Divergent RNA structures support accurate splicing of the SF3B1-sensitive MAP3K7 intron

Through high-throughput mutagenesis and SHAPE-MAP analysis of the MAP3K7 intron, this study reveals that while branchpoint mutations are the primary drivers of cryptic splice-site activation in the context of SF3B1 mutations, accurate splicing is maintained by extensive RNA structural variability rather than a single conserved structure, except within specific RNA-binding protein hotspots where structural similarity correlates with splicing outcomes.

The Gist

Imagine your DNA is a massive instruction manual for building a human body. To read the right instructions, the cell has to cut out the "junk" parts (introns) and stitch the "good" parts (exons) together. This process is called splicing, and it's like a very precise editor cutting and pasting text.

This paper investigates what happens when that editor makes a mistake. Specifically, it looks at a gene called MAP3K7, which acts like a stress alarm for the cell.

The Problem: A Sneaky "Ghost" Instruction

Inside the MAP3K7 instruction manual, there is a hidden, fake instruction called a cryptic splice site. Normally, the cell's editor ignores this fake instruction and follows the real one. However, if a specific part of the editing machine (a protein called SF3B1) gets mutated—specifically changing a letter from "K" to "E" (the K700E mutation)—the editor gets confused. It starts using that fake instruction instead of the real one, leading to a broken product. This confusion is linked to certain types of cancer.

The Experiment: A "Choose Your Own Adventure" of Mutations

To figure out why the editor gets confused, the researchers created a massive library of 249 different versions of the MAP3K7 gene. They treated this like a giant "Choose Your Own Adventure" book where they changed:

• The punctuation marks (nucleotide composition).
• The binding spots for other helpers (RNA-binding protein motifs).
• The way the text folds up (RNA structures).

They then watched how these changes affected the editing process in two scenarios: when the editor was working normally, and when the editor had the "K700E" glitch.

The Discovery: It's Not Just About the Shape

The researchers expected that if the gene's shape (its 3D structure) looked different from the original, the editor would get confused. They used a special chemical "flashlight" (SHAPE-MAP) to take pictures of how the RNA folded.

Here is what they found:

1. The Punctuation Matters Most: The biggest reason the editor started using the fake instruction was when they messed up the branch point. Think of the branch point as the specific "glue spot" where the cut needs to happen. If you mess up the glue spot, the editor panics and grabs the nearest fake instruction.
2. Shape Isn't Everything: Surprisingly, changing the overall shape of the RNA didn't automatically cause errors. In fact, many different shapes worked just fine.
3. The "Hotspot" Exception: There was one specific area where the RNA folds and interacts with helper proteins (like U2AF2 and SRSF2). In this specific zone, if the shape stayed similar to the original, the editor worked correctly. But if the shape changed there, it caused problems.

The Big Takeaway

The main conclusion is a bit like a flexible origami crane. You don't need the paper to be folded in one exact, rigid way for the crane to work. The cell's editing machine is surprisingly flexible; it can handle a wide variety of different shapes and structures as long as the critical "glue spots" (branch points) are correct.

However, if you mess with the specific area where the helpers grab on, the shape becomes very important. The paper shows that while some parts of the RNA need to be rigid and specific, other parts can be "divergent"—meaning they can look very different from each other and still allow the cell to splice the gene accurately.

Technical Summary

Technical Summary: Divergent RNA Structures Support Accurate Splicing of the SF3B1-Sensitive MAP3K7 Intron

Problem Statement
Pre-mRNA splicing is regulated by the interplay between the spliceosome and specific sequence and structural elements within the precursor RNA. However, the precise relative contributions of RNA sequence versus RNA structural elements to splicing fidelity remain unclear. This uncertainty is particularly relevant in the context of SF3B1 mutations, which are prevalent in various cancers and are known to promote aberrant splicing. Specifically, the SF3B1 K700E mutation increases the usage of a cryptic 3' splice site within the MAP3K7 intron, a gene encoding a stress-response kinase. Understanding how RNA structure influences the selection of splice sites, especially under conditions of spliceosomal dysfunction, is critical for elucidating the mechanisms of splicing errors in disease.

Methodology
To systematically dissect the determinants of splicing, the authors employed a high-throughput mutagenesis approach using a MAP3K7 intron reporter system.

• Library Design: A pooled library comprising 249 distinct MAP3K7 mutants was generated. These mutations targeted specific functional regions, including branch points, RNA-binding protein (RBP) motifs, nucleotide composition, and predicted structural elements.
• Experimental Conditions: The impact of these mutations on splicing efficiency was quantified in two distinct cellular contexts: normal expression and expression of the oncogenic SF3B1 K700E mutant.
• Structural Analysis: To correlate splicing outcomes with RNA conformation, the authors performed parallel in vitro high-throughput SHAPE-MAP (Selective 2'-Hydroxyl Acylation analyzed by Primer Extension and Mutational Profiling) chemical probing. This allowed for the assessment of RNA structural changes across the mutant library.

Key Results
The study yielded several distinct findings regarding the relationship between sequence, structure, and splicing outcomes:

1. Branch Point Dominance: Mutations within the branch point sequence were identified as the primary drivers of increased cryptic splice-site usage, exerting the strongest effect on splicing fidelity.
2. Lack of Global Structural Correlation: The researchers observed no overall correlation between the degree of cryptic splice-site usage and the structural similarity of the mutant RNA to the wild-type MAP3K7 RNA. This suggests that global structural conservation is not a strict prerequisite for accurate splicing in this context.
3. RBP Hotspot and Structural Variability: A specific region identified as an RNA-binding protein hotspot (containing binding sites for U2AF2, U2AF1, KHSRP, and SRSF2) showed a distinct pattern. Mutants within this hotspot were associated with cryptic splice-site use and retained structural similarity to the wild-type RNA. However, these structural changes were linked to increased ensemble diversity, indicating a dynamic range of conformations.
4. Divergent Structures: The data revealed that while certain regions of the RNA maintain specific structures, other regions exhibit extensive variability. Crucially, these divergent RNA structures were found to be compatible with accurate splicing.

Significance and Claims
The paper concludes that accurate splicing of the MAP3K7 intron does not rely on a single, rigid RNA structure. Instead, the results demonstrate that while key structured regions exist, there is substantial flexibility in the RNA landscape. The authors claim that divergent RNA structures can successfully support accurate splicing, challenging the notion that a single, conserved structural conformation is necessary for spliceosome function. This finding provides a more nuanced view of how RNA structural plasticity interacts with sequence determinants to regulate splicing, particularly in the context of SF3B1-associated splicing errors.