Tyrosinemia type 1
van Ginkel, Wiggert
DOI:10.33612/diss.137426908
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van Ginkel, W. (2020). Tyrosinemia type 1: Remaining challenges after introduction of NTBC. University of Groningen. https://doi.org/10.33612/diss.137426908
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Modifi ed version of:
“Long-Term Outcomes and Practical Considerations in the Pharmacological Management of Tyrosinemia Type 1”. Willem G. van Ginkel, Iris L. Rodenburg, Cary O. Harding, Carla E. M. Hollak, M. Rebecca Heiner-Fokkema, Francjan J. van Spronsen.. Paediatr Drugs. 2019 Dec;21(6):413-426
CHAPTER
General introduction and
outline thesis
Historical overview and introduction to
Tyrosinemia type 1
Tyrosinemia type 1 (TT1 McKusick 27670), also called hepatorenal tyrosinemia or hereditary Tyrosinemia type 1, is an autosomal recessively inherited defect in amino acid metabolism, caused by mutations in the gene encoding the enzyme fumarylacetoacetate hydrolase (FAH). The enzyme FAH is the last enzyme in the catabolic pathway of the amino acid tyrosine, and it converts fumarylacetoacetate (FAA) into fumarate and acetoacetate in the liver and kidneys as shown in figure 1 (Figure 1).
The first report of a TT1 patient was published in 1956. This patient presented at 9 months with liver cirrhosis, renal tubular defects and vitamin D resistant rickets, although the exact diagnosis was not clear at that time1. First, the primary
enzyme defect was considered to be a defect of 4-hydroxyphenylpyruvate dioxygenase (4HPPD)2. Some years later, it became apparent that the primary
enzyme deficiency was located more downward in the catabolic pathway of tyrosine (Fig 1)3,4.
The only existing treatment at that time was dietary restriction of tyrosine and its precursor phenylalanine. When only treated with a phenylalanine-tyrosine restricted diet, the outcome was extremely poor. Many TT1 patients did not survive the initial period when they presented with severe liver failure and its associated problems, including ascites and bleeding5. If patients survived this period, many died shortly after due to the development of hepatocellular carcinoma (HCC) or respiratory failure caused by porphyria-like syndrome5-7.
As a consequence, orthotopic liver transplantation (OLT) was long considered the only definitive option to treat the metabolic as well as the oncological problem8-10. Therefore, at that time, research mainly focused on the timing of
Figure 1. Phenylalanine-tyrosine catabolic pathway with associated enzymes and diseases.
TT1 is caused by a fumarylacetoacetate hydrolase defi ciency leading to increased concentrations of fumarylacetoacetate, succinylacetone and consequentely delta-aminolevulinic acid that all contribute to the clinical pathology that characterizes TT1.The other metabolic disorders mentioned in the fi gure are respectively: phenylketonuria (PKU), tyrosinemia type 2 (TT2), tyrosinemia type 3 (TT3), hawkinsinuria, alkaptonuria (AKU) and maleylacetoacetate isomerase defi ciency (MAAID).
This all changed after the introduction of 2-(2-nitro-4-trifl uoromethylbenzoyl)-1,3-cyclohexanedione (NTBC, also known as Nitisinone) as a new treatment option in 199212. NTBC showed to be a potent inhibitor of the enzyme 4HPPD that
was fi rst thought to be responsible for the disease. In this way, NTBC prevents the production of downstream toxic metabolites FAA, maleylacetoacetate, succinylacetoacetate, and succinylacetone (SA) and thereby substantially improves the clinical outcome13,14. However, as a consequence of 4HPPD
inhibition by NTBC treatment, tyrosine concentrations increase, making dietary restriction of tyrosine and its precursor phenylalanine again part of the treatment of TT115,16.
It has been more than 25 years since NTBC was introduced as a treatment option. Since then, many TT1 patients have been treated with NTBC, most of them in combination with a phenylalanine-tyrosine restricted diet. At first sight, this treatment seemed to resolve all clinical problems. However, recent research indicated that some of the “old” challenges remained and “new” challenges arose. The remainder of this introduction will review the different clinical problems that previously characterized TT1 and will highlight remaining and new challenges in the treatment of TT1 patients. Some of these challenges will be addressed in this thesis and will be specifically highlighted in the remainder of the introduction.
