splice site mutation

**Normal splicing background:** Pre-mRNA introns are removed by the spliceosome (snRNPs: U1, U2, U4, U5, U6), which recognizes: (1) 5' splice site (donor) with conserved GU at the start of the intron; (2) 3' splice site (acceptor) with conserved AG at the end of the intron; (3) branch point adenine (A) located upstream of the 3' site, with a polypyrimidine tract. Mechanism: the branch point A attacks the 5' GU, forming a lariat intermediate; the 3' end of the upstream exon then joins the downstream exon. Introns are removed, exons are ligated. Anti-Smith (anti-snRNP) and anti-U1 RNP antibodies target spliceosome components (SLE, mixed connective tissue disease).

**Pathophysiology of mutations:** Point mutations, small deletions, or insertions at the invariant GU/AG dinucleotides, the branch point, or exonic/intronic splicing enhancers and silencers can cause: - Exon skipping (the most common outcome) - Intron retention - Activation of a cryptic splice site (either within an exon or intron), which causes partial exon loss or partial intron inclusion - Frameshift and/or premature stop codon, which can trigger nonsense-mediated decay (NMD) of the mRNA and lead to loss of function - In-frame deletions or insertions, which may yield a truncated or dysfunctional protein

Because mutations can be silent at the protein level yet still affect splicing, a synonymous or "silent" exonic mutation near a splice junction can still be pathogenic.

**Clinical associations (high-yield):** - **Beta-thalassemia:** the classic board example. Splice site mutations at the β-globin gene cause β0 (no protein, e.g., mutations at the GT/AG consensus) or β+ (reduced protein, e.g., cryptic splice site activation within intron 1 or 2). This leads to decreased β-chain synthesis, excess α-chains precipitating in RBCs, ineffective erythropoiesis, microcytic hypochromic anemia, target cells, and HbA2 and HbF elevation. Remember that the "β-thal" mutations are often at promoter or splicing sites. - **Duchenne/Becker muscular dystrophy (dystrophin):** splice mutations can disrupt the reading frame (Duchenne) or preserve it (Becker). Exon-skipping antisense oligonucleotides (eteplirsen, golodirsen) restore the reading frame and give a milder Becker-like phenotype. - **Spinal muscular atrophy:** SMN2 has a C→T change in exon 7 that causes exon 7 skipping, so most SMN2 protein is nonfunctional. Nusinersen (an antisense oligonucleotide) and risdiplam (an oral splicing modifier) promote exon 7 inclusion. - **Cystic fibrosis (CFTR):** some splicing mutations (e.g., 3849+10kb C→T) cause a milder disease. - **Others:** Tay-Sachs (HEXA, e.g., in Ashkenazi Jews, a 4-bp insertion plus a splice mutation), familial dysautonomia (IKBKAP/ELP1 intron 20 mutation causing exon 20 skipping), neurofibromatosis type 1, BRCA1/BRCA2 (hereditary breast and ovarian cancer), and Marfan syndrome and others.

**Key distinctions for boards:** - Splice site mutations are a subtype of point mutation, but their effect on the protein is variable and typically more severe than a missense change. - Not the same as a nonsense mutation, which changes a codon directly to a stop codon, although splice mutations may generate a premature stop indirectly by frameshift. - Detecting these needs mRNA/cDNA analysis or splicing prediction software, since exon-only sequencing may miss deep intronic variants. - Because introns are not translated, mutations deep in an intron are usually benign, but mutations at the conserved splice sequences are pathogenic.

**Therapeutic angle:** Antisense oligonucleotides (nusinersen, eteplirsen) and small-molecule splicing modifiers (risdiplam) can correct or modulate splicing, demonstrating the clinical importance of this mechanism.