Comparing lamin proteins post-translational relative stability using a 2A peptide-based system reveals elevated resistance of progerin to cellular degradation

Development of an accelerated cellular model for early changes in Alzheimer's disease

Alzheimer's Disease (AD) is a leading cause of dementia characterized by amyloid plaques and neurofibrillary tangles, and its pathogenesis remains unclear. Current cellular models for AD often require several months to exhibit phenotypic features due to the lack of an aging environment in vitro. Lamin A is a key component of the nuclear lamina. Progerin, a truncated protein resulting from specific lamin A mutations, causes Hutchinson-Gilford Progeria Syndrome (HGPS), a disease that prematurely ages individuals. Studies have reported that lamin A expression is induced in the brains of AD patients, and overlapping cellular phenotypes have been observed between HGPS and AD cells. In this study, we investigated the effects of exogenous progerin expression on neural progenitor cells carrying familial AD mutations (FAD). Within three to four weeks of differentiation, these cells exhibited robust AD phenotypes, including increased tau phosphorylation, amyloid plaque accumulation, and an elevated Aβ42 to Aβ40 ratio. Additionally, progerin expression significantly increased AD cellular phenotypes such as cell death and cell cycle re-entry. Our results suggest that progerin expression could be used to create an accelerated model for AD development and drug screening.

Gene-specific nonsense-mediated mRNA decay targeting for cystic fibrosis therapy

Low CFTR mRNA expression due to nonsense-mediated mRNA decay (NMD) is a major hurdle in developing a therapy for cystic fibrosis (CF) caused by the W1282X mutation in the CFTR gene. CFTR-W1282X truncated protein retains partial function, so increasing its levels by inhibiting NMD of its mRNA will likely be beneficial. Because NMD regulates the normal expression of many genes, gene-specific stabilization of CFTR-W1282X mRNA expression is more desirable than general NMD inhibition. Synthetic antisense oligonucleotides (ASOs) designed to prevent binding of exon junction complexes (EJC) downstream of premature termination codons (PTCs) attenuate NMD in a gene-specific manner. We describe cocktails of three ASOs that specifically increase the expression of CFTR-W1282X mRNA and CFTR protein upon delivery into human bronchial epithelial cells. This treatment increases the CFTR-mediated chloride current. These results set the stage for clinical development of an allele-specific therapy for CF caused by the W1282X mutation.

The W1282X nonsense mutation in the CFTR gene causes cystic fibrosis by reducing its mRNA and functional protein levels. Here the authors developed antisense-oligonucleotide cocktails that restore CFTR protein function by gene-specific stabilization of CFTR mRNA.

Exon-skipping antisense oligonucleotides for cystic fibrosis therapy

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene cause cystic fibrosis (CF), and the CFTR-W1282X nonsense mutation causes a severe form of CF. Although Trikafta and other CFTR-modulation therapies benefit most CF patients, targeted therapy for patients with the W1282X mutation is lacking. The CFTR-W1282X protein has residual activity but is expressed at a very low level due to nonsense-mediated messenger RNA (mRNA) decay (NMD). NMD-suppression therapy and read-through therapy are actively being researched for CFTR nonsense mutants. NMD suppression could increase the mutant CFTR mRNA, and read-through therapies may increase the levels of full-length CFTR protein. However, these approaches have limitations and potential side effects: because the NMD machinery also regulates the expression of many normal mRNAs, broad inhibition of the pathway is not desirable, and read-through drugs are inefficient partly because the mutant mRNA template is subject to NMD. To bypass these issues, we pursued an exon-skipping antisense oligonucleotide (ASO) strategy to achieve gene-specific NMD evasion. A cocktail of two splice-site-targeting ASOs induced the expression of CFTR mRNA without the premature-termination-codon-containing exon 23 (CFTR-Δex23), which is an in-frame exon. Treatment of human bronchial epithelial cells with this cocktail of ASOs that target the splice sites flanking exon 23 results in efficient skipping of exon 23 and an increase in CFTR-Δex23 protein. The splice-switching ASO cocktail increases the CFTR-mediated chloride current in human bronchial epithelial cells. Our results set the stage for developing an allele-specific therapy for CF caused by the W1282X mutation.

A targeted antisense therapeutic approach for Hutchinson-Gilford progeria syndrome

Hutchinson-Gilford progeria syndrome (HGPS) is a rare accelerated aging disorder characterized by premature death from myocardial infarction or stroke. It is caused by de novo single-nucleotide mutations in the LMNA gene that activate a cryptic splice donor site, resulting in the production of a toxic form of lamin A, which is termed progerin. Here we present a potential genetic therapeutic strategy that utilizes antisense peptide-conjugated phosphorodiamidate morpholino oligomers (PPMOs) to block pathogenic splicing of mutant transcripts. Of several candidates, PPMO SRP-2001 provided the most significant decrease in progerin transcripts in patient fibroblasts. Intravenous delivery of SRP-2001 to a transgenic mouse model of HGPS produced significant reduction of progerin transcripts in the aorta, a particularly critical target tissue in HGPS. Long-term continuous treatment with SRP-2001 yielded a 61.6% increase in lifespan and rescue of vascular smooth muscle cell loss in large arteries. These results provide a rationale for proceeding to human trials.

