Abstract 3805: Expression of tryptophan -2, 3-dioxygense (TDO) in metastatic uveal melanoma
Authors: Terai Mizue , Link Emma , Link Eric , Lam Bao , Orloff Marlana , Sato Takami 
Published: August 2018

Uveal (eye) melanoma (UM) is the most common primary eye malignancy in adults. Despite effective treatments for primary UM, up to 50% of patients subsequently develop systemic metastasis, predominantly in the liver. Once hepatic metastasis develops, the survival of UM patients is generally short and currently available treatments fail to show meaningful improvement of their survival. Metastatic uveal melanoma (MUM) is highly resistant to traditional systemic therapies, including immunotherapy. Immune checkpoint blockades approved for metastatic cutaneous melanoma (MCM) such as anti-CTLA4 antibody and anti-PD-1 antibody have shown only marginal clinical benefit in MUM (response rates &lt;5%). We have reported that, while PD-1 expression was seen in both MCM and MUM, there was a stark difference in PD-L1 expression between these two types of melanoma (MCM vs. MUM, 20.8% vs. 0%) (ASCO 2016, A Javed, et al). It is of note that, of 6 MCM specimens obtained from hepatic metastasis, none expressed PD-L1. These results indicate the presence of unique immune microenvironment in the liver, harboring melanoma metastasis. One of the factors dominant in the liver is tryptophan 2,3-dioxygenase (TDO). TDO and indoleamine 2,3-dioxygenase (IDO) catalyze tryptophan and produce kynurenine, a metabolite that reportedly inhibits anti-tumor T cell immune responses. Recent studies have revealed that TDO is constitutively expressed in a wide variety of cancer cells. Investigation on TCGA data indicates expression of TDO2 in 62% of primary UM. 42% of samples showed increased TDO2 expression (RPKM &gt;= 1 RPKM), while TDO2 message is not present in 1/3 of the samples. Expression of TDO2 in primary UM correlated to poor survival (Cox regression hazard ratio 0.7, p=0.04). Moreover, TDO2 expression correlated to patients with BAP1 mutations (p=0.00071) and differences between patients with monosomy 3 and disomy 3 (p=0.00017). We further investigated whether MUM cell lines and MUM tumor specimens express TDO. We detected TDO protein in all of 4 MUM cell lines by Western blotting. Although inducible by exogenous IFN-gamma, IDO1 protein was not detected in any of these MUM cell lines without stimulation. TDO mRNA was increased 3.5 fold by stimulating MUM cells with recombinant human TNF-alpha. All MUM cell lines possess TNF-alpha receptors. We have also investigated TDO and IDO1 proteins in 10 MUM specimens obtained from hepatic metastasis by immuno-histochemical staining. TDO was positive in both normal hepatocytes as well as tumor themselves in all MUM specimens. Interestingly, the intensity of TDO staining is much stronger in MUM, compared to the surrounding liver tissues. On the other hand, IDO1 protein was not positive in any of MUM tissues obtained from liver metastasis. These results indicate that expression of TDO by MUM cells might be one of the key mechanisms for escape from T-cell immune surveillance in hepatic metastasis from primary UM.

Citation Format: Mizue Terai, Emma Link, Eric Link, Bao Lam, Marlana Orloff, Takami Sato. Expression of tryptophan -2, 3-dioxygense (TDO) in metastatic uveal melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3805.