Lysine intake and neurotoxicity in glutaric aciduria type I: towards a rationale for therapy?
Received March 24, 2006. Accepted April 25, 2006.
Glutaric aciduria type I (GA-I) is a rare cerebral organic acid disorder caused by inherited deficiency of glutaryl-CoA dehydrogenase (GCDH; EC 1.3.99.7
[EC]
), a mitochondrial flavoprotein catalysing the oxidative decarboxylation of glutaryl-CoA to crotonyl-CoA in the final catabolic pathways of the amino acids L-lysine, L-hydroxylysine and L-tryptophan (Goodman et al., 1975
). Biochemically, GA-I is characterized by an accumulation of the dicarboxylic acids glutaric acid (GA) and 3-hydroxyglutaric acid (3-OH-GA) as well as glutarylcarnitine (Baric et al., 1999). Clinically, the disease course is complicated by striatal injury during an acute encephalopathic crisis, which is usually precipitated by a catabolic state (e.g. infectious diseases) in infancy or early childhood (Strauss et al., 2003; Kölker et al., 2006). If treated before the onset of irreversible neurological symptoms, the encephalopathic crises can be prevented in the majority of children (Strauss et al., 2003; Naughten et al., 2004; Kölker et al., 2006). In particular, maintenance treatment with L-carnitine supplementation and lysine restriction is beneficial for pre-symptomatically diagnosed children (Kölker et al., 2006).
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Lysine excess and neurotoxicity |
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 Top Lysine excess and neurotoxicity Lack of elevated 3...Dicarboxylic acids and the...BBB dysfunction--a missing link?Lysine restriction and...References |
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An animal model of GA-I has been developed to study pathophysiology. A detailed analysis of the histopathology, biochemistry, bioenergetics and behaviour of these mice has previously been published (Koeller et al., 2002
; Sauer et al., 2005). In spite of high levels of GA and 3-OH-GA, these Gcdh–/– mice do not develop striatal degeneration either spontaneously or following a number of triggers, inducing a catabolic state (Koeller et al., 2002), which contrasts with the association between catabolism and encephalopathic crises in GA-I patients. Recently, Zinnanti et al. reported in Brain (2006) that excessive dietary intake of lysine or protein produced a phenotype in Gcdh–/– mice that resembled some important clinical and neuroradiological aspects of the acute encephalopathic crises seen in human patients. Significantly, they demonstrate an age-dependent onset of acute striatal necrosis, which closely mimics what is seen in patients. The rationale for these studies was that increased protein intake, and specifically lysine, is predicted to increase levels of GA and 3-OH-GA. It is noteworthy, however, that a high protein or a high lysine diet has not been reported to precipitate encephalopathic crises in humans, nor is there a correlation between blood levels of GA and disease severity (Christensen et al., 2004; Kölker et al., 2006), which was observed in the present study (Zinnanti et al., 2006). Thus, this study opens new facets to the pathophysiology of GA-I but also, at first sight, unresolved discrepancies to previous reports. |
Lack of elevated 3-hydroxyglutaric acid, the key metabolite of glutaric aciduria type I |
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TopLysine excess and neurotoxicityLack of elevated 3...Dicarboxylic acids and the...BBB dysfunction--a missing link?Lysine restriction and...References |
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Beyond considerable discrepancies in trigger factors inducing acute encephalopathic crises, there are important biochemical differences between the diet-induced mouse model and affected patients. Unexpectedly, the recent study failed to demonstrate significant accumulation of 3-OH-GA in serum or brain samples of Gcdh–/– mice on either a normal or high lysine diet, in contrast to previous reports of elevated 3-OH-GA concentrations in these mice (Koeller et al., 2002
; Sauer et al., 2006). Furthermore, increased urinary concentrations of 3-OH-GA are usually found in GA-I patients, even in patients with a mild biochemical phenotype (Baric et al., 1999; Christensen et al., 2004). The lack of elevated 3-OH-GA in the diet-induced mouse model of neurotoxicity appears important for toxicological considerations, since 3-OH-GA was shown to induce cell damage in cultivated neurons (Kölker et al., 2004), immature oligodendroglial cells (Gerstner et al., 2005) and human dermal microvascular endothelial cells (Mühlhausen et al., 2006). |
