Protein destabilization and loss of protein-protein interaction are fundamental mechanisms in cblA-type methylmalonic aciduria.

Mutations in the human MMAA gene cause the metabolic disorder cblA-type methylmalonic aciduria (MMA), although knowledge of the mechanism of dysfunction remains lacking. MMAA regulates the incorporation of the cofactor adenosylcobalamin (AdoCbl), generated from the MMAB adenosyltransferase, into the destination enzyme methylmalonyl-CoA mutase (MUT). This function of MMAA depends on its GTPase activity, which is stimulated by an interaction with MUT. Here, we present 67 new patients with cblA-type MMA, identifying 19 novel mutations. We biochemically investigated how missense mutations in MMAA in 22 patients lead to disease. About a third confer instability to the recombinant protein in bacterial and human expression systems. All 15 purified mutant proteins demonstrated wild-type like intrinsic GTPase activity and only one (p.Asp292Val), where the mutation is in the GTP binding domain, revealed decreased GTP binding. However, all mutations strongly decreased functional association with MUT by reducing GTPase activity stimulation upon incubation with MUT, while nine mutant proteins additionally lost the ability to physically bind MUT. Finally, all mutations interfered with gating the transfer of AdoCbl from MMAB to MUT. This work suggests loss of functional interaction between MMAA and MUT as a disease-causing mechanism that impacts processing and assembly of a cofactor to its destination enzyme.

Renal involvement in a patient with cobalamin A type (cblA) methylmalonic aciduria: a 42-year follow-up.

Chronic renal failure is a well-known long-term complication of methylmalonic aciduria (MMA-uria), occurring even under apparently optimal metabolic management. The onset of renal dysfunction seems to be dependent on the type of defect and vitamin B12-responsiveness. We report on a patient with a vitamin B12-responsive cobalamin A type (cblA) MMA-uria caused by a homozygous stop mutation (p.R145X) in the cobalamin A gene (MMAA). She was diagnosed with chronic kidney disease (CKD) stage III at the age of 12 years. Following re-evaluation, the patient received vitamin B12 (hydroxocobalamin) treatment, resulting in a significant decrease in the concentration of methylmalonic acid (MMA) in urine and plasma. Until age 29 years glomerular filtration rate remained stable probably due to hydroxocobalamin treatment slowing down progression to end-stage renal failure. Kidney biopsies showed non-specific manifestations of chronic interstitial inflammation. The patient received a renal transplant at age 35 years. Under continuous treatment with hydroxocobalamin there is no evidence of kidney damage due to MMA-uria until the last follow-up 6 years after transplantation. This case report illustrates (i) a long-term follow-up of a patient with MMA-uria due to cblA deficiency, (ii) the involvement of the kidney as a target organ and (iii) the importance of early and adequate vitamin B12 substitution in responsive patients. Further investigation will be necessary to prove the protective effect of hydroxocobalamin in the kidney in vitamin B12-responsive patients.

The human B12 trafficking chaperones: CblA, ATR, CblC and CblD.

Mammals rely on an elaborate intracellular trafficking pathway for processing and delivering vitamin B12 to two client enzymes. CblC (also known as MMACHC) is postulated to receive the cofactor as it enters the cytoplasm and converts varied B12 derivatives to a common cob(II)alamin intermediate. CblD (or MMADHC) reacts with CblC-bound cob(II)alamin forming an interprotein thiolato-cobalt coordination complex and, by a mechanism that remains to be elucidated, transfers the cofactor to methionine synthase. In the mitochondrion, CblB (also known as MMAB or adenosyltransferase) synthesizes AdoCbl from cob(II)alamin and ATP in the presence of an electron donor. CblA (or MMAA), a GTPase, gates cofactor loading from CblB to methylmalonyl-CoA mutase and off-loading of cob(II)alamin in the reverse direction. This chapter focuses on assays for measuring the activities of the four B12 chaperones CblA-D.

Clinical picture and treatment effects in 5 patients with Methylmalonic aciduria related to MMAA mutations.

INTRODUCTION: Methylmalonic Aciduria (MMA) is a heterogeneous group of rare diseases leading to accumulation of methylmalonic acid in body fluids. One of the causes of the disease is the methylmalonic aciduria, cblA type (cblA - type MMA), conditioned by a mutation in the MMAA gene, which is essential for the proper functioning of a cofactor of the methylmalonyl-CoA mutase. The symptoms of the disease, depending on the cause, may manifest themselves at different ages. Most patients are sensitive to high doses of hydroxycobalamin, which is associated with better prognosis. MATERIAL AND METHOD: The purpose of the study was to retrospectively analyze the clinical picture and effects of treatment of patients with methylmalonic aciduria related to mutation in the MMAA gene. RESULTS: Five patients with diagnosed cblA - type MMA were presented. At the time of diagnosis the median of age was 18.8 months, but the symptoms had already appeared since infancy, as recurrent vomiting and delayed psychomotor development. Significant excretion of methylmalonic acid in urine and metabolic acidosis traits with significantly increased anionic gap were observed in all patients. All of them were sensitive to the treatment with vitamin B12. The median of therapy duration and observation is 12.2 years. During the treatment, good metabolic control was achieved in all patients, but their cognitive development is delayed. Three patients have renal failure and pharmacologically treated arterial hypertension. CONCLUSIONS: Patients with a mutation in the MMAA gene are sensitive to treatment with hydroxocobalamine, but the inclusion of appropriate treatment does not protect against neurodevelopmental disorders and chronic kidney disease.

