Multicentric Malignant Melanoma in a Grey Mare: A Case Report.

Overview

• This research article reports a detailed case study of a 26-year-old grey mare diagnosed with widespread malignant melanoma linked to genetic factors associated with the grey coat color.
• The study combines clinical observations, pathological findings, and genomic analysis to understand melanoma progression and spread, including previously under-recognized cardiac metastases.

Background and Significance

• The grey coat color in horses arises from a specific genetic variation — a 4.6 kb copy number variation (CNV) within the syntaxin 17 (STX17) gene, also called the Grey locus.
• This genetic variant functions as a melanocyte-specific enhancer, which means it influences pigment-producing cells and leads to progressive hair depigmentation (turning grey) over time.
• Importantly, horses carrying this genetic variant have a much higher risk of developing melanoma, a malignant tumor of melanocytic origin.
• Understanding the genetic and pathological basis of melanoma in grey horses helps in early diagnosis, management, and potentially improving animal welfare.

Case Description

• The subject was a 26-year-old grey mare with a distinctive “flea-bitten” coat appearance, which signals dispersed pigmentation spots amidst greying hair.
• Clinically, the mare showed multiple melanocytic tumors affecting skin, lymph nodes, and internal organs, especially the heart.
• Progressive symptoms included worsening breathing difficulties (dyspnea) and congestive heart failure, prompting humane euthanasia.
• Following euthanasia, a necropsy was conducted to investigate the extent and nature of melanoma dissemination.

Pathological Findings

• Necropsy identified widespread melanocytic tumors beyond the skin and lymph nodes, also involving visceral organs—most notably the pericardium (the heart’s outer lining) and the right atrium.
• The discovery of cardiac metastases is significant because such involvement has been underdiagnosed in equine melanoma cases.
• The cardiac metastasis likely contributed directly to the mare’s heart failure symptoms.

Genetic Analysis and Interpretation

• Genotyping of normal tissue confirmed the mare bore a G1/G3 allele combination at the Grey locus, indicating increased melanoma risk and faster greying.
• Tumor tissues exhibited additional somatic copy number variations, with amplifications of the Grey locus resulting in up to six total copies of this genomic region.
• This amplification suggests that beyond inherited risk, further genetic changes within tumors may drive malignant transformation and aggressiveness.
• The evidence supports a model where CNV expansion at the Grey locus amplifies the pathological process in melanoma development.

Conclusions and Implications

• This case reinforces the link between CNV expansions at the Grey locus and malignant melanoma progression in grey horses.
• The presence of cardiac metastases highlights the need for more thorough cardiovascular assessments in horses suspected of having advanced melanoma.
• It mirrors similar under-recognized metastatic patterns observed in human and canine melanomas, suggesting broader comparative relevance.
• Early genetic screening for high-risk grey horse genotypes could improve monitoring, diagnosis, and clinical outcomes.
• The work advocates integrating genomic data with clinical and pathological assessments to enhance understanding of melanoma biology and guide management.
• Future research with larger populations is needed to confirm these findings and to develop targeted interventions.