J Inherit Metab Dis (2014) 37:687–698 DOI 10.1007/s10545-014-9742-3
ICIEM SYMPOSIUM 2013
Inborn errors of pyrimidine metabolism: clinical update and therapy Shanti Balasubramaniam & John A. Duley & John Christodoulou
Received: 7 April 2014 / Revised: 5 June 2014 / Accepted: 24 June 2014 / Published online: 17 July 2014 # SSIEM 2014
Abstract Inborn errors involving enzymes essential for pyrimidine nucleotide metabolism have provided new insights into their fundamental physiological roles as vital constituents of nucleic acids as well as substrates of lipid and carbohydrate metabolism and in oxidative phosphorylation. Genetic aberrations of pyrimidine pathways lead to diverse clinical manifestations including neurological, immunological, haematological, renal impairments, adverse reactions to analogue therapy and association with malignancies. Maintenance of cellular nucleotides depends on the three aspects of metabolism of pyrimidines: de novo synthesis, catabolism and recycling of these metabolites. Of the ten recognised disorders of pyrimidine metabolism treatment is currently restricted to only two disorders: hereditary orotic aciduria (oral uridine therapy) and
Communicated by: Ron A. Wevers Presented at the 12th International Congress of Inborn Errors of Metabolism, Barcelona, Spain, September 3-6, 2013 S. Balasubramaniam Metabolic Unit, Princess Margaret Hospital, Roberts Road, Subiaco, Perth, WA 6008, Australia S. Balasubramaniam School of Paediatrics and Child Health, The University of Western Australia, Perth, WA 6009, Australia J. A. Duley School of Pharmacy and the Mater Research Institute, The University of Queensland, Brisbane QLD 4102, Australia J. Christodoulou (*) Western Sydney Genetics Program, Children’s Hospital at Westmead, Locked Bag 4001, Westmead, Sydney, NSW 2145, Australia e-mail: [email protected] J. Christodoulou Disciplines of Paediatrics and Child Health and Genetic Medicine, University of Sydney, Sydney, NSW 2006, Australia
mitochondrial neurogastrointestinal encephalomyopathy (MNGIE; allogeneic hematopoetic stem cell transplant and enzyme replacement). The ubiquitous role that pyrimidine metabolism plays in human life highlights the importance of improving diagnostic evaluation in suggestive clinical settings, which will contribute to the elucidation of new defects, future development of novel drugs and therapeutic strategies. Limited awareness of the expanding phenotypic spectrum, with relatively recent descriptions of newer disorders, compounded by considerable genetic heterogeneity has often contributed to the delays in the diagnosis of this group of disorders. The lack of an easily recognisable, easily measurable end product, akin to uric acid in purine metabolism, has contributed to the under-recognition of these disorders.This review describes the currently known inborn errors of pyrimidine metabolism, their variable phenotypic presentations, established diagnostic methodology and recognised treatment options.
Introduction Pyrimidines are aromatic heterocyclic organic compounds including the nucleobases cytosine, thymine and uracil, and are crucial for key functions in cell physiology. Pyrimidines form nucleotides that are the vital building blocks for nucleic acids DNA (cytosine and thymine) and RNA (uracil and cytosine), the basic elements of the cell programming machinery. Pyrimidine-activated sugars are involved in polysaccharide and phospholipid synthesis, glycosylation of proteins and lipids (Loeffler and Zameitat 2004), and have a functional vasoregulatory role through novel endothelium-derived vasoactive dinucleotides (Jankowski et al 2005). Pyrimidine nucleotides are synthesised endogenously by either the multistep pathway of de novo synthesis, or by the recycling (via the salvage pathway) of the nucleosides derived from catabolism
688
during the normal process of cell turnover, or from pyrimidines in the diet (Cameron et al 1993). Defects in the metabolic pathways of these important molecules, although rare, can have devastating or life-threatening consequences.
Definition and epidemiology The actual prevalence of purine and pyrimidine disorders is presently unknown and probably underestimated. Less than 1000 patients had been diagnosed with purine or related pyrimidine disorders in the last survey in 1999, from a population of 435 million, spanning 18 European countries (van Gennip 1999), of which 70 % had been detected in only three countries where adequate laboratory facilities were available (Simmonds et al 1997). The identification of 29 patients (28 purine and one pyrimidine defect) within a relatively short period of 3 years through selective screening for purine and pyrimidine defects in Poland, suggests that these defects are not as rare as previously considered (Jurecka et al 2008). The clinical spectrum of defects of purine and pyrimidine metabolism is diverse, and even within these defects there is considerable clinical heterogeneity.
