Mutations in mut methylmalonic acidemia: clinical and enzymatic correlations

Compound Heterozygosis in AADC Deficiency and Its Complex Phenotype in Terms of AADC Protein Population

Aromatic amino acid decarboxylase (AADC) deficiency is a rare monogenic disease due to mutations in the ddc gene producing AADC, a homodimeric pyridoxal 5′-phosphate-dependent enzyme. The disorder is often fatal in the first decade and is characterized by profound motor impairments and developmental delay. In the last two years, there has been a net rise in the number of patients and variants identified, maybe also pushed by the ongoing gene therapy trials. The majority of the identified genotypes are compound heterozygous (about 70%). Efforts are underway to reach early diagnosis, find possible new markers/new fast methods, and predict clinical outcome. However, no clear correlation of genotype-to-phenotype exists to date. Nevertheless, for homozygous patients, reliable results have been obtained using genetic methods combined with available computational tools on crystal structures corroborated by biochemical investigations on recombinant homodimeric AADC variants that have been obtained and characterized in solution. For these variants, the molecular basis for the defect has been suggested and validated, since it correlates quite well with mildness/severity of the homozygous phenotype. Instead, prediction for compound heterozygous patients is more difficult since complementation effects could happen. Here, by analyzing the existing literature on compound heterozygosity in AADC deficiency and other genetic disorders, we highlight that, in order to assess pathogenicity, the measurement of activity of the AADC heterodimeric variant should be integrated by bioinformatic, structural, and functional data on the whole protein constellation theoretically present in such patients. A wider discussion on symptomatic heterozygosity in AADC deficiency is also presented.

Toxic Metabolites and Inborn Errors of Amino Acid Metabolism: What One Informs about the Other

In inborn errors of metabolism, such as amino acid breakdown disorders, loss of function mutations in metabolic enzymes within the catabolism pathway lead to an accumulation of the catabolic intermediate that is the substrate of the mutated enzyme. In patients of such disorders, dietarily restricting the amino acid(s) to prevent the formation of these catabolic intermediates has a therapeutic or even entirely preventative effect. This demonstrates that the pathology is due to a toxic accumulation of enzyme substrates rather than the loss of downstream products. Here, we provide an overview of amino acid metabolic disorders from the perspective of the ‘toxic metabolites’ themselves, including their mechanism of toxicity and whether they are involved in the pathology of other disease contexts as well. In the research literature, there is often evidence that such metabolites play a contributing role in multiple other nonhereditary (and more common) disease conditions, and these studies can provide important mechanistic insights into understanding the metabolite-induced pathology of the inborn disorder. Furthermore, therapeutic strategies developed for the inborn disorder may be applicable to these nonhereditary disease conditions, as they involve the same toxic metabolite. We provide an in-depth illustration of this cross-informing concept in two metabolic disorders, methylmalonic acidemia and hyperammonemia, where the pathological metabolites methylmalonic acid and ammonia are implicated in other disease contexts, such as aging, neurodegeneration, and cancer, and thus there are opportunities to apply mechanistic or therapeutic insights from one disease context towards the other. Additionally, we expand our scope to other metabolic disorders, such as homocystinuria and nonketotic hyperglycinemia, to propose how these concepts can be applied broadly across different inborn errors of metabolism and various nonhereditary disease conditions.

Compound heterozygosis in AADC deficiency: A complex phenotype dissected through comparison among heterodimeric and homodimeric AADC proteins

Compound heterozygosis is the most diffuse and hardly to tackle condition in aromatic amino acid decarboxylase (AADC) deficiency, a genetic disease leading to severe neurological impairment. Here, by using an appropriate vector, we succeeded in obtaining high yields of AADC protein and characterizing two new heterodimers, T69M/S147R and C281W/M362T, detected in two AADC deficiency patients. We performed an extensive biochemical characterization of the heterodimeric recombinant proteins and of the related homodimers, by a combination of dichroic and fluorescence spectroscopy and activity assays together with bioinformatic analyses. We found that T69M/S147R exhibits negative complementation in terms of activity but it is more stable than the average of the homodimeric counterparts. The heterodimer C281W/M362T retains a nearly good catalytic efficiency, whereas M362T homodimer is less affected and C281W homodimer is recovered as insoluble. These results, which are consistent with the related phenotypes, and the data emerging from previous studies, suggest that the severity of AADC deficiency is not directly explained by positive or negative complementation phenomena, but rather depends on: i) the integrity of one or both active sites; ii) the structural and functional properties of the entire pool of AADC proteins expressed. Overall, this integrated and cross-sectional approach enables proper characterization and depicts the functional result of subunit interactions in the dimeric structure and will help to elucidate the physio-pathological mechanisms in AADC deficiency.

1-13C-propionate breath testing as a surrogate endpoint to assess efficacy of liver-directed therapies in methylmalonic acidemia (MMA)

PURPOSE

To develop a safe and noninvasive in vivo assay of hepatic propionate oxidative capacity.

METHODS

A modified 1-¹³C-propionate breath test was administered to 57 methylmalonic acidemia (MMA) subjects, including 19 transplant recipients, and 16 healthy volunteers. Isotopomer enrichment (¹³CO₂/¹²CO₂) was measured in exhaled breath after an enteral bolus of sodium-1-¹³C-propionate, and normalized for CO₂ production. 1-¹³C-propionate oxidation was then correlated with clinical, laboratory, and imaging parameters collected via a dedicated natural history protocol.

RESULTS

Lower propionate oxidation was observed in patients with the severe mut⁰ and cblB subtypes of MMA, but was near normal in those with the cblA and mut^- forms of the disorder. Liver transplant recipients demonstrated complete restoration of 1-¹³C-propionate oxidation to control levels. 1-¹³C-propionate oxidation correlated with cognitive test result, growth indices, bone mineral density, renal function, and serum biomarkers. Test repeatability was robust in controls and in MMA subjects (mean coefficient of variation 6.9% and 12.8%, respectively), despite widely variable serum methylmalonic acid concentrations in the patients.

CONCLUSION

Propionate oxidative capacity, as measured with 1-¹³C-propionate breath testing, predicts disease severity and clinical outcomes, and could be used to assess the therapeutic effects of liver-targeted genomic therapies for MMA and related disorders of propionate metabolism.

