Nuclear Rupture In A Mouse Model Of Lmna-related Cardiomyopathy Causes Cytoplasmic Exposure Of The Proinflammatory Signaling Protein Hmgb1

Cardiomyopathies caused by mutations in LMNA, encoding nuclear Lamin A/C, are highly malignant and prevalent. How LMNA mutations cause cardiomyopathies remains unknown. We characterized cellular, molecular, and pathological evolution of mouse models of LMNA -related cardiomyopathy and provide evidence for a model in which nuclear rupture generates nuclear-localized proinflammatory signaling as a candidate molecular mechanism underlying disease pathogenesis. We observed that cardiomyocyte-specific, tamoxifen-inducible deletion of Lmna in adult mice ( Lmna CMKO ) caused a gradual reduction of Lamin A/C protein at the nuclear lamina, reflecting the slow turnover of Lamin A/C. A modest reduction of Lamin A/C in Lmna CMKO was sufficient to cause extensive fibrosis, reduced ejection fraction, and chamber dilation by 3 weeks after Lmna gene deletion. Lmna CMKO cardiomyocytes exhibited localized rupture of the nuclear envelope 2 weeks prior to the development of fibrosis and reduction of ejection fraction. Nuclear rupture in Lmna CMKO was immediately followed by an extensive upregulation of pro-inflammatory gene expression programs. We hypothesized that nuclear rupture might expose nuclear DNA to the cytoplasm thereby activating the pro-inflammatory cGas-STING cytosolic DNA sensing pathway. However, we did not observe localization of the cytosolic DNA sensor cGas to cytoplasmic DNA protruded from the ruptured nuclei in Lmna CMKO cardiomyocytes. Instead, we found that HMGB1, a potent proinflammatory protein normally sequestered in the nucleus, was released from the ruptured nuclei in Lmna CMKO cardiomyocytes. Mass spectrometry identified a strong interaction between Lamin A/C and HMGB1 in normal human fibroblast cells. Our data suggested that Lamin A/C tethers HMGB1 to the nuclear periphery by direct interaction and that reduction of Lamin A/C unleashes HMGB1 to the cytoplasm upon nuclear rupture. Future work will examine the hypothesis that cytoplasmic HMGB1 triggers pathogenic sterile inflammation leading to dilated cardiomyopathies in Lmna CMKO mice. In conclusion, we identified the nuclear rupture-induced cytoplasmic release of HMGB1 as a candidate mechanism underlying LMNA -related cardiomyopathies.