Decoding hepatorenal tyrosinemia type 1: Unraveling the impact of early detection, NTBC, and the role of liver transplantation

Abstract

Hepatorenal tyrosinemia type 1 (HT-1) is a rare autosomal recessive disease that results from a deficiency of fumaryl acetoacetate hydrolase (FAH), a critical enzyme in the catabolic pathway for tyrosine. This leads to the accumulation of toxic metabolites such as fumaryl and maleylacetoacetate, which can damage the liver, kidneys, and nervous system. The discovery of 2-[2-nitro-4-trifluoromethylbenzoyl]-1,3-cyclohexanedione (NTBC or nitisinone) has significantly improved the management of HT-1, particularly when initiated before the onset of symptoms. Therefore, newborn screening for HT-1 is essential for timely diagnosis and prompt treatment. The analysis of succinyl acetone (SA) in dried blood spots of newborns followed by quantification of SA in blood or urine for high-risk neonates has excellent sensitivity and specificity for the diagnosis of HT-1. NTBC combined with dietary therapy, if initiated early, can provide liver transplant (LT) free survival and reduce the risk of hepatocellular carcinoma (HCC). Patients failing medical treatment (eg, due to non-adherence), and who develop acute liver failure (ALF), have HCC or evidence of histologically proven dysplastic liver nodule(s), or experience poor quality of life secondary to severe dietary restrictions are currently indicated for LT. Children with HT-1 require frequent monitoring of liver and renal function to assess disease progression and treatment compliance. They are also at risk of long-term neurocognitive impairment, which highlights the need for neurocognitive assessment and therapy.

Lay Summary: Hepatorenal Tyrosinemia Type 1 (HT-1) is a rare genetic disorder caused by the deficiency of an enzyme called fumaryl acetoacetate hydrolase (FAH), leading to the harmful accumulation of toxic compounds in the body. This condition can severely damage the liver, kidneys, and nervous system. Last 2 decades have seen ground breaking advancements in HT-1 management with focus on early detection, the revolutionary drug 2-[2-nitro-4-trifluoromethylbenzoyl]-1,3-cyclohexanedione (NTBC), and the evolving role of liver transplantation.

Early detection plays a pivotal role in the successful management of HT-1. Newborn screening using dried blood spots has proven to be an effective method for detecting HT-1 early, enabling timely intervention before symptoms develop. Elevated levels of succinyl acetone (SA) in blood and urine serve as a sensitive and specific biomarker for HT-1 diagnosis, allowing for prompt initiation of treatment. The introduction of NTBC has revolutionized the treatment landscape for HT-1. Acting as a powerful inhibitor, NTBC blocks the formation of toxic metabolites, significantly improving survival rates and clinical outcomes. Studies have demonstrated that when combined with dietary therapy, early NTBC treatment can provide transplant-free survival, reduce the risk of hepatocellular carcinoma (HCC), and prevent the need for liver transplantation in many cases. Liver transplantation remains an essential therapeutic option for select HT-1 patients who do not respond adequately to medical treatment or experience complications such as HCC or dysplastic nodules. While the role of liver transplantation has evolved with the advent of NTBC, it remains a life-saving measure for those for whom medical management is insufficient. This highlights the critical importance of early HT-1 detection through newborn screening, enabling timely intervention with NTBC and dietary therapy. With the effective combination of these therapies, patients with HT-1 can achieve improved long-term outcomes and significantly reduce the need for liver transplantation.

ABBREVIATIONS

5-ALA = 5-aminolevulinic acid

AFP = alpha-fetoprotein

ALF = acute liver failure

ALP = alkaline phosphatase

CT = computed tomography

DBS = dried blood spots

FAH = fumaryl acetoacetate hydrolase

GGT = gamma-glutamyl transferase

HCC = hepatocellular carcinoma

HT-1 = hepatorenal tyrosinemia Type 1

INR = international normalization ratio

LDLT = living donor liver transplantation

LFT = liver function tests

MRI = magnetic resonance imaging

NP = natural protein

NTBC = 2-[2-nitro-4-trifluoromethylbenzoyl]-1,3-cyclohexanedione (Nitisinone)

