Fine mapping of Dyscalc1, the major genetic determinant of dystrophic cardiac calcification in mice.

Calcification of severely dystrophic muscle is occasionally observed in targeted mouse models of muscular dystrophy and cardiomyopathy. Intracellular calcium deposition occurs in necrotic myocytes in the absence of plasma calcium and phosphate imbalances. In the heart, this recessive trait is referred to as dystrophic cardiac calcinosis (DCC). We identified previously Dyscalc1, a major genetic determinant of DCC, in a 15.2-Mbp region on proximal chromosome 7. We report now further steps toward the identification of the Dyscalc1 gene by reverse genetics. Transferring the Dyscalc1 locus from susceptible mouse strain C3H/He onto a resistant C57BL/6 strain background, we generated congenic inbred strains B6.C3-(D7Mit56-D7Mit230) and B6.C3-(D7Nds5-D7Mit230). Three days after myocardial freeze-thaw injury, both strains exhibited calcification of necrotic lesions, confirming the pathogenetic relevance of Dyscalc1. Analysis of two (129S1 x C57BL/6) x 129S1 backcrosses allowed mapping of Dyscalc1 more precisely to a region spanning 0.76 Mbp between genes Fgf21 (39.70 Mbp) and Myod1 (40.46 Mbp). This interval contains 31 known and putative genes in three large, ancestral haplotypes shared by susceptible strains C3H/He, 129S1, and DBA/2. Thus we were able to exclude previously proposed candidate genes Bax and Hrc. Instead, a potential candidate may be the gene encoding the ATP-binding cassette C6. Mutations in the orthologous human ABCC6 gene cause pseudoxanthoma elasticum, or Gronblad-Strandberg syndrome, an elastic tissue disorder with cardiovascular calcifications.

Calcification of myocardial necrosis is common in mice.

Independent of the severity, phenotypes and clinical outcomes of myocardial infarction may vary considerably in patients, suggesting a strong genetic influence on healing and adaptive processes. Since little is known about these genetic determinants, we examined the tissue response to myocardial injury in seven inbred mouse strains, including those employed for gene targeting or transgenic overexpression. Myocardial necrosis was produced by non-ischemic, trans-diaphragmal freeze-thaw injury in strains C57BL/6, C3H/He, DBA/2, BALB/c, 129S1, FVB/n and A/J. Two days after injury, necrotic cardiomyocytes calcified in C3H/He, DBA/2, BALB/c and 129S1, a phenotype known as dystrophic cardiac calcinosis (DCC). The susceptibility to DCC of 129S1 was determined by Dyscalc1, a locus on chromosome 7, which was identified previously in C3H/He and DBA/2. DCC was also observed in C3H/He following ischemic injury by permanent coronary artery ligation, indicating that DCC was independent of the mode of injury. In contrast, strains C57BL/6, FVB and A/J were resistant to DCC, showing formation of a fibrous scar without calcification. The development of DCC was studied in detail in C3H/He and C57BL/6. In both strains, no calcium deposition and only little structural disintegration were noted in necrotic myocardium 24 h after injury upon calcium-sensitive fluorescence staining. Ultrastructural examination revealed calcified mitochondria in C3H/He that may have served later as a nidus for rapid intracellular calcification of cardiomyocytes. We concluded that the susceptibility to calcification of myocardial necrosis may be common among inbred strains and should be recognised as a strong genetic modifier of experimental myocardial injury.