David Watkins

McGill University, Montréal, Quebec, Canada

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Publications (49)277.67 Total impact

  • 38th Annual Meeting of the Society-for-Inherited-Metabolic-Disorders; 03/2015
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    ABSTRACT: In the folate cycle MTHFD1, encoded by MTHFD1, is a trifunctional enzyme containing 5,10-methylenetetrahydrofolate dehydrogenase, 5,10-methenyltetrahydrofolate cyclohydrolase and 10-formyltetrahydrofolate synthetase activity. To date, only one patient with MTHFD1 deficiency, presenting with hyperhomocysteinemia, megaloblastic anaemia, hemolytic uremic syndrome (HUS) and severe combined immunodeficiency, has been identified (Watkins et al J Med Genet 48:590-2, 2011). We now describe four additional patients from two different families. The second patient presented with hyperhomocysteinemia, megaloblastic anaemia, HUS, microangiopathy and retinopathy; all except the retinopathy resolved after treatment with hydroxocobalamin, betaine and folinic acid. The third patient developed megaloblastic anaemia, infection, autoimmune disease and moderate liver fibrosis but not hyperhomocysteinemia, and was successfully treated with a regime that included and was eventually reduced to folic acid. The other two, elder siblings of the third patient, died at 9 weeks of age with megaloblastic anaemia, infection and severe acidosis and had MTFHD1 deficiency diagnosed retrospectively. We identified a missense mutation (c.806C > T, p.Thr296Ile) and a splice site mutation (c.1674G > A) leading to exon skipping in the second patient, while the other three harboured a missense mutation (c.146C > T, p.Ser49Phe) and a premature stop mutation (c.673G > T, p.Glu225*), all of which were novel. Patient fibroblast studies revealed severely reduced methionine formation from [(14)C]-formate, which did not increase in cobalamin supplemented culture medium but was responsive to folic and folinic acid. These additional cases increase the clinical spectrum of this intriguing defect, provide in vitro evidence of disturbed methionine synthesis and substantiate the effectiveness of folic or folinic acid treatment.
    Journal of Inherited Metabolic Disease 01/2015; 38(5). DOI:10.1007/s10545-015-9810-3 · 3.37 Impact Factor
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    ABSTRACT: An inborn error of metabolism associated with mutations in the human methylenetetrahydrofolate dehydrogenase 1 (MTHFD1) gene has been identified. The proband presented with SCID, megaloblastic anemia, and neurologic abnormalities, but the causal metabolic impairment is unknown. SCID has been associated with impaired purine nucleotide metabolism, whereas megaloblastic anemia has been associated with impaired de novo thymidylate (dTMP) biosynthesis. MTHFD1 functions to condense formate with tetrahydrofolate and serves as the primary entry point of single carbons into folate-dependent one-carbon metabolism in the cytosol. In this study, we examined the impact of MTHFD1 loss of function on folate-dependent purine, dTMP, and methionine biosynthesis in fibroblasts from the proband with MTHFD1 deficiency. The flux of formate incorporation into methionine and dTMP was decreased by 90% and 50%, respectively, whereas formate flux through de novo purine biosynthesis was unaffected. Patient fibroblasts exhibited enriched MTHFD1 in the nucleus, elevated uracil in DNA, lower rates of de novo dTMP synthesis, and increased salvage pathway dTMP biosynthesis relative to control fibroblasts. These results provide evidence that impaired nuclear de novo dTMP biosynthesis can lead to both megaloblastic anemia and SCID in MTHFD1 deficiency.
