Molecular Neurodegeneration(2023) 18:16
Mehta et al. Molecular Neurodegeneration
https://doi.org/10.1186/s13024-023-00608-5

Open Access

REVIEW

The era of cryptic exons: implications
for ALS‑FTD
Puja R. Mehta1, Anna‑Leigh Brown1, Michael E. Ward2 and Pietro Fratta1*   

Abstract
TDP-43 is an RNA-binding protein with a crucial nuclear role in splicing, and mislocalises from the nucleus to the cyto‑
plasm in a range of neurodegenerative disorders. TDP-43 proteinopathy spans a spectrum of incurable, heterogene‑ous, and increasingly prevalent neurodegenerative diseases, including the amyotrophic lateral sclerosis and fronto‑
temporal dementia disease spectrum and a significant fraction of Alzheimer’s disease. There are currently no directeddisease-modifying therapies for TDP-43 proteinopathies, and no way to distinguish who is affected before death. It
is now clear that TDP-43 proteinopathy leads to a number of molecular changes, including the de-repression andinclusion of cryptic exons. Importantly, some of these cryptic exons lead to the loss of crucial neuronal proteins andhave been shown to be key pathogenic players in disease pathogenesis (e.g., STMN2), as well as being able to modify
disease progression (e.g., UNC13A). Thus, these aberrant splicing events make promising novel therapeutic targets torestore functional gene expression. Moreover, presence of these cryptic exons is highly specific to patients and areasof the brain affected by TDP-43 proteinopathy, offering the potential to develop biomarkers for early detection andstratification of patients. In summary, the discovery of cryptic exons gives hope for novel diagnostics and therapeutics
on the horizon for TDP-43 proteinopathies.
Keywords Motor neuron disease, Amyotrophic lateral sclerosis, Frontotemporal dementia, TDP-43 proteinopathies,
Cryptic exons, Splicing, Biomarkers, Therapeutics, UNC13A, STMN2
Background‘TDP-43 proteinopathy’ refers to human diseases pathologically defined by cellular nuclear-to-cytoplasmic
mislocalisation and aggregation of TAR DNA-binding
protein-43 (TDP-43), an RNA-binding protein with acrucial role in RNA metabolism and splicing. TDP-43
proteinopathy encompasses a wide spectrum of progressive neurodegenerative diseases and phenotypes. It is a
key feature in ~ 97% of people with amyotrophic lateral*Correspondence:
Pietro Frattap.fratta@ucl.ac.uk
1
Department of Neuromuscular Diseases, UCL Queen Square Instituteof Neurology, UCL Queen Square Motor Neuron Disease Centre,
London WC1N 3BG, UK
2National Institute of Neurological Disorders and Stroke, NIH, Bethesda,
MD, USAsclerosis (ALS) and ~ 45% of people with frontotemporaldementia (FTD), a seminal discovery made in 2006 [1,2]. Subsequently, TDP-43 proteinopathy has been foundas a co-pathology in a large fraction of Alzheimer’s disease, as well as inclusion body myositis and Paget diseaseof bone [3–5]. Furthermore, TDP-43 mislocalisation canbe trigged by traumatic stimuli, such as physical stressor brain injury, which are themselves increasingly recognised as risk factors for a number of the above describedneurodegenerative disorders [6–9].
Common to TDP-43 proteinopathies is a lack of
directed disease-modifying therapies, and an inability toconfirm and stratify individuals that have TDP-43 pathology ante-mortem. The disease that is most frequently
characterised by TDP-43 mislocalisation is ALS, which
is a rapidly progressive paralysing illness with eventual
death on average 3 years from symptom onset [10]. Up
to 15% of people with ALS can experience associated© The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/. The Creative Commons Public Domain Dedication waiver (http://​creat​iveco​
mmons.​org/​publi​cdoma​in/​zero/1.​0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.