TRIAL REGISTRATION

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

Mutation analysis of genes related to methylmalonic acidemia: identification of eight novel mutations

Methylmalonic acidemia (MMA), an inherited metabolic disease, results from genetic defects in methylmalonyl-CoA mutase or any of the proteins involved in adenosylcobalamin synthesis. This enzyme is classified into several complementation groups and genotypic classes. In this work we explain the biochemical, structural and genetic analysis of 25 MMA patients, from Iran. The diagnosis was established by the measurement of propionylcarnitine in blood using tandem mass spectrometry and confirmed using a gas chromatography-flame ionization detector. Using clinical, biochemical, structural and molecular analyses we identified 15 mut MMA, three cblA, one cblB, and four cblC-deficient patients. Among mutations identified in the MUT gene (MUT) only one, the c.1874A>C (p.D625A) variant, is likely a mut^- mutation. The remaining mutations are probably mut⁰. Here, we present the first molecular analysis of MMA in Iranian patients and have identified eight novel mutations. Four novel mutations (p.D625A, p.R326G, p.V157F, p.F379L) were seen exclusively in patients from northern Iran. One novel splice site mutation (c.2125-3C>G) in MUT and two novel mutation (p.N225M and p.A99P) in the MMAA gene were associated with patients from eastern Iran. The rs184829210 SNP was recognized only in patients with the novel c.958G>A (p.A320T) mutation. This study confirms pathogenesis of deficient enzyme activity in MUT, MMAA, MMAB, and MMACHC as previous observations. These results could act as a basis for the performance of pharmacological therapies for increasing the activity of proteins derived from these mutations.

Hyperammonaemia in classic organic acidaemias: a review of the literature and two case histories

Background

The ‘classic’ organic acidaemias (OAs) (propionic, methylmalonic and isovaleric) typically present in neonates or infants as acute metabolic decompensation with encephalopathy. This is frequently accompanied by severe hyperammonaemia and constitutes a metabolic emergency, as increased ammonia levels and accumulating toxic metabolites are associated with life-threatening neurological complications. Repeated and frequent episodes of hyperammonaemia (alongside metabolic decompensations) can result in impaired growth and intellectual disability, the severity of which increase with longer duration of hyperammonaemia. Due to the urgency required, diagnostic evaluation and initial management of patients with suspected OAs should proceed simultaneously. Paediatricians, who do not have specialist knowledge of metabolic disorders, have the challenging task of facilitating a timely diagnosis and treatment. This article outlines how the underlying pathophysiology and biochemistry of the organic acidaemias are closely linked to their clinical presentation and management, and provides practical advice for decision-making during early, acute hyperammonaemia and metabolic decompensation in neonates and infants with organic acidaemias.

Clinical management

The acute management of hyperammonaemia in organic acidaemias requires administration of intravenous calories as glucose and lipids to promote anabolism, carnitine to promote urinary excretion of urinary organic acid esters, and correction of metabolic acidosis with the substitution of bicarbonate for chloride in intravenous fluids. It may also include the administration of ammonia scavengers such as sodium benzoate or sodium phenylbutyrate. Treatment with N-carbamyl-L-glutamate can rapidly normalise ammonia levels by stimulating the first step of the urea cycle.

Conclusions

Our understanding of optimal treatment strategies for organic acidaemias is still evolving. Timely diagnosis is essential and best achieved by the early identification of hyperammonaemia and metabolic acidosis. Correcting metabolic imbalance and hyperammonaemia are critical to prevent brain damage in affected patients.

Metabolic etiologies in West syndrome

West syndrome (WS) is an early life epileptic encephalopathy associated with infantile spasms, interictal electroencephalography (EEG) abnormalities including high amplitude, disorganized background with multifocal epileptic spikes (hypsarrhythmia), and often neurodevelopmental impairments. Approximately 64% of the patients have structural, metabolic, genetic, or infectious etiologies and, in the rest, the etiology is unknown. Here we review the contribution of etiologies due to various metabolic disorders in the pathology of WS. These may include metabolic errors in organic molecules involved in amino acid and glucose metabolism, fatty acid oxidation, metal metabolism, pyridoxine deficiency or dependency, or acidurias in organelles such as mitochondria and lysosomes. We discuss the biochemical, clinical, and EEG features of these disorders as well as the evidence of how they may be implicated in the pathogenesis and treatment of WS. The early recognition of these etiologies in some cases may permit early interventions that may improve the course of the disease.

Hepatic Effects of Pharmacological Doses of Hydroxy-Cobalamin[c-lactam] in Mice

The vitamin B₁₂ analog hydroxy-cobalamin[c-lactam] (HCCL) impairs hepatic mitochondrial protein synthesis and function of the electron transport chain in rats. We aimed to establish an in vivo model for mitochondrial dysfunction in mice, which could be used to investigate hepatotoxicity of mitochondrial toxicants. In a first step, we performed a dose-finding study in mice treated with HCCL 0.4 mg/kg and 4 mg/kg i.p. for two to four weeks. The plasma methylmalonate concentration was strongly increased at 4 mg/kg starting at three weeks of treatment. We subsequently treated mice daily with 4 mg/kg HCCL i.p. for three weeks and characterized liver function and histology as well as liver mitochondrial function. We found an increase in liver weight in HCCL-treated mice, which was paralleled by hepatocellular accumulation of triglycerides. In liver homogenate of HCCL-treated mice, the complex I activity of the electron transport chain was reduced, most likely explaining hepatocellular triglyceride accumulation. The activity of CPT1 was not affected by methylmalonyl-CoA in isolated liver mitochondria. Despite impaired complex I activity, mitochondrial superoxide anion production was not increased and the hepatocellular glutathione (GSH) pool was maintained. Finally, the mitochondrial DNA content was not altered with HCCL treatment. In conclusion, treatment of mice with HCCL is associated with increased liver weight explained by hepatocellular triglyceride accumulation. Hepatocellular fat accumulation is most likely a consequence of impaired activity of the mitochondrial electron transport chain. The impairment of complex I activity is not strong enough to result in ROS accumulation and reduction of the GSH stores.

Methylmalonic and propionic acidemias: clinical management update

PURPOSE OF REVIEW

Recent clinical studies and management guidelines for the treatment of the organic acidopathies methylmalonic acidemia (MMA) and propionic acidemia address the scope of interventions to maximize health and quality of life. Unfortunately, these disorders continue to cause significant morbidity and mortality due to acute and chronic systemic and end-organ injury.

