Reduced brain choline in homocystinuria due to remethylation defects

Clinical and neuroimaging review of monogenic cerebral small vessel disease from the prenatal to adolescent developmental stage

Cerebral small vessel disease (cSVD) refers to a group of pathological processes with various etiologies affecting the small vessels of the brain. Most cases are sporadic, with age-related and hypertension-related sSVD and cerebral amyloid angiopathy being the most prevalent forms. Monogenic cSVD accounts for up to 5% of causes of stroke. Several causative genes have been identified. Sporadic cSVD has been widely studied whereas monogenic cSVD is still poorly characterized and understood. The majority of cases of both the sporadic and monogenic types, including cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), typically have their onset in adulthood. Types of cSVD with infantile and childhood onset are rare, and their diagnosis is often challenging. The present review discusses the clinical and neuroimaging findings of monogenic cSVD from the prenatal to adolescent period of development. Early diagnosis is crucial to enabling timely interventions and family counseling.

Neuroimaging findings of inborn errors of metabolism: urea cycle disorders, aminoacidopathies, and organic acidopathies

Although there are many types of inborn errors of metabolism (IEMs) affecting the central nervous system, also referred to as neurometabolic disorders, individual cases are rare, and their diagnosis is often challenging. However, early diagnosis is mandatory to initiate therapy and prevent permanent long-term neurological impairment or death. The clinical course of IEMs is very diverse, with some diseases progressing to acute encephalopathy following infection or fasting while others lead to subacute or slowly progressive encephalopathy. The diagnosis of IEMs relies on biochemical and genetic tests, but neuroimaging studies also provide important clues to the correct diagnosis and enable the conditions to be distinguished from other, more common causes of encephalopathy, such as hypoxia-ischemia. Proton magnetic resonance spectroscopy (¹H-MRS) is a powerful, non-invasive method of assessing neurological abnormalities at the microscopic level and can measure in vivo brain metabolites. The present review discusses neuroimaging findings, including those of ¹H-MRS, of IEMs focusing on intoxication disorders such as urea cycle disorders, aminoacidopathies, and organic acidopathies, which can result in acute life-threatening metabolic decompensation or crisis.

Lessons Learned from Inherited Metabolic Disorders of Sulfur-Containing Amino Acids Metabolism

The metabolism of sulfur-containing amino acids (SAAs) requires an orchestrated interplay among several dozen enzymes and transporters, and an adequate dietary intake of methionine (Met), cysteine (Cys), and B vitamins. Known human genetic disorders are due to defects in Met demethylation, homocysteine (Hcy) remethylation, or cobalamin and folate metabolism, in Hcy transsulfuration, and Cys and hydrogen sulfide (H2S) catabolism. These disorders may manifest between the newborn period and late adulthood by a combination of neuropsychiatric abnormalities, thromboembolism, megaloblastic anemia, hepatopathy, myopathy, and bone and connective tissue abnormalities. Biochemical features include metabolite deficiencies (e.g. Met, S-adenosylmethionine (AdoMet), intermediates in 1-carbon metabolism, Cys, or glutathione) and/or their accumulation (e.g. S-adenosylhomocysteine, Hcy, H2S, or sulfite). Treatment should be started as early as possible and may include a low-protein/low-Met diet with Cys-enriched amino acid supplements, pharmacological doses of B vitamins, betaine to stimulate Hcy remethylation, the provision of N-acetylcysteine or AdoMet, or experimental approaches such as liver transplantation or enzyme replacement therapy. In several disorders, patients are exposed to long-term markedly elevated Met concentrations. Although these conditions may inform on Met toxicity, interpretation is difficult due to the presence of additional metabolic changes. Two disorders seem to exhibit Met-associated toxicity in the brain. An increased risk of demyelination in patients with Met adenosyltransferase I/III (MATI/III) deficiency due to biallelic mutations in the MATIA gene has been attributed to very high blood Met concentrations (typically >800 μmol/L) and possibly also to decreased liver AdoMet synthesis. An excessively high Met concentration in some patients with cystathionine β-synthase deficiency has been associated with encephalopathy and brain edema, and direct toxicity of Met has been postulated. In summary, studies in patients with various disorders of SAA metabolism showed complex metabolic changes with distant cellular consequences, most of which are not attributable to direct Met toxicity.

