A novel myopathy with autophagic vacuoles associated with biallelic variants in <i>CLN8</i>

Autophagic vacuoles in muscle fibers are a characteristic finding in several muscle diseases and usually indicate perturbed lysosomal protein degradation. Some of these are associated with defects in proteins directly involved in autophagy and lysosomal degradation. The gene CLN8 encodes an endoplasmic reticulum transmembrane protein, previously associated with childhood-onset neuronal ceroid lipofuscinosis (NCL), a group of lysosomal storage diseases. We describe the clinical features and results from pathology, genetic, and proteomic investigations in an adult-onset myopathy with autophagic vacuoles associated with biallelic variants in CLN8. A 40-year-old woman presented with seizures followed by transient muscle weakness and myalgia. Creatine kinase and myoglobin levels were moderately elevated. Over time, she developed progressive muscle weakness and cognitive fatigue. Muscle biopsy showed an autophagic vacuolar myopathy with fat tissue replacement and increased interstitial connective tissue. There was a marked immunohistochemical increase of markers of autophagy such as lysosomal-associated membrane protein 2 (LAMP2), microtubule-associated protein 1A/1B-light chain 3 (LC3), and sequestosome1/p62, as well as lysosomal deposition of curvilinear-like, autofluorescent material containing subunit c of mitochondrial adenosine triphosphate (ATP) synthase (mitochondrial ATP synthase membrane subunit c locus 3 [ATP5MC3/SCMAS]), typical for some forms of NCLs, including CLN8. Blood lymphocytes showed typical fingerprint inclusions. Genetic analysis revealed biallelic CLN8 variants, c.511C>T; p.P171S and c.536T>A; p.L179H. Proteomic analysis demonstrated upregulation of proteins involved in autophagy, muscle regeneration, and protein turnover. Proteins associated with oxidative phosphorylation were downregulated, except for ATP5MC3/SCMAS, which showed accumulation. In conclusion, we describe a novel myopathy with autophagic vacuoles and characteristic features of ceroid lipofuscinosis, including autophagosomal/lysosomal deposition of curvilinear-like, autofluorescent material containing ATP5MC3/SCMAS. This disease appears to be an unusual adult-onset form of CLN8.