Clinical problems that previously characterized
TT1
Liver problems
Especially FAA, that accumulates prior to the enzymatic defect, has been shown to be cytotoxic and mutagenic. It causes glutathione depletion, oxidative stress, chromosomal instability, cell cycle arrest and apoptosis in the cells where it is generated, primarily hepatocytes17-19. As a consequence, TT1 is characterised
by progressive liver disease. The majority of the patients presented (very) early with severe acute liver failure, and associated pronounced coagulopathy, ascites due to low albumin concentrations, and hypoglycemia. In particular these early presenting patients had a poor outcome. If not transplanted, only 10-30% of the patients was still alive 2 years after diagnosis5,20. When patients
survived the initial period, there was a high risk for developing chronic liver disease, cirrhosis and eventually HCC when treated with a phenylalanine-tyrosine restricted diet only5,6,21. Therefore, OLT was long considered as the only
definitive option to prevent the metabolic and oncological problems8-10,22-27.
In most patients, introduction of NTBC resulted in a quick recovery of liver function12. Yet, about 10% of the patients suffering from (acute) liver failure are
still considered not to respond to the treatment13,28-31, most likely because of the
end-stage liver disease that cannot be effectively treated anymore. However, when NTBC treatment is combined with pre-clinical diagnosis (for example by population based newborn screening) even the initial symptomatology
with liver failure can be prevented14. Therefore, the remaining questions these
days do not focus on the initial liver-associated symptomatology but mainly on how to adequately prevent long-term liver disease such as HCC. It is thought that the risk for liver cancer (mainly HCC, although hepatoblastoma may occur32) is still increased when NTBC is initiated “late” due to delayed
diagnosis or unavailability of NTBC or when adherence to treatment is poor12-14,33-35. However, among others due to the heterogeneity of the patient
population and the rarity of the disease, the true risk for HCC is still unknown. Consequently, the protocol for TT1 treatment monitoring and HCC screening is still under debate.
Renal problems
FAA does not only affect hepatocytes, but tubular cells of the kidney as well. FAH can cause oxidative stress, acute apoptosis and cellular death in proximal tubular cells just as in hepatocytes36. In addition, SA has been shown to reduce
sugar and amino acid uptake in the proximal tubulus leading to renal Fanconi syndrome and secondary hypophosphataemic (vitamin D resistant) rickets may develop37. Dietary restriction of phenylalanine and tyrosine seemed to
improve renal tubular defects at least in some patients. However, only after the introduction of NTBC a quick improvement, usually with normalisation of kidney function is seen12,29,38-41. When patients are pre-clinically diagnosed
and treated with NTBC, clinically significant renal problems seem not to occur anymore14,42.
Neurological problems
Recurrent neurological crises were present in up to 40% of the TT1 patients merely treated with diet, and were a main cause of hospitalization or even death5,7. The neurological crises were usually provoked by a minor infection
and presented as a peripheral neuropathy with hypertonia, paralytic ileus with vomiting, or muscle weakness that could progress to paralysis or even respiratory failure that could mimic the progressive weakness in Guillain-Barre7.
Treatment with NTBC causes a rapid decline in SA, resulting in an increase in porphobilinogen synthase activity and as a consequence normalisation
of delta-aminolevulinic acid (5-ALA) concentrations. Increased 5-ALA concentrations are thought to be responsible for the neurological crises or so-called porphyria-like-syndrome7,12. These neurological symptoms have
completely disappeared after the start of NTBC treatment13,14,31, and only seem
to reappear after discontinuation of NTBC for 1-8 weeks43,44. This stresses the
importance of continuation of NTBC in short term to prevent neurological problems, and in long term to prevent the development of liver cancer.
