Serum Cytochrome C as a Tumor Marker in Leukemia and Lymphoma.
Authors: Osaka Akemi , Sawada Takashi , Yamada Yasuaki , Aoyama Muneo , Hasegawa Hiroo , Sasaki Daisuke , Tsuruda Kazuto , Inokuchi Naoko , Kamihira Shimeru 
Published: April 2020

Abstract
When cells become apoptotic, cytochrome c (Cyt c) is translocated from the mitochondria to the cytosol. It has been reported that Cyt c released to the cytosol is further released from the cells into the extracellular medium. Since tumor cells show an accelerated cell cycle with rapid cell growth accompanied by various grades of cell death including apoptosis, sera from patients with malignancies may contain increased levels of Cyt c. We thus investigated serum Cyt c as a tumor marker in hematological malignancies. Cyt c concentration was measured by our original electrochemiluminescence immunoassay. The measurement range of this assay was 5 to 3,000 ng/ml in serum. We confirmed that cell lines triggered by inducers such as anti-Fas/CD95 or TNF-related apoptosis-inducing ligand (TRAIL) released Cyt c into the culture medium within several hours. The serum Cyt c concentration in healthy adults showed a distribution from 14.6 to 36.1 ng/ml (median value: 25.1). Unexpectedly, there was no significant difference between healthy adults and patients with AML, CML, ALL, CLL, multiple myeloma (MM), non-Hodgkin Lymphoma (NHL) or adult T cell leukemia/lymphoma (ATLL), and the median values in these diseases were distributed between 19.7 and 42.4. Of note, however, there were some patients whose Cyt c concentrations were extremely high up to 2,533.2 ng/ml. Values over 50ng/ml were observed in three of the 10 patients with AML (30%), three of the 6 patients with ALL (50%), five of the 21 patients with ATLL (23.8%) and one of the 6 patients with NHL (16.7%). In contrast, there were no patients with CML, CLL or MM who showed values higher than 50 ng/ml. On subtype-analysis in ATLL, the five patients who showed more than 50 ng/ml all had aggressive subtypes, and none of the patients with indolent subtypes showed values higher than 50 ng/ml There was a significant positive correlation between Cyt c concentration and absolute ATLL cell counts in the peripheral blood. More interestingly, parallel analysis using serum and bone marrow specimens in AML and ALL indicated that bone marrow contained more than 10 times higher Cyt c than serum. These results indicate that certain rapidly growing tumor cells exhibit increased spontaneous apoptotic cell death, and serum and/or bone marrow Cyt c concentration can be a reliable marker to estimate the extent. Then, we examined serial serum Cyt c levels during chemotherapy to see whether they reflect the effect of chemotherapy. There was a clear correlation between serum Cyt c and the effect of chemotherapy; Cyt c increased several times soon after the start of chemotherapy, then returned to normal levels after completion. Although serum LDH activity also showed similar variation, the response of serum Cyt c was much quicker and sharper than that of LDH activity. In conclusion, serum Cyt c serves as a marker not only for spontaneous tumor cell death but also for monitoring chemotherapeutic response. Monitoring Cyt c provides new insight for understanding the pathophysiology of hematological malignancies.