Our results suggest that besides energy deficiency, mitochondrial biogenesis per se is a maladaptive response in MIC and, possibly, in other metabolic cardiomyopathies.
Human parvovirus B19 infection is occasionally associated with acute lymphocytic myocarditis (ALM). Three infants with B19 virus-associated ALM were followed up clinically, histologically, and immunovirologically. Each infant had B19 virus DNA in the blood or B19 virus-specific IgM antibodies. Two infants with postnatal infection recovered after immunosuppressive therapy. The third infant with possible prenatal infection developed chronic persistent myocarditis associated with persistent B19 virus DNA in the blood. All 3 infants had increased levels of interferon-gamma, tumor necrosis factor-alpha, and interleukins -6 and -8. Four newborns with congenital B19 virus infection and 4 infants and children who had postnatally acquired B19 virus infection without myocarditis all had normal levels of these cytokines. These observations suggest that B19 virus infection in infancy causes ALM in some infants and children.
The genetic and epigenetic factors underlying the variable penetrance of homoplasmic mitochondrial DNA mutations are poorly understood. We investigated a 16-year-old patient with hypertrophic cardiomyopathy harboring a homoplasmic m.4277T>C mutation in the mt-tRNA(Ile) (MTTI) gene. Skeletal muscle showed multiple respiratory chain enzyme abnormalities and a decreased steady-state level of the mutated mt-tRNA(Ile). Transmitochondrial cybrids grown on galactose medium demonstrated a functional effect of this mutation on cell viability, confirming pathogenicity. These findings were reproduced in transmitochondrial cybrids, harboring a previously described homoplasmic m.4300A>G MTTI mutation. The pathogenic role of the m.4277T>C mutation may be ascribed to misfolding of the mt-tRNA molecule, as demonstrated by the altered electrophoretic migration of the mutated mt-tRNA. Indeed, structure and sequence analyses suggest that thymidine at position 4277 of mt-tRNA(Ile) is involved in a conserved tertiary interaction with thymidine at position 4306. Interestingly, the mutation showed variable penetrance within family members, with skeletal muscle from the patient's clinically unaffected mother demonstrating normal muscle respiratory chain activities and steady-state levels of mt-tRNA(Ile), while homoplasmic for the m.4277T>C mutation. Analysis of mitochondrial isoleucyl-tRNA synthetase revealed significantly higher expression levels in skeletal muscle and fibroblasts of the unaffected mother when compared with the proband, while the transient over-expression of the IARS2 gene in patient transmitochondrial cybrids improved cell viability. This is the first observation that constitutively high levels of aminoacyl-tRNA synthetases (aaRSs) in human tissues prevent the phenotypic expression of a homoplasmic mt-tRNA point mutation. These findings extend previous observations on aaRSs therapeutic effects in yeast and human.
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