RECENT FINDINGS

Dietary management with medical foods has been a mainstay of therapy for decades, yet well controlled patients can manifest growth, development, cardiac, ophthalmological, renal, and neurological complications. Patients with organic acidopathies suffer metabolic brain injury that targets specific regions of the basal ganglia in a distinctive pattern, and these injuries may occur even with optimal management during metabolic stress. Liver transplantation has improved quality of life and metabolic stability, yet transplantation in this population does not entirely prevent brain injury or the development of optic neuropathy and cardiac disease.

SUMMARY

Management guidelines should identify necessary screening for patients with methylmalonic acidemia and propionic acidemia, and improve anticipatory management of progressive end-organ disease. Liver transplantation improves overall metabolic control, but injury to nonregenerative tissues may not be mitigated. Continued use of medical foods in these patients requires prospective studies to demonstrate evidence of benefit in a controlled manner.

Functional characterization and categorization of missense mutations that cause methylmalonyl-CoA mutase (MUT) deficiency

Methylmalonyl-CoA mutase (MUT) is an essential enzyme in propionate catabolism that requires adenosylcobalamin as a cofactor. Almost 250 inherited mutations in the MUT gene are known to cause the devastating disorder methylmalonic aciduria; however, the mechanism of dysfunction of these mutations, more than half of which are missense changes, has not been thoroughly investigated. Here, we examined 23 patient missense mutations covering a spectrum of exonic/structural regions, clinical phenotypes, and ethnic populations in order to determine their influence on protein stability, using two recombinant expression systems and a thermostability assay, and enzymatic function by measuring MUT activity and affinity for its cofactor and substrate. Our data stratify MUT missense mutations into categories of biochemical defects, including (1) reduced protein level due to misfolding, (2) increased thermolability, (3) impaired enzyme activity, and (4) reduced cofactor response in substrate turnover. We further demonstrate the stabilization of wild-type and thermolabile mutants by chemical chaperones in vitro and in bacterial cells. This in-depth mutation study illustrates the tools available for MUT enzyme characterization, guides future categorization of further missense mutations, and supports the development of alternative, chaperone-based therapy for patients not responding to current treatment.

Growth retardation, general hypotonia, and loss of acquired neuromotor skills in the infants of mothers with cobalamin deficiency and the possible role of succinyl-CoA and glycine in the pathogenesis

Vitamin B12 (cobalamin, Cbl) deficiency can cause metabolic, hematological, and neurological abnormalities. Adequate levels of succinyl-coenzyme A (CoA) cannot be synthesized from methylmalonyl-CoA because of the decreased activity of the methylmalonyl-CoA mutase enzyme that uses Cbl as the cofactor. Succinyl-CoA synthesis deficiency leads to decreased heme synthesis and gluconeogenesis. The reason of growth retardation can be gluconeogenesis deficiency together with heme synthesis deficiency whereas the reason of the neurological abnormalities can be glycine increase in the tissue due to decreased heme synthesis. We present 7 infants diagnosed with severe nutritional Cbl deficiency and discuss the role of succinyl-CoA and glycine in the possible pathogenesis in this article. Patients brought to our clinic with a complaint of growth retardation and diagnosed with nutritional Cbl deficiency were included in the study. There were 5 females and 2 males. The mean age was 11 ± 2.30 (range 6-13) months. All patients had general muscular hypotonia and 4 had growth retardation. Neuromotor growth retardation was found in 4 of the children who had previously shown normal neuromotor development for age. The mean Cbl level was 83.8 ± 27.6 (45.6-114) pg/mL. The mean Cbl level of the mothers was 155 ± 56.6 (88-258) pg/mL. Six of the patients had anemia and 1 had thrombocytopenia. Mean corpuscular volume value was 91.5 ± 12.2 fL. Following treatment, the muscle tonus of the patients improved, the anemia and growth retardation decreased, and the lost neuromotor abilities were recovered. Severe nutritional Cbl deficiency is an important nutritional disease where complications can be prevented with early treatment. When evaluating the pathogenesis, it should be noted that nutritional Cbl deficiency is a succinyl-CoA synthesis deficiency.

Mutation analysis of methylmalonyl CoA mutase gene exon 2 in Egyptian families: Identification of 25 novel allelic variants

Methylmalonic aciduria (MMA) is an autosomal recessive disorder of methylmalonate and cobalamin (cbl; vitamin B₁₂) metabolism. It is an inborn error of organic acid metabolism which commonly results from a defect in the gene encoding the methylmalonyl-CoA mutase (MCM) apoenzyme. Here we report the results of mutation study of exon 2 of the methylmalonyl CoA mutase (MUT) gene, coding MCM residues from 1 to 128, in ten unrelated Egyptian families affected with methylmalonic aciduria. Patients were presented with a wide-anion gap metabolic acidosis. The diagnosis has established by the measurement of C3 (propionylcarnitine) and C3:C2 (propionylcarnitine/acetylcarnitine) in blood by using liquid chromatography–tandem mass spectrometry (LC/MS–MS) and was confirmed by the detection of an abnormally elevated level of methylmalonic acid in urine by using gas chromatography–mass spectrometry (GC/MS) and isocratic cation exchange high-performance liquid-chromatography (HPLC). Direct sequencing of gDNA of the MUT gene exon 2 has revealed a total of 26 allelic variants: ten of which were intronic, eight were located upstream to the exon 2 coding region, four were novel modifications predicted to affect the splicing region, three were novel mutations within the coding region: c.15G > A (p.K5K), c.165C > A (p.N55K) and c.7del (p.R3EfsX14), as well as the previously reported mutation c.323G > A (p.R108H).

Novel B(12)-dependent acyl-CoA mutases and their biotechnological potential

The recent spate of discoveries of novel acyl-CoA mutases has engendered a growing appreciation for the diversity of 5'-deoxyadenosylcobalamin-dependent rearrangement reactions. The prototype of the reaction catalyzed by these enzymes is the 1,2 interchange of a hydrogen atom with a thioester group leading to a change in the degree of carbon skeleton branching. These enzymes are predicted to share common architectural elements: a Rossman fold and a triose phosphate isomerase (TIM)-barrel domain for binding cofactor and substrate, respectively. Within this family, methylmalonyl-CoA mutase (MCM) is the best studied and is the only member found in organisms ranging from bacteria to man. MCM interconverts (2R)-methylmalonyl-CoA and succinyl-CoA. The more recently discovered family members include isobutyryl-CoA mutase (ICM), which interconverts isobutyryl-CoA and n-butyryl-CoA; ethylmalonyl-CoA mutase, which interconverts (2R)-ethylmalonyl-CoA and (2S)-methylsuccinyl-CoA; and 2-hydroxyisobutyryl-CoA mutase, which interconverts 2-hydroxyisobutyryl-CoA and (3S)-hydroxybutyryl-CoA. A variant in which the two subunits of ICM are fused to a G-protein chaperone, IcmF, has been described recently. In addition to its ICM activity, IcmF also catalyzes the interconversion of isovaleryl-CoA and pivalyl-CoA. This review focuses on the involvement of acyl-CoA mutases in central carbon and secondary bacterial metabolism and on their biotechnological potential for applications ranging from bioremediation to stereospecific synthesis of C2-C5 carboxylic acids and alcohols, and for production of potential commodity and specialty chemicals.

