Transcriptional Readthrough at Atf4 Locus Suppresses Rps19bp1 and Impairs Heart Development

Activating Transcription Factor 4 (ATF4) functions as a transcriptional regulator in various cell types and tissues under both physiological and pathological conditions. While previous studies have linked ATF4 activation with promoting cardiomyocyte (CM) death in dilated cardiomyopathy (DCM), atrial fibrillation, and heart failure, its role in developing CMs remains unexplored.

We generated multiple distinct CM-specific (Atf4 ^cKO(e2/3/pA) and Atf4 ^cKO(e2) ) and global Atf4 knockout (Atf4 ^7del/7del and Atf4 ^1ins/1ins ) mouse models targeting different Atf4 regions, as well as cardiomyocyte-specific deletion of Rps19bp1 to study cardiac phenotypes. Detailed morphological and molecular analyses were performed.

Atf4 ^cKO(e2/3/pA) (targeting exon 2-3 including the polyadenylation signal (polyA)) mice exhibited severe cardiac defects and died before E17.5, likely due to ectopic activation of p53 signaling pathway resulting from Rps19bp1 downregulation, a potent suppressor of p53. Further investigation revealed that deleting the polyA signal of Atf4 in Atf4 ^cKO(e2/3/pA) mice led to transcriptional readthrough, resulting in the formation of an Atf4-Cacna1i fusion transcript and Rps19bp1 downregulation. To avoid readthrough while abolishing ATF4 function, we introduced small indels into exon 3 of Atf4 in mice (Atf4 ^7del/7del and Atf4 ^1ins/1ins ), which showed normal Rps19bp1 expression and cardiac morphology. Importantly, CM-specific deletion of Rps19bp1 recapitulated the cardiac defects and transcriptional change seen in Atf4 ^cKO(e2/3/pA) mice.

We found that the downregulation of Rps19bp1, not loss of ATF4 function, underlying the cardiac phenotypes in Atf4 ^cKO(e2/3/pA) mice. The reduced expression of Rps19bp1 in Atf4 ^cKO(e2/3/pA) mice is likely due to the unintentional deletion of Atf4 polyA signal and subsequent transcriptional readthrough, underscoring the essential role of RPS19BP1, not ATF4, in cardiac development. Consistent Rps19bp1 downregulation has been observed in other tissue-specific Atf4 knockout models utilizing the Atf4 ^fl(e2/3/pA) allele, suggesting that previously reported Atf4 KO phenotypes may result from Atf4 transcriptional readthrough effects. These findings reveal a locus-dependent transcriptional interference mechanism and emphasize the importance of avoiding confounding cis effects in genetically engineered models.

Our findings clarify ATF4's role in heart development by showing that cardiac defects in cardiomyocyte-specific ATF4 knockout mice-using a widely employed floxed ATF4 line-result from unintended downregulation of RPS19BP1 caused by transcriptional readthrough. This shifts the focus from ATF4 to RPS19BP1, a key regulator of p53 activity, as a potential driver of cardiac developmental abnormalities. Clinically, these insights caution against misinterpretation of genetic knockout models and highlight RPS19BP1 as a promising target for congenital heart disease and related cardiac dysfunctions, with potential implications for future therapies.