The Spectrum of Small Heat Shock Protein B8 (HSPB8)-Associated Neuromuscular Disorders

Abstract

The heat shock protein B8 (HSPB8) is one of the small heat shock proteins (sHSP or HSPB) and is a ubiquitous protein in various organisms, including humans. It is highly expressed in skeletal muscle, heart, and neurons. It plays a crucial role in identifying misfolding proteins and participating in chaperone-assisted selective autophagy (CASA) for the removal of misfolded and damaged, potentially cytotoxic proteins. Mutations in HSPB8 can cause distal hereditary motor neuropathy (dHMN), Charcot-Marie-Tooth (CMT) disease type 2L, or myopathy. The disease can manifest from childhood to mid-adulthood. Most missense mutations in the N-terminal and α-crystallin domains of HSPB8 lead to dHMN or CMT2L. Frameshift mutations in the C-terminal domain (CTD), resulting in elongation of the HSPB8 C-terminal, cause myopathy with myofibrillar pathology and rimmed vacuoles. Myopathy and motor neuropathy can coexist. HSPB8 frameshift mutations in the CTD result in HSPB8 mutant aggregation, which weakens the CASA ability to direct misfolded proteins to autophagic degradation. Cellular and animal models indicate that HSPB8 mutations drive pathogenesis through a toxic gain-of-function mechanism. Currently, no cure is available for HSPB8-associated neuromuscular disorders, but numerous therapeutic strategies are under investigation spanning from small molecules to RNA interference to exogenous HSPB8 delivery.

Keywords: CASA; CMT2L; CTM2; HMN; HSPB8; dHMN; myofibrillar myopathy; myopathy; rimmed vacuoles.

Figure 1

Patient with HSPB8-myopathy. Patient manifested with proximal lower limb muscle weakness that later extended to upper limb and axial muscles. (A) He had difficulty elevating the arms and scoliosis. (B) Quadriceps is atrophic and weak as suggested by his difficulty extending the knees. (C) Calf muscles, although atrophic, had preserved strength, and he was able to stand on toes. (This patient was previously reported: Nicolau et al. [24]).

Figure 2

Quadriceps biopsy from patient with HSBP8-myopathy (p.Thr194Serfs∗23). (A) Many fibers harboring rimmed vacuoles (arrows) unevenly distributed across the muscle section and a regenerating fiber (arrowhead) (hematoxylin-eosin). (B) A necrotic fiber invaded by macrophages (arrow; trichrome). (C) In a region of the specimen, a few fibers showing a focal loss (arrow) or focal increase of nicotinamide adenine dinucleotide tetrazolium reductase (NADH) enzyme reactivity. (D) One (arrow) of the rare fibers containing two small congophilic inclusions (bright red; Congo red). A few fibers with myotilin aggregates (E), punctuate desmin ((F), arrow) or TIA1 ((G), arrow) positivity. (H) A fiber with dystrophin aggregates. (I) Lack of fiber type grouping pointing away from coexisting reinnervation (ATPase pH 4.3, type 1 fibers stain brown). (This patient was previously reported: Nicolau et al. [24]). Magnification: 40× (A–H); 10× (I).

Figure 3

Schematic representation of HSPB8, protein domains, and mutations causing neuromuscular diseases. p.P90L, p.N138T, and p.K141M have been associated with dHMN; p.K141T has been associated with CMT2L; p.K141N has been associated with dHMN and CMT2L; p.K141E has been reported in neuromyopathy. p.Q170Gfs*45, p.P173Sfs*43, p.T176Wfs*38, p.T194Sfs*23, and p.G192Afs*55 in CTD have been associated with myopathy; p.P173Sfs*43 has been associated with neuromyopathy.

Figure 4

Schematic representation of HSPB8’s structural and functional role in chaperone-assisted selective autophagy (CASA). Hydrophobic (HP) residues are exposed on a misfolded protein and recognized by the alpha-crystallin domain of HSPB8, which is modulated by phosphorylation of key residues T87, S27, S24, and S57. HSPB8 recruits BAG3 through interaction with its IV-X-IV (IPV) motif. HSPA (HSP70) binds the BAG domain of BAG3 and recognizes the KFERQ motif on the misfolded substrate, allowing for further stabilization of the misfolded protein. STUB1 is recruited to the complex, interacting with HSPA through its tetratricopeptide repeat (TPR) domain, and polyubiquitinates the misfolded protein. SQSTM1 recognizes the ubiquitinated substrate and serves as an adaptor protein linking the misfolded protein to the autophagic membrane through interaction with LC3. The misfolded protein subsequently undergoes autophagic degradation.