RACK1, a clue to the diagnosis of cutaneous melanomas in horses

The Impact of Excision Interval on Equine Melanoma Progression: Time Matters?

Equine melanomas are a common neoplasm in gray horses. However, scientific knowledge about their progression over time is quite scarce. Some owners and veterinarians still believe that early intervention is not necessary, stating that tumors evolve very slowly and intervention could worsen the animal's condition. This work aims to identify clinical and histological differences that may exist between equine melanomas with different excision intervals (time between tumor detection and surgical excision). A total of 42 tumors (13 benign and 29 malignant) from 34 horses were included in this study. There was a statistically significant association between excision interval and tumor size (p = 0.038), with tumors excised later being significantly larger than the ones excised sooner. The excision interval was also statistically associated with the number of tumors (p = 0.011), since the horses that carried a tumor for longer seemed to be prone to have multiple tumors. Furthermore, there was an association between excision interval and malignancy (p = 0.035), with tumor excised later being fives times more likely to be malignant. This study provides evidence of delayed excision's effect on the progression of equine melanomas. Additionally, it reinforces the importance of the early excision of these tumors.

Cyclooxygenase-2 (COX-2) Expression in Equine Melanocytic Tumors

Equine melanocytic tumors are common and have an unusual benign behavior with low invasiveness and metastatic rates. However, tumoral mass growth is usually a concern that can have life-threatening consequences. COX-2 is related to oncogenesis, promoting neoplastic cell proliferation, invasion, and metastasis. The aim of this study was to evaluate the immunohistochemical expression of COX-2 in equine melanocytic tumors. Through extension and intensity of labeling, 39 melanocytomas and 38 melanomas were evaluated. Of the malignant tumors, 13.2% were negative and 63.2% presented a low COX-2 expression. Only 6 malignant tumors presented >50% of labeled cells, 18 malignant and 8 benign had an expression between 21 and 50%, 8 malignant and 3 benign tumors had an expression between 6 and 20%, 1 malignant tumor had an expression between 1 and 5%, and 5 malignant and 28 benign tumors had no expression. Malignant tumors showed higher COX-2 expression than did benign tumors, with statistically significant differences. The low levels of COX-2 may be one of the molecular reasons for the presence of expansive mass growth instead of the invasive pattern of other species, which is related to high COX-2 levels.

Programmed Cell Death-Ligand 1 (PD-L1) Immunohistochemical Expression in Equine Melanocytic Tumors

Currently available treatments for equine melanocytic tumors have limitations, mainly due to mass localization and dimension, or the presence of metastases. Therefore, a search for new therapies is necessary. Programmed cell death-ligand 1 (PD-L1) is expressed by several tumors, blocking T cell-mediated elimination of the tumor cells by binding to programmed cell death protein 1 (PD-1). A novel therapeutic approach using PD-1/PD-L1 blockade in human melanoma resulted in tumor regression and prolonged tumor-free survival. This study aimed to evaluate the immunohistochemical expression of PD-L1 in equine melanocytic tumors. A total of 77 melanocytic tumors were classified as benign or malignant and evaluated by extension of labeling. A total of 59.7% of the tumors showed >50% of immunolabeled cells. Regarding malignant tumors, 24/38 tumors presented >50% of labeled cells, 13 tumors presented between 25-50% and one tumor presented <10%. Regarding benign tumors, 22/39 tumors presented >50% of labeled cells, nine tumors presented 25-50%, three tumors presented 10-25%, two tumors presented <10% and three tumors did not present expression. Our results suggest that PD-L1 blockade may be a potential target for immunotherapy in equine melanocytic tumors and that future clinical research trials into the clinical efficacy of the anti-PD-L1 antibody are necessary.

E-Cadherin Immunostaining in Equine Melanocytic Tumors

Melanocytic tumors are an important neoplastic disease in human and veterinary medicine, presenting large differences regarding tumor behavior between species. In horses, these tumors present a prolonged benign behavior, with rare invasiveness and metastases. In humans and small animals, invasion and metastasis have been associated with an Epithelial-Mesenchymal Transition, where the loss of E-cadherin expression plays a key role in tumor progression. This process and the role of E-cadherin have not yet been evaluated in equine melanocytic tumors. This study aimed to assess the immunolabeling of E-cadherin in equine melanocytic tumors and relate this with clinicopathological variables. A total of 72 equine melanocytic tumors were classified as benign and malignant and evaluated by immunohistochemistry for E-cadherin expression. A different pattern of immunostaining was found, contrasting with other species. A total of 69.4% of tumors presented raised immunolabeling of E-cadherin, with 70.7% of melanomas remaining with high expression. The typical loss of immunostaining was not seen in malignant melanomas and no differences were found between benign and malignant melanomas regarding E-cadherin immunostaining. The high immunolabeling of E-cadherin may contribute to the low invasiveness of these tumors, and it is in accordance with the benign behavior of equine melanoma and with the genetic factors associated with its development.

