A fluid biomarker reveals loss of TDP-43 splicing repression in presymptomatic ALS–FTD

Abstract

Although loss of TAR DNA-binding protein 43 kDa (TDP-43) splicing repression is well documented in postmortem tissues of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), whether this abnormality occurs during early-stage disease remains unresolved. Cryptic exon inclusion reflects loss of function of TDP-43, and thus detection of proteins containing cryptic exon-encoded neoepitopes in cerebrospinal fluid (CSF) or blood could reveal the earliest stages of TDP-43 dysregulation in patients. Here we use a newly characterized monoclonal antibody specific to a TDP-43-dependent cryptic epitope (encoded by the cryptic exon found in HDGFL2) to show that loss of TDP-43 splicing repression occurs in ALS–FTD, including in presymptomatic C9orf72 mutation carriers. Cryptic hepatoma-derived growth factor-like protein 2 (HDGFL2) accumulates in CSF at significantly higher levels in familial ALS–FTD and sporadic ALS compared with controls and is elevated earlier than neurofilament light and phosphorylated neurofilament heavy chain protein levels in familial disease. Cryptic HDGFL2 can also be detected in blood of individuals with ALS–FTD, including in presymptomatic C9orf72 mutation carriers, and accumulates at levels highly correlated with those in CSF. Our findings indicate that loss of TDP-43 cryptic splicing repression occurs early in disease progression, even presymptomatically, and that detection of the HDGFL2 cryptic neoepitope serves as a potential diagnostic biomarker for ALS, which should facilitate patient recruitment and measurement of target engagement in clinical trials.

HDGFL2 cryptic proteins report presence of TDP-43 pathology in neurodegenerative diseases

“…By developing a sensitive and specific immunoassay to detect HDGFL2-CE proteins, we observed that HDGFL2-CE is significantly increased in brain regions with TDP-43 pathology in FTLD-TDP and AD-TDP, associates with pTDP-43 burden, and can distinguish individuals with TDP-43 pathology from those without. In line with our findings, Irwin et al used their HDGFL2-CE antibody to perform immunofluorescent staining of motor cortex and hippocampus tissues from patients with ALS-FTD demonstrating that HDGFL2-CE proteins accumulate in cells exhibiting pTDP-43 pathology [ 11 ]. They additionally found that, compared to controls, CSF HDGFL2-CE was statistically significantly higher in individuals likely to have TDP-43 pathology, namely presymptomatic or symptomatic C9orf72 repeat expansion carriers and patients with sporadic ALS [ 11 ].…”

“…In line with our findings, Irwin et al used their HDGFL2-CE antibody to perform immunofluorescent staining of motor cortex and hippocampus tissues from patients with ALS-FTD demonstrating that HDGFL2-CE proteins accumulate in cells exhibiting pTDP-43 pathology [ 11 ]. They additionally found that, compared to controls, CSF HDGFL2-CE was statistically significantly higher in individuals likely to have TDP-43 pathology, namely presymptomatic or symptomatic C9orf72 repeat expansion carriers and patients with sporadic ALS [ 11 ].…”

“…Consequently, CEs are anomalously included in critical transcripts such as STMN2 and UNC13A [ 2 – 9 ], which can produce truncated or destabilized RNAs and lead to a loss of their function. Recently, we and others demonstrated that some transcripts with in-frame CEs produce stable CE-containing novel proteins detectable in cerebrospinal fluid (CSF) from patients with FTLD-TDP or ALS [ 10 , 11 ]. One notable example is a cryptic protein derived from the gene hepatoma-derived growth factor-like protein 2 (HDGFL2-CE), a histone-binding protein expressed throughout the brain.…”

“…One notable example is a cryptic protein derived from the gene hepatoma-derived growth factor-like protein 2 (HDGFL2-CE), a histone-binding protein expressed throughout the brain. Accordingly, CSF HDGFL2-CE could potentially serve as a sensitive biomarker of TDP-43 pathology, illuminating the contribution of pathological TDP-43 to the clinical variability in TDP-43 proteinopathies [ 11 ]. But testing whether CSF HDGFL2-CE is indicative of TDP-43 pathology is hampered by the lack of robust methods to measure pathological TDP-43 in biofluids.…”

“…Retention of CEs in mRNAs owing to TDP-43 dysfunction is well-documented in FTLD-TDP, ALS and AD-TDP [ 2 – 8 ], but the identification that certain in-frame CEs generate stable cryptic proteins is new [ 8 , 10 , 11 ]. Here, we explored the recently identified HDGFL2-CE protein.…”