Hearts require mitochondrial fusion mediated by outer mitochondrial membrane mitofusins (Mfn) 1 and 2 and inner mitochondrial membrane optic atrophy 1 (Opa1). Cardiomyocyte-targeted genetic ablation of Mfn1 and Mfn2 or of Opa1 induces severe cardiomyopathy. Paradoxically, damaging human MFN2 mutations that systemically interrupt mitochondrial fusion cause a hereditary peripheral neuropathy (Charcot Marie Tooth disease type 2A; CMT), but are not associated with cardiac disease. We hypothesized that monoallelic CMT mutations have minor systemic effects which can be compensated for in hearts by normal Mfn1 and Mfn2, suggesting that more severe mutations have been overlooked.
Results: - DNA sequencing of 684 patients with dilated (DCM) and concentric (HCM) cardiomyopathies identified 11 nonsynonymous mutations of MFN2. Polyphen2 classified R400Q, detected in 2 HCM and 1 DCM patients (0.31%), as “probably damaging”. The Q400 mutation is reported in 8/60,706 ExAC database subjects (0.01%; P=0.0002 vs heart disease). Alignment of Mfn2 amino acid sequences from 39 species revealed 100% conservation at this site from human to frog. In cultured cells, recombinantly expressed Mfn2 Q400 lacked intrinsic fusion activity and dominantly inhibited mitochondrial fusion without impacting mitophagy. A cell-permeant peptide Mfn agonist (Franco, Nature 2016) restored Mfn2 Q400 function in vitro. Mfn2 Q400 CRISPR knock-in mice exhibited fetal and perinatal lethality. Cardiomyocyte-directed transgenic expression of Mfn2 Q400 in mice induced progressive cardiac hypertrophy (~40% > LV mass, cardiomyocyte CSA) over 16 weeks that stabilized for 2 years; LV ejection fraction and fetal gene expression were normal. Mfn2 Q400 cardiomyocyte mitochondria were abnormally small from suppressed fusion, but respiration was normal and there was no mitochondrial stress. A therapeutic trial of a genetically-encoded Mfn agonist to prevent MFN2 Q400-mediated cardiac hypertrophy is underway.
Conclusion: – Genetic defects that interrupt mitochondrial fusion have multi-system effects; in hearts they cause cardiac hypertrophy. This and other mutations that impair mitochondrial fusion may underlie or modify unexplained genetic cardiac hypertrophy.