Background: Peripartum cardiomyopathy (PPCM) is a life-threatening disease in women without known cardiovascular disease and is characterized by a sudden onset of heart failure before or after delivery. Previous studies suggest impaired endothelial to cardiomyocyte crosstalk as a major pathomechanism of the disease. Notch signaling is essential for this crosstalk, and its role in the pathophysiology of PPCM was analyzed.

Methods and results: Serum (sampled at diagnosis) from PPCM patients reduced Notch1 expression in induced pluripotent stem cell derived cardiomyocytes (iPSC-CM, -20%, P<0.05) compared to serum from postpartum-matched healthy controls (PP-Ctrl). MicroRNA profiling in plasma from PPCM patients revealed significant upregulation of miRNAs (miR-146a, 6.8-fold, P<0.01), described to target Notch1 signaling compared to PP-Ctrl. Cardiomyocyte (CM) Notch1 expression is reduced in the experimental PPCM mouse model (cardiac-specific STAT3-knockout (STAT3-KO)) in part due to 16-kDa prolactin-mediated upregulation of miRNA-146a. Treatment with the prolactin blocker bromocriptine (BR) prevented Notch1 reduction in postpartum STAT3-KO mice. Mice with a cardiomyocyte-restricted knockout of Notch1 (aMHC-Cre^tg/+;Notch1^flox/flox, N1-CKO) are born at the expected Mendelian ratio and nulli pari (NP) N1-CKO female mice do not show a distinct cardiac phenotype at the age of 3 to 4 months compared to WT female mice (%FS WT: 41±5 vs N1-CKO: 37±6; n= 9-12). However, they develop mild heart failure at the age of 6 months (%FS WT: 44±4 vs N1-CKO: 31±8; n=11-14; P<0.01). Mated N1-CKO females displayed normal cardiac function and LV dimensions in the last week of the first pregnancy. After 3 pregnancies and nursing periods (average age 6 months), N1-CKO developed severe heart failure (%FS WT PP: 30±5 vs N1-CKO PP: 16±5; n=8-12; P<0.01), and show enhanced mortality (P<0.01). N1-CKO-NP and N1-CKO-PP displayed significant increase in the cardiac expression of the stress markers ANP and BNP, and in Col1A1 compared to respective WT-NP and WT-PP. RNA-Seq analysis of N1-CKO-PP revealed 1380 transcripts (adj. P<0.01) differently regulated genes, among them down-regulation of ErbB4 and dystrophin (Dmd). Treatment with BR was not able to improve survival and to rescue cardiac function and LV dimensions in postpartum N1-CKO females (WT: 37±4, N1-CKO: 23±10; n=7-8). Moreover, RNA-Seq analysis revealed that BR had no effect on the dysregulated cardiac mRNA.

In conclusion, the pathophysiology of PPCM is associated with reduced cardiac Notch1 expression, which results in part from 16-kDa-prolactin mediated upregulation of miRNAs targeting Notch1. The reduced cardiac Notch1 expression contributes to PPCM as indicated with severe postpartum heart failure and high mortality in N1-CKO mice. Inhibition of prolactin secretion with BR failed to rescue the development of PPCM in N1-CKO mice suggesting that deregulated Notch1 is downstream of the 16-kDa prolactin-miRNA pathophysiology. Whether genetic alterations in Notch1 signaling and its downstream targets represent a novel subgroup of PPCM is currently analyzed in our exome sequencing project of PPCM patients.