JPC SYSTEMIC PATHOLOGY

NERVOUS SYSTEM

January 2023

N-M16 (NP)

 

Signalment (JPC #2320366): A young Gunn rat

 

HISTORY: This rat was jaundiced prior to death.

 

HISTOPATHOLOGIC DESCRIPTION: Medulla oblongata and cerebellum: Multifocally, Purkinje cells and neurons within brainstem nuclei are often either swollen with multiple variably sized, clear, intracytoplasmic vacuoles (neuronal degeneration); shrunken with hypereosinophilic cytoplasm, variable presence of intracytoplasmic vacuoles, and pyknotic or karyolytic nuclei (necrosis); or Purkinje cells are absent leaving remaining well-defined, round to oval, clear foci that contain scant cellular and karyorrhectic debris (empty baskets). There is mild brainstem gliosis. 

 

MORPHOLOGIC DIAGNOSIS: Medulla oblongata, brainstem nuclei neurons; cerebellum, Purkinje cells: Neuronal degeneration, necrosis, and loss, multifocal, moderate, Gunn rat, rodent.

 

ETIOLOGIC DIAGNOSIS: Hereditary unconjugated hyperbilirubinemic neuropathy 

 

CAUSE: Uridine diphosphate glucuronyltransferase (UGT1A1) deficiency 

 

GENERAL DISCUSSION:  

• Hereditary unconjugated hyperbilirubinemic neuropathy is a disease that occurs due to a lack of the hepatic enzyme uridine diphosphate glucuronyltransferase (UGT1A1) which results in accumulation of toxic levels of bilirubin and neuronal necrosis (kernicterus)
• Autosomal recessive deficiency of glucuronyl transferase due to a frame shift deletion of a single guanine residue
  • Homozygous animals have increased susceptibility to displacement of albumin-bound bilirubin after administration of sulfonamides or salicylates resulting in increased unconjugated bilirubin
  • Heterozygous animals have 50% glucuronyl transferase activity and are not jaundiced
• Enzyme deficient rats develop alternate pathways for excreting bilirubin into the intestine and urine
• The Gunn rat is a mutant strain of the Wistar rat and is the only animal in which bilirubin-induced brain damage occurs spontaneously

 

PATHOGENESIS:  

• Bilirubin is the end product of heme metabolism and is considered a cytotoxic agent
• Under normal conditions, almost all bilirubin secreted in the bile is conjugated and cannot pass the blood brain barrier
• Lack of the hepatic enzyme required to conjugate bilirubin and glucuronic acid (UDPGT) allows unconjugated bilirubin to accumulate and cross the blood-brain barrier where it is toxic to neurons
  • Movement of bilirubin into the bile canaliculus for excretion is via Mrp-2 (as is reduced glutathione); Mrp-2 mutation may also play a role in hyperbilirubinemia in rats (EHBR (Eisai hyperbilirubinemic rats) and TR¯)
• Increased bilirubin causes decreased cell respiration, uncoupling of oxidative phosphorylation, inhibition of mitochondrial enzymes, and impairment of protein synthesis; other changes include disruption of plasma and mitochondrial membranes and changes in intracellular calcium concentration
• Increased levels of bilirubin also cause medullary necrosis in the kidney

 

TYPICAL CLINICAL FINDINGS:  

• Icterus, stunted growth, ataxia and a slight normocytic anemia
• Cerebellar hypoplasia develops postnatally
• Bilirubin levels are slightly elevated to normal at birth
• By seven days of age many cells are affected (especially Purkinje cells), and there is concurrent neuromotor ataxia, weight loss, and incoordination

 

TYPICAL GROSS FINDINGS:  

• Kernicterus: Macroscopic canary yellow staining of susceptible nuclei in the brain (cerebellum, especially Purkinje cells; deep cerebellar nuclei; nuclear floor of the fourth ventricle; basal ganglion; hippocampus; subthalamic nuclei; cranial nerve nuclei) 
  • Also caused by bilirubin encephalopathy in neonatal domestic animals 
• Cerebellar hypoplasia
• Mild yellow discoloration of the kidneys and fat