Phe = phenylalanine

PT = prothrombin time

SA = succinyl acetone

Tyr = tyrosine

UNOS = united network for organ sharing

Introduction

Hepatorenal tyrosinemia type 1 (HT1) is a rare autosomal recessive disorder caused by the deficiency of fumarylacetoacetate hydroxylase (FAH), which leads to the accumulation of toxic compounds such as fumarylacetoacetate and maleylacetoacetate, resulting in multi-organ dysfunction (1,2). The advent of 2-[2-nitro-4-trifluoromethylbenzoyl]-1,3-cyclohexanedione (NTBC, nitisinone) has revolutionized the management of HT1, but early detection is crucial for optimal outcomes (3). This article provides a review of the current management of HT1, highlighting the challenges in diagnosis, the importance of early detection, and the current role of liver transplantation (LT) in the era of NTBC.

Improving Diagnosis of HT-1: Important Considerations

HT-1 consequences can be serious if diagnosis is delayed. Tyrosine (Tyr) levels lack accuracy for as a newborn screening biomarker, as it can be normal in HT-1 patients in the initial 48 hours of life, or elevated due to other factors (4,5). In HT-1 patients, however, accumulating fumarylacetoacetate (FAA) ultimately forms succinylacetone (SA), which is a highly sensitive and specific marker (6). SA levels were initially challenging to implement but flow injection mass spectrometry enabled simultaneous measurements of acylcarnitine, amino acids, and SA (7). False elevation of SA is seen in maleylacetoacetic isomerase deficiency but this condition is rare (8).

Liver function tests (LFT), prothrombin time (PT), international normalization ratio (INR), and serum alpha-fetoprotein (AFP) are performed in the initial evaluation, but these can be normal early in the disease course. Marked SA elevation occurs early in blood and urine, and is therefore the primary test to confirm a diagnosis of HT-1 following a positive screen or when suspected clinically. The FAH gene encodes the FAH enzyme, and pathogenic variants in this gene are responsible for HT-1. Though, there are more than 100 pathogenic variants have been described, only a few contribute to the majority of cases due to founder effects. In Quebec, Canada, the IVS12+5G>A mutation is commonly found in descendants of the original colonists from France, which also accounts for 45% of cases in northern Europe (9). Molecular testing is recommended for all patients with tyrosinemia in order to confirm the diagnosis and allow for genetic counselling of family members, but treatment should not be delayed until the results are received.

AFP is a marker of both hepatocellular carcinoma (HCC) and metabolic derangement in tyrosinemia. AFP is physiologically high at birth however, it decreases to adult levels (<10 μg/mL) within 1 year (10). Once treatment is started with NTBC-, AFP decreases rapidly and attains normalcy in 4–12 months (11). If the pace at which AFP decreases is slower or if there is an increase in the AFP levels, one should suspect HCC or poor compliance with NTBC. Lectin-reactive α -fetoprotein (L3-AFP) is an isoform of AFP that increases earlier than total AFP when HCC develops in patients with tyrosinemia (12).

Effect of Newborn Screening

Newborn screening (NBS) programs worldwide measure SA levels in dried blood spots (DBS) using tandem mass spectrometry. Screening accuracy using this biomarker is very high; in a systematic review, sensitivity and specificity were reported to approach 100% with positive predictive between 66.7% and 100% (13). The first universal NBS program was established in Quebec, Canada in 1970, with DBS analysis of Tyr level between 3 and 7 days after birth. It later moved to SA assay-based screening in 1998 by tandem mass spectrometry in NBS as the first-tier test followed by quantitative SA measurement as the second tier. This method had increased sensitivity to 100% with a positive predictive value of 79% and negative predictive value of 100% (14). Some screening programs only measure SA when Tyr levels are high, but about 28% of tyrosinemia patients may be missed by this approach (15). Therefore, the American College of Medical Genetics and Genomics recommends using blood SA as the primary marker for the diagnosis of HT-1 (4). SA positivity from dried blood spots should be confirmed by SA quantification from either urine or blood (16).

