# Clinical Cases: Nucleotide Metabolism

## Case 1: Gout (Hyperuricemia and Urate Crystal Deposition)

### Clinical Image
![Gout Tophi](case_01_image.jpg)
*Source: [Wikipedia - Gout](https://en.wikipedia.org/wiki/Gout) - CC BY-SA 3.0*

### Case Presentation
A 55-year-old man presents to the emergency department at 3 AM with excruciating pain, swelling, and redness of his right great toe. The pain awakened him from sleep and is so severe he cannot tolerate even a bedsheet touching his foot. He attended a work dinner last night with rich food and several alcoholic drinks. He has a history of hypertension treated with hydrochlorothiazide. Physical examination reveals an erythematous, swollen, exquisitely tender first metatarsophalangeal joint (podagra). Laboratory studies show serum uric acid of 10.2 mg/dL (elevated). Arthrocentesis yields cloudy synovial fluid with WBC count of 42,000/mm3 and negatively birefringent needle-shaped crystals under polarized microscopy, confirming monosodium urate crystals. He is diagnosed with acute gouty arthritis. The biochemistry is explained: uric acid is the end product of purine degradation in humans; we lack uricase, which converts uric acid to the more soluble allantoin in most other mammals. When serum urate exceeds solubility (~6.8 mg/dL), crystals precipitate in joints and trigger intense inflammation. He is treated with colchicine and NSAIDs for acute relief, and allopurinol (a xanthine oxidase inhibitor) is later started for long-term urate lowering. Thiazide diuretics are changed as they decrease uric acid excretion.

### Key Learning Points
- Uric acid is the final product of purine catabolism in humans; xanthine oxidase converts hypoxanthine to xanthine and xanthine to uric acid
- Hyperuricemia results from either overproduction (increased purine synthesis or turnover) or underexcretion (the more common cause); thiazides, alcohol, and high-purine foods can elevate urate levels
- Allopurinol and febuxostat inhibit xanthine oxidase, reducing uric acid production; uricosurics (probenecid) increase renal excretion; newer agents like pegloticase provide recombinant uricase

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## Case 2: Lesch-Nyhan Syndrome (HGPRT Deficiency)

### Clinical Image
![Lesch-Nyhan Self-Injury](case_02_image.jpg)
*Source: [Wikipedia - Lesch-Nyhan syndrome](https://en.wikipedia.org/wiki/Lesch%E2%80%93Nyhan_syndrome) - CC BY-SA 3.0*

### Case Presentation
A 3-year-old boy is referred for evaluation of developmental delay, abnormal movements, and self-injurious behavior. He was noted to have hypotonia and developmental delay in infancy. He began biting his fingers at 18 months, requiring protective hand restraints. Physical examination reveals dystonia, choreoathetosis, and spasticity with hyperreflexia. His parents point out scarring on his lips and fingers from self-biting, and note he has had two episodes of kidney stones. Laboratory studies reveal markedly elevated serum uric acid at 12.5 mg/dL and elevated urinary uric acid excretion. 24-hour urine uric acid to creatinine ratio is significantly elevated. The clinical picture strongly suggests Lesch-Nyhan syndrome. Enzymatic assay confirms absent hypoxanthine-guanine phosphoribosyltransferase (HGPRT) activity, and genetic testing identifies a hemizygous mutation in HPRT1 on the X chromosome. Without HGPRT, salvage of purines is impossible, leading to accumulation of PRPP which drives excessive de novo purine synthesis. The resulting massive uric acid production causes gout and nephrolithiasis in childhood. The neurological manifestations (self-mutilation, dystonia, cognitive impairment) may relate to dopaminergic dysfunction, though the precise mechanism remains incompletely understood. He is treated with allopurinol for hyperuricemia and requires ongoing behavioral and physical management.

### Key Learning Points
- HGPRT catalyzes the salvage of hypoxanthine and guanine to IMP and GMP; its deficiency causes accumulation of PRPP, which activates de novo purine synthesis
- The resulting purine overproduction leads to severe hyperuricemia with childhood gout and uric acid nephrolithiasis; allopurinol controls the metabolic consequences but not the neurological manifestations
- The compulsive self-injurious behavior (particularly biting lips and fingers) is a hallmark feature whose neurobiological basis likely involves abnormal dopamine neurotransmission

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## Case 3: Methotrexate Toxicity (Antifolate Therapy)

### Clinical Image
![Methotrexate Mucositis](case_03_image.jpg)
*Source: [Wikipedia - Mucositis](https://en.wikipedia.org/wiki/Mucositis) - CC BY-SA 3.0*

### Case Presentation
A 62-year-old woman receiving high-dose methotrexate for osteosarcoma develops severe mucositis, diarrhea, and pancytopenia one week after her chemotherapy cycle. She was supposed to receive leucovorin rescue starting 24 hours after methotrexate but missed several doses due to nausea. Physical examination reveals oral ulcerations with white plaques, diffuse abdominal tenderness, and pallor. Laboratory studies show WBC 1,200/mm3, hemoglobin 8.5 g/dL, platelets 45,000/mm3, and elevated creatinine at 2.1 mg/dL. Serum methotrexate level is elevated at 0.8 micromolar (should be <0.1 at this timepoint). She has methotrexate toxicity exacerbated by renal impairment. Methotrexate inhibits dihydrofolate reductase (DHFR), blocking regeneration of tetrahydrofolate (THF) required for one-carbon transfers in purine and thymidylate synthesis. Without THF, cells cannot synthesize DNA, particularly affecting rapidly dividing cells (bone marrow, GI mucosa). She is treated with high-dose leucovorin (folinic acid, which bypasses DHFR), IV fluids for hydration, and urinary alkalinization to enhance methotrexate excretion. Her counts recover over the following week.

### Key Learning Points
- Methotrexate inhibits dihydrofolate reductase (DHFR), preventing regeneration of tetrahydrofolate required for de novo synthesis of purines (N10-formyl-THF) and thymidylate (N5,N10-methylene-THF)
- Rapidly dividing cells are most sensitive to folate antagonists; toxicity manifests as mucositis (GI epithelium) and myelosuppression (bone marrow)
- Leucovorin (folinic acid) rescue provides reduced folate that bypasses DHFR; timing and adequate dosing are critical, especially when methotrexate clearance is impaired by renal dysfunction