    Proceedings of the National Academy of Sciences 12/2014; 112(2). DOI:10.1073/pnas.1414555112 · 9.67 Impact Factor
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    ABSTRACT: Background We report a 6.5 year-old female with a homozygous missense mutation in ZFYVE20, encoding Rabenosyn-5 (Rbsn-5), a highly conserved multi-domain protein implicated in receptor-mediated endocytosis. The clinical presentation includes intractable seizures, developmental delay, microcephaly, dysostosis, osteopenia, craniofacial dysmorphism, macrocytosis and megaloblastoid erythropoiesis. Biochemical findings include transient cobalamin deficiency, severe hypertriglyceridemia upon ketogenic diet, microalbuminuria and partial cathepsin D deficiency.Methods and resultsWhole exome sequencing followed by Sanger sequencing confirmed a rare (frequency:0.003987) homozygous missense mutation, g.15,116,371 G¿>¿A (c.1273G¿>¿A), in ZFYVE20 resulting in an amino acid change from Glycine to Arginine at position 425 of the Rbsn protein (p.Gly425Arg), as the only mutation segregating with disease in the family. Studies in fibroblasts revealed expression and localization of Rbsn-5G425R in wild-type manner, but a 50% decrease in transferrin accumulation, which is corrected by wild-type allele transfection. Furthermore, the patient¿s fibroblasts displayed an impaired proliferation rate, cytoskeletal and lysosomal abnormalities.Conclusion These results are consistent with a functional defect in the early endocytic pathway resulting from mutation in Rbsn-5, which secondarily disrupts multiple cellular functions dependent on endocytosis, leading to a severe multi-organ disorder.
    Orphanet Journal of Rare Diseases 09/2014; 9(1):141. DOI:10.1186/s13023-014-0141-5 · 3.36 Impact Factor
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    ABSTRACT: Disorders of intracellular cobalamin (vitamin B12) metabolism result from deficient synthesis of the coenzymes derived from vitamin B12: adenosylcobalamin and methylcobalamin. Disturbances of cobalamin-cofactor synthesis result in elevated levels of homocysteine and/or methylmalonic acid. Nine defects of intracellular cobalamin metabolism have been defined. The most common of these disorders is cblC (combined methylmalonic aciduria and homocystinuria). The cblD disorder is rare with fewer than twenty cases reported in the literature. Some cblD patients have combined methylmalonic aciduria and homocystinuria (referred to as "cblD original," "cblD-combined," or herein "cblD-MMA/HC"); some have isolated homocystinuria (referred to as "cblD-variant 1" or herein "cblD-HC"); and others have isolated methylmalonic aciduria (called "cblD-variant 2" or herein "cblD-MMA"). Only six cases of cblD-HC have been defined thus far. We report the 7th case of cblD-HC. The clinical manifestations, biochemical profile, genetic mutation, and plausible ancestry are discussed.
    08/2014; 17. DOI:10.1007/8904_2014_340
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    ABSTRACT: Background: 5,10-Methylenetetrahydrofolate reductase (MTHFR) deficiency is an inborn error of the folate-recycling pathway that affects the remethylation of homocysteine to methionine. The clinical presentation of MTHFR deficiency is highly variable ranging from early neurological deterioration and death in infancy to a mild thrombophilia in adults. Patient and methods: We describe an adolescent girl with a history of mild learning disabilities who presented at age 14 years with an epilepsy syndrome initially thought to be juvenile myoclonic epilepsy. She later developed intractable epilepsy with myoclonus, leg weakness, cognitive decline, and ataxia consistent with the syndrome of progressive myoclonic epilepsy. This prompted further evaluation that revealed elevated plasma homocysteine and decreased plasma methionine. The diagnosis of MTHFR deficiency was confirmed based on extremely reduced fibroblast MTHFR activity (0.3 nmol CHO/mg prot/hr) as well as mutation analysis that revealed two variants in the MTHFR gene, a splice site mutation p (IVS5-1G>A), as well as a missense mutation (c.155 G>A; p. Arg52Gln). Therapy with folinic acid, betaine, and methionine has produced significant clinical improvement, including improved strength, less severe ataxia, and decreased seizure frequency, as well as improvements in her electroencephalography and electromyography. Conclusion: This patient demonstrates the importance of considering MTHFR deficiency in the differential diagnosis of progressive myoclonic epilepsy because it is one of the few causes for which specific treatment is available.