The human and animals' malignant melanoma: comparative tumor models and the role of microbiome in dogs and humans

Currently, the most progressively occurring incident cancer is melanoma. The mouse is the most popular model in human melanoma research given its various benefits as a laboratory animal. Nevertheless, unlike humans, mice do not develop melanoma spontaneously, so they need to be genetically manipulated. In opposition, there are several reports of other animals, ranging from wild to domesticated animals, that spontaneously develop melanoma and that have cancer pathways that are similar to those of humans. The influence of the gut microbiome on health and disease is being the aim of many recent studies. It has been proven that the microbiome is a determinant of the host's immune status and disease prevention. In human medicine, there is increasing evidence that changes in the microbiome influences malignant melanoma progression and response to therapy. There are several similarities between some animals and human melanoma, especially between canine and human oral malignant melanoma as well as between the gut microbiome of both species. However, microbiome studies are scarce in veterinary medicine, especially in the oncology field. Future studies need to address the relevance of gut and tissue microbiome for canine malignant melanoma development, which results will certainly benefit both species in the context of translational medicine.

Equine Melanocytic Tumors: A Narrative Review

Adult grey horses have a high incidence of melanocytic tumors. This article narratively reviews the role of some genetic features related to melanoma formation in horses, such as STX17 mutation, ASIP or MITF alterations, and the link between the graying process and the development of these tumors. A clear system of clinical and pathological classification of melanocytic tumors in naevus, dermal melanoma, dermal melanomatosis and anaplastic malignant melanoma is provided. Clinical and laboratorial methods of diagnosing are listed, with fine needle aspiration and histopathology being the most relevant. Relevance is given to immunohistochemistry, describing potentially important diagnostic biomarkers such as RACK1 and PNL2. Different therapeutical options available for equine practitioners are mentioned, with surgery, chemotherapy and electroporation being the most common. This article also elucidatesnew fields of research, perspectives, and new therapeutic targets, such as CD47, PD-1 and COX-2 biomarkers.

Interventions for treatment of cutaneous melanoma in horses: a structured literature review

Several therapies have been developed to treat equine cutaneous melanoma, but formal comparisons among different treatment options are currently unavailable. It was our intent to assess the efficacy of different treatment protocols and the quality of the studies based on the original published data, and summarize the knowledge concerning the outcome after equine cutaneous melanoma management. This structured review followed PRISMA procedure to search for treatment protocols on equine cutaneous melanoma published from 1960 until June 2021. Studies were assessed for the risk of bias. A descriptive analysis was performed, considering the disease control rate, the recurrence rate of the tumor, comorbidities, need for anesthesia, and horses' welfare. Twenty-three studies were included, from which the treatment outcomes of 173 horses were assessed. The homogeneity of the included trials was low. The percentages of each treatment arm achieving a partial response and curative effects accounted for 93.1% (surgical intervention), 90% (medication), and 39.4% (immunotherapies), respectively. A variable efficacy of different therapies of equine cutaneous melanoma was observed. Surgical intervention performed the best from the perspective of local antitumor effects alone. This literature review and descriptive analysis can serve as a source to assist in designing quality therapy research and can potentially aid in providing a clinical treatment reference for equine cutaneous melanoma.

Spontaneously occurring melanoma in animals and their relevance to human melanoma

In contrast to other cancer types, melanoma incidence has been increasing over the last 50 years, and while it still represents less than 5% of all cutaneous malignancies, melanoma accounts for the majority of skin cancer deaths, due to its propensity to metastasise. Whilst melanoma most commonly affects the skin, it can also arise in mucosal surfaces, the eye, and the brain. For new therapies to be developed, a better understanding of the genetic landscape, signalling pathways, and tumour–microenvironmental interactions is needed. This is where animal models are of critical importance. The mouse is the foremost used model of human melanoma. Arguably this is due to its plethora of benefits as a laboratory animal; however, it is important to note that unlike humans, melanocytes are not present at the dermal–epidermal junction in mice and mice do not develop melanoma without genetic manipulation. In contrast, there are numerous reports of animals that spontaneously develop melanoma, ranging from sharks and parrots to hippos and monkeys. In addition, several domesticated and laboratory‐bred animals spontaneously develop melanoma or UV‐induced melanoma, specifically, fish, opossums, pigs, horses, cats, and dogs. In this review, we look at spontaneously occurring animal ‘models’ of melanoma and discuss their relevance to the different types of melanoma found in humans. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland..

