Development of a method for isolation of melanin from archival FFPE tissues of human melanoma for structural studies by pyrolysis-gas chromatography-tandem mass spectrometry

Epidemiological data indicate that people with light skin exhibit higher risk for developing cutaneous melanoma than dark-skinned individuals [1]. The constitutive color of human skin depends on various genetically determined factors, including the amount and type of melanin, a polymeric pigment synthesized in epidermis by melanocytes. These specialized skin cells produce two structurally and functionally distinct pigments: eumelanin, which consists of indole-type monomers, and pheomelanin, composed of sulfur-containing units of benzothiazine-type and benzothiazole-type [2, 3]. Eumelanin is a sunscreen, antioxidant, and free radical scavenger, and its major physiological role is the protection of skin cells against damage induced by UV radiation [4, 5, 6]. By contrast, pheomelanin – as well as some of the inter-mediates of its biosynthesis – can act as an endogenous photosensitizer that generates reactive oxygen species and highly mutagenic DNA photoproducts after UV exposure, and that also has UV-independent pro-oxidative activity [5, 7, 8, 9, 10, 11]. A few studies suggest that the elevated proportion of potentially harmful pheomelanin in human skin may be involved in the promotion and progression of cutaneous melanoma. Structural study revealed that dysplastic nevi, one of risk factors for melanoma, contain significantly higher amounts of pheomelanin than common nevi [12]. The proportion of pheomelanin in melanoma cells was also found to be elevated compared to normal melanocytes [13]. However, it is not known whether there is any correlation between the type and/or clinical stage of cutaneous melanoma and the content of pheomelanin in the tumor tissue. To address this problem, a large-scale study is needed, and we believe that archival formalin-fixed, paraffin-embedded (FFPE) melanoma tissues could be a good source of melanin for future comparative structural study.

Previously, we developed a sensitive method that enables fast and reliable differentiation of pheomelanin and eumelanin, and that allows for detection and quantitation of a pheomelanin component in melanin pigments of various origin [14]. Our method is based on gas chromatography / tandem mass spectrometry (GC/MS/MS) identification of the marker products formed during thermal degradation (pyrolysis, Py) of melanin. The pheomelanin markers are identified in the pigment pyrolysate by simultaneous measuring of a set of their characteristic precursor ion / product ion pairs with a triple quadrupole mass spectrometer operating in a multiple reaction monitoring (MRM) mode. The relative pheomelanin content is then calculated from a standard curve, created with the use of a series of synthetic melanin pigments with known percentages of incorporated pheomelanin. Using the above approach, we have quantified intracellular pheomelanin produced by cultured human melanoma cells [15], and by normal melanocytes with various levels of constitutive pigmentation [16].

The aim of this work was to develop a method for isolation and purification of melanin from archival FFPE specimens of primary melanoma and the tumor lymph node metastases. Melanin isolation in this kind of solid tissue sample is particularly challenging. The pigment itself is insoluble in most solvents, and the extracellular matrix of both tumor tissue and metastatic lymph nodes is abundant in fibrous structural proteins that are resistant to hydrolytic cleavage by common proteases. To assess the suitability of the isolation protocol for the planned structural studies, the pigment obtained from FFPE melanoma samples was analyzed for pheomelanin content by the Py-GC/MS/MS method.

Fig. 1 displays reconstructed total ion current MRM chromatograms of the marker pyrolysis products (MRM pyrograms) of melanin isolated from the FFPE specimens. As shown in Table 1 (Material and Method section), we designed MRM settings to monitor the two distinct groups of products in each pyrolysate: heterocyclic compounds with 1,3-thiazole or 1,4 thiazine rings, which are characteristic of thermally degraded pheomelanin (the pheomelanin markers, P) and the products devoid of sulfur, such as toluene, phenol, methylphenol, indole and methylindole. The latter group of compounds are always formed in high yields during the pyrolysis of melanin, irrespective of its structural type and origin, and therefore we use them for normalization of the pyrolytic data, which is essential for pheomelanin quantitation [16].