Mehta et al. Molecular Neurodegeneration(2023) 18:16

cognitive, language, and behavioural deficits, consistent
with a diagnosis of FTD, comprising the ALS-FTD spectrum [11]. Pathogenic mutations in TARDBP, the genewhich encodes TDP-43, are rare, occurring in < 1% of
ALS cases, but underscore TDP-43’s central role in disease pathogenesis [12–14]. Altered subcellular localisation of TDP-43 is relevant – either via cytoplasmic toxic
gain of function, nuclear loss of function, or both. However, the precise molecular mechanisms downstream of
TDP-43 mislocalisation remain incompletely understood.
In this review, we will focus on a key ‘on-off ’ consequence of nuclear TDP-43 loss of function, which has
emerged over recent years – the occurrence of splicing
defects leading to the erroneous inclusion of intronic
sequences into mature mRNA, forming so-called ‘cryptic
exons’ [15]. With a spotlight on ALS-FTD, we will review
their relevance to disease pathogenesis, and discuss
exciting and emerging opportunities on the horizon forbiomarker development and therapeutics in the field of
TDP-43 proteinopathies (Fig. 1).

Page 2 of 9

TDP‑43 depletion leads to de‑repression and inclusionof cryptic exons

TDP-43 was identified in 1995 as a suppressor of HIVgene expression. It was later discovered that TDP-43
expression promotes skipping of exon 9 in the CFTR
gene, first realising TDP-43’s role as an RNA-binding
protein and regulator of alternative splicing [16]. Since
then, an increasing number of genes have been identified for which the splicing of annotated, conservedexons is regulated by TDP-43; as such, altered splicing in
POLDIP3, SORT1, and PFKP have all been used as reliable readouts for nuclear TDP-43 function in models
with TDP-43 knockout, overexpression, and for studying
TDP-43 ALS-causing mutations [17–27].
TDP-43’s consensus binding motif comprises “UG”rich dinucleotide repeats. High-throughput sequencing, combined with cross-linking immunoprecipitation
experiments to discover direct TDP-43 mRNA targets,revealed that the majority of TDP-43’s binding sites in
pre-RNAs were in introns [26, 27]. A number of these
intronic binding sites were later revealed to be sites

Fig. 1 Schematic summarising how cryptic exons (CEs) arise in the context of TDP-43 mislocalisation from the nucleus to the cytoplasm (owing
to de-repression of splicing in intronic regions), possible downstream consequences (loss of functional protein, nonsense-mediated decay (NMD)of the cryptic-containing transcript, or translation of a novel cryptic peptide), and opportunities that CEs provide for better understanding diseasemechanisms (STMN2, UNC13A, other genes yet to be explored), biomarker development (RNA and protein biomarkers), and therapeutics (viarestoration of protein levels, or splicing modification)

Mehta et al. Molecular Neurodegeneration(2023) 18:16

where TDP-43 binding acts as a splicing repressor.
When, as a result of TDP-43 depletion, non-conserved,
intronic sequences are erroneously included in mature
RNA, they are called cryptic exons [15]. Importantly,
these cryptic exons can be found in patient tissues frompeople affected by ALS and FTD, Alzheimer’s disease,and inclusion body myositis [15, 23, 28–34]. While TDP43 regulation of annotated splicing leads to shifts in theratio of known isoforms, TDP-43 cryptic splicing produces novel RNA isoforms. These changes include novel
cassette exons, skipping of canonical splicing products, 3′or 5′ extensions of annotated exons, novel transcription
start sites, and novel poly-adenylation events [15, 35].
What are the possible consequences that result fromthe insertion of novel non-conserved sequences inmRNA (Fig. 1)? Most cryptic exons are predicted to leadto destabilisation and degradation of mRNA, thereby
resulting in a reduction in functional levels of the corresponding protein. This occurs primarily throughnonsense-mediated decay (NMD) due to introductionof frameshifts and premature termination codons [32,36]. Conceptually, such cryptic exons could lead to deleterious downstream physiological consequences if they
reside in transcripts encoding critical proteins. Occasionally, cryptic exons lead to an in-frame change to the
nucleotide sequence with no premature stop codons,
thereby escaping NMD and resulting in a predictedtranslation of novel cryptic peptides. Alternatively, they
can result in premature polyadenylation, potentially leading to truncated proteins. It is conceivable that these proteins resulting from mis-spliced RNA could acquire gainof function changes in their biology, be toxic, or serve as
detectable biomarkers for monitoring of disease activity,or for stratification of subtypes in ALS-FTD and otherneurodegenerative disorders.
Relevance to disease
Impact on neuronal health – STMN2

