Development of low-cost in-house tetra-ARMS-PCR assay for the screening of five CBS mutations found in Pakistani homocystinuria patients

BACKGROUND

Classical homocystinuria is an inborn amino acid metabolism disorder resulting from mutations in the Cystathionine-β-Synthase (CBS) gene. These mutations lead to elevated homocysteine and methionine levels and reduced cysteine levels in the blood. Typically, diagnosis occurs after patients display symptoms, and various lab methods confirm it. DNA sequencing is the best option for early detection of genetic variants in asymptomatic suspected individuals. Unfortunately, its high cost can hinder its use, especially in low-income countries like Pakistan.

OBJECTIVE

Aim of this study was to devise a robust low-cost diagnostic/screening assay based on Tetra-ARMS-PCR for five prevalent genetic variants found in Pakistani classical homocystinuria patients.

MATERIALS AND METHODS

In the current study, T-ARMS-PCR assays were developed for five mutations (c.975G > C, c.770C > T, c.752T > C, c.1039 + 1G > T, c.451 + 1GG > TA), which were characterized previously in classical homocystinuria patients. These low-cost T-ARMS-PCR assays were then used to screen the affected individuals and their family members to identify their genotypes for pathogenic variations in the asymptomatic patients and carriers in their respective families.

RESULTS

The outcomes were entirely consistent with those obtained from Sanger DNA sequencing, confirming the sensitivity, specificity, and reliability of the T-ARMS-PCR assay for detecting CBS mutations.

CONCLUSION

T-ARMS-PCR has wide applications for low-income countries for the screening and early diagnosis of asymptomatic patients and carriers in the homocystinuria affected families as well as other inherited diseases.

Dental complications in homocystinurias

Background

Cystathionine beta synthase deficiency (causing classical homocystinuria) has been associated with high-arched palates and crowded teeth, but little has been said about other oral health complications. Other homocystinurias (e.g., the remethylation defects) also have had little reported in terms of oral health. Individuals with the homocystinurias have been described as having bone density issues which can correlate with oral health. Moreover, elevations in homocysteine have a theoretical impact on tooth health and the paucity of clinical reports of oral health issues in homocystinuria may be the consequence of lack of attention by the medical community.

Significance

Oral health is essential to overall health. If inadequate attention is paid to the oral health complications which can be seen in homocystinurias, then appropriate referrals and attention in therapeutic guidelines will not reflect the importance of oral health.

Specific aims/research question

What oral health complications are reported by individuals with homocystinurias? Do these differ according to diagnosis?

Methods

Data were collected from patients with homocystinurias by a series of questionnaires using the RARE-X platform. All subjects were consented prior to the collection of their data. All research was performed in accordance with the Declaration of Helsinki. Demographic data were collected as the initial questionnaire and other data were collected via the oral health questionnaire.

Analysis

Questionnaires were opened to the community in mid-2022 and collection of data for this study ended with data submitted up to November 2022. Descriptive statistics were done. Due to the small size of the cohort, additional statistical analyses were not attempted.

Results

Patients with homocystinuria, not related to cystathionine beta synthase deficiency, are reporting some tooth structure differences. The cohort taken as a whole does not have increased risk for gingivitis, but there appears to be a risk for long-term gum disease possibly due to the rate of osteoporosis/osteopenia in this population. A large number of individuals have malalignment and malocclusion of the teeth. These data highlight oral health as an important component of care in individuals with the homocystinurias as is true of the general population at large.

An orally administered enzyme therapeutic for homocystinuria that suppresses homocysteine by metabolizing methionine in the gastrointestinal tract

Classical homocystinuria (HCU) is a rare inborn error of amino acid metabolism characterized by accumulation of homocysteine, an intermediate product of methionine metabolism, leading to significant systemic toxicities, particularly within the vascular, skeletal, and ocular systems. Most patients require lifelong dietary therapy with severe restriction of natural protein to minimize methionine intake, and many patients still struggle to maintain healthy homocysteine levels. Since eliminating methionine from the diet reduces homocysteine levels, we hypothesized that an enzyme that can degrade methionine within the gastrointestinal (GI) tract could help HCU patients maintain healthy levels while easing natural protein restrictions. We describe the preclinical development of CDX-6512, a methionine gamma lyase (MGL) enzyme that was engineered for stability and activity within the GI tract for oral administration to locally degrade methionine. CDX-6512 is stable to low pH and intestinal proteases, enabling it to survive the harsh GI environment without enteric coating and to degrade methionine freed from dietary protein within the small intestine. Administering CDX-6512 to healthy non-human primates following a high protein meal led to a dose-dependent suppression of plasma methionine. In Tg-I278T Cbs-/- mice, an animal model that recapitulates aspects of HCU disease including highly elevated serum homocysteine levels, oral dosing of CDX-6512 after a high protein meal led to suppression in serum levels of both methionine and homocysteine. When animals received a daily dose of CDX-6512 with a high protein meal for two weeks, the Tg-I278T Cbs-/- mice maintained baseline homocysteine levels, whereas homocysteine levels in untreated animals increased by 39%. These preclinical data demonstrate the potential of CDX-6512 as an oral enzyme therapy for HCU.

Clinical and Molecular Spectrum of Patients with Methylmalonic Acidemia

OBJECTIVES

To study the clinical and molecular spectrum of Methylmalonic acidemia (MMA).

METHODS

In this retrospective study, the records of 30 MMA patients were evaluated for their phenotype, biochemical abnormalities, genotype, and outcomes.

RESULTS

Thirty patients with MMA (age range 0-21 y) from 27 unrelated families were enrolled. Family history and consanguinity were noted in 10/27 (37%) and 11/27 (41%) families respectively. Acute metabolic decompensation was more common (57%) than chronic presentation. Biochemical work-up was suggestive of isolated MMA (n = 18) and MMA with homocystinuria (n = 9) respectively. Molecular testing in 24 families showed 21 pathogenic or likely pathogenic variants with MMA cblC as the commonest molecular subtype (n = 8). B12 responsiveness, an important determinant of long-term outcome, was observed in eight patients [MMAA (n = 3) and MMACHC (n = 5)]. Mortality was 30% (n = 9/30) with a high proportion of early-onset severe disease and fatal outcome in isolated MMA mut⁰ (4/4) and MMA cblB (3/3), as compared to MMA cblA (1/5) and MMA cblC (1/10).

CONCLUSIONS

This study cohort had MMA cblC subtype as the most common type of MMA followed by the MMA mutase defect. Outcomes in MMA are influenced by the type of molecular defect, age, and severity of presentation. Early detection and management is likely to result in better outcomes.

Current and Novel Therapeutical Approaches of Classical Homocystinuria in Childhood With Special Focus on Enzyme Replacement Therapy, Liver-Directed Therapy and Gene Therapy

Classical homocystinuria is a hereditary defect of the enzyme cystathionine beta synthase, which is produced in the liver. If this enzyme fails, the synthesis pathway of cysteine from methionine is interrupted, leading to the accumulation of homocysteine in the blood plasma and homocysteine in the urine. After birth, the children are unremarkable except for the characteristic laboratory findings. Symptoms rarely appear before the second year of life. The most common symptom is a prolapse of the crystalline lens. This finding is seen in 70% of untreated 10-year-old affected individuals. As the earliest symptom, psychomotor retardation occurs in the majority of patients already during the first two years of life. Limiting factors in terms of life expectancy are thromboembolism, peripheral arterial disease, myocardial infarction, and stroke. These symptoms are due to the damage to the vessels caused by the elevated amino acid levels. About 30% suffer a thromboembolic event by the age of 20, about half by the age of 30. This review focus on present and new therapeutical approaches like the role of enzyme replacement with presentation of different novel targets in research like pegtibatinase, pegtarviliase, CDX-6512, erymethionase, chaperones, proteasome inhibitors and probiotic treatment with SYNB 1353. Furthermore, we analyze the role of liver-directed therapy with three dimensional (3D) bioprinting, liver bioengineering of liver organoids in vitro and liver transplantation. The role of different gene therapy options to treat and cure this extremely rare disease in childhood will be discussed.

Intracardiac amorphous tumor presenting in a patient with homocystinuria; a case report with literature review

Homocystinuria is a rare genetic disease with autosomal recessive pattern. It is reported to be highest in Arabian descend and could cause thrombosis, but mainly peripherally. Cardiac amorphous tumor has been recognized in the past 20 years and it is also a very rare cause primary benign tumor of the heart. Most of the cases reported to be associated with end-stage renal disease. Homocystinuria associated with Cardiac Amorphous tumor is extremely rare. Up to our knowledge, there has been only one other case has been reported. Our patient is a 14-year-old female known case of homocystinuria presented with dyspnea and leg edema. On workup was found to have a mass in the right atrium extending to superior vena cava and inferior cava. Surgery undertaken on cardiopulmonary bypass partial resection of the mass was done and result came back as cardiac amorphous tumor. We assume the cause of this sinister complication of her primary illness is calcification of thrombus as stated in literature. And also recommend further studies regarding issue on hand.

