Breed-related expression patterns of Ki67, γH2AX, and p21 during ageing in the canine liver.

Cellular senescence is a molecular hallmark of ageing that is associated with multiple pathologies, and DNA damage marker γH2AX, together with cell cycle inhibitor p21, have been used as senescence markers in multiple species including dogs. Idiopathic canine chronic hepatitis has recognised breed-related differences in predisposition and prognosis, but reasons behind this are poorly understood. This retrospective study using archived post mortem tissue aimed to provide insight into liver ageing in 51 microscopically normal canine livers across seven breed categories, including those with and without increased risk of chronic hepatitis. Immunohistochemistry was conducted for γH2AX, p21, and cell proliferation marker Ki67, and the mean number of positive hepatocytes per high power field was determined. All three markers were strongly correlated to each other, but no age-dependent expression was seen in the combined study population. Overall expression levels were low in most dogs, with median values representing less than 1.5% of hepatocytes, but this increased to 20-30% in individual dogs at the upper end of the range. Individual breed differences were noted in two breeds that have increased risk of chronic hepatitis, with English Springer Spaniels having lower expression of Ki67 than other dogs, and Labradors having higher expression of Ki67 and γH2AX than other dogs. These results warrant further investigation in these breeds and highlight a need to validate reliable markers of cellular senescence in dogs.