Variable dietary management of methylmalonic acidemia: metabolic and energetic correlations

BACKGROUND

Isolated methylmalonic acidemia (MMA) is managed by dietary protein restriction and medical food supplementation. Resting energy expenditure (REE) can be depressed in affected individuals for undefined reasons.

OBJECTIVE

The objective was to document the spectrum of nutritional approaches used to treat patients with MMA, measure REE, and analyze the dependence of REE on body composition, biochemical, and nutritional variables.

DESIGN

Twenty-nine patients with isolated MMA (22 mut, 5 cblA, 2 cblB; 15 males, 14 females; age range: 2-35 y) underwent evaluation. REE was measured with open-circuit calorimetry and compared with predicted values by using age-appropriate equations.

RESULTS

Nutritional regimens were as follows: protein restriction with medical food (n = 17 of 29), protein restriction with medical food and supplemental isoleucine or valine (n = 5 of 29), or the use of natural protein alone for dietary needs (n = 7 of 29). Most mut patients had short stature and higher percentage fat mass compared with reference controls. Measured REE decreased to 74 ± 13.6% of predicted (P < 0.001) in the ≤ 18-y group (n = 22) and to 83 ± 11.1% (P = 0.004) in patients aged >18 y (n = 7). Linear regression modeling suggested that age (P = 0.001), creatinine clearance (P = 0.01), and height z score (P = 0.04) accounted for part of the variance of measured REE per kilogram of fat-free mass (model R² = 0.66, P < 0.0001).

CONCLUSIONS

There is wide variation in the dietary treatment of MMA. Standard predictive equations overestimate REE in this population primarily due to their altered body composition and decreased renal function. Defining actual energy needs will help optimize nutrition and protect individuals from overfeeding. This trial is registered at clinicaltrials.gov as NCT00078078.

Genetic disorders of vitamin B₁₂ metabolism: eight complementation groups--eight genes

Vitamin B12 (cobalamin, Cbl) is an essential nutrient in human metabolism. Genetic diseases of vitamin B12 utilisation constitute an important fraction of inherited newborn disease. Functionally, B12 is the cofactor for methionine synthase and methylmalonyl CoA mutase. To function as a cofactor, B12 must be metabolised through a complex pathway that modifies its structure and takes it through subcellular compartments of the cell. Through the study of inherited disorders of vitamin B12 utilisation, the genes for eight complementation groups have been identified, leading to the determination of the general structure of vitamin B12 processing and providing methods for carrier testing, prenatal diagnosis and approaches to treatment.

Advances in the understanding of cobalamin assimilation and metabolism

The haematological and neurological consequences of cobalamin deficiency define the essential role of this vitamin in key metabolic reactions. The identification of cubilin-amnionless as the receptors for intestinal absorption of intrinsic factor-bound cobalamin and the plasma membrane receptor for cellular uptake of transcobalamin bound cobalamin have provided a clearer understanding of the absorption and cellular uptake of this vitamin. As the genes involved in the intracellular processing of cobalamins and genetic defects of these pathways are identified, the metabolic disposition of cobalamins and the proteins involved are being recognized. The synthesis of methylcobalamin and 5'-deoxyadenosylcobalamin, their utilization in conjunction with methionine synthase and methylmalonylCoA mutase, respectively, and the metabolic consequences of defects in these pathways could provide insights into the clinical presentation of cobalamin deficiency.

Metabolic phenotype of methylmalonic acidemia in mice and humans: the role of skeletal muscle

Background

Mutations in methylmalonyl-CoA mutase cause methylmalonic acidemia, a common organic aciduria. Current treatment regimens rely on dietary management and, in severely affected patients, liver or combined liver-kidney transplantation. For undetermined reasons, transplantation does not correct the biochemical phenotype.

Methods

To study the metabolic disturbances seen in this disorder, we have created a murine model with a null allele at the methylmalonyl-CoA mutase locus and correlated the results observed in the knock-out mice to patient data. To gain insight into the origin and magnitude of methylmalonic acid (MMA) production in humans with methylmalonyl-CoA mutase deficiency, we evaluated two methylmalonic acidemia patients who had received different variants of combined liver-kidney transplants, one with a complete liver replacement-kidney transplant and the other with an auxiliary liver graft-kidney transplant, and compared their metabolite production to four untransplanted patients with intact renal function.

Results

Enzymatic, Western and Northern analyses demonstrated that the targeted allele was null and correctable by lentiviral complementation. Metabolite studies defined the magnitude and tempo of plasma MMA concentrations in the mice. Before a fatal metabolic crisis developed in the first 24–48 hours, the methylmalonic acid content per gram wet-weight was massively elevated in the skeletal muscle as well as the kidneys, liver and brain. Near the end of life, extreme elevations in tissue MMA were present primarily in the liver. The transplant patients studied when well and on dietary therapy, displayed massive elevations of MMA in the plasma and urine, comparable to the levels seen in the untransplanted patients with similar enzymatic phenotypes and dietary regimens.

Conclusion

The combined observations from the murine metabolite studies and patient investigations indicate that during homeostasis, a large portion of circulating MMA has an extra-heptorenal origin and likely derives from the skeletal muscle. Our studies suggest that modulating skeletal muscle metabolism may represent a strategy to increase metabolic capacity in methylmalonic acidemia as well as other organic acidurias. This mouse model will be useful for further investigations exploring disease mechanisms and therapeutic interventions in methylmalonic acidemia, a devastating disorder of intermediary metabolism.

