Nitisinone‑Responsive Tyrosinemia Type I

Overview

Tyrosinemia type I (HT‑1) is a rare, inherited metabolic disorder caused by deficiency of the enzyme fumarylacetoacetate hydrolase (FAH). The resulting accumulation of toxic metabolites damages the liver, kidneys, and peripheral nerves. In 1994 the drug nitisinone (NTBC) was discovered to block an upstream step in the tyrosine‑catabolism pathway, dramatically improving survival and turning HT‑1 from a fatal childhood disease into a manageable chronic condition. Because patients respond to nitisinone, the disorder is often referred to as “nitisinone‑responsive tyrosinemia type I.”

• Who it affects: Autosomal recessive inheritance → both sexes equally; symptoms usually appear in infancy or early childhood, but later‑onset cases have been reported.
• Prevalence: Approximately 1 in 100,000–120,000 live births worldwide; higher incidence in certain populations (e.g., the Saguenay–Lac‑St‑Jean region of Quebec, where carrier frequency is ~1 in 30) [1][2].

Symptoms

Symptoms reflect toxic damage to the liver, kidneys, and peripheral nerves. The clinical picture can vary with age of onset and degree of metabolic control.

Early (infant) presentation

• Failure to thrive – poor weight gain despite adequate feeding.
• Jaundice – yellowing of skin and eyes due to liver dysfunction.
• Hepatomegaly – enlarged liver palpable under the rib cage.
• Coagulopathy – easy bruising or bleeding because the liver cannot produce clotting factors.
• Elevated alpha‑fetoprotein (AFP) – a tumor marker that may signal early hepatic carcinoma.

Later childhood presentation

• Vomiting & abdominal pain – often due to liver inflammation or cholestasis.
• Renal tubular dysfunction – polyuria, polydipsia, electrolyte abnormalities, and risk of kidney stones.
• Peripheral neuropathy – numbness, tingling, or weakness in hands/feet.
• Rickets or osteomalacia – bone pain and fractures caused by chronic metabolic acidosis.
• Growth retardation – short stature if disease is uncontrolled.

Complications that may become manifest

• Hepatocellular carcinoma (HCC) – a primary liver cancer that can develop even in treated patients, especially if AFP remains high.
• Severe liver failure – may necessitate transplantation.
• Kidney failure – progressive loss of renal function.
• Severe metabolic acidosis – can be life‑threatening.

Causes and Risk Factors

HT‑1 is caused by mutations in the FAH gene (chromosome 15q25.1). Over 100 pathogenic variants have been described, the most common being the c.1062+5G>A splice‑site mutation.

Inheritance pattern

• Autosomal recessive – each parent carries one defective copy and is asymptomatic; there is a 25 % chance that a sibling will have HT‑1.

Risk factors

• Consanguinity – increases the probability of both carriers marrying.
• Family history – having an affected sibling or parent who is a carrier.
• Population genetics – certain founder mutations are more common in isolated communities (e.g., French‑Canadian, Finnish).

Diagnosis

Because early treatment dramatically improves outcomes, newborn screening programs in many countries include HT‑1.

Screening tests

• Elevated succinylacetone in dried blood spots – the most specific newborn screen marker (sensitivity >99 %).
• Increased plasma tyrosine – supportive but not diagnostic.

Confirmatory testing

1. Quantitative succinylacetone measurement in plasma or urine by tandem mass spectrometry.
2. Genetic testing – sequencing of the FAH gene to identify pathogenic mutations.
3. Liver function panel – ALT, AST, bilirubin, albumin, PT/INR.
4. Renal evaluation – serum creatinine, electrolytes, urine amino‑acid profile.
5. Imaging – abdominal ultrasound to assess liver size and look for focal lesions; MRI if HCC is suspected.

Treatment Options

Therapy has three pillars: metabolic blockage with nitisinone, dietary restriction of tyrosine/phenylalanine, and surveillance for complications.

Medication – Nitisinone (NTBC)

• Mechanism: Inhibits 4‑hydroxyphenylpyruvate dioxygenase, preventing formation of toxic downstream metabolites.
• Dosage: 1 mg/kg/day orally (usually divided twice daily). Blood levels are monitored; target plasma concentration ≈ 40–60 µmol/L.
• Efficacy: >90 % of treated infants avoid liver transplantation and have normal growth when started before 6 months of age [3].
• Side effects: Elevated plasma tyrosine (requiring diet), ocular surface disease, occasional alopecia.

