Metabolic Evaluation of Epilepsy: A Diagnostic Algorithm With Focus on Treatable Conditions

Although inborn errors of metabolism do not represent the most common cause of seizures, their early identification is of utmost importance, since many will require therapeutic measures beyond that of common anti-epileptic drugs, either in order to control seizures, or to decrease the risk of neurodegeneration. We translate the currently-known literature on metabolic etiologies of epilepsy (268 inborn errors of metabolism belonging to 21 categories, with 74 treatable errors), into a 2-tiered diagnostic algorithm, with the first-tier comprising accessible, affordable, and less invasive screening tests in urine and blood, with the potential to identify the majority of treatable conditions, while the second-tier tests are ordered based on individual clinical signs and symptoms. This resource aims to support the pediatrician, neurologist, biochemical, and clinical geneticists in early identification of treatable inborn errors of metabolism in a child with seizures, allowing for timely initiation of targeted therapy with the potential to improve outcomes

Vitamin B6 and Related Inborn Errors of Metabolism

Vitamin B6 (vitB6) is a generic term that comprises six interconvertible pyridine compounds. These vitB6 compounds (also called vitamers) are pyridoxine (PN), pyridoxamine (PM), pyridoxal (PL) and their 5′-phosphorylated forms pyridoxine 5′-phosphate (PNP), pyridoxamine 5′-phosphate (PMP) and pyridoxal 5′-phosphate (PLP). VitB6 is an essential nutrient for all living organisms, but only microorganisms and plants can carry out de novo synthesis of this vitamin. Other organisms obtain vitB6 from dietary sources and interconvert its different forms according to their needs via a biochemical pathway known as the salvage pathway. PLP is the biologically active form of vitB6 which is important for maintaining the biochemical homeostasis of the body. In the human body, PLP serves as a cofactor for more than 140 enzymatic reactions, mainly associated with synthesis, degradation and interconversion of amino acids and neurotransmitter metabolism. PLP-dependent enzymes are also involved in various physiological processes, including biologically active amine biosynthesis, lipid metabolism, heme synthesis, nucleic acid synthesis, protein and polyamine synthesis and several other metabolic pathways. PLP is an important vitamer for normal brain function since it is required as a coenzyme for the synthesis of several neurotransmitters including D-serine, D-aspartate, L-glutamate, glycine, γ-aminobutyric acid (GABA), serotonin, epinephrine, norepinephrine, histamine and dopamine. Intracellular levels of PLP are tightly regulated and conditions that disrupt this homeostatic regulation can cause disease. In humans, genetic and dietary (intake of high doses of vitB6) conditions leading to increase in PLP levels is known to cause motor and sensory neuropathies. Deficiency of PLP in the cell is also implicated in several diseases, the most notable example of which are the vitB6-dependent epileptic encephalopathies. VitB6-dependent epileptic encephalopathies (B6EEs) are a clinically and genetically heterogeneous group of rare inherited metabolic disorders. These debilitating conditions are characterized by recurrent seizures in the prenatal, neonatal, or postnatal period, which are typically resistant to conventional anticonvulsant treatment but are well-controlled by the administration of PN or PLP. In addition to seizures, children affected with B6EEs may also suffer from developmental and/or intellectual disabilities, along with structural brain abnormalities. Five main types of B6EEs are known to date, these are: PN-dependent epilepsy due to ALDH7A1 (antiquitin) deficiency (PDE-ALDH7A1) (MIM: 266100), hyperprolinemia type 2 (MIM: 239500), PLP-dependent epilepsy due to PNPO deficiency (MIM: 610090), hypophosphatasia (MIM: 241500) and PLPBP deficiency (MIM: 617290). This chapter provides a review of vitB6 and its different vitamers, their absorption and metabolic pathways in the human body, the diverse physiological roles of vitB6, PLP homeostasis and its importance for human health. Finally, the chapter reviews the inherited neurological disorders affecting PLP homeostasis with a special focus on vitB6-dependent epileptic encephalopathies (B6EEs), their different subtypes, the pathophysiological mechanism underlying each type, clinical and biochemical features and current treatment strategies

Untreated PKU patients without intellectual disability: what do they teach us?

