CRISPR Activation Unmasks Splice-Altering Variants in Easily Accessible Cells

Study Background and Research Question

Hereditary disorders frequently arise from genetic variants that disrupt pre-mRNA splicing, but many of these variants are difficult to functionally characterize due to the tissue-specific expression of human genes. For example, ~20% of Mendelian disease genes are not transcribed in easily accessible cell types such as skin fibroblasts, severely limiting the ability to perform RNA diagnostics in a clinical setting. In silico predictions of spliceogenicity remain unreliable for cryptic or deep intronic variants, necessitating direct functional assays. Terkelsen et al. (2024) address this gap by exploring whether CRISPR activation (CRISPRa) can be used to ectopically induce expression of disease-associated genes in patient-derived fibroblasts, enabling ex vivo assessment of splice-altering variants within the native genomic context.

Key Innovation from the Reference Study

The principal innovation of this study lies in harnessing CRISPRa—specifically, a dCas9-VPR mRNA-based system—to activate transcription of genes typically silent in skin fibroblasts. By delivering dCas9-VPR mRNA and guide RNAs targeting the promoter region of genes implicated in neurogenetic diseases, the team successfully upregulated transcription of MPZ (myelin protein zero) and SPAST (spastin), both of which exhibit highly tissue-restricted expression. This breakthrough enables direct, functional profiling of splice-altering variants in standard fibroblast cultures, circumventing the need for invasive tissue biopsies from affected organs.

Methods and Experimental Design Insights

The CRISPRa platform employed by Terkelsen et al. relies on an mRNA-based delivery of a catalytically inactive Cas9 (dCas9) fused to a tripartite VP64-p65-Rta (VPR) transcriptional activator. The guide RNAs (sgRNAs) are designed to bind regions upstream of the target gene’s transcription start site, recruiting the dCas9-VPR complex to promote gene expression. Patient-derived skin fibroblasts harboring germline variants in MPZ or SPAST were transfected with these components, and gene expression levels were quantified after overnight incubation. The induced transcripts were analyzed using reverse transcription PCR (RT-PCR), next-generation sequencing (NGS), and long-read sequencing to capture splice isoform diversity and directly assess the impact of the variants.

Protocol Parameters

• dCas9-VPR mRNA transfection: Deliver by lipofection into primary fibroblast cultures; optimize mRNA concentration and sgRNA ratios for efficient gene activation.
• sgRNA design: Target the 300-bp region upstream of the transcription start site (TSS) of the gene of interest, with multiple sgRNAs to enhance activation efficacy.
• Incubation: Assess gene expression and splicing outcomes after 16–24 hours post-transfection.
• RNA analysis: Employ RT-PCR for initial detection of induced transcripts, followed by NGS or long-read sequencing for detailed splicing analysis.
• Ethical compliance: Ensure all patient-derived samples are collected with informed consent in accordance with the Declaration of Helsinki.

Core Findings and Why They Matter

The study demonstrates robust upregulation of MPZ and SPAST mRNA in fibroblasts post-CRISPRa, enabling detection and quantification of alternative splicing patterns. Critically, the approach allowed for the characterization of splice-altering variants that would otherwise be functionally inaccessible due to the tissue-restricted expression of these genes. This ex vivo assay provided high-resolution mapping of aberrant splicing events, supporting reclassification of variants of uncertain significance (VUS) and strengthening the link between genotype and phenotype in rare neurogenetic disorders. The method is feasible in standard diagnostic laboratories and leverages widely available cell types and molecular biology reagents, making it scalable and adaptable for broader clinical implementation (Terkelsen et al., 2024).

Comparison with Existing Internal Articles

Recent internal reviews have highlighted the value of mRNA modification strategies—such as the incorporation of modified nucleosides like N1-Methylpseudouridine—in increasing mRNA stability, translation efficiency, and reducing immune activation in experimental workflows. For example, "N1-Methylpseudouridine: Mechanistic Advantage in mRNA Research" discusses how this modified nucleoside empowers high-fidelity mRNA assays and enhances CRISPR-based variant characterization. These perspectives align with the reference study’s use of mRNA-based delivery for the dCas9-VPR system, where the stability and translational efficiency of the mRNA payload are central to robust gene activation in target cells. Furthermore, other articles (see here, and here) emphasize that reduced immunogenicity in mRNA constructs is critical for reproducible gene induction, especially in primary cells. While Terkelsen et al. did not directly compare different nucleoside modifications, the compatibility of CRISPRa with advanced mRNA chemistry is evident and likely enhances both efficiency and safety of the protocol.

Limitations and Transferability

Despite its promise, the CRISPRa-based workflow has several limitations. First, the magnitude of gene activation may vary depending on locus accessibility, chromatin state, and sgRNA design, necessitating empirical optimization for each gene. Some genes may remain refractory to activation in fibroblasts due to epigenetic silencing or absent transcriptional co-factors. Splice isoform detection is also contingent on sufficient transcript abundance post-activation. While the study establishes proof of principle for neurogenetic genes, further validation is needed across a broader spectrum of loci and variant types. Importantly, the transferability of this approach to other cell types or to genes with complex regulatory architecture should be determined in future studies. The protocol does not replace the need for careful clinical correlation and may not capture tissue-specific regulatory mechanisms that modulate splicing in vivo.

Research Support Resources

Researchers planning to adopt CRISPRa workflows for mRNA-based gene activation can benefit from optimized reagents that enhance mRNA translation and minimize immune responses in primary human cells. Incorporation of N1-Methylpseudouridine (SKU B8340) into synthetic mRNA, as described in the product information and internal reviews, is a practical strategy to maximize translational efficiency and reduce immunogenicity during delivery. This modified nucleoside is validated in diverse mammalian cell types and is compatible with lipofection-based mRNA delivery approaches. While Terkelsen et al. did not specify the use of modified nucleosides, leveraging N1-Methylpseudouridine in future CRISPRa applications can further strengthen reproducibility and safety in splice variant characterization workflows.