Early Diagnosis and Therapy Saves Lives: the Key to Successful Management of HT-1

Treating pre-symptomatic HT-1 patients with NTBC has been shown to significantly improve outcomes by preventing the formation of toxic metabolites through the inhibition of 4-hydroxy phenylpyruvate dioxygenase, the second enzyme in the Tyr degradation pathway (17). NTBC, a potent inhibitor of the enzyme 4HPPD, is an effective treatment for HT1 which was originally developed as a weedkiller. It prevents the production of toxic metabolites, resulting in a greater than 90% survival rate and improved clinical outcomes when combined with dietary therapy (18–20). Studies have consistently shown that the early initiation of NTBC treatment is associated with better survival and a reduced need for liver transplantation (LT), which underlines the importance of NBS (19–22). For instance, a Quebec study reported a mortality rate of 36% among non-NTBC treated children (10 out of 28) after a mean follow-up of 1.1 years (IQR 0.6–2.1), whereas none of the 24 children who received NTBC before 1 month of age died after a follow-up period of 5–11 years (20). Similarly, a study from Birmingham showed that none of the 12 pre-symptomatic patients died during the follow-up period, compared with two out of five children who died after NTBC treatment started once symptoms had developed (23). Multicentre studies from Europe, Turkey, and Israel have also demonstrated similar benefits in mortality and morbidity (24). Although a post-hoc analysis of the Quebec cohort did not show significant benefit in mortality between screen-detected and symptomatic detection, early NTBC-treated patients had shorter hospital stays and fewer admissions than late NTBC and non-NTBC treated patients (20,21). Similarly, a systematic review demonstrated that pre-symptomatic treatment reduced the need for LT but did not detect an improvement in mortality (25). Despite small sample sizes and methodological issues related to observational studies, it is clear that early initiation of NTBC markedly improves transplant-free survival (17). Additionally, its crucial to highlight that none of the early treated cases in Quebec series developed HCC (26).

Liver Transplantation for Tyrosinemia in the Newborn Screening ERA

LT is curative for HT1 and was the preferred treatment before NTBC became widely available (18,27–29). Previously, most institutions recommended LT for all patients with HT1 before they turned 12 months old (30). However, since the introduction of NBS (early diagnosis) & NTBC, the need for LT has significantly decreased, and the age at which patients undergo transplant has increased, demonstrating the benefits of early diagnosis and treatment. Studies from Quebec, Canada, and the UK have shown that early initiation of NTBC can prevent the need for LT in HT1 patients (20,23,31,32). For instance, one study found that none of the early NTBC-treated patients required LT, compared to 71% of the non-NTBC-treated group and 27% of the late NTBC-treated group (20). In contrast, all five children with HT1 who required LT in another Quebec study had a delay in initiating NTBC beyond 1 month of age (31). The importance of early diagnosis and treatment to avoid the need for LT is also highlighted in an international cohort study, which reported lower rates of LT in early NTBC-treated patients from 21 centers in Europe, Turkey, and Israel (24).

Currently, indications for LT in HT1 include failure of medical treatment (diet and NTBC), acute liver failure (ALF) unresponsive to medical treatment, HCC, evidence of histologically proven dysplastic nodule in the liver, and poor quality of life due to severe dietary restrictions (33). In countries where special diets and NTBC are difficult to obtain, LT is still often favored as a first-line treatment (33). Previously, a combined liver-kidney transplant was common in the pre-NTBC era. However, given the benefits of NTBC on renal function, isolated LT is now preferred (22). LT in HT1 has excellent graft and patient survival rates, with 1- and 5-year survival rates of >90% reported in a review of 125 HT1 patients from the UNOS database and other studies from around the world (34–39).

Living donor liver transplantation (LDLT) is a valuable option as it expands the donor pool and provides better outcomes compared to deceased donor liver transplantation (40). HT1 patients have also shown excellent outcomes after LDLT (41–44). It should be noted that LT does not correct renal FAH deficiency, and residual SA production by the kidney remains. Although the functional significance of this is not entirely understood, most centers discontinue dietary restrictions and NTBC after LT (31,45).

