Fig. 5: Dosage-dependent changes in delB pacemaker cardiomyocytes and concomitant gain of PITX2 expression and loss of ISL1 expression.

a Fetal (E17.5) delB/delB SAN nuclei exhibit variable ISL1 and PITX2 expression levels. Scale is 100 μm and panel scale is 10 μm. b Violin plots showing the quantification of normalized mean gray value per nucleus in wild-type (N = 3; n = 223), delB/+ (N = 3; n = 176) and delB/delB (N = 3; n = 452) fetal SANs (two-sided Kolmogorov-Smirnov test, p < 0.0001). There is an overall incremental gain in PITX2 and reduced ISL1 expression levels in both the delB/+ and delB/delB SAN, and a progressive shift in PITX2:ISL1 expression balance in the delB/+ and delB/delB fetal SAN towards less ISL1 and more PITX2. Red line indicates median. Two-sided Kolmogorov-Smirnov test p < 0.0001. c Relative expression levels of genes of interest show stepwise, Pitx2-correlated changes in the expression levels of pacemaker- and working myocardium-associated transcription factors and ion channels in the delB/+ (n = 6) and delB/delB (n = 3) SAN compared to the wild-type (n = 3) SAN. Wald test corrected for multiple comparisons using the Benjamini-Hochberg method. d Scatterplot showing the relative expression of genes in the delB/+ and delB/delB HCN4^low subdomains compared to the wild-type HCN4+ SAN demonstrates that while the delB/+ HCN4^low subdomains are less severely affected than their delB/delB counterparts, differentially expressed genes behave similarly. Wald test corrected for multiple comparisons using the Benjamini-Hochberg method. e Variable PITX2 expression levels in the delB/delB HCN4^high and HCN4^low subdomain at E17.5. Scale is 100 μm, and panel scale is 10  μm. f Fraction of total nuclei in HCN4^high and HCN4^low subdomain in equivalent sections of the delB/+ (n = 6) and delB/delB (n = 6) fetal SAN head domain. Two-tailed paired T test. Source data are provided in the Source Data file. E embryonic day, SAN sinus node, L2FC Log₂Fold-change.