Human vault RNAs exhibit diverse expression patterns and inter-locus compensation

Technical Summary: Human Vault RNAs Exhibit Diverse Expression Patterns and Inter-locus Compensation

Problem Statement
Vault RNAs (vtRNAs) are small noncoding RNAs discovered in 1986 associated with the vault ribonucleoprotein (RNP) particle. While the vault particle itself has been linked to multidrug resistance, cancer, and viral infection, the functional role of the vtRNAs remains enigmatic. A significant portion of vtRNA molecules exist independently of the vault particle in vivo. Furthermore, humans express four vtRNA paralogs from two distinct loci on chromosome 5: three clustered at the VTRNA1 locus (1-1, 1-2, 1-3) and one at the VTRNA2 locus (2-1). There is ongoing debate regarding whether these paralogs share functional connections or should be classified together, particularly given that VTRNA2-1 (often called nc886) lacks the external regulatory elements found in VTRNA1 genes and shows distinct expression patterns. Previous studies have reported conflicting results regarding vtRNA roles in proliferation, apoptosis, and viral defense, often with paralog-specific or cell-type-specific claims that lack consistency.

Methodology
The authors employed a multi-faceted approach using human cell lines (HEK293T, HeLa, U2OS, MRT, iPSCs, and BJ fibroblasts) to characterize vtRNA expression and function:

• Quantification: vtRNA copy numbers per cell (cpc) were measured using quantitative reverse transcription PCR (RT-qPCR) and northern blotting.
• Genetic Editing: CRISPR-Cas9 was used to generate HEK293T cell lines with single knockouts (KO) of VTRNA1-1, VTRNA1-2, or VTRNA1-3, as well as triple knockouts (TKO) of all three VTRNA1 paralogs. A "CTRLdel" line with flanking sequence deletions but intact genes was also isolated.
• Sequence Analysis: PCR amplification and sequencing of the VTRNA loci and flanking regions were performed across cell lines to check for genetic polymorphisms.
• Functional Assays:
  • Growth/Viability: Competitive growth assays (co-culturing KO with wild-type cells) and flow cytometry (Annexin-V/PI staining) assessed proliferation and apoptosis.
  • Stress Response: Cells were treated with menadione (vitamin K3) to induce oxidative RNA damage and transfected with SLFN11 (an endoribonuclease) to test RNA repair pathways.
  • Viral Infection: HEK293T cells were infected with Human Coronavirus OC43 (HCoV-OC43) to assess viral replication and cell survival.
  • Transcriptomics: RNA sequencing (both poly-A enriched and rRNA depleted) was performed on wild-type, single-KO, triple-KO, and CTRLdel lines to identify differentially expressed genes and pathways.
• Ectopic Expression: Plasmids expressing VTRNA2-1 and VTRNA1-1 (with and without specific deletions) were transfected to test for epigenetic suppression and promoter sensitivity.

Key Contributions and Results

1. Variable Expression and Epigenetic Regulation:

   • VTRNA1 paralogs (1-1, 1-2, 1-3) showed relatively consistent expression levels (few thousand copies per cell) across cell types.
   • VTRNA2-1 expression was highly variable, ranging from undetectable in HEK293T and U2OS to ~1,000 cpc in other lines.
   • Sequence analysis revealed no genetic polymorphisms in the VTRNA genes or flanking regions across cell lines. However, ectopic expression of VTRNA2-1 in HEK293T cells was robust, indicating that the lack of endogenous expression in these cells is due to epigenetic silencing (likely DNA methylation) rather than a lack of trans-acting factors.

2. Inter-locus Compensation:

   • Knockout of one or more VTRNA1 paralogs did not trigger compensatory upregulation of the remaining VTRNA1 paralogs.
   • Crucially, the authors observed a consistent and significant upregulation of VTRNA2-1 (ranging from 3-fold to 27-fold) in VTRNA1 single- and triple-knockout lines. This suggests a novel inter-locus regulatory mechanism where the loss of VTRNA1 family members leads to increased expression of VTRNA2-1, implying a functional connection between the two loci despite their structural and regulatory differences.

3. Flanking Sequence Sensitivity:

   • CRISPR editing resulted in off-target deletions in the flanking sequences of the VTRNA1 genes. These deletions, located between the TATA box and transcription start sites, correlated with reduced expression of the targeted genes.
   • Transfection assays confirmed that these upstream deletions reduce transcription efficiency, suggesting that precise spacing of regulatory elements is critical for VTRNA1 expression.

4. Lack of Phenotype in Standard Conditions:

   • Growth and Apoptosis: The VTRNA1 TKO HEK293T cells showed no significant basal growth deficit or increased apoptosis compared to wild-type cells, contradicting some prior reports of vtRNA roles in proliferation.
   • Viral Infection: Infection with HCoV-OC43 did not reveal a significant difference in viral replication or cell survival between wild-type and VTRNA1 TKO cells.
   • RNA Damage: While VTRNA1 TKO cells showed a modest (~35% lower LC50) increase in sensitivity to menadione-induced oxidative stress, they did not show sensitivity to SLFN11 overexpression. This suggests a minor, non-specific role in oxidative stress tolerance rather than a defined role in the SLFN11 RNA repair pathway.

5. Transcriptomic Analysis:

   • RNA-seq revealed broad transcriptomic changes in knockout lines, but no single gene or biological process met the authors' stringent criteria for being consistently affected across all triple-knockout clones and single-knockout lines while remaining unchanged in the CTRLdel control.
   • Relaxing criteria implicated a few genes (e.g., CRYAB, H1-4) and processes related to sensory perception and smooth muscle differentiation, but the authors note these findings are not definitive and may be influenced by batch effects or clonal variation.

Significance and Claims
The paper concludes that while human vtRNA paralogs exhibit distinct expression patterns and may operate via different molecular mechanisms, they are not functionally independent. The consistent upregulation of VTRNA2-1 upon loss of VTRNA1 paralogs points to a regulatory connection between the two loci. However, the study challenges the generalizability of previously reported vtRNA functions in proliferation, viral defense, and apoptosis within the HEK293T model system. The authors assert that the biological roles of vtRNAs remain enigmatic and require further critical study, particularly to resolve discrepancies between cell types and to understand the mechanism of the observed inter-locus compensation. They suggest that future work should focus on generating quadruple-knockout cell lines in cell types where VTRNA2-1 is naturally expressed (e.g., MRT cells) to fully dissect the functional overlap between the paralogs.