3-Hydroxyisobutyrate aciduria and mutations in the ALDH6A1 gene coding for methylmalonate semialdehyde dehydrogenase

3-hydroxyisobutyric aciduria is an organic aciduria with a poorly understood biochemical basis. It has previously been assumed that deficiency of 3-hydroxyisobutyrate dehydrogenase (HIBADH) in the valine catabolic pathway is the underlying enzyme defect, but more recent evidence makes it likely that individuals with 3-hydroxyisobutyryic aciduria represent a heterogeneous group with different underlying mechanisms, including respiratory chain defects or deficiency of methylmalonate semialdehyde dehydrogenase. However, to date methylmalonate semialdehyde dehydrogenase deficiency has only been demonstrated at the gene level for a single individual. We present two unrelated patients who presented with developmental delay and increased urinary concentrations of 3-hydroxyisobutyric acid. Both children were products of consanguineous unions and were of European or Pakistani descent. One patient developed a febrile illness and subsequently died from a hepatoencephalopathy at 2 years of age. Further studies were initiated and included tests of the HIBADH enzyme in fibroblast homogenates, which yielded normal activities. Sequencing of the ALDH6A1 gene (encoding methylmalonate semialdehyde dehydrogenase) suggested homozygosity for the missense mutation c.785 C > A (S262Y) in exon 7 which was not found in 210 control alleles. Mutation analysis of the ALDH6A1 gene of the second patient confirmed the presence of a different missense mutation, c.184 C > T (P62S), which was also identified in 1/530 control chromosomes. Both mutations affect highly evolutionarily conserved amino acids of the methylmalonate semialdehyde dehydrogenase protein. Mutation analysis in the ALDH6A1 gene can reveal a cause of 3-hydroxyisobutyric aciduria, which may present with only slightly increased urinary levels of 3-hydroxyisobutyric acid, if a patient is metabolically stable.