“New” challenges after introduction of NTBC
Neuropsychological and behavioral problems
No consistent cognitive or behavioral deficiencies were reported in TT1 patients prior to the introduction of NTBC. In fact, intellectual development and school performance were considered to be normal even in patients suffering from recurrent neurological crises7. However, recently, years after
the introduction of NTBC, several studies reported a non-optimal cognitive development in TT1 patients. In 2008, 35% of French TT1 patients had school problems31. Later research, that predominately focussed on IQ, showed a
lower-than-average IQ45,46 and even regression of IQ over time47. All studies
were however performed retrospectively and/or in a small sample. Regarding behavior, even fewer studies have been done. Only one study investigated the behavioral outcome of a small number of TT1 patients showing that they have a tendency to show attention problems48. Considering the small sample size
of the studies and limited amount of instruments that have been used, a more extensive exploration of the cognitive-behavioral phenotype in TT1 patients is required. This will be one of the main focus points in this thesis. Next to this, studies investigating associations between neuropsychological problems and metabolic control are currently lacking. This is another issue that will be addressed in the following chapters.
Practical management of NTBC treatment
Dose optimization of NTBC can be based on several (indirect) parameters, such as: 1) doses individually adjusted to mg/kg body weight, 2) porphobilinogen synthase activity in erythrocytes, 3) 5-ALA concentrations, 4) Blood NTBC
concentrations or 5) urine, plasma or blood spot SA concentrations12,28.
Nowadays, the recommendation in Europe, US and Canada is to treat patients with 1 mg/kg/day NTBC13,15,49,50. However, some clinicians advice to start
with 2 mg/kg/day in case of acute liver failure15, while chronic treatment
in stable patients is sometimes given at much lower NTBC doses around 0.36-0.6 mg/kg/day51-53. Research has shown that NTBC doses based on
body weight resulted in a large inter-individual variability in blood NTBC concentrations. Furthermore, recommendations for target blood NTBC concentrations are hampered by lack of standardization of NTBC assays53-59.
Therefore, target blood NTBC concentrations vary mostly between 30 and 60 µmol/L and concentrations ranging between 20 and 150 µmol/L have been reported15,35,37,51,54. Alternatively, a detectable or increased SA concentration in
blood spots or plasma, or its excretion in urine is considered to be a sensitive indicator for suboptimal NTBC treatment and reason for adjustment of therapy. New techniques for SA measurement have led to a clear improvement in sensitivity60. This made it possible to detect and quantify SA concentrations,
even in subjects without TT161.
It could be hypothesized that long-lasting suboptimal use of NTBC is the most important reason for later development of HCC, as has been reported in TT1 mice62. Therefore, monitoring NTBC treatment is a main focus point.
Further investigation of cut-off values for blood NTBC concentrations and its association with blood SA concentrations are consequently essential.
Dietary treatment
Without NTBC, dietary restriction of tyrosine and its precursor phenylalanine could improve renal function, but could not prevent a fatal outcome8,12,63.
When treated with NTBC, the clinical outcome improved tremendously, but due to inhibition of 4HPPD, tyrosine concentrations increase significantly. As increased tyrosine concentrations are associated with secondary development of ocular problems and possibly with neuropsychological problems as well, NTBC treatment has since its introduction been combined with a tyrosine and phenylalanine restricted diet12,30,64. This diet consists of protein restriction,
which is combined with the intake of a synthetic mixture of amino acids without phenylalanine and tyrosine to prevent a deficient intake of all
phenylalanine and tyrosine amino acids.
Currently, treatment recommendations vary between different centers and countries, with upper tyrosine concentrations varying between 400 and 600 µmol/L13,15,30,49,65, although higher levels up to 800 µmol/L are sometimes
accepted in practice37. With the combination of NTBC and dietary treatment
it turned out to be difficult to keep tyrosine concentrations within target range, while preventing low phenylalanine concentrations66,67. These low
phenylalanine concentrations have been associated with growth retardation, neurological impairments and skin problems in an infant with TT168.
Phenylalanine supplementation has been suggested to prevent these low phenylalanine concentrations, although the exact dosage and its effect on phenylalanine (and tyrosine) concentrations is not clear yet66,68. Moreover,
no uniform consensus guidelines about target phenylalanine concentrations exist, but the usual advice is to keep phenylalanine concentrations within the normal range (38-78 µmol/L)69.