Phenylketonuria (PKU) management is aimed at preventing neurocognitive and psychosocial dysfunction by keeping plasma phenylalanine concentrations within the recommended target range. It can be questioned, however, whether universal plasma phenylalanine target levels would result in optimal neurocognitive outcomes for all patients, as similar plasma phenylalanine concentrations do not seem to have the same consequences to the brain for each PKU individual. To better understand the inter-individual differences in brain vulnerability to high plasma phenylalanine concentrations, we aimed to identify untreated and/or late-diagnosed PKU patients with near-normal outcome, despite high plasma phenylalanine concentrations, who are still alive. In total, we identified 16 such cases. While intellectual functioning in these patients was relatively unaffected, they often did present other neurological, psychological, and behavioral problems. Thereby, these "unusual" PKU patients show that the classical symptomatology of untreated or late-treated PKU may have to be rewritten. Moreover, these cases show that a lack of intellectual dysfunction despite high plasma phenylalanine concentrations does not necessarily imply that these high phenylalanine concentrations have not been toxic to the brain. Also, these cases may suggest that different mechanisms are involved in PKU pathophysiology, of which the relative importance seems to differ between patients and possibly also with increasing age. Further research should aim to better distinguish PKU patients with respect to their cerebral effects to high plasma phenylalanine concentrations

Untreated PKU Patients without Intellectual Disability: What Do They Teach Us?

Phenylketonuria (PKU) management is aimed at preventing neurocognitive and psychosocial dysfunction by keeping plasma phenylalanine concentrations within the recommended target range. It can be questioned, however, whether universal plasma phenylalanine target levels would result in optimal neurocognitive outcomes for all patients, as similar plasma phenylalanine concentrations do not seem to have the same consequences to the brain for each PKU individual. To better understand the inter-individual differences in brain vulnerability to high plasma phenylalanine concentrations, we aimed to identify untreated and/or late-diagnosed PKU patients with near-normal outcome, despite high plasma phenylalanine concentrations, who are still alive. In total, we identified 16 such cases. While intellectual functioning in these patients was relatively unaffected, they often did present other neurological, psychological, and behavioral problems. Thereby, these "unusual" PKU patients show that the classical symptomatology of untreated or late-treated PKU may have to be rewritten. Moreover, these cases show that a lack of intellectual dysfunction despite high plasma phenylalanine concentrations does not necessarily imply that these high phenylalanine concentrations have not been toxic to the brain. Also, these cases may suggest that different mechanisms are involved in PKU pathophysiology, of which the relative importance seems to differ between patients and possibly also with increasing age. Further research should aim to better distinguish PKU patients with respect to their cerebral effects to high plasma phenylalanine concentrations

Timing of therapy and neurodevelopmental outcomes in 18 families with pyridoxine-dependent epilepsy

Background: Seventy-five percent of patients with pyridoxine-dependent epilepsy due to a-aminoadipic semialdehyde dehydrogenase deficiency (PDE-ALDH7A1) suffer intellectual developmental disability despite pyridoxine treatment. Adjunct lysine reduction therapies (LRT), aimed at lowering putative neurotoxic metabolites, are associated with improved cognitive outcomes. However, possibly due to timing of treatment, not all patients have normal intellectual function. Methods: This retrospective, multi-center cohort study evaluated the effect of timing of pyridoxine monotherapy and pyridoxine with adjunct LRT on neurodevelopmental outcome. Patients with confirmed PDE-ALDH7A1 with at least one sibling with PDE-ALDH7A1 and a difference in age at treatment initiation were eligible and identified via the international PDE registry, resulting in thirty-seven patients of 18 families. Treatment regimen was pyridoxine monotherapy in ten families and pyridoxine with adjunct LRT in the other eight. Primary endpoints were standardized and clinically assessed neurodevelopmental outcomes. Clinical neurodevelopmental status was subjectively assessed over seven domains: overall neurodevelopment, speech/language, cognition, fine and gross motor skills, activities of daily living and behavioral/psychiatric abnormalities. Results: The majority of early treated siblings on pyridoxine monotherapy performed better than their late treated siblings on the clinically assessed domain of fine motor skills. For siblings on pyridoxine and adjunct LRT, the majority of early treated siblings performed better on clinically assessed overall neurodevelopment, cognition, and behavior/psychiatry. Fourteen percent of the total cohort was assessed as normal on all domains. Conclusion: Early treatment with pyridoxine and adjunct LRT may be beneficial for neurodevelopmental outcome. When evaluating a more extensive neurodevelopmental assessment, the actual impairment rate may be higher than the 75% reported in literature. Take- home message: Early initiation of lysine reduction therapies adjunct to pyridoxine treatment in patients with PDE-ALDH7A1 may result in an improved neurodevelopmental outcome. (C) 2022 Published by Elsevier Inc

