3-Hydroxyisobutyric acid dehydrogenase deficiency: Expanding the clinical spectrum and quantitation of D- and L-3-Hydroxyisobutyric acid by an LC-MS/MS method.

A deficiency of 3-hydroxyisobutyric acid dehydrogenase (HIBADH) has been recently identified as a cause of primary 3-hydroxyisobutyric aciduria in two siblings; the only previously recognized primary cause had been a deficiency of methylmalonic semialdehyde dehydrogenase, the enzyme that is immediately downstream of HIBADH in the valine catabolic pathway and is encoded by the ALDH6A1 gene. Here we report on three additional patients from two unrelated families who present with marked and persistent elevations of urine L-3-hydroxyisobutyric acid (L-3HIBA) and a range of clinical findings. Molecular genetic analyses revealed novel, homozygous variants in the HIBADH gene that are private within each family. Evidence for pathogenicity of the identified variants is presented, including enzymatic deficiency of HIBADH in patient fibroblasts. This report describes new variants in HIBADH as an underlying cause of primary 3-hydroxyisobutyric aciduria and expands the clinical spectrum of this recently identified inborn error of valine metabolism. Additionally, we describe a quantitative method for the measurement of D- and L-3HIBA in plasma and urine and present the results of a valine restriction therapy in one of the patients.

Cerebral edema in maple syrup urine disease despite newborn screening diagnosis and early initiation of treatment.

A 7-day-old girl had an elevated leucine level on newborn screen drawn at 2 days of age and was suspected of having maple syrup urine disease (MSUD). When reported, the patient was immediately admitted to hospital, and started on a modified diet involving high calories with reduced branched chain amino acid (BCAA) formula. Clinical exam was normal at initial assessment. Despite rapid initiation of treatment, the baby became lethargic and somnolent over the next day. Diet was stopped and infusions of 12.5% dextrose and 20% intravenous lipids at 2 g/kg per day were immediately started. Lethargy improved within 3 h of intravenous therapy initiation. Brain magnetic resonance imaging demonstrated diffuse cerebral edema, and symmetric restricted diffusion in bilateral cerebellar white matter, dorsal brainstem, thalami, globi pallidi, posterior limbs of internal capsules, and corona radiata. Plasma leucine was 1.98 mmol/L on admission (normal 0.05-0.17 mmol/L), decreasing to 1.34 mmol/L with diet, however clinical deterioration occurred despite improving leucine levels.Cerebral edema in MSUD is thought secondary to a combination of increased cerebral BCAA levels, and depleted levels of other essential amino acids, as well as neurotransmitters. Our case illustrates that newborns can develop encephalopathy with cerebral edema despite treatment with special formula initiated while asymptomatic. These findings suggest decompensation may begin early on, so that early introduction of high dextrose infusion and intravenous lipids, in combination with reduced BCAA formula, should be initiated for any patient with a positive newborn screen for MSUD.