Tracheobronchomegaly (Mounier-Kuhn syndrome) and Bronchiectasis as rare manifestations of Homocystinuria

Homocystinuria (HCU) is a rare autosomal recessive inherited disorder usually diagnosed in childhood. It is characterized by a deficiency of the enzyme that converts homocysteine to cystathionine. The accumulation of homocysteine leads to abnormalities in the ocular, skeletal, cardiovascular, and central nervous systems. HCU shares several clinical features with Marfan syndrome; however, respiratory system involvement in HCU is uncommon and rarely reported. Bronchiectasis has been previously reported in a few cases of HCU, and it was attributed mainly to fibrillin deficiency. This case describes a young girl diagnosed with classical HCU since childhood who presented with a chronic productive cough and was initially misdiagnosed as bronchial Asthma. However, upon further evaluation, she was eventually diagnosed with tracheobronchomegaly (TBM), or Mounier-Kuhn Syndrome, and bronchiectasis based on the computed tomography (CT) scan of chest findings. To our knowledge, this is the first reported case of TBM and bronchiectasis in HCU. We believe that fibrillin degeneration may be the key to understanding this unusual association in HCU.

Novel compound heterozygous mutations of MTHFR Gene in a Chinese family with homocystinuria due to MTHFR deficiency

BACKGROUND

Homocystinuria due to methylenetetrahydrofolate reductase (MTHFR) deficiency is a rare autosomal recessive disorder. The purpose of this study is to expand the mutation site of the MTHFR gene and provide genetic counseling for this family.

METHODS

A couple came to our hospital for pre-pregnancy genetic counseling. We collected the family history and detailed clinical information, then performed whole-exome sequencing, and analyzed the pathogenicity of the candidate mutations.

RESULTS

We found that the father of the proband had homocystinuria, the proband and his brother had low blood methionine levels at birth, and the brain MRI showed brain dysplasia. The third fetus was found to have a broadened triangle of the bilateral ventricle at 19 weeks of pregnancy. The compound heterozygous variants of c.602 A > C (p.His201Pro) and c.1316T > C (p.Leu439Pro) of the MTHFR gene in the first three fetuses were found by whole-exome sequencing. The heterozygous c.602 A > C variant of the MTHFR gene is a novel missense variant that has been submitted to the ClinVar with Variation ID 992,662.

CONCLUSION

In consideration of the clinical phenotype, family history, and result of genetic testing, we speculated that both patients may have homocystinuria due to MTHFR deficiency. Homocystinuria due to MTHFR deficiency caused by compound heterozygous mutations composed of the MTHFR gene in this family may be associated with cerebral atrophy and cerebral dysplasia. The novel compound heterozygous mutations broaden the mutation spectrum of the MTHFR gene and enhance the application of genetic counseling and carrier screening in rare diseases.

Association of selected genetic variants in CBS and MTHFR genes in a cohort of children with homocystinuria in Sri Lanka

BACKGROUND

Homocystinuria is an inherited, inborn error of homocysteine metabolism, which leads to the abnormal accumulation of homocysteine and its metabolites in blood and urine, resulting in various complications. Variants in the cystathionine β-synthase (CBS) and methylenetetrahydrofolate reductase (MTHFR) genes interrupt the formation of the corresponding enzymes and prevent homocysteine from being metabolised; hence, the homocysteine levels in plasma increase than the optimum levels.

MATERIALS AND METHODS

In the current study, eight clinically confirmed children with homocystinuria were detected to study the chosen variants in the CBS gene (c.833 T>C and c.19del) and in the MTHFR gene (c.665 C>T, c.1286 A>C) using SNaPshot mini-sequencing and direct sequencing.

RESULTS

After screening eight patients, none had the c.833T>C, but four patients were in the homozygous state for the c.19del variant in the CBS gene. Furthermore, seven were heterozygous for c.1286A>C, while one patient was heterozygous for c.665C>T in the MTHFR gene.

CONCLUSION

According to the results, c.19del is common in the studied cohort of Sri Lankan children, while c.833T>C is absent, whereas c.1286A>C was more frequent than c.665C>T. To our knowledge, the current study was the first report to discuss the genetic impact of homocystinuria in Sri Lanka; further comprehensive studies are necessary with a larger sample size to establish the association of these variants with the disease in Sri Lanka, which can be beneficial in enhanced patient care and for prospective studies.

H2S biogenesis by cystathionine beta-synthase: mechanism of inhibition by aminooxyacetic acid and unexpected role of serine

Cystathionine beta-synthase (CBS) is a pivotal enzyme of the transsulfuration pathway responsible for diverting homocysteine to the biosynthesis of cysteine and production of hydrogen sulfide (H₂S). Aberrant upregulation of CBS and overproduction of H₂S contribute to pathophysiology of several diseases including cancer and Down syndrome. Therefore, pharmacological CBS inhibition has emerged as a prospective therapeutic approach. Here, we characterized binding and inhibitory mechanism of aminooxyacetic acid (AOAA), the most commonly used CBS inhibitor. We found that AOAA binds CBS tighter than its respective substrates and forms a dead-end PLP-bound intermediate featuring an oxime bond. Surprisingly, serine, but not cysteine, replaced AOAA from CBS and formed an aminoacrylate reaction intermediate, which allowed for the continuation of the catalytic cycle. Indeed, serine rescued and essentially normalized the enzymatic activity of AOAA-inhibited CBS. Cellular studies confirmed that AOAA decreased H₂S production and bioenergetics, while additional serine rescued CBS activity, H₂S production and mitochondrial function. The crystal structure of AOAA-bound human CBS showed a lack of hydrogen bonding with residues G305 and Y308, found in the serine-bound model. Thus, AOAA-inhibited CBS could be reactivated by serine. This difference may be important in a cellular environment in multiple pathophysiological conditions and may modulate the CBS-inhibitory activity of AOAA. In addition, our results demonstrate additional complexities of using AOAA as a CBS-specific inhibitor of H₂S biogenesis and point to the urgent need to develop a potent, selective and specific pharmacological CBS inhibitor.

Genetic regulatory element based whole-cell biosensors for the detection of metabolic disorders

Clinicians require simple, and cost-effective diagnostic tools for the quantitative determination of amino acids in physiological fluids for the detection of metabolic disorder diseases. Besides, amino acids also act as biological markers for different types of cancers and cardiovascular diseases. Herein, we applied an in-silico based approach to identify potential amino acid-responsive genetic regulatory elements for the detection of metabolic disorders in humans. Identified sequences were further transcriptionally fused with GFP, thus generating an optical readout in response to their cognate targets. Screening of genetic regulatory elements led us to discover two promoter elements (pmetE::GFP and ptrpL::GFP) that showed a significant change in the fluorescence response to homocysteine and tryptophan, respectively. The developed biosensors respond specifically and sensitively with a limit of detection of 3.8 μM and 3 μM for homocysteine and tryptophan, respectively. Furthermore, the clinical utility of this assay was demonstrated by employing it to identify homocystinuria and tryptophanuria diseases through the quantification of homocysteine and tryptophan in plasma and urine samples within 5 h. The precision and accuracy of the biosensors for disease diagnosis were well within an acceptable range. The general strategy used in this system can be expanded to screen different genetic regulatory elements present in other gram-negative and gram-positive bacteria for the detection of metabolic disorders.

Combined Methylmalonic Aciduria and Homocystinuria Presenting as Pulmonary Hypertension

Combined methylmalonic aciduria and homocystinuria, cblC type, (MAHCC) is a rare autosomal recessive metabolic disorder of remethylation caused due to mutations in the MMACHC (metabolism of cobalamin associated C) gene with predominant neurological involvement. Microvascular, renal, and cardiovascular complications are also known to occur. However, the disease presenting primarily with a cardiovascular phenotype without any neurological involvement is a rare entity. We report a case of developmentally normal 23-mo-old female child, who presented with pulmonary arterial hypertension (PAH) and succumbed to cardiac failure. Extensive workup for PAH was inconclusive. Posthumous trio whole-exome sequencing revealed pathogenic compound heterozygous variants in the MMACHC. Diagnosis of MAHCC should be considered as a differential diagnosis for unexplained PAH in children. An elevated plasma homocysteine level can serve as a simple screening modality for this disorder. Accurate diagnosis has paramount therapeutic implications, as management with hydroxocobalamin and betaine may lead to partial or complete remission of PAH in these patients.