Variable phenotypes and outcomes associated with the MMACHC c.609G>A homologous mutation: long term follow-up in a large cohort of cases

Background

Cobalamin C deficiency (cblC) caused by the MMACHC mutations is the most common type of the disorders of intracellular cobalamin metabolism. While the c.609G > A mutation is most frequent in Chinese cblC patients, its correlation with phenotype has not been delineated. Here we aim to investigate the factors affecting variable phenotypes and outcomes associated with the MMACHC c.609G > A homologous mutation in 149 Chinese cases to have implications for treatment and prevention.

Methods

We assessed 149 cblC patients caused by MMACHC c.609G > A homozygous mutation. The clinical manifestations, complications, treatment, and outcomes were evaluated; 120 patients were followed-up till December 2019.

Results

Two patients (1.3%) were prenatally diagnosed, treated after birth and consequently showed normal development. In 15 patients (10.1%) detected by newborn screening, 10 were treated at the age of 2 weeks and showed normal development, while the other 5 were treated after onset and showed neurologic disorders. All 132 clinically diagnosed patients (88.6%) developed symptoms at age from few minutes after birth to 72 months. Among them, 101 (76.5%) had early-onset (before the age of 12 months) and 31 (23.5%) had late-onset (after the age of 12 months). Totally 5 patients died and 24 were lost to follow-up. Of the 132 clinical diagnosed patients, 92 (69.7%) presented with developmental delay, 65 (49.2%) had seizures, 37 (28.0%) had anemia, 24 (18.2%) had feeding difficulty, 23 (17.4%) had ocular problems, and 22 (16.7%) had hydrocephalus. Compared with the non-developmental delay group, the onset age, the age at treatment initiation and the time from onset to treatment initiation were later in the developmental delay group. Seizure group showed significantly higher urinary methylmalonic acid concentration. During long-term follow-up, plasma total homocysteine (tHcy) levels were significantly higher in patients in the uncontrolled group than those in the seizure-free group.

Conclusions

Most cblC patients caused by MMACHC c.609G > A homozygous mutation showed early-onset. The clinically diagnosed patients usually showed the presence of irreversible brain disorders. Patients treated from the pre-symptomatic stage showed favorable outcomes. Therefore, newborn screening, prenatal diagnosis and early treatment are crucial and the c.609G > A mutant allele should be listed in the pre-pregnancy carrier screening panel in China.

The clinical presentation of cobalamin-related disorders: From acquired deficiencies to inborn errors of absorption and intracellular pathways

This review gives an overview of clinical characteristics, treatment and outcome of nutritional and acquired cobalamin (Cbl; synonym: vitamin B12) deficiencies, inborn errors of Cbl absorption and intracellular trafficking, as well as methylenetetrahydrofolate dehydrogenase (MTHFD1) and methylene tetrahydrofolate reductase (MTHFR) deficiencies, which impair Cbl-dependent remethylation. Acquired and inborn Cbl-related disorders and MTHFR deficiency cause multisystem, often severe disease. Failure to thrive, neurocognitive or psychiatric symptoms, eye disease, bone marrow alterations, microangiopathy and thromboembolic events are characteristic. The recently identified MTHFD1 defect additionally presents with severe immune deficiency. Deficient Cbl-dependent enzymes cause reduced methylation capacity and metabolite toxicity. Further net-effects of perturbed Cbl function or reduced Cbl supply causing oxidative stress, altered cytokine regulation or immune functions are discussed.

Functional assessment of creatine transporter in control and X-linked SLC6A8-deficient fibroblasts

Creatine transporter is currently the focus of renewed interest with emerging roles in brain neurotransmission and physiology, and the bioenergetics of cancer metastases. We here report on amendments of a standard creatine uptake assay which might help clinical chemistry laboratories to extend their current range of measurements of creatine and metabolites in body fluids to functional enzyme explorations. In this respect, short incubation times and the use of a stable-isotope-labeled substrate (D₃-creatine) preceded by a creatine wash-out step from cultured fibroblast cells by removal of fetal bovine serum (rich in creatine) from the incubation medium are recommended. Together, these measures decreased, by a first order of magnitude, creatine concentrations in the incubation medium at the start of creatine-uptake studies and allowed to functionally discriminate between 4 hemizygous male and 4 heterozygous female patients with X-linked SLC6A8 deficiency, and between this cohort of eight patients and controls. The functional assay corroborated genetic diagnosis of SLC6A8 deficiency. Gene anomalies in our small cohort included splicing site (c.912G > A [p.Ile260_Gln304del], c.778-2A > G and c.1495 + 2 T > G), substitution (c.407C > T) [p.Ala136Val] and deletion (c.635_636delAG [p.Glu212Valfs*84] and c.1324delC [p.Gln442Lysfs*21]) variants with reduced creatine transporter function validating their pathogenicity, including that of a previously unreported c.1324delC variant. The present assay adaptations provide an easy, reliable and discriminative manner for exploring creatine transporter activity and disease variations. It might apply to drug testing or other evaluations in the genetic and metabolic horizons covered by the emerging functions of creatine and its transporter, in a way, however, requiring and completed by additional studies on female patients and blood-brain barrier permeability properties of selected compounds. As a whole, the proposed assay of creatine transporter positively adds to currently existing measurements of this transporter activity, and determining on a large scale the extent of its exact suitability to detect female patients should condition in the future its transfer in clinical practice.

