Tyrosinemia (Types I, II, and III)
| Type | Deficient Enzyme | Accumulated Toxins | Key Clinical Signatures |
|---|---|---|---|
| Type I Hepatorenal |
Fumarylacetoacetate Hydrolase (FAH) | Fumarylacetoacetate, Succinylacetone |
Severe liver failure, cirrhosis, hepatocellular carcinoma (HCC), and renal Fanconi syndrome. Cabbage-like odor. |
| Type II Oculocutaneous |
Tyrosine Aminotransferase (TAT) | Tyrosine, Tyrosine crystals |
Painful palmar/plantar hyperkeratosis, corneal ulcerations, and photophobia. Mild intellectual disability. |
| Type III Neurological |
4-Hydroxyphenylpyruvate Dioxygenase | 4-Hydroxyphenylpyruvate | Neurological abnormalities, ataxia, seizures, and developmental delay. No liver or skin involvement. |
High-Yield Core Realities:
- The Succinylacetone Trapped Marker: In Type I tyrosinemia, the block at the final step of tyrosine breakdown causes fumarylacetoacetate to divert into alternative pathways, creating succinylacetone. Succinylacetone is a potent chemical toxin that serves as the definitive biomarker for diagnosing Type I disease.
- ALAD Invalidation & Porphyria Mimicry: Elevated succinylacetone structurally mimics and competitively blocks δ-aminolevulinic acid dehydratase (ALAD), an essential enzyme in heme biosynthesis. This secondary block leads to the accumulation of ALA, triggering acute neurological crises characterized by abdominal pain, peripheral neuropathy, and psychiatric symptoms that mimic acute intermittent porphyria.
- Renal Fanconi Syndrome Dynamics: Accumulation of alkylating metabolites inside renal proximal convoluted tubule cells causes profound mitochondrial dysfunction. This destroys tubular resorptive capacity, causing a full Fanconi syndrome presenting with glucosuria, phosphaturia, aminoaciduria, and bicarbonate wasting, leading to hypophosphatemic rickets.
- The Cabbage-Like Volatile Odor: The characteristic cabbage-like or rancid butter odor observed in infants presenting with Type I tyrosinemia is driven by the co-accumulation and transamination products of tyrosine and secondary alterations in methionine catabolism.
- Pharmacological Shunt Management (Nitisinone): Modern clinical stabilization relies on Nitisinone (NTBC). NTBC blocks 4-hydroxyphenylpyruvate dioxygenase (the Type III enzyme), completely preventing the formation of upstream fumarylacetoacetate and succinylacetone. While this intentionally shifts the patient into a Type III metabolic phenotype with higher plasma tyrosine levels, it protects the liver and kidneys from malignant transformation. Management requires pairing NTBC with strict dietary restriction of phenylalanine and tyrosine.