Functional inhibition of redox regulated heme proteins: A novel mechanism towards oxidative stress induced by homocysteine

Homocysteine (Hcy) is a sulfur containing non-protein toxic amino acid synthesized from methionine. Elevated level of Hcy is associated with cardiovascular complications and neurodegeneration. Hcy is believed to induce organ damage and apoptosis via oxidative stress. The pro-oxidant nature of Hcy is considered to originate from the metal-induced oxidation of thiol group-containing molecules forming disulfides (Hcy-Hcy, Hcy-cysteine, Hcy-glutathione, etc) or with free cysteine residues of proteins (a process called protein S-homocysteinylation). Formation of such disulfides indeed results in the generation of reactive oxygen species (ROS) which eventually leads to loss of cellular integrity. In the present manuscript, we performed systematic investigation of the effect of Hcy on iron containing proteins. We discover a novel mechanism of Hcy toxicity wherein Hcy oxidation is linked with the functional loss of the protein with iron as cofactors. Our results indicate that redox regulated heme proteins might be primarily involved in the Hcy toxicity and associated oxidative stress.

Recurrent dislocation of binocular crystal lenses in a patient with cystathionine beta-synthase deficiency

Background

Ectopia lentis is the common ocular manifestation of homocystinuria resulting from cystathionine beta-synthase (CBS) deficiency which has a high risk of thromboembolic complications.

Case presentation

The present study reports the case of a teenager with recurrent lens dislocation and glaucoma. He was diagnosed with CBS deficiency according to a high level of serum homocysteine and compound heterozygous mutations at two different positions on the CBS gene. Antiglaucoma eyedrops and a mydriatic successfully controlled the intraocular pressure, while oral pyridoxine and betaine uptake lowered the serum homocysteine level effectively.

Conclusions

Children with CBS deficiency may suffer from ectopia lentis, glaucoma and/or amblyopia. We firstly discovered a new mutation of CBS c. 697 T > G which had not been reported before. The patient was pyridoxine responsive and well controlled by medicine.

Implementation of second-tier tests in newborn screening for the detection of vitamin B12 related acquired and genetic disorders: results on 258,637 newborns

BACKGROUND

Alteration of vitamin B₁₂ metabolism can be genetic or acquired, and can result in anemia, failure to thrive, developmental regression and even irreversible neurologic damage. Therefore, early diagnosis and intervention is critical. Most of the neonatal cases with acquired vitamin B₁₂ deficiency have been detected by clinical symptoms and only few of them trough NBS programs. We aim to assess the usefulness of the second-tier test: methylmalonic acid (MMA), methylcitric acid (MCA) and homocysteine (Hcys) in our newborn screening program and explore the implications on the detection of cobalamin (vitamin B₁₂) related disorders, both genetic and acquired conditions.

METHODS

A screening strategy using the usual primary markers followed by the analysis of MMA, MCA and Hcys as second tier-test in the first dried blood spot (DBS) was developed and evaluated.

RESULTS

During the period 2015-2018 a total of 258,637 newborns were screened resulting in 130 newborns with acquired vitamin B₁₂ deficiency (incidence 1:1989), 19 with genetic disorders (incidence 1:13,613) and 13 were false positive. No false negatives were notified. Concerning the second-tier test, the percentage of cases with MMA above the cut-off levels, both for genetic and acquired conditions was very similar (58% and 60%, respectively). Interestingly, the percentage of cases with increased levels of Hcys was higher in acquired conditions than in genetic disorders (87% and 47%, respectively). In contrast, MCA was high only in 5% of the acquired conditions versus in 53% of the genetic disorders, and it was always very high in all patients with propionic acidemia.

CONCLUSIONS

When screening for methylmalonic acidemia and homocystinuria, differential diagnosis with acquired vitamin B₁₂ deficiency should be done. The results of our strategy support the inclusion of this acquired condition in the NBS programs, as it is easily detectable and allows the adoption of corrective measures to avoid the consequences of its deficiency.

Holistic Approach in the Management of Skeletal Deformity in a Case of Homocystinuria

INTRODUCTION

Homocystinuria has a wide range of clinical presentations ranging from near normal intelligence and appearance with just lens dislocation and minimal skeletal deformities to severe mental retardation with gross skeletal deformities. In this background, we describe one such case with skeletal deformity managed comprehensively.

CASE REPORT

A 17-year-old boy presented with complaints of deformity of the left lower limb since childhood more evident for the past 5 years along with a history of blurring of vision. On examination the pubis-heal length > crown-pubis length along with genu valgum of left lower-limb with 16 cm intermalleolar distance. He also had a superolateral subluxation of the lens in both eyes. Valgus angle was 16° on the left leg compared to 6° on the right. The diagnosis of homocystinuria was confirmed by biochemical investigations. The left side genu valgum was addressed with medial closing wedge osteotomy and fixed with distal femur locking compression plating. Lens subluxation was treated with posterior chamber intra-ocular lens surgery. He was also given medical treatment and on regular monitoring of his homocysteine levels. The patient had good functional outcome at 2-year follow-up.

CONCLUSION

Homocystinuria is a rare disease that needs early identification and effective management to avoid complications. Skeletal complications are common and include genu valgum, pes cavus, chest wall deformities, and skeletal deformities such as kyphosis and scoliosis. Skeletal deformities can be avoided when identified early and associated osteoporosis which is managed effectively. A holistic approach is needed in the management of such patients with inter-departmental coordination to bring quality to the life of patients with homocystinuria.

MMADHC premature termination codons in the pathogenesis of cobalamin D disorder: Potential of translational readthrough reconstitution

Mutations in the MMADHC gene cause cobalamin D disorder (cblD), an autosomal recessive inborn disease with defects in intracellular cobalamin (cbl, vitamin B12) metabolism. CblD patients present methylmalonic aciduria (MMA), homocystinuria (HC), or combined MMA/HC, and usually suffer developmental delay and cognitive deficits. The most frequent MMADHC genetic alterations associated with disease generate MMADHC truncated proteins, in many cases due to mutations that create premature termination codons (PTC). In this study, we have performed a comprehensive and global characterization of MMADHC protein variants generated by all annotated MMADHC PTC mutations in cblD patients, and analyzed the potential of inducible translational PTC readthrough to reconstitute MMADHC biosynthesis. MMADHC protein truncation caused by disease-associated PTC differentially affected the alternative usage of translation initiation sites, protein abundance, and subcellular localization of MMADHC. Aminoglycoside compounds induced translational PTC readthrough of MMADHC truncated variants, allowing the biosynthesis of full-length MMADHC in a PTC-specific manner. Our results suggest that translational PTC readthrough-based interventions could complement current therapies for cblD patients carrying specific MMADHC PTC mutations.

Derangement of hepatic polyamine, folate, and methionine cycle metabolism in cystathionine beta-synthase-deficient homocystinuria in the presence and absence of treatment: Possible implications for pathogenesis

Cystathionine beta-synthase deficient homocystinuria (HCU) is a life-threatening disorder of sulfur metabolism. Our knowledge of the metabolic changes induced in HCU are based almost exclusively on data derived from plasma. In the present study, we present a comprehensive analysis on the effects of HCU upon the hepatic metabolites and enzyme expression levels of the methionine-folate cycles in a mouse model of HCU. HCU induced a 10-fold increase in hepatic total homocysteine and in contrast to plasma, this metabolite was only lowered by approximately 20% by betaine treatment indicating that this toxic metabolite remains unacceptably elevated. Hepatic methionine, S-adenosylmethionine, S-adenosylhomocysteine, N-acetlymethionine, N-formylmethionine, methionine sulfoxide, S-methylcysteine, serine, N-acetylserine, taurocyamine and N-acetyltaurine levels were also significantly increased by HCU while cysteine, N-acetylcysteine and hypotaurine were all significantly decreased. In terms of polyamine metabolism, HCU significantly decreased spermine and spermidine levels while increasing 5'-methylthioadenosine. Betaine treatment restored normal spermine and spermidine levels but further increased 5'-methylthioadenosine. HCU induced a 2-fold induction in expression of both S-adenosylhomocysteine hydrolase and methylenetetrahydrofolate reductase. Induction of this latter enzyme was accompanied by a 10-fold accumulation of its product, 5-methyl-tetrahydrofolate, with the potential to significantly perturb one‑carbon metabolism. Expression of the cytoplasmic isoform of serine hydroxymethyltransferase was unaffected by HCU but the mitochondrial isoform was repressed indicating differential regulation of one‑carbon metabolism in different sub-cellular compartments. All HCU-induced changes in enzyme expression were completely reversed by either betaine or taurine treatment. Collectively, our data show significant alterations of polyamine, folate and methionine cycle metabolism in HCU hepatic tissues that in some cases, differ significantly from those observed in plasma, and have the potential to contribute to multiple aspects of pathogenesis.

