Age-related testicular degeneration can be defined as the progressive deterioration of the testis that typically occurs in middle-aged or older males and that leads to diminished testicular function and subfertility. In the equine breeding industry, genetically valuable males maintain their value as breeding animals well into old age. Because testicular degeneration is common in middle-aged and older stallions, the disease often has a significant negative impact on a stallion’s breeding career and leads to economic losses in the horse breeding industry.
Because testicular degeneration is a tissue autologous disease in the horse, the objective of this study was to use whole-transcriptome sequencing to compare the testicular transcriptomes of normal, fertile stallions to those of stallions affected by age-related testicular degeneration in order to better understand the pathophysiology of the disease.
Cross sectional.
Testicular tissue samples from clinical castrations or euthanasia were collected from normal healthy (n=3) or older subfertile (n=4) stallions. Samples were processed and sequenced on an Illumina HiSeq™ 2000 Sequencing System. Bioinformatic analysis of the data was performed in R/RStudio, and the transcriptomes were compared between the two groups. Genes were considered to be differentially expressed between healthy and diseased tissue if they demonstrated at least a ±1.5x fold change difference and had an FDR-adjusted P value < 0.05. Gene ontology analysis was performed using Ingenuity® IPA.
Analyses of differential expression of individual genes, as well as computer-based gene ontology analysis, identified upregulation of cytokine-mediated inflammatory pathways in testes from stallions affected with testicular degeneration. This upregulation of inflammation was associated with upregulation of cell survival pathways, inhibition of apoptotic pathways and increases in collagen formation.
There are unavoidable confounding factors (e.g. differences in breed, management, environment, age) that could create non disease-related genetic variation between our normal and affected samples. In addition, there are practical limitations to applying computer-based gene ontology analysis to equine samples. Gene ontology software relies on published information (mostly non-equine), and some biological processes (e.g. apoptosis and inflammation) are more commonly studied than others and so are over-represented in the literature and therefore more likely to be identified by computer algorithms. Caution should be taken when interpreting the data, as alterations in gene expression can be the cause of disease processes or can be the result of disease processes.
These results suggest that chronic, low-grade inflammation may be involved in the pathophysiology of age-related testicular degeneration in stallions.

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