Abstract
Background.
Isolated methylmalonic acidemia refers to a group of inborn errors of metabolism characterized by elevated methylmalonic acid concentrations in the blood and urine. It occurs in approximately one to three out of every 100 thousand Chinese newborns. Mutations in the MMAA gene cause isolated methylmalonic acidemia.
Case presentation.
A 13-month-old boy was diagnosed with isolated methylmalonic acidemia. We identified two mutations in the MMAA gene in this case: c.491G>A and c.650T>A. The c.491G>A is a novel mutation in the MMAA gene. The boy is a heterozygous carrier of both mutations. The boy was treated with intravenous sodium benzoate and fluids. His sensorium gradually improved and he recovered from the acute illness. Other family members are heterozygous carriers of either mutations but with no symptoms.
Conclusions.
We identified a novel c.491G>A mutation in the MMAA gene. Heterozygous carriers of both c.491G>A and c.650T>A mutations are associated with isolated methylmalonic acidemia.
Keywords: Isolated Methylmalonic Acidemia, Methylmalonyl-CoA Mutase, MMAA
INTRODUCTION
Isolated methylmalonic acidemia is a group of inborn errors of metabolism characterized by elevated methylmalonic acid concentration in the blood and urine due to the failure of converting methylmalonyl-CoA into succinyl-CoA during propionyl-CoA metabolism (1). Mutations in the human MMAA gene caused methylmalonic acidemia type cb1A (1). Here we reported a 13-month-old boy with isolated methylmalonic acidemia. We described the natural history and clinical outcomes. We identified a novel mutation in the MMAA gene in this case.
Clinical Data
A 13-month-old boy, born of non-consanguineous parents, presented with sudden onset of protracted vomiting and unconsciousness for two days. He did not have a fever. He was asymptomatic until then. He reached developmental milestones normally. He has a weight of 8.5 kg and length of 71 cm, both below the fifth percentile for his age. His Glasgow Coma Scale score was five. He had hypotonia on four limbs with brisk deep tendon reflexes. Bilateral plantar reflexes were extensors. There were no signs of meningeal irritation. Other systems were within normal limits. Laboratory tests revealed his hemoglobin was12 g%, total leucocyte count was 9300/mm3, and platelet count was 295,000/mm3. Neutrophil count was higher at 80% while lymphocyte count was lower at 15%. His blood glucose concentration was 108 mg/dL (60-150 mg/dL); blood urea was 40 mg/dL (8-32 mg/dL); serum creatinine was 0.4 mg/dL (0.6-1.6 mg/dL), serum sodium was 118 mEq/L (130-143 mEq/L); serum potassium was 5 mEq/dL (3.5-5 mEq/L). Liver enzymes were normal: aspartate transaminase (ALT) was 36 U/L (5-40 U/L), alanine transaminase (AST) was 34 U/L (5-40 U/L), and alkaline phosphatase was195 U/L (40-140 U/L). Cerebrospinal fluid analysis showed no cells on microscopy and Gram stain. Blood cultures were negative. Cerebrospinal Fluid(CSF) tests showed that CSF glucose was 100 mg/dL and CSF protein was 55 mg/dL (10-20 mg/dL). The results of arterial blood gas analysis are as follows: pH was 7.07 (7.35-7.45); PaO2 was 212 mmHg; PaCO2 was 8.2 mmHg (35-45 mmHg); HCO3- was 2.2 mmol/L (20-28 mmol/L). These data suggest severe metabolic acidosis with a high anion gap. His serum ammonia was 2834 μg% (25-94 μg%) suggesting severe hyperammonemia. His serum lactate was 26 mg/dL (5-22 mg/dL). The thin-layer chromatography (TLC) of amino acid from the urine showed moderate organic aciduria with severe ketonuria and excretion of valine and alanine. He was treated with intravenous sodium benzoate and fluids. Gradually his sensorium improved. He recovered from the acute illness. He was continuously treated with oral sodium benzoate, thiamine, biotin and carnitine.
