Changes in protein function underlie the disease spectrum in patients with CHIP mutations

Abstract

Monogenetic disorders that cause cerebellar ataxia are characterized by defects in gait and atrophy of the cerebellum; however, patients often suffer from a spectrum of disease, complicating treatment options. Spinocerebellar ataxia autosomal recessive 16 (SCAR16) is caused by coding mutations in STUB1, a gene that encodes the multifunctional enzyme CHIP (C terminus of HSC70-interacting protein). The disease spectrum of SCAR16 includes a varying age of disease onset, cognitive dysfunction, increased tendon reflex, and hypogonadism. Although SCAR16 mutations span the multiple functional domains of CHIP, it is unclear whether the location of the mutation and the change in the biochemical properties of CHIP contributes to the clinical spectrum of SCAR16. In this study, we examined relationships between the clinical phenotypes of SCAR16 patients and the changes in biophysical, biochemical, and functional properties of the corresponding mutated protein. We found that the severity of ataxia did not correlate with age of onset; however, cognitive dysfunction, increased tendon reflex, and ancestry were able to predict 54% of the variation in ataxia severity. We further identified domain-specific relationships between biochemical changes in CHIP and clinical phenotypes and specific biochemical activities that associate selectively with either increased tendon reflex or cognitive dysfunction, suggesting that specific changes to CHIP–HSC70 dynamics contribute to the clinical spectrum of SCAR16. Finally, linear models of SCAR16 as a function of the biochemical properties of CHIP support the concept that further inhibiting mutant CHIP activity lessens disease severity and may be useful in the design of patient-specific targeted approaches to treat SCAR16.

CHIP mutations affect the heat shock response differently in human fibroblasts and iPSC-derived neurons

“…This provides a disease model with endogenous protein levels and the patient's own genetic background. Although mutations in STUB1_1 and STUB1_3 do not alter STUB1 transcript levels, protein stability is strongly reduced by the mutations, leading to a lower CHIP level ( Heimdal et al, 2014 ; Ronnebaum et al, 2014 ; Pakdaman et al, 2017 ; Kanack et al, 2018 ; Shi et al, 2018 ; Madrigal et al, 2019 ). STUB1_2 shows reduced transcript and protein levels.…”

“…reduced by the mutations, leading to a lower CHIP level (Heimdal et al, 2014;Ronnebaum et al, 2014;Pakdaman et al, 2017;Kanack et al, 2018;Shi et al, 2018;Madrigal et al, 2019). STUB1_2 shows reduced transcript and protein levels.…”

Clinical and pathologic phenotype of a large family with heterozygous STUB1 mutation

“…27 The U-box domain is especially important, as mutations in this domain have a stronger effect on the overall loss of CHIP function and strongly associate with cognitive dysfunction. 28 Indeed, most frameshift mutations identified so far in SCA48 are located in the U-box domain (table e-1, links.lww.com/NXG/A251). 9,11 Whether this results in nonsense mediated RNA decay or a truncated protein (with a possible dominant negative effect) remains to be elucidated.…”

CHIP as a therapeutic target for neurological diseases

“…Several studies regarding the function of mutated CHIP have reported that some STUB1 mutations mediate disease by affecting the CHIP E3 ubiquitin ligase interactions and function through modification of its oligomeric states and structural stability, which indicates that change in structure may lead to gain-or loss-of-function and thus cause disease. However, we should also note that numerous mutations have limited effects on the structure and ubiquitination activity of CHIP, suggesting that these mutations may affect CHIP function though other mechanisms [13][14][15]118 . The most intensively studied mutation site in vitro and in vivo was T246M, and these studies could serve as grounds for further research of other CHIP mutation sites.…”

“…However, it remains unclear whether there is a relationship between the location of the mutation and changes in the biological function of the corresponding mutated protein with the clinical spectrum of SCAR16. Madrigal et al reported that the mutations located in the U-box domain strongly induce loss of CHIP function and are intensely related to cognitive impairment in patients with SCAR16 118 ; TPR and CC mutations mildly affect CHIP function and are associated with an increased tendon reflex. They also reported that inhibiting the interaction between mutant CHIP and HSP70 can lead to later age of onset and less severe ataxia 118 .…”