In 2019, a breakthrough discovery from two independentgroups found that TDP-43 loss of function leads to theinclusion of a cryptic exon within the STMN2 gene [23,30]. This gene encodes for stathmin-2, a microtubuleassociated protein needed for axonal growth and repair
in neurons. Occurring after exon 1 and containing a stopcodon and polyadenylation site, the cryptic exon leads topremature transcript termination and predicted formation of a truncated non-functional protein. Using in vitro
neuronal models of TDP-43 depletion via knockdown,
induction of TDP-43 mislocalisation, and pathological loss of function TDP-43 mutations known to cause
ALS, the research groups demonstrated a resultant dramatic reduction in levels of mature STMN2 mRNA andfunctional protein. Crucially, the STMN2 cryptic exon

Page 3 of 9

was confirmed to be detectable also in ALS-FTD brains
and spinal cords where TDP-43 pathology is present andspecifically in patient post-mortem neurons with TDP43 pathology isolated by laser capture or nuclear sorting[30–32, 37]. Without TDP-43, motor neuron axons wereunable to regenerate after an axotomy. Remarkably, they
were able to rescue axonal regrowth by restoring levelsof STMN2 protein, thereby demonstrating that STMN2
is critical to the health of motor neurons, and that its
TDP-43 dependent loss can be a key pathogenic player in
ALS. Following this pivotal discovery, the hunt for othercryptic exons that might contribute to or exacerbate ALS
pathology continued.
Impact on ALS disease progression – UNC13A

UNC13A is one of the top genetic risk factors for ALS,
first reported in a genome-wide association study in2009 [38]. UNC13A protein plays an important role inneurotransmitter release and synaptic transmission, andmice that lack this gene (called Munc13–1 in mice) havesynaptic impairments and die soon after birth [39]. In
humans, rare cases with homozygous nonsense mutations in UNC13A result in fatal microcephaly, cortical
hyperexcitability, and myasthenia [40]. Single nuclear
polymorphisms (SNPs) in UNC13A have been shown toincrease risk of ALS and also shorten survival in patients.
However, the underlying mechanism by which these
UNC13A variants increase ALS risk was not known until
earlier this year when, along with the Gitler and Petrucelli groups, we independently found that TDP-43 depletion resulted in inclusion of a cryptic exon between exons
20 and 21 of UNC13A, leading to reduced expression.
Similarly to the STMN2 cryptic exon, this event can also
be detected in post-mortem brain and spinal cord tissue
from patients with ALS-FTD, exclusively where TDP-43
pathology is present, but not in healthy controls.
Intriguingly, two of the associated UNC13A SNPs,
rs12608932 (A > C) and rs12973192 (C > G), were foundto be located within the intron containing the cryptic
exon, with the former SNP being located within the cryptic exon region itself. Especially given the large size of the
UNC13A gene (~ 87 kb), the proximity of the SNPs to thecryptic exon led us to hypothesise a direct link between
the SNPs and UNC13A cryptic splicing. Given that thecryptic exon is not present in RNA-sequencing data fromhealthy controls, even when homozygous for the risk
SNPs (approximately 10% of Caucasians are homozygous for both risk SNPs), they seem not to be sufficient
to cause cryptic exon inclusion on their own. Thus, it was
hypothesised that rather than increasing ALS or FTD risk
directly, the SNPs act by potentiating the expression of acryptic exon which is dependent on TDP-43 depletion.