SAMMY-seq reveals early alteration of heterochromatin and deregulation of bivalent genes in Hutchinson-Gilford Progeria Syndrome

Hutchinson-Gilford progeria syndrome is a genetic disease caused by an aberrant form of Lamin A resulting in chromatin structure disruption, in particular by interfering with lamina associated domains. Early molecular alterations involved in chromatin remodeling have not been identified thus far. Here, we present SAMMY-seq, a high-throughput sequencing-based method for genome-wide characterization of heterochromatin dynamics. Using SAMMY-seq, we detect early stage alterations of heterochromatin structure in progeria primary fibroblasts. These structural changes do not disrupt the distribution of H3K9me3 in early passage cells, thus suggesting that chromatin rearrangements precede H3K9me3 alterations described at later passages. On the other hand, we observe an interplay between changes in chromatin accessibility and Polycomb regulation, with site-specific H3K27me3 variations and transcriptional dysregulation of bivalent genes. We conclude that the correct assembly of lamina associated domains is functionally connected to the Polycomb repression and rapidly lost in early molecular events of progeria pathogenesis.

Diminished Canonical β-Catenin Signaling During Osteoblast Differentiation Contributes to Osteopenia in Progeria

Patients with Hutchinson-Gilford progeria syndrome (HGPS) have low bone mass and an atypical skeletal geometry that manifests in a high risk of fractures. Using both in vitro and in vivo models of HGPS, we demonstrate that defects in the canonical WNT/β-catenin pathway, seemingly at the level of the efficiency of nuclear import of β-catenin, impair osteoblast differentiation and that restoring β-catenin activity rescues osteoblast differentiation and significantly improves bone mass. Specifically, we show that HGPS patient-derived iPSCs display defects in osteoblast differentiation, characterized by a decreased alkaline phosphatase activity and mineralizing capacity. We demonstrate that the canonical WNT/β-catenin pathway, a major signaling cascade involved in skeletal homeostasis, is impaired by progerin, causing a reduction in the active β-catenin in the nucleus and thus decreased transcriptional activity, and its reciprocal cytoplasmic accumulation. Blocking farnesylation of progerin restores active β-catenin accumulation in the nucleus, increasing signaling, and ameliorates the defective osteogenesis. Moreover, in vivo analysis of the Zmpste24-/- HGPS mouse model demonstrates that treatment with a sclerostin-neutralizing antibody (SclAb), which targets an antagonist of canonical WNT/β-catenin signaling pathway, fully rescues the low bone mass phenotype to wild-type levels. Together, this study reveals that the β-catenin signaling cascade is a therapeutic target for restoring defective skeletal microarchitecture in HGPS. © 2018 American Society for Bone and Mineral Research.

Human CTL-based functional analysis shows the reliability of a munc13-4 protein expression assay for FHL3 diagnosis

Familial hemophagocytic lymphohistiocytosis (FHL) is the major form of hereditary hemophagocytic lymphohistiocytosis (HLH); as such, it requires prompt and accurate diagnosis. We previously reported that FHL type 3 (FHL3) can be rapidly screened by detecting munc13-4 expression in platelets using flow cytometry; however, the reliability of the munc13-4 expression assay for FHL3 diagnosis is unclear. Regardless of the type of UNC13D mutation, all reported FHL3 cases examined for the munc13-4 protein showed significantly reduced expression. However, the translated munc13-4 protein of some reportedly disease-causing UNC13D missense variants has not been assessed in terms of expression or function; therefore, their clinical significance remains unclear. The aim of this study was to determine the reliability of a munc13-4 expression assay for screening FHL3. Between 2011 and 2016, 108 HLH patients were screened by this method in our laboratory, and all 15 FHL3 patients were diagnosed accurately. To further elucidate whether munc13-4 expression analysis can reliably identify FHL3 patients harboring missense mutations in UNC13D, we developed an alloantigen-specific cytotoxic T lymphocyte (CTL) line and a CTL line immortalized by Herpesvirus saimiri derived from FHL3 patients. We then performed a comprehensive functional analysis of UNC13D variants. Transient expression of UNC13D complementary DNA constructs in these cell lines enabled us to determine the pathogenicity of the reported UNC13D missense variants according to expression levels of their translated munc13-4 proteins. Taken together with previous findings, the results presented herein show that the munc13-4 protein expression assay is a reliable tool for FHL3 screening.

Lamin A and microtubules collaborate to maintain nuclear morphology

Lamin A (LA) is a critical structural component of the nuclear lamina. Mutations within the LA gene (LMNA) lead to several human disorders, most striking of which is Hutchinson-Gilford Progeria Syndrome (HGPS), a premature aging disorder. HGPS cells are best characterized by an abnormal nuclear morphology known as nuclear blebbing, which arises due to the accumulation of progerin, a dominant mutant form of LA. The microtubule (MT) network is known to mediate changes in nuclear morphology in the context of specific events such as mitosis, cell polarization, nucleus positioning and cellular migration. What is less understood is the role of the microtubule network in determining nuclear morphology during interphase. In this study, we elucidate the role of the cytoskeleton in regulation and misregulation of nuclear morphology through perturbations of both the lamina and the microtubule network. We found that LA knockout cells exhibit a crescent shape morphology associated with the microtubule-organizing center. Furthermore, this crescent shape ameliorates upon treatment with MT drugs, Nocodazole or Taxol. Expression of progerin, in LA knockout cells also rescues the crescent shape, although the response to Nocodazole or Taxol treatment is altered in comparison to cells expressing LA. Together these results describe a collaborative effort between LA and the MT network to maintain nuclear morphology.