    Pediatric Neurology 08/2014; 51(2):266-270. DOI:10.1016/j.pediatrneurol.2014.04.005 · 1.70 Impact Factor
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    ABSTRACT: Importance: Hereditary spastic paraplegia is a highly heterogeneous group of neurogenetic disorders with pure and complicated clinical phenotypes. No treatment is available for these disorders. We identified 2 unrelated families, each with 2 siblings with severe methylenetetrahydrofolate reductase (MTHFR) deficiency manifesting a complicated form of adult-onset hereditary spastic paraparesis partially responsive to betaine therapy. Observations: Both pairs of siblings presented with a similar combination of progressive spastic paraparesis and polyneuropathy, variably associated with behavioral changes, cognitive impairment, psychosis, seizures, and leukoencephalopathy, beginning between the ages of 29 and 50 years. By the time of diagnosis a decade later, 3 patients were ambulatory and 1 was bedridden. Investigations have revealed severe hyperhomocysteinemia and hypomethioninemia, reduced fibroblast MTHFR enzymatic activity (18%-52% of control participants), and 3 novel pathogenic MTHFR mutations, 2 as compound heterozygotes in one family and 1 as a homozygous mutation in the other family. Treatment with betaine produced a rapid decline of homocysteine by 50% to 70% in all 4 patients and, over 9 to 15 years, improved the conditions of the 3 ambulatory patients. Conclusions and relevance: Although severe MTHFR deficiency is a rare cause of complicated spastic paraparesis in adults, it should be considered in select patients because of the potential therapeutic benefit of betaine supplementation.
    05/2014; 71(7). DOI:10.1001/jamaneurol.2014.116
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    ABSTRACT: Transcobalamin (TC) transports cobalamin from blood into cells. TC deficiency is a rare autosomal recessive disorder usually presenting in early infancy with failure to thrive, weakness, diarrhoea, pallor, anemia, and pancytopenia or agammaglobulinemia. It can sometimes resemble neonatal leukemia or severe combined immunodeficiency disease. Diagnosis of TC deficiency is suspected based on megaloblastic anemia, elevation of total plasma homocysteine, and blood or urine methylmalonic acid. It is confirmed by studying the synthesis of TC in cultured fibroblasts, or by molecular analysis of the TCN2 gene. TC deficiency is treatable with supplemental cobalamin, but the optimal type, route and frequency of cobalamin administration and long term patient outcomes are unknown. Here we present a series of 30 patients with TC deficiency, including an update on multiple previously published patients, in order to evaluate the different treatment strategies and provide information about long term outcome. Based on the data presented, current practice appears to favour treatment of individuals with TC deficiency by intramuscular injections of hydroxy- or cyanocobalamin. In most cases presented, at least weekly injections (1 mg IM) were necessary to ensure optimal treatment. Most centres adjusted the treatment regimen based on monitoring CBC, total plasma homocysteine, plasma and urine methylmalonic acid, as well as, clinical status. Finally, continuing IM treatment into adulthood appears to be beneficial.