J Inherit Metab Dis (2014) 37:687–698
Aetiology and pathogenesis The metabolism of the pyrimidine nucleotides can be divided into three principal routes (Fig. 1) (van den Berghe et al 2012): 1) the biosynthetic, de novo pathway begins with the formation of carbamoylphosphate by cytosolic carbamylphosphate synthetase II (as distinct from the mitochondrial form of this enzyme), the first step of a tri-functional protein CAD (carbamoylphosphate synthetase/ATCase/dihydroorotase) that synthesises dihydroorotate (DHO). Orotic acid is then formed by dihydroorotate dehydrogenase (DHODH). Synthesis of the key nucleotide uridine 5′-monophosphate (UMP) from orotic acid is achieved by UMP synthase; 2) the catabolic pathway of pyrimidine nucleotides starts with the nucleobases uracil and thymine and yields β-alanine and βaminoisobutyrate respectively, which are converted into intermediates of the citric acid cycle (Berg et al 2002); 3) the salvage pathway, composed of a number of kinases, recycles the pyrimidine nucleosides cytidine, uridine and thymidine into their corresponding mononucleotides. Pyrimidine salvage, which re-utilises nucleosides, thus differs from purine salvage, which recycles mainly nucleobases.
Inborn errors of pyrimidine metabolism Pyrimidine de novo biosynthesis Inheritance All ten recognised pyrimidine defects exhibit autosomal recessive inheritance.
Clinical presentation Previously considered to have only a paediatric onset, pyrimidine disorders are now increasingly being recognised in adults with partial deficiencies, and so may present from birth onwards (Simmonds and van Gennip 2003). The ubiquitous presence of pyrimidine-derived compounds underlines the heterogeneity in clinical expression, even within families, thus often making recognition difficult. Clinical presentation may include unexplained anaemia (megaloblastic, haemolytic or aplastic), delayed development, epilepsy, neonatal fitting, hypertonicity (or hypotonicity), microcephaly, mental retardation, dysmorphic features, neurogastrointestinal symptoms, ophthalmoplegia, malabsorption, muscle atrophy and polyneuropathy (van Gennip and van Kuilenburg 2000). The disorders of pyrimidine metabolism, their genetic defects, their chromosomal localisation, the broad spectrum of clinical presentations, diagnostic metabolites and therapeutic options are listed in Table 1.
Dihydroorotate dehydrogenase (DHODH) deficiency (Miller syndrome; Genée–Wiedemann; Wildervanck–Smith syndrome; postaxial acrofacial dysostosis syndrome [POADS]) DHODH catalyses the oxidation of DHO to orotic acid (OA). DHODH was identified as the cause of Miller syndrome, being the first Mendelian disorder to be identified using a whole exome sequencing strategy (Ng et al 2010). The clinical characteristics of Miller syndrome include severe micrognathia, cleft lip and/or palate, hypoplasia or aplasia of the posterior elements of the limbs, coloboma of the eyelids and supernumerary nipples. Evidence supporting the loss of the enzymatic activity of DHODH as the cause of Miller syndrome is provided by the teratogenic effects (wide range of limb and craniofacial defects including exencephaly, cleft palate and ‘open eye’ or failure of the eyelid to close) in mouse embryos exposed to leflunomide, a widely-used immunosuppressive agent and specific inhibitor of DHODH. TNF-α production is repressed by the direct inhibition of NF-κB activity in the liver of mice treated with leflunomide (Imose et al 2004), providing a causal link to mutations in DHODH (Ng et al 2010). These phenotypic characteristics recapitulate the malformations observed in individuals with Miller syndrome which could be
Enzyme defect
Pyrimidine catabolism Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE),
Deficiency of thymidine phosphorylase (TP).