Dicarboxylic acids and the brain—a lesson from the ‘deep’ metabolic compartment |
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TopLysine excess and neurotoxicityLack of elevated 3...Dicarboxylic acids and the...BBB dysfunction--a missing link?Lysine restriction and...References |
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Another important biochemical difference in comparison with previous studies is that serum GA concentrations massively increased following high lysine intake in symptomatic 4-week-old Gcdh–/– mice, reaching a mean concentration of
2500 µmol/l (Zinnanti et al., 2006), which is 10–1000-fold higher than that seen in GA-I patients (Baric et al., 1999; Strauss et al., 2003). In contrast to serum, brain concentrations of GA and 3-OH-GA are massively increased in patients with either a classic (high excretors) or a mild biochemical phenotype (low excretors; Funk et al., 2005). Notably, cerebral GA concentrations demonstrated in these post-mortem studies (1000 µmol/l) were similar to Gcdh–/– mice fed on a normal diet (Sauer et al., 2006; Zinnanti et al., 2006). The marked cerebral accumulation and steep blood-to-brain gradient of GA was unexpected, given the low level of cerebral GCDH activity (McMillan et al., 1988; Woontner et al., 2000). This has raised questions about the role of the blood–brain barrier (BBB) and intracerebral de novo synthesis of GA and 3-OH-GA (Sauer et al., 2006). The BBB metabolically isolates the brain from the circulation and protects it against fluctuations of nutrient delivery and intoxication; >20 specific transport systems act as biochemical gatekeepers for key solutes (Tamai and Tsuji, 2000). For dicarboxylic acids, such as GA and 3-OH-GA, the transport across the BBB is strongly limited, and thus increased cerebral de novo synthesis of dicarboxylic acids would facilitate the cerebral accumulation of these metabolites. A recent study has hypothesized the involvement of such mechanisms in the pathophysiology of GA-I, demonstrating that brain concentrations of GA and 3-OH-GA only slightly or transiently increased in mice with isolated hepatic GCDH deficiency or following intraperitoneal loading with deuterated GA or 3-OH-GA (Sauer et al., 2006). The same authors also demonstrated limited flux of GA and 3-OH-GA across cultivated porcine brain capillary endothelial cells and excluded significant dysfunction of the endothelial monolayer induced by GA or 3-OH-GA (up to 1 mmol/l). The separating role of the BBB for GA and 3-OH-GA is further supported by the fruit-eating bat Rousettus aegypticus, a naturally occurring animal model with isolated hepatic GCDH deficiency. Although urinary excretion of GA (2300–7900 mmol/mol creatinine) in these animals resembles that of high excretors and significantly increases (7000–21 000 mmol/mol creatinine) following oral loading with L-lysine (100 mg/kg body weight), these bats do not produce neurological abnormalities (McMillan et al., 1988). |
BBB dysfunction—a missing link? |
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TopLysine excess and neurotoxicityLack of elevated 3...Dicarboxylic acids and the...BBB dysfunction--a missing link?Lysine restriction and...References |
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In contrast to these findings, the recent study of Zinnanti et al. (2006)
hypothesized disruption of the BBB as an early event following the administration of a high lysine diet causing cerebral oedema and subarachnoid bleedings. However, a clear-cut differentiation between vasogenic and cytotoxic oedema has not been shown. Furthermore, subarachnoid bleeding is uncommon, whereas subdural bleedings are frequently found in GA-I patients (Strauss et al., 2003). BBB breakdown induced by 3-OH-GA was first hypothesized by Strauss and Morton (2003). A recent study has demonstrated that only at millimolar concentrations 3-OH-GA affects the morphology of vascular endothelial growth factor (VEGF) induced endothelial tubes, disrupts the actin skeleton of cultivated endothelial cells and induces vascular dilatation and haemorrhage in chick chorioallantoic membranes (Mühlhausen et al., 2006). However, these 3-OH-GA-induced effects are unlikely to be involved in the diet-induced model of Zinnanti et al. (2006) owing to normal 3-OH-GA concentrations in brain and serum.