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:  

• Ballooning and loss of Purkinje cells
• Neuronal cell swelling with pyknotic eccentric nuclei (neuronal necrosis in the specific sites / nuclei cited above)
• Affected cells contain bright yellow granules within the cytoplasm in frozen tissue  
• +/- Alzheimer type II astrocytes 

 

ULTRASTRUCTURE:  

 

DIFFERENTIAL DIAGNOSIS:  

For icterus:

• Hepatic disease 
• Intravascular hemolysis

 

 

 

COMPARATIVE PATHOLOGY:  

Causes of kernicterus or hyperbilirubinemia:

• Gunn rat is the animal model for Crigler-Najjar syndrome type I in humans which causes severe icterus, kernicterus and death
• Corriedale sheep (and humans): Dubin-Johnson-like syndrome is reported; results in an abnormality in excretion of conjugated bilirubin (idiopathic icterus) and phylloerythrin (photosensitization); suspected Mrp-2 mutation 
• Southdown sheep (and humans): Gilbert’s-like syndrome is reported; results in unconjugated hyperbilirubinemia due to impaired hepatic uptake; no icterus due to partial bilirubin excretion
• Horses: acute hepatic dysfunction/obstruction is the most common cause of icterus
• Foals, kittens, rarely in puppies:  Neonatal isoerythrolysis may result in bilirubin encephalopathy (kernicterus) in some cases if bilirubin conjugation is overwhelmed by excess hemolysis (i.e. in foals that consume anti-RBC antibodies in colostrum)
• Cat:  Feline hepatic steatosis in obese female cats (fatty liver), infectious anemias (Cytauxzoon felis, Mycoplasma spp., Babesia spp.), oxidative anemia ("Heinz body” hemolytic anemia)
• Dog: infectious anemias (Babesia canis, B. gibsoni)
• Cattle, sheep, goats; Lantana camara poisoning (severe icterus and photosensitization), infectious anemias (Anaplasma spp., Theileria spp.)
• Golden lion tamarins: hereditary deficiency of bilirubin transport

 

References:  

1. Car BD, Eng VM, Everds NE, Bounous DI. Clinical Pathology of the Rat. In: Suckow MA, ed. The Laboratory Rat. 2^nd ed. San Diego, CA: Elsevier Inc; 2006:137.
2. Cardona JC, Johnsrude JD, McManus PM, MacWilliams PS. The Spleen. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and Hematology of the Dog and Cat. 5th ed. St. Louis, MO: Elsevier Mosby; 2014:358-359.
3. Cullen JM, Stalker MJ. Liver and biliary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Saunders Elsevier; 2016: 277, 293-294, 338.
4. Lehman-McKeeman LD, Ruepp SU. Biochemical and Molecular Basis of Toxicity. In: Walling MA, ed. Fundamentals of Toxicologic Pathology. 3rd ed. Cambridge, MA: Elsevier; 2018:24. 
5. Meyer DJ. The Liver. In: Raskin RE, Meyer DJ, eds. Canine and Feline Cytology: A Color Atlas and Interpretation Guide. 3rd ed. St. Louis, MO: Elsevier; 2016:270.
6. Miller MA, Zachary JF. Mechanisms and Morphology of Cellular Injury, Adaptation, and Death. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7^th ed. St. Louis, MO: Mosby Elsevier; 2017:39.
7. Stacy NI, Hollinger C. Introduction to Comparative Clinical Pathology. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. London, UK: Academic Press; 2018:87. 
8. Stockham SL, Scott MA. Fundamentals of Veterinary Clinical Pathology. 2nd ed. Hoboken, NJ: Wiley; 2013: 177-187, 684. 
9. Van Wettere AJ, Brown DL. Hepatobiliary System and Exocrine. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:498-499. 

Click the slide to view.

---