    Journal of Inherited Metabolic Disease 12/2013; 37(3). DOI:10.1007/s10545-013-9664-5 · 3.37 Impact Factor
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    ABSTRACT: Derivatives of vitamin B12 (cobalamin) are essential cofactors for enzymes required in intermediary metabolism. Defects in cobalamin metabolism lead to disorders characterized by the accumulation of methylmalonic acid and/or homocysteine in blood and urine. The most common inborn error of cobalamin metabolism, combined methylmalonic acidemia and hyperhomocysteinemia, cblC type, is caused by mutations in MMACHC. However, several individuals with presumed cblC based on cellular and biochemical analysis do not have mutations in MMACHC. We used exome sequencing to identify the genetic basis of an X-linked form of combined methylmalonic acidemia and hyperhomocysteinemia, designated cblX. A missense mutation in a global transcriptional coregulator, HCFC1, was identified in the index case. Additional male subjects were ascertained through two international diagnostic laboratories, and 13/17 had one of five distinct missense mutations affecting three highly conserved amino acids within the HCFC1 kelch domain. A common phenotype of severe neurological symptoms including intractable epilepsy and profound neurocognitive impairment, along with variable biochemical manifestations, was observed in all affected subjects compared to individuals with early-onset cblC. The severe reduction in MMACHC mRNA and protein within subject fibroblast lines suggested a role for HCFC1 in transcriptional regulation of MMACHC, which was further supported by the identification of consensus HCFC1 binding sites in MMACHC. Furthermore, siRNA-mediated knockdown of HCFC1 expression resulted in the coordinate downregulation of MMACHC mRNA. This X-linked disorder demonstrates a distinct disease mechanism by which transcriptional dysregulation leads to an inborn error of metabolism with a complex clinical phenotype.
    The American Journal of Human Genetics 09/2013; 93(3):506-14. DOI:10.1016/j.ajhg.2013.07.022 · 10.93 Impact Factor
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    ABSTRACT: The cblG and cblC disorders of cobalamin (Cbl) metabolism are two inherited causes of megaloblastic anemia. In cblG, mutations in methionine synthase (MTR) decrease conversion of HOCbl to MeCbl, while in cblC, mutations in MMACHC disrupt formation of cob(II)alamin (detected as HOCbl). Cases with undetectable methionine synthase (MS) activity are extremely rare and classified as 'cblG-variant'. In 4 'cblG-variant' cases we observed a decreased conversion of CNCbl to HOCbl that is also seen in cblC cases. To explore this observation, we studied the gene defects, splicing products and expression of MS, as well as MS/MMACHC protein interactions in cblG-variant, cblG, cblC and control fibroblasts. We observed a full size MS encoded by MTR-001 and a 124 kDa truncated MS encoded by MTR-201 in cblG, cblC, control fibroblasts, and HEK cells, but only the MTR-201 transcript and inactive truncated MS in cblG-variant cells. Co-immunoprecipitation and proximity ligation assay showed interaction between truncated MS and MMACHC in cblG-variant cells. This interaction decreased 2.2, 1.5 and 5.0-fold in proximity ligation assay of cblC cells with p.R161Q and p.R206W mutations, and HEK cells with knock down expression of MS by siRNA respectively, when compared to control cells. In 3-D modelling and docking analysis, both truncated and full size MS provide a loop anchored to MMACHC, which makes contacts with R-161 and R-206 residues. Our data suggest that the interaction of MS with MMACHC may play a role in the regulation of the cellular processing of cobalamins that is required for Cbl cofactor synthesis.
    Human Molecular Genetics 07/2013; 22(22). DOI:10.1093/hmg/ddt308 · 6.39 Impact Factor
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    ABSTRACT: A neonate, who was found to have an elevated C3/C2 ratio and minimally elevated propionylcarnitine on newborn screening, was subsequently identified as having the rare cblF inborn error of vitamin B12 (cobalamin) metabolism. This disorder is characterized by the retention of unmetabolized cobalamin in lysosomes such that it is not readily available for cellular metabolism. Although cultured fibroblasts from the patient did not show the expected functional abnormalities of the cobalamin-dependent enzymes, methylmalonyl-CoA mutase and methionine synthase, they did show reduced synthesis of the active cobalamin cofactors adenosylcobalamin and methylcobalamin. Mutation analysis of LMBRD1 established that the patient had the cblF disorder. Treatment was initiated promptly, and the patient showed a robust response to regular injections of cyanocobalamin, and she was later switched to hydroxocobalamin. Currently, at 3 years of age, the child is clinically well, with appropriate development. Adjusted newborn screening cutoffs in Ontario allowed detection of a deficiency that might not have otherwise been identified, allowing early treatment and perhaps preventing the adverse sequelae seen in some untreated patients.
