A CMT2 family carrying the P7R mutation in the N- terminal region of the HSPB1 gene

Charcot-Marie-Tooth disease type 2F associated with biallelic HSPB1 mutations

Objective

This work aims to expand knowledge regarding the genetic spectrum of HSPB1‐related diseases. HSPB1 is a gene encoding heat shock protein 27, and mutations in HSPB1 have been identified as the cause of axonal Charcot–Marie–Tooth (CMT) disease type 2F and distal hereditary motor neuropathy (dHMN).

Methods

Two patients with axonal sensorimotor neuropathy underwent detailed clinical examinations, neurophysiological studies, and next‐generation sequencing with subsequent bioinformatic prioritization of genetic variants and in silico analysis of the likely causal mutation.

Results

The HSPB1 p.S135F and p.R136L mutations were identified in homozygosis in the two affected individuals. Both mutations affect the highly conserved alpha‐crystallin domain and have been previously described as the cause of severe CMT2F/dHMN, showing a strictly dominant inheritance pattern.

Interpretation

Thus, we report for the first time two cases of biallelic HSPB1 p.S135F and p.R136L mutations in two families.

A novel HSPB1 mutation associated with a late onset CMT2 phenotype: Case presentation and systematic review of the literature

Mutations in the HSPB1 gene are associated with Charcot-Marie-Tooth (CMT) disease type 2F (CMT2F) and distal hereditary motor neuropathy type 2 (dHMN2). More than 18 pathogenic mutations spanning across the whole HSPB1 gene have been reported. Three family members with a novel p.P57S (c.169C>T) HSPB1 mutation resulting in a late onset axonal neuropathy with heterogeneous clinical and electrophysiological features are detailed. We systematically reviewed published case reports and case series on HSPB1 mutations. While a genotype-phenotype correlation was not obvious, we identified a common phenotype, which included adult onset, male predominance, motor more frequently than sensory involvement, distal and symmetric distribution with preferential involvement of plantar flexors, and a motor and axonal electrophysiological picture.

Mutations in HspB1 and hereditary neuropathies

Charcot-Marie-Tooth (CMT) disease is major hereditary neuropathy. CMT has been linked to mutations in a range of proteins, including the small heat shock protein HspB1. Here we review the properties of several HspB1 mutants associated with CMT. In vitro, mutations in the N-terminal domain lead to a formation of larger HspB1 oligomers when compared with the wild-type (WT) protein. These mutants are resistant to phosphorylation-induced dissociation and reveal lower chaperone-like activity than the WT on a range of model substrates. Mutations in the α-crystallin domain lead to the formation of yet larger HspB1 oligomers tending to dissociate at low protein concentration and having variable chaperone-like activity. Mutations in the conservative IPV motif within the C-terminal domain induce the formation of very large oligomers with low chaperone-like activity. Most mutants interact with a partner small heat shock protein, HspB6, in a manner different from that of the WT protein. The link between the altered physico-chemical properties and the pathological CMT phenotype is a subject of discussion. Certain HspB1 mutations appear to have an effect on cytoskeletal elements such as intermediate filaments and/or microtubules, and by this means damage the axonal transport. In addition, mutations of HspB1 can affect the metabolism in astroglia and indirectly modulate the viability of motor neurons. While the mechanisms of pathological mutations in HspB1 are likely to vary greatly across different mutations, further in vitro and in vivo studies are required for a better understanding of the CMT disease at molecular level.

Small heat shock proteins in neurodegenerative diseases

Small heat shock proteins are ubiquitously expressed chaperones, yet mutations in some of them cause tissue-specific diseases. Here, we will discuss how small heat shock proteins give rise to neurodegenerative disorders themselves while we will also highlight how these proteins can fulfil protective functions in neurodegenerative disorders caused by protein aggregation. The first half of this paper will be focused on how mutations in HSPB1, HSPB3, and HSPB8 are linked to inherited peripheral neuropathies like Charcot-Marie-Tooth (CMT) disease and distal hereditary motor neuropathy (dHMN). The second part of the paper will discuss how small heat shock proteins are linked to neurodegenerative disorders like Alzheimer's, Parkinson's, and Huntington's disease.