Defects in polyol metabolism
Introduction
Polyols, or polyhydric alcohols, are sugar-derived metabolites. The origins, metabolic fates and functions of most of the polyols are unknown. They are particularly abundant within the central nervous system (1).The only known inborn metabolic disorder with accumulation of a polyol was galactosemia. However, world-wide metabolic screening does not include the assessment of polyols in body fluids, and patients may be missed. Recently, we described a patient with a leukoencephalopathy and peripheral neuropathy of unknown origin, in whom we found highly elevated levels of D-arabitol and ribitol in all body fluids, especially the CSF, and even higher levels in the brain (2). Since the detection of the first patient, we have started to evaluate and re-evaluate sugar and polyol profiles in body fluids of patients strongly suspected of an inborn error of metabolism, in whom extensive metabolic screening is unrevealing. Since the introduction of this policy, other patients have been identified.
Clinical symptoms and MRI
Patient 1 has been described in detail before (2). He was born in 1984 as the only child of healthy, unrelated parents. He had a psychomotor retardation from early on. At the age of 4 years, he developed epilepsy. From the age of 7 years onwards, slow neurological deterioration occurred with prominent cerebellar ataxia, some spasticity, optic atrophy and a mild sensory-motor neuropathy. There were no signs of dysfunction of internal organs; there was no organomegaly. Magnetic Resonance Imaging (MRI) at the ages of 11 and 14 years revealed extensive abnormalities of the cerebral white matter with most prominent involvement of the U-fibers. The abnormal white matter had a slightly swollen appearance, suggesting a spongiform leukoencephalopathy. Motor and sensory nerve conduction velocities were mildly decreased, confirming the presence of a peripheral polyneuropathy. Routine chemistry panel, including liver function, was normal. Extensive metabolic studies did not reveal the cause of the disease. Magnetic resonance spectroscopy (MRS) at the age of 14 years revealed the presence of several very high unknown resonances between 3.5 and 4.0 ppm, the sugar region of the spectrum. These resonances could be assigned to D-arabitol and ribitol. Analysis of sugars and polyols in urine, plasma and CSF revealed that the patient had highly elevated levels of D-arabitol, ribitol and to a lesser extent xylitol, xylulose, treitol, arabinose, ribulose, and ribose (2). The levels were consistently much more elevated in CSF than in plasma.
Patient 2 is a boy born in 1996 as the first child of healthy, nonconsanguineous parents. His younger brother was normal. He had a psychomotor retardation from early on. At the age of 4 years, he could walk unsupportedly. He could speak in two-word sentences. He had signs of autism with a limited social interaction. He had a mild facial dysmorphism with down-slanting eyelids. Neurological examination revealed a generalized hypotonia, but no other abnormalities. At physical examination there were no signs of dysfunction of internal organs; there was no organomegaly. MRI of the brain at the age of 16 months showed some delay in myelination, but no other white matter abnormalities. Nerve conduction velocity was normal. Slight elevations of liver functions were found: ASAT 157 U/l (normal 10-40), ALAT 196 U/l (normal 4.5-45), and LDH 761 U/l (normal 220-590), but normal gGT. Urinary glucose levels were consistently elevated, ranging between 1212 and 3831 mmol/mol creatinine (normal 22-43). Further analysis of sugars and polyols in urine showed elevated levels of arabitol, ribitol, myo-inositol, mannitol and galactose. Plasma levels of arabitol were consistently mildly elevated, but the levels of sugars and other polyols were all normal, including normal glucose levels. In CSF arabitol and ribitol were mildly elevated. MRS of the brain did not reveal abnormal metabolites between 3.5 and 4 ppm.
Patient 3 is a girl born in 1989 as the first child of consanguineous parents (3). She was born at term as a dysmature baby. Soon after birth, she underwent surgery for a coarctatio aortae. In the course of a few months she developed a hepatosplenomegaly. Psychomotor development remained normal. At the age of 10 years, height and weight for height were in the low percentiles. Physical examination revealed numerous teleangiectasies and an unchanged hepatosplenomegaly. She had a peculiar face. Laboratory tests were performed because of the hepatosplenomegaly, revealing normal ASAT, ALAT, LDH and g-GT. A bleeding tendency was found. No evidence was found of a metabolic disorder or a viral cause. A liver biopsy revealed nodular liver cirrhosis without signs of cholestasis, viral hepatitis or storage disease. Repeated analysis of sugars and polyols in urine revealed consistent elevations of arabitol, xylulose, ribitol, ribose and erythritol. In plasma only arabitol, ribitol and erythritol were elevated. In CSF a minor elevation of ribitol was the only abnormal finding.