A high frequency and geographical distribution of MMACHC R132* mutation in children with cobalamin C defect

Cobalamin C defect is caused by pathogenic variants in the MMACHC gene leading to impaired conversion of dietary vitamin B₁₂ into methylcobalamin and adenosylcobalamin. Variants in the MMACHC gene cause accumulation of methylmalonic acid and homocysteine along with decreased methionine synthesis. The spectrum of MMACHC gene variants differs in various populations. A total of 19 North Indian children (age 0-18 years) with elevated methylmalonic acid and homocysteine were included in the study, and their DNA samples were subjected to Sanger sequencing of coding exons with flanking intronic regions of MMACHC gene. The genetic analysis resulted in the identification of a common pathogenic nonsense mutation, c.394C > T (R132*) in 85.7% of the unrelated cases with suspected cobalamin C defect. Two other known mutations c.347T > C (7%) and c.316G > A were also detected. Plasma homocysteine was significantly elevated (> 100 µmol/L) in 75% of the cases and methionine was decreased in 81% of the cases. Propionyl (C3)-carnitine, the primary marker for cobalamin C defect, was found to be elevated in only 43.75% of cases. However, the secondary markers such as C3/C2 and C3/C16 ratios were elevated in 87.5% and 100% of the cases, respectively. Neurological manifestations were the most common in our cohort. Our findings of the high frequency of a single MMACHC R132* mutation in cases with combined homocystinuria and methylmalonic aciduria may be proven helpful in designing a cost-effective and time-saving diagnostic strategy for resource-constraint settings. Since the R132* mutation is located near the last exon-exon junction, this is a potential target for the read-through therapeutics.

Plasma phospholipid dysregulation in patients with cystathionine-β synthase deficiency

BACKGROUND & AIMS

Patients with cystathionine β-synthase deficiency (CBSD) exhibit high circulating levels of homocysteine and enhanced lipid peroxidation. We have characterized the plasma lipidome in CBSD patients and related lipid abnormalities with reactions underlying enhanced homocysteine levels.

METHODS AND RESULTS

Using an ultra-high-performance liquid chromatography-electrospray ionization-quadrupole-time of flight-mass spectrometry method, plasma lipids were determined with an untargeted lipidomics approach in 11 CBSD patients and 11 matched healthy subjects (CTRL). Compared to CTRL, CBSD patients had a higher medium and long-chain polyunsaturated fatty acids (PUFA) content in phosphatidylethanolamine (PE) and lysophosphatidylethanolamine (LPE) species (p < 0.02), and depletion of phosphatidylcholine (PC; p = 0.02) and of lysophosphatidylcholine (LPC; p = 0.003) species containing docosahexaenoic acid (DHA), suggesting impaired phosphatidylethanolamine-N-methyltransferase (PEMT) activity. PEMT converts PE into PC using methyl group by S-adenosylmethionine (SAM) thus converted in S-adenosylhomocysteine (SAH). Whole blood SAM and SAH concentrations by liquid chromatography tandem mass spectrometry were 1.4-fold (p = 0.015) and 5.3-fold (p = 0.003) higher in CBSD patients than in CTRL. A positive correlation between SAM/SAH and PC/PE ratios (r = 0.520; p = 0.019) was found.

CONCLUSIONS

A novel biochemical abnormality in CBSD patients consisting in depletion of PC and LPC species containing DHA and accumulation of PUFA in PE and LPE species is revealed by this lipidomic approach. Changes in plasma SAM and SAH concentrations are associated with such phospholipid dysregulation. Given the key role of DHA in thrombosis prevention, depletion of PC species containing DHA in CBSD patients provides a new direction to understand the poor cardiovascular outcome of patients with homocystinuria.

Laboratory evaluation of homocysteine remethylation disorders and classic homocystinuria: Long-term follow-up using a cohort of 123 patients

The homocystinurias, caused by defects of remethylation and cystathionine-beta-synthase (CBS) deficiency, are characterized by elevated homocysteine and abnormal methionine levels. Various treatments, including injectable hydroxycobalamin and oral betaine, aim to reduce homocysteine toxicity and normalize methionine, but only limited biochemical data has been reported assessing biochemical response to treatment. We analyzed laboratory results in 812 plasma samples from 56 patients with remethylation disorders and 67 patients with CBS deficiency. Total plasma homocysteine (tHcys) decreased with therapy, but rarely normalized regardless of treatment, with highest levels seen in CBS (116 ± 79 μmol/L) and MTHFR (102 ± 56 μmol/L) deficiencies. In CBS deficiency, tHcys correlated positively with methionine (rs = 0.51, p < 0.0001) and inversely with cystine (rs = -0.57, p < 0.0001) consistent with a metabolic block downstream of homocysteine. In patients with remethylation disorders, methionine was mostly normal on therapy, and inversely correlated with tHcys (rs = -0.57, p < 0.0001) demonstrating effectiveness of hydroxycobalamin and/or betaine in stimulating tHcys remethylation. Betaine also significantly increased sarcosine from its pre-treatment level on average 19-fold in remethylation disorders and 3-fold in CBS deficiency, with sarcosine > 5 μmol/L being 97% sensitive and 95% specific for betaine therapy. These results show that existing therapies improve sulfur amino acid metabolism without completely normalizing it and that sarcosine can determine compliance to betaine supplementation.

Estimated prevalence of moderate to severely elevated total homocysteine levels in the United States: A missed opportunity for diagnosis of homocystinuria?

Classical homocystinuria (HCU) is a genetic disorder caused by mutations in the cystathionine beta synthase gene, which results in impaired metabolism of the sulfur-bearing amino acid homocysteine and its accumulation in blood and tissues. Classical HCU can be detected via newborn screening in the United States, but the test is widely acknowledged to miss many patients. While severely elevated homocysteine levels (>100 μmol /L) frequently lead to a classical HCU diagnosis, intermediate levels (>30 to 100 μmol /L), though linked to many of the known complications of HCU, are not always recognized as associated with HCU. We aimed to identify and describe potentially undiagnosed classical HCU patients using a nationally-representative database of administrative claims and laboratory results. We estimated the national prevalence of patients with homocysteine >30 μmol /L, and compared their demographic and clinical characteristics to those of patients with homocysteine levels ≤30 μmol/L. Among 57,580 patients with a homocysteine test result, 1.8% had a value >30 μmol /L. Patients with homocysteine >30 μmol /L were more frequently diagnosed with hypothyroidism (39.2% vs. 20.7%, p < .001) and renal disease (9.7% vs. 5.5%, p < .001), and were more likely to have a prescription for an anxiolytic/antidepressant (44.5% vs. 38.9%), opioid (58.4% vs. 53.1%), steroid (46.4% vs. 42.5%), or thyroid hormone (38.8% vs. 18.8%), compared to patients with homocysteine ≤30 μmol /L (all p < .05). Both groups were equally likely to have a diagnosis of homocystinuria or another disorder of sulfur-bearing amino acid metabolism (3.8% vs. 4.0%, p = .752). The age-adjusted national prevalence of homocysteine >30 μmol /L was estimated at 33,068 (95% CI: 1033 - 35,104). These findings suggest that thousands of people in the US may be living with intermediate to severely elevated homocysteine levels and may require further evaluation for the presence of classical HCU.

The correlation between the evolution of bilateral basal ganglia hemorrhage using MR imaging and neurological damage recovery in an infant with methylmalonic aciduria

PURPOSE

We report on one patient with methylmalonic acidemia (MMA) who presented with symmetrical hemorrhage of the caudate nucleus accompanied by severe ventricular dilatation, follow-up magnetic resonance imaging (MRI) findings from one year later, and the clinical manifestations, neuropsychological scores, genetic test results, urine and blood laboratory results and evolution of the disorder.

MATERIALS AND METHODS

This study describes the recent and remote neuropathologic findings, reviews the literature, and discusses the possible pathogenetic mechanisms of these central nervous system lesions.

RESULTS

Patients with MMA may have signs of basal ganglia hemorrhage during acute onset, and the hemorrhage may disappear after treatment. During the treatment, both laboratory examination indexes and neuropsychological scores improved. There was a correlation between the evolution of bilateral basal ganglia hemorrhage using MRI(magnetic resonance imaging) and neurological damage recovery in this infant with MMA.

CONCLUSION

There was a correlation between the evolution of the bilateral basal ganglia hemorrhage using MR imaging and neurological damage recovery in an infant with MMA. We recommend performing conventional MR and diffusion-weighted imaging (DWI) examinations in patients with MMA who present with neurological symptoms.

Prevalence, characteristics, and costs of diagnosed homocystinuria, elevated homocysteine, and phenylketonuria in the United States: a retrospective claims-based comparison

Background

Classical homocystinuria (HCU), an inborn error of homocysteine metabolism, has previously been estimated to affect approximately 1 in 100,000–200,000 people in the United States (US). HCU is poorly detected by newborn screening, resulting in underestimates of its prevalence. This study compared characteristics, healthcare use and costs, and projected prevalence between patients with diagnosed HCU, elevated total homocysteine (tHcy), and diagnosed phenylketonuria (PKU).

