Abstract
Oral mucosal melanomas (OMMs) are aggressive and resistant cancers of high importance in veterinary oncology. Amelanotic OMM produces comparatively less melanin and is considered to be more aggressive than melanotic OMM. Global DNA methylation profiles with hypomethylated or hypermethylated patterns have both been associated with aggressive neoplasms; however, global DNA hypomethylation seems to correlate to higher aggressiveness. Accordingly, global DNA methylation in peripheral blood leukocytes has been investigated to understand the role of systemic or environmental factors in cancer development. This study aimed to quantify global DNA methylation in canine melanotic and amelanotic OMM samples and in the peripheral blood leukocytes of the same dogs. Tumor tissue samples were collected from 38 dogs, of which 19 were melanotic and 19 were amelanotic OMM. These were submitted to immunohistochemistry (IHC) with anti-5-methylcytosine (5mC) and anti-Ki67 primary antibodies. Ki67- and 5mC-positive nuclei were manually scored with the help of an image analysis system. Peripheral blood samples were collected from 18 among the 38 OMM-bearing dogs and from 7 additional healthy control dogs. Peripheral blood leukocytes were isolated from the 25 dogs, and DNA was extracted and analyzed by high-performance liquid chromatography (HPLC) for global DNA methylation. The pattern of global DNA methylation in both canine melanotic and amelanotic OMM indicated higher percentages of weakly or negatively stained nuclei in most of the OMM cells, presuming predominant global DNA hypomethylation. In addition, Ki67 counts in amelanotic OMM were significantly higher than those in melanotic OMM (p < 0.001). Global DNA methylation different immunostaining patterns (strong, weak or negative) correlated with Ki67 scores. Global DNA methylation in circulating leukocytes did not differ between the 9 melanotic and 9 amelanotic OMM or between the 18 OMM-bearing dogs and the 7 healthy dogs. This study provides new information on canine melanotic and amelanotic OMM based on global DNA methylation and cell proliferation.
Highlights
Melanoma is the most common malignant tumor in the oral cavity of dogs, accounting for 30–40% of all oral cavity cancers in dogs and 7% of all types of cancer in this species [1, 2]
Amelanotic and melanotic OMM samples (9 samples of each) were obtained from the animals during the surgical procedures for removal of tumors, or electrochemotherapy procedures, and blood samples from the same dogs were obtained for the quantification of leukocyte global DNA methylation
The dog breeds with melanotic OMM were Mongrels (8/19), Poodles (4/19), Beagles (1/19), Lhasa Apsos (1/19), Golden Retriever (2/19), Schnauzer (2/19) and Yorkshier (1/19), while those with amelanotic OMM were Mongrels (11/19), and Yorkshires (2/19), Pischer (1/19), Pug (1/19), Teckel (2/18), Cocker Spaniel (1/18)
Summary
Melanoma is the most common malignant tumor in the oral cavity of dogs, accounting for 30–40% of all oral cavity cancers in dogs and 7% of all types of cancer in this species [1, 2]. Breeds with higher mucosal pigmentation have a higher risk of developing oral melanoma, the Scottish Terrier, Golden Retriever, Dachshund, Cocker Spaniels, and Poodles. An epidemiological study involving 2,350 affected dogs revealed that poodles are at a high risk of developing oral melanomas [3]. 40% of OMM cases without lymphadenomegaly already have lymph node metastasis. The presence of metastatic lymph nodes at the time of diagnosis accounts for ∼25% [2, 4]. Goldschmidt et al [5] reported that melanomas in canines could be classified in melanotic (or melanocytic), when they kept the capacity to produce melanin, and amelanotic (or amelanocytic) when this capacity was totally or partially lost
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