Can a variant affect splicing without changing the protein-coding sequence?

Yes. Variants in introns, at splice-site signals, or in regulatory elements can alter how exons are joined and therefore change the mature transcript and protein, even when they lie outside the coding sequence or appear synonymous.

Why is RNA testing sometimes done for splice-site variants?

Because computational tools predict splicing impact imperfectly, examining the actual transcript (for example by RNA sequencing or reverse-transcription assays) can confirm whether an exon is skipped, an intron retained, or a cryptic site used.

Splice-Site Mutations

Splice-site mutations disrupt the signals that direct removal of introns and joining of exons during pre-messenger RNA processing. By altering a splice donor or acceptor site, or creating or activating a cryptic site, they can cause exon skipping, intron retention, or use of an aberrant boundary, changing the mature transcript and the resulting protein even though the coding sequence itself may look intact.

Definition

A splice-site mutation is a sequence change that disrupts the signals controlling pre-mRNA splicing — typically the conserved dinucleotides at the intron-exon (acceptor) or exon-intron (donor) boundaries, or nearby regulatory and cryptic sites — leading to aberrant splicing such as exon skipping, intron retention, or use of an alternative boundary.

Scope

This topic covers sequence changes that affect RNA splicing: variants at the conserved splice donor and acceptor sites, changes in the surrounding splicing signals and branch point, and variants that create or activate cryptic splice sites. It addresses the consequences for the mature transcript and the challenges of predicting and confirming splicing effects. It is a reference entry on the mechanism, not clinical management guidance.

Key concepts

Splice donor (5') and acceptor (3') sites
Canonical GT-AG dinucleotides
Branch point and polypyrimidine tract
Exon skipping
Intron retention
Cryptic splice-site activation
Splicing enhancers and silencers
RNA-level confirmation of splicing effects

Mechanisms

Splicing removes introns and ligates exons under the direction of the spliceosome, guided by conserved sequence signals: the 5' donor site (usually beginning GT), the 3' acceptor site (usually ending AG), the branch point, and the polypyrimidine tract, plus auxiliary enhancer and silencer elements. A variant in these signals can abolish recognition of a site, causing the spliceosome to skip an exon or retain an intron, or can create or strengthen a cryptic site that is used in place of the normal one. The resulting transcript may carry an altered reading frame and a premature stop, may delete or add residues in frame, or may be degraded; the precise outcome often differs from naive prediction, so splicing effects are frequently confirmed at the RNA level (Scotti & Swanson, 2015; Sibley et al., 2016).

Clinical relevance

Splicing variants are an important and sometimes under-recognised cause of genetic disease, including changes outside the canonical dinucleotides and deep within introns that are easy to miss with coding-focused analysis. Because in silico prediction of splicing impact is imperfect, functional assessment of the transcript is often used to clarify significance. This topic describes how such variants act and are evaluated and is not a basis for individual diagnostic or treatment decisions.

Evidence & guidelines

The ACMG/AMP framework (Richards et al., 2015) addresses how predicted and demonstrated splicing effects contribute to variant classification, and HGVS nomenclature (den Dunnen et al., 2016) provides conventions for describing variants at the DNA and, where established, RNA level.

Debates

How reliably can splicing impact be predicted from sequence?: Variants at the canonical splice dinucleotides are usually disruptive, but the effect of changes in the wider splicing signals, deep-intronic regions, and potential cryptic sites is harder to predict; functional RNA studies are often needed to confirm whether and how splicing is altered.

Seminal works

scotti-2015
sibley-2016

Frequently asked questions

Can a variant affect splicing without changing the protein-coding sequence?: Yes. Variants in introns, at splice-site signals, or in regulatory elements can alter how exons are joined and therefore change the mature transcript and protein, even when they lie outside the coding sequence or appear synonymous.
Why is RNA testing sometimes done for splice-site variants?: Because computational tools predict splicing impact imperfectly, examining the actual transcript (for example by RNA sequencing or reverse-transcription assays) can confirm whether an exon is skipped, an intron retained, or a cryptic site used.