Methods

Patients in the MarketScan® Research Databases were identified with strictly-defined HCU (> 2 diagnoses, including 1 ICD-10), broadly-defined HCU (> 1 ICD-10), elevated tHcy (> 20 μmol/L) without an HCU diagnosis, or > 1 ICD-9/ICD-10 PKU diagnosis during 1/1/2010–12/31/2016 (first qualifying claim = index). Demographics and healthcare utilization and costs per patient per month (PPPM) were compared between all cohorts, frequencies of comorbidities and medications were compared between HCU and elevated tHcy patients, and healthcare provider types were assessed among HCU patients. The prevalence of patients meeting each cohort definition was projected to the United States (US) population.

Results

Patients with strictly-defined (N = 2450) and broadly-defined (N = 6613) HCU, and with elevated tHcy (N = 2017), were significantly older than PKU patients (N = 5120) (57 vs. 56 vs. 53 vs. 18 years; p < 0.05). Vitamin D deficiency, hyperlipidemia, folic acid/B vitamins, and lipid-lowering medications, among others, were more common among diagnosed HCU patients vs. those with elevated tHcy (all p < 0.05). Rates of healthcare utilization were generally higher among HCU and elevated tHcy patients, compared to PKU, though total healthcare costs were similar between groups. Most HCU patients (~ 38%) received their index diagnosis from a primary care physician; very few (~ 1%) had any claim from a geneticist during their enrollment. The age-adjusted national prevalence of HCU was projected at 31,162 (95% CI: 30,411 – 31,913; ~ 1 in 10,000 of the US population) using the broad definition.

Conclusions

The actual prevalence of HCU may be > 10 times prior estimates, at 1 in 10,000 in the US, and this study suggests that HCU is not being diagnosed until later in life. Improvements to newborn screening, detection in young children, and physician education regarding HCU among patients may be necessary to alleviate the burden of this genetic disease.

Methylmalonic Acidemia Complicated by Homocystinuria Diseases: a Report of Three Cases

This study aims to improve our understanding of methylmalonic acidemia (MMA) complicated by homocystinuria disease by analyzing the clinical characteristics, treatment response and prognosis of three patients. Hyperhomocysteinemia and developmental retardation were present in all patients, epilepsy was present in one patient, and hemolytic uremic syndrome was present in one patient. The conditions of two patients were complicated by pulmonary arterial hypertension, one patient by left pulmonary vein ectopic drainage to the coronary sinus and the other by noncompaction of the ventricular myocardium. The two MMA patients with the complication of severe pulmonary arterial hypertension died because of late diagnosis and irregular treatment of MMA. Echocardiography is necessary for patients with combined MMA and homocystinuria, and these patients are susceptible to cardiovascular disease. When a patient with combined MMA and homocystinuria has the complication of severe pulmonary arterial hypertension, the prognosis is poor.

Betaine anhydrous in homocystinuria: results from the RoCH registry

Background

The Registry of Adult and Paediatric Patients Treated with Cystadane® – Homocystinuria (RoCH) is a non-interventional, observational, multi-centre, post-authorization safety study that aimed to identify safety of betaine anhydrous (Cystadane®) in the treatment of patients with inborn errors of homocysteine metabolism (homocystinuria) in order to minimise the treatment associated risks and establish better knowledge on its clinical use. The registry included patients of all ages with homocystinuria who were treated with betaine anhydrous in conjunction with other therapies. Clinical data were collected retrospectively from 2007 to 2013, then prospectively up to February 2014. All adverse events (AEs) reported during the study were recorded. The clinical and biological status of patients was monitored at least once a year.

Results

A total of 125 patients with homocystinuria (adults [> 18 years]: 50; paediatric [≤18 years]: 75) were enrolled at 29 centres in France and Spain. Patients were treated with betaine anhydrous for a mean duration of 7.4 ± 4.3 years. The median total daily dose of betaine anhydrous at the first and last study visits was 6 g/day for cystathionine β-synthase (CBS)-deficient vitamin B6 responders and 9 g/day for methylenetetrahydrofolate reductase-deficient patients, while the median daily dose increased in CBS-deficient B6 non-responders (from 6 to 9 g/day) and cobalamin metabolism-defective patients (from 3 to 6 g/day) between the first and last visits. Treatment caused a mean overall reduction of 29% in plasma homocysteine levels in the study population. A total of 277 AEs were reported during the study, of which two non-serious AEs (bad taste and headache) and one serious AE (interstitial lung disease) were considered to be drug related. Overall, betaine anhydrous was well tolerated with no major safety concerns.

Conclusions

Data from the RoCH registry provided real-world evidence on the clinical safety and efficacy of betaine anhydrous in the management of homocystinuria in paediatric and adult patients.

Electronic supplementary material

The online version of this article (10.1186/s13023-019-1036-2) contains supplementary material, which is available to authorized users.

In silico and in vivo models for Qatari-specific classical homocystinuria as basis for development of novel therapies

Homocystinuria is a rare inborn error of methionine metabolism caused by cystathionine β-synthase (CBS) deficiency. The prevalence of homocystinuria in Qatar is 1:1,800 births, mainly due to a founder Qatari missense mutation, c.1006C>T; p.R336C (p.Arg336Cys). We characterized the structure-function relationship of the p.R336C-mutant protein and investigated the effect of different chemical chaperones to restore p.R336C-CBS activity using three models: in silico, ΔCBS yeast, and CRISPR/Cas9 p.R336C knock-in HEK293T and HepG2 cell lines. Protein modeling suggested that the p.R336C induces severe conformational and structural changes, perhaps influencing CBS activity. Wild-type CBS, but not the p.R336C mutant, was able to restore the yeast growth in ΔCBS-deficient yeast in a complementation assay. The p.R336C knock-in HEK293T and HepG2 cells decreased the level of CBS expression and reduced its structural stability; however, treatment of the p.R336C knock-in HEK293T cells with betaine, a chemical chaperone, restored the stability and tetrameric conformation of CBS, but not its activity. Collectively, these results indicate that the p.R336C mutation has a deleterious effect on CBS structure, stability, and activity, and using the chemical chaperones approach for treatment could be ineffective in restoring p.R336C CBS activity.

Reversible Cerebral White Matter Abnormalities in Homocystinuria

Striking MRI brain changes resembling leukoencephalopathy are rarely seen in classical homocystinuria. Our case suggests that reversible white matter changes (WMC) are linked to elevated plasma methionine levels arising during treatment.A 6-year-old boy with learning difficulties and a normal MRI brain scan was diagnosed with homocystinuria (initial total homocysteine 344 μmol/L and methionine 64 μmol/L). At the age of 6.5 years, he developed superior sagittal sinus (SSS) thrombosis. Antithrombotic and homocysteine-lowering treatments were started. Due to poor dietary compliance and betaine treatment, his methionine level reached 1,285 μmol/L, and left side weakness developed. Repeat MRI scan revealed new confluent WMC in previously myelinated brain areas. Further 3-month treatment with tighter dietary control significantly dropped his methionine level (233 μmol/L) with resolution of his neurological deficit and of radiological changes.We suggest a reversible toxicity from hypermethioninaemia as a possible source of cerebral WMC (secondary to a demyelinating process) in patients with homocystinuria. It highlights the importance of homocysteine-lowering treatment as a prevention and complete resolution of neurological complications. It also demonstrates the need to consider homocystinuria in a differential diagnosis of paediatric leukoencephalopathy.

Cobalamin D Deficiency Identified Through Newborn Screening

Cobalamin D deficiency (cblD) is one of the least common cobalamin metabolism disorders. It may result in isolated homocystinuria, isolated methylmalonic aciduria, or combined methylmalonic aciduria and homocystinuria (cblD-combined). Only seven cases of the combined cblD form have been reported to date. Due to the rarity of this disorder, the presentation and symptoms are not well described. We present an eighth case of the cblD-combined subtype, who had a positive newborn screen (NBS) on day of life 3. She was symptomatic and developed lethargy and poor oral intake at 8 days of life. She was treated with 10% dextrose, folinic acid, intramuscular hydroxocobalamin, and betaine. Despite the early initiation of treatment, she developed complications of the disease and was found to have abnormal brain imaging findings at 17 days of age and macular atrophy at 3 months of age and has global developmental delay. We provide detailed description of her presentation, her treatment, and her complications to aid in the understanding of this rare disorder, which is very similar to the more common cobalamin C disorder (cblC).

Improving the diagnosis of cobalamin and related defects by genomic analysis, plus functional and structural assessment of novel variants

Background

Cellular cobalamin defects are a locus and allelic heterogeneous disorder. The gold standard for coming to genetic diagnoses of cobalamin defects has for some time been gene-by-gene Sanger sequencing of individual DNA fragments. Enzymatic and cellular methods are employed before such sequencing to help in the selection of the gene defects to be sought, but this is time-consuming and laborious. Furthermore some cases remain undiagnosed because no biochemical methods have been available to test for cobalamin absorption and transport defects.