MRI and (1)H-MRS in adenosine kinase deficiency

INTRODUCTION

Adenosine kinase deficiency (ADK deficiency) is a recently described disorder of methionine and adenosine metabolism resulting in a neurological phenotype with developmental delay, muscular hypotonia, and epilepsy as well as variable systemic manifestations. The underlying neuropathology is poorly understood. We have investigated MRI and (1)H-MRS changes in ADK deficiency in order to better understand the in vivo neuropathologic changes of ADK deficiency.

METHODS

Systematic evaluation of 21 MRIs from eight patients (age range 9 days-14.6 years, mean 3.9 years, median 2.7 years) including diffusion-weighted imaging in six and (1)H-MRS in five patients.

RESULTS

Brain maturation was delayed in the neonatal period and in infancy (6/6), but ultimately complete. White matter changes occurring in five of eight patients were discrete, periventricular, and unspecific (4/5), or diffuse with sparing of optic radiation, corona radiata, and pyramidal tracts (1/5). Choline was low in white matter spectra (3/3), while there was no indication of low creatine in white matter or basal ganglia (5/5), and diffusion was variably decreased or increased. Central tegmental tract hyperintensity was a common finding (6/8), as was supratentorial atrophy (6/8).

CONCLUSIONS

MRI changes in ADK deficiency consist of delayed but ultimately completed brain maturation with later onset of mostly unspecific white matter changes and potentially transient central tegmental tract hyperintensity. Immaturity on neonatal MRI is consistent with prenatal onset of disease and reduced choline with lower membrane turnover resulting in delayed myelination and deficient myelin maintenance.

A critical reappraisal of dietary practices in methylmalonic acidemia raises concerns about the safety of medical foods. Part 2: cobalamin C deficiency

PURPOSE

Cobalamin C (cblC) deficiency impairs the biosynthesis of adenosyl- and methylcobalamin resulting in methylmalonic acidemia combined with hyperhomocysteinemia and hypomethioninemia. However, some patients with cblC deficiency are treated with medical foods, devoid of methionine and high in leucine content, that are formulated for patients with isolated propionate oxidative defects. We examined the effects of imbalanced branched-chain amino acid intake on growth outcomes in cblC patients.

METHODS

Dietary intake was correlated with biochemical, anthropometric, body composition measurements and other disease parameters in a cohort of 28 early-onset cblC patients.

RESULTS

Protein restricted diets were followed by 21% of the patients, while 32% received medical foods. Patients on protein-restricted diets had lower height-for-age Z-score (P=0.034), while patients consuming medical foods had lower head-circumference Z-scores (P=0.037), plasma methionine concentrations (P=0.001) and predicted methionine influx through the blood brain barrier Z-score (−1.29 vs. −0.0617, P=0.007). The combination of age of diagnosis, a history of seizures and the leucine/valine dietary intake ratio best predicted head circumference Z-score based on multiple regression modeling (R²= 0.945).

CONCLUSIONS

Patients with cblC deficiency treated with medical foods designed for isolated methylmalonic acidemia are at risk for iatrogenic methionine deficiency that could adversely affect brain growth and development.

TRIAL REGISTRATION

This clinical study is registered in www.clinicaltrials.gov with the ID: NCT00078078. Study URL: http://clinicaltrials.gov/ct2/show/NCT00078078

Reversal of respiratory failure in both neonatal and late onset isolated remethylation disorders

Respiratory failure is a well-documented potential presentation of inherited isolated remethylation disorders (IRDs). It appears to be a combination of both central and peripheral neuropathy and has previously often been considered to herald an irreversible neurological decline. We present three patients, one with methionine synthase (cblG) and two with methyltetrahydrofolate reductase deficiency (MTHFR). One patient with MTHFR presented in infancy, and other patients in later childhood. All three patients required intubation for respiratory failure but in all three, this was totally reversed by the initiation of medical therapy. This consisted of betaine and folinic acid supplementation in all three, methionine in two and cobalamin supplementation in two. The rate of respiratory improvement was variable, though two of the cases were successful extubated within a week of commencement of medical therapy. We document their subsequent clinical, biochemical and electrophysiological progress and review the potential pathological mechanisms underlying respiratory failure in these disorders.