Genetics of Skin Disease in Horses

Equine skin diseases are common, causing increased costs and reduced welfare of affected horses.Genetic testing, if available, can complement early detection, disease diagnosis, and clinical treatment and offers horse breeders the possibility to rule out carrier status. The mechanisms of complex disease can be investigated by using the latest state-of-the-art genomic technologies. Genome-based strategies may also serve as an efficient and cost-effective strategy for the management of the disease severity levels, with particular interest in complex traits such as insect bite hypersensitivity, chronic progressive lymphedema, and melanoma.

Melanoma-Bearing Libechov Minipig (MeLiM): The Unique Swine Model of Hereditary Metastatic Melanoma

National cancer databases document that melanoma is the most aggressive and deadly cutaneous malignancy with worldwide increasing incidence in the Caucasian population. Around 10% of melanomas occur in families. Several germline mutations were identified that might help to indicate individuals at risk for preventive interventions and early disease detection. More than 50% of sporadic melanomas carry mutations in Ras/Raf/mitogen-activated protein kinase (MAPK/MEK) pathway, which may represent aims of novel targeted therapies. Despite advances in targeted therapies and immunotherapies, the outcomes in metastatic tumor are still unsatisfactory. Here, we review animal models that help our understanding of melanoma development and treatment, including non-vertebrate, mouse, swine, and other mammal models, with an emphasis on those with spontaneously developing melanoma. Special attention is paid to the melanoma-bearing Libechov minipig (MeLiM). This original swine model of hereditary metastatic melanoma enables studying biological processes underlying melanoma progression, as well as spontaneous regression. Current histological, immunohistochemical, biochemical, genetic, hematological, immunological, and skin microbiome findings in the MeLiM model are summarized, together with development of new therapeutic approaches based on tumor devitalization. The ongoing study of molecular and immunological base of spontaneous regression in MeLiM model has potential to bring new knowledge of clinical importance.

The MeLiM Minipig: An Original Spontaneous Model to Explore Cutaneous Melanoma Genetic Basis

Melanoma is the deadliest skin cancer and is a major public health concern with a growing incidence worldwide. As for other complex diseases, animal models are needed in order to better understand the mechanisms leading to pathology, identify potential biomarkers to be used in the clinics, and eventually molecular targets for therapeutic solutions. Cutaneous melanoma, arising from skin melanocytes, is mainly caused by environmental factors such as UV radiation; however a significant genetic component participates in the etiology of the disease. The pig is a recognized model for spontaneous development of melanoma with features similar to the human ones, followed by a complete regression and a vitiligo-like depigmentation. Three different pig models (MeLiM, Sinclair, and MMS-Troll) have been maintained through the last decades, and different genetic studies have evidenced a complex inheritance of the disease. As in humans, pigmentation seems to play a prominent role, notably through MC1R and MITF signaling. Conversely, cell cycle genes as CDKN2A and CDK4 have been excluded as predisposing for melanoma in MeLiM. So far, only sparse studies have focused on somatic changes occurring during oncogenesis, and have revealed major cytological changes and a potential dysfunction of the telomere maintenance system. Finally, the spontaneous tumor progression and regression occurring in these models could shed light on the interplay between endogenous retroviruses, melanomagenesis, and adaptive immune response.

RACK1 cooperates with NRASQ61K to promote melanoma in vivo

Melanoma is the deadliest skin cancer. RACK1 (Receptor for activated protein kinase C) protein was proposed as a biological marker of melanoma in human and domestic animal species harboring spontaneous melanomas. As a scaffold protein, RACK1 is able to coordinate the interaction of key signaling molecules implicated in both physiological cellular functions and tumorigenesis. A role for RACK1 in rewiring ERK and JNK signaling pathways in melanoma cell lines had been proposed. Here, we used a genetic approach to test this hypothesis in vivo in the mouse. We show that Rack1 knock-down in the mouse melanoma cell line B16 reduces invasiveness and induces cell differentiation. We have developed the first mouse model for RACK1 gain of function, Tyr::Rack1-HA transgenic mice, targeting RACK1 to melanocytes in vivo. RACK1 overexpression was not sufficient to initiate melanomas despite activated ERK and AKT. However, in a context of melanoma predisposition, RACK1 overexpression reduced latency and increased incidence and metastatic rate. In primary melanoma cells from Tyr::Rack1-HA, Tyr::NRas^Q61K mice, activated JNK (c-Jun N-terminal kinase) and activated STAT3 (signal transducer and activator of transcription 3) acted as RACK1 oncogenic partners in tumoral progression. A sequential and coordinated activation of ERK, JNK and STAT3 with RACK1 is shown to accelerate aggressive melanoma development in vivo.