Diagnostic Yield and Treatment Impact of Targeted Exome Sequencing in Early-Onset Epilepsy

Targeted whole-exome sequencing (WES) is a powerful diagnostic tool for a broad spectrum of heterogeneous neurological disorders. Here, we aim to examine the impact on diagnosis, treatment and cost with early use of targeted WES in early-onset epilepsy. WES was performed on 180 patients with early-onset epilepsy (≤5 years) of unknown cause. Patients were classified as Retrospective (epilepsy diagnosis >6 months) or Prospective (epilepsy diagnosis <6 months). WES was performed on an Ion Proton™ and variant reporting was restricted to the sequences of 620 known epilepsy genes. Diagnostic yield and time to diagnosis were calculated. An analysis of cost and impact on treatment was also performed. A molecular diagnoses (pathogenic/likely pathogenic variants) was achieved in 59/180 patients (33%). Clinical management changed following WES findings in 23 of 59 diagnosed patients (39%) or 13% of all patients. A possible diagnosis was identified in 21 additional patients (12%) for whom supporting evidence is pending. Time from epilepsy onset to a genetic diagnosis was faster when WES was performed early in the diagnostic process (mean: 145 days Prospective vs. 2,882 days Retrospective). Costs of prior negative tests averaged $8,344 per patient in the Retrospective group, suggesting savings of$5,110 per patient using WES. These results highlight the diagnostic yield, clinical utility and potential cost-effectiveness of using targeted WES early in the diagnostic workup of patients with unexplained early-onset epilepsy. The costs and clinical benefits are likely to continue to improve. Advances in precision medicine and further studies regarding impact on long-term clinical outcome will be important

The clinical application of genome-wide sequencing for monogenic diseases in Canada: Position statement of the Canadian College of medical geneticists

Purpose and scope: The aim of this Position Statement is to provide recommendations for Canadian medical geneticists, clinical laboratory geneticists, genetic counsellors and other physicians regarding the use of genome-wide sequencing of germline DNA in the context of clinical genetic diagnosis. This statement has been developed to facilitate the clinical translation and development of best practices for clinical genome-wide sequencing for genetic diagnosis of monogenic diseases in Canada; it does not address the clinical application of this technology in other fields such as molecular investigation of cancer or for population screening of healthy individuals. Methods of statement development: Two multidisciplinary groups consisting of medical geneticists, clinical laboratory geneticists, genetic counsellors, ethicists, lawyers and genetic researchers were assembled to review existing literature and guidelines on genome-wide sequencing for clinical genetic diagnosis in the context of monogenic diseases, and to make recommendations relevant to the Canadian context. The statement was circulated for comment to the Canadian College of Medical Geneticists (CCMG) membership-at-large and, following incorporation of feedback, approved by the CCMG Board of Directors. The CCMG is a Canadian organisation responsible for certifying medical geneticists and clinical laboratory geneticists, and for establishing professional and ethical standards for clinical genetics services in Canada. Results and conclusions: Recommendations include (1) clinical genome-wide sequencing is an appropriate approach in the diagnostic assessment of a patient for whom there is suspicion of a significant monogenic disease that is associated with a high degree of genetic heterogeneity, or where specific genetic tests have failed to provide a diagnosis; (2) until the benefits of reporting incidental findings are established, we do not endorse the intentional clinical analysis of disease-associated genes other than those linked to the primary indication; and (3) clinicians should provide genetic counselling and obtain informed consent prior to undertaking clinical genome-wide sequencing. Counselling should include discussion of the limitations of testing, likelihood and implications of diagnosis and incidental findings, and the potential need for further analysis to facilitate clinical interpretation, including studies performed in a research setting. These recommendations will be routinely reevaluated as knowledge of diagnostic and clinical utility of clinical genome-wide sequencing improves. While the document was developed to direct practice in Canada, the applicability of the statement is broader and will be of interest to clinicians and health jurisdictions internationally