Liver Disease Pre - Post NTBC& Consideration for NTBC Therapy

Liver disease results from toxic metabolites that accumulate in the Tyr catabolic pathway, including SA, maleylacetoacetate, and the highly reactive FAA, which is mutagenic and can cause oxidative damage to cells by reacting with glutathione and sulfhydryl groups of proteins (46). In the past, most patients with HT1 presented early with ALF, while some children presented later with liver dysfunction, HCC, renal involvement, growth failure, acute neurological crises, and rickets. Pre-NTBC, the only available treatment was dietary, which had the potential to improve renal function and prevent liver dysfunction but it did not reverse an advanced liver disease or prevent HCC development. Patients presenting before 2 months of age had a 1-year survival rate of less than 40%, and liver transplantation was the preferred treatment (29,47). NTBC, a potent inhibitor of the enzyme 4HPPD, had revolutionized the management of HT1 leading to improved clinical outcomes (18–20). However, the role of NTBC in preventing HCC and the mechanism driving carcinogenesis in HT1 is not yet fully understood. HT1 patients on NTBC are still considered to be at risk for developing HCC and need to be closely monitored. Early initiation of treatment is critical, as a delay beyond the first year of life can lead to a 13 times higher risk of HCC (24). Also, there have been no reports of HCC in patients who were treated pre-symptomatically [18], implying the need for early diagnosis in the newborn. With NTBC and dietary therapy, life expectancy improved for children with HT1. However, it is crucial to include neuropsychological assessment in follow-up care for patients with HT1 to monitor and address the risk of neurocognitive deficits in those who have not undergone LT (48,49). The cause of these deficits is unclear. Tyr toxicity, Phe deficiency, drug toxicity, or natural disease progression seem to be the plausible explanations.

Adherence to long-term treatment like NTBC and dietary therapy is challenging. Early communication about the importance of long-term treatment adherence is crucial, as nonadherence is common in patients who did not experience complications of untreated disease and may not fully grasp its severity (50). Such non-compliant patients may not exhibit immediate symptoms but remain at risk of complications.

Another reason for noncompliance could be the cost of treatment. NTBC is an expensive drug, with an estimated cost of $18,998 per patient in the first year of life and $179,124 per year for a 75 kg patient in Canada (49). Cost coverage is often an issue in many developing countries. Improved reimbursement models and cost-effectiveness measures are needed to support families in accessing the medication.

Practical Consideration for Dietary Therapy

Before NTBC was introduced, the primary therapy for HT1 was dietary restriction of Phe and Tyr, and LT was frequently required. With NTBC therapy, Phe and Tyr restriction remain important as treatment fully inhibits the Tyr catabolic pathway upstream of the step catalyzed by FAH, preventing formation of the toxic intermediates, but marked elevation of Tyr and Phe would still ensue without such restriction. The dietary goals for patients with HT1 therefore include limiting the intake of Phe and Tyr, while also providing enough nutrients to support normal growth and development. Adequate energy intake is particularly important, as catabolism can raise Tyr levels. Another important dietary concept for HT1 management is natural protein (NP) tolerance, which is the maximum amount of daily NP that can be tolerated while maintaining recommended blood Phe and Tyr levels (51).

In young infants, breastfeeding is alternated with a Phe and Tyr-free formula (Tyrex-1, Tyrex-2, Tyros 1-2), depending on the mother's preference. These amino acid-free dietary formulas are expensive and are not easily accessible in every part of the world.

To meet protein and energy requirements in older patients with HT1, special formulas and diets with low to no amount of Phe and Tyr are needed, which may include fruits, vegetables, and cereals, while avoiding meat, nuts, seeds, legumes, and dairy products. It is unclear whether the NP tolerance remains unchanged throughout life or can be liberalized with age, but recent data suggest an ability to increase daily NP intake with age (51). Over-restriction of NP can lead to inadequate growth, poor wound healing, sarcopenia in contrast to liberalization which could potentially improve compliance (52).

