The melanoma proteome
Malignant melanoma is the leading cause of skin-related death in Caucasians, but rare in populations with darker pigmented skin color. The incidence has increased dramatically during the last decades, with an almost four-fold increase in the Nordic countries in the time period 1964-2003. The increased incidence reflects in part different UV exposure behavior in the population, but hereditary risk factors including skin type are also important. Short intermittent exposure with sunburns appears to be the main risk factor. Other important risk factors include the number of melanocytic nevi and the number of dysplastic melanocytic nevi.
Primary cutaneous malignant melanoma is thought to develop in a multi-step process. Precursor lesions, such as dysplastic melanocytic nevi develop into a melanoma in situ stage and further into invasive melanoma and eventually metastatic melanoma. Invasive malignant melanoma is traditionally divided into four principal histopathological subgroups based on the microscopical appearance: superficial spreading melanoma (SSM), nodular malignant melanoma (NMM), lentigo maligna melanoma (LMM) and acral lentiginous melanoma (ALM).
Here, we explore the melanoma proteome using TCGA transcriptomics data and antibody-based protein data. 208 genes are suggested as prognostic based on transcriptomics data from 102 patients; 163 genes are associated with unfavorable prognosis and 45 genes are associated with favorable prognosis.
TCGA data analysis
In this metadata study, we used data from TCGA where transcriptomics data was available from 102 patients with skin cutaneous melanoma in total. The total dataset included 42 females and 60 males. Most of the patients (73 patients) were still alive at the time of data collection. The stage distribution was stage i) 2 patients, stage i/ii NOS) 1 patient, stage ii) 65 patients, stage iii) 27 patients, stage iv) 32 patients, and 4 patients with missing stage information.
Unfavorable prognostic genes in melanoma
For unfavorable genes, higher relative expression levels at diagnosis give significantly lower overall survival for the patients. There are 163 genes associated with an unfavorable prognosis in melanoma. In Table 1, the top 20 most significant genes related to an unfavorable prognosis are listed.
MCM6 is a gene associated with an unfavorable prognosis in melanoma. The best separation is achieved by an expression cutoff at 16.5 fpkm which divides the patients into two groups with 0% 3-year survival for patients with high expression versus 62% for patients with low expression, p-value: 1.09e-5. Immunohistochemical staining using an antibody targeting MCM6 (HPA004818) shows a differential expression pattern in melanoma samples.
TIMELESS is another gene associated with an unfavorable prognosis in melanoma. The best separation is achieved by an expression cutoff at 10.0 fpkm which divides the patients into two groups with 0% 3-year survival for patients with high expression versus 48% for patients with low expression, p-value: 1.06e-5. Immunohistochemical staining using an antibody targeting TIMELESS (HPA060655) shows a differential expression patterns in melanoma samples.
Table 1. The 20 genes with highest significance associated with an unfavorable prognosis in melanoma.
Favorable prognostic genes in melanoma
For favorable genes, higher relative expression levels at diagnosis give significantly higher overall survival for the patients. There are 45 genes associated with a favorable prognosis in melanoma. In Table 2, the top 20 most significant genes related to a favorable prognosis are listed.
WIPI1 is a gene associated with a favorable prognosis in melanoma. The best separation is achieved by an expression cutoff at 14.0 fpkm which divides the patients into two groups with 52% 3-year survival for patients with high expression versus 0% for patients with low expression, p-value: 3.11e-4. Immunohistochemical staining using an antibody targeting WIPI1 (HPA007493) shows a differential expression pattern in melanoma samples.
Table 2. The 20 genes with highest significance associated with a favorable prognosis in melanoma.
The melanoma transcriptome
The transcriptome analysis shows that 68% (n=13794) of all human genes (n=20162) are expressed in melanoma. All genes were classified according to the melanoma-specific expression into one of five different categories, based on the ratio between mRNA levels in melanoma compared to the mRNA levels in the other 16 analyzed cancer tissues.
Figure 1. The distribution of all genes across the five categories based on transcript abundance in melanoma as well as in all other cancer tissues.
319 genes show some level of elevated expression in melanoma compared to other cancers (Figure 1). The elevated category is further subdivided into three categories as shown in Table 3.
Table 3. The number of genes in the subdivided categories of elevated expression in melanoma.
The histopathological features of malignant melanomas vary widely. A typical feature, often facilitating the diagnosis of melanoma, is the occurrence of a pagetoid growth pattern, characterized by the growth of melanoma cells in the upper layers of the epidermis from below, as opposed to the localization of normal melanocytes in basal layers of the epidermis. Melanoma cells can be large and rich in cytoplasm, small or even spindly. In a subset of melanomas, there are areas with abundant melanin and in other melanomas, melanin pigment is scarce or absent. Tumor cell nuclei are enlarged, often with a prominent nucleolus and mitoses are present at a variable degree.
Patients that present with an advanced tumor stage at the time for diagnosis and patients with relapse of melanoma after surgical removal generally have a poor prognosis. In patients diagnosed with localized disease, the most important prognostic indicator is tumor thickness at the time of diagnosis. The tumor thickness is measured in mm, according to a system initially described by Breslow, and is the dominating parameter for determining the tumor stage (T-stage) of melanoma. Other prognostic factors include mitotic rate and ulceration, as well as clinical factors such as age, gender and the anatomic site of the primary tumor. In Sweden, the 5-year melanoma-specific survival rate is 98% for patients in stage IA (tumor thickness <1mm without ulceration, and 49% for patients in stage IVB (tumor thickness >4 mm with ulceration).
Immunohistochemistry is often used to distinguish malignant melanoma from other tumor types. Traditionally, antibodies towards different S-100 proteins have been used as markers of melanocytes, however, these antibodies also stain in e.g. Langerhans cells and nerve fibers. Other markers, such as Melan-A (MART-1) and tyrosinase (TYR), stain melanocytes more specifically and are useful to determine if a tumor is of melanocytic origin. Proliferation markers are also widely used in the differential diagnostics of melanocytic lesions with uncertain malignant potential. The most accepted marker for determining mitotic cells is Ki-67 (MKI67). The presence of Ki-67 positive melanocytic cells is often used in routine pathology to distinguish malignant melanoma from benign melanocytic tumors.
Relevant links and publications
Uhlen M et al., A pathology atlas of the human cancer transcriptome. Science. (2017)