Health Care for Mitochondrial Disorders in Canada: A Survey of Physicians

Background: An improved understanding of diagnostic and treatment practices for patients with rare primary mitochondrial disorders can support benchmarking against guidelines and establish priorities for evaluative research. We aimed to describe physician care for patients with mitochondrial diseases in Canada, including variation in care. Methods: We conducted a cross-sectional survey of Canadian physicians involved in the diagnosis and/or ongoing care of patients with mitochondrial diseases. We used snowball sampling to identify potentially eligible participants, who were contacted by mail up to five times and invited to complete a questionnaire by mail or internet. The questionnaire addressed: personal experience in providing care for mitochondrial disorders; diagnostic and treatment practices; challenges in accessing tests or treatments; and views regarding research priorities. Results: We received 58 survey responses (52% response rate). Most respondents (83%) reported spending 20% or less of their clinical practice time caring for patients with mitochondrial disorders. We identified important variation in diagnostic care, although assessments frequently reported as diagnostically helpful (e.g., brain magnetic resonance imaging, MRI/MR spectroscopy) were also recommended in published guidelines. Approximately half (49%) of participants would recommend mitochondrial cocktails for all or most patients, but we identified variation in responses regarding specific vitamins and cofactors. A majority of physicians recommended studies on the development of effective therapies as the top research priority. Conclusions: While Canadian physicians\u27 views about diagnostic care and disease management are aligned with published recommendations, important variations in care reflect persistent areas of uncertainty and a need for empirical evidence to support and update standard protocols

The genotypic and phenotypic spectrum of MTO1 deficiency.

BACKGROUND: Mitochondrial diseases, a group of multi-systemic disorders often characterized by tissue-specific phenotypes, are usually progressive and fatal disorders resulting from defects in oxidative phosphorylation. MTO1 (Mitochondrial tRNA Translation Optimization 1), an evolutionarily conserved protein expressed in high-energy demand tissues has been linked to human early-onset combined oxidative phosphorylation deficiency associated with hypertrophic cardiomyopathy, often referred to as combined oxidative phosphorylation deficiency-10 (COXPD10). MATERIAL AND METHODS: Thirty five cases of MTO1 deficiency were identified and reviewed through international collaboration. The cases of two female siblings, who presented at 1 and 2years of life with seizures, global developmental delay, hypotonia, elevated lactate and complex I and IV deficiency on muscle biopsy but without cardiomyopathy, are presented in detail. RESULTS: For the description of phenotypic features, the denominator varies as the literature was insufficient to allow for complete ascertainment of all data for the 35 cases. An extensive review of all known MTO1 deficiency cases revealed the most common features at presentation to be lactic acidosis (LA) (21/34; 62% cases) and hypertrophic cardiomyopathy (15/34; 44% cases). Eventually lactic acidosis and hypertrophic cardiomyopathy are described in 35/35 (100%) and 27/34 (79%) of patients with MTO1 deficiency, respectively; with global developmental delay/intellectual disability present in 28/29 (97%), feeding difficulties in 17/35 (49%), failure to thrive in 12/35 (34%), seizures in 12/35 (34%), optic atrophy in 11/21 (52%) and ataxia in 7/34 (21%). There are 19 different pathogenic MTO1 variants identified in these 35 cases: one splice-site, 3 frameshift and 15 missense variants. None have bi-allelic variants that completely inactivate MTO1; however, patients where one variant is truncating (i.e. frameshift) while the second one is a missense appear to have a more severe, even fatal, phenotype. These data suggest that complete loss of MTO1 is not viable. A ketogenic diet may have exerted a favourable effect on seizures in 2/5 patients. CONCLUSION: MTO1 deficiency is lethal in some but not all cases, and a genotype-phenotype relation is suggested. Aside from lactic acidosis and cardiomyopathy, developmental delay and other phenotypic features affecting multiple organ systems are often present in these patients, suggesting a broader spectrum than hitherto reported. The diagnosis should be suspected on clinical features and the presence of markers of mitochondrial dysfunction in body fluids, especially low residual complex I, III and IV activity in muscle. Molecular confirmation is required and targeted genomic testing may be the most efficient approach. Although subjective clinical improvement was observed in a small number of patients on therapies such as ketogenic diet and dichloroacetate, no evidence-based effective therapy exists