Regular articleClinical, biochemical, mitochondrial, and metabolomic aspects of methylmalonate semialdehyde dehydrogenase deficiency: Report of a fifth case
Introduction
Methylmalonate semialdehyde dehydrogenase deficiency (MMSDD) is a rare autosomal recessive defect in valine and pyrimidine catabolism. Four molecularly confirmed cases have been reported with these displaying considerable clinical and biochemical variability [[1], [2], [3], [4]]. The first reported patient was serendipitously identified due to an increased methionine concentration on newborn screening; MMSDD was identified in the course of follow-up metabolic evaluation [1]. The patient was nondysmorphic with normal development; however, a persistent urine biochemical phenotype included the excretion of β-alanine, β-aminoisobutyrate, 3-hydroxypropionate, and 3-hydroxyisobutyrate [1]. The three subsequently reported patients were developmentally delayed, displayed dysmorphic features, and had altered brain morphology including thin corpus callosum, delayed myelination, and cortical micro-calcification [[2], [3], [4]]. All had increased urinary excretion of 3-hydroxyisobutyrate; however, the increased presence of β-aminoisobutyrate, β-alanine, 3-hydroxypropionate, and other analytes were variable [[2], [3], [4]]. In all four cases, unique ALDH6A1 mutations were identified [[1], [2], [3], [4]].
Valine catabolism (Fig. 1) generates S-methylmalonic semialdehyde, while the terminal steps of thymine degradation leads to R-3-aminoisobutyric acid, which is transaminated to R-methylmalonic semialdehyde. Racemic S- and R-methylmalonic semialdehyde is metabolized to propionyl-CoA by MMSD. Thus MMSDD is predicted to lead to accumulation of both S- and R-methylmalonic semialdehyde. Accumulation of β-alanine and 3-hydroxypropionic from methylmalonic semialdehyde and R- and S-3-aminoisobutyric acid, and S- and R-3-hydroxyisobutyric acid from methylmalonic semialdehyde likely occurs due to reversible reactions of transaminases and 3-hydroxyisobutyric acid dehydrogenase.
We present a fifth molecularly confirmed case of MMSDD, demonstrating additional clinical, biochemical, and molecular heterogeneity. In addition, functional mitochondrial assessment and metabolomic profiling demonstrate MMSDD influences energy production and metabolite homeostasis respectively.
A female was born near term by spontaneous vaginal delivery to non-consanguineous parents of East African origin after an uncomplicated pregnancy. Birth weight was 2730 g. The patient was admitted to the NICU for sepsis evaluation based on neonatal hypothermia and maternal chorioamnionitis. The patient underwent a 48-h antibiotic course, which was discontinued after reassuring lab work and negative cultures. Newborn screening was normal. She was discharged on day 10 of life with a weight of 2549 g, but was re-admitted on day 14 of life due to continued weight loss (weight 2300 g on admission) and non-bloody, non-bilious emesis after feeding, felt to be consistent with gastroesophageal reflux. Stooling, urination, and appetite were normal. Ultrasound showed poor gastric emptying and likely pyloric spasms. Family history was unremarkable. Physical exam identified no apparent dysmorphology. The liver and spleen were of normal size. Brain MRI, MR spectroscopy, and echocardiogram were unremarkable. The neurologic exam identified mild generalized hypotonia.
Routine tests including lactate, ammonia, liver enzymes, glucose, and bilirubin were normal. Metabolic evaluation included plasma amino acids, urine amino acids, acylcarnitines, free/total carnitine, sialo-transferrin phenotyping, plasma methylmalonate, hemoglobin phenotyping, and very long chain fatty acids. Hemoglobin chromatography showed sickle cell trait. Table 1 provides initial diagnostic results for relevant analytes in urine and blood. Clinical ALDH6A1 sequencing identified a homozygous c.1261C > T (p.Pro421Ser) variant of unknown significance. Clinical single nucleotide polymorphism microarray analysis identified neither a deletion within the ALDH6A1 locus nor genomic loss of heterozygosity in any other regions. Because of the suspected diagnosis of MMSDD, the patient was placed on valine restriction. During the course of admission, weight gain improved following valine restriction. In house clinical observation also reported reduced gastroesophageal reflux after valine restriction. Valine intake was subsequently adjusted based on plasma amino acid concentrations. After discharge, the patient was seen regularly as an outpatient with valine restriction and weight gain continued. At 10 weeks of life a hiatal hernia was identified, which was surgically repaired along with G-tube placement. Approximately a month after surgery, daytime feeds were almost entirely oral and weight gain continued. At ~3 months of age, dietary valine was gradual increased. At ~5 months of age, she again experienced post-feeding emesis, which resolved with ranitidine. At ~7 months of age, valine restriction was discontinued, and the patient continued to gain weight. However, at a current age of 23 months, her rate of linear growth has declined and she demonstrates excessive oral secretions. Development, initially delayed, is presently felt to be normal but further monitoring will be necessary to assure continue normal development.
Section snippets
Establishing a MMSDD fibroblast cell line
A dermal punch biopsy was clinically collected from the patient. A fibroblast cell line was established in Dulbecco's modified Eagles medium (DMEM; Corning Life Sciences, Manassas, VA) containing 4.5 g/L glucose and supplemented with 10% fetal bovine serum, 4 mM glutamine, 100 IU penicillin and 100 μg/mL streptomycin, grown at 37 °C in a 5% CO2 humidified atmosphere. Control fibroblasts were obtained from the American Type Culture Collection (ATTC.org) or Coriell Institutute (Coriell.org);
Clinical metabolite analysis
Biochemical genetic assessments utilized clinical laboratory facilities and procedures established at the UPMC Children's Hospital of Pittsburgh Biochemical Genetics laboratory. MMSDD was strongly suggested by the pattern of analytes observed in routine biochemical testing. 3-hydroxypropionate and 3-hydroxyisobutyrate were over-represented in urine organic acids, elevated concentrations of β-aminoisobutyrate and β-alanine were observed in urine amino acids while plasma amino acids displayed an
Discussion
We report the fifth case of MMSDD, describing further clinical and biochemical heterogeneity in this rare disorder. Western blotting experiments on patient fibroblasts showed absence of MMSD protein, which in conjunction with characteristic metabolites in patient and cells confirms pathogenicity of the previously unidentified ALD6A1 variant. Unlike some cases, MRI studies in our patients identified normal brain structure with hypotonia being the only neurologic finding. Our patient presented in
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