CRISPR/Cas9-Mediated Constitutive Loss of VCP (Valosin-Containing Protein) Impairs Proteostasis and Leads to Defective Striated Muscle Structure and Function In Vivo

Valosin-containing protein (VCP) acts as a key regulator of cellular protein homeostasis by coordinating protein turnover and quality control. Mutations in VCP lead to (cardio-)myopathy and neurodegenerative diseases such as inclusion body myopathy with Paget’s disease of the bone and frontotemporal dementia (IBMPFD) or amyotrophic lateral sclerosis (ALS). To date, due to embryonic lethality, no constitutive VCP knockout animal model exists. Here, we generated a constitutive CRISPR/Cas9-induced vcp knockout zebrafish model. Similar to the phenotype of vcp morphant knockdown zebrafish embryos, we found that vcp-null embryos displayed significantly impaired cardiac and skeletal muscle function. By ultrastructural analysis of skeletal muscle cells and cardiomyocytes, we observed severely disrupted myofibrillar organization and accumulation of inclusion bodies as well as mitochondrial degeneration. vcp knockout was associated with a significant accumulation of ubiquitinated proteins, suggesting impaired proteasomal function. Additionally, markers of unfolded protein response (UPR)/ER-stress and autophagy-related mTOR signaling were elevated in vcp-deficient embryos, demonstrating impaired proteostasis in VCP-null zebrafish. In conclusion, our findings demonstrate the successful generation of a stable constitutive vcp knockout zebrafish line that will enable characterization of the detailed mechanistic underpinnings of vcp loss, particularly the impact of disturbed protein homeostasis on organ development and function in vivo.

Spatial and Functional Distribution of MYBPC3 Pathogenic Variants and Clinical Outcomes in Patients With Hypertrophic Cardiomyopathy

Supplemental Digital Content is available in the text.

Pathogenic variants in MYBPC3, encoding cardiac MyBP-C (myosin binding protein C), are the most common cause of familial hypertrophic cardiomyopathy. A large number of unique MYBPC3 variants and relatively small genotyped hypertrophic cardiomyopathy cohorts have precluded detailed genotype-phenotype correlations.

HSC70 is a chaperone for wild-type and mutant cardiac myosin binding protein C

Cardiac myosin binding protein C (MYBPC3) is the most commonly mutated gene associated with hypertrophic cardiomyopathy (HCM). Haploinsufficiency of full-length MYBPC3 and disruption of proteostasis have both been proposed as central to HCM disease pathogenesis. Discriminating the relative contributions of these 2 mechanisms requires fundamental knowledge of how turnover of WT and mutant MYBPC3 proteins is regulated. We expressed several disease-causing mutations in MYBPC3 in primary neonatal rat ventricular cardiomyocytes. In contrast to WT MYBPC3, mutant proteins showed reduced expression and failed to localize to the sarcomere. In an unbiased coimmunoprecipitation/mass spectrometry screen, we identified HSP70-family chaperones as interactors of both WT and mutant MYBPC3. Heat shock cognate 70 kDa (HSC70) was the most abundant chaperone interactor. Knockdown of HSC70 significantly slowed degradation of both WT and mutant MYBPC3, while pharmacologic activation of HSC70 and HSP70 accelerated degradation. HSC70 was expressed in discrete striations in the sarcomere. Expression of mutant MYBPC3 did not affect HSC70 localization, nor did it induce a protein folding stress response or ubiquitin proteasome dysfunction. Together these data suggest that WT and mutant MYBPC3 proteins are clients for HSC70, and that the HSC70 chaperone system plays a major role in regulating MYBPC3 protein turnover.

Abstract 385: Cardiac Myosin Binding Protein-C Mutants Disrupt Ubiquitin Proteasome System and Hsc70 Functions

The most commonly mutated gene in hypertrophic cardiomyopathy (HCM) is cardiac myosin binding protein C (MYBPC3). Over 90% of MYBPC3 mutations are nonsense, but whether these mutations manifest in loss- or gain-of-function is unresolved. Evidence suggests MYBPC3 mutants impact protein quality control mechanisms. The objective of this study was to evaluate interactions of MYBPC3 with proteostatic systems and test the hypothesis that these interactions affect protein homeostasis in cardiomyocytes.

