Idiopathic inflammatory myopathies: Pathogenic mechanisms of muscle weakness

Idiopathic inflammatory myopathies (IIMs) are a heterogenous group of complex muscle diseases of unknown etiology. These diseases are characterized by progressive muscle weakness and damage, together with involvement of other organ systems. It is generally believed that the autoimmune response (autoreactive lymphocytes and autoantibodies) to skeletal muscle-derived antigens is responsible for the muscle fiber damage and muscle weakness in this group of disorders. Therefore, most of the current therapeutic strategies are directed at either suppressing or modifying immune cell activity. Recent studies have indicated that the underlying mechanisms that mediate muscle damage and dysfunction are multiple and complex. Emerging evidence indicates that not only autoimmune responses but also innate immune and non-immune metabolic pathways contribute to disease pathogenesis. However, the relative contributions of each of these mechanisms to disease pathogenesis are currently unknown. Here we discuss some of these complex pathways, their inter-relationships and their relation to muscle damage in myositis. Understanding the relative contributions of each of these pathways to disease pathogenesis would help us to identify suitable drug targets to alleviate muscle damage and also improve muscle weakness and quality of life for patients suffering from these debilitating muscle diseases

Idiopathic inflammatory myopathies: pathogenic mechanisms of muscle weakness

Idiopathic inflammatory myopathies (IIMs) are a heterogenous group of complex muscle diseases of unknown etiology. These diseases are characterized by progressive muscle weakness and damage, together with involvement of other organ systems. It is generally believed that the autoimmune response (autoreactive lymphocytes and autoantibodies) to skeletal muscle-derived antigens is responsible for the muscle fiber damage and muscle weakness in this group of disorders. Therefore, most of the current therapeutic strategies are directed at either suppressing or modifying immune cell activity. Recent studies have indicated that the underlying mechanisms that mediate muscle damage and dysfunction are multiple and complex. Emerging evidence indicates that not only autoimmune responses but also innate immune and non-immune metabolic pathways contribute to disease pathogenesis. However, the relative contributions of each of these mechanisms to disease pathogenesis are currently unknown. Here we discuss some of these complex pathways, their inter-relationships and their relation to muscle damage in myositis. Understanding the relative contributions of each of these pathways to disease pathogenesis would help us to identify suitable drug targets to alleviate muscle damage and also improve muscle weakness and quality of life for patients suffering from these debilitating muscle diseases

Multiple novel prostate cancer susceptibility signals identified by fine-mapping of known risk loci among Europeans

Genome-wide association studies (GWAS) have identified numerous common prostate cancer (PrCa) susceptibility loci. We have fine-mapped 64 GWAS regions known at the conclusion of the iCOGS study using large-scale genotyping and imputation in 25 723 PrCa cases and 26 274 controls of European ancestry. We detected evidence for multiple independent signals at 16 regions, 12 of which contained additional newly identified significant associations. A single signal comprising a spectrum of correlated variation was observed at 39 regions; 35 of which are now described by a novel more significantly associated lead SNP, while the originally reported variant remained as the lead SNP only in 4 regions. We also confirmed two association signals in Europeans that had been previously reported only in East-Asian GWAS. Based on statistical evidence and linkage disequilibrium (LD) structure, we have curated and narrowed down the list of the most likely candidate causal variants for each region. Functional annotation using data from ENCODE filtered for PrCa cell lines and eQTL analysis demonstrated significant enrichment for overlap with bio-features within this set. By incorporating the novel risk variants identified here alongside the refined data for existing association signals, we estimate that these loci now explain ∼38.9% of the familial relative risk of PrCa, an 8.9% improvement over the previously reported GWAS tag SNPs. This suggests that a significant fraction of the heritability of PrCa may have been hidden during the discovery phase of GWAS, in particular due to the presence of multiple independent signals within the same regio