    PEDIATRICS 06/2013; 132(1). DOI:10.1542/peds.2013-0105 · 5.47 Impact Factor
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    ABSTRACT: Isolated methylmalonic aciduria (MMA) results either from a defect in the mitochondrial enzyme methylmalonylCoA mutase (MCM), or in the intracellular conversion of vitamin B12 (cobalamin) into its active coenzyme adenosylcobalamin (AdoCbl). Mutations in the MMAB gene affect the function of the enzyme ATP:cob(I)alamin adenosyltransferase (ATR) and the production of AdoCbl. Measurement of MCM function in cultured patient fibroblasts, followed by somatic cell complementation analysis in cases where MCM function is decreased, has classically been used to diagnose the cblB cobalamin disorder. A patient with persistent MMA, who could not be diagnosed using traditional somatic cell studies, was subsequently shown by sequencing in a clinical laboratory to contain two variants in the MMAB gene. This observation brings into question whether somatic cell studies have failed to diagnose other cblB patients with mild cellular phenotypes. A high resolution melting analysis (HRMA) assay was developed for the MMAB gene. It was used to scan 96 reference samples and two cohorts of patients: 42 patients diagnosed with cblB by complementation studies; and 181 patients with undiagnosed MMA. MMAB mutations, including one novel nonsense mutation (c.12 C>A [p.C4X]), were identified in all members of the cblB cohort. Four patients with undiagnosed MMA, including the index case described above, were found to contain variants in the MMAB gene: c.185C>T (p.T62M), c.394T>C (p.C132R), c.398C>T (p.S133F), c.521C>T (p.S174L), c.572G>A (p.R191Q). Only the index case was found to have two variants, suggesting that somatic cell studies diagnose almost all cblB patients.
    Molecular Genetics and Metabolism 05/2013; 110(1-2). DOI:10.1016/j.ymgme.2013.04.020 · 2.63 Impact Factor
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    ABSTRACT: Background: The cblE disorder is an inherited disorder of vitamin B12 metabolism that results in elevated levels of homocysteine and decreased methionine in body fluids. Renal complications have been reported in patients with cblC disease, but not in those with cblE disease. The renal complications of cblC disease include thrombotic microangiopathy (TMA), neonatal hemolytic uremic syndrome, chronic renal failure, tubulointerstitial nephritis and proximal renal tubular acidosis. Previously, we reported a patient with cblC disease who had an atypical glomerulopathy that manifested with proteinuria and progressive renal insufficiency. Case-diagnosis/treatment: Studies were done on cultured fibroblasts. Renal biopsy tissue was examined by light and electron microscopy. There was decreased incorporation of labeled methyltetrahydrofolate and decreased synthesis of methylcobalamin. Complementation analysis placed the patient into the cblE complementation group. The findings from the histological and ultrastructural studies of renal biopsy were similar, but not identical, to those of idiopathic membranoproliferative glomerulonephritis (MPGN) and overlapped with those of TMA. Conclusions: We describe a patient with cblE disease who had an atypical glomerulopathy similar to MPGN. Additional findings included migraine headaches, hypothyroidism and livedo reticularis.
    Pediatric Nephrology 03/2013; 28(7). DOI:10.1007/s00467-013-2443-6 · 2.86 Impact Factor
  • David Watkins · David S Rosenblatt
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    ABSTRACT: Background: Since 1975 cells lines from patients with suspected inborn errors of vitamin B12 metabolism have been referred to our laboratory because of elevations of homocysteine, methylmalonic acid, or both. Design: Cultured fibroblasts from patients were subjected to a battery of tests: incorporation of labelled propionate and methyltetrahydrofolate into cellular macromolecules, to test the functional integrity of methylmalonyl-CoA mutase and methionine synthase, respectively; uptake of labelled cyanocobalamin and synthesis of adenosylcobalamin and methylcobalamin; and, where applicable, complementation analysis. Results: This approach has allowed for the discovery of novel steps in the cellular transport and metabolism of vitamin B12, including those involving cellular uptake, the efflux of vitamin B12 from lysosomes, and the synthesis of adenosylcobalamin and methylcobalamin. For all of these disorders, the responsible genes have been discovered. Conclusion: The study of highly selected patients with suspected inborn errors of metabolism has consistently resulted in the discovery of previously unknown metabolic steps and has provided new lessons in biology.
