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Difficulty: MediumCellular Injury and Adaptation

A 42-year-old man with no history of tobacco use is referred to a pulmonologist for progressive exertional dyspnea that has worsened over the past two years. Pulmonary function tests demonstrate an obstructive pattern that does not improve with inhaled bronchodilators. High-resolution computed tomography of the chest reveals widespread emphysematous changes predominantly affecting the lower lung zones. A comprehensive metabolic panel is notable for mildly elevated aspartate and alanine aminotransferases. To further evaluate the liver abnormalities, a percutaneous liver biopsy is performed. Microscopic examination of the tissue reveals round, eosinophilic globules within the cytoplasm of periportal hepatocytes. These inclusions stain intensely with periodic acid-Schiff (PAS) and resist digestion with diastase. Which of the following best describes the fundamental cellular mechanism responsible for the hepatocyte inclusions seen in this patient?

  1. Defective polypeptide folding leading to failure of transport from the rough endoplasmic reticulumAnswer
  2. B
    Excessive cytosolic storage of complex carbohydrates secondary to an inherited lysosomal hydrolase deficiency
  3. C
    Impaired intracellular coupling of newly synthesized lipids with specific apolipoproteins for export
  4. D
    Accumulation of partially degraded microbial cell wall components within the phagolysosomes of macrophages
  5. E
    Intracellular aggregation of indigestible lipid-protein crosslinks resulting from chronic free radical injury

Answer

Defective polypeptide folding leading to failure of transport from the rough endoplasmic reticulum
This patient's clinical presentation of early-onset, lower-lobe predominant emphysema combined with liver dysfunction is classic for alpha-1 antitrypsin (AAT) deficiency. The disease is most commonly caused by the PiZZ mutation, which results in a substitution of lysine for glutamate. This single amino acid change causes the newly synthesized AAT protein to misfold. The quality control mechanisms within the cell recognize the misfolded protein, preventing it from leaving the rough endoplasmic reticulum (RER) for the Golgi apparatus. Over time, these misfolded proteins aggregate, leading to the formation of PAS-positive, diastase-resistant globules in the cytoplasm of hepatocytes. The accumulation causes direct cellular stress and injury to the liver, potentially progressing to cirrhosis.

Step-by-Step Solution

1
Analyze the patient's clinical presentation and imaging findings.
Early-onset, lower-lobe predominant panacinar emphysema in a non-smoker is highly suspicious for alpha-1 antitrypsin (AAT) deficiency.
Identifying the systemic clinical syndrome correctly frames the differential diagnosis for the concurrent hepatic abnormalities.
2
Interpret the liver biopsy histopathology.
The presence of PAS-positive, diastase-resistant globules in periportal hepatocytes confirms the diagnosis of AAT deficiency.
These specific histochemical staining characteristics successfully differentiate AAT aggregates from glycogen, which is diastase-sensitive.
3
Link the established diagnosis to the underlying mechanism of cellular injury.
In AAT deficiency, a genetic mutation causes the protein to misfold, preventing its exit from the rough endoplasmic reticulum and leading to pathological intracellular aggregation.
This directly answers the prompt by identifying the precise subcellular etiology of the observed hepatocyte inclusions.

Key Concept

Alpha-1 antitrypsin deficiency causes hepatocellular injury through the accumulation of misfolded proteins within the rough endoplasmic reticulum, leading to PAS-positive, diastase-resistant globules.
Estimated Time:1m 30s
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Topics

Cellular Injury and Adaptation

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