De novo pyrimidine synthesis Miller syndrome/Genée– Deficiency of dihydroorotate Wiedemann/ dehydrogenase (DHODH); Wildervanck–Smith protein localised in syndrome/postaxial mitochondria, with ubiquinone acrofacial dysostosis electron acceptor (Loeffler et al syndrome (POADS) 2004) DHODH, 16q22.2 (126064) (Ng et al 2010; Rainger et al 2012) Hereditary orotic aciduria, UMPS, 3q21.2 (258900) - Type I Type I: Deficiency of orotate phosphoribosyl-transferase (OPRT) (Bailey 2009), part of bifunctional Uridine monophosphate synthase (UMPS) - Type II, hereditary orotic Type II: Deficiency of orotidylic aciduria decarboxylase (ODC), part of bifunctional protein UMPS (Fox et al 1973). - Type III, orotic aciduria Type III: Deficiency of OPRT without anaemia (Besley et al 2000). (OAWA) Pyrimidine salvage Mitochondrial DNA Deficiency of thymidine kinase-2 depletion syndrome-2 (TK2) (MDS-2, myopathic type), TK2, 16q21 (609560)
Disorder, gene symbol, chromosomal location, (OMIM)
Table 1 Inborn errors of pyrimidine metabolism Diagnosis/metabolic tests
Gross urinary orotic acid excretion, ratio of Type I: Oral uridine therapy, 50– urinary orotate/orotidine >10; absent 300 mg/kg/day (Webster et al 2001) erythrocyte OPRT activity.
Not established.
Treatments
Progressive ptosis and external ophthalmoparesis, gastrointestinal dysmotility, cachexia, leukoencephalopathy, peripheral
Raised urinary and plasma thymidine and deoxy-uridine (Spinazzola et al 2002; Fairbanks et al 2002), raised urinary thymine and uracil (Spinazzola et al
Severe infantile myopathy with motor Marked increase in creatine kinase, an regression and early death (Mancuso et al unusual finding in mitochondrial 2002); milder phenotypes include spinal myopathies; mitochondrial dysfunction, muscle atrophy type 3-like presentation lactic acidosis; abnormal muscle fibre (Oskoui et al 2006), rigid spine syndrome staining on biopsy; myopathic patternand milder myopathic presentation with EMG; marked mtDNA depletion/ longer survival (Oskoui et al 2006); multiple mtDNA deletions in some patients with autosomal recessive progressive external ophthalmoplegia (Tyynismaa et al 2012).
Hemodialysis and platelet infusions transiently reduce thymidine overload (Lara et al 2006); trials of allogeneic hematopoetic stem cell transplant (Hirano
Not established.
Single case report (Fox et al 1973) appears Increased urine orotic acid, ratio of urinary Type II: As above clinically indistinguishable from type I orotate/ orotidine
ICIEM SYMPOSIUM 2013
Inborn errors of pyrimidine metabolism: clinical update and therapy Shanti Balasubramaniam & John A. Duley & John Christodoulou
Received: 7 April 2014 / Revised: 5 June 2014 / Accepted: 24 June 2014 / Published online: 17 July 2014 # SSIEM 2014
Abstract Inborn errors involving enzymes essential for pyrimidine nucleotide metabolism have provided new insights into their fundamental physiological roles as vital constituents of nucleic acids as well as substrates of lipid and carbohydrate metabolism and in oxidative phosphorylation. Genetic aberrations of pyrimidine pathways lead to diverse clinical manifestations including neurological, immunological, haematological, renal impairments, adverse reactions to analogue therapy and association with malignancies. Maintenance of cellular nucleotides depends on the three aspects of metabolism of pyrimidines: de novo synthesis, catabolism and recycling of these metabolites. Of the ten recognised disorders of pyrimidine metabolism treatment is currently restricted to only two disorders: hereditary orotic aciduria (oral uridine therapy) and
Communicated by: Ron A. Wevers Presented at the 12th International Congress of Inborn Errors of Metabolism, Barcelona, Spain, September 3-6, 2013 S. Balasubramaniam Metabolic Unit, Princess Margaret Hospital, Roberts Road, Subiaco, Perth, WA 6008, Australia S. Balasubramaniam School of Paediatrics and Child Health, The University of Western Australia, Perth, WA 6009, Australia J. A. Duley School of Pharmacy and the Mater Research Institute, The University of Queensland, Brisbane QLD 4102, Australia J. Christodoulou (*) Western Sydney Genetics Program, Children’s Hospital at Westmead, Locked Bag 4001, Westmead, Sydney, NSW 2145, Australia e-mail: [email protected] J. Christodoulou Disciplines of Paediatrics and Child Health and Genetic Medicine, University of Sydney, Sydney, NSW 2006, Australia
mitochondrial neurogastrointestinal encephalomyopathy (MNGIE; allogeneic hematopoetic stem cell transplant and enzyme replacement). The ubiquitous role that pyrimidine metabolism plays in human life highlights the importance of improving diagnostic evaluation in suggestive clinical settings, which will contribute to the elucidation of new defects, future development of novel drugs and therapeutic strategies. Limited awareness of the expanding phenotypic spectrum, with relatively recent descriptions of newer disorders, compounded by considerable genetic heterogeneity has often contributed to the delays in the diagnosis of this group of disorders. The lack of an easily recognisable, easily measurable end product, akin to uric acid in purine metabolism, has contributed to the under-recognition of these disorders.This review describes the currently known inborn errors of pyrimidine metabolism, their variable phenotypic presentations, established diagnostic methodology and recognised treatment options.
