Forensic significance of intracardiac heme oxygenase-1 expression in acute myocardial ischemia

Heme oxygenase-1 (HO-1), an inducible stress-response protein, exerts anti-oxidant and anti-apoptotic effects. However, its significance in forensic diagnosis of acute ischemic heart diseases (AIHD) such as myocardial infarction (MI) is still unknown. We examined the immunohistochemical expression of HO-1 in the heart samples to discuss their forensic significance to determine acute cardiac ischemia. The heart samples were obtained from 23 AIHD cases and 33 non-AIHD cases as controls. HO-1 positive signals in cardiomyocyte nuclear were detected in 78.2% of AIHD cases, however, that were detected in only 24.2% control cases with statistical difference between AIHD and non-AIHD groups. In contrast to HO-1 protein expression, there was no significant difference in the appearance of myoglobin pallor regions and leukocyte infiltration in the hearts between AIHD and non-AIHD groups. From the viewpoints of forensic pathology, intracardiac HO-1 expression would be considered a valuable marker to diagnose AIHD as the cause of death.

Ischemic heart disease is the leading cause of death in the worldwide and is the most common cause of sudden cardiac death (SCD) 1,2 . In the cases of SCD, the postmortem diagnosis of acute myocardial ischemia is an important issue for both clinical physicians and forensic pathologists if death occurs within a short period of time after the onset of ischemic heart attack.
Myocardial infarction (MI), pathologically defined as myocardial cell death due to prolonged ischemia, is most representative acute ischemic heart disease (AIHD). After the onset of myocardial ischemia, histological cell death does not occur immediately, and it takes a finite period of time to develop-as little as 20 min, or less in some animal models [3][4][5] . Depending on the sensitivity of cardiomyocytes, macroscopic or microscopic postmortem examination takes several hours to identify myocardial necrosis. Thus, the evaluation of early myocardial damage using routine histological examination such as HE staining is only possible in death cases at least several hours passed after the onset of ischemic damage.
To identify early signs of cardiac ischemia, histochemical techniques have been proposed [6][7][8][9][10][11][12] . However, these methods are not suitable for routine use, and their specificity and sensitivity are controversial because positive results may be sometimes associated with agonal factors and postmortem changes 13,14 . Recent works have immunohistochemically investigated, some markers such as fibronectin, C5b-9 and myoglobin which accumulate in or leak from cardiomyocytes after ischemia. However, these molecules almost failed to be detected in the very early phase of myocardial ischemia 5,[15][16][17][18] .
Heme oxygenase-1 (HO-1, HSP32) is a vital heme degradative enzyme and inducible stress protein [19][20][21] . Bilirubin and carbon monoxide (CO) that are reaction products of HO-1 are protective against ischemia-induced injury such as myocardial infarction, ischemia-reperfusion injury, and post-infarct structural remodeling [22][23][24] . Oxidative stress associated with over-vasoconstriction is the major pathogenic mechanism in the various heart pathologies. Nuclear translocation of HO-1 induced by oxidative stress resulted in the promoted interaction of HO-1 with the redox-sensitive transcription factor Nrf2 25 . The observations described previously suggest that HO-1 had a role in the pathogenesis of AIHD. However, the knowledge of their involvement in AIHD and their significance in forensic diagnose are insufficient. Therefore, in the present study, we examined intracardiac Intracardiac leukocyte infiltration. Intracardiac leukocyte recruitment is also one of the hallmarks in AIHD. Thus, we examined the degrees of intracardiac leukocyte recruitment. Subsequently, immunohistochemical analyses demonstrated that few MPO + neutrophils ( Fig. 2a and Supplemental Fig. 3) and macrophages ( Fig. 2b and Supplemental Fig. 4) were present in the control hearts. In AIHD hearts, neutrophils and macrophages were observed to a similar extent as non-AIHD (Fig. 2). These observations implied that the evaluation of leukocyte recruitment would not be powerful clue for the diagnosis of early phase of AIHD.
Enhanced intracardiac HO-1 expression in AIHD group. We could observe intracardiac HO-1 expression in both groups. The immunohistochemical expression of HO-1 was more intense and diffuses in the nuclei of cardiomyocytes in AIHD group, compared with non-AIHD group ones ( Fig. 3a and Supplemental  Fig. 5). The comparison between AIHD and non-AIHD groups showed significant difference of the number of HO-1 expressed case (Fig. 3b). There were no significant differences on age, gender, or PMI for HO-1 protein expressions (Fig. 4).

