Anne-Marie Koop

7 259 RESULTS Cardiac-specific deletion of Hand2 augments RV susceptibility to increased pressure overload induced by PAB To assess the contribution of Hand2 to the response of the RV to pressure overload, we induced deletion of a floxed Hand2 ( Hand2 F/F ) allele using a tamoxifen-inducible Cre recombinase protein fused to two mutant oestrogen-receptor ligand-binding domains [F] under control of the cardiac-specific a-myosin heavy chain promoter (MCM- Hand2 F/F ) in adult mice, as described by us before. 19 Hand2 F/F (control) and MCM- Hand2 F/F (knockout) micewere subjected to shamor pulmonary artery banding (PAB) surgery for 6 weeks, a period during which cardiac function was assessed by echocardiography at day 5 and MRI at weeks 2 and 6 ( figure 1a ). Whereas 5 days after PAB surgery, both Hand2 F/F and MCM-Hand2 F/F mice displayed a similar increase in PA gradients ( figure 1b ), the knockout mice showed a slight decrease in pressure 6 weeks after banding, even though it was not statistically significant. In control mice, RV pressure overload induced myocardial Hand2 expression by 3.5-fold in the RV and 3-fold in the LV ( figure 1c ). Elevated expression of Hand2 in control mice subjected to PAB, is accompanied by impaired RV function as reflected by functional parameters, assessed by MRI at week 6 ( figure 1d-1k, table 1 ). These results suggest that lowering Hand2 expression could protect the heart from RV pressure overload, as also shown previously for mouse models of LV pressure overload. 19 Hand2 silencing, however, resulted in further impairment of RV remodelling and function as reflected by increased RV end-diastolic and end-systolic volumes (EDV and ESV, respectively; figure 1e and 1f, table 1 ) and a subsequent decrease in RV ejection fraction (RV EF; figure 1g, table 2 ). Impaired RV function was also associated with increased RVmass ( figure 1i and 1j, table 1 ). These data suggest an increase in sensitivity of the RV to pressure overload when Hand2 is silenced. Although elevated expression of Hand2 in controlmicesubjectedtoPAB isaccompanied by impaired RV function, Hand2 silencing resulted in further impairment of right ventricular remodelling and function.