Impairment of one-carbon metabolism during pregnancy, either due to nutritional deficiencies in B9 or B12 vitamins or caused by specific genetic defects, is often associated with neurological defects, including cognitive dysfunction that persists even after vitamin supplementation. Animal nutritional models do not allow for conclusions regarding the specific brain mechanisms that may be modulated by systemic compensations. Using the Cre-lox system associated to the neuronal promoter Thy1.2, a knock-out model for the methionine synthase specifically in the brain was generated. Our results on the neurobehavioral development of offspring show that the absence of methionine synthase did not lead to growth retardation, despite an effective reduction of both its expression and the methylation status in brain tissues. Behaviors were differently affected according to their functional outcome. Only temporary retardations were recorded in the acquisition of vegetative functions during the suckling period, compared to a dramatic reduction in cognitive performance after weaning. Investigation of the glutamatergic synapses in cognitive areas showed a reduction of AMPA receptors phosphorylation and clustering, indicating an epigenomic effect of the neuronal deficiency of methionine synthase on the reduction of glutamatergic synapses excitability. Altogether, our data indicate that cognitive impairment associated with methionine synthase deficiency may not only result from neurodevelopmental abnormalities, but may also be the consequence of alterations in functional plasticity of the brain.

Methionine synthase deficiency (cblG complementation group) is a rare inborn error of metabolism affecting the homocysteine re-methylation pathway. It leads to a biochemical phenotype of hyperhomocysteinemia and hypomethioninemia. The clinical presentation of cblG is variable, ranging from seizures, encephalopathy, macrocytic anemia, hypotonia, and feeding difficulties in the neonatal period to onset of psychiatric symptoms or acute neurologic changes in adolescence or adulthood. Given the variable and nonspecific symptoms seen in cblG, the diagnosis of affected patients is often delayed. Medical management of cblG includes the use of hydroxocobalamin, betaine, folinic acid, and in some cases methionine supplementation. Treatment has been shown to lead to improvement in the biochemical profile of affected patients, with lowering of total homocysteine levels and increasing methionine levels. However, the published literature contains differing conclusions on whether treatment is effective in changing the natural history of the disease. Herein, we present five patients with cblG who have shown substantial clinical benefit from treatment with objective improvement in their neurologic outcomes. We demonstrate more favorable outcomes in our patients who were treated early in life, especially those who were treated before neurologic symptoms manifested. Given improved outcomes from treatment of presymptomatic patients, cblG warrants inclusion in newborn screening.

In order to propose a course of action to be taken in the face of any hyperhomocysteinemia, we have reported for the first time in a French journal the recommendations made within the framework of the European E-HOD project for the diagnosis and treatment of remethylation disorders. The remethylation route ensures homocysteine-methionine conversion. It is linked to the folate cycle and the intracellular metabolism of cobalamins. Remethylation disorders can be classified into three groups: 1) isolated disorders (cblD-HC, cblE, cblG) corresponding to an isolated deficit in the production of methylcobalamin, cofactor of methionine synthase; 2) combined disorders (cblC, cblD-MMA/HC, cblF, cblJ) corresponding to an alteration of the transport and intracellular metabolism of cobalamins, which causes a defect in the synthesis of the two functional forms of cobalamin: methylcobalamin and adenosylcobalamin, a cofactor for methyl malonylCoA mutase; 3) MTHFR deficit, an abnormality of the folate cycle. The biological anomalies observed are hyperhomocysteinemia and hypomethioninaemia associated in the case of disorders combined with increased urinary excretion of methylmalonic acid. The clinical presentation is however heterogeneous according to the remethylation disorder but also for the same pathology according to the age. Given the large number of pathologies grouped together in remethylation disorders, this point is illustrated by only two clinical cases concerning the same deficit (deficit in MTHFR) but with different discovery circumstances: a neonatal form and a late form.

Kasapkara ÇS, Yılmaz-Keskin E, Özbay-Hoşnut F, Akçaboy M, Polat E, Olgaç A, Zorlu P. An infant with an extremely rare cobalamin disorder: Methionine synthase deficiency and importance of early diagnosis and treatment. Turk J Pediatr 2019; 61: 282-285. Functional methionine synthase deficiency can be separated into two classes, cobalamin (Cbl) deficiency type E (CblE) and type G (CblG), which are the result of mutations that affect methionine synthase reductase or methionine synthase, respectively. Deficiency of methionine synthase activity may result in megaloblastic anemia without methylmalonic aciduria and neuromuscular abnormality of varying severity. Delayed milestones, ataxia, cerebral atrophy, muscular hypotonia, neonatal seizures, and blindness have been reported as the associated clinical findings. Early diagnosis and treatment are crucial for a more favorable diagnosis of the affected cases. Herein we report a three-month-old boy with CblG disease who presented with failure to thrive, chronic diarrhea, feeding intolerance, oral ulcers, microcephaly and hypotonia, and showed a dramatic response to treatment. In the first few months of life, megaloblastic anemia accompanied by apparent neurological involvement should direct physicians to order examinations like measurement of total homocysteine and methylmalonic acid levels to detect possible forms of inherited Cbl intracellular metabolism disorders.

The molecular consequences of inborn errors of vitamin B12 or cobalamin metabolism are far from being understood. Moreover, innovative therapeutic strategies are needed for the treatment of neurological outcomes that are usually resistant to conventional treatments. Our previous findings suggest a link between SIRT1, cellular stress and RNA binding proteins (RBP) mislocalization in the pathological mechanisms triggered by impaired vitamin B12 metabolism. The goal of this study was to investigate the effects of the pharmacological activation of SIRT1 using SRT1720 on the molecular mechanisms triggered by impaired methionine synthase activity. Experiments were performed in vitro with fibroblasts from patients with the cblG and cblC inherited defects of vitamin B12 metabolism and in vivo with an original transgenic mouse model of methionine synthase deficiency specific to neuronal cells. Subcellular localization of the RBPs HuR, HnRNPA1, RBM10, SRSF1 and Y14 was investigated by immunostaining and confocal microscopy in patient fibroblasts. RBPs methylation and phosphorylation were studied by co-immunoprecipitation and proximity ligation assay. Cognitive performance of the transgenic mice treated with SRT1720 was measured with an aquatic maze. Patient fibroblasts with cblC and cblG defects of vitamin B12 metabolism presented with endoplasmic reticulum stress, altered methylation, phosphorylation and subcellular localization of HuR, HnRNPA1 and RBM10, global mRNA mislocalization and increased HnRNPA1-dependent skipping of IRF3 exons. Incubation of fibroblasts with cobalamin, S-adenosyl methionine and okadaic acid rescued the localization of the RBPs and mRNA. The SIRT1 activating compound SRT1720 inhibited ER stress and rescued RBP and mRNA mislocalization and IRF3 splicing. Treatment with this SIRT1 agonist prevented all these hallmarks in patient fibroblasts but it also improved the deficient hippocampo-dependent learning ability of methionine synthase conditional knock-out mice. By unraveling the molecular mechanisms triggered by inborn errors of cbl metabolism associating ER stress, RBP mislocalization and mRNA trafficking, our study opens novel therapeutic perspectives for the treatment of inborn errors of vitamin B12 metabolism.