Although BBB disruption would be an elegant way to explain the rapid development of striatal lesions during encephalopathic crises, elevated serum concentrations of leucine, isoleucine and valine, which occurred secondary to lysine excess only in symptomatic Gcdh–/– mice (Zinnanti et al., 2006), should be considered as an alternative cause for cerebral oedema and brain damage. Elevated serum leucine concentrations have never been described in GA-I patients but are characteristic for maple syrup urine disorder (MSUD). Notably, untreated patients with MSUD in metabolic decompensation develop a progressive encephalopathy resulting in lethargy, cerebral oedema and death in response to the increase of leucine (Morton et al., 2002). Metabolic intoxication becomes apparent with relatively mild increases in leucine concentrations (Korein et al., 1994). Notably, in the symptomatic Gcdh–/– mice serum leucine concentrations were highly elevated, that is, in the range known from patients with metabolically decompensated MSUD. We suggest that the MSUD-like biochemical phenotype in symptomatic Gcdh–/– mice is induced by lysine loading itself. A likely mechanism is glutaryl-CoA-induced inhibition of the branched-chain alpha-keto acid dehydrogenase complex (BCKDH)—in analogy to the recently described inhibition of alpha-ketoglutarate dehydrogenase and pyruvate dehydrogenase complexes (Sauer et al., 2005). In fact, we found decreased activity (mean ± standard deviation = 45 ± 9% of control activity; n = 6 experiments) of purified bovine BCKDH (Globozyme, Carlsbad, CA, USA) after incubation with glutaryl-CoA (1 mmol/l; Sigma-Aldrich, Taufkirchen, Germany). However, it remains to be elucidated whether this mechanism is of relevance for GA-I because of the high spare capacity of BCKDH (Harris et al., 2005).
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Lysine restriction and neuroprotection |
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TopLysine excess and neurotoxicityLack of elevated 3...Dicarboxylic acids and the...BBB dysfunction--a missing link?Lysine restriction and...References |
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Although many questions remain unanswered and further characterization of the diet-induced mouse model seems necessary, the recent study of Zinnanti et al. (2006)
is an important step towards the development of an animal model that allows the study of major mechanisms underlying acute striatal damage in GA-I and to evaluate neuroprotective strategies. In particular, the role of the BBB for this disease could be a clue to the understanding of cerebral injury and the development of a treatment rationale, including a well-balanced reduction of lysine flux across the BBB by dietary lysine restriction (Kölker et al., 2006) or supplementation with arginine and/or ornithine (competing with lysine for transporter y+), trans-stimulation of organic acid transporters (e.g. OAT-3; Sauer et al., 2006) to stimulate the efflux of accumulating cerebral dicarboxylic acids and prevention of BBB disruption and vascular dysfunction (Mühlhausen et al., 2006; Zinnanti et al., 2006).
1 Department of General Pediatrics, Division of Inborn Metabolic Diseases University Children's Hospital, Heidelberg, Germany 2 Departments of Pediatrics, Molecular and Medical Genetics Oregon Health and Science University, Portland, OR, USA
Correspondence to: Stefan Kölker, MD, Department of General Pediatrics, Division of Inborn Metabolic Diseases, University Children's Hospital, Im Neuenheimer Feld 150, D-69120 Heidelberg, Germany E-mail: Stefan_Koelker{at}med.uni-heidelberg.de
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References |
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TopLysine excess and neurotoxicityLack of elevated 3...Dicarboxylic acids and the...BBB dysfunction--a missing link?Lysine restriction and...References |
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