Basic Science and Pathogenesis

Repetitive head impacts that occur in contact/collision sports, military service, and physical violence are associated with TDP-43 pathology, and TDP-43 inclusions are frequently present in the hippocampus and frontal cortex in chronic traumatic encephalopathy (CTE). Individuals with CTE and TDP-43 inclusions are more likely to show severe cognitive impairment; however, the underlying mechanism is unknown. In amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with TDP-43 inclusions (FTLD-TDP), TDP-43 inclusions are associated with widespread gene mis-splicing and cryptic exon expression. We hypothesized that TDP-43 pathology in CTE similarly contributes to gene mis-splicing, leading to selective protein loss and neurodegeneration.

A total of 212 brain donors with a history of repetitive head impacts were examined for CTE and the presence and distribution of TDP-43 inclusions. Clinical outcomes, including measures of cognitive impairment, were obtained from informants and medical records. Bulk RNA sequencing and SomaScan 7K aptamer-based proteomics were performed on the dorsolateral prefrontal cortex. After excluding those with AD, FTLD-TDP, or ALS, variable splicing events were determined using LeafCutter in those with CTE and TDP-43 inclusions in the hippocampus or frontal cortex (CTE-TDP) compared to CTE without TDP-43 in those regions (CTE).

Out of 142 with CTE, 72 (51%) had TDP-43 inclusions within the hippocampus, frontal cortex, or both (CTE-TDP). Differential analysis revealed altered splicing within 836 genes in CTE-TDP compared to CTE with a false discovery rate <0.01. Of these, 23 overlapped with previously identified mis-spliced genes in ALS and FTLD-TDP. Seven of these genes had protein levels measured with SomaScan proteomics, including CAMK2B, DLGAP4, EPB41L1, NCAM1, PTPRD, RAP1GAP, and SH3KBP1. Notably, protein levels of erythrocyte membrane protein band 4.1 like 1 (EPB41L1) and RAP1 GTPase-activating protein (RAP1GAP) were significantly reduced in CTE-TDP frontal cortex compared to CTE (p's <0.01) and negatively correlated with both amygdala and hippocampal TDP-43 inclusions. Furthermore, decreased levels of EPB41L1 and RAP1GAP were associated with dementia adjusting for age (p's <0.05).

Both EPB41L1 and RAP1GAP are involved in synaptic plasticity and signaling. Mis-splicing and decreased protein expression of these synapse-related genes may partially underlie the neurodegeneration and cognitive impairments in CTE with TDP-43 inclusions.