Does PERM1 Regulate Systolic Cardiac Function? A Game of Numbers.

Abstract

We and others demonstrated that PERM1 is a positive regulator of mitochondrial bioenergetics in the heart. However, discrepant results have emerged with regard to whether PERM1 loss-of-function affect cardiac contractility. In order to exclude the possibility that the reported negative results can be due to insufficient power of statistical test, we conducted a more robust echocardiography (Echo) analysis by increasing the sample size. We used Perm1-KO and their respective wildtype (WT) littermates, which were destined to tissue harvest. This yielded 84 WT mice and 88 Perm1-KO mice. We analyzed Echo-derived parameters of left ventricular (LV) systolic function. At the end of the study, ejection fraction (EF) was 65.43 {+/-} 7.13 in WT vs. 53.98 {+/-} 8.80 in Perm1-KO yielding p < 0.00000000000000004. Other parameters which reached statistically significant difference between WT and Perm1-KO (at p < 0.05) included LV fractional shortening (FS), LV diastolic and systolic diameters, LV anterior and posterior systolic wall thickness, LV posterior wall systolic thickening, stroke volume, and cardiac output (CO). Retrospectively, a p value < 0.05 was consistently achieved in assessment of EF only after average N per group reached 13. Larger minimal N per group were required for other parameters. Of interest, in both groups there were no correlation between EF% and CO. At the same time, in both groups EF strongly inversely correlated with LV diastolic diameter. This led us to a speculation that low EF may be in part compensated by an increased LV circumference, for the purpose of maintaining invariant CO. Indeed, the intergroup difference in CO (6%) was much smaller than the intergroup difference in EF (18%). We conclude that PERM1 does regulate cardiac mechanics. Changes caused by constitutive Perm1-KO can be conceptualized as reduced contractility partially compensated by increased LV circumference. This study underscores the importance of sufficiently large sample size for detecting significant differences in Echo data.