Truncated small chaperone HSPB1 in peripheral neuropathy: molecular mechanisms and altered cellular pathways

Abstract

HSPB1 is a member of a family; small heat shock proteins (sHSP). These sHSP have a common conserved α-crystallin domain (ACD), flanked by variable N- and C- termini, whose functions are poorly understood. More than 20 disease causing mutations in sHSPs has been described till date, most of them being dominant missense mutations clustered around the ACD. The protein is ubiquitously expressed, participates in a number of cellular processes, and different pathogenic mechanisms have been proposed. In this study, we investigated the cellular, transcriptional and biochemical consequences of a novel heterozygous frame shift mutation in HSPB1 p.(M169Cfs2X) predicting C-terminal truncation of the protein in a patient with dominantly inherited motor predominant axonal Charcot-Marie-Tooth disease. We show that the truncated protein is stable in primary fibroblasts, proving that the mutant mRNA is able to escape nonsense mediated decay. Moreover, the truncated protein binds wild type HSPB1 and interferes with its dimerization. The mutant protein translocates to nucleus and impairs the heat tolerance of patient primary fibroblasts. However, the mutant HSPB1 does not alter the change in global transcriptional response to heat. We suggest that ablation of the HSPB1 C-terminus might have caused neuropathy by a dominant negative mechanism that prevents normal HSPB1 dimerization. The impairment of the cellular growth is mediated by the inability of truncated HSPB1 to chaperone proteins under heat stress.