Mehta et al. Molecular Neurodegeneration(2023) 18:16

Indeed, the risk SNPs enhance cryptic splicing and alter
direct TDP-43 binding, thereby reducing its ability tophysiologically repress cryptic exon inclusion. In keepingwith this, patients homozygous for either SNP had more
UNC13A cryptic exon-containing transcripts in a doseresponsive manner compared to heterozygotes and noncarriers. This discovery provides a biological explanation
for how presence of the UNC13A risk SNPs in ALS-FTD
patients with TDP-43 pathology modifies disease outcomes, negatively affecting survival from symptom onset[32, 33, 41–47]. Further work needs to be done to understand the biology connecting the above molecular eventsand disease progression, by specifically investigating thefunctional consequences of synaptic dysfunction associated with TDP-43 and UNC13A loss.
Future directions
Opportunities for novel biomarkers

Biomarkers for early diagnosis, disease stratification ofsubtypes and clinical stages, prognostication, and monitoring treatment response are vitally important, bothfor clinical trials and patient care. Fluid biomarkers ofgeneral neurodegeneration exist, most notably serum
neurofilament light chain, cerebrospinal fluid (CSF)
phosphorylated neurofilament heavy chain, and urinary­p75ECD [48–50]. However, biomarkers specific for ALSFTD and other TDP-43 proteinopathies are not currently
available, nor are there readouts of TDP-43 function inliving patients.
Unlike canonical TDP-43 regulated splicing, the ‘onoff ’ expression of cryptic exons only under TDP-43
nuclear loss of function makes them potential potent
biomarker candidates for TDP-43 pathology. This proof
of concept was first shown with the STMN2 cryptic exon
in post-mortem frontal cortex samples, in which its presence was able to discriminate between patients withFTD-TDP, progressive supranuclear palsy, and healthy
control participants [31].
A key future direction will be to exploit this characteristic to develop methods that detect TDP-43 pathology
ante-mortem in patient biofluids, such as in blood, CSF,
urine, or tissue. Assays could involve either direct RNA
detection of the cryptic exon itself, or detection of novel
cryptic peptides for transcripts that escape NMD. An
ideal biomarker would have a reproducibly high and linear correlation with disease activity, high sensitivity andspecificity, stability throughout the day, and easy accessto obtain a sample [51].
With regards to biomarker detection of RNA, methods
have been developed that can sensitively detect circulating RNA in the blood of individuals with Alzheimer’s
disease that reflect disease-specific neural transcriptomechanges [52]. However, there are challenges. For example,

