Objective—To determine the tissue-restricted expression pattern of tyrosinase mRNA in canine and equine melanocytic tumors and relative tyrosinase and major histocompatibility complex (MHC) I mRNA expression in variants of melanocytic tumors.
Sample—39 canine and 8 equine tumor samples and 10 canine and 6 equine normal tissue samples.
Procedures—RNA was isolated from formalin-fixed, paraffin-embedded tissues. Real-time PCR assays were designed to amplify canine and equine tyrosinase, S18 ribosomal RNA, and major histocompatibility complex I transcripts. Relative expression was determined by use of S18 as a reference and comparison with pigmented and nonpigmented normal tissues.
Results—High tyrosinase expression was found in all melanocytic tumors, compared with normal tissues, and expression had no correlation with presence or absence of tumor pigmentation. No significant difference in tyrosinase expression was found among histologic variants of melanocytic tumors. No correlation was found between MHC I and tyrosinase expression or tissue histologic classification.
Conclusions and Clinical Relevance—In the present study, the methods used were highly sensitive and specific for detection of tyrosinase expression in equine and canine tumors, and overexpression of this transcript in melanomas was detected. This suggested that a DNA vaccine developed for use in dogs with melanoma that targets tyrosinase may be considered for use in other affected species, such as horses.
A 25-year-old 4.4-kg male aquarium-hatched African penguin (Spheniscus demersus) was evaluated because of a raised 1.5 × 0.5-cm pigmented mass extending from within the right naris noted 2 days earlier.
The penguin had a raised pigmented mass extending out from the right naris and onto the upper beak. Histologic examination of excisional biopsy specimens confirmed a diagnosis of malignant melanoma. A treatment plan including administration of meloxicam, radiation therapy, and immunotherapy was initiated.
TREATMENT AND OUTCOME
Treatment with meloxicam (0.2 mg/kg, PO, q 24 h) was initiated and continued for a total of 45 weeks; however, the medication was discontinued for a period of 6 weeks because of the risk of toxic effects in the chick that the penguin was feeding at that time. The penguin underwent local hypofractionated radiation therapy and received 4 once weekly 8-Gy fractions of radiation (total radiation dose, 32 Gy). The penguin was administered a canine melanoma vaccine transdermally every other week for 4 doses, with a booster injection given 7 months after the first dose. Treatment with the vaccine appeared to have no adverse effects. The penguin’s pre- and postvaccination tyrosinase-specific antibody titers were measured with an anti–human tyrosinase-specific ELISA, and a 3-fold titer increase indicated a positive humoral immune response to the canine melanoma vaccination. The penguin died of unrelated causes 54 weeks after initial diagnosis, and there was no evidence of metastasis on necropsy.
These case findings suggested that vaccination with a canine melanoma vaccine may be a safe and useful adjunct treatment for management of malignant melanoma in penguins.