Fig. 1

MRM pyrograms of melanin isolated from FFPE tissue of both primary melanoma (Fig. 1, panels A and D) and lymph node metastases (panels B and E) were quite similar, and were dominated by the non-sulfur-containing products. However, all the monitored compounds characteristic of pheomelanin structural units were present in the pyrolysates. The most abundant pheomelanin markers were thiazole (P1, 5–10% of relative intensity) and hydroxybenzothiazole (P3, 5–6%). The abundance of other pheomelanin markers (P4–P6) were substantially lower. The MRM pyrolytic profiles of the pigments isolated from FFPE specimens were similar to the profile of synthetic melanin standard with 10% of incorporated pheomelanin (Fig. 1, panels C and F). However, interestingly enough, the ratio of the relative content of P4′ isomer of 2,3-dihydro-5H-1,4-benzothiazin-5-one to the P4″ isomer in both pyrolysates of melanoma melanin was different, as compared to the pyrolysate of the synthetic melanin. The results of our recent Py-GC/MS/MS studies of synthetic eumelanin/pheomelanin copolymers show that eluted later from the GC column P4″ isomer of 2,3-dihydro-5H-1,4-benzothiazin-5-one is the pyrolytic marker of the pheomelanin monomer units formed via oxidative polymerization of 5-S-cysteinyl-3,4-dihydroxyphenylalanine (5-S-cysteinyl-DOPA), while the P4′ isomer arises from the units derived oxidatively from 2-S-cysteinyl-DOPA. 5-S-cysteinyl-DOPA is regarded as the major precursor of human epidermal pheomelanin, and indeed, our previous study shows that the pyrolysate of such a pigment, produced by normal melanocytes, contains mainly the P4″ isomer [16]. The results presented here strongly suggest that malignant melanocytes may preferentially use 2-S-cysteinyl conjugate of DOPA for pheomelanin synthesis. We intend to address this issue in our next research.

To obtain a standard curve for pheomelanin quantitation in the pigment isolated from FFPE specimens, the ratio of the total peak area of all pheomelanin markers and the total peak area of all non-sulfur-containing pyrolysis products, taken from the MRM programs of a series of synthetic eumelanin/pheomelanin copolymers, was plotted against the percentage of the pheomelanin component in the melanin standard (0.5–20 %), and a high degree of linearity was achieved (R² > 0.995). Using the linear regression equation we calculated that the pigments from the FFPE primary melanoma and FFPE melanoma lymph node metastases contain 6.6 and 7.5 % of pheomelanin, respectively. For obvious reasons (only two samples tested, each one from different donor), it is difficult to assess the significance of the obtained results of the quantitative analysis, and that was not our goal in this work. Moreover, the only pheomelanin percentage data available in the literature are for normal skin. For example, according to Del Bino et al. [17], human epidermis comprises about 26% of pheomelanin, regardless of the constitutive skin color. Similar results (26–36%, depending on the skin phototype) were reported recently by Lerche et al. [18]. Importantly, the above findings are based on an analytical method different from that used in our study: on chemical degradation of unfixed skin sections or punch biopsy specimens taken from healthy individuals, without previous isolation of melanin pigment. Reported here, the pheomelanin content of melanin from melanoma, less than one would expect in light of the published results related to normal skin, could also be a characteristic feature of the particular kind of tumors whose FFPE tissues were examined. There is some evidence that neoplastic transformation of melanocytes is accompanied by abnormal melanogenesis. It seems that these alterations include not only the total melanin content (primary melanoma lesions are usually hyperpigmented, rarely amelanotic), but also its quality in terms of the proportion of pheomelanin-type structural units. Furthermore, our previous Py-GC/MS/MS study revealed that cultured human melanoma G-361 cells, which were found to be completely eumelanotic, start to produce pheomelanin in a dose-dependent manner when exposed to valproic acid and 5,7-dimethoxycoumarin, the potential chemopreventive agents that are capable of inducing the differentiation process of tumor cells, thus converting them to normal melanocytes [15]. This suggests that at least some kinds of cutaneous melanoma tumors, as undifferentiated or poorly differentiated cells, may contain much less pheomelanin than normal skin.

The developed multistep procedure of paraffin removal, tissue rehydration, homogenization and enzymatic digestion allows for the isolation of melanin from FFPE melanoma specimens. The pigment can be successfully studied for pheomelanin content by the Py-GC/MS/MS method. The results of our study show that archival FFPE tissues could be used as a source of melanin in future research aimed at shedding more light on the role of pheomelanin in the induction and progression of cutaneous melanoma.