# Clinical Cases: Translation and Protein Synthesis ## Case 1: Diphtheria (EF-2 Inhibition) ### Clinical Image ![Diphtheria Pseudomembrane](case_01_image.jpg) *Source: [Wikipedia - Diphtheria](https://en.wikipedia.org/wiki/Diphtheria) - CC BY-SA 3.0* ### Case Presentation A 4-year-old unvaccinated child from an underserved community presents with fever, sore throat, and difficulty breathing for 3 days. He has not received routine childhood vaccinations. Physical examination reveals cervical lymphadenopathy ("bull neck"), low-grade fever, and an adherent grayish-white pseudomembrane covering his tonsils and pharynx that bleeds when an attempt is made to remove it. He also has a hoarse voice suggesting laryngeal involvement. Nasopharyngeal culture grows Corynebacterium diphtheriae. He is immediately treated with diphtheria antitoxin (to neutralize circulating toxin) and antibiotics (erythromycin). The mechanism of diphtheria toxin is explained: it is an AB toxin where the B subunit binds heparin-binding EGF-like growth factor receptor, enabling entry into cells. The A subunit catalyzes ADP-ribosylation of elongation factor 2 (EF-2), specifically modifying a unique amino acid called diphthamide. This irreversibly inactivates EF-2, blocking translocation during protein synthesis and halting all protein production in the cell. Each toxin molecule can inactivate many EF-2 molecules catalytically, making the toxin extraordinarily potent. Heart block develops on day 5 (myocarditis from toxin effect), requiring temporary pacing. He survives but requires long-term cardiac monitoring. ### Key Learning Points - Diphtheria toxin ADP-ribosylates elongation factor 2 (EF-2), inactivating it and blocking the translocation step of protein synthesis; this affects all cells but is particularly devastating to high-metabolic tissues like heart and nerve - EF-2 is essential for translocation, the step that moves the ribosome one codon along the mRNA after peptide bond formation; without functional EF-2, protein synthesis halts completely - The modification is at diphthamide, a unique post-translationally modified histidine residue found only in EF-2, explaining the toxin's specificity; antitoxin must be given early before toxin enters cells --- ## Case 2: Aminoglycoside-Induced Ototoxicity (Ribosomal Inhibition) ### Clinical Image ![Aminoglycoside Ototoxicity](case_02_image.jpg) *Source: [Wikipedia - Aminoglycoside](https://en.wikipedia.org/wiki/Aminoglycoside) - CC BY-SA 3.0* ### Case Presentation A 68-year-old man with infective endocarditis due to Enterococcus faecalis is treated with 6 weeks of ampicillin plus gentamicin. During week 4, he reports progressive hearing loss and tinnitus, initially attributed to his age and illness. Formal audiometry reveals bilateral high-frequency sensorineural hearing loss. Gentamicin trough levels are checked and found to be elevated. The antibiotic is discontinued, but hearing loss persists and proves to be permanent. The mechanism is explained: aminoglycosides bind to the 30S ribosomal subunit, causing misreading of the genetic code and premature termination. While this preferentially affects bacterial ribosomes (explaining the antibiotic effect), human mitochondrial ribosomes are more similar to bacterial ribosomes than cytoplasmic ribosomes are. Individuals with certain mitochondrial DNA mutations (m.1555A>G in 12S rRNA) are particularly susceptible to aminoglycoside ototoxicity, even with single doses. Hair cells of the inner ear depend heavily on mitochondrial function and, once damaged, cannot regenerate. Genetic testing reveals he carries the m.1555A>G mutation. His family members are counseled to avoid aminoglycosides. ### Key Learning Points - Aminoglycosides bind the 30S ribosomal subunit decoding site, causing misreading and premature termination; they preferentially affect bacterial ribosomes but can also impact mitochondrial ribosomes - Human mitochondrial ribosomes resemble bacterial 70S ribosomes more than cytoplasmic 80S ribosomes, making mitochondria vulnerable to some antibiotics; the m.1555A>G mutation in mitochondrial 12S rRNA increases this vulnerability - Ototoxicity from aminoglycosides affects cochlear hair cells which have high metabolic demands and cannot regenerate; the hearing loss is typically irreversible --- ## Case 3: I-Cell Disease (Protein Trafficking Defect) ### Clinical Image ![I-Cell Disease](case_03_image.jpg) *Source: [Wikipedia - Inclusion-cell disease](https://en.wikipedia.org/wiki/Inclusion-cell_disease) - CC BY-SA 3.0* ### Case Presentation A 10-month-old boy is evaluated for coarse facial features, skeletal abnormalities, and developmental delay. He was noted at birth to have an umbilical hernia and has since developed hepatosplenomegaly, limited joint mobility, and progressive developmental regression. Physical examination reveals coarse facial features, gingival hyperplasia, and claw hand deformities. Radiographs show dysostosis multiplex. Fibroblast culture reveals numerous cytoplasmic inclusions (giving the disease its name, "I-cell disease"). Lysosomal enzyme activities are markedly reduced in fibroblasts but elevated in serum, indicating that lysosomal enzymes are being secreted rather than targeted to lysosomes. The diagnosis of mucolipidosis type II (I-cell disease) is confirmed by demonstrating deficient N-acetylglucosamine-1-phosphotransferase activity. This enzyme normally adds mannose-6-phosphate (M6P) tags to lysosomal enzymes in the Golgi apparatus. Without M6P tags, newly synthesized lysosomal enzymes cannot be recognized by M6P receptors and are secreted instead of being delivered to lysosomes. Lysosomes cannot degrade substrates, leading to accumulation of undegraded material. The prognosis is poor, with most patients dying in the first decade from cardiorespiratory complications. ### Key Learning Points - Lysosomal enzymes are tagged with mannose-6-phosphate (M6P) in the Golgi; M6P receptors recognize this tag and direct enzymes to lysosomes via the late endosomal pathway - I-cell disease results from deficiency of the enzyme that adds the M6P tag (N-acetylglucosamine-1-phosphotransferase); without this signal, lysosomal enzymes are secreted rather than delivered to lysosomes - This illustrates the critical importance of post-translational modifications and sorting signals in protein trafficking; the name "I-cell" (inclusion cell) refers to the accumulated undegraded material visible as inclusions