Cryptic exon splicing is a pathological RNA processing event in which normally repressed "cryptic" exonic sequences are aberrantly included in mature mRNA transcripts. In the context of neurodegeneration, cryptic exon inclusion has emerged as a central downstream consequence of [TDP-43](/proteins/tdp-43-protein) nuclear depletion — the defining pathological feature of [ALS](/diseases/amyotrophic-lateral-sclerosis) and the majority of [FTD](/diseases/behavioral-variant-ftd) cases. When [TDP-43](/mechanisms/tdp-43-proteinopathy) is lost from the nucleus, it can no longer suppress the inclusion of cryptic exons in its target pre-mRNAs, leading to aberrant protein products, nonsense-mediated mRNA decay, and loss of essential proteins.
This mechanism has transformed our understanding of how TDP-43 proteinopathy causes neurodegeneration. Rather than a simple toxic gain-of-function from cytoplasmic TDP-43 aggregates, the loss of TDP-43's nuclear splicing function — and the resulting cryptic exon inclusion — appears to be a primary driver of neuronal dysfunction and death. [@snyder2024]
What Are Cryptic Exons?
Cryptic exons are intronic sequences that contain potential splice sites but are not normally recognized by the spliceosome. They are "cryptic" because they are hidden from the splicing machinery under normal conditions. Several features distinguish cryptic exons: [@baghel2024]
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Cryptic Exon Splicing in TDP-43 Proteinopathy
Overview
Cryptic exon splicing is a pathological RNA processing event in which normally repressed "cryptic" exonic sequences are aberrantly included in mature mRNA transcripts. In the context of neurodegeneration, cryptic exon inclusion has emerged as a central downstream consequence of [TDP-43](/proteins/tdp-43-protein) nuclear depletion — the defining pathological feature of [ALS](/diseases/amyotrophic-lateral-sclerosis) and the majority of [FTD](/diseases/behavioral-variant-ftd) cases. When [TDP-43](/mechanisms/tdp-43-proteinopathy) is lost from the nucleus, it can no longer suppress the inclusion of cryptic exons in its target pre-mRNAs, leading to aberrant protein products, nonsense-mediated mRNA decay, and loss of essential proteins.
This mechanism has transformed our understanding of how TDP-43 proteinopathy causes neurodegeneration. Rather than a simple toxic gain-of-function from cytoplasmic TDP-43 aggregates, the loss of TDP-43's nuclear splicing function — and the resulting cryptic exon inclusion — appears to be a primary driver of neuronal dysfunction and death. [@snyder2024]
What Are Cryptic Exons?
Cryptic exons are intronic sequences that contain potential splice sites but are not normally recognized by the spliceosome. They are "cryptic" because they are hidden from the splicing machinery under normal conditions. Several features distinguish cryptic exons: [@baghel2024]
Weak splice sites: Cryptic exons have suboptimal splice donor and acceptor sequences that are normally insufficient for recognition
Repressor-dependent silencing: They are kept silent by RNA-binding proteins (especially TDP-43) that bind nearby to block splice site recognition
Evolutionary non-conservation: Most TDP-43-dependent cryptic exons are not conserved across species, suggesting they arose from random intronic mutations
Pathological consequences: When included, cryptic exons typically introduce premature stop codons, causing nonsense-mediated decay (NMD) or truncated, non-functional proteins
TDP-43 as a Cryptic Exon Repressor
Normal Function
[TDP-43](/proteins/tdp-43-protein) (TAR DNA-binding protein 43) is a nuclear RNA-binding protein that regulates RNA splicing, stability, and transport. Its role in cryptic exon repression involves: [@udine2024]
UG-repeat binding: TDP-43 recognizes UG-rich sequences in introns flanking cryptic exons through its two RNA recognition motifs (RRM1 and RRM2)
Spliceosome blocking: By binding to these UG-rich regions, TDP-43 sterically blocks the spliceosome from recognizing the cryptic splice sites
Co-factor recruitment: TDP-43 recruits other splicing factors (hnRNPs, SR proteins) to reinforce cryptic exon skipping
Genome-wide repression: TDP-43 represses hundreds of cryptic exons across the transcriptome
Loss-of-Function in Disease
Mermaid diagram (expand to render)
When TDP-43 mislocalizes to the cytoplasm (as occurs in ALS/FTD): [@cheng2024]
Nuclear TDP-43 levels fall below the threshold needed for cryptic exon repression
Cryptic exons are included in hundreds of target transcripts
Most inclusions introduce premature stop codons -> nonsense-mediated decay -> protein loss
Some inclusions produce aberrant proteins with novel, potentially toxic sequences
Key Cryptic Exon Targets
STMN2 (Stathmin-2)
The most clinically significant TDP-43-dependent cryptic exon target is STMN2[@snyder2024].
Normal function: [STMN2](/genes/stmn2) (also called SCG10) is essential for axonal regeneration and maintenance in motor [neurons](/entities/neurons)
Cryptic exon: TDP-43 loss leads to inclusion of a cryptic exon in STMN2 intron 1, introducing a premature polyadenylation signal
Consequence: Truncated STMN2 mRNA that produces no functional protein
Disease relevance: STMN2 protein is dramatically reduced in ALS/FTD patient motor neurons
Therapeutic target: Restoring STMN2 expression (via ASOs that block the cryptic exon) is being pursued as a therapy for ALS
Biomarker potential: Truncated STMN2 transcripts detectable in patient-derived neurons
UNC13A
A critical genetic modifier of ALS/FTD disease course[@cheng2024].
