The RNA-binding protein Quaking is essential for cardiac homeostasis and function by regulating Morf4l2 splicing.

[BACKGROUND] Lower levels of Qki were reported in human and mouse-failing hearts, implicating its involvement in cardiac diseases. However, the molecular and functional effects of its downregulation in adult myocardium remain largely unknown.

[OBJECTIVE] We aim to uncover the effects of Qki knockdown in adult hearts.

[METHODS & RESULTS] Here we show that AAV9-mediated knockdown of Qki by shRNAs in the hearts of adult BALB/c mice led to cardiac malfunction, atrophy, apoptosis, heart failure, and death within two weeks. Global transcriptomic analysis of Qki knockdown hearts revealed significant dysregulation of 996 alternative splicing events upon Qki knockdown. Mechanistically, we discovered that loss of Qki promotes the exclusion of the third exon of Morf4l2, leading to higher expression of exon three excluded variant (Morf4l2Δex3). Like rodents, the RNA-seq dataset from 108 human hearts revealed a lower splice junction count of MORF4L2 exon three in hearts with low levels of QKI compared to subjects with higher QKI levels. Specific knockdown of Morf4l2Δex3 rescues Qki knockdown-induced cardiac cachexia and improves cardiac function. Moreover, Morf4l2Δex3 was increased in the colon cancer-induced cardiac cachexia mouse model, and its inhibition prevented cardiac cachexia and improved cardiac function. Mechanistically, exon three of Morf4l2 lies in the 5'UTR, and its exclusion leads to higher expression of MORF4L2 upon Qki knockdown due to the lack of a G2-quadruplex. Importantly, MORF4L2 protein sequence and localization were not affected by alternative splicing as exon three lies in the 5'UTR. We found that MORF4L2 is a chromatin-bound protein and regulates H3K27ac.

[CONCLUSION] Qki knockdown in the adult heart leads to cardiac cachexia due to the alteration of Morf4l2 splicing. Inhibition of Morf4l2Δex3 inhibits cancer-induced cardiac cachexia, demonstrating it as a potential therapeutic target.