    Biochimie 02/2013; 95(5). DOI:10.1016/j.biochi.2013.01.013 · 2.96 Impact Factor
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    ABSTRACT: Folate and vitamin B(12) metabolism are essential for de novo purine synthesis, and several defects in these pathways have been associated with immunodeficiency. Here we describe the occurrence of severe combined immunodeficiency (SCID) with megaloblastic anemia, leukopenia, atypical hemolytic uremic syndrome, and neurologic abnormalities in which hydroxocobalamin and folate therapy provided partial immune reconstitution. Whole exome sequencing identified compound heterozygous mutations in the MTHFD1 gene, which encodes a trifunctional protein essential for processing of single-carbon folate derivatives. We now report the immunologic details of this novel genetic cause of SCID and the response to targeted metabolic supplementation therapies. This finding expands the known metabolic causes of SCID and presents an important diagnostic consideration given the positive impact of therapy.
    PEDIATRICS 01/2013; 131(2). DOI:10.1542/peds.2012-0899 · 5.47 Impact Factor
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    ABSTRACT: MMACHC and MMADHC are the genes responsible for cblC and cblD defects of vitamin B(12) metabolism, respectively. Patients with cblC and cblD defects present with various combinations of methylmalonic aciduria (MMA) and homocystinuria (HC). Those with cblC mutations have both MMA and HC whereas cblD patients can present with one of three distinct biochemical phenotypes: isolated MMA, isolated HC, or combined MMA and HC. Based on the subcellular localization of these enzymatic pathways it is thought that MMACHC functions in the cytoplasm while MMADHC functions downstream of MMACHC in both the cytoplasm and the mitochondrion. In this study we determined the subcellular location of MMACHC and MMADHC by immunofluorescence and subcellular fractionation. We show that MMACHC is cytoplasmic while MMADHC is both mitochondrial and cytoplasmic, consistent with the proposal that MMADHC acts as a branch point for vitamin B(12) delivery to the cytoplasm and mitochondria. The factors that determine the distribution of MMADHC between the cytoplasm and mitochondria remain unknown. Functional complementation experiments showed that retroviral expression of the GFP tagged constructs rescued all biochemical defects in cblC and cblD fibroblasts except propionate incorporation in cblD-MMA cells, suggesting that the endogenous mutant protein interferes with the function of the transduced wild type construct.
    Molecular Genetics and Metabolism 12/2012; 108(2). DOI:10.1016/j.ymgme.2012.11.284 · 2.63 Impact Factor
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    ABSTRACT: Inborn errors of vitamin B(12) (cobalamin) metabolism are characterized by decreased production of active cobalamin cofactors and subsequent deficiencies in the activities of methionine synthase and methylmalonyl-CoA mutase. With the recent discovery of the cblJ defect in two patients with phenotypes mimicking the cblF defect, there are nine genes known to be involved in cobalamin metabolism. The new defect is caused by mutations in the ABCD4 gene, encoding an ABC transporter. At the moment, there is no clear distinction between the cblJ and cblF defects either clinically or biochemically, and both defects result in blocks in the transport of cobalamin from the lysosome to the cytoplasm. A patient was diagnosed with hyperhomocysteinemia and methylmalonic aciduria at the age of 8years. Incorporations of both [(14)C]propionate and [(14)C]methyltetrahydrofolate in cultured fibroblasts were within reference ranges and thus too high to allow for complementation analysis. We observed decreased synthesis of both adenosylcobalamin and methylcobalamin and accumulation of unmetabolized cyanocobalamin. Exome sequencing was performed to identify causative mutation(s) and Sanger re-sequencing was performed to validate segregation of mutation in the family. By this approach, a homozygous mutation, c.423C>G, in the ABCD4 gene was identified. Here, we report the successful application of exome sequencing for diagnosis of a rare inborn error of vitamin B(12) metabolism in a patient whose unusual presentation precluded diagnosis using standard biochemical and genetic approaches. The patient represents only the third known patient with the cblJ disorder.