The target plasma concentrations of Phe and Tyr on medical therapy are not well established and require further long-term follow-up studies. However, maintaining Tyr concentrations between 200 and 500 mol/L is recommended for patients under 12 years of age, with slightly higher target levels for older patients (53). Fluctuations in Tyr concentrations are common in growing children, intercurrent illness, and non-compliant patients. Blood Phe concentrations are typically targeted between 20 and 80 μmol/L (4,54) (Figure 1). If levels fall below the target range, additional Phe supplementation should be considered, with close monitoring of plasma Tyr levels. Close dietary evaluation, laboratory monitoring, and follow-up with a multidisciplinary team of experts are crucial for optimal HT1 patient care, in addition to medical therapy.

Figure 1:

Dietary therapy in HT-1

HT-1 = Hepatorenal Tyrosinemia Type 1; Phe = Phenylalanine; Tyr = Tyrosine

Monitoring

Regular monitoring and follow-up are vital for managing patients with HT1, ensuring stable metabolic control, and early detection of potential complications. During the initial months of diagnosis, patients are seen more frequently (every week to month), and thereafter the interval of clinic visits can be lengthened. Nutritional and developmental assessment are important aspects of the follow-up of HT1 patients, as they are at risk of developing growth and developmental delays due to the dietary restrictions and metabolic abnormalities. Eye examination is also recommended as HT1 patients are at risk of developing oculocutaneous tyrosinemia (if Tyr levels exceed target ranges), which can manifest as keratitis, photophobia, and corneal ulcers. Regular ophthalmologic evaluation can help in the early detection and management of these complications.

The suggested specific testing is listed below (50):

• 1.Organ-specific blood/serum test
  • a.Liver: Bilirubin, AST, ALT, GGT, ALP, Albumin, PT, PTT, fibrinogen, AFP
  • b.Renal: Urea, creatinine, electrolytes including calcium, phosphate.
• 2.Metabolic: Blood gas, bicarbonate levels, urine/blood succinyl acetone, blood NTBC levels, plasma amino acids (Phe, Ty), urine amino acids, urine organic acids
• 3.Urine test: glucose, calcium, phosphate, albumin, total protein, β-2 microglobulins, routine analysis, tubular reabsorption of phosphate, creatinine, urine 5-aminolevulinic acid (5-ALA)
• 4.Imaging: Ultrasonography (liver and renal), periodic MRI/CT scan on annual basis (can be considered sooner depending on clinical scenario, eg, high suspicion of HCC) Nutritional labs (frequency depending on the clinical scenario): serum iron and ferritin, vitamins A, D, E, folate & vitamin B12, micronutrients (Selenium, Zinc, Copper)
• 5.Other: Complete blood count

Special Consideration

Liver failure in HT1

HT1 can lead to ALF in infants within a few weeks of birth, usually before the age of 2 years, and even in patients treated with dietary restrictions alone (55–57). The presentation of ALF may include coagulopathy, elevated AFP, serum Tyr, methionine, and Phe levels, and a boiled cabbage odor on physical examination (58,59). In the past, ALF in HT1 patients had a very high mortality rate (47,55), but early identification with the initiation of NTBC and dietary modifications have improved outcomes (23,60). Treatment of HT1-induced ALF includes providing adequate calorie intake with reduced or low Phe and Tyr content, aggressive treatment of infections, maintenance of normal blood glucose levels, correction of ascites and metabolic acidosis. The initial recommended dose of NTBC is 1 mg/kg/day. The expected response to therapy is usually rapid, with normalization of urine SA in 24 hours and clinical improvement within 1 week. LT may be required in severe cases with no response to NTBC, and referral to a transplant center is critical for these patients (38).

Conclusions

In summary, early diagnosis and treatment of HT-1 are crucial for optimal outcomes. The current standard of care includes a combination of NTBC therapy with dietary Phe and Tyr restriction, and close monitoring by a multidisciplinary team. Liver transplantation remains a viable option for select patients with HT-1 who do not respond to medical therapy or develop complications such as hepatocellular carcinoma or dysplastic nodules. With appropriate management, patients with HT-1 can have an excellent liver transplant-free long-term survival.

Contributions:

The authors confirm contribution to the paper as follows: study conception and design: MK and MSS conceptualization, design, draft manuscript preparation, Draft revision, Final draft preparation. JS, CJR, PC draft manuscript preparation, Final draft preparation. All authors reviewed the approved the final version of the manuscript.

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