WT and mutant MYBPC3 constructs were expressed in neonatal rat ventricular myocytes (NRVMs) via adenovirus. Mutant MYBPC3 induced ubiquitin proteasome system reporter GFPu accumulation (fold increase in GFPu-positive cells vs control: WT 138±14.0%, mutant 198±27.2%, mean±SEM, p<0.05 vs control and WT), indicating proteasome dysfunction. Affinity purification/mass spectrometry identified molecular chaperones Hsp70 and Hsc70 as prominent interactors with MYBPC3. We observed MYBPC3 degradation by cycloheximide chase in response to Hsc70 siRNA knockdown or pharmacological treatment with Hsp70 activator YM-1. Hsc70 knockdown slowed degradation of WT and mutant MYBPC3 (WT control t ½ =5.47±0.70 hr, WT Hsc70 knockdown t ½ =13.5±1.62 hr; mutant control t ½ =3.42±0.61 hr, mutant Hsc70 knockdown t ½ =9.87±0.95 hr), while YM-1 treatment accelerated degradation (WT DMSO t ½ =10.2±3.28 hr, WT YM-1 t ½ =3.16±0.61 hr; mutant DMSO t ½ =11.7±2.67 hr, mutant YM-1 t ½ =1.37±0.16 hr). We then evaluated whether transferrin uptake via clathrin mediated endocytosis, a critical Hsc70-dependent activity, was affected by mutant MYBPC3. Transferrin uptake was significantly decreased in NRVMs expressing mutant MYBPC3 compared to WT and untreated controls (transferrin-positive cells: control 22.93±3.34%, WT 17.47±0.70%, mutant 9.30±1.63%, mean±SEM, p<0.05 vs control and WT).

In conclusion, we have demonstrated that Hsp70 chaperones interact with MYBPC3 in cardiomyocytes and affect MYBPC3 degradation, suggesting MYBPC3 is a client of Hsp70 and Hsc70. Additionally, expression of mutant MYBPC3 causes ubiquitin proteasome impairment and interferes with normal Hsc70 function. These results support our hypothesis that mutant MYBPC3 affects protein homeostasis in HCM.

Sarcomere mutation-specific expression patterns in human hypertrophic cardiomyopathy

BACKGROUND

Heterozygous mutations in sarcomere genes in hypertrophic cardiomyopathy (HCM) are proposed to exert their effect through gain of function for missense mutations or loss of function for truncating mutations. However, allelic expression from individual mutations has not been sufficiently characterized to support this exclusive distinction in human HCM.

METHODS AND RESULTS

Sarcomere transcript and protein levels were analyzed in septal myectomy and transplant specimens from 46 genotyped HCM patients with or without sarcomere gene mutations and 10 control hearts. For truncating mutations in MYBPC3, the average ratio of mutant:wild-type transcripts was ≈1:5, in contrast to ≈1:1 for all sarcomere missense mutations, confirming that nonsense transcripts are uniquely unstable. However, total MYBPC3 mRNA was significantly increased by 9-fold in HCM samples with MYBPC3 mutations compared with control hearts and with HCM samples without sarcomere gene mutations. Full-length MYBPC3 protein content was not different between MYBPC3 mutant HCM and control samples, and no truncated proteins were detected. By absolute quantification of abundance with multiple reaction monitoring, stoichiometric ratios of mutant sarcomere proteins relative to wild type were strikingly variable in a mutation-specific manner, with the fraction of mutant protein ranging from 30% to 84%.

CONCLUSIONS

These results challenge the concept that haploinsufficiency is a unifying mechanism for HCM caused by MYBPC3 truncating mutations. The range of allelic imbalance for several missense sarcomere mutations suggests that certain mutant proteins may be more or less stable or incorporate more or less efficiently into the sarcomere than wild-type proteins. These mutation-specific properties may distinctly influence disease phenotypes.