In vivo MRI characterization of progressive cardiac dysfunction in the mdx mouse model of muscular dystrophy

Abstract

Aims: The mdx mouse has proven to be useful in understanding the cardiomyopathy that frequently occurs in muscular dystrophy patients. Here we employed a comprehensive array of clinically relevant in vivo MRI techniques to identify early markers of cardiac dysfunction and follow disease progression in the hearts of mdx mice.

Methods and results: Serial measurements of cardiac morphology and function were made in the same group of mdx mice and controls (housed in a non-SPF facility) using MRI at 1, 3, 6, 9 and 12 months after birth. Left ventricular (LV) and right ventricular (RV) systolic and diastolic function, response to dobutamine stress and myocardial fibrosis were assessed. RV dysfunction preceded LV dysfunction, with RV end systolic volumes increased and RV ejection fractions reduced at 3 months of age. LV ejection fractions were reduced at 12 months, compared with controls. An abnormal response to dobutamine stress was identified in the RV of mdx mice as early as 1 month. Late-gadolinium-enhanced MRI identified increased levels of myocardial fibrosis in 6, 9 and 12-month-old mdx mice, the extent of fibrosis correlating with the degree of cardiac remodeling and hypertrophy.

Conclusions: MRI could identify cardiac abnormalities in the RV of mdx mice as young as 1 month, and detected myocardial fibrosis at 6 months. We believe these to be the earliest MRI measurements of cardiac function reported for any mice, and the first use of late-gadolinium-enhancement in a mouse model of congenital cardiomyopathy. These techniques offer a sensitive and clinically relevant in vivo method for assessment of cardiomyopathy caused by muscular dystrophy and other diseases.

Figure 1. Serial measurements of cardiac function and mortality.

A–E; Graphs of left and right ventricular morphology and function measured in vivo in mdx and WT mice from 1 to 12 months. F; Kaplein-Meier graph of mouse survival during the study indicating earlier mortality in mdx mice.

Figure 2. Cardiac MRI.

In vivo short axis cine-MR images of the same wild type and mdx mouse imaged at 1, 3, 6, 9 and 12 months.

Figure 3. Left ventricular filling and ejection rates in control mouse.

Time-volume curve (upper) and rate of change of volume curve (lower) acquired before and after bolus injection of 1.5 mg/kg dobutamine. The dobutamine induced an increase in peak ejection rate, but a reduction in peak filling rate, with lengthening of the high velocity filling phase. Mdx and control mice showed similar responses.

Figure 4. Late gadolinium enhancement in 6 and 12 month mdx mice.

Upper left; late gadolinium enhancement in 3 of 9 mdx mice at 6 months (arrows). Lower panel; late gadolinium enhancement was present in the septum of all mdx mice imaged at 12 months in both long axis (left) and short axis (right) views as well as RV wall (arrows). Upper right; no late gadolinium enhancement was present in wild type mice (WT) at any time point. Far right; Sirius red staining of cryosections confirmed presence of fibrosis in the septum and RV wall of mdx mouse hearts.

Figure 5. Greater cardiac remodeling in hearts with more late gadolinium enhancement.

End diastolic volumes (left) and end systolic volumes (middle) and left ventricular mass (right) were increased in hearts with more segments showing late gadolinium enhancement.