Expanding the Molecular and Pathologic Spectrum of HSPB8 Myopathy and Distal Motor Neuropathy

Figures

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  Figure 1. Patient 1's Muscle Biopsies Supraspinatus biopsy (A–G). (A and B) Hematoxylin-eosin–stained sections showing internalized nuclei (A, asterisk), rare necrotic (A, arrowhead) fibers, rimmed vacuoles (B, arrows) in atrophic and nonatrophic fibers, and increased endomysial connective tissue (B, double dagger). (C) Gomori trichrome–stained section showing a fiber (asterisk) with a rimmed vacuole (arrow), which overreacts for p62 (D, arrow) and TDP43 (not shown). (E) Small p62-positive aggregates also occurring in nonvacuolated fibers (arrow). (F) Small punctuate TIA positivity occurring in the sarcoplasm of the same vacuolated fiber shown in (C). (G) Congo red–stained section viewed under rhodamine optics, demonstrating a small congophilic inclusion (arrow). No Z-disk myofibril protein aggregates were observed; exceedingly rare vacuolated fibers were faintly and diffusely overreacting for myotilin and alpha-B crystallin (not shown). (H) Tibialis anterior biopsy. ATPase-stained section (pH 9.4) showing marked predominance and grouping of type 1 fibers with only a few type 2 fibers (arrows). Magnification: ×40 (A–K) and ×10 (L).

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  Figure 2. Patient 2's Vastus Lateralis Biopsy (A) Hematoxylin-eosin–stained section showing a fiber with rimmed vacuoles (asterisk) and atrophic fibers (small fibers). (B) Congo red–stained section demonstrating small congophilic inclusions (bright red) within a vacuolated fiber (asterisk). No large congophilic inclusions, as often seen in classic myofibrillar myopathy, were seen. (C) Gomori trichrome–stained section highlighting amorphous, dark, small (asterisks), and large (arrowhead) inclusions and a rimmed vacuole (arrow), which (D) overreacts for TIA1. (E) TDP-43 positivity occurring in the large inclusion (arrowhead) and vacuole (arrow). (F) A different fiber in the Gomori trichrome–stained section, with inclusions conferring a lace-like appearance (arrowhead) overreacting for (G) TIA1 and (H) p62, but not for TPD-43 (not shown). (I) In other fibers, the amorphous areas (arrow) seen in Gomori trichrome–stained sections, overreact for (J) NADH dehydrogenase, (K) myotilin, and (L) alpha-B crystallin. Magnification: ×40.

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  Figure 3. Schematic Representation of HSPB8 Gene, Protein, Previously Reported and Novel Variants (A), and Molecular Dynamics Simulations of the HSBP8 Homodimer Structure (B–E) (A). The top panel represents the gene with its 3 exons [exon 1: amino acid (aa) 1–123, exon 2: aa 123–144, exon 3: aa 144–196] while the bottom panel is a schematic representation of the protein structure. The distinct colors indicate the associated phenotype. The novel variant in patient 1 is indicated in red. NTD = N-terminal domain; ACD = alpha-crystallin domain; CTD = C-terminal domain. (B). Structural model of the HSBP8 homodimer generated using AlphaFold 3. The 2 chains are shown in green and cyan. Residue Gly62 is highlighted as a red sphere. The close-up view on the right emphasizes the hydrogen bonds formed by Lys141, Leu31, and Asp33. (C) Backbone RMS deviation of HSBP8 wild type and p.Lys141Glu mutant during an 800-ns MD simulation. (D). Total Coulombic energy and (E) total van der Waals energy of the well-folded region of HSBP8 (residues 93–170), calculated over the 400–800 ns segment of the MD simulations. Wild type (WT) is shown in black; the p.Lys141Glu mutant is shown in red.