The Mef2 transcription network is disrupted in myotonic dystrophy heart tissue, dramatically altering miRNA and mRNA expression

Abstract

Cardiac dysfunction is the second leading cause of death in myotonic dystrophy type 1 (DM1), primarily because of arrhythmias and cardiac conduction defects. A screen of more than 500 microRNAs (miRNAs) in a DM1 mouse model identified 54 miRNAs that were differentially expressed in heart. More than 80% exhibited downregulation toward the embryonic expression pattern and showed a DM1-specific response. A total of 20 of 22 miRNAs tested were also significantly downregulated in human DM1 heart tissue. We demonstrate that many of these miRNAs are direct MEF2 transcriptional targets, including miRNAs for which depletion is associated with arrhythmias or fibrosis. MEF2 protein is significantly reduced in both DM1 and mouse model heart samples, and exogenous MEF2C restores normal levels of MEF2 target miRNAs and mRNAs in a DM1 cardiac cell culture model. We conclude that loss of MEF2 in DM1 heart causes pathogenic features through aberrant expression of both miRNA and mRNA targets.

Figure 1. Mis-regulation of a subset of miRNAs in a heart-specific mouse model for DM1

(A) Schematic of an inducible heart-specific DM1 mouse model. EpA960 mice contain a transgene containing DMPK exon 15 with 960 CTG interrupted repeats that were crossed with Mer Cre Mer (MCM) (Sohal et al., 2001)mice to generate heart-specific and tamoxifen-inducible expression of CUG^exp RNA. (B) Expression profiling using quantitative real-time qRT-PCR based TaqMan arrays of >500 miRNAs in EpA960; MCM vs. MCM control mouse heart 1 wk after tamoxifen injection. The volcano plot shows up or downregulated miRNAs in DM1 mice compared to MCM controls. Data is normalized relative to U6 small nuclear (sn) RNA (n=3). (C) Adult-to-embryonic shift in miRNA expression in DM1 mice upon CUG^exp RNA expression. Pie chart summarizing miRNAs that are differentially expressed and exhibit a developmental shift towards the embryonic pattern. (D) Heat map showing developmental up regulation of 42 miRNAs during normal mouse heart development, which are down regulated at 72h and 1 wk after repeat RNA expression. (E) Reduced miRNA expression in DM1 heart tissue. Each bar represents fold change in individual miRNA expression (mean ± SD) from heart samples of adults with DM1 (n=8) relative to heart samples from unaffected individuals (n=4).(F) Developmentally regulated miRNAs do not show a coordinate reduction in expression in two distinct models of heart disease: calcineurin transgenic nA-Tg) mice and (G) wild type mice 8 wks after Transverse Aortic Constriction (TAC). Each bar represents fold change in individual miRNA expression (mean ± SD) from heart samples of CnA-Tg mice relative to littermate controls (n=3) or from mice that underwent TAC surgery relative to shams (n=3). *P< 0.05.

Figure 2. Altered miRNA expression identified in DM1 is not reproduced by loss of Mbnl1 or gain of CELF1

(A) qRT-PCR analysis of miRNA expression in hearts of tet-inducible and heart-specific CELF1 transgenic (TgCELF1) mice. Each bar represents fold change in individual miRNA expression (mean ± SD) in TgCELF1 mice relative to MHC-rtTA controls given doxycycline (dox). Data is normalized relative to U6 snRNA (n=3). (B) RT-PCR analysis monitoring percent spliced in (PSI) of two CELF1-regulated alternative splicing events, Mtmr3 exon 16 and Mfn2 exon 3 in MHCrtTA or TgCELF1 mice given dox or Mbnl1 ^{3/ 3} mice. (C) miRNA expression in hearts of Mbnl1 ^{3/ 3} relative to Mbnl1^+/+ mice showing fold change in individual miRNA expression (mean ± SD). Data is normalized relative to U6 snRNA (n=3). (D) RT-PCR analysis monitoring PSI of two Mbnl1-regulated alternative splicing events, Mbnl1 exon 5 and Tnnt2 exon 5 in Mbnl1^+/+, TgCELF1 + dox, or Mbnl1^{••3/••3} mice. (E) Reduced expression of ten primary (pri-) miRNA transcripts at 72h and 1 wk after CUG^exp RNA induction in DM1 mice. Each bar represents fold change in individual pri-miRNAs in DM1 mice relative to MCM controls at 72h or 1 wk after tamoxifen injection. (F) Reduced steady state levels of pri-miR-1-1 and pri-miR-1-2 transcripts in human heart samples from DM1 patients relative to unaffected individuals (n=3). *P< 0.05.

