Background/Purpose: The aim of this study was to investigate whether in immune-mediated necrotising myopathy (IMNM), anti-HMGCR antibodies interfere with HMGCR activity and have a myopathic effect.

Methods: To obtain polyclonal anti-HMGCR autoantibodies, the 6 peptides identified as the epitopes of anti-HMGCR antibodies were used to immunise a white New Zealand rabbit. Antibodies from a non-immunised animal served as control. Plasma of anti-HMGCR (n=5) and anti-SRP (n=2) IMNM patients were obtained from plasmapheresis eluates. Rabbit and human anti-HMGCR antibodies were purified by affinity chromatography. Inhibition of HMGCR was measured in vitro by spectrophotometry in the presence of different concentrations of autoantibodies. Pravastatin was used as a negative control. Human autoantibodies were electroporated into human myotubes: creatine-kinase levels (CK) were measured in the supernatant, and hematoxylin/eosin (H&E) as well as oil red-O stainings were performed.Thirty-four patients with inflammatory myopathies (IM) according to the EULAR/ACR 2017 criteria, not taking immunomodulators or statins, were included (anti-HMGCR: n=10; other IM: n=24) as well as 3 patients without neuromuscular diseases (no NMD). Histological sections of deltoid muscle taken at the time of diagnosis were immunostained with an anti-human IgG antibody. Oil red-O staining was used to study the accumulation of lipid droplets, quantified (extension score 0-4) by two myopathologists blinded to the diagnosis.

Results: To test whether anti-HMGCR antibodies inhibit the function of their target, HMGCR enzymatic activity was assessed in vitro in the presence of anti-HMGCR, control antibodies and pravastatin. A dose-dependent inhibition of HMGCR activity was observed in the presence of anti-HMGCR (Figure 1a). Rabbit anti-HMGCR 5 µg/ml and human anti-HMGCR 2 µg/ml had similar effects to that of pravastatin 0.5 μM (Figure 1a, 1b). IgG from the non-immunised rabbit and plasma from anti-SRP patients did not affect HMGCR activity (Figure 1a, 1b). To test whether anti-HMGCR internalisation in myofibres exerts a myopathic effect, human myotubes were electroporated with human purified anti-HMGCR, control IgG or pravastatin. Antibody presence in the cytoplasm 4 days after electroporation was confirmed. CK levels were 2.4-fold higher in the supernatant of myotubes electroporated with anti-HMGCR compared to those electroporated with control IgG (p=0.0001). H&E staining showed necrotic myotubes after electroporation with purified anti-HMGCR and pravastatin, but not with control IgG. Oil red-O staining revealed a lipid droplet accumulation in myotubes electroporated with purified anti-HMGCR and pravastatin, but not with control IgG (Figure 1c). Lipid droplet accumulation score was 10-fold higher in anti-HMGCR IMNM patients compared to other IM and no NMD (3.2 ± 0.6 vs. 0.3 ± 0.5; vs. 0.3 ± 0.6, respectively, p=0.0001) and a score ≥ 2 was a hallmark of anti-HMGCR IMNM (Figure 1d, 1e).

Conclusion: Together, these data demonstrate that anti-HMGCR antibodies inhibit HMGCR activity, leading to the sarcoplasmic accumulation of lipid droplets and myofibres necrosis. These findings could have implications for both the diagnosis and treatment of IMNM.

« Back to ACR Convergence 2025

ACR Meeting Abstracts - https://acrabstracts.org/abstract/in-immune-mediated-necrotising-myopathy-anti-hmgcr-antibodies-inhibit-hmgcr-activity-leading-to-the-sarcoplasmic-accumulation-of-lipid-droplets-and-myofibres-necrosis/