Background: Hypertrophic cardiomyopathy (HCM) is the most common genetically inherited cardiovascular disease that affects one in 500 people.1 Variants in MYBPC3 result in premature termination codons, ultimately causing haploinsufficiency in MyBP-C protein–a hallmark of familial HCM pathophysiology.2,3,4,5 Recent GWAS reports identify risk alleles for HCM that are concordant with left ventricular (LV) functional traits of decreased left ventricular chamber volumes and increased ejection fraction, and in contrast, are protective alleles for dilated cardiomyopathy.6 Several of the top HCM risk alleles encode for co-chaperones of HSP70 (Fig. 1).7,8,9,10 However, how these co-chaperones modulate sarcomeric protein turnover and HCM pathophysiology is unknown.

Hypothesis: I hypothesize that HSP70 co-chaperones identified by recent GWAS can modulate cardiac contractility by regulating sarcomeric and Z-disc steady state expression.

Methods and Results: Human induced pluripotent stem cell cardiomyocytes (hiPSC-CMs) were transduced with GFP-tagged adenovirus (AdV) expressing shRNA targeted against BAG3, DNAJC18, HSPB7, or scrambled shRNA (control) with >70% transduction efficiency and no cellular toxicity.11 BAG3, DNAJC18, and HSPB7 were reduced by ≥50% at both the transcript and protein levels as measured via RT-qPCR and western blot.12 BAG3 KD reduced MyBP-C protein levels (p<0.0001) by 53%, along with most other profiled sarcomeric and Z-disc proteins (p<0.05). DNAJC18 had no significant effect on MyBP-C protein levels, but caused a decrease in tropomyosin (p<0.0001) and myosin light chain 3 (p<0.0001). HSPB7 KD increased MyBP-C protein levels (p<0.01) by 46%. Myosin (p<0.01), tropomyosin (p<0.05), and myosin light chain 2 (p<0.05) also showed increased expression upon HSPB7 knockdown. Co-chaperone KD also caused transcriptome-wide changes identified by RNA-seq. BAG3 was overexpressed (OE) via an adeno-associated virus (AAV) resulted in an increase in MyBP-C (p<0.01).

Conclusions/Future Directions: GWAS-identified HSP70 co-chaperones BAG3, DNAJC18, and HSPB7 regulate steady state expression of sarcomeric and Z-disc proteins. Further studies will be needed to elucidate how whether therapeutic modulation of these co-chaperones can be leveraged to stabilize MyBP-C and other sarcomeric and Z-disc proteins in patients with familial HCM.