    Molecular Genetics and Metabolism 10/2012; 107(4). DOI:10.1016/j.ymgme.2012.10.005 · 2.63 Impact Factor
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    ABSTRACT: The gene product of MMAA is required for the intracellular metabolism of cobalamin (Cbl). Mutations in this gene lead to the cblA class of disorders, characterized by isolated methylmalonic aciduria. We have been concerned that somatic cell methods of diagnosis may miss patients with mild cellular phenotypes. A high resolution melting analysis (HRMA) assay was developed to rapidly scan the coding exons and flanking intronic regions of the MMAA gene for variants. DNA was scanned by HRMA from 96 unaffected reference individuals, 72 cblA patients confirmed by complementation, and 181 patients with isolated elevated methylmalonic acid, who could not be diagnosed using complementation analysis. Suspected variants were confirmed by Sanger sequencing. In the cblA cohort, HRMA correctly identified all previously known mutations as well as an additional 22 variants, 10 of which had not been previously reported. Novel variants included one duplication (c.551dupG, p.C187LfsX3), one deletion (c.387delC, p.Y129YfsX13), one splice site mutation (c.440-2A>G, splice site), 4 missense mutations (c.748G>A, p.E520K; c.820G>A, p.G274S; c.627G>T, p.R209S; c.826A>G, p.K276E), and 3 nonsense mutations (c.960G>A, p.W320X; c.1075C>T, p.E359X; c.1084C>T, p.Q362X). All novel missense variants affect highly conserved residues and are predicted to be damaging. Scanning of MMAA in the 181 undiagnosed samples revealed a single novel heterozygous missense change (c.821G>A, p.G274D).
    Molecular Genetics and Metabolism 09/2012; 107(3). DOI:10.1016/j.ymgme.2012.09.012 · 2.63 Impact Factor
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    ABSTRACT: Inherited disorders of vitamin B(12) (cobalamin) have provided important clues to how this vitamin, which is essential for hematological and neurological function, is transported and metabolized. We describe a new disease that results in failure to release vitamin B(12) from lysosomes, which mimics the cblF defect caused by LMBRD1 mutations. Using microcell-mediated chromosome transfer and exome sequencing, we identified causal mutations in ABCD4, a gene that codes for an ABC transporter, which was previously thought to have peroxisomal localization and function. Our results show that ABCD4 colocalizes with the lysosomal proteins LAMP1 and LMBD1, the latter of which is deficient in the cblF defect. Furthermore, we show that mutations altering the putative ATPase domain of ABCD4 affect its function, suggesting that the ATPase activity of ABCD4 may be involved in intracellular processing of vitamin B(12).
    Nature Genetics 08/2012; 44(10):1152-5. DOI:10.1038/ng.2386 · 29.35 Impact Factor
  • David S. Rosenblatt · David Watkins
    35th Annual Meeting of the Society-for-Inherited-Metabolic-Disorders; 03/2012

Publication Stats

1k Citations
277.67 Total Impact Points


  • 2004–2014
    • McGill University
      • Department of Human Genetics
      Montréal, Quebec, Canada
  • 2000–2013
    • McGill University Health Centre
      Montréal, Quebec, Canada
  • 2005
    • The University of Calgary
      • Department of Biochemistry and Molecular Biology
      Calgary, Alberta, Canada
  • 2003
    • New York Methodist Hospital
      New York City, New York, United States