This case report illustrates the clinical impact of low ALT activity gene variants in dogs and the interpretation of routine biochemical testing. Patients with this trait may be underdiagnosed or diagnosed later in the course of necroinflammatory hepatic conditions. Additionally, monitoring ALT in these cases may not be as suitable a surrogate biomarker for hepatocellular injury as it is in dogs with normal ALT activity genotypes.
Evidence of hepatic damage in CuCH typically manifests as increased serum ALT activity, histologic changes, or both when hepatic copper concentrations exceed 1000 µg/g dw and almost always occurs at concentrations over 1500 µg/g dw. In this case, ALT concentrations were normal despite the histologic presence of multifocal inflammation and necrotic hepatocytes. At least two possibilities for this observation exist. First, this degree of necroinflammatory disease was insufficient to result in increased serum ALT activity in this dog and unrelated to the CFA13 locus genotype. The second possibility is that this dog’s genotype contributed to its discordantly normal serum ALT activity. The normal serum AST activities measured in this case may support the former assertion. However, in one study, the median AST:ALT ratios in dogs with hepatic fibrosis and necroinflammatory changes were approximately 0.25 (7). Thus, dogs with mild ALT increases would frequently have normal serum AST activities. The original study documenting the low ALT activity gene variant did not include serum AST in their model. In the primary author’s dissertation defense, AST was analyzed, and they found the opposite trend compared to ALT between AST activities and the A allele at the CFA13 locus. Interestingly, however, many heterozygous dogs with hepatic disease had AST activities in the normal range (8). It is impossible to irrefutably explain this patient’s normal ALT activity by its genotype alone. However, given the histologic findings and the reduction in ALT after treatment, we suspect the dogs’ low ALT activity genotype is the most likely explanation for this patient’s discordant clinical pathology results and liver histologic lesions.
Discordance and variability between histologic disease severity, serum hepatic enzyme activities, or hepatic copper concentrations have been documented previously in canine liver diseases (3, 7, 9–11). Chronic hepatitis can be present in dogs with normal serum liver enzyme activities, despite being the most common biochemical abnormality in dogs with chronic hepatitis (4). In one study of dogs with CuCH, ALT activity was increased in all cases where it was measured; however, hepatic copper concentrations were not measured, precluding more direct comparisons (12). This case represents an additional challenge as serum ALT activity is frequently used as a surrogate marker for hepatocellular injury to guide therapy in dogs with CuCH (4). Serum concentrations of some microRNAs, such as miR-122, are increased in dogs with hepatocellular injury (13–16). MicroRNA biomarkers may be helpful in screening and monitoring cases of chronic hepatitis with low ALT activity genotypes but, unfortunately, are not available for routine diagnostic testing. This dog’s increased ALP activity aligns with the histologically observed vacuolar changes and may be partly reactive to CuCH. Therefore, monitoring ALP activity may provide some metric of treatment response, although it may be less specific for copper chelation efficacy than ALT. Ultimately, liver biopsy is the gold standard measure of copper chelation efficacy. However, treatment success is often gauged by ALT monitoring. Thus, there may be a greater need to consider post-treatment liver biopsy to monitor chelation efficacy in dogs with low ALT activity genotypes.
This case illustrates the potential impact of genotype on the interpretation of serum ALT activity in dogs with necroinflammatory hepatic diseases. Genetic testing may guide diagnostic and monitoring decisions that frequently rely on ALT as a surrogate marker of hepatocellular injury. Further investigation of ALT allele variants in dogs with necroinflammatory hepatic disease may help explain some of the discordance observed between the histologic severity of hepatocellular injury and serum ALT activities. This case also illustrates the need for additional clinically available biomarkers for hepatic necroinflammatory conditions, such as CuCH in the dog.