Mutations in the molecular co-chaperone Bcl2-associated athanogene 3 (BAG3) are found to cause dilated cardiomyopathy (DCM), resulting in systolic dysfunction and heart failure, as well as myofibrillar myopathy (MFM), which is characterized by protein aggregation and myofibrillar disintegration in skeletal muscle cells. Here, we generated a CRISPR/Cas9-induced Bag3 knockout zebrafish line and found the complete preservation of heart and skeletal muscle structure and function during embryonic development, in contrast to morpholino-mediated knockdown of Bag3. Intriguingly, genetic compensation, a process of transcriptional adaptation which acts independent of protein feedback loops, was found to prevent heart and skeletal muscle damage in our Bag3 knockout model. Proteomic profiling and quantitative real-time PCR analyses identified Bag2, another member of the Bag protein family, significantly upregulated on a transcript and protein level in bag3^-/- mutants. This implied that the decay of bag3 mutant mRNA in homozygous bag3^-/- embryos caused the transcriptional upregulation of bag2 expression. We further demonstrated that morpholino-mediated knockdown of Bag2 in bag3^-/- embryos evoked severe functional and structural heart and skeletal muscle defects, which are similar to Bag3 morphants. However, Bag2 knockdown in bag3^+/+ or bag3^+/- embryos did not result in (cardio-)myopathy. Finally, we found that inhibition of the nonsense-mediated mRNA decay (NMD) machinery by knockdown of upf1, an essential NMD factor, caused severe heart and skeletal muscle defects in bag3^-/- mutants due to the blockade of transcriptional adaptation of bag2 expression. Our findings provide evidence that genetic compensation might vitally influence the penetrance of disease-causing bag3 mutations in vivo.

发现分子伴侣伴侣Bcl2相关的致癌基因3（BAG3）中的突变会引起扩张型心肌病（DCM），导致收缩功能障碍和心力衰竭，以及以蛋白质聚集和肌原纤维为特征的肌原纤维肌病（MFM）。在骨骼肌细胞中解体。在这里，我们生成了一个由CRISPR / Cas9诱导的Bag3敲除斑马鱼品系，并发现在胚胎发育过程中心脏和骨骼肌的结构和功能得到了完全保留，这与吗啉代介导的Bag3敲除相反。有趣的是，在我们的Bag3基因敲除模型中，发现遗传补偿是一种转录适应过程，其独立于蛋白质反馈环而起作用，可防止心脏和骨骼肌的损伤。蛋白质组分析和实时定量PCR分析确定了Bag2，bag3 ^-/-突变体。这暗示纯合的bag3 ^-/-胚胎中bag3突变体mRNA的衰减导致bag2表达的转录上调。我们进一步证明了吗啡介导的bag3 ^-/-胚胎中Bag2的敲低引起了严重的功能和结构性心脏和骨骼肌缺陷，与Bag3 morphant相似。然而，袋2的敲低在BAG3 ^{+ / +}或BAG3 ^+/-胚胎没有导致（治疗心）肌病。最后，我们发现通过敲低upf1对无义介导的mRNA衰变（NMD）机制的抑制NMD是必需的NMD因子，由于对bag2表达的转录适应的阻断，导致bag3 ^-/-突变体中出现严重的心脏和骨骼肌缺陷。我们的发现提供了证据，表明遗传补偿可能会在体内严重影响引起疾病的bag3突变的渗透性。