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Transposable Elements in FTLD-TDP and ALS-TDP

Amyotrophic Lateral Sclerosis (ALS) is a devastating neurodegenerative disease involving the rapid and progressive loss of motor neurons. While a number of inherited mutations have been identified as causing ALS, the vast majority of cases are sporadic with no family history of disease. This has led to the model that ALS may be a complex syndrome with an unknown number of causal molecular pathways. However, nearly all ALS patient tissues display aggregates of the RNA binding protein TDP-43 (TARDBP/TAR DNA-binding protein), and mutations in the gene encoding this protein are known to cause ALS. We show that one of the normal functions of TDP-43 is to bind and regulate retrotransposons, repetitive sequences in the genome that are normally silent, due in part to the function of TDP-43. Furthermore, we demonstrate that many ALS patients with TDP-43 pathology also display de-silencing of retrotransposon RNAs, consistent with a loss of normal TDP-43 function. Those ALS patients that exhibit high levels of retrotransposon expression form a distinct subset of ALS subjects that also show other correlated alterations to the transcriptome.

Using unsupervised machine learning algorithms, we identified three distinct molecular subtypes from the whole genomes and transcriptomes of a large cohort of 148 ALS patient samples. The retrotransposon re-activated subset formed 20% of ALS samples; two additional groups displayed signatures of oxidative stress (61%), and glial dysfunction (19%), respectively. Together these results demonstrate that retrotransposon re-activation is common in a subset of ALS patients with TDP-43 pathology.