Spectrum of mutations in mut methylmalonic acidemia and identification of a common Hispanic mutation and haplotype

Cobalamin nonresponsive methylmalonic acidemia (MMA, mut complementation class) results from mutations in the nuclear gene MUT, which codes for the mitochondrial enzyme methylmalonyl CoA mutase (MCM). To better elucidate the spectrum of mutations that cause MMA, the MUT gene was sequenced in 160 patients with mut MMA. Sequence analysis identified mutations in 96% of disease alleles. Mutations were found in all coding exons, but predominantly in exons 2, 3, 6, and 11. A total of 116 different mutations, 68 of which were novel, were identified. Of the 116 different mutations, 53% were missense mutations, 22% were deletions, duplications or insertions, 16% were nonsense mutations, and 9% were splice-site mutations. Sixty-one of the mutations have only been identified in one family. A novel mutation in exon 2, c.322C>T (p.R108C), was identified in 16 of 27 Hispanic patients. SNP genotyping data demonstrated that Hispanic patients with this mutation share a common haplotype. Three other mutations were seen exclusively in Hispanic patients: c.280G>A (p.G94R), c.1022dupA, and c.970G>A (p.A324T). Seven mutations were seen almost exclusively in black patients, including the previously reported c.2150G>T (p.G717V) mutation, which was identified in 12 of 29 black patients. Two mutations were seen only in Asian patients. Some frequently identified mutations were not population-specific and were identified in patients of various ethnic backgrounds. Some of these mutations were found in mutation clusters in exons 2, 3, 6, and 11, suggesting a recurrent mutation.

Mutation and haplotype analyses of the MUT gene in Japanese patients with methylmalonic acidemia

Methylmalonic acidemia (MMA) is caused by a deficiency in the activity of L: -methylmalonyl-CoA mutase (MCM), a vitamin B12 (or cobalamin, Cbl)-dependent enzyme. Apoenzyme-deficient MMA (mut MMA) results from mutations in the nuclear gene MUT. Most of the MUT mutations are thought to be private or restricted to only a few pedigrees. Our group elucidated the spectrum of mutations of Japanese mut MMA patients by performing mutation and haplotype analyses in 29 patients with mut MMA. A sequence analysis identified mutations in 95% (55/58) of the disease alleles. Five mutations were relatively frequent (p.E117X, c.385 + 5G > A, p.R369H, p.L494X, and p.R727X) and four were novel (p.M1V, c.753_753 + 5delGGTATA, c.1560G > C, and c.2098_2099delAT). Haplotype analysis suggested that all of the frequent mutations, with the exception of p.R369H, were spread by the founder effect. Among 46 Japanese patients investigated in the present and previous studies, 76% (70/92) of the mutations were located in exons 2, 6, 8, and 13. This finding - that a limited number of mutations account for most of the mutations in Japanese mut MMA patients - is in contrast with results of a previous study in Caucasian patients.

Assembly and protection of the radical enzyme, methylmalonyl-CoA mutase, by its chaperone

MeaB is a recently described P-loop GTPase that plays an auxiliary role in the reaction catalyzed by the radical B12 enzyme, methylmalonyl-CoA mutase. Defects in the human homologue of MeaB result in methylmalonic aciduria, but the role of this protein in coenzyme B12 assimilation and/or utilization is not known. Methylmalonyl-CoA mutase catalyzes the isomerization of methylmalonyl-CoA to succinyl-CoA that uses reactive radical intermediates that are susceptible to oxidative inactivation. In this study, we have examined the influence of MeaB on the kinetics of the reaction catalyzed by methylmalonyl-CoA mutase and on the thermodynamics of cofactor binding. MeaB alone has a modest effect on the affinity of the mutase for the 5'-deoxyadenosylcobalamin (AdoCbl) cofactor, increasing it 2-fold from 404 +/- 71 to 210 +/- 22 nM. However, in the presence of GDP, the affinity for the cofactor decreases 5-fold to 1.89 +/- 0.33 microM, while in the presence of guanosine 5'(beta-gamma imino)triphosphate, a nonhydrolyzable analogue of GTP, the binding of AdoCbl to the mutase is not detected. Protection against oxidative inactivation of the mutase by MeaB is dependent upon the presence of nucleotides with the MeaB/GDP and MeaB/GTP complexes decelerating the rate of formation of oxidized cofactor by 3- and 15-fold, respectively. This study suggests that MeaB functions in the GTP-dependent assembly of holomethylmalonyl-CoA mutase and subsequent protection of radical intermediates during catalysis.

Quantitative analysis of mitochondrial protein expression in methylmalonic acidemia by two-dimensional difference gel electrophoresis

Isolated methylmalonic acidemia (MMA) is a rare metabolic disease due to the deficient activity of L-methylmalonyl-CoA mutase (MCM). This mitochondrial enzyme converts L-methylmalonyl-CoA to succinyl-CoA using adenosylcobalamin (Adocbl) as cofactor. Isolated MMA is subdivided into five forms: mut MMA associated with MCM deficiency, three different defects related to mitochondrial Adocbl formation (cblA, cblB, and cblH), and cblD variant 2. We performed proteomic analysis on mitochondria from an individual with cblH/cblD disorder using 2-D DIGE to identify differentially expressed proteins in this disease. Comparative analysis of control/patient mitochondrial proteome allowed us to identify differential expression of 10 proteins. The most notable groups included proteins involved in apoptosis (cytochrome c), oxidative stress (manganese superoxide dismutase) and cell metabolism (succinyl-CoA ligase (GDP forming) and mitochondrial glycerophosphate dehydrogenase). Immunoblot analysis further validated 2-D DIGE results of two of these proteins in multiple MMA patients, suggesting that the differences in expression are a general effect in this disorder. It is feasible that the differential proteins identified in this study have a biological significance and might be related to the pathophysiology of MMA.

New insights into the pathophysiology of cobalamin deficiency

Cobalamin-deficient (Cbl-D) central neuropathy in the rat is associated with a locally increased expression of neurotoxic tumour necrosis factor-alpha (TNF-alpha) and a locally decreased expression of neurotrophic epidermal growth factor (EGF). These recent findings suggest that cobalamin oppositely regulates the expression of TNF-alpha and EGF, and raise the possibility that these effects might be independent of its coenzyme function. Furthermore, adult Cbl-D patients have high levels of TNF-alpha and low levels of EGF in the serum and cerebrospinal fluid. Serum levels of TNF-alpha and EGF of cobalamin-treated patients normalize concomitantly with haematological disease remission. These observations suggest that cobalamin deficiency induces an imbalance in TNF-alpha and EGF levels in biological fluids that might have a role in the pathogenesis of the damage caused by pernicious anaemia.