Introduction Pyrimidines are aromatic heterocyclic organic compounds including the nucleobases cytosine, thymine and uracil, and are crucial for key functions in cell physiology. Pyrimidines form nucleotides that are the vital building blocks for nucleic acids DNA (cytosine and thymine) and RNA (uracil and cytosine), the basic elements of the cell programming machinery. Pyrimidine-activated sugars are involved in polysaccharide and phospholipid synthesis, glycosylation of proteins and lipids (Loeffler and Zameitat 2004), and have a functional vasoregulatory role through novel endothelium-derived vasoactive dinucleotides (Jankowski et al 2005). Pyrimidine nucleotides are synthesised endogenously by either the multistep pathway of de novo synthesis, or by the recycling (via the salvage pathway) of the nucleosides derived from catabolism
688
during the normal process of cell turnover, or from pyrimidines in the diet (Cameron et al 1993). Defects in the metabolic pathways of these important molecules, although rare, can have devastating or life-threatening consequences.
Definition and epidemiology The actual prevalence of purine and pyrimidine disorders is presently unknown and probably underestimated. Less than 1000 patients had been diagnosed with purine or related pyrimidine disorders in the last survey in 1999, from a population of 435 million, spanning 18 European countries (van Gennip 1999), of which 70 % had been detected in only three countries where adequate laboratory facilities were available (Simmonds et al 1997). The identification of 29 patients (28 purine and one pyrimidine defect) within a relatively short period of 3 years through selective screening for purine and pyrimidine defects in Poland, suggests that these defects are not as rare as previously considered (Jurecka et al 2008). The clinical spectrum of defects of purine and pyrimidine metabolism is diverse, and even within these defects there is considerable clinical heterogeneity.
J Inherit Metab Dis (2014) 37:687–698
Aetiology and pathogenesis The metabolism of the pyrimidine nucleotides can be divided into three principal routes (Fig. 1) (van den Berghe et al 2012): 1) the biosynthetic, de novo pathway begins with the formation of carbamoylphosphate by cytosolic carbamylphosphate synthetase II (as distinct from the mitochondrial form of this enzyme), the first step of a tri-functional protein CAD (carbamoylphosphate synthetase/ATCase/dihydroorotase) that synthesises dihydroorotate (DHO). Orotic acid is then formed by dihydroorotate dehydrogenase (DHODH). Synthesis of the key nucleotide uridine 5′-monophosphate (UMP) from orotic acid is achieved by UMP synthase; 2) the catabolic pathway of pyrimidine nucleotides starts with the nucleobases uracil and thymine and yields β-alanine and βaminoisobutyrate respectively, which are converted into intermediates of the citric acid cycle (Berg et al 2002); 3) the salvage pathway, composed of a number of kinases, recycles the pyrimidine nucleosides cytidine, uridine and thymidine into their corresponding mononucleotides. Pyrimidine salvage, which re-utilises nucleosides, thus differs from purine salvage, which recycles mainly nucleobases.
Inborn errors of pyrimidine metabolism Pyrimidine de novo biosynthesis Inheritance All ten recognised pyrimidine defects exhibit autosomal recessive inheritance.
Clinical presentation Previously considered to have only a paediatric onset, pyrimidine disorders are now increasingly being recognised in adults with partial deficiencies, and so may present from birth onwards (Simmonds and van Gennip 2003). The ubiquitous presence of pyrimidine-derived compounds underlines the heterogeneity in clinical expression, even within families, thus often making recognition difficult. Clinical presentation may include unexplained anaemia (megaloblastic, haemolytic or aplastic), delayed development, epilepsy, neonatal fitting, hypertonicity (or hypotonicity), microcephaly, mental retardation, dysmorphic features, neurogastrointestinal symptoms, ophthalmoplegia, malabsorption, muscle atrophy and polyneuropathy (van Gennip and van Kuilenburg 2000). The disorders of pyrimidine metabolism, their genetic defects, their chromosomal localisation, the broad spectrum of clinical presentations, diagnostic metabolites and therapeutic options are listed in Table 1.