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most cryptic exon-containing transcripts, such as for
UNC13A, undergo NMD and are therefore not stable.
Moreover, if transcripts are at low levels, highly sensitive
tools would be required. Detection of a panel of relevant
cryptic exons deemed to be pathogenic players could theoretically increase sensitivity.
For cryptic exons that produce cryptic peptides, specific antibodies could be designed to detect these via
immunohistochemistry, or ELISA and SIMOA-based
assays. This has its own challenges, whether it be due to
NMD of cryptic-containing transcripts, low expression,or instability of the peptides. For STMN2, even though
the cryptic-containing transcript is highly expressedand escapes NMD, attempts to detect the putative 16
amino acid cryptic peptide have not yet been successful. However, antibody generation to specifically detect
pathological proteins is evolving, including in the field of
ALS-FTD, with antibodies now available to detect dipeptide repeats that occur in the C9orf72 subtype of ALSFTD. These have not only furthered our understanding of
disease mechanisms, but are also being utilised to assesstherapies [53–56].
Excitingly, two new pre-print publications this year
have independently reported the detection of cryptic
peptides in the CSF of patients with ALS [57, 58]. Irwin
et al. demonstrated that a newly characterised monoclonal antibody, specific to a TDP-43-dependent cryptic
epitope encoded by the cryptic exon found in HDGFL2,
detects the cryptic peptide in C9orf72-associated ALS.
Strikingly, this includes pre-symptomatic mutation carriers [57]. In Seddighi et al., our groups first demonstrated
the presence of de novo cryptic peptides in iPSC-derived
neurons with TDP-43 knockdown, and then used a novel
targeted proteomics assay to confirm the presence of
cryptic peptides in CSF of patients with ALS-FTD [58].
Further work will be needed to assess the specificity andsensitivity of cryptic peptides as biomarkers, but, taken
together, these discoveries are encouraging steps towards
facilitating earlier diagnosis of ALS, and also providing a
way of measuring target engagement in clinical trials for
new therapies aimed at restoring TDP-43 function.
Given the cell-type specificity of TDP-43 controlled
cryptic exons, it is also possible that biomarkers towards
cryptic exons could be designed to assess the flow of
TDP-43 proteinopathy across the nervous tissue [25,35, 59, 60]. While TDP-43 proteinopathy in cortical andmotor neurons has received much attention, TDP-43
proteinopathy is also present in glia and Schwann cells[2, 60–63]. Indeed, TDP-43 depletion in mouse Schwann
cells showed that TDP-43 regulates the inclusion of acell-type specific cryptic exon in Neurofascin in thesecells, highlighting how, if certain TDP-43 cryptic exons
are expressed only by given cells, they could allow tracingMehta et al. Molecular Neurodegeneration(2023) 18:16

of TDP-43 proteinopathy. Furthermore, recent work
showing that pTDP-43 and loss of nuclear TDP-43 can bedetected years before ALS disease presentation in central
nervous system and non-central nervous system tissue
suggests that detection of TDP-43 cryptic exons could
possibly pre-date disease onset [64, 65].
Another interesting avenue is the use of model systems
to determine the timing of the emergence of cryptic exon
inclusion events. For instance, studies assessing the presence of cryptic exons at different levels of TDP-43 knockdown in vitro, or in patients with or post-mortem tissue
from varying clinical stages of ALS-FTD, could allow
curation of a database reflecting which cryptic exons
appear early and late in disease. Further analysis of this
could provide a greater mechanistic understanding of
cryptic exon biology and what factors make certain genes
more sensitive and susceptible to the inclusion of cryptic exons with TDP-43 loss. Such a stratification may also
have implications for biomarker studies and early diagnosis and stratification of people with ALS-FTD.
Further work is necessary to utilise the specificity ofcryptic exons to develop appropriate biomarkers, which
could be transformative to the field of ALS-FTD andother TDP-43 proteinopathies. Particularly with newtherapies on the horizon for neurodegenerative diseases,
early diagnosis of individuals who are or will become
affected and a readout for treatment response will be
critical in halting the disease process at an early enoughstage to pre-empt the point of no return in the ALS disease cascade [66].
Opportunities for therapeutics

The discovery that certain cryptic exons, and potentiallymore that remain to be explored, underpin the disease
process in ALS – either by impairing neuronal health,increasing ALS risk, or worsening disease progression –makes them exciting novel therapeutic targets. If early
diagnosis can be achieved, the hope would be to restore
expression of critical proteins to normal functional levels, or use splicing therapies to modulate and prevent
aberrant splicing from occurring altogether in TDP-43
proteinopathies.
The STMN2 cryptic exon leads to a reduction in
STMN2 expression. Thus, one strategy would be torestore normal STMN2 levels using a viral vector containing the correct DNA sequence, such as via adenoassociated viral (AAV) or lentiviral delivery. This proof
of concept was shown when transduction of TDP-43
depleted iPSC-derived motor neurons with lentivirus carrying STMN2 restored STMN2 levels and also,
importantly, restored axonal regeneration after axotomy [30]. It was also recently shown in mice deficient of
Stmn2 (Stmn2−/−) that the introduction of the humanPage 5 of 9