Discussion
The post-mortem diagnosis of SCD can be difficult if it is based only on routine investigation as heart macroscopic findings and HE analyses. As a method for detecting myocardial ischemia, visualization of molecular change by immunohistochemical examination has been proposed, but its application depends on the availability of a specific and sensitive marker 15,26,27 . Until now, several lines of accumulating evidence implied the possibility Representative results from non-AIHD (Drowning) and AIHD groups were shown here. HO-1 is a rapidly inducible protein which degrades heme to biliverdin, ferrous iron, and CO in order to protect cells from oxidative stress [19][20][21] . HO-1 mitigate cellular injury by its anti-oxidant, -apoptotic, and -inflammatory effects 21,32-34 . HO-1-mediated cytoprotection reflects the effects of its catalytic products especially CO because CO can rescue the injurious effects of HO-1 deficiency 21,32-35 . Although short-term HO-1-mediated cytoprotective effects in heart of animals have been reported 36,37 , it was unknown whether HO-1 could be an useful indicator of postmortem diagnosis of human AIHD. Novo et al. 38 determined serum levels of HO-1 in acute MI patients to assess their clinical significance and potential prognostic value. They found that serum HO-1 level was increased in patients of coronary artery diseases, which is in accordance with the cardioprotective role of HO-1 39 . In the present study, we found that the intranuclear HO-1 expression in cardiomyocytes were detected in 78.2% (18 cases/23 cases) of AIHD group suggesting that HO-1 expression can be a marker of early ischemia.
In individuals with an ischemic injury for several hours, the myocardial alterations can be detected as follows: necrotic clotting, wavy fibers, hypereosinophilic anucleated myocytes, and early inflammatory infiltration 40 . The contraction bands also have been considered as a sign of ischemia and a histological hallmark of adrenergic stress and/or reperfusion injury [41][42][43] . In fact, although the frequency was low, about half of AIHD group (12 cases/23 cases, 52.1%) had contraction bands with a significant difference, compared with non-AIHD one (3 cases/33 cases, 9.0%).
Activation of inflammation is important to clear the damaged myocardium. Diffuse leukocyte infiltration is first observed after 9 h from the onset of myocardial ischemia 44,45 . Our data showed few neutrophils and macrophages in the both AIHD and non-AIHD hearts with no significant difference, suggesting that detection of leukocytes in the hearts would not be suited for postmortem diagnosis of early phase of AIHD.
The early loss of myoglobin within 6 h from the onset of the symptoms is clearly documented in immunohistochemical studies on human samples 26,27,46,47 . This marker has been tested enough sensitive and specific for the detection of AIHD lasting for several hours, however, may not be suitable for detection of very early myocardial ischemic damage.
C5b-9 is most commonly used to support the final diagnosis of SCD because it can reveal areas of myocardial necrosis 5,15,27,30 . This property of C5b-9 is due to its direct involvement in the complement cascade. Indeed, it has been reported that the C5b-9 complex, which is known to form under ischemic conditions, may directly contribute to cardiomyocyte damage 48 . In addition, immunohistochemical demonstrations of fibronectin have been shown to be useful in the localization and diagnosis of early ischemic myocardial necrosis 5,15,27,31 . Fibronectin leaks from damaged capillaries and extracellular matrix to damaged myocytes during ischemia. Hu et al. reported experimentally observing a positive reaction of fibronectin in the cytoplasm of necrotic myocytes 30-60 min after coronary artery occlusion 49 .
For the immunohistochemical evaluation of C5b-9 and fibronectin in the previous studies, positive staining was graded 5,15,27,31 . On the other hand, the interpretation of HO-1 staining in our study is evaluated as positive or negative, which imply that results may be less influenced by each investigator, resulting in less variation of the interpretation. This suggests that our study would be more simple and advantageous in forensic practices, compared with the previous studies.
In this study, we proposed a useful marker HO-1 in the forensic diagnosis of acute myocardial ischemia. In forensic practices, it is difficult to make definite evaluation of the cause of death, wound ages or wound vitality using a single marker. In line with this, no single immunohistochemical reaction is ideal for diagnosis of early myocardial ischemia in the aspects of forensic safety. Taken together, the combined analysis using several different markers such as HO-1 as well as C5b-9 and fibronectin could provide more objective and accurate information for the evaluation of early myocardial damage, eventually contributing to the advance of postmortem diagnosis of AIHD when macroscopic or microscopic evidence is insufficient.   Morphometrical analysis. Morphometrical analysis was performed with immunohistochemical findings.

Methods
Cases in which HO-1-positive cells were uniformly observed in myocardial samples were defined as HO-1 + cases. These analyses were performed by two investigators with no prior knowledge of the samples.
Statistical analysis. Comparison of two groups was performed using two-side unpaired Student's t test to identify significant difference. P < 0.05 was considered as significant. All statistical analyses were performed using Statcel3 software under the supervision of a medical statistician.