Results

This paper reports the use of massive parallel sequencing of DNA (exome analysis) for the accurate and rapid genetic diagnosis of cobalamin-related defects in a cohort of affected patients. The method was first validated in an initial cohort with different cobalamin defects. Mendelian segregation, the frequency of mutations, and the comprehensive structural and functional analysis of gene variants, identified disease-causing mutations in 12 genes involved in the absorption and synthesis of active cofactors of vitamin B₁₂ (22 cases), and in the non-cobalamin metabolism-related genes ACSF3 (in four biochemically misdiagnosed patients) and SUCLA2 (in one patient with an unusual presentation). We have identified thirteen new variants all classified as pathogenic according to the ACGM recommendation but four were classified as variant likely pathogenic in MUT and SUCLA2. Functional and structural analysis provided evidences to classify them as pathogenic variants.

Conclusions

The present findings suggest that the technology used is sufficiently sensitive and specific, and the results it provides sufficiently reproducible, to recommend its use as a second-tier test after the biochemical detection of cobalamin disorder markers in the first days of life. However, for accurate diagnoses to be made, biochemical and functional tests that allow comprehensive clinical phenotyping are also needed.

Electronic supplementary material

The online version of this article (10.1186/s13023-018-0862-y) contains supplementary material, which is available to authorized users.

The effects of low protein products availability on growth parameters and metabolic control in selected amino acid metabolism disorders patients

BACKGROUND

In Saudi Arabia, a diet for life policy has been adopted in the management of amino acid metabolism disorders for years. However, the specially designed low protein products/medical foods - which are one of the important treatment tools - were not available up until several years ago in Saudi Arabia (SA). Our aim was to measure the compliance and quality of life in patients affected with these disorders followed in the metabolic nutrition clinic at King Faisal Specialist Hospital & Research Centre (KFSH&RC), Riyadh, SA.

METHODOLOGY

We used a non-randomized retrospective/prospective study which utilized the growth parameters, biochemical data of patients plus questionnaires collected from patients and their family/caregivers. A total of n = 182 patients affected with selected amino acid metabolism disorders were enrolled. Some were excluded n = 84 for various reasons. Sample analyzed were: Phenylketonuria (PKU) (44), Maple Syrup Urine Disease (MSUD) (30), Tyrosinemia (TYR) (17) and Homocystinuria (HCU) (7). Tandem Mass Spectrometry (TMS) used to quantitate plasma amino acid concentrations. Data was obtained using (COMPLE) Microsoft-Access which was designed by the metabolic nutrition clinic at KFSH&RC-Riyadh. Student's paired t-test was used to investigate relationship between variables.

RESULTS

The main findings were the improvement of selected amino acid levels pre and post the usage of medical foods. In PKU patients, the TMS Phenylalanine (PHE) levels post usage was significantly decreased (P value < .0001). This was also the case in MSUD patients with significant decrease in Leucine & Isoleucine levels (P value .0008) but not in Valine levels (P value .1148) as 36.7% of them received Valine supplements while enrolled in the study.

CONCLUSION

Low protein products availability was successful in improving outcomes for selected amino acid metabolic disorders. However, due to compliance issues and impracticality of the diet, the results were not significant in all enrolled patients.

Late-onset cobalamin C deficiency Chinese sibling patients with neuropsychiatric presentations

The Cobalamin C deficiency (cblC), characterized with elevated methylmalonic acidemia and homocystinuria in plasma, is an inborn error of cobalamin metabolism. The late-onset cblC siblings patients were rarely reported. In this study, we analyzed the clinical presentations and treatment outcomes of late-onset cblC in Chinese sibling patients with neuropsychiatric presentations. The clinical data of four pairs of Chinese patients were retrospectively analyzed. Serum homocysteine, urine organic acids measurements, neuroimaging exams and gene analysis were carried out in all patents. Patients were reevaluated after treatments with cobalamin, folate, betaine, L-carnitine and compound vitamin B. The mean age at disease onset was 13.7 (range 2-19) years. The neuropsychiatric disturbances including cognitive decline (3/8), psychiatric disturbances (4/8), gait instability (2/8), lower extremity weakness and numbness (3/8) and thromboembolic events (1/8). Two patients suffered nephropathy. The mean serum homocysteine when patients were diagnosed was 109.4 (range 69.5-138) μM/L. The abnormal radioimaging included scoliosis by X-ray (5/6), cerebral atrophy (4/6) and spinal cord atrophy (3/6) by MRI scan. Three pairs of siblings showed heterozygous mutations of MMACHC gene including c.482G > A (4/6), c.354G > C (2/6), c.570insT (2/6), c.445_446del (2/6) and c.656_4658del (2/6). The other two siblings showed homozygous mutation with c.452A > G in MMACHC gene. After treatments, the psychiatric symptoms were obviously relieved in all the patients. In Chinese siblings with late-onset cblC, the main clinic manifestation and abnormal radioimaging were cognitive decline and cerebral atrophy respectively. The most common gene mutation was c.482G > A of MMACHC gene. The patients responded well to the treatments.

Eight novel mutations of CBS gene in nine Chinese patients with classical homocystinuria

BACKGROUND

Classical homocystinuria (homocysteinemia type 1, MIM# 236200) is a rare inherited disorder in Mainland China. This study aimed to identify mutations in the cystathionine β-synthase (CBS) gene which are associated with classical homocystinuria in nine Chinese patients.

METHODS

Nine Chinese patients were diagnosed at the age of 5 years 4 months to 18 years by plasma total homocysteine and blood methionine determination. CBS gene analysis was performed for the patients and their families.

RESULTS

All nine patients had significantly increased plasma total homocysteine (142-500 μmol/L vs. the normal range of 0-15 μmol/L) and blood methionine (144.3-500 μmol/L vs. the normal range of 0-50 μmol/L). None of the patients was pyridoxine responsive. Eleven mutations in CBS gene were identified in the nine patients. Eight mutations (IVS3+1G>A, p.Thr493fsX46, p.Thr236Asn, p.Leu230Gln, p.Lys72Ile, p.Ser201ProfsX36, p.Met337IlefsX115, and IVS14-1G>C) were novel. Three mutations (p.Arg125Gln, p.Thr257Met and p.Gly116Arg) had been previously reported.

CONCLUSIONS

In this study, eight novel mutations in CBS were identified in nine Chinese patients with classical homocystinuria. None of the hotspot mutations reported in other regions previously was found. These data indicated that Chinese maybe had different CBS mutation spectrum from other population. The identification of mutations not only confirms the diagnosis but also enables accurate genetic counselling and prenatal diagnosis for the fetuses of the families.

Crystal structure of cystathionine β-synthase from honeybee Apis mellifera

Cystathionine β-synthase (CBS), the key enzyme in the transsulfuration pathway, links methionine metabolism to the biosynthesis of cellular redox controlling molecules. CBS catalyzes the pyridoxal-5'-phosphate-dependent condensation of serine and homocysteine to form cystathionine, which is subsequently converted into cysteine. Besides maintaining cellular sulfur amino acid homeostasis, CBS also catalyzes multiple hydrogen sulfide-generating reactions using cysteine and homocysteine as substrates. In mammals, CBS is activated by S-adenosylmethionine (AdoMet), where it can adopt two different conformations (basal and activated), but exists as a unique highly active species in fruit fly Drosophila melanogaster. Here we present the crystal structure of CBS from honeybey Apis mellifera, which shows a constitutively active dimeric species and let explain why the enzyme is not allosterically regulated by AdoMet. In addition, comparison of available CBS structures unveils a substrate-induced closure of the catalytic cavity, which in humans is affected by the AdoMet-dependent regulation and likely impaired by the homocystinuria causing mutation T191M.

Identification of ABC transporters acting in vitamin B12 metabolism in Caenorhabditis elegans

Vitamin B₁₂ (cobalamin, Cbl) is a micronutrient essential to human health. Cbl is not utilized as is but must go through complex subcellular and metabolic processing to generate two cofactor forms: methyl-Cbl for methionine synthase, a cytosolic enzyme; and adenosyl-Cbl for methylmalonyl-CoA mutase, a mitochondrial enzyme. Some 10-12 human genes have been identified responsible for the intracellular conversion of Cbl to cofactor forms, including genes that code for ATP-binding cassette (ABC) transporters acting at the lysosomal and plasma membranes. However, the gene for mitochondrial uptake is not known. We hypothesized that ABC transporters should be candidates for other uptake and efflux functions, including mitochondrial transport, and set out to screen ABC transporter mutants for blocks in Cbl utilization using the nematode roundworm Caenorhabditis elegans. Thirty-seven mutant ABC transporters were screened for the excretion of methylmalonic acid (MMA), which should result from loss of Cbl transport into the mitochondria. One mutant, wht-6, showed elevated MMA excretion and reduced [¹⁴C]-propionate incorporation, pointing to a functional block in methylmalonyl-CoA mutase. In contrast, the wht-6 mutant appeared to have a normal cytosolic pathway based on analysis of cystathionine excretion, suggesting that cytosolic methionine synthase was functioning properly. Further, the MMA excretion in wht-6 could be partially reversed by including vitamin B₁₂ in the assay medium. The human ortholog of wht-6 is a member of the G family of ABC transporters. We propose wht-6 as a candidate for the transport of Cbl into mitochondria and suggest that a member of the corresponding ABCG family of ABC transporters has this role in humans. Our ABC transporter screen also revealed that mrp-1 and mrp-2 mutants excreted lower MMA than wild type, suggesting they were concentrating intracellular Cbl, consistent with the cellular efflux defect proposed for the mammalian MRP1 ABC transporter.