METHODS
Isolated methylmalonic acidemia was diagnosed by analyzing the organic acids in plasma and/or urine using gas-liquid chromatography and tandem mass spectrometry. The subtype of methylmalonic acidemia was established by cellular biochemical studies including 14°C propionate incorporation, B12 responsiveness, complementation analysis, cobalamin distribution assays and molecular genetic testing. Identification of biallelic pathogenic variants in MUT, MMAA, MMAB, MCEE, and MMADHC genes and confirmation of carriers in parents can establish the diagnosis (2-4). In our case the MMAA mutations in the proband were detected by targeted sequencing. The mutations in the proband and his family members were validated by Sanger sequencing.
RESULTS
The analysis of organic acids and amino acids in the plasma by tandem mass spectrometry showed C3: 14.50 μM (0.30-5.00), C3/C2: 0.67(0.02-0.25), Leu: 328.15 μM (40-250), Val: 441.52 μM (50-280). The analysis of organic acids in urine by gas-liquid chromatography showed Methyl malonic acid: 95.6 μM (0-4), Methyl citrate: 5.4 μM (0-0.7), 3-hydroxypropionic acid 19.2 μM (0-4).
For the patient (I), in MMAA gene, a G to A substitution at position 491 was found (NM_172250.2; c.491G>A). This is a novel mutation in MMAA gene.
His father (II) and elder sister (IV) had the same mutation. However, his mother (III) did not have this mutation (Fig. 1). This mutation leads to the substitution of glycine by glutamic acid at position 164 (p.Gly164Glu).
Figure 1.
MMAA; NM_172250.2; c.491G>A; p.Gly164Glu. I: the patient; II: his father; III: his mother; IV: his elder sister.
In the other allele of this patient, a T to A substitution at position 650 was found (NM_172250.2; c.650T>A). His mother showed the same mutation in a heterozygous pattern. However, his father and elder sister did not have this mutation (Fig. 2). This mutation has been reported to cause nonsense mutation at position 217 (p.Leu217*)(5-6).
Figure 2.
MMAA; NM_172250.2; c.650T>A; p.Leu217*. I: the patient; II: his father; III: his mother; IV: his elder sister.
DISCUSSION
Isolated methylmalonic acidemia is an autosomal recessive disorder (7). At conception, each sib of an affected individual has a 25% chance of being affected, a 25% chance of being unaffected and 50% chance of being an asymptomatic carrier (7). Identification of the MMAA gene mutations in the proband and the family members enables evaluation of the severity of the mutations and helps with prenatal diagnosis for subsequent pregnancies. In some circumstances, enzyme analysis and metabolite measurements on cultured fetal cells can facilitate prenatal diagnosis for pregnancies at high risk (8).
Mutations in the human MMAA gene cause the cblA-type methylmalonic acidemia (9-10). The long-term effect of MMA depends on where the mutations are and on the severity of the mutations. The MMAA protein regulates the incorporation of the cofactor adenosylcobalamin (AdoCbl), 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 (10). Mutations in the MMAA gene can affect either its GTPase activity or the interaction with MUT (10). In our case, the boy with symptoms is a heterozygous carrier of both mutations. On one allele, he has a previously reported nonsense p.Leu217* mutation (5-6). On the other allele, he has a novel p.Gly164Glu point mutation. His family members, who carry either mutations on one allele, have no symptoms. This novel mutation is likely to be pathogenic. However, the mechanisms of dysfunction of this mutant require further investigation.
In conclusion, we identified a novel c.491G>A mutation in the MMAA gene. Heterozygous carriers of both c.491G>A and c.650T>A mutations are associated with isolated methylmalonic acidemia.
Conflict of interest
The authors declare that they have no conflict of interest.
Consent for publication
The guardians of this patient gave informed consent to the publication of this study. The guardians of this patient gave informed consent to the study.
Funding
This work was supported by the National Science Foundation of China under Grant No. 81773444.