Genetic and genomic systems to study methylmalonic acidemia

Methylmalonic acidemia (MMAemia) is the biochemical hallmark of a group of genetic metabolic disorders that share a common defect in the ability to convert methylmalonyl-CoA into succinyl-CoA. This disorder is due to either a mutant methylmalonyl-CoA mutase apoenzyme or impaired synthesis of adenosylcobalamin, the cofactor for this enzyme. In this article, we will provide an overview of the pathways disrupted in these disorders, discuss the known metabolic blocks with a particular focus on molecular genetics, and review the use of selected model organisms to study features of methylmalonic acidemia.

Cobalamin (vitamin B12) in subacute combined degeneration and beyond: traditional interpretations and novel theories

Subacute combined degeneration (SCD) is a neuropathy due to cobalamin (Cbl) (vitamin B(12)) deficiency acquired in adult age. Hitherto, the theories advanced to explain the pathogenesis of SCD have postulated a causal relationship between SCD lesions and the impairment of either or both of two Cbl-dependent reactions. We have identified a new experimental model, the totally gastrectomized rat, to reproduce the key morphological features of the disease [spongy vacuolation, intramyelinic and interstitial edema of the white matter of the central nervous system (CNS), and astrogliosis], and found new mechanisms responsible for the pathogenesis of SCD: the neuropathological lesions in TGX rats are not only due to mere vitamin withdrawal but also to the overproduction of the myelinolytic tumor necrosis factor (TNF)-alpha and the reduced synthesis of the two neurotrophic agents, epidermal growth factor (EGF) and interleukin-6. This deregulation of the balance between TNF-alpha and EGF synthesis induced by Cbl deficiency has been verified in the sera of patients with pernicious anemia (but not in those with iron-deficient anemia), and in the cerebrospinal fluid (CSF) of SCD patients. These new functions are not linked to the coenzyme functions of the vitamin, but it is still unknown whether they involve genetic or epigenetic mechanisms. Low Cbl levels have also been repeatedly observed in the sera and/or CSF of patients with Alzheimer's disease or multiple sclerosis, but whether Cbl deficit plays a role in the pathogenesis of these diseases is still unclear.

Genetic analysis of three genes causing isolated methylmalonic acidemia: identification of 21 novel allelic variants

Isolated methylmalonic aciduria (MMA) is an inborn error of metabolism due to the impaired isomerization of l-methylmalonyl-CoA to succinyl-CoA. This reaction is catalyzed by the mitochondrial protein methylmalonyl-CoA mutase (MCM, EC 5.4.99.2), an adenosylcobalamin-dependent enzyme. Four different forms of isolated MMA have been described: mut MMA associated with defects in the MCM apoenzyme, and phenotypically divided into two subtypes mut- and mut0 MMA, and three different defects involved in the synthesis of the active form of the cofactor adenosylcobalamin, termed cbl MMA, and classified into three different complementation groups cblA, cblB, and cblH associated with defects in the MMAA and MMAB genes and with an unidentified protein, respectively. In this work we describe the genetic analysis of 25 MMA patients, mainly from Spain. Using biochemical and cellular approaches our patients have been classified, identifying 13 mut MMA, 7 cblA, 2 cblB, and 3 noncblA, noncblB deficient patients. cDNA and genomic DNA sequence analysis of the MUT, MMAA, and MMAB genes have allowed us to identify 27 different changes, 21 novel ones. Among the missense mutations identified in the MUT gene only one, the c.970G>A (p.A324T) variant located in the substrate binding domain is likely a mut- mutation. The remaining missense mutations c.326A>G (p.Q109R), c.983T>C (p.L328P), c.1846C>T (p.R616C), and c.1850T>G (p.L617R) are probably mut0. In the MMAA patients analyzed, frameshift mutations are prevalent. We have explored the genotype-phenotype correlation for this clinically heterogeneous disease.

14C-propionate incorporation assay by rapid filtration in multiwell plates

BACKGROUND

Methylmalonic aciduria (MMA) is divided into two groups according to cobalamin dependency, and this classification is important for treatment. Unfortunately, there has been no rapid and reliable method for the evaluation of cobalamin dependency.

METHODS

[14C]-propionate incorporation into intact cells in the presence of either media alone or media containing various amounts of cobalamin was measured using a 96-well filtration plate and vacuum manifold. Incorporation of radioactivity was measured by direct microplate scintillation.

RESULTS

Using peripheral white blood cells from normal individuals, we obtained a linear relationship between the rate of 14C-propionate incorporation and the number of cells over a broad range (10,000 to 100,000 cells/well). 14C-propionate incorporation in cells from eight patients was 1% to 13% of parallel controls.

CONCLUSIONS

In this report, we describe a rapid, sensitive and reliable method for evaluating the cobalamin dependency of methylmalonic aciduria.

Polymorphisms within the vitamin B12 dependent methylmalonyl-coA mutase are not risk factors for neural tube defects

Methionine synthase and methylmalonyl-CoA mutase (mutase) are the only two known vitamin B(12) (B(12)) dependent enzymes in humans. A lower level of B(12) has been shown to be an independent maternal risk factor for neural tube defects (NTDs) prompting an investigation of common genetic variants within B(12) dependent enzymes. To investigate the role of methylmalonyl-CoA mutase variants we studied 279 complete NTD triads (NTD affected case and both parents) and 256 controls. Based on case-control and family based (transmission disequilibrium test) analyses we did not find an association between the mutase single nucleotide polymorphisms (SNPs) K212K (636A-->G), H532R (1595A-->G) and V671I (2011G-->A) and NTDs. However, there was a significant difference in the frequencies of these polymorphisms between a group of African Americans and American Caucasians (K212K, P=0.002; H532R, P</=0.001; V671I, P=0.006). In conclusion, common variants in the mutase gene do not appear to be risk factors for NTDs but their allele frequencies are significantly different between ethnic groups.

Novel mutations in a Thai patient with methylmalonic acidemia

A Thai patient with methylmalonic acidemia (MMA) and no methylmalonyl-CoA mutase (MCM, EC 5.4.99.2) activity in leukocytes in the presence of deoxyadenosyl cobalamin (mut(0)) was found to be heterozygous for two novel mutations: 1048delT and 1706_1707delGGinsTA (G544X), inherited from her mother and father, respectively. The proband was also heterozygous for the polymorphism, A499T, which did not affect the activity of recombinant MCM.