Homocystinuria with Stroke and Positive Familial History

Homocystinuria is the second most common treatable aminoacidopathy. Clinically, affected patients present with eye, skeleton, central nervous system, and most importantly, vascular system abnormalities. This autosomal recessive disorder leads to accumulation of homocysteine and its metabolites in the blood and urine. In this report, we present a case with clinical and biochemical findings of homocystinuria with stroke and a positive familial history of the disease in her brother. A 4-year-old girl was admitted to pediatric emergency ward because of acute onset of right hemiparesis and subsequent generalized tonic–clonic seizures. Cranial magnetic resonance imaging revealed acute infarct areas in the left cerebral hemisphere. Metabolic screening revealed elevated concentrations of serum homocysteine and methionine and a normal serum concentration of vitamin B12. These findings, along with a positive familial history led to the diagnosis of homocystinuria. In any child who presents with stroke, some rare condition such as homocystinuria should be considered in diagnosis.

Ocular Pseudoexfoliation Syndrome Linkage to Cardiovascular Disease

PURPOSE OF REVIEW

Pseudoexfoliation syndrome (PEX) is a common cause of open-angle glaucoma that is characterized by stress-induced elastic microfibrillopathy related to an accumulation of matrix metalloproteinases. The accumulation of matrix metalloproteinases increases deposition of protein substance within ocular structures and other organs including the heart. Many studies have associated the presence of cardiovascular disease with pseudoexfoliation syndrome, but much debate exists between studies in terms of significant relationships. The following meta-analysis aims to relate pseudoexfoliation syndrome with certain cardiovascular events and disorders. A thorough literature review was performed to acquire information concerning PEX patients with certain cardiovascular disorders. Diseases considered included myocardial infarction, ischemic heart disease, angina, congestive heart failure, cardiomyopathy, aortic aneurysm, hypertension, and homocystinuria. Patients without evidence of pseudoexfoliation disease were the controls of our study. Multiple forest plots were created to compile and analyze collected data for statistical comparison.

RECENT FINDINGS

From a literature review, 18 studies were selected for our analysis. Cardiovascular disorders that had a statistically significant association (within a 95 % confidence interval) with PEX included ischemic heart disease, aortic aneurysms, and homocystinuria. The association between ischemic heart disease and PEX was statistically significant (p = 0.045). Myocardial infarction, chronic ischemic heart disease, angina, and hypertension did not show a correlation of relationship with the presence of pseudoexfoliation. Patients with PEX are prone to present with ischemic heart disease in addition to abdominal aortic aneurysms and homocystinuria. Patients that present with PEX should be screened for these detrimental cardiovascular disorders.

Cerebral venous sinus thrombosis in homocystinuria: Dietary intervention in conjunction with anticoagulation

The presentation of cerebral venous sinus thrombosis may be acute or chronic with a progressive clinical course. The diagnosis can be challenging, and there are several clinical syndromes associated with the disease. It is also an uncommon but recognised complication of homocystinuria. We describe a case where early anticoagulation, together with dietary intervention, was associated with a favourable clinical outcome.

Cystathionine β-synthase deficiency: Of mice and men

Cystathionine β-synthase (CBS) deficiency (Online Mendelian Inheritance in Man [OMIM] 236,200) is an autosomal recessive disorder that is caused by mutations in the CBS gene. It is the most common inborn error of sulfur metabolism and is the cause of classical homocystinuria, a condition characterized by very high levels of plasma total homocysteine and methionine. Although recognized as an inborn error of metabolism over 60years ago, these is still much we do not understand related to how this specific metabolic defect gives rise to its distinct phenotypes. To try and answer these questions, several groups have developed mouse models on CBS deficiency. In this article, we will review various mouse models of CBS deficiency and discuss how these mouse models compare to human CBS deficient patients.

Cystathionine beta-synthase deficiency alters hepatic phospholipid and choline metabolism: Post-translational repression of phosphatidylethanolamine N-methyltransferase is a consequence rather than a cause of liver injury in homocystinuria

Classical homocystinuria (HCU) due to inactivating mutation of cystathionine β-synthase (CBS) is a poorly understood life-threatening inborn error of sulfur metabolism. A previously described cbs-/- mouse model exhibits a semi-lethal phenotype due to neonatal liver failure. The transgenic HO mouse model of HCU exhibits only mild liver injury and recapitulates multiple aspects of the disease as it occurs in humans. Disruption of the methionine cycle in HCU has the potential to impact multiple aspect of phospholipid (PL) metabolism by disruption of both the Kennedy pathway and phosphatidylethanolamine N-methyltransferase (PEMT) mediated synthesis of phosphatidylcholine (PC). Comparative metabolomic analysis of HO mouse liver revealed decreased levels of choline, and choline phosphate indicating disruption of the Kennedy pathway. Alterations in the relative levels of multiple species of PL included significant increases in PL degradation products consistent with enhanced membrane PL turnover. A significant decrease in PC containing 20:4n6 which primarily formed by the methylation of phosphatidylethanolamine to PC was consistent with decreased flux through PEMT. Hepatic expression of PEMT in both the cbs-/- and HO models is post-translationally repressed with decreased levels of PEMT protein and activity that inversely-correlates with the scale of liver injury. Failure to induce further repression of PEMT in HO mice by increased homocysteine, methionine and S-adenosylhomocysteine or depletion of glutathione combined with examination of multiple homocysteine-independent models of liver injury indicated that repression of PEMT in HCU is a consequence rather than a cause of liver injury. Collectively, our data show significant alteration of a broad range of hepatic PL and choline metabolism in HCU with the potential to contribute to multiple aspects of pathogenesis in this disease.

Combined methylmalonic acidemia and homocysteinemia presenting predominantly with late-onset diffuse lung disease: a case series of four patients

Combined methylmalonic acidemia (MMA) and homocysteinemia are a group of autosomal recessive disorders caused by inborn errors of cobalamin metabolism, including CblC, D, F, and J, with cblC being the most common subtype. The clinical manifestations of combined MMA and homocysteinemia vary, but typically include neurologic, developmental and hematologic abnormalities.

We report 4 children with combined MMA and homocysteinemia who presented predominantly with late-onset diffuse lung diseases (DLD). Of these, 3 accompanied by pulmonary arterial hypertension (PAH), 1 accompanied by hypertension, and 2 accompanied by renal thrombotic microangiopathy (TMA), which was confirmed by renal biopsy. This confirms combined MMA and homocysteinemia should be considered in the differential diagnosis of DLD with or without PAH or renal TMA.

Oxidative Stress in Homocystinuria Due to Cystathionine ß-Synthase Deficiency: Findings in Patients and in Animal Models

Homocystinuria is an inborn error of amino acid metabolism caused by deficiency of cystathionine ß-synthase (CBS) activity, biochemically characterized by homocysteine (Hcy) and methionine (Met) accumulation in biological fluids and high urinary excretion of homocystine. Clinical manifestations include thinning and lengthening of long bones, osteoporosis, dislocation of the ocular lens, thromboembolism, and mental retardation. Although the pathophysiology of this disease is poorly known, the present review summarizes the available experimental findings obtained from patients and animal models indicating that oxidative stress may contribute to the pathogenesis of homocystinuria. In this scenario, several studies have shown that enzymatic and non-enzymatic antioxidant defenses are decreased in individuals affected by this disease. Furthermore, markers of lipid, protein, and DNA oxidative damage have been reported to be increased in blood, brain, liver, and skeletal muscle in animal models studied and in homocystinuric patients, probably as a result of increased free radical generation. On the other hand, in vitro and in vivo studies have shown that Hcy induces reactive species formation in brain, so that this major accumulating metabolite may underlie the oxidative damage observed in the animal model and human condition. Taken together, it may be presumed that the disruption of redox homeostasis may contribute to the tissue damage found in homocystinuria. Therefore, it is proposed that the use of appropriate antioxidants may represent a novel adjuvant therapy for patients affected by this disease.

Homocystinuria with Cerebral Venous Sinus Thrombosis: Excellent Recovery with Intravenous Recombinant Tissue Plasminogen Activator

Hyperhomocysteinemia can cause cerebral venous thrombosis. Recombinant tissue plasminogen activator is one of the treatment options for cerebral venous thrombosis in selected cases. We present here a 7-year-old boy with homocysteinuria with stroke. MRI of brain showed cerebral venous sinus thrombosis. We successfully treated with intravenous recombinant tissue plasminogen activator. He recovered completely without any complications. Recombinant tissue plasminogen activator can be considered one of the treatment options in cerebral venous thrombosis in homocystinura.