Allosteric Regulation of Oligomerization by a B12 Trafficking G-Protein Is Corrupted in Methylmalonic Aciduria.

Allosteric regulation of methylmalonyl-CoA mutase (MCM) by the G-protein chaperone CblA is transduced via three "switch" elements that gate the movement of the B12 cofactor to and from MCM. Mutations in CblA and MCM cause hereditary methylmalonic aciduria. Unlike the bacterial orthologs used previously to model disease-causing mutations, human MCM and CblA exhibit a complex pattern of regulation that involves interconverting oligomers, which are differentially sensitive to the presence of GTP versus GDP. Patient mutations in the switch III region of CblA perturb the nucleotide-sensitive distribution of the oligomeric complexes with MCM, leading to loss of regulated movement of B12 to and/or from MCM and explain the molecular mechanism of the resulting disease.

Newborn Screening Programmes in Europe, Arguments and Efforts Regarding Harmonisation: Focus on Organic Acidurias.

BACKGROUND: The state of newborn screening (NBS) programmes for organic acidurias in Europe was assessed by a web-based questionnaire in the EU programme of Community Action in Public Health 2010/2011 among the - at that time - 27 EU member states, candidate countries, potential candidates and three EFTA countries. RESULTS: Thirty-seven data sets from 39 target countries were analysed. Newborn screening for glutaric aciduria type I (GA-I) was performed in ten, for isovaleric aciduria (IVA) in nine and for methylmalonic aciduria including cblA, cblB, cblC and cblD (MMACBL) as well as for propionic aciduria (PA) in seven countries. Samples were obtained at a median age of 2.5 days and laboratory analysis began at median age of 4.5 days. Positive screening results were mostly confirmed in specialised centres by analysis of organic acids in urine. Confirmation of a positive screening result usually did not start before the second week of life (median ages: 9.5 days [IVA], 9 days [GA-I], 8.5 days [PA, MMACBL]) and was completed early in the third week of life (median ages: 15 days [IVA, PA, MMA], 14.5 days [GA-I]). Treatment was initiated in GA-I and IVA at a median age of 14 days and in MMACBL and PA at a median age of 15 days. CONCLUSION: NBS for organic acidurias in Europe is variable and less often established than for amino acid disorders. While for GA-I its benefit has already been demonstrated, there is room for debate of NBS for IVA and especially PA and MMACBL.

Spectrum of ocular manifestations in cobalamin C and cobalamin A types of methylmalonic acidemia.

BACKGROUND: Cobalamin C disease (cblC), which leads to methylmalonic acidemia with homocystinuria, is the most common inherited disorder of vitamin B12 metabolism. Reported ocular findings associated with cblC have been maculopathy, pigmentary retinopathy, and optic nerve atrophy. Cobalamin A disease (cblA) which causes an isolated methylmalonic acidemia without homocystinuria is rarer than cblC. This is the first detailed report of the ocular findings associated with cblA. We also describe the spectrum of ocular findings in our cblC patients. MATERIALS AND METHODS: A case series describing the ophthalmologic clinical course of six patients with a diagnosis of cobalamin C type and one patient with cobalamin A type of methylmalonic acidemia. Patients were diagnosed through biochemical laboratory testing and genetic analysis was conducted on most patients. Longitudinal fundus findings, optical coherence tomography (OCT), autofluorescence, and electrophysiology were followed in the patients. RESULTS: The cblA patient demonstrated a relatively mild ocular phenotype with late-onset and slowly progressing temporal disc pallor and peripapillary atrophy in the second decade of life. The patient maintained good visual acuity and central vision, without evidence of maculopathy. The six cblC patients demonstrated a range of ocular findings from unremarkable and mild phenotypes to significant retinopathy, including bull's eye maculopathy, severe maculopathy with punched out chorioretinal atrophy, peripheral bone spicules, and optic nerve atrophy. CONCLUSIONS: The spectrum of ocular manifestations seen with inherited disorders of cobalamin metabolism is wide, ranging from mild optic nerve atrophy to severe macular or retinal degeneration. This heterogeneity may in part reflect the associated biochemical phenotype, such as that observed between our cblA and cblC patients. We also observed heterogeneity within the cblC type in agreement with previous reports.