Homocysteine and methylmalonic acid in diagnosis and risk assessment from infancy to adolescence

The concentration of total homocysteine (tHcy) in serum and plasma is elevated in both folate and cobalamin deficiencies, whereas methylmalonic acid (MMA) in serum, plasma, or urine is a specific marker of cobalamin function. The combined measurement of both metabolites is useful for the diagnosis and follow-up of these deficiency states. In addition, tHcy is elevated under various pathologic states (eg, renal failure), and hyperhomocysteinemia is associated with an increased risk of cardiovascular disease, cognitive dysfunction, and adverse pregnancy outcomes. The diagnostic utility of tHcy and MMA concentrations as markers of folate and cobalamin deficiencies in healthy and diseased children has been documented. This article briefly summarizes the biochemical background of tHcy and MMA and the associations of tHcy and MMA with various disease states and focuses on novel data obtained in infants, children, and adolescents, with emphasis on cobalamin status in infants. The utility of tHcy and MMA as indicators of cobalamin and folate deficiencies in adults can be extended to infants and older children. Furthermore, as in adults, tHcy is related to unhealthy lifestyle factors and is a risk factor for vascular disease. High MMA concentrations in newborns, occasionally denoted as benign methylmalonic aciduria, may reflect impaired cobalamin function.

Hyperhomocysteinemia and B-Vitamin Deficiencies in Infants and Children

Measurement of total homocysteine (tHcy) in healthy and diseased children has documented the utility of this marker in pediatric research and diagnostics. This article focuses on novel data obtained in infants, children and adolescents, with emphasis on cobalamin status in infants. In children, determinants of plasma tHcy are similar to those established in adults, and include age, gender, nutrition, B-vitamin status, and some drugs interfering with B-vitamin function. In infants (age < 1 year), tHcy is moderately elevated and related to serum cobalamin, whereas in older children and throughout childhood, plasma tHcy is low (about 60% of adult levels), and folate status becomes a strong tHcy determinant. As in adults, hyperhomocysteinemia in childhood is a risk factor for stroke, and folate-responsive hyperhomocysteinemia has been detected in children with renal failure. tHcy seems to be a sensitive indicator of folate deficiency in children on a poor diet, in HIV-infected children, and in children treated with anti-folate drugs. In children at increased risk of cobalamin deficiency, which includes children born to vegetarian mothers or children in developing countries on a poor diet, tHcy and methylmalonic acid are responsive indicators of a deficiency state. In newborns and infants born to mothers with an adequate nutrition, there are consistent observations of low cobalamin, elevated tHcy and methylmalonic acid, and reduction of both metabolites by cobalamin supplementation. These data have raised the question whether cobalamin deficiency may be widespread and undetected in babies born to non-vegetarian women on a Westernized diet.

N219Y, a new frequent mutation among mut(degree) forms of methylmalonic acidemia in Caucasian patients

Mutations in the MUT locus encoding for the methylmalonyl-CoA mutase (MCM) apoenzyme are responsible for the mut forms of methylmalonic acidemia (MMA). To date, 49 different mutations have been identified in mut MMA. Only two frequent mutations have been reported in the Japanese population and in African-Americans. Here we report a new missense mutation N219Y (731 A-->T) which we found in five unrelated families of French and Turkish descent. All the patients exhibited a severe mut(degree) phenotype and three of them were homozygotes for N219Y. Direct involvement of the mutation in the loss of enzyme activity was demonstrated by mutagenesis and transient expression study. Mapping of the mutation onto a three-dimensional model of human MCM constructed by homology with the Propionibacterium shermanii enzyme shows that it lies in a highly conserved secondary structure motif and might suggest impaired folding and/or poor stability compatible with the mut(degree) phenotype. Finally, a 1% N219Y carrier frequency was observed in a French anonymous control population. Thus, N219Y is the first frequent mut mutation to be reported in the Caucasian population.

Mutation analysis of the MCM gene in Israeli patients with mut(0) disease

Three novel mutations (IVS8+3a --> g, N219Y, and E414X) were identified in 6 unrelated patients with mut(0) methylmalonic aciduria. The presence of a wild-type along with rearranged fragments in homozygotes for the IVS8+3a --> g mutation may contribute to their later age of onset (3-11 months of age). Nonetheless, delayed onset was not associated with better neurological outcome and prolonged survival. The large number of undiagnosed dead sibs in most families suggests that the disease is largely underdiagnosed in this region.

Molecular and structural analysis of two novel mutations in a patient with mut(-) methylmalonyl-CoA deficiency

Inherited defects in the gene encoding the methylmalonyl-CoA mutase (MCM) result in the mut forms of methylmalonic aciduria (MMA). Twelve mutations have been identified associated with the mut(-) phenotype. We report two novel mutations (K621N and D156N) in a compound heterozygote mut(-) patient. These two mutations and three previously published ones (H627N, A191E, Y231N) were mapped onto a three-dimensional homology model of the human MCM constructed from the crystal structure of the Propionibacterium shermanii enzyme.

Seven novel mutations in mut methylmalonic aciduria

Methylmalonic aciduria (MMA) is an autosomal recessive inborn error of metabolism that results from functional defects in methylmalonyl CoA mutase (MCM), a nuclear-encoded, mitochondrial enzyme that uses the vitamin B12 derivative, adenosylcobalamin (AdoCbl) as a cofactor. To date, 23 mutations have been identified at the MUT locus on the short arm of chromosome 6, causing the mut forms of MMA (mut complementation group; mut MMA, McKusick #251000). We now report seven novel mutations. Three were found inmut0 patients: R228Q (c759G-->A) was found as a heterozygous change; G312V (c1011G-->T) and 346delL (c1112delCTT) were both found as homozygous changes. Four mutations were found in mut patients: A191E (c648C-->A) and V633G (c1974T-->G) were found in the same patient; 684insL (c2128insCTC) and L685R (c2130T-->G) were both found as homozygous changes. The recent modelling of the human methylmalonyl CoA mutase allowed for an interpretation of the identified mutations.

Prolonged exclusive breast-feeding from vegan mother causing an acute onset of isolated methylmalonic aciduria due to a mild mutase deficiency

We describe a case of methylmalonic aciduria (MMA) occurred in a 22-month-old boy whose diet was exclusively based upon breast-feeding from a mother following a long-lasting strict vegetarian diet. Clinical picture showed a dramatic onset, with a profound drowsiness associated with a severe metabolic acidosis, hyperammonemia, macrocytic anemia, ketonuria, and massive methylmalonic aciduria without homocystinuria. Both symptoms and biochemical findings quickly improved thanks to prompt vitamin B(12)parenteral therapy. Biochemical and enzymatic findings allowed a diagnosis of mild mutase deficiency, which only and inadequate dietary B(12)contribution might have revealed. Our case highlights the risk of a prolonged strictly vegetarian diet of lactating mother for providing inadequate amounts of some nutrients to the breast-fed baby. Moreover, such a dietary behaviour could dramatically unmask otherwise clinically unapparent metabolic defects in the baby.