Polyol metabolic pathways
Polyols are ubiquitous metabolites formed by reduction of sugars. They are subdivided on the basis of the number of carbon atoms in the C-chain. Examples of acyclic hexitols (C6 polyols) are sorbitol, derived from glucose, and galactitol, derived from galactose. Myo-inositol, a cyclic hexitol, is the most abundant polyol in the brain (1). Acyclic pentitols (C5 polyols) include arabitol, derived from xylulose, and ribitol, derived from ribose. The pentose precursors of these pentitols are formed from glucose through the pentose phosphate cycle. The pentose phosphate cycle is an alternative pathway for the oxidation of glucose. This cycle has two major functions: the generation of NADPH for reductive syntheses, and the provision of ribose for nucleotide and nucleic acid biosynthesis. The glucuronic acid pathway, which is important for the excretion of metabolites and xenobiotics as glucuronides in urine, generates glucuronic acid from glucose. The end product of the glucuronic acid pathway is xylulose-5-phosphate, which is further metabolized in the pentose phosphate cycle. The metabolic pathways involving polyols have been mainly studied in fungi. In contrast, little is known about the metabolism of polyols in humans. Most enzymes that catalyze the formation of pentitols from pentoses have not been characterized. Even less is known about polyol transport. The levels of many polyols, including myo-inositol, mannitol, sorbitol, galactitol, ribitol and arabitol are normally higher in CSF than in blood (1, 4). One explanation could be that there may be a selective transport of polyols from blood over the blood-brain-barrier into the brain and CSF. Another explanation could be that polyols are mainly produced in brain tissue (1).
When starting screening for abnormalities in polyol levels in body fluids, we were aware of the fact that polyols are probably end products of different metabolic pathways and that their changes would be a marker of various metabolic defects in different pathways.
Novel assays for detection of polyols
Considering the changes in metabolites in patient 1, a xylulokinase deficiency was thought to be most likely. This enzyme reversibly converts D-xylulose and probably also D-ribulose into D-xylulose-5-phosphate. We developed an assay for D-xylulokinase in erythrocytes and did not find a decreased enzyme activity in the patient (2). We also excluded deficiencies of transketolase and transaldolase. The changes in polyol levels in patient 2 are difficult to explain, in particular in view of the consistent glucosuria. We studied polyol levels in the urine of a patient with renal glucosuria (SGLT2 deficiency) and found no abnormalities. There is evidence that there are several renal glucose transporters (5, 6). Little is known about their function. It is possible that patient 2 has a defect in one of these transporters. The changes in sugar and polyol levels in patient 3 were thought to be suggestive of a transaldolase deficiency. We developed an assay to assess the activities of transketolase and transaldolase in erythrocytes. Ribose-5-phosphate was used as a substrate, which is rapidly converted into ribulose-5-phosphate and xylulose-5-phosphate by the cell system, providing the second substrate for transaldolase. A deficient activity of transaldolase was found in the patient as compared to controls (3).
Both the differences in clinical symptomatology and the differences in sugar and polyol profiles among the three patients suggest that we are dealing with three different disorders caused by different defects in the metabolism or transport of sugars and polyols. Two organs are involved in these patients: the brain and the liver. A comparable situation is found in galactosemia, a disorder of galactose metabolism. This disease is biochemically characterized by accumulation of the sugars galactose and galactose-1-phosphate and the polyol galactitol. The liver, the brain, and the eye are the organs mainly affected. There is evidence that the involvement of the eyes and the brain is related to the accumulation of galactitol (7, 8), whereas the hepatic dysfunction is ascribed to the toxicity of galactose-1-phosphate (7,8). There is also evidence for neurotoxicity of sorbitol in diabetes mellitus (9-11). The significance of polyols in the pathophysiology of nervous tissue damage is substantiated by studies showing that aldose reductase inhibitors, which have no effect on glucose and galactose levels in nervous tissue, but reduce sorbitol and galactose accumulation, ameliorate injury (10, 12).
Patient 1 had only neurological problems, no liver problems. Patient 3 had only liver problems and no neurological problems. Patient 2 had both neurological problems and biochemical evidence of some hepatic dysfunction. In patient 1, there is a strong brain: CSF: plasma gradient of ribitol and D-arabitol, with the highest levels in the brain. This finding suggest that the patient has a defect involving an enzyme that is normally exclusively of mainly functional in nervous tissue and that the accumulated polyols are toxic for nervous tissue. In patients 2 and 3, we could not detect abnormally elevated polyols in the brain using MRS. In patient 2, CSF ribitol and arabitol levels were mildly elevated; in patient 3 CSF analysis revealed only a borderline elevation of ribitol. Patient 2 had a considerable mental deficiency and signs of autism, whereas patient 3 was neurologically entirely normal. Both had signs of liver problems, with mild biochemical abnormalities in patient 2 and liver cirrhosis in patient 3. In analogy to galactosemia, it is most likely that the accumulated sugar phosphates are hepatotoxic rather than the polyols.
It is clear that we have only just started to explore a new area of metabolism and possible metabolic defects related to sugars and polyols, and that we have no idea about the range of possible phenotypes related to various possible defects in metabolism and transport. At present, the safest policy seems to start broad screening for abnormalities in sugar and polyol metabolism and decide after several years whether this policy is justified. Especially patients with an encephalopathy or liver problem of unkown origin should be considered candidates for screening.
References
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