Neurologic and neurodevelopmental phenotypes in young children with early-treated combined methylmalonic acidemia and homocystinuria, cobalamin C type

Abnormal neurodevelopment has been widely reported in combined methylmalonic aciduria (MMA) and homocystinuria, cblC type (cblC disease), but neurodevelopmental phenotypes in cblC have not previously been systematically studied. We sought to further characterize developmental neurology in children with molecularly-confirmed cblC. Thirteen children at our center with cblC, born since implementation of expanded newborn screening in New York State, undertook standard-of-care evaluations with a pediatric neurologist and pediatric ophthalmologist. At most recent follow-up (mean age 50 months, range 9-84 months), of twelve children with early-onset cblC, three (25%) had a history of clinical seizures and two (17%) meet criteria for microcephaly. A majority of children had hypotonia and nystagmus. Twelve out of thirteen (92%) underwent neurodevelopmental evaluation (mean age 41 months; range 9-76 months), each child tested with standardized parental interviews and, where possible, age- and disability-appropriate neuropsychological batteries. All patients showed evidence of developmental delay with the exception of one patient with a genotype predictive of attenuated disease and near-normal biochemical parameters. Neurodevelopmental deficits were noted most prominently in motor skills, with relative preservation of socialization and communication skills. Nine children with early-onset cblC underwent magnetic resonance imaging and spectroscopy (MRI/MRS) at mean age of 47 months (range 6-81 months); common abnormalities included callosal thinning, craniocaudally short pons, and increased T2 FLAIR signal in periventricular and periatrial white matter. Our study further characterizes variable neurodevelopmental phenotypes in treated cblC, and provides insights into the etiopathogenesis of disordered neurodevelopment frequently encountered in cblC. Plasma homocysteine and MMA, routinely measured at clinical follow-up, may be poor predictors for neurodevelopmental outcomes. Additional data from large, prospective, multi-center natural history studies are required to more accurately define the role of these metabolites and others, as well as that of other genetic and environmental factors in the etiopathogenesis of the neurologic components of this disorder.

Combined methylmalonic acidemia and homocystinuria, cblC type. II. Complications, pathophysiology, and outcomes

Combined methylmalonic acidemia and homocystinuria, cblC type, is stated to be the most common inborn error of intracellular cobalamin metabolism. The disorder can display a wide spectrum of clinical manifestations, spanning the prenatal period through late adulthood. While increased homocysteine concentrations and impaired methyl group metabolism may contribute to disease-related complications, the characteristic macular and retinal degeneration seen in many affected patients appears to be unique to cblC disease. The early detection of cblC disease by newborn screening mandates a careful assessment of therapeutic approaches and provides a new opportunity to improve the outcome of affected patients. The following article reviews the current knowledge on the complications, pathophysiology, and outcome of cblC disease in an effort to better guide clinical practice and future therapeutic trials.

Intact recovery from early 'acquired methylmalonic aciduria' secondary to maternal atrophic gastritis

AIM

A 6-month-old infant with severe hyporegenerative anaemia, muscular hypotonia and developmental delay is reported, and the metabolic, diagnostic and therapeutic implications of this case are discussed.

RESULTS

Diagnostic work-up disclosed vitamin B12 depletion with an elevated excretion of methylmalonic acid (MMA), but a normal plasma total homocysteine. MRI showed fronto-temporal atrophy and a delay in myelinization. The boy's disease was attributable to a maternal atrophic gastritis. After initiation of vitamin B12 supplementation, he quickly recovered regarding haematopoiesis and MMA excretion. His neurological development completely normalized during 18 months of follow-up including assessment by Bayley scores.

CONCLUSION

As the majority of reported patients with this acquired form of methylmalonic aciduria show a persistent neurological deficit, early diagnosis of this condition is mandatory and should include sensitive markers of vitamin B12 depletion, namely MMA formation and plasma homocysteine.

Brainstem metabolites in multiple system atrophy of cerebellar type: 3.0-T magnetic resonance spectroscopy study

BACKGROUND

The aim of this study was to find biomarkers of disease severity in multiple system atrophy of cerebellar type by imaging disease specific regions using proton magnetic resonance spectroscopy on a 3.0 T system.