Dihydroorotate dehydrogenase (DHODH) deficiency (Miller syndrome; Genée–Wiedemann; Wildervanck–Smith syndrome; postaxial acrofacial dysostosis syndrome [POADS]) DHODH catalyses the oxidation of DHO to orotic acid (OA). DHODH was identified as the cause of Miller syndrome, being the first Mendelian disorder to be identified using a whole exome sequencing strategy (Ng et al 2010). The clinical characteristics of Miller syndrome include severe micrognathia, cleft lip and/or palate, hypoplasia or aplasia of the posterior elements of the limbs, coloboma of the eyelids and supernumerary nipples. Evidence supporting the loss of the enzymatic activity of DHODH as the cause of Miller syndrome is provided by the teratogenic effects (wide range of limb and craniofacial defects including exencephaly, cleft palate and ‘open eye’ or failure of the eyelid to close) in mouse embryos exposed to leflunomide, a widely-used immunosuppressive agent and specific inhibitor of DHODH. TNF-α production is repressed by the direct inhibition of NF-κB activity in the liver of mice treated with leflunomide (Imose et al 2004), providing a causal link to mutations in DHODH (Ng et al 2010). These phenotypic characteristics recapitulate the malformations observed in individuals with Miller syndrome which could be
Enzyme defect
Pyrimidine catabolism Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE),
Deficiency of thymidine phosphorylase (TP).
De novo pyrimidine synthesis Miller syndrome/Genée– Deficiency of dihydroorotate Wiedemann/ dehydrogenase (DHODH); Wildervanck–Smith protein localised in syndrome/postaxial mitochondria, with ubiquinone acrofacial dysostosis electron acceptor (Loeffler et al syndrome (POADS) 2004) DHODH, 16q22.2 (126064) (Ng et al 2010; Rainger et al 2012) Hereditary orotic aciduria, UMPS, 3q21.2 (258900) - Type I Type I: Deficiency of orotate phosphoribosyl-transferase (OPRT) (Bailey 2009), part of bifunctional Uridine monophosphate synthase (UMPS) - Type II, hereditary orotic Type II: Deficiency of orotidylic aciduria decarboxylase (ODC), part of bifunctional protein UMPS (Fox et al 1973). - Type III, orotic aciduria Type III: Deficiency of OPRT without anaemia (Besley et al 2000). (OAWA) Pyrimidine salvage Mitochondrial DNA Deficiency of thymidine kinase-2 depletion syndrome-2 (TK2) (MDS-2, myopathic type), TK2, 16q21 (609560)
Disorder, gene symbol, chromosomal location, (OMIM)
Table 1 Inborn errors of pyrimidine metabolism Diagnosis/metabolic tests
Gross urinary orotic acid excretion, ratio of Type I: Oral uridine therapy, 50– urinary orotate/orotidine >10; absent 300 mg/kg/day (Webster et al 2001) erythrocyte OPRT activity.
Not established.
Treatments
Progressive ptosis and external ophthalmoparesis, gastrointestinal dysmotility, cachexia, leukoencephalopathy, peripheral
Raised urinary and plasma thymidine and deoxy-uridine (Spinazzola et al 2002; Fairbanks et al 2002), raised urinary thymine and uracil (Spinazzola et al
Severe infantile myopathy with motor Marked increase in creatine kinase, an regression and early death (Mancuso et al unusual finding in mitochondrial 2002); milder phenotypes include spinal myopathies; mitochondrial dysfunction, muscle atrophy type 3-like presentation lactic acidosis; abnormal muscle fibre (Oskoui et al 2006), rigid spine syndrome staining on biopsy; myopathic patternand milder myopathic presentation with EMG; marked mtDNA depletion/ longer survival (Oskoui et al 2006); multiple mtDNA deletions in some patients with autosomal recessive progressive external ophthalmoplegia (Tyynismaa et al 2012).
Hemodialysis and platelet infusions transiently reduce thymidine overload (Lara et al 2006); trials of allogeneic hematopoetic stem cell transplant (Hirano
Not established.
Single case report (Fox et al 1973) appears Increased urine orotic acid, ratio of urinary Type II: As above clinically indistinguishable from type I orotate/ orotidine