Discovery of Novel Drugs to Alleviate Muscle Weakness in Myositis

Idiopathic inflammatory myopathies (myositis) often exhibit refractory muscle weakness, which persists despite elimination of infiltrating inflammatory cells with immunosuppressive drugs. Myositis patients and our mouse model of myositis have been reported to acquire a deficiency of the metabolic enzyme AMP deaminase 1 (AMPD1), which catalyzes the rate-limiting step of the purine nucleotide cycle. In humans, a congenital AMPD1 deficiency due to a loss-of-function allele is associated with easy fatigability, weakness, and cramping. We hypothesize that an acquired AMPD1 deficiency contributes to muscle weakness in myositis, so we sought to find drugs that increased expression of AMPD1 to alleviate this refractory weakness. We created a coincidental reporter HEK293 cell line that used the AMPD1 promoter to drive the expression of a bicistronic transcript for the renilla and firefly luciferase genes intervened by a ribosomal skipping sequence, designed to reduce false positives by assessing the readout from two non-homologous reporters. The resulting cell line was utilized for quantitative high throughput screening (qHTS) of 3 libraries, totaling 4194 compounds at 7 to 11 dilutions each. Actives were then validated in cultured myotubes by RT-qPCR and an enzymatic activity assay of endogenous AMPD1. Regulation of the coincidental reporter in HEK293 cells was similar to that in muscle cells and the qHTS assay conditions were optimized for 1536 well-plates (Z’\u3e0.5). The commercially available LOPAC1280 and FDA approved libraries identified microtubule polymerization inhibitors, such as podophyllotoxin and colchicine, as the most active compounds. Podophyllotoxin was verified to up-regulate endogenous AMPD1 mRNA and increase enzymatic activity in cultured myotubes. Additionally, qHTS of a 50 compound library of aza-podophyllotoxin analogues identified novel actives that are being pursued as lead compounds. Most of the active drugs identified by qHTS were microtubule polymerization inhibitors, suggesting a mechanistic link between microtubule polymerization and AMPD1 transcription. Active compounds identified by these assays will undergo preclinical testing in our mouse model of myositis for their ability to up-regulate AMPD1 and alleviate muscle weakness

Morpholino-induced exon skipping stimulates cell-mediated and humoral responses to dystrophin in mdx mice.

Exon skipping is a promising genetic therapeutic strategy for restoring dystrophin expression in the treatment of Duchenne muscular dystrophy (DMD). The potential for newly synthesized dystrophin to trigger an immune response in DMD patients, however, is not well established. We have evaluated the effect of chronic phosphorodiamidate morpholino oligomer (PMO) treatment on skeletal muscle pathology and asked whether sustained dystrophin expression elicits a dystrophin-specific autoimmune response. Here, two independent cohorts of dystrophic mdx mice were treated chronically with either 800 mg/kg/month PMO for 6 months (n = 8) or 100 mg/kg/week PMO for 12 weeks (n = 11). We found that significant muscle inflammation persisted after exon skipping in skeletal muscle. Evaluation of humoral responses showed serum-circulating antibodies directed against de novo dystrophin in a subset of mice, as assessed both by Western blotting and immunofluorescent staining; however, no dystrophin-specific antibodies were observed in the control saline-treated mdx cohorts (n = 8) or in aged (12-month-old) mdx mice with expanded ‘revertant’ dystrophin-expressing fibers. Reactive antibodies recognized both full-length and truncated exon-skipped dystrophin isoforms in mouse skeletal muscle. We found more antigen-specific T-cell cytokine responses (e.g. IFN-g, IL-2) in dystrophin antibody-positive mice than in dystrophin antibody-negative mice. We also found expression of major histocompatibility complex class I on some of the dystrophin-expressing fibers along with CD8+ and perforin-positive T cells in the vicinity, suggesting an activation of cell-mediated damage had occurred in the muscle. Evaluation of complement membrane attack complex (MAC) deposition on the muscle fibers further revealed lower MAC deposition on muscle fibers of dystrophin antibody-negative mice than on those of dystrophin antibody-positive mice. Our results indicate that de novo dystrophin expression after exon skipping can trigger both cell-mediated and humoral immune responses in mdx mice. Our data highlights the need to further investigate the autoimmune response and its long-term consequences after exon-skipping therapy. Copyright © 2019 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd