Amino acid synthesis deficiencies.

In recent years the number of disorders known to affect amino acid synthesis has grown rapidly. Nor is it just the number of disorders that has increased: the associated clinical phenotypes have also expanded spectacularly, primarily due to the advances of next generation sequencing diagnostics. In contrast to the "classical" inborn errors of metabolism in catabolic pathways, in which elevated levels of metabolites are easily detected in body fluids, synthesis defects present with low values of metabolites or, confusingly, even completely normal levels of amino acids. This makes the biochemical diagnosis of this relatively new group of metabolic diseases challenging. Defects in the synthesis pathways of serine metabolism, glutamine, proline and, recently, asparagine have all been reported. Although these amino acid synthesis defects are in unrelated metabolic pathways, they do share many clinical features. In children the central nervous system is primarily affected, giving rise to (congenital) microcephaly, early onset seizures and varying degrees of mental disability. The brain abnormalities are accompanied by skin disorders such as cutis laxa in defects of proline synthesis, collodion-like skin and ichthyosis in serine deficiency, and necrolytic erythema in glutamine deficiency. Hypomyelination with accompanying loss of brain volume and gyration defects can be observed on brain MRI in all synthesis disorders. In adults with defects in serine or proline synthesis, spastic paraplegia and several forms of polyneuropathy with or without intellectual disability appear to be the major symptoms in these late-presenting forms of amino acid disorders. This review provides a comprehensive overview of the disorders in amino acid synthesis.

Genome-wide association study of NMDA receptor coagonists in human cerebrospinal fluid and plasma.

The N-methyl-D-aspartate receptor (NMDAR) coagonists glycine, D-serine and L-proline play crucial roles in NMDAR-dependent neurotransmission and are associated with a range of neuropsychiatric disorders. We conducted the first genome-wide association study of concentrations of these coagonists and their enantiomers in plasma and cerebrospinal fluid (CSF) of human subjects from the general population (N=414). Genetic variants at chromosome 22q11.2, located in and near PRODH (proline dehydrogenase), were associated with L-proline in plasma (ß=0.29; P=6.38 × 10(-10)). The missense variant rs17279437 in the proline transporter SLC6A20 was associated with L-proline in CSF (ß=0.28; P=9.68 × 10(-9)). Suggestive evidence of association was found for the D-serine plasma-CSF ratio at the D-amino-acid oxidase (DAO) gene (ß=-0.28; P=9.08 × 10(-8)), whereas a variant in SRR (that encodes serine racemase and is associated with schizophrenia) constituted the most strongly associated locus for the L-serine to D-serine ratio in CSF. All these genes are highly expressed in rodent meninges and choroid plexus, anatomical regions relevant to CSF physiology. The enzymes and transporters they encode may be targeted to further construe the nature of NMDAR coagonist involvement in NMDAR gating. Furthermore, the highlighted genetic variants may be followed up in clinical populations, for example, schizophrenia and 22q11 deletion syndrome. Overall, this targeted metabolomics approach furthers the understanding of NMDAR coagonist concentration variability and sets the stage for non-targeted CSF metabolomics projects.

Neurometabolic disorders are treatable causes of dystonia.

A broad range of rare inherited metabolic disorders can present with dystonia. For clinicians, it is important to recognize dystonic features, but it can be complicated by the mixed and complex clinical picture seen in many neurometabolic patients. Careful phenotyping is the first step towards the diagnosis of the underlying condition and subsequent targeted treatment, further supported by imaging, biochemical diagnostics and the availability of modern diagnostic techniques such as next generation sequencing. As several neurometabolic disorders are treatable causes of dystonia, these should have priority in the diagnostic process. In the symptomatic treatment of dystonia, several therapeutic options are available. Awareness for the occurrence and optimal treatment of dystonia and other movement disorders in neurometabolic conditions is important because these symptoms can have a substantial impact on the quality of life and daily functioning; this effect is not only exerted by the dystonia itself, but also by the frequently associated non-motor features. In this paper, the highlights and key concepts of neurometabolic forms of dystonia are discussed, with a focus on phenomenology, the diagnostic approach, the most important neurometabolic aetiologies, co-occurring non-motor features and therapeutic options.

Somatic mosaicism in a mother of two children with Pitt-Hopkins syndrome.

Pitt-Hopkins syndrome (PTHS) is a neurodevelopmental disorder characterized by intellectual disability, unusual face and breathing abnormalities and can be caused by haploinsufficiency of TCF4. The majority of cases are sporadic. Somatic mosaicism was reported infrequently. We report on a proband with typical manifestations of PTHS and his younger brother with a less striking phenotype. In both, a heterozygous frameshift mutation (c.1901_1909delinsA, p.Ala634AspfsX67) was found in exon 19 of TCF4. The same mutation was found at low levels in DNA extracted from the mother's blood, urine and saliva. This report of familial recurrence with somatic mosaicism in a healthy mother has important consequences for genetic counseling. We suggest careful studies in parents of other patients with PTHS to determine the frequency of germline and somatic mosaicism for TCF4 mutations.

An update on serine deficiency disorders.

Serine deficiency disorders are caused by a defect in one of the three synthesising enzymes of the L-serine biosynthesis pathway. Serine deficiency disorders give rise to a neurological phenotype with psychomotor retardation, microcephaly and seizures in newborns and children or progressive polyneuropathy in adult patients. There are three defects that cause serine deficiency of which 3-phosphoglycerate dehydrogenase (3-PGDH) deficiency, the defect affecting the first step in the pathway, has been reported most frequently. The other two disorders in L-serine biosynthesis phosphoserine aminotransferase (PSAT) deficiency and phosphoserine phosphatase (PSP) deficiency have been reported only in a limited number of patients. The biochemical hallmarks of all three disorders are low concentrations of serine in cerebrospinal fluid and plasma. Prompt recognition of affected patients is important, since serine deficiency disorders are treatable causes of neurometabolic disorders. The use of age-related reference values for serine in CSF and plasma can be of great help in establishing a correct diagnosis of serine deficiency, in particular in newborns and young children.

Mutations in MVK, encoding mevalonate kinase, cause hyperimmunoglobulinaemia D and periodic fever syndrome.

Hyperimmunoglobulinaemia D and periodic fever syndrome (HIDS; MIM 260920) is an autosomal recessive disorder characterized by recurrent episodes of fever associated with lymphadenopathy, arthralgia, gastrointestinal dismay and skin rash. Diagnostic hallmark of HIDS is a constitutively elevated level of serum immunoglobulin D (IgD), although patients have been reported with normal IgD levels. To determine the underlying defect in HIDS, we analysed urine of several patients and discovered increased concentrations of mevalonic acid during severe episodes of fever, but not between crises. Subsequent analysis of cells from four unrelated HIDS patients revealed reduced activities of mevalonate kinase (MK; encoded by the gene MVK), a key enzyme of isoprenoid biosynthesis. Sequence analysis of MVK cDNA from the patients identified three different mutations, one of which was common to all patients. Expression of the mutant cDNAs in Escherichia coli showed that all three mutations affect the activity of the encoded proteins. Moreover, immunoblot analysis demonstrated a deficiency of MK protein in patient fibroblasts, indicating a protein-destabilizing effect of the mutations.

Expanding the clinical spectrum of 3-phosphoglycerate dehydrogenase deficiency.

UNLABELLED: 3-Phosphoglycerate dehydrogenase (3-PGDH) deficiency is considered to be a rare cause of congenital microcephaly, infantile onset of intractable seizures and severe psychomotor retardation. Here, we report for the first time a very mild form of genetically confirmed 3-PGDH deficiency in two siblings with juvenile onset of absence seizures and mild developmental delay. Amino acid analysis showed serine values in CSF and plasma identical to what is observed in the severe infantile form. Both patients responded favourably to relatively low dosages of serine supplementation with cessation of seizures, normalisation of their EEG abnormalities and improvement of well-being and behaviour. These cases illustrate that 3-PGDH deficiency can present with mild symptoms and should be considered as a treatable disorder in the differential diagnosis of mild developmental delay and seizures. SYNOPSIS: we present a novel mild phenotype in patients with 3-PGDH deficiency.

L-serine synthesis in the central nervous system: a review on serine deficiency disorders.

The de novo synthesis of the amino acid L-serine plays an essential role in the development and functioning of the central nervous system (CNS). L-serine displays many metabolic functions during different developmental stages; among its functions providing precursors for amino acids, protein synthesis, nucleotide synthesis, neurotransmitter synthesis and L-serine derived lipids. Patients with congenital defects in the L-serine synthesizing enzymes present with severe neurological abnormalities and underscore the importance of this synthetic pathway. In this review, we will discuss the cellular functions of the L-serine pathway, structure and enzymatic properties of the enzymes involved and genetic defects associated with this pathway.

Hydrolysed formula is a risk factor for vitamin K deficiency in infants with unrecognised cholestasis.

OBJECTIVES: Vitamin K deficiency (VKD) may cause life-threatening haemorrhages, especially in breast-fed infants with unrecognised cholestasis. Interestingly, hypoallergenic formulas appear overrepresented in reported cases of VKD bleeding (VKDB) in formula-fed infants. We therefore assessed whether the risk of VKD in formula-fed infants with cholestasis is associated with hypoallergenic formulas. PATIENTS AND METHODS: Infants born in the Netherlands between January 1991 and December 2006 with cholestatic jaundice due to biliary atresia (BA) or to α-1-antitrypsin deficiency (A1ATD) were identified in the Netherlands Study Group for Biliary Atresia Registry and the A1ATD registry, respectively. The relative risk (RR) of VKDB in patients with BA or A1ATD was calculated for different formula types. The influence of prior or ongoing breast-feeding on the RR of VKDB was also assessed. RESULTS: A total of 179 infants with either BA (139) or A1ATD (40) were included. One hundred eighteen infants were formula fed; 8 presented with VKD. Six of these 8 infants (75%) received hypoallergenic formula (whey-based hydrolysate in 4). One infant on whey-based hydrolysed formula presented with VKDB. Risk factor analysis revealed that infants receiving hydrolysed, especially whey-based, formula, had a strongly increased risk of VKD (RR 25.0 [6.4-97.2], P < 0.001)) compared with infants receiving regular formula. Prior or ongoing breast-feeding was not significantly associated with VKD. CONCLUSIONS: Infants with cholestasis receiving (whey-based) hydrolysed formula are at increased risk of developing VKD, compared with infants receiving regular formula. Because VKD may lead to serious haemorrhages, infants receiving whey-based hydrolysed formulas may need additional vitamin K supplementation.

Novel mutations in 3-phosphoglycerate dehydrogenase (PHGDH) are distributed throughout the protein and result in altered enzyme kinetics.

Three-phosphoglycerate dehydrogenase (3-PGDH) deficiency is a rare recessive inborn error in the biosynthesis of the amino acid L-serine characterized clinically by congenital microcephaly, psychomotor retardation, and intractable seizures. The biochemical abnormalities associated with this disorder are low concentrations of L-serine, D-serine, and glycine in cerebrospinal fluid (CSF). Only two missense mutations (p.V425M and p.V490M) have been identified in PHGDH, the gene encoding 3-PGDH, but it is currently unclear how these mutations in the carboxy-terminal regulatory domain of the protein affect enzyme function. We now describe five novel mutations in five patients with 3-PGDH deficiency; one frameshift mutation (p.G238fsX), and four missense mutations (p.R135W, p.V261M, p.A373T, and p.G377S). The missense mutations were located in the nucleotide binding and regulatory domains of 3-PGDH and did not affect steady-state expression, protein stability, and protein degradation rates. Patients' fibroblasts displayed a significant, but incomplete, reduction in maximal enzyme activities associated with all missense mutations. In transient overexpression studies in HEK293T cells, the p.A373T, p.V425M, and p.V490M mutations resulted in almost undetectable enzyme activities. Molecular modeling of the p.R135W and p.V261M mutations onto the partial crystal structure of 3-PGDH predicted that these mutations affect substrate and cofactor binding. This prediction was confirmed by the results of kinetic measurements in fibroblasts and transiently transfected HEK293T cells, which revealed a markedly decreased V(max) and an increase in K(m) values, respectively. Taken together, these data suggest that missense mutations associated with 3-PGDH deficiency either primarily affect substrate binding or result in very low residual enzymatic activity.

Musculoskeletal manifestations of lysosomal storage disorders.

Lysosomal storage disorders (LSDs), a heterogeneous group of inborn metabolic disorders, are far more common than most doctors presume. Although patients with a severe LSD subtype are often readily diagnosed, the more attenuated subtypes are frequently missed or diagnosis is significantly delayed. The presenting manifestations often involve the bones and/or joints and therefore these patients are frequently under specialist care by (paediatric) rheumatologists, receiving inadequate treatment. Since effective disease-specific treatments, including enzyme replacement therapy and stem cell transplantation, have become available for certain LSDs and timely initiation of these treatments is necessary to prevent the development of severe, disabling and irreversible manifestations, early diagnosis has become essential. The challenge is to raise awareness for better recognition of the presenting signs and symptoms of LSDs by all doctors who may encounter these patients, including rheumatologists.

A clinical diagnostic algorithm for early onset cerebellar ataxia.

Early onset cerebellar Ataxia (EOAc) comprises a large group of rare heterogeneous disorders. Determination of the underlying etiology can be difficult given the broad differential diagnosis and the complexity of the genotype-phenotype relationships. This may change the diagnostic work-up into a time-consuming, costly and not always rewarding task. In this overview, the Childhood Ataxia and Cerebellar Group of the European Pediatric Neurology Society (CACG-EPNS) presents a diagnostic algorithm for EOAc patients. In seven consecutive steps, the algorithm leads the clinician through the diagnostic process, including EOA identification, application of the Inventory of Non-Ataxic Signs (INAS), consideration of the family history, neuro-imaging, laboratory investigations, genetic testing by array CGH and Next Generation Sequencing (NGS). In children with EOAc, this algorithm is intended to contribute to the diagnostic process and to allow uniform data entry in EOAc databases.

A survey of natural protein intake in Dutch phenylketonuria patients: insight into estimation or measurement of dietary intake.

This study investigated which methods patients and parents used to determine phenylalanine (Phe) intake and the relationship between the methods applied, age, and blood Phe concentration, as this practice had not been studied before in relation to metabolic control. A questionnaire was sent to 327 Dutch phenylketonuria patients (age 0-29 years) to investigate the method used to determine Phe intake (either by estimation, exact measurement, or a combination of both). Mean blood Phe concentration of each individual patient was related to the method reported to be used. Three different age groups (<10 years, > or =10-15 years, and > or =16 years) were distinguished. The response rate for the questionnaires was 73%. In these 188 patients, data for both Phe concentrations and questionnaires could be used. Of these, 75 used exact measurement, 75 used estimation, and 38 used both methods. The number of patients that estimated Phe intake clearly increased with age. Whatever method was used, an increase in Phe concentrations was seen with age. During childhood, exact measurement was used more frequently, and from adolescence on estimation was used more frequently. The method (exact measurement and/or estimation) did not result in statistically different Phe concentrations in any of the three age groups, although blood Phe concentration tended to be lower in adolescence using exact measurement. Data suggest that estimation and exact measurement of Phe intake are both reliable methods. Therefore, in addition to exact measurement, patients should be instructed in both methods at an early age, so that both methods can be used adequately.

[Umbilical cord blood from an unrelated donor as source for stem cell transplantations in inborn errors of metabolism]. / Navelstrengbloed van een onverwante donor als bron voor stamceltransplantaties bij aangeboren stofwisselingsziekten.

In certain inborn errors of metabolism, an allogeneic stem cell transplantation is able to prevent disease progression. This is only possible when the stem cell transplantation (SCT) is performed early in life, before cerebral involvement has occurred. In addition to bone marrow and peripheral blood, unrelated umbilical cord blood appears to be an effective stem cell source as well. Important advantages of umbilical blood as stem cell source are: the time between diagnosis and SCT can be considerably reduced; there is a greater chance of finding a suitable donor and the risk of graft-versus-host disease and viral transmission is decreased. By far the most common disease in the group of inborn metabolic errors for which SCTs are performed is Hurler's disease. In these patients, the percentage of successful transplantations is considerably higher after the use ofunrelated cord blood than when bone marrow or peripheral blood is used as a stem cell source. In addition, donor chimerism occurred significantly more often in those patients who had received unrelated umbilical cord blood. There are also potential disadvantages attached to the use of umbilical blood as stem cell source: the possibility of only one donation per donor and less adaptive immunity following umbilical blood SCT with an increased risk of reactivation of a previous viral infection. However, these disadvantages are less applicable to young children with inborn errors of metabolism. The improvement of transplantation techniques and the availability of this new stem cell source could improve the success rate of this procedure and consequently the prognosis of these severely affected patients.

Renal Fanconi syndrome with ultrastructural defects in lysinuric protein intolerance.

Renal Fanconi syndrome developed rapidly in a 3-year-old Moroccan girl with established lysinuric protein intolerance. She was hospitalized because of lowered consciousness, uncoordinated movements and hepatosplenomegaly after a febrile period. Laboratory investigations revealed plasma ammonia 270 micromol/L (normal <70 micromol/L), ferritin 159 micromol/L (normal 2-59 micromol/L), LDH 1180 U/L (normal 26-534 U/L). LPI was diagnosed based on the findings of reduced plasma ornithine, arginine and lysine, and an increased level of glutamine. Urinary orotic acid (645 micromol/mmol creatinine; normal <3.6) was strongly increased. A defect in the SLC7A7 amino acid transporter was established (homozygous c.726G > A mutation). Detailed renal function tests including an acid challenge test, bicarbonate loading, and tubular maximal reabsorption of glucose showed complex tubular dysfunction. No evidence of respiratory chain defects was found in muscle or kidney tissue. No morphological abnormalities were demonstrated in the mitochondria. Ultrastructural analysis of proximal tubular cells showed vacuolization and sloughing of the apical brush border (Fig. 1). Renal involvement in LPI has only been described in a few reports; however, no detailed studies of the renal acidification mechanism were performed. Our patient had evidence of a full-blown Fanconi syndrome. Surprisingly, a metabolic acidosis was found with a moderately increased serum anion gap combined with repeatedly normal plasma organic acid values. This finding is in contrast with the diagnosis of renal tubular acidosis. Patients with hyperlysinaemia have a similar heavy load on the renal tubules; they never develop a renal Fanconi syndrome. Therefore, we consider the intratubular accumulation of lysine an unlikely candidate for the development of the renal Fanconi syndrome.

Reviewing the role of the genes G72 and DAAO in glutamate neurotransmission in schizophrenia.

We review the role of two susceptibility genes; G72 and DAAO in glutamate neurotransmission and the aetiology of schizophrenia. The gene product of G72 is an activator of DAAO (D-amino acid oxidase), which is the only enzyme oxidising D-serine. D-serine is an important co-agonist for the NMDA glutamate receptor and plays a role in neuronal migration and cell death. Studies of D-serine revealed lower serum levels in schizophrenia patients as compared to healthy controls. Furthermore, administration of D-serine as add-on medication reduced the symptoms of schizophrenia. The underlying mechanism of the involvement of G72 and DAAO in schizophrenia is probably based on decreased levels of D-serine and decreased NMDA receptor functioning in patients. The involvement of this gene is therefore indirect support for the glutamate dysfunction hypothesis in schizophrenia.

Cranial ultrasound in metabolic disorders presenting in the neonatal period: characteristic features and comparison with MR imaging.

BACKGROUND AND PURPOSE: Brain imaging is an integral part of the diagnostic work-up for metabolic disorders, and the bedside availability of cranial ultrasonography (cUS) allows very early brain imaging in symptomatic neonates. Our aim was to investigate the role and range of abnormalities seen on cUS in neonates presenting with metabolic disorders. A secondary aim, when possible, was to address the question of whether brain MR imaging is more informative by comparing cUS to MR imaging findings. MATERIALS AND METHODS: Neonates with a metabolic disorder who had at least 1 cUS scan were eligible. cUS images were reviewed for anatomic and maturation features, cysts, calcium, and other abnormalities. When an MR imaging scan had been obtained, both sets of images were compared. RESULTS: Fifty-five infants (35 also had MR imaging) were studied. The most frequent findings were in oxidative phosphorylation disorders (21 cUS and 12 MR imaging): ventricular dilation (11 cUS and 6 MR imaging), germinolytic cysts (GLCs; 7 cUS and 5 MR imaging), and abnormal white matter (7 cUS and 6 MR imaging); in peroxisomal biogenesis disorders (13 cUS and 9 MR imaging): GLCs (10 cUS and 6 MR imaging), ventricular dilation (10 cUS and 5 MR imaging), abnormal cortical folding (8 cUS and 7 MR imaging), and lenticulostriate vasculopathy (8 cUS); in amino acid metabolism and urea cycle disorders (14 cUS and 11 MR imaging): abnormal cortical folding (9 cUS and 4 MR imaging), abnormal white matter (8 cUS and 8 MR imaging), and hypoplasia of the corpus callosum (7 cUS and 6 MR imaging); in organic acid disorders (4 cUS and 2 MR imaging): periventricular white matter echogenicity (2 cUS and 1 MR imaging); and in other disorders (3 cUS and 1 MR imaging): ventricular dilation (2 cUS and 1 MR imaging). cUS findings were consistent with MR imaging findings. cUS was better for visualizing GLCs and calcification. MR imaging was more sensitive for subtle tissue signal intensity changes in the white matter and abnormality in areas difficult to visualize with cUS, though abnormalities of cortical folding suggestive of polymicrogyria were seen on cUS. CONCLUSION: A wide range of abnormalities is seen using cUS in neonatal metabolic disorders. cUS is a reliable bedside tool for early detection of cysts, calcium, structural brain abnormalities, and white matter echogenicity, all suggestive of metabolic disorders.

[From gene to disease; Menkes disease: copper deficiency due to an ATP7A-gene defect]. / Van gen naar ziekte; de ziekte van menkes: koperdeficiëntie door een ATP7A-gendefect.

Menkes disease is an X-linked recessive disorder characterized by neurological deterioration, failure to thrive, peculiar hair and death in childhood, secondary to mutations in the ATP7A gene. The ATP7A gene encodes for a copper transporting P-type ATPase (ATP7A), which is ubiquitously expressed. A defect of the ATP7A protein leads to both a reduced transport of copper from the intestine into the circulation and into the central nervous system, as well as reduced transport of copper into the Golgi apparatus for incorporation into various copper-dependent enzymes. This results in a systemic copper deficiency as well as reduced activity of various copper-dependent enzymes. The reduced activity of these copper-dependent enzymes accounts for most of the characteristic features ofMenkes disease patients.

Non-motor symptoms and quality of life in dopa-responsive dystonia patients.

BACKGROUND: In patients with GTP-cyclohydrolase deficient dopa-responsive dystonia (DRD) the occurrence of associated non-motor symptoms (NMS) is to be expected. Earlier studies report conflicting results with regard to the nature and severity of NMS. The aim of our study was to investigate the prevalence of psychiatric disorders, sleep problems, fatigue and health-related quality of life (HR-QoL) in a Dutch DRD cohort. METHODS: Clinical characteristics, motor symptoms, type and severity of psychiatric co-morbidity, sleep problems, fatigue and HR-QoL were assessed in DRD patients with a confirmed GCH1 mutation and matched controls. RESULTS: Twenty-eight patients were included (18 adults and 10 children), from 10 families. Dystonia symptoms were well-controlled in all patients. According to the DSM IV patients significantly more often met the criteria for a lifetime psychiatric disorder than controls (61% vs. 29%, p < 0.05). In particular the frequencies of generalized anxiety and agoraphobia were higher in patients (both 29% vs. 4%, p < 0.05). Patients scored significantly higher on daytime sleepiness than controls (ESS, 11.2 vs 5.7, p < 0.05). Adult patients had significantly lower scores on the mental component of the HR-QoL (47 vs. 54, p < 0.05) than controls mainly associated with (worse) quality of sleep. CONCLUSION: NMS were highly prevalent in our cohort of DRD patients, despite adequate treatment of motor symptoms. Our findings support the accumulating evidence of an important non-motor phenotype in DRD, with possible involvement of serotonergic mechanisms. This highlights the need to address NMS and the underlying neurobiology in patients with DRD.

Treatment with amino acids in serine deficiency disorders.

Serine deficiency disorders are rare defects in the biosynthesis of the amino acid L-serine. At present two disorders have been reported: 3-phosphoglycerate dehydrogenase deficiency and 3-phosphoserine phosphatase deficiency. These enzyme defects lead to severe neurological symptoms such as congenital microcephaly and severe psychomotor retardation and in addition in patients with 3-phosphoglycerate dehydrogenase deficiency to intractable seizures. These symptoms respond to a variable degree to treatment with L-serine, sometimes combined with glycine. In this paper the current practice of amino acid treatment with L-serine and glycine in serine deficiency is reviewed.