References
- 1.Matsui SM, Mahoney MJ, Rosenberg LE. The natural history of the inherited methylmalonic acidemias. N Engl J Med. 1983;308(15):857–861. doi: 10.1056/NEJM198304143081501. [DOI] [PubMed] [Google Scholar]
- 2.Dobson CM, Wai T, Leclerc D, Wilson A, Wu X, Dore C, Hudson T, Rosenblatt DS, Gravel RA. Identification of the gene responsible for the cblA complementation group of vitamin B12-responsive methylmalonic acidemia based on analysis of prokaryotic gene arrangements. Proc Natl Acad Sci USA. 2002;99(24):15554–15559. doi: 10.1073/pnas.242614799. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Dobson CM, Wai T, Leclerc D, Kadir H, Narang M, Lerner-Ellis JP, Hudson TJ, Rosenblatt DS, Gravel RA. Identification of the gene responsible for the cblB complementation group of vitamin B12-dependent methylmalonic aciduria. Hum Mol Genet. 2002;11(26):3361–3369. doi: 10.1093/hmg/11.26.3361. [DOI] [PubMed] [Google Scholar]
- 4.Han LS, Huang Z, Han F, Ye J, Qiu WJ, Zhang HW, Wang Y, Gong ZW, Gu XF. Clinical features and MUT gene mutation spectrum in Chinese patients with isolated methylmalonic acidemia: identication of ten novel allelic variants. World Journal of Pediatrics. 2015;11(4):358–365. doi: 10.1007/s12519-015-0043-1. [DOI] [PubMed] [Google Scholar]
- 5.Yang X, Sakamoto O, Matsubara Y, Kure S, Suzuki Y, Aoki Y, Suzuki Y, Sakura N, Takayanagi M, Iinuma K, Ohura T. Mutation analysis of the MMAA and MMAB genes in Japanese patients with vitamin B 12-responsive methylmalonic acidemia: identification of a prevalent MMAA mutation. Molecular genetics and metabolism. 2004;82(4):329–333. doi: 10.1016/j.ymgme.2004.05.002. [DOI] [PubMed] [Google Scholar]
- 6.Richards CS, Bale S, Bellissimo DB, Das S, Grody WW, Hegde MR, Lyon E, Ward BE. ACMG recommendations for standards for interpretation and reporting of sequence variations: Revisions 2007. Genet Med. 2008;10(4):294–300. doi: 10.1097/GIM.0b013e31816b5cae. [DOI] [PubMed] [Google Scholar]
- 7.Splinter K, Niemi AK, Cox R, Platt J, Shah M, Enns GM, Kasahara M, Bernstein JA. Impaired health-related quality of life in children and families affected by methylmalonic acidemia. Journal of Genetic Counseling. 2016;25(5):n936–944. doi: 10.1007/s10897-015-9921-x. [DOI] [PubMed] [Google Scholar]
- 8.Devi ARR, Naushad SM. Targeted exome sequencing for the identication of complementation groups in methylmalonic aciduria: a south Indian experience. Clinical Biochemistry. 2017;50(1):68–72. doi: 10.1016/j.clinbiochem.2016.08.016. [DOI] [PubMed] [Google Scholar]
- 9.Imtiaz F, Al-Mubarak BM, Al-Mostafa A, Al-Hamed M, Allam R, Al-Hassnan Z, Al-Owain M, Al-Zaidan H, Rahbeeni Z, Qari A, Faqeih EA, Alasmari A, Al-Mutairi F, Alfadhel M, Eyaid WM, Rashed MS, Al-Sayed M. Spectrum of mutations in 60 Saudi patients with mut methylmalonic acidemia. JIMD Reports. 2016;(29):39–46. doi: 10.1007/8904_2014_297. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Plessl T, Burer C, Lutz S, Yue WW, Baumgartner MR, Froese DS. Protein destabilization and loss of protein-protein interaction are fundamental mechanisms in cblA-type methylmalonic aciduria. Human Mutation. 2017;38(8):988–1001. doi: 10.1002/humu.23251. [DOI] [PubMed] [Google Scholar]