STMN2 gene was able to rescue motor deficits [67].
However, translating this method to humans poses challenges, including discerning what the target level should
be and at what point to treat in order to avoid missing
the effective therapeutic window but also avoid overexpression and potential gain of function toxicity, as wasobserved in spinal muscular atrophy (SMA) studies as aresult of SMN overexpression [68]. Moreover, it would
be important that such therapies are directed to the correct anatomical site with appropriate tissue and cell-type
specificity to prevent off-target effects, and also that any
harmful immunogenic responses to the viral vector itself
are avoided [69]. Another obstacle would be the limitation in packaging capacity of the viral vector and therefore the size of the gene needing to be restored. As such,
UNC13A would be too large to be introduced via thismethodology, as opposed to STMN2, which would theoretically be more amenable.
Other important gene-therapy strategies involve theprevention of erroneous RNA splicing. Compared to theabove described gene delivery methods, these approaches
aim to correct endogenous splicing and therefore avoid
problems deriving from overexpression and toxicity. One
method is via antisense oligonucleotide therapies (ASOs),
which are short synthetic DNA sequences that aredesigned to selectively bind to pre-mRNA in cells. ASOs
have come to the forefront as a new tool for numerous
neurodegenerative diseases, including toxic gain of function gene-specific ALS subtypes, where they have beenused to degrade mutant transcripts. ASOs designed toreduce protein produced from mutant SOD1 and C9orf72
have been tested in ALS clinical trials (BIIB067 and
BIIB078, respectively). The former showed very encouraging results, but failed to reach significance in achieving
primary clinical endpoints in a phase 3 trial, and the latter phase 1 trial showed no difference from placebo forclinical endpoints and in fact trended towards greater
decline at the higher dose. These results highlight someof the caveats of ASO therapies [70, 71]. ASOs can also
be used to directly introduce splicing modulation, andthis has been used to improve functional splicing in disorders, such as SMA, or to limit the effect of deleterious
mutations by skipping specific exons, such as in Duchenne muscular dystrophy (DMD) [72–75]. One drawbackof ASO therapies is the need for repeated dosing for amaintained response, which is particularly undesirable
for patients if the delivery method is invasive, such as via
intrathecal administration.
Other effective splice switching strategies include systems that target pre-mRNA, but are stably expressed in
the long-term, thereby preventing the need for repeateddosages, whilst also aiming to circumvent the above
concerns regarding overexpression. These include

Mehta et al. Molecular Neurodegeneration(2023) 18:16

CRISPR-Cas ribonuclease programming, such as via
RfxCas13d (also named CasRx, from Ruminococcus flavefaciens), and using modified uridine-rich small nuclear
RNA (snRNA) gene therapy, such as via U7 snRNA delivery, which has shown promise in DMD, and has enteredits first clinical trial [76–78]. In addition to the above
benefits, both methods involve relatively small-sized
effectors, such that they can be packaged into a viral vector, and theoretically may even allow for multiple cryptic
exons to be targeted in parallel.
Finally, small molecule delivery has been developed asa further tool to alter splicing. A clear benefit of this therapeutic platform is the ability to deliver the treatment topatients orally. The first small molecule splicing modifier
to be approved was Risdiplam for oral delivery in SMA. It
was shown to increase SMN protein in the central nervous system and peripheral tissues in mice, and to restore
functional SMN protein in patients with SMA [79–82].
Homing in on correcting cryptic exon-related missplicing, a phase I clinical trial is planned to assess an
ASO designed to restore STMN2 levels [83]. Commonlyin the field of therapeutics, mouse models are used prior
to testing in humans; however, human cryptic exons havebeen found to share very little overlap with mouse cryptic exons, making this a problem [15]. The cryptic exonsin both STMN2 and UNC13A that have been reported inhumans are not present in mice. In fact, and interestingly,
different UNC13A cryptic exons are present in mice [15].
Therefore, in vivo models are needed to test splicing therapies that target specific cryptic exons. One possibility is
the development of humanised mouse models containing the relevant human cryptic exons [84, 85]. Anotherissue arising from the use of human iPSC models alone
to study therapeutics in the field of cryptic exons is thatcryptic exon signatures in TDP-43 depletion have beenshown to be highly variable between cell types [25, 35].
Whilst there was some commonality between stem cells,
neurons, and myocytes, most non-conserved cryptic
exons were cell-type specific. Thus, TDP-43 loss of function may impair cell-type specific pathways, which hasmechanistic and treatment-evaluation implications thatwould need to be considered. With the development
of better models for assessing human-relevant cryptic
exons, there is significant promise for novel therapies on
the horizon.
Whilst blocking TDP-43 cryptic exons to restore gene
expression appears to have a relatively clear mechanistic benefit, it remains to be seen how many andwhich cryptic exons need to be targeted for an effective therapy. Future experiments may seek to determine
which cryptic exon inclusion events are pathologically relevant, impacting on a biological pathway that