Targeted exome sequencing for the identification of complementation groups in methylmalonic aciduria: A south Indian experience

OBJECTIVES

In view of high incidence of methylmalonic aciduria (MMA) among South Indians, we have performed clinical, biochemical and molecular genetic evaluation of fifteen patients.

DESIGN AND METHODS

Targeted exome sequencing was performed for a panel of MMA causing genes i.e. MUT, ABCD4, ACSF3, CD320, LMBRD1, MCEE, MMAA, MMAB, MMACHC, MMADHC.

RESULTS

Methylmalonyl-CoA mutase (MUT), MMAB and MMAA genetic variants were found to contribute towards 40%, 33.3% and 6.6% etiology, respectively. Early onset of the disease (during the neonatal period) and presence of MUT and MMAB genetic variants was shown to be associated with higher mortality. The patients with MMAA variants had a milder disease. Among the identified mutations, 66% were already known. Three novel mutations, i.e.MUTp.Ala376Serfs, MMAB p.Glu112* and MMAA p.Tyr24* were identified. We also report three novel variants with predicted pathogenicity, MMAA intron 3 c.562+1_562+2insT, p.Ala668Pro in exon 12 of one of the alleles of the MUT gene and c.519+1G>A in intron 6 of one of the alleles in MMAB gene. We performed prenatal diagnosis in five of these families.

CONCLUSIONS

MMA among South Indian patients is genetically heterogeneous, caused by different complementation groups. Both B12-responsive and non-responsive patients were diagnosed. In biochemically diagnosed patients, targeted exome sequencing is cost effective to identify different MMA causing mutations and facilitate genetic counseling.

The remarkable S. Harvey Mudd - A reminiscence

Harvey Mudd was the father of methionine metabolic disorders. Beginning with his identification of the enzyme defect in homocystinuria, he co-discovered cobalamin C disorder as the first known human disorder of vitamin B12 metabolism, thereby extending our concept of homocystinuria as a key feature of related disorders rather than a single disease, and identified new disorders that produce hypermethioninemia. He had no equal in our understanding of how critical methionine metabolism is to human homeostasis.

Homocystinuria: Therapeutic approach

Homocystinuria is a disorder of sulfur metabolism pathway caused by deficiency of cystathionine β-synthase (CBS). It is characterized by increased accumulation of homocysteine (Hcy) in the cells and plasma. Increased homocysteine results in various vascular and neurological complications. Present strategies to lower cellular and plasma homocysteine levels include vitamin B6 intake, dietary methionine restriction, betaine supplementation, folate and vitamin B12 administration. However, these strategies are inefficient for treatment of homocystinuria. In recent years, advances have been made towards developing new strategies to treat homocystinuria. These mainly include functional restoration to mutant CBS, enhanced clearance of Hcy from the body, prevention of N-homocysteinylation-induced toxicity and inhibition of homocysteine-induced oxidative stress. In this review, we have exclusively discussed the recent advances that have been achieved towards the treatment of homocystinuria. The review is an attempt to help clinicians in developing effective therapeutic strategies and designing novel drugs against homocystinuria.

Low bone mineral density is a common finding in patients with homocystinuria

Homocystinuria (HCU) due to deficiency of cystathionine beta-synthetase is associated with increased plasma levels of homocysteine and methionine and is characterized by developmental delay, intellectual impairment, ocular defects, thromboembolism and skeletal abnormalities. HCU has been associated with increased risk for osteoporosis in some studies, but the natural history of HCU-related bone disease is poorly understood. The objective of this study was to characterize bone mineral density (BMD) measured by dual energy X-ray absorptiometry (DXA) in a multi-center, retrospective cohort of children and adults with HCU. We identified 19 subjects (9 males) aged 3.5 to 49.2 years who had DXA scans performed as a part of routine clinical care from 2002-2010. The mean lumbar spine (LS) BMD Z-score at the time of first DXA scan in this cohort was -1.2 (± SD of 1.3); 38% of participants had low BMD for age (as defined by a Z-score ≤-2). Homocysteine and methionine were positively associated with LS BMD Z-score in multiple linear regression models. Our findings suggest that low BMD is common in both children and adults with HCU and that routine assessment of bone health in this patient population is warranted. Future studies are needed to clarify the relationship between HCU and BMD.

Molecular and biochemical investigations of patients with intermediate or severe hyperhomocysteinemia

A discrepancy has been identified between numbers of expected and identified patients with homocystinuria due to cystathionine beta-synthase (CBS) deficiency. Patients homozygous for the frequent c.833T>C (p.I278T) are most often responsive to vitamin B6, and can present with a total-homocysteine (tHcy) <100 μM on a normal diet. In Denmark, patients with tHcy <100 μM are not routinely sequenced for CBS(2) mutations. This study investigated the prevalence of CBS mutations and the common methylenetetrahydrofolate reductase (MTHFR) c.677C>T polymorphism in patients with tHcy ≥ 50 μM and the association with clinical manifestations. We studied a cohort of patients with intermediate and severe hyperhomocysteinemia (tHcy ≥ 50 μM) determined between 1996 and 2011. Among the 413 eligible patients, 184 (45%) patients agreed to participate in the present follow-up study. A MTHFR(3)c.677TT genotype was found in 49% of the patients. Eight patients were found to have mutations in CBS(2). Of those, two were homozygous for c.833T>C (p.I278T), and four were compound heterozygous for c.833T>C. One c.833T>C (p.I278T) compound heterozygote was identified by lowering the threshold for sequencing from tHcy at 100 μM to 50 μM. The most prominent clinical presentation among patients with a CBS(2) mutation was thrombosis presenting at a median age of 25 years. In case of arterial or venous thrombosis without any explanation in individuals below 40 years, tHcy should be part of the thrombophilia screening. When tHcy is between 50 and 100 μM genotyping for the MTHFR(3) c.677TT is relevant, and when tHcy >100 μM CBS should be genotyped.

MMACHC gene mutation in familial hypogonadism with neurological symptoms

Recent studies have convincingly documented that hypogonadism is a component of various hereditary disorders and is often recognized as an important clinical feature in combination with various neurological symptoms, yet, the causative genes in a few related families are still unknown. High-throughput sequencing has become an efficient method to identify causative genes in related complex hereditary disorders. In this study, we performed exome sequencing in a family presenting hypergonadotropic hypogonadism with neurological presentations of mental retardation, epilepsy, ataxia, and leukodystrophy. After bioinformatic analysis and Sanger sequencing validation, we identified compound heterozygous mutations: c.482G>A (p.R161Q) and c.609G>A (p.W203X) in MMACHC gene in this pedigree. MMACHC was previously confirmed to be responsible for methylmalonic aciduria (MMA) combined with homocystinuria, cblC type (cblC disease), a hereditary vitamin B12 metabolic disorder. Biochemical and gas chromatography-mass spectrometry (GC-MS) examinations in this pedigree further supported the cblC disease diagnosis. These results indicated that hypergonadotropic hypogonadism may be a novel clinical manifestation of cblC disease, but more reports on additional patients are needed to support this hypothesis.

Sex-specific dysregulation of cysteine oxidation and the methionine and folate cycles in female cystathionine gamma-lyase null mice: a serendipitous model of the methylfolate trap

In addition to its role in the endogenous synthesis of cysteine, cystathionine gamma-lyase (CGL) is a major physiological source of the vasorelaxant hydrogen sulfide. Cgl null mice are potentially useful for studying the influence of this compound upon vascular tone and endothelial function. Here, we confirm a previous report that female Cgl null mice exhibit an approximate 45-fold increase in plasma total homocysteine compared to wild type controls. This level of homocysteine is approximately 3.5-fold higher than that observed in male Cgl null mice and is essentially equivalent to that observed in mouse models of cystathionine beta synthase deficient homocystinuria. Cgl null mice of both sexes exhibited decreased expression of methylenetetrahydrofolate reductase and cysteinesulfinate decarboxylase compared to WT controls. Female Cgl null mice exhibited a sex-specific induction of betaine homocysteine S-methyltransferase and methionine adenosyltransferase 1, alpha and a 70% decrease in methionine synthase expression accompanied by significantly decreased plasma methionine. Decreased plasma cysteine levels in female Cgl null mice were associated with sex-specific dysregulation of cysteine dioxygenase expression. Comparative histological assessment between cystathionine beta-synthase and Cgl null mice indicated that the therapeutic potential of cystathionine against liver injury merits possible further investigation. Collectively, our data demonstrates the importance of considering sex when investigating mouse models of inborn errors of metabolism and indicate that while female Cgl null mice are of questionable utility for studying the physiological role of hydrogen sulfide, they could serve as a useful model for studying the consequences of methionine synthase deficiency and the methylfolate trap.