METHODS

We performed proton magnetic resonance spectroscopy separately in the pons and medulla on 12 multiple system atrophy of cerebellar type patients and 12 age and gender matched control subjects. The metabolite concentrations were estimated from single-voxel proton magnetic resonance spectra measured by point resolved spectroscopy, which were then correlated with clinical severity using Part I, II, and IV of the unified multiple system atrophy rating scale.

RESULTS

Proton magnetic resonance spectroscopy showed that myo-inositol concentrations in both the pons and medulla were significantly higher in multiple system atrophy of cerebellar type patients compared to those of the control subjects (P < 0.05). By contrast, total N-acetylaspartate (the sum of N-acetylaspartate and N-acetylaspartylglutamate) and total choline compounds concentrations in both the pons and medulla were significantly lower in multiple system atrophy of cerebellar type patients compared to control subjects (P < 0.05). Creatine concentration in the pons was significantly higher in multiple system atrophy of cerebellar type patients compared to the control subjects (P < 0.05). Furthermore, a significant correlation was found between the myo-inositol/creatine ratio in the pons and clinical severity, defined by the sum score of unified multiple system atrophy rating scale (I+II+IV) (r = 0.76, P < 0.01).

CONCLUSION

Proton magnetic resonance spectroscopy, in conjunction with a 3.0 T system, can be feasible to detect part of pathological changes in the brainstem, such as gliosis and neuronal cell loss, and the metabolites can be used as biomarkers of clinical severity in multiple system atrophy of cerebellar type patients.

Isolated remethylation disorders: do our treatments benefit patients?

Deficiency of 5,10-methylenetetrahydrofolate reductase (MTHFR), the very rare methionine synthase reductase (CblE) and methionine synthase (CblG) defects, and the recently identified CblD-variant-1 defect are primary remethylation defects characterized by an isolated defect in methionine synthesis without methylmalonic aciduria. The clinical signs are mainly neurological, and hematological signs are seen in CblE, CblG, and CblD-variant-1 defects. Patients with neonatal or early-onset disease exhibit acute neurological distress. Infants and children have unspecific mental retardation, often with acquired microcephaly. Without appropriate therapy, they may experience acute or rapidly progressive neurological deterioration, which may be fatal. Adolescents and adults show normal development or mild developmental delay initially and then experience rapid neurological or behavioral deterioration. A few patients may have signs of subacute combined degeneration of the spinal cord. Adults may be asymptomatic or present with isolated thromboembolism. All patients with suspected remethylation disorders should receive emergency treatment with parenteral administration of hydroxocobalamin and folate supplements combined with betaine orally. The long-term treatment of CblE, CblG, and CblD-variant-1 defects consists of parenterally administered hydroxocobalamin and orally administered folate and betaine supplements, whereas patients with MTHFR deficiency require long-term oral folate and betaine supplements. Long-term oral methionine therapy should also be considered. Early treatment may lead to a favorable outcome with developmental recovery and prevention of further neurological deterioration. In contrast, most late-treated patients have severe and irreversible neuromotor impairments. Hematological abnormalities are easily corrected.

High prevalence of structural heart disease in children with cblC-type methylmalonic aciduria and homocystinuria

OBJECTIVE

To characterize the frequency and nature of cardiovascular defects in patients with CblC-type methylmalonic aciduria and homocystinuria (cblC), an inborn error of cobalamin (vitamin B12) metabolism resulting in accumulation of methylmalonic acid and homocysteine.

STUDY DESIGN

A retrospective observational study was conducted investigating 10 patients with cblC ranging in age from 2 weeks to 24 years (mean 4.4 years +/- 7.5 years, median 0.6 years). All patients underwent a complete 2-D echocardiogram including quantitative assessment of left ventricular systolic function.

RESULTS

Structural heart defects were detected in 50% of patients with cblC. Heart defects included left ventricular (LV) non-compaction (3), secundum atrial septal defect (2), muscular ventricular septal defect (1), dysplastic pulmonary valve without pulmonary stenosis (1) and mitral valve prolapse with mild mitral regurgitation (1). One patient had resolved cor pulmonale and right heart failure secondary to pulmonary embolism. All patients had quantitatively normal LV systolic function.

CONCLUSIONS

Diverse and clinically significant structural heart defects appear to be highly prevalent in cblC, perhaps due to abnormal DNA and histone methylation during embryogenesis. The specific cardiac defects detected in our cohort were variable, and studies with a larger number of patients are needed to establish which forms are most common. Routine and periodic cardiovascular evaluation may be indicated in patients with cblC.