Page 6 of 9

contributes to disease, as this may impact the development of therapies and potential ways to restore theexpression of multiple mRNA targets, as opposed to
one at a time.

Conclusions
The past decade has seen leaps forward for people withgene-specific ALS-FTD, now with gene-based therapies coming into play. However, these would not benefit
people with sporadic disease, making up the majorityof patients with ALS-FTD, or those that have different
pathogenic genes that are not amenable to treatment
with available gene-based therapies. Given that almost
all patients with ALS have TDP-43 pathology, therecent breakthroughs revealing that TDP-43 dependent mis-splicing of mRNA plays a major role in disease
pathogenesis provides a fresh injection of hope into
the field, not only for the ability to better understandmolecular mechanisms, but also for the transformative potential with regard to biomarker development
and therapeutics, desperately needed for patients livingwith this fatal disease. Important focuses going forward
will be to establish which other cryptic exons, if any, arepathogenic players, and for the knowledge gained about
cryptic exons to be transferred to and explored in other
TDP-43 proteinopathies.
Abbreviations
ALS	Amyotrophic lateral sclerosis
FTD	Frontotemporal dementia
CE	Cryptic exon
NMD	Nonsense-mediated decay
ASO	Antisense oligonucleotide
snRNA	Small nuclear RNA
SMA	Spinal muscular atrophy
DMD	Duchenne muscular dystrophy
AAV	Adeno-associated virus
SNP	Single nucleotide polymorphism
Acknowledgements
Not applicable.
Authors’ contributions
PRM wrote the first draft of the manuscript with contribution from AL-B. MEW
and PF reviewed and edited the manuscript. All authors approved the final
version.
Funding
PRM is supported by a Wellcome Trust Clinical Training Fellowship(102186/B/13/Z). PF is supported by a UK Medical Research Council Senior
Clinical Fellowship, Lady Edith Wolfson Fellowship (MR/M008606/1 and MR/
S006508/1), and the UCLH NIHR Biomedical Research Centre; MEW is sup‑ported by the NIH Intramural Research Program of the National Institutes of
Neurological Disorders and Stroke. This work is also supported by grants fromNIH U54NS123743 (PF), and The Robert Packard Center for ALS Research (MEWand PF).
Availability of data and materials
Not applicable.

Mehta et al. Molecular Neurodegeneration(2023) 18:16

Declarations
Ethics approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Competing interests
A patent application related to this work has been filed. The technology
described in this work has been protected in the patent PCT/EP2021/084908
and UK patent 2117758.9 (patent applicant, UCL Business Ltd. and NIH; statuspending), in which AL-B, MEW, and PF are named as inventors. The otherauthors declare no competing interests.
Received: 15 December 2022 Accepted: 17 February 2023

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