Abstract 1473: Cardiac Targeted Transgenic Models of Mutations in TK2 or Pol γ Genes Result in Ultrastructural Changes in Mitochondria and Cardiac Hypertrophy

Abstract

Background: Mitochondrial (mt) biogenesis and homeostasis of precursor pools are critical to normal cardiac performance. Two important enzymes are involved in mtDNA replication. DNA polymerase gamma (Pol γ) is responsible for replication of mtDNA. Thymidine kinase 2 (TK2) is responsible for intramitochondrial monophosphorylation of pyrimidines. Point mutations in Pol γ and TK2 identified in mitochondrial genetic diseases include human chronic progressive external opthalmoplegia (CPEO) associated with a Pol γ Y955C mutation, while His121Asn (HIS) and Ile212Asn (ILE) mutations in TK2 result in heritable mtDNA depletion syndromes with organ dysfunction. Because nucleoside reverse transcriptase inhibitors (NRTI) can also disrupt mtDNA replication, we examined the impact of cardiac-targeted transgenic overexpressors of Pol γ or TK2 mutants in combination with NRTI-based HAART on cardiac function. Methods: Targeted transgenic mice (TG) were generated that overexpress Y955C, HIS or ILE in cardiac tissues driven by the α-MyHC promoter. Changes in cardiac and mitochondrial structure and function were examined by echocardiography (ECHO), transmission electron microscopy (EM), mtDNA abundance, 8-OH2dG and SDH enzyme histochemistry quantitation. TGs and wild-type (WT) littermates were treated with vehicle or NRTI-based HAART (ziduvodine, lamivudine, and indinavir, 35 days). Results: Y955C TGs had decreased cardiac mtDNA abundance, increased mitochondrial oxidative stress (increased 8-OH2dG), and mitochondrial destruction together with increased LV mass. HIS TGs (both treated and untreated) demonstrated increased LV mass and a 2-fold increase in mtDNA abundance compared to WT. Untreated ILE TGs had no analogous impact, but the addition of HAART treatment resulted in increased LV mass and mtDNA abundance. Conclusions: Inefficient mtDNA replication and mitochondrial oxidative stress contribute to mtDNA depletion. Pol γ Y955C and TK2 HIS each alters mtDNA replication. ILE had minimal impact on mitochondrial ultrastructure or cardiac function, but together with HAART had an effect. These pathogenic point mutations in genes involved in mtDNA replication may increase risk for cardiac hypertrophy compounded by NRTI treatment.