SH3 domain or replacing it with GFP or other protein relieves this inhibition, allowing spontaneous activation independent of TCR stimulation. Our study also indicates the sensitivity of the TCR signaling pathway to the level of activation and hyperactivation is detrimental. To validate the function of VAV1_SH2-GFP in normal T cells, we have also successfully edited primary CD4+ T cells with VAV1_SH2-GFP and the TCR signaling pathway in edited primary CD4+ T cell is currently being evaluated with and without anti-CD3/CD28 stimulation.