In the past, monitoring dietary treatment was usually done by measuring plasma phenylalanine and tyrosine concentrations at the outpatient clinic. Nowadays, dried blood spots (DBS) are often used to measure phenylalanine and tyrosine concentrations. DBS have the advantage that blood samples can be collected at home. Consequently, blood samples can be taken more frequently, making dietary monitoring and adjustment easier. However, the validity of the DBS is subject to an ongoing debate. Differences between lithium heparin plasma and DBS phenylalanine concentrations, and differences in dried blood spot phenylalanine concentrations between various laboratories are thought to play a role.
Research questions and outline of thesis
Nowadays, it is no longer the goal to keep TT1 patients alive but to enable them to grow and develop into adulthood as healthy as possible. Notwithstanding this progress in the treatment of TT1, some challenges that previously characterized TT1 remained, while new challenges have emerged. This thesis investigated some of these challenges and focussed on the following research questions:
1. How to monitor the risk of HCC development?
2. What is the cognitive-behavioral phenotype of TT1 patients?
3. Which factors contribute to the cognitive dysfunction that has been reported?
4. How to improve metabolic control monitoring?
The remaining risk for HCC
Severe liver problems have always characterized TT1. After the introduction of NTBC, acute and long-term liver problems faded away, especially when NTBC treatment is combined with pre-clinical diagnoses by population based newborn screening. Therefore, we hypothesize that in such low risk patients, imaging with ultrasound and/or magnetic resonance imaging is no longer necessary, and monitoring could be done with AFP instead. In Chaper 2, a case report will be presented to illustrate this hypothesis. However, one important condition necessary to have a low risk for HCC development, is optimal NTBC treatment. Sub-optimal NTBC treatment is namely considered to be one of the main risk factors for HCC development. We hypothesize that monitoring NTBC treatment can be done exclusively by measuring SA. This is studied measuring daily variation of NTBC and its association with SA concentrations in Chapter 2.
The cognitive-behavioral phenotype of TT1 patients
So far, research has shown unspecific neurocognitive issues, such as school problems and a lower IQ. Those problems have been retrospectively reported in small samples of patients, usually without clear correlations to metabolic control. We hypothesize that the cognitive-behavioral outcome is a true concern in TT1 patients and involves more than IQ.
To test this hypothesis, a observational cross-sectional international study is performed that investigated several cognitive domains and behavioral outcomes of TT1 patients. To cross validate our data, results of TT1 patients are not only compared to age- and gender matched controls, but to norm scores obtained from a reference population and to phenylketonuria (PKU) patients as well. PKU is another metabolic disease that affects phenylalanine-tyrosine metabolism (Figure 1) and is also treated with a protein restricted diet. The
comparison between TT1 and PKU patients might be very interesting, as these diseases possibly share some pathophysiological mechanisms that affect brain functioning. In this way, knowledge on one disease can help us understand the other. These studies and their results on the cognitive-behavioral profile of TT1 patients are shown in Chapter 3.
Pathophysiology of brain dysfunction
Research into the pathophysiology of brain dysfunction in TT1 is limited, probably due to the rarity of TT1 and the fact that the neuropsychological issues have only recently emerged. Based on the neurocognitive and biochemical similarities between TT1 and PKU patients, We hypothesize that changes in blood phenylalanine and tyrosine concentrations affect brain biochemistry and consequently brain function in TT1 patients in a similar way as in PKU patients. This is investigated in several ways. Correlational analyses is performed in Chapter 3, to study whether high tyrosine and/or low phenylalanine concentrations are related to the cognitive and behavioral outcome of TT1 patients. However, as blood amino acid concentrations might not reflect brain biochemistry adequately, brain biochemistry is studied in
Chapter 4 using both, a theoretical model and TT1 mice.
Monitoring metabolic control
As changes in phenylalanine and tyrosine concentrations are thought to play a detrimental role in the cognitive-behavioral outcome, frequent and adequate monitoring is necessary. With home monitoring getting increasingly important, DBS analyses have gained in popularity. However, in PKU patients, the validity of the DBS is subject to an on-going debate. We hypothesize that in TT1, the time of blood sampling is more important than the method of analyses, especially when low phenylalanine concentrations are the main focus. This hypothesis will be investigated in Chapter 5, by comparing different methods of blood sampling and by studying the diurnal variation of phenylalanine and tyrosine concentrations with and without phenylalanine supplementation.
The general discussion will address the different research questions and hypotheses posed in this introduction in Chapter 6.
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