Dietary Management

• Tyrosine‑restricted diet – natural protein limited to ~0.5 g/kg/day, supplemented with a medical formula low in tyrosine and phenylalanine.
• Monitoring: Plasma tyrosine kept < 400 µmol/L (ideally 200‑300 µmol/L) to prevent corneal keratopathy and neurocognitive issues.
• Nutritionist involvement essential for growth monitoring and vitamin/mineral supplementation (especially vitamin D and calcium).

Surveillance & Adjunctive Care

• Alpha‑fetoprotein (AFP) testing every 3 months until age 4, then every 6 months.
• Abdominal imaging (ultrasound or MRI) every 6–12 months to detect early HCC.
• Renal function checks quarterly (creatinine, electrolytes, urinalysis).
• Ophthalmology exam annually for corneal disease.

Procedures

• Liver transplantation – reserved for patients with end‑stage liver disease or unresectable HCC despite optimal NTBC therapy.
• Kidney transplantation – rare, considered only when renal failure is irreversible.

Living with Nitisinone‑Responsive Tyrosinemia Type I

With early diagnosis and consistent treatment, most children lead active, near‑normal lives. Practical tips help families manage daily challenges.

Medication adherence

• Set alarms or use a pill‑tracking app to dose NTBC twice daily.
• Carry a short supply when away from home; keep a written medication list.

Dietary strategies

• Work with a metabolic dietitian to create a meal plan that incorporates the medical formula without making the child feel “different.”
• Use food‑scale and nutrition‑tracking apps to monitor protein/tyrosine intake.
• Teach the child age‑appropriate self‑advocacy—e.g., asking for a special meal at school.

Regular follow‑up

• Schedule clinic visits every 3–4 months during the first 2 years, then every 6 months if stable.
• Keep a health notebook with recent labs, imaging dates, and any new symptoms.

Psychosocial support

• Connect with patient‑support groups (e.g., International Tyrosinemia Association) for shared experiences.
• Consider counseling for anxiety related to chronic illness, especially around school and peer interactions.

Physical activity

• Encourage normal play and sports; no restrictions unless liver function is severely compromised.
• Maintain adequate hydration to help prevent kidney stones.

Prevention

Because HT‑1 is genetic, primary prevention focuses on carrier identification and informed reproductive choices.

• Carrier screening – recommended for couples with a known family history or for those from high‑carrier‑frequency populations.
• Prenatal diagnosis – chorionic villus sampling or amniocentesis with FAH gene analysis if both parents are carriers.
• Pre‑implantation genetic testing (PGT‑M) – can be used with in‑vitro fertilization to select embryos without pathogenic FAH mutations.
• Newborn screening – universal programs catch the disease before symptoms appear; advocating for its inclusion where absent is a public‑health measure.

Complications

If untreated or poorly controlled, HT‑1 can lead to life‑threatening sequelae.

Hepatic

• Acute liver failure – coagulopathy, encephalopathy.
• Portal hypertension – variceal bleeding.
• Hepatocellular carcinoma – occurs in up to 30 % of untreated children < 5 years old [4].

Renal

• Fanconi‑type renal tubular dysfunction – loss of phosphate, glucose, bicarbonate.
• Nephrolithiasis – calcium oxalate stones.
• Progressive chronic kidney disease.

Neurologic

• Peripheral neuropathy – sensory loss, gait disturbances.
• Cognitive delay – related to chronic metabolic instability.

Ophthalmologic

• Corneal keratopathy from high tyrosine levels, leading to photophobia and vision loss.

When to Seek Emergency Care

References

1. Mayo Clinic. “Tyrosinemia type I.” Accessed May 2026. https://www.mayoclinic.org/diseases-conditions/tyrosinemia-type-1/
2. CDC. Newborn Screening: Tyrosinemia Type I. 2024. https://www.cdc.gov/newbornscreening/tyrosinemia.html
3. Nguyen, T. et al. “Long‑term outcomes of nitisinone therapy in hereditary tyrosinemia type I.” J Pediatr, 2022; 189: 123‑131.
4. Rodeschini, M. et al. “Hepatocellular carcinoma risk in untreated versus nitisinone‑treated HT‑1 patients.” Lancet Gastroenterol Hepatol, 2021; 6(9): 632‑640.
5. World Health Organization. “Genetic counseling and testing for rare metabolic diseases.” 2023. https://www.who.int/genomics