The number of X chromosomes influences protection from cardiac ischaemia/reperfusion injury in mice: one X is better than two

Sex differences in coronary heart disease have been attributed to sex hormones, whereas the potential role of the sex chromosomes has been ignored so far. Here, we investigated the role of the sex chromosomes in causing sex differences in myocardial ischaemia/reperfusion (I/R) injury. We used two unique mouse models, the 'four core genotypes' [XX mice with ovaries (XXF) or testes (XXM) and XY mice with ovaries (XYF) or testes (XYM)] and XY* (gonadal male or female mice with one or two X chromosomes). All mice were gonadectomized (GDX). In vivo or isolated Langendorff-perfused hearts were subjected to I/R injury. The in vivo infarct size in XY mice was significantly smaller than XX mice regardless of their gonadal type (24.5 ± 4.1% in XYF and 21.8 ± 3.3% in XYM vs. 37.0 ± 3.2% in XXF and 35.5 ± 2.1% in XXM, P < 0.01). Consistent with the results in vivo, the infarct size was markedly smaller and cardiac functional recovery was significantly better in XY mice compared with XX ex vivo. The mitochondrial calcium retention capacity was significantly higher in XY compared with XX mice (nmol/mg protein: XXF = 126 ± 9 and XXM = 192 ± 45 vs. XYF = 250 ± 56 and XYM = 286 ± 51, P < 0.05). In XY* mice, mice with 2X chromosomes had larger infarct size (2X females = 41.4 ± 8.9% and 2X males = 46.3 ± 9.5% vs. 1X females = 23.7 ± 3.9% and 1X males = 26.6 ± 6.9%, P < 0.05) and lower heart functional recovery, compared with those with 1X chromosome. Several X genes that escape X inactivation (Eif2s3x, Kdm6a, and Kdm5c) showed higher expression in XX than in XY hearts. XX mice have higher vulnerability to I/R injury compared with XY mice, which is due to the number of X chromosomes rather than the absence of the Y chromosome.