Fig. 2. Histological findings of hnRNPA2B1 myopathy.

a Hematoxylin and eosin (H&E) image of right rectus femoris muscle biopsy obtained at 8 years of age from patient 1 with c.992delG, p.(G331Efs*28) showing myofiber atrophy, fiber size variability, and internalized nuclei with several fibers with rimmed subsarcolemmal vacuoles (arrow). b Modified Gömöri trichrome (GT) staining highlights similar findings and rimmed vacuoles (arrow) (patient 1 muscle biopsy). c–f Vacuole contents (arrow) do not stain positive with NADH, suggesting they are devoid of mitochondria (c) and do not show increased non-specific esterase (NSE) activity (d). The vacuoles (arrow) do not contain glycoproteins based on periodic acid–Schiff (PAS) stain (f) but have increased acid phosphatase (AcidPh) activity (arrow) (e), suggesting they are lysosomal or autophagic in origin. NSE stain (d) also highlights a few angular atrophic fibers, suggestive of mild acute neurogenic atrophy. c–f are from patient 2 muscle biopsy. g, h Electron microscopy (EM) studies showing marked autophagic changes (black arrow) and vacuoles containing membranous myelin-like whorls. i, j Many myofibers contain areas with ~15–20-nm thick, tubulofilamentous inclusions (i, white arrow and inset), which on occasion were also seen within the nuclei (Nu) near the vacuoles (j, white arrow). g–i are from patient 2 muscle biopsy, j is from patient 8 muscle biopsy. k–o Immunofluorescence staining of muscle biopsy of patient 1 with c.992delG, p.(G331Efs*28) showing cytoplasmic hnRNPA2/B1-positive inclusions that partially co-localize with P62 (k), ubiquilin 2 (m), TIA1 (n), and TDP-43 (o). Co-localization with ubiquitin (l) is restricted to very few perinuclear and cytoplasmic puncta (arrows). Scale bars: a–f 25 µm; g 2 µm; h–j 500 nm; k–o 20 µm. The micrographs shown are representative images of a single diagnostic muscle biopsy in each indicated patient. No independent replicates were performed.