Exome sequencing was used to identify novel variants linked to amyotrophic lateral sclerosis (ALS), in a family without mutations in genes previously linked to ALS. A F115C mutation in the gene MATR3 was identified, and further examination of other ALS kindreds identified an additional three mutations in MATR3; S85C, P154S and T622A. Matrin 3 is an RNA/DNA binding protein as well as part of the nuclear matrix. Matrin 3 interacts with TDP-43, a protein that is both mutated in some forms of ALS, and found in pathological inclusions in most ALS patients. Matrin 3 pathology, including mislocalization and rare cytoplasmic inclusions, was identified in spinal cord tissue from a patient carrying a mutation in Matrin 3, as well as sporadic ALS patients. In an effort to determine the mechanism of Matrin 3 linked ALS, the protein interactome of wild-type and ALS-linked MATR3 mutations was examined. Immunoprecipitation followed by mass spectrometry experiments were performed using NSC-34 cells expressing human wild-type or mutant Matrin 3. Gene ontology analysis identified a novel role for Matrin 3 in mRNA transport centered on proteins in the TRanscription and EXport (TREX) complex, known to function in mRNA biogenesis and nuclear export. ALS-linked mutations in Matrin 3 led to its re-distribution within the nucleus, decreased co-localization with endogenous Matrin 3 and increased co-localization with specific TREX components. Expression of disease-causing Matrin 3 mutations led to nuclear mRNA export defects of both global mRNA and more specifically the mRNA of TDP-43 and FUS. Our findings identify ALS-causing mutations in the gene MATR3, as well as a potential pathogenic mechanism attributable to MATR3 mutations and further link cellular transport defects to ALS.

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the deterioration of motor neurons. ALS affects about 1 in 20,000 people and leads to death within 2 to 5 years after diagnosis. There is currently no cure for ALS, but there are many genes known to be associated with ALS, such as SOD 1 and C9orf72. Recently, mutations in Matrin 3 were linked to ALS. While 15 mutations in Matrin 3 have been discovered, this study focuses on the four initial mutations, which are the Ser85Cys, Phe115Cys, Pro154Ser, and Thr622Ala mutations. This study attempts to understand the mechanism of how these mutations lead to ALS. The first aim focuses on the role of Matrin mutations in the mislocalization of TDP-43 from the nucleus to the cytoplasm, a pathological hallmark of ALS. We hypothesized expression of mutant Matrin 3 would lead to TDP-43 mislocalization, however the data did not support that hypothesis. The second aim of this study focuses on the mislocalization of TRanscription EXport (TREX) complex proteins within the nucleus. TREX proteins were studied based off of previous experiments suggesting that proteins within this complex bind to Matrin 3. The results showed differences in co-localization between each of these proteins and wild-type and mutant Matrin 3, confirming our earlier results. These findings can help increase our understanding of the mechanism of ALS while also setting the framework for future studies.