Cross-species models of human melanoma

Although transformation of melanocytes to melanoma is rare, the rapid growth, systemic spread, as well as the chemoresistance of melanoma present significant challenges for patient care. Here we review animal models of melanoma, including murine, canine, equine, and zebrafish models, and detail the immense contribution these models have made to our knowledge of human melanoma development, and to melanocyte biology. We also highlight the opportunities for cross-species comparative genomic studies of melanoma to identify the key molecular events that drive this complex disease.

Constitutive activation of the ERK pathway in melanoma and skin melanocytes in Grey horses

Background

Constitutive activation of the ERK pathway, occurring in the vast majority of melanocytic neoplasms, has a pivotal role in melanoma development. Different mechanisms underlie this activation in different tumour settings. The Grey phenotype in horses, caused by a 4.6 kb duplication in intron 6 of Syntaxin 17 (STX17), is associated with a very high incidence of cutaneous melanoma, but the molecular mechanism behind the melanomagenesis remains unknown. Here, we investigated the involvement of the ERK pathway in melanoma development in Grey horses.

Methods

Grey horse melanoma tumours, cell lines and normal skin melanocytes were analyzed with help of indirect immunofluorescence and immunoblotting for the expression of phospho-ERK1/2 in comparison to that in non-grey horse and human counterparts. The mutational status of BRAF, RAS, GNAQ, GNA11 and KIT genes in Grey horse melanomas was determined by direct sequencing. The effect of RAS, RAF and PI3K/AKT pathways on the activation of the ERK signaling in Grey horse melanoma cells was investigated with help of specific inhibitors and immunoblotting. Individual roles of RAF and RAS kinases on the ERK activation were examined using si-RNA based approach and immunoblotting.

Results

We found that the ERK pathway is constitutively activated in Grey horse melanoma tumours and cell lines in the absence of somatic activating mutations in BRAF, RAS, GNAQ, GNA11 and KIT genes or alterations in the expression of the main components of the pathway. The pathway is mitogenic and is mediated by BRAF, CRAF and KRAS kinases. Importantly, we found high activation of the ERK pathway also in epidermal melanocytes, suggesting a general predisposition to melanomagenesis in these horses.

Conclusions

These findings demonstrate that the presence of the intronic 4.6 kb duplication in STX17 is strongly associated with constitutive activation of the ERK pathway in melanocytic cells in Grey horses in the absence of somatic mutations commonly linked to the activation of this pathway during melanomagenesis. These findings are consistent with the universal importance of the ERK pathway in melanomagenesis and may have valuable implications for human melanoma research.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2407-14-857) contains supplementary material, which is available to authorized users.

Equine melanocytic tumors

Melanomas are among the most common skin tumors in horses, with prevalence rates reaching as high as 80% in adult gray horses. Most melanocytic tumors are benign at initial presentation; however, if left untreated, up to two-thirds can progress to overt malignant behavior. Standard local treatment options can be used to treat solitary early-stage lesions but do not address the underlying risk of recurrent tumor formation or the transformation to a malignant phenotype. An understanding of the specific molecular genetic factors associated with tumor formation should lead to targeted therapies that can be used to treat or ideally prevent disease.

Canine melanoma diagnosis: RACK1 as a potential biological marker

Melanoma diagnosis in dogs can be challenging due to the variety of histological appearances of canine melanocytic neoplasms. Markers of malignancy are needed. Receptor for activated C-kinase 1 (RACK1) was found to characterize melanomas in other mammals. We investigated the value of RACK1 detection in the classification of 19 cutaneous and 5 mucosal melanocytic neoplasms in dogs. These tumors were categorized as melanocytomas or benign and melanomas or malignant after evaluation of their morphology, mitotic index, and Ki-67 growth fraction. Using immunofluorescence, we confirmed microphthalmia-associated transcription factor (MITF) as a marker of normal and transformed melanocytic cells in dog tissues. All control (n = 10) and tumoral (n = 24) samples stained positively for MITF (34/34, 100%). Whereas RACK1 was not detected in healthy skin melanocytes, melanocytic lesions were all positive for RACK1 signal (24/24, 100%). RACK1 cytoplasmic staining appeared with 2 distinct distribution patterns: strong, diffuse, and homogeneous or granular and heterogeneous. All melanoma samples (13/13, 100%) stained homogeneously for RACK1. All melanocytomas (11/11, 100%) stained heterogeneously for RACK1. Immunohistochemistry was less consistent than immunofluorescence for all labelings in melanocytic lesions, which were often very pigmented. Thus, the fluorescent RACK1-MITF labeling pattern helped to distinguish melanomas from melanocytomas. Furthermore, RACK1 labeling correlated with 2 of 11 morphological features linked to malignancy: cell and nuclear size. These results suggest that RACK1 may be used as a marker in dog melanomas.