Figure 3. DM1 heart mouse model shows large-scale shift in gene expression that identifies disrupted Mef2 network

(A) Gene expression profiling in mouse heart development and adult DM1 mice shows a developmental reversion in mRNA expression. Heat map representation of transcripts over expressed (yellow) and under expressed (blue) in hearts of wild type adult mice, wild type embryonic day 14 (E14), and DM1 mice induced to express CUG^exp RNA for 72 h and 1 wk, when compared to MCM controls (P< 0.01, fold change >1.5). Rows, transcripts (values centered on MCM group); columns, profiled samples.(B) Ingenuity pathway analysis identified Mef2 as a key regulator of both miRNA and mRNA with altered expression in heart tissue expressing CUG^exp RNA. Cardiovascular gene function categories with P<1E-05 are highlighted in the figure.

Figure 4. Disruption of Mef2 transcription program in DM1

(A) Reduced Mef2a and Mef2c expression in heart tissue from the EpA960; MCM DM1 mouse model (n=3) and individuals with DM1 (n=8) or normal controls (n=4). Gata4 mRNA levels are not affected. Data is normalized to ribosomal protein L30 (Rpl30).(B) Western blot showing reduction in steady state MEF2A protein levels in human DM1 heart samples. CELF1 protein levels are up regulated in these samples, as previously described(Savkur et al., 2001; Timchenko et al., 2001). Quantification of relative band intensities, normalized to GAPDH levels are shown below. (C) Decreased Mef2a and Mef2c expression affects mRNA steady-state levels of Mef2 target genes in mouse DM1 heart tissue. Representative Mef2 target genes show a significant reduction in expression (light red bars) whereas Gata4 target genes are unaffected (light blue bars). *P< 0.05.

Figure 5. Identification of Mef2-regulated miRNAs in cardiac cells

RNAi-based Mef2 knockdowns coupled with chromatin immunoprecipitation (ChIP) assays identify miRNAs directly regulated by Mef2. (A)Knockdown efficiency of Mef2a and Mef2c siRNAs in HL-1 cardiac cells was determined by qRT-PCR in three independent experiments. Reduced steady state levels of Mef2 mRNA targets Myocardin (Myocd), Myomesin (Myom1) and α-T-catenin (Ctnna3) in response to Mef2A, Mef2C individual or double knockdowns (mean ± SD; n=3). (B) Reduced steady state levels of pri-miRNAs in Mef2 knockdowns. All data is plotted relative to a luciferase control siRNA and expression is normalized to Rpl30. *P< 0.05, N.D, not detected. (C) Reduced interaction of Mef2 with its primary miRNA and mRNA gene targets in DM1 mice. Quantification of genomic DNA in chromatin immunoprecipitates using Mef2 antibody in heart tissue of MCM controls and DM1 mice. The primers used for qRT-PCR assays span the Mef2 binding sites in target primary miRNAs or mRNAs. Each bar represents mean ± SD of the fraction of input detected in the Mef2 precipitates normalized to IgG precipitates (n=3).

Figure 6. Rescued expression of Mef2 miRNA and mRNAs targets in a cardiac cell DM1 model by exogenous Mef2c

(A) Experimental schematic of Mef2c rescue in CUG^exp RNA expressing cardiac cells. (B) GFP expression was detected by indirect fluorescence using anti-GFP antibody. RNA foci containing DT960 RNA were detected by FISH using Cy3-labeled probes. Nuclei were counterstained with DAPI. GFP expression or RNA foci formation was not detected in the absence of dox. All images were taken at the same exposure time. Scale bars: 20 m. (C) Induction of DT0 and DT960 containing DMPK mRNA after dox treatment. Each bar represents fold change in expression relative to the DT0 control without dox treatment. Positions of qRT-PCR primers used to quantitate mRNA expression are indicated with red arrows. Data is normalized relative to Rpl30. *P< 0.05. Reduced (D) Mef2A and Mef2C; (E) Myocd and Myom1; and (F) pri-miR-1-1, 1-2, 133a-1 and 133a-2 steady state levels in response to CUG^exp RNA expression. Exogenous Mef2c restores the mRNA and miRNA expression in CUG^exp RNA expressing cells. Each bar represents fold change in expression relative to DT0 control. a, significantly different from DT0 (infected with control virus). b, significantly different from DT960 (infected with control virus). (G) Model of Mef2-miRNA circuitry in normal and DM1 cardiac cells. In healthy adult cardiomyocytes, Mef2 proteins coordinate expression of cardiac-enriched mRNA and miRNA genes. miRNAs post-transcriptionally down regulate expression of multiple gene targets to maintain adult cardiac gene expression program. This transcriptional circuit becomes defective in DM1 (black panel); Mef2 levels are reduced in response to CUG^exp RNA expression, which results in loss of expression of Mef2-driven cardiac genes and miRNAs resulting in developmental reprogramming of gene expression.