Normal function: [UNC13A](/genes/unc13a) is essential for synaptic vesicle release and neurotransmission
Cryptic exon: TDP-43 loss causes inclusion of a cryptic exon in UNC13A intron 20-21
Genetic link: Common SNPs (rs12608932, rs12973192) in the UNC13A cryptic exon region are among the strongest ALS risk modifiers in GWAS
Mechanism: These SNPs create stronger cryptic splice sites, making UNC13A more sensitive to TDP-43 loss
Consequence: Loss of UNC13A protein impairs synaptic function
Clinical impact: UNC13A risk variants associate with shorter survival in ALS patients
KALRN (Kalirin)
Rho-GEF protein critical for dendritic spine morphology and synaptic plasticity
TDP-43-dependent cryptic exon in KALRN leads to loss of functional protein
May contribute to cognitive/synaptic dysfunction in FTD
AGRN (Agrin)
Essential for neuromuscular junction formation and maintenance
Cryptic exon inclusion disrupts agrin protein production
May contribute to NMJ dysfunction in ALS
SYF2
Splicing factor involved in pre-mRNA processing
Loss amplifies splicing defects in a feed-forward manner
Creates a cascade of splicing disruption
Species-Specific Cryptic Exons
A remarkable feature of TDP-43-dependent cryptic exons is their lack of evolutionary conservation[@zeng2024].
Mouse and human TDP-43 perform the same function (cryptic exon repression)
But the specific cryptic exons are different between species
The STMN2 cryptic exon exists in humans but NOT in mice
This means mouse models of TDP-43 pathology do not recapitulate the loss of STMN2
Humanized mouse models (carrying the human STMN2 locus) have been developed to address this
This species specificity has major implications for preclinical drug development
Detection Methods
RNA Sequencing
Standard RNA-seq can detect cryptic exon inclusion by identifying novel splice junctions
Quantitative RT-PCR measures the ratio of cryptic vs. normal transcripts
Can be applied to patient-derived iPSC neurons and post-mortem tissue
Immunohistochemistry
Antibodies against cryptic exon-encoded peptide sequences (e.g., truncated STMN2)
Enables spatial mapping of cryptic exon inclusion in tissue sections
Confirms correlation between TDP-43 mislocalization and cryptic exon inclusion at single-cell level
Therapeutic Strategies
Antisense Oligonucleotides (ASOs)
ASOs that block cryptic exon inclusion are a leading therapeutic approach[@cheng2024].
STMN2-targeted ASOs: Designed to bind the cryptic exon splice site in STMN2, preventing its inclusion and restoring normal STMN2 protein expression
UNC13A-targeted ASOs: Block the cryptic exon in UNC13A to maintain synaptic function
Splice-switching ASOs do not degrade the target mRNA — they redirect splicing
Multiple ASO candidates in preclinical development for ALS
TDP-43 Nuclear Restoration
Rather than targeting individual cryptic exons, restoring nuclear TDP-43 could correct all downstream splicing defects[@zeng2024].
Nuclear import enhancers
Reducing cytoplasmic TDP-43 aggregation
[Chaperone-based therapies](/proteins/hsp70-protein) to maintain TDP-43 solubility
Challenge: once TDP-43 forms cytoplasmic aggregates, nuclear restoration may be difficult
Gene Therapy
AAV-mediated delivery of STMN2 to motor neurons could bypass the cryptic exon problem
CRISPR-based editing to strengthen the normal splice sites and weaken cryptic splice sites
Challenges include delivery to motor neurons and long-term expression
Cryptic Exons in Other Proteinopathies
While TDP-43-dependent cryptic exons are best characterized, analogous mechanisms may operate in other diseases:
FUS-Dependent Cryptic Exons
[FUS](/genes/fus) also functions as a splicing regulator, and FUS depletion leads to its own set of cryptic exon inclusions, relevant to [FUS-ALS](/mechanisms/als-fus-pathway)
hnRNP-Dependent Cryptic Exons
[hnRNPA1](/genes/hnrnpa1) and [hnRNPA2B1](/genes/hnrnpa2b1) mutations cause multisystem proteinopathy; loss of these proteins may cause cryptic exon inclusion in their target transcripts
PTBP1/PTBP2 and Neuronal Splicing
Polypyrimidine tract binding proteins regulate neuronal-specific splicing programs. Their dysfunction could contribute to cryptic exon events in neurodegeneration
Clinical and Diagnostic Implications
Biomarker development: Detection of truncated STMN2 or UNC13A transcripts as biomarkers for TDP-43 nuclear depletion
Patient stratification: Genotyping UNC13A risk SNPs to identify patients who may benefit most from UNC13A-targeted therapies
Disease staging: Measuring the extent of cryptic exon inclusion could indicate disease progression
Companion diagnostics: Monitoring cryptic exon levels during ASO clinical trials as a pharmacodynamic readout
Research Questions
How many cryptic exons does TDP-43 repress across the entire transcriptome, and which are the most pathologically consequential?
Can targeting just 2-3 key cryptic exons (STMN2, UNC13A) provide meaningful therapeutic benefit, or must nuclear TDP-43 function be globally restored?
How do cell type-specific transcriptomic differences affect cryptic exon vulnerability?
What is the temporal sequence — does cryptic exon inclusion begin before or after clinical symptom onset?
Can cryptic exon-derived peptides serve as neoantigens recognized by the immune system, contributing to [neuroinflammation](/mechanisms/neuroinflammation-pathway)?