20% of dilated cardiomyopathy patients carry mutations in the giant protein titin. Mutations are predominant in A band but also occur in I band, a domain that regulates passive tension and myocyte signaling. A recessive mouse mutation in titin I band N2A region (mdm) causes early onset muscular dystrophy with myositis and death. We assessed cardiac morphology, function, and transcriptional profiles (RNAseq) in mdm mice.
Young homozygous mdm mice (n>6) have reduced body weight (7gms) vs. heterozygous (20gm) or WT (17gm) littermates, with severe skeletal muscle dystrophy. Four-week old homozygous mdm mice have higher left ventricular (LV): body weight ratios. Echocardiography revealed thinner LV posterior wall and septum (LVPWd and IVSd) and normal LV diameter (LVDd); when normalized for body weight, cardiac dimensions were increased compared to WT or heterozygous mdm mice. Fractional shortening was reduced in homozygous Mdm mice (35%) vs. WT (40-41%, p<0.01); histology showed neither overt pathology nor fibrosis. Titin gels showed lack of difference in cardiac titin isoform pattern, consistent with RNAseq, which showed the mdm titin transcript excluded exons 107 and 108, deleting in frame 48 amino acids. 240 transcripts (0.8%) were differentially expressed (fold change >1.5 and <0.75, p<0.001) in homozygous vs. heterozygous mdm hearts; ANP and BNP were mildly upregulated (2- and 1.2-fold). Altered transcripts participated in extracellular and immune signaling pathways. Among titin binding partners, only calpain-3 that interacts with N2A was changed (0.6-fold), consistent with previous reports in skeletal muscle. As humans have heterozygous mutations, we stressed adult heterozygous mdm and WT mice (2 weeks of angiotensin II infusion): both had comparable hypertrophic responses (increased LVPWd and IVSd). Aged (89 week old) unstressed heterozygous mdm mice had normal cardiac dimensions and function.
The N2A region, I-band titin mdm mutation causes minimal cardiac dysfunction in mice, unlike the severe skeletal muscle phenotype. Human I-band mutations are unlikely to cause dilated cardiomyopathy.