Tyrosinemia arises from inherited deficiency of one or more enzymes participating in the stepwise degradation of tyrosine. Each recognized subtype corresponds to impairment at a distinct enzymatic step, producing differing downstream metabolite accumulation patterns and organ involvement profiles. For Tyrosinemia Type II specifically, the GeneReviews chapter identifies deficiency of the enzyme tyrosine aminotransferase as the underlying metabolic mechanism, producing markedly elevated plasma tyrosine concentrations—typically exceeding 500 µmol/L and sometimes surpassing 1,000 µmol/L—and elevated urinary concentrations of tyrosine metabolites including 4-hydroxyphenylpyruvate, 4-hydroxyphenyllactate, and 4-hydroxyphenylacetate (GeneReviews). A distinguishing biochemical feature of Type II relative to Type I is the presence of elevated plasma tyrosine without succinylacetone accumulation. Founder pathogenic variants have been documented in populations from northern Italy, Tunisia, Lebanon, Palestine, and Gran Canaria, and consanguinity among parents of affected individuals is frequently reported in those populations (GeneReviews). Causative mechanisms specific to Tyrosinemia Type I, Type III, and transient forms are not separately documented in the certified data available for this packet.

According to the GeneReviews chapter for Tyrosinemia Type II, no consensus clinical diagnostic criteria have been published. Diagnosis is established by identification of biallelic pathogenic variants in the gene encoding the enzyme tyrosine aminotransferase on molecular genetic testing, or—in limited instances—by measurement of significantly reduced enzyme activity in liver tissue. GeneReviews describes two diagnostic scenarios: an abnormal newborn screening (NBS) result showing elevated tyrosine without succinylacetone, which GeneReviews identifies as an indicator for further investigation of Type II; and presentation in a symptomatic individual with ocular or skin manifestations accompanied by elevated plasma tyrosine and characteristic urinary metabolite patterns. Supporting laboratory findings include markedly elevated plasma tyrosine (typically ≥500 µmol/L), elevated urinary 4-hydroxyphenyl metabolites, normal serum transaminase levels, and absence of succinylacetone in blood and urine. Elevated plasma tyrosine is not specific to Type II and may reflect liver immaturity, hepatic disease, or other tyrosine metabolism disorders; additional testing is required to confirm the diagnosis (GeneReviews). Tyrosinemia Type II is not typically included in standard NBS panels, though it may be detected incidentally when screening for Type I is performed.

For Tyrosinemia Type II, the GeneReviews chapter describes dietary restriction of tyrosine and phenylalanine as the principal therapeutic intervention. No clinical practice guidelines have been published; management described in GeneReviews is based on clinical experience at specialized metabolic centers. The therapeutic objective is reduction of plasma tyrosine concentrations through a low-protein diet supplemented with age-appropriate tyrosine- and phenylalanine-free amino acid preparations, vitamins, and minerals. GeneReviews describes treatment as lifelong, initiated with prescribed infant formula in infancy and transitioning to a low-protein dietary regimen after weaning. Dietary intervention has been associated with significant improvement in ocular and skin manifestations in most individuals within the first week of treatment initiation; early dietary restriction is also associated with prevention of intellectual disability (GeneReviews). Supportive care described in GeneReviews includes lubricating eye drops and ointments, ocular surgery for bilateral corneal ulcers unresponsive to dietary treatment, and developmental and educational support for individuals with neurodevelopmental involvement. No approved pharmacological therapies or orphan drug designations are documented in the structured drug fields of this packet. Management approaches specific to other tyrosinemia subtypes are not detailed in the certified data available.

According to GeneReviews (Tyrosinemia Type II), prognosis is substantially influenced by the timing of diagnosis and initiation of dietary treatment. Individuals diagnosed and treated from early infancy typically demonstrate normal neurodevelopmental outcomes and may present with only mild or absent ocular and skin manifestations. Corneal and skin findings have improved or resolved in most individuals within the first week of dietary treatment, though recurrence following discontinuation of dietary restrictions has been documented in at least one case (GeneReviews). Intellectual disability, which occurs in approximately 60% of untreated individuals, is described as largely preventable through early dietary intervention. The natural history of untreated Tyrosinemia Type II is characterized by progressive ocular, cutaneous, and neurodevelopmental involvement. Prognosis specific to Tyrosinemia Type I, Type III, or transient tyrosinemia of the newborn is not documented in the certified data available for this packet.

As of the packet data, two clinical studies are listed among active or recruiting trials associated with tyrosinemia (ClinicalTrials.gov). NCT03655223 ("Early Check: Expanded Screening in Newborns") is an active, non-recruiting study sponsored by RTI International, initiated in October 2018 with an anticipated completion date of December 2026; this study evaluates expanded newborn screening across multiple conditions including tyrosinemia. NCT06941532 ("GMP Powdered Substitutes in PKU and TYR") is a recruiting study (Phase NA) sponsored by Nutricia UK Ltd, initiated in May 2025 with an anticipated completion date of December 2026; it investigates glycomacropeptide-based powdered nutritional substitutes as metabolic dietary management tools for phenylketonuria and tyrosinemia. The broader trial registry search at ClinicalTrials.gov reflects the full active trial landscape for this disease group.