An iPSCs Model of FUS mutation Carriers Reveals Pathways that Protect Against ALS
Principle Investigator: Ziyuan Guo, PhD, Organoid Center
Amyotrophic lateral sclerosis (ALS) is a complex neurodegenerative disease with approximately 10 to 15% familial cases caused by genetic mutations in an autosomal dominant fashion. Although rare, ALS can also develop before the age of 25; the median age of developing Juvenile ALS is 6.5 years. ALS patients may become paralyzed between age 12 to age 60. Since familial and sporadic ALS are clinically indistinguishable, studies of familial ALS will facilitate understanding of ALS etiology in general. Among the ALS genes identified, several encode RNA binding proteins including Fused in Sarcoma (FUS). FUS functions in multiple RNA metabolic pathways. Mutant FUS protein is mis-localized to the cytoplasm where it forms granules and inclusions, a pathological hallmark of ALS. We and others have studied the FUS protein under physiological and pathological conditions in various models. Strikingly, we recently identified five individuals (three of them are second cousins) in an extended ALS kindred who carry the ALS-linked FUS R521G mutation but live well beyond their 60s without developing ALS (Unaffected Mutation Carriers, UMCs). Our discovery of incomplete penetrance in this extended FUS-ALS pedigree is the first of its kind.
We hypothesize that, despite carrying a disease-causing FUS mutation, UMCs have protective genetic modifiers preventing disease development. The determination of such modifiers and the underlying mechanisms will point to novel therapeutic targets. Patient-derived induced pluripotent stem cells (iPSCs) and their differentiated motor neurons (MNs) facilitate mechanistic studies in target cells in a relevant human genetic background. Taking advantage of newly generated iPSC lines derived from the unique ALS pedigree with multiple UMCs, we are identifying protective genetic modifiers to determine the underlying protective molecular/biological pathways. We are identifying de novo variants and mapping genetic modifiers in UMCs using whole genome sequencing analysis. Also, we are deciphering the molecular/biological pathways responsible for preserving normal functions in UMCs iPSC-MNs using RNASeq. We are differentiating MNs from iPSC lines of UMCs, ALS patients, and healthy controls to characterize the pathophysiological dysfunctions at different differentiation stages using immunocytochemistry and electrophysiology. In addition, we are comparing the cellular and functional features of iPSC-MNs from UMCs with ALS patients and healthy controls, which will reveal which cellular and biochemical features are critical to protecting UMCs. The integration of genomic, transcriptomic and phenotypic data will provide in-depth understanding of how putative genetic modifiers function through molecular pathways to yield protective phenotypes in UMCs. Completion of the proposed studies will provide novel insights into naturally occurring genetic modifiers that protect UMCs from contracting ALS. The compensatory mechanism identified in this project will lay the foundation for future studies to develop future ALS therapies.