Mitochondrial dysfunction may explain the cardiomyopathy of chronic iron overload

Abstract

In patients with hemochromatosis, cardiac dysfunction may appear years after they have reached a state of iron overload. We hypothesized that cumulative iron-catalyzed oxidant damage to mitochondrial DNA (mtDNA) might explain the cardiomyopathy of chronic iron overload. Mice were given repetitive injections of iron dextran for a total of 4weeks after which the iron-loaded mice had elevated cardiac iron, modest cardiac hypertrophy, and cardiac dysfunction. qPCR amplification of near-full-length ( approximately 16kb) mtDNA revealed >50% loss of full-length product, whereas amounts of a qPCR product of a nuclear gene (13kb region of beta globin) were unaffected. Quantitative rtPCR analyses revealed 60-70% loss of mRNA for proteins encoded by mtDNA with no change in mRNA abundance for nuclear-encoded respiratory subunits. These changes coincided with proportionate reductions in complex I and IV activities and decreased respiration of isolated cardiac mitochondria. We conclude that chronic iron overload leads to cumulative iron-mediated damage to mtDNA and impaired synthesis of mitochondrial respiratory chain subunits. The resulting respiratory dysfunction may explain the slow development of cardiomyopathy in chronic iron overload and similar accumulation of damage to mtDNA may also explain the mitochondrial dysfunction observed in slowly progressing diseases such as neurodegenerative disorders.

Figure 1

Thin section (5 µM) electron micrographs of cardiomyocytes from control mice (A) and mice given repetitive injections of iron dextran for 4 weeks (B). M: mitochondria; MF: myofibrils; H: electron dense iron deposits.

Figure 2

Decreased PCR-amplifiable full length (16 kb) cardiac mtDNA in mice given repetitive injections of iron dextran for 4 weeks. Total DNA was isolated and amplified by QPCR as described under Materials and Methods. A: Full length mtDNA product was significantly decreased in hearts from iron treated mice while no changes were observed in the short (0.3 kb) PCR product (a control for input of mtDNA). B: No change in the amounts of either a long (13 kb) segment of the beta-globin gene or short (223 bp) segment after iron loading. *p<0.01 (Student's t test, two-tailed; n = 3 in each case).

Figure 3

Decreased respiration of mitochondria from hearts of mice given repetitive injections of iron dextran for 4 weeks. A: Maximal oxygen consumption rates in the presence of the uncoupler CCCP were significantly decreased following iron loading. B: Similar iron-mediated decrements in respiration were also observed in the absence of an uncoupler; p<0.01 untreated vs. iron treated mice in both cases (Student's t test, two-tailed; n = 3 separate mitochondrial preparations in each case.

Figure 4

Cardiac NADH-DH and COX activities in iron treated mice and untreated treated mice. The results are shown as a ratio of NADH-DH/citrate synthase activities (A) and as a ratio of COX/citrate synthase activities (B). In both cases, *p<0.01 (Student’s t test, two-tailed) (n = 5 and 7, respectively).