Cardiovascular response to beta-adrenergic blockade or activation in 23 inbred mouse strains

Abstract

We report the characterisation of 27 cardiovascular-related traits in 23 inbred mouse strains. Mice were phenotyped either in response to chronic administration of a single dose of the beta-adrenergic receptor blocker atenolol or under a low and a high dose of the beta-agonist isoproterenol and compared to baseline condition. The robustness of our data is supported by high trait heritabilities (typically H(2)>0.7) and significant correlations of trait values measured in baseline condition with independent multistrain datasets of the Mouse Phenome Database. We then focused on the drug-, dose-, and strain-specific responses to beta-stimulation and beta-blockade of a selection of traits including heart rate, systolic blood pressure, cardiac weight indices, ECG parameters and body weight. Because of the wealth of data accumulated, we applied integrative analyses such as comprehensive bi-clustering to investigate the structure of the response across the different phenotypes, strains and experimental conditions. Information extracted from these analyses is discussed in terms of novelty and biological implications. For example, we observe that traits related to ventricular weight in most strains respond only to the high dose of isoproterenol, while heart rate and atrial weight are already affected by the low dose. Finally, we observe little concordance between strain similarity based on the phenotypes and genotypic relatedness computed from genomic SNP profiles. This indicates that cardiovascular phenotypes are unlikely to segregate according to global phylogeny, but rather be governed by smaller, local differences in the genetic architecture of the various strains.

Figure 1. Phenotypic monitoring and timing in inbred mice.

Animals were trained on the Visitech BP-2000 tail-cuff apparatus on a daily basis from days 1 to 5 and 8 to 9. SBP and HR were effectively recorded on days 10, 11, and 12. Osmotic mini-pumps loaded with the appropriate drugs were implanted sub-cutaneously on day 3 under anaesthesia. ECGs were recorded on day 15 under halothane anaesthesia. Mice were sacrificed by decapitation on day 17.

Figure 2. Means and standard deviations of ten selected phenotypes in 23 inbred mouse strains.

White bars: ctr; blue bars: ate; orange bars: iso1; red bars: iso10. Strains are ranked by increasing HR (TC) means of ctr mice. See Table 1 for abbreviations.

Figure 3. Conservation of baseline phenotypes across independent studies.

Five comparisons of ctr CV-PGX strain means with means of selected MPD projects are illustrated. The dashed line is the diagonal of identity and the red line is the best fit of (x∶y) pairs. Data are presented as means±SD. See Table 1 for abbreviations. r2: squared coefficient of correlation (Pearson); p∶ p-value.

Figure 4. Intra-strain significance of drug treatments.

Data are presented for the phenotypes selected in Figure 2. Intra-strain p-values of phenotypic data recorded in treated vs untreated groups (Wilcoxon ranksum statistics) are presented as bar graphs on a -log10 scale. The threshold of significance is indicated by red lines (p = 0.05) and -log p-values are signed according to the directionality of the effect induced by the drugs. When significant, p-values for testing responses under iso10 vs iso1 are indicated by coloured stars (i.e. *: p<0.05; **: p<0.01; ***: p<0.001; red star: phenotypic mean under iso10> phenotypic mean under iso1; blue star: phenotypic mean under iso10

Figure 5. Patterns of phenotype and strain correlations under drug treatment.

For each phenotype (rows) and combination (columns) of a strain and a treatment (ate, iso1, iso10) the significance (signed -log10 value of Wilcoxon ranksum test, as used in Figure 4) of the phenotypic response with respect to the ctr group is shown using a colour code. Rows and columns are clustered according to pattern similarity. The branches of the dendrograms illustrating the clusters are plotted with the same colour as long as the average linkage distance is less than 20% of the maximal distance. AC: ate vs ctr; I1C: iso1 vs ctr; I10C: iso10 vs ctr. See Table 1 for abbreviations. HR std: standard deviation of HR (TC) strain means; SBP std: standard deviation of SBP strain means.

Figure 6. Patterns of phenotype and strain correlations under drug treatment.

For each strain (rows) and combination (columns) of a phenotype and a treatment (ate, iso1, iso10) the significance (signed -log10 value of Wilcoxon ranksum test, as used in Figure 4) of the phenotypic response with respect to the ctr group is shown using a colour code. Rows and columns are clustered according to pattern similarity. The branches of the dendrograms illustrating the clusters are plotted with the same colour as long as the average linkage distance is less than 50% of the maximal distance. AC: ate vs ctr; I1C: iso1 vs ctr; I10C: iso10 vs ctr. See Table 1 for abbreviations. HR std: standard deviation of HR (TC) strain means; SBP std: standard deviation of SBP strain means.

Figure 7. Comparison of strain relatedness based on phenotypes or genotypes.

Along the main diagonal, five clustered correlation matrices (where k = ctr, ate, iso1, iso10, or G) across all phenoypes or SNPs (k = G) are shown. Similar strains are placed close to each other and their order is indicated by the coloured circles and dendrograms displayed on the right as determined by standard hierarchical clustering. Above these five matrices ten composite matrices are shown whose lower left part consists of the correlation matrix shown on the main diagonal to the left, while their upper right contains the correlations from the dataset indicated on top, using the same order of strains (i.e. the rows of all adjacent correlation matrices correspond to a fixed strain). The degrees of correlations are indicated by a colour code. In the scatter plots below the diagonal, each pair of strains (s,s') is represented by a dot whose coordinates are given by the correlations and where k and k' are indicated on the left and top, respectively. The correlation r2 between these dots is indicated and a red line shows the best linear fit. Blue arrow: C3H/HeJ vs CBA/J, red arrow: 129S1/SvImJ vs LP/J, green arrow: C57BL76J vs C57BLKS/J.