Towards metabolic sink therapy for mut methylmalonic acidaemia: correction of methylmalonyl-CoA mutase deficiency in T lymphocytes from a mut methylmalonic acidaemia child by retroviral-mediated gene transfer

The pathology associated with mut methylmalonic acidaemia (MMA) is caused by systemic accumulation of methylmalonate. Therefore, removal of methylmalonate from the circulation of affected individuals by an engineered metabolic system is proposed as a potential treatment. The haematopoietic cell is a potential site for such a metabolic system because of its direct contact with the accumulated metabolite and the demonstrated safety and ease in utilizing this cell. In this study, we assessed the feasibility of developing a haematopoietic cell-based methylmalonate sink by analysing propionate/methylmalonate metabolism in a variety of haematopoietic cells. The results show that propionate metabolism and methylmalonyl-CoA mutase (MCM) activity are intact in primary T cells, EBV-B cells, and CD34+ haematopoietic stem cell-derived granulocytes, whereas they are defective in those from a mut MMA child. Moreover, normal T and EBV-B cells clear methylmalonate from the medium at a significant rate. Transduction of MCM-deficient T cells with a recombinant retrovirus encoding the human MCM cDNA results in correction of propionate metabolism. These results establish the basis for developing haematopoietic cell-based metabolic sink therapy for mut MMA by T lymphocyte/haematopoietic stem cell-directed gene transfer.

Outcome of individuals with low-moderate methylmalonic aciduria detected through a neonatal screening program

BACKGROUND

The clinical spectrum of methylmalonic aciduria (MMAuria) ranges from severe, neonatal acidosis to benign asymptomatic organic aciduria. In 1975, screening for MMAuria was established in the province of Quebec. Although newborn screening programs facilitate presymptomatic detection and treatment and also detect asymptomatic variants, uncertainties about potential long-term hazards of mild to moderate elevations of MMA create concern. The objective of this study was to examine the outcome of individuals excreting low to intermediate quantities of MMA, ascertained by a newborn screening program.

RESULTS AND STUDY DESIGN

One hundred and thirty-six individuals with elevations of urinary MMA were initially identified by the screening program; 122 individuals were noted to have excretion of urinary MMA <1400 micromol/mmol creatinine. At follow-up assessment at 1 year of age, in 65 of these 122 individuals, the MMA excretion had resolved. Of the remaining individuals, 9 were lost to follow-up, 13 had symptoms, and the remaining 35 were free of symptoms. Among the 35 individuals with asymptomatic persistent MMAuria, MMA excretion has resolved in 13 over 1 year; 22 individuals exhibit persistent low-moderate MMAuria (range, 210 to 1133 micromol/mmol creatinine).

CONCLUSION

Follow-up examination of individuals in the latter asymptomatic cohort with persistent low-moderate MMAuria indicates normal somatic and cognitive outcomes.

Ionizing radiation and genetic risks IX. Estimates of the frequencies of mendelian diseases and spontaneous mutation rates in human populations: a 1998 perspective

This paper is focused on baseline frequencies of mendelian diseases and the conceptual basis for calculating doubling doses both of which are relevant for the doubling dose method of estimating genetic risks of exposure of human populations to ionizing radiation. With this method, the risk per unit dose is obtained as a product of three quantities, namely, the baseline frequency of the disease class under consideration, the relative mutation risk (which is the reciprocal of the doubling dose, which in turn, is calculated as a ratio of spontaneous and induction rates of mutations) and mutation component, i.e., the responsiveness of the disease class to an increase in mutation rate. The estimates of baseline frequencies of mendelian diseases that are currently used in risk estimation date back to the late 1970s. Advances in human genetics during the past two decades now permit an upward revision of these estimates. The revised estimates are 150 per 10(4) livebirths for autosomal dominants (from the earlier estimate of 95 per 10(4)), 75 per 10(4) livebirths for autosomal recessives (from 25 per 10(4)) and to 15 per 10(4) livebirths for X-linked diseases (from 5 per 10(4)). The revised total frequency of mendelian diseases is thus 240 per 10(4) livebirths and is about twice the earlier figure of 125 per 10(4) livebirths. All these estimates, however, pertain primarily to Western European and Western European-derived populations. The fact that in several population isolates or ethnic groups, some of these diseases (especially the autosomal recessives) are more common as a result of founder effects and/or genetic drift is well known and many more recent examples have come to light. These data are reviewed and illustrated with data from studies of the Ashkenazi Jewish, Finnish, French Canadian, Afrikaner and some other populations to highlight the need for caution in extrapolating radiation risks between populations. The doubling dose of 1 Gy that has been used for the past 20 years for risk estimation is based on mouse data for both spontaneous and induction rates of mutations. In extrapolating the mouse-data-based doubling dose to humans, it is assumed that the spontaneous rates in mice and humans are similar. This assumption is incorrect because of the fact that in humans, for several well-studied mendelian diseases, the mutation rate differs between the two sexes and it increases with paternal age. In estimates of spontaneous mutation rates in humans (which represent averages over both sexes), however, paternal age effects are automatically incorporated. In the mouse, these effects are expected to be much less (if they exist at all), but the problem has not been specifically addressed. The complexities and uncertainties associated with assessing the potential impact of spontaneous mutations which arise as germinal mosaics (and which can result in clusters of mutations in the following generation) on mutation rate estimates (in the mouse) and on mutation rate estimates and disease frequencies (in humans) are discussed. In view of (i) the lack of comparability of spontaneous mutation rates in mice and humans and (ii) the fact that these estimates for human genes already include both paternal age effects and correction for clusters (if they had occurred), it is suggested that a prudent procedure now is to base doubling dose calculations on spontaneous mutation rates of human genes (and induction rates of mouse genes, in the absence of a better alternative). This concept, however, is not new and was used by the US National Academy's Committee on the Biological Effects of Ionizing Radiation in its 1972 report.

The Yin-Yang of cobalamin biochemistry

The cobalamins are B12 cofactors with a reactive cobalt-carbon bond at their core and support the activity of two mammalian enzymes that are both medically important. The reactive organometallic bond of the cofactor can be cleaved either homolytically or heterolytically, but what determines how the enzymes control the fate of this bond?