Implications of Mutant SOD1 on RNA Processing and Interferon Responses in Amyotrophic Lateral Sclerosis: Omics Data Analysis.

INTRODUCTION: Cytoplasmic inclusions are observed in motor neurons in amyotrophic lateral sclerosis (ALS) associated with the Cu/Zn superoxide dismutase mutation (mtSOD1). Although these inclusions are a hallmark of the disorder, degeneration is not necessarily initiated in the cytoplasm, nor are these structures the culprit of ALS. The nucleus stores genetic material and acts as the cell's control center, and a small fraction of mtSOD1 is reported to be distributed in the nucleus. We hypothesized that mtSOD1 in the nucleus contributes to motor neuron degeneration. METHODS: We explored the roles of mtSOD1 in relation to nuclear proteins, chromosomal DNA, and mRNA expression. An immortalized cell line derived from a transgenic ALS mouse model expressing mtSOD1-L126delTT with a FLAG was used for stable immunoprecipitation of mtSOD1-binding molecules using shotgun proteomics and chromatin immunoprecipitation-sequencing (ChIP-seq). We also examined mRNA expression by silencing whole SOD1 (innate mouse Sod1 and mtSOD1) or mtSOD1 alone and compared these patterns against those in non-silenced counterparts. RESULTS: We identified 392 mtSOD1-interacting proteins in the nucleus. Gene ontology (GO) revealed these proteins to be enriched for "mRNA processing." Notably, more than 11% of mtSOD1-interacting proteins were expressed concurrently with previously reported wild-type TAR DNA-binding protein 43 (TDP-43)-interacting proteins. ChIP-seq revealed that mtSOD1-interacting DNA portions showed a preference for zinc finger protein-binding motifs. GO analysis of the ChIP-seq data revealed that "mRNA processing" was again enriched among the genes harboring mtSOD1-binding domains. RNA expression analyses revealed that the presence of mouse Sod1 and mtSOD1 induced the overexpression of molecules related to "type 1 IFN responses." CONCLUSIONS: We revealed that mtSOD1 interacted with nuclear proteins and specific DNA segments and that RNA expression was notably altered when mouse Sod1 and mtSOD1 were silenced. These interactions could play a pivotal role in motor neuron degeneration.