Pathology and nuclear abnormalities in hearts of transgenic mice expressing M371K lamin A encoded by an LMNA mutation causing Emery-Dreifuss muscular dystrophy

Mutations in LMNA, which encodes nuclear lamins A and C, cause a broad range of diseases, including autosomal dominant Emery-Dreifuss muscular dystrophy (EDMD) and related disorders with a predominant cardiomyopathy. Homozygous Lmna model "knock-in" and null mice develop cardiomyopathy, whereas heterozygous mice do not. Overexpression of lamin A mutants that cause cardiomyopathy in cultured cells induces morphological abnormalities in the nuclear envelope and lamina; however, effects on tissue and organ pathology have not been determined. We used the heart-selective alpha-myosin heavy chain promoter to drive expression in transgenic mice of human wild-type and M371K lamin A, which causes EDMD. Mice expressing M371K lamin A were born at approximately 0.07 of the expected frequency and those born typically died at 2-7 weeks of age. Histological analysis showed increased eosinophilia and fragmentation of cardiomyofibrils, nuclear pyknosis and edema without fibrosis or significant inflammation, indicative of acute or subacute injury. Mice expressing human wild-type lamin A were born at only slightly less than the expected frequency and had normal life spans. Confocal immunofluorescence microscopy demonstrated abnormal nuclear envelopes with intranuclear foci of lamins in cardiac cells expressing M371K lamin A. Electron microscopy revealed extensively convoluted nuclear envelopes, intranuclear inclusions and chromatin clumps in cardiomyocyte nuclei. These results demonstrate that expression of a lamin A mutant that induces alterations in nuclear morphology can cause tissue and organ damage in mice with a normal complement of wild-type lamins.