Muscle Ring Finger 1 (MuRF1) and MuRF2 Regulate Gene Expression Mediated by the E2F Transcription Factors and are Necessary but Functionally Redundant During Developmental Cardiac Growth In Vivo

We recently reported MuRF1 and MuRF2 are functionally redundant during cardiac development. Specifically, mice missing all four MuRF1 and MuRF2 alleles [MuRF1/MuRF2 double null (DN)] were born with a massive spontaneous hypertrophic cardiomyopathy and heart failure; mice that were null for one of the genes, but heterozygous for the other (i.e. MuRF1−/− //MuRF2+/− or MuRF1+/−//MuRF2−/−) were phenotypically identical to wild type mice. Microarray analysis of genes differentially expressed between MuRF1/MuRF2 DN, mice missing 3 of the four alleles, and wild type mice revealed a significant enrichment of genes regulated by the E2F transcription factor family. Over 85% of the differentially expressed genes had E2F promoter regions (E2f:DP; p<0.001). Western analysis of E2F revealed no differences between MuRF1/MuRF2 DN hearts and wild type hearts; however, chromatin IP studies revealed that MuRF1/MuRF2 DN hearts had significantly less binding of E2F1 in the promoter regions of genes previously defined to be regulated by E2F1 (p21, Brip1, and PDK4, p<0.01). These studies suggest that MuRF1 and MuRF2 play a redundant role in regulating developmental physiologic hypertrophy, by regulating E2F transcription factors essential for normal cardiac development by supporting E2F localization to the nucleus, but not through a process that degrades the transcription factor.