Shared Molecular Features of Inherited and Sporadic ALS/FTD

• Main Author: Conlon, Erin Grace
• Language: English
• Published: 2018
• Subjects: Biochemistry; Genetics; Neurosciences; Amyotrophic lateral sclerosis; Frontotemporal dementia; Molecular biology
• Online Access: https://doi.org/10.7916/D8W6831W

Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD) are two devastating neurodegenerative diseases in urgent need of therapeutic intervention. The last seven years has been a period of great progress in understanding these disorders separately and as a disease spectrum. Most notable is the discovery of the hexanucleotide GGGGCC expansion in the C9ORF72 (C9) gene, which is the greatest known cause of inherited and sporadic forms of these two diseases. In response to this groundbreaking discovery, we set out to elucidate the molecular mechanisms of C9 pathogenesis with a focus on the expanded RNA transcripts derived from the C9 expansion. Our two primary goals have been to contribute to the worldwide efforts to understand the primary toxic insults of this mutation that will ultimately shape therapeutic development, and to identify molecular criteria that can be used to define new links between these diseases and undetermined genetic factors. In the introduction, we review the broad conceptual links between RNA binding proteins (RBPs), mRNA regulation, and neurodegeneration. This review contains substantial discussion of ALS, FTD, and C9, as well as related neurodegenerative, neuromuscular and repeat expansion diseases. In addition to providing a detailed history of molecular mechanisms proposed for these disorders, this section serves as a justification for our focus on the C9 RNA, RBP sequestration, and altered splicing that we describe in the following chapters. Chapter two consists of our 2016 Elife paper on sequestration of the RBP hnRNP H and resulting splicing changes in C9ALS-FTD afflicted individuals. In this paper, we sought to identify the most biochemically sound candidate for the proposed RBP sequestration hypothesis. We found that the splicing factor hnRNP H binds with high affinity to the repeat sequence and likely has a role in regulating the transition of the repeat RNA from linear to G-quadruplex (G-Q) conformation. Importantly, we identified functional deficiency of this protein in patient brains, as evidenced by dysregulation of known hnRNP H splicing targets, and loss of soluble hnRNP H. Chapter three consists of recently submitted work on the molecular links between C9ALS/FTD, and sporadic ALS/FTD at large. Building upon our findings in C9ALS-FTD, we have sought to ask whether the changes to hnRNP H we predicted would occur in C9 expansion carriers as a result of the repeat RNA might also occur independent of this expansion. We found that indeed half of all patients in a cohort of 50 sporadic ALS, ALS-FTD, and FTD brains demonstrated hnRNP H sequestration and accompanying splicing changes, a pattern we refer to as like-C9. Like-C9 patients may be thought of as phenocopies of C9 expansion carriers, in that they not only present with similar clinical symptoms, but also possess remarkably similar molecular signatures of RBP dysfunction. While the genomic origins of like-C9 remain unknown, we propose that they are suggestive of repeat expansions analogous to C9, much like what is seen in DM1 and DM2, and HD and HDL2 (discussed in Ch. 1). This work has provided the foundation for our ongoing search for novel genomic expansions that confer increased susceptibility to ALS/FTD.