3-Hydroxyisobutyric acid dehydrogenase deficiency (3-hydroxyisobutyric aciduria) is an extremely rare inborn error of metabolism (IEM), potentially caused by numerous mechanisms. It is currently thought to be autosomal recessively inherited. At least two cases of 3-hydroxyisobutyric aciduria were determined to be caused by a mutation in the ALDH6A1 gene, which encodes acylating methylmalonate-semialdehyde dehydrogenase. This enzyme converts 2-methyl-3-oxopropanoate, CoA and water into propanoyl-CoA, using NAD+ as an oxidizing agent, and producing a hydrogen ion and hydrogencarbonate as byproducts. Other forms of 3-hydroxyisobutyric aciduria may be caused by a mutation in the gene encoding 3-hydroxyisobutyrate dehydrogenase, which forms (S)-methylmalonic acid semialdehyde from (S)-3-hydroxyisobutyric acid. This mutation leads to an accumulation of (S)-3-hydroxyisobutyric acid, as no other processes in the pathway use it. 3-hydroxyisobutyric aciduria is characterized by elevated levels of 3-hydroxyisobutyric acid excreted in the urine. Symptoms of the disorder include dysmorphic features, developmental delays and intellectual disabilities. Treatments are not currently well researched due to the rarity of the condition, but protein-restricted diets may be helpful. It is estimated that 3-hydroxyisobutyric aciduria affects less than 1 in 1,000,000 people, with only 12 cases having been reported by 2006.
Loupatty FJ, van der Steen A, Ijlst L, Ruiter JP, Ofman R, Baumgartner MR, Ballhausen D, Yamaguchi S, Duran M, Wanders RJ: Clinical, biochemical, and molecular findings in three patients with 3-hydroxyisobutyric aciduria. Mol Genet Metab. 2006 Mar;87(3):243-8. doi: 10.1016/j.ymgme.2005.09.019. Epub 2006 Feb 8.
Sass JO, Walter M, Shield JP, Atherton AM, Garg U, Scott D, Woods CG, Smith LD: 3-Hydroxyisobutyrate aciduria and mutations in the ALDH6A1 gene coding for methylmalonate semialdehyde dehydrogenase. J Inherit Metab Dis. 2012 May;35(3):437-42. doi: 10.1007/s10545-011-9381-x. Epub 2011 Aug 24.
Valine, Leucine, and Isoleucine Degradation References
Lehninger, A.L. Lehninger principles of biochemistry (4th ed.) (2005). New York: W.H Freeman.
Salway, J.G. Metabolism at a glance (3rd ed.) (2004). Alden, Mass.: Blackwell Pub.
Wanders RJ, Duran M, Loupatty FJ: Enzymology of the branched-chain amino acid oxidation disorders: the valine pathway. J Inherit Metab Dis. 2012 Jan;35(1):5-12. doi: 10.1007/s10545-010-9236-x. Epub 2010 Nov 23.
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