Canine chronic hepatitis (CCH) is characterised histologically by hepatocellular apoptosis or necrosis, a variable mononuclear or mixed inflammatory infiltrate, regeneration and fibrosis. Life expectancy following diagnosis is unpredictable, ranging from days to years. Reported prognostic indicators (hyperbilirubinaemia, hypoalbuminaemia and presence of ascites or cirrhosis) are unreliable, particularly in end-stage disease. Hepatocyte expression of p21, a universal cell-cycle inhibitor and marker of cellular senescence, is strongly negatively correlated with outcome in humans with alcoholic- and non-alcoholic-related liver disease and is a better prognostic marker than histological or clinical scoring systems. P21 expression has not been investigated in CCH.

This study investigated whether hepatocyte p21 expression is increased in CCH and if expression is associated with survival. Cases of CCH were retrieved from the pathology database (2004–2016) and liver biopsy samples were reviewed using routine stains (haematoxylin and eosin, rhodanine, and Sirius Red). Immunohistochemistry was performed using monoclonal mouse anti-human p21 on all selected cases and four control liver samples with normal histology and no history of liver disease. P21 expression was manually quantified by scoring p21-positive and p21-negative hepatocytes in eight high-power fields for each case by two blinded observers and expressed as a percentage of total hepatocyte number. Kendall's tau correlation coefficients were used to assess relationships between p21 expression and survival time or age; significance was set at p<0.05 (2-tailed).

Twenty-two dogs with CCH were selected. There was a significant negative correlation between hepatocellular p21 expression (median 83.17%; range 15.30–99.40) and survival (460 days; range 12–1559), but no correlation between p21 expression and age (median 7.3 years; range 2.0–12.0). Inclusion or exclusion of cases with copper-associated disease gave similar results. Three of four control samples from young dogs (<6.5 y) displayed negligible p21 expression as expected (<10%). The fourth control sample from a 16.5-year-old dog had a high percentage of p21-positive hepatocytes (85%).

This study demonstrates that p21 expression in normal dogs increases with age as expected but is upregulated in CCH, indicating a state of cellular senescence irrespective of age. The median percentage of senescent hepatocytes in CCH was higher than in human hepatitis. These cells are indistinguishable from normal hepatocytes using routine stains but are unable to perform normal hepatic functions. P21 quantification in CCH may therefore provide additional information regarding hepatic function. Furthermore, a negative correlation between p21 expression and survival was identified, suggesting a role for p21 quantification in determining prognosis in CCH.

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