Association Between Matrix Metalloproteinase-9 (MMP-9) Immunohistochemical Expression and Clinicopathological Features in Equine Melanoma.

Overview

• This study investigated the expression of the protein matrix metalloproteinase-9 (MMP-9) in equine melanomas and its relationship to various clinical and pathological features of the tumors in horses.
• It found that higher MMP-9 expression is linked to more aggressive tumor characteristics and larger tumor size, providing new insights into the behavior of melanoma in horses.

Background and Objective

• Equine melanoma is a common and significant cancer in horses, especially in gray horses, and behaves differently compared to melanomas in other species.
• MMP-9 is an enzyme involved in the degradation of the extracellular matrix, facilitating tumor invasion and metastasis, and has been linked to tumor aggressiveness in humans and dogs.
• The role and expression pattern of MMP-9 in equine melanoma had not been previously described.
• The study aimed to evaluate MMP-9 immunohistochemical expression in equine melanomas and to analyze its association with clinicopathological features such as tumor malignancy, size, and other histological characteristics.

Methods

• Samples: 68 formalin-fixed, paraffin-embedded melanoma tissue samples from 48 gray horses were analyzed.
• Technique: Immunohistochemistry was used to detect and quantify MMP-9 protein expression within the tumor tissues.
• Clinicopathological variables assessed included:
  • Histological classification (benign vs malignant melanoma)
  • Tumor size (small, medium, etc.)
  • Excision interval (time between tumor detection and surgical removal)
  • Ulceration presence
  • Tumor circumscription (well-defined or infiltrative borders)
  • Tumor localization (anatomical site)
  • Stromal immunolabeling (presence of MMP-9 in surrounding connective tissue)
  • Horse’s age and number of tumors
• Statistical analysis was performed to identify significant associations, with a p-value threshold of 0.05 for significance.

Results

• MMP-9 Expression:
  • 69.1% of tumors showed low MMP-9 expression
  • 27.9% showed high MMP-9 expression
  • 2.9% were negative for MMP-9
• Malignant melanomas had significantly higher MMP-9 expression compared to benign melanomas (p = 0.04), suggesting a link between MMP-9 levels and tumor aggressiveness.
• Medium-sized tumors exhibited significantly higher MMP-9 expression than small tumors (p = 0.02), indicating expression correlates with tumor growth.
• MMP-9 expression positively correlated with the excision interval (p = 0.03), meaning longer periods before tumor removal were associated with higher MMP-9 levels.
• Longer excision intervals were also significantly associated with histological malignancy (p < 0.001), reinforcing that slower treatment may align with more aggressive tumors.
• No significant relationships were found between MMP-9 expression and ulceration, how sharply the tumor edges were defined, the horse’s age, tumor location, or number of tumors.

Conclusions

• This is the first study to characterize MMP-9 expression in equine melanoma tissue using immunohistochemistry.
• MMP-9 expression tends to be low in equine melanoma but increases significantly with malignancy and intermediate tumor sizes.
• The positive correlation between MMP-9 and longer excision intervals suggests that delayed tumor removal may be associated with enhanced tumor aggressiveness.
• The lack of association with some other clinicopathological features implies that MMP-9’s role may be more specifically tied to malignancy and growth rather than all tumor characteristics.
• Understanding MMP-9 expression patterns helps expand the clinicopathological profile of equine melanoma, potentially contributing to better prognostic assessments and therapeutic strategies in veterinary oncology.