Cracking the cryptic code in amyotrophic lateral sclerosis and frontotemporal dementia: Towards therapeutic targets and biomarkers

Abstract

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are two devastating human neurodegenerative diseases. A hallmark pathological feature of both diseases is the depletion of the RNA-binding protein TDP-43 from the nucleus in the brain and spinal cord of patients. A major function of TDP-43 is to repress the inclusion of cryptic exons during RNA splicing. When it becomes depleted from the nucleus in disease, this function is lost, and recently, several key cryptic splicing targets of TDP-43 have emerged, including STMN2, UNC13A, and others. UNC13A is a major ALS/FTD risk gene, and the genetic variations that increase the risk for disease seem to do so by making the gene more susceptible to cryptic exon inclusion when TDP-43 function is impaired. Here, we discuss the prospects and challenges of harnessing these cryptic splicing events as novel therapeutic targets and biomarkers. Deciphering this new cryptic code may be a touchstone for ALS and FTD diagnosis and treatment.

Keywords: ALS; FTD; TDP-43; UNC13A.

FIGURE 1

TDP‐43 is a repressor of cryptic RNA splicing. In healthy neurons, TDP‐43 localizes to the nucleus. Cryptic exons (yellow) are sequences located within introns that should not be included in the mature messenger RNA (mRNA) following splicing. As one of its major normal functions, nuclear TDP‐43 represses the inclusion of cryptic exons. In the early stages of disease, the depletion of TDP‐43 from the nucleus occurs before the appearance of cytoplasmic TDP‐43 aggregates. Nuclear TDP‐43 depletion leads to the inclusion of cryptic exons in mRNAs. These cryptic exons can destabilize the RNA, leading to its degradation or resulting in the production of aberrant peptides (cryptic peptides). In later stages of disease, TDP‐43 disappears from the nucleus and forms aggregates in the cytoplasm. Thus, both the loss of nuclear TDP‐43 and its cytoplasmic aggregation are associated with neurodegeneration in amyotrophic lateral sclerosis/frontotemporal dementia (ALS/FTD)

FIGURE 2

Cryptic splicing events as novel therapeutic targets and biomarker candidates. (A) Therapeutic strategies to prevent cryptic splicing events of key TDP‐43 targets could include CRISPR‐based genome editing of cryptic splice donor and acceptor sites, modified antisense oligonucleotides (ASOs) that trigger splice skipping, or gene therapy viral vectors (e.g., adeno‐associated virus [AAV]) to restore the expression of proteins that are downregulated by cryptic splicing events. (B) Cryptic splicing events can also serve as novel biomarker candidates, which could serve as sensitive readouts of TDP‐43 function. These biomarkers could be RNA‐based or protein‐based and could be detected in patient fluids (e.g., plasma or cerebrospinal fluid [CSF]). These biomarkers (or panels of multiple biomarkers) will aid in both early disease detection and the assessment of the efficacy of experimental therapeutics