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Major progress occurred in understanding inborn errors of ketone body transport and metabolism between the International Congresses on Inborn Errors of Metabolism in Barcelona (2013) and Rio de Janeiro (2017). These conditions impair either ketogenesis (presenting as episodes of hypoketotic hypoglycemia) or ketolysis (presenting as ketoacidotic episodes); for both groups, immediate intravenous glucose administration is the most critical and (mHGGCS, HMGCS2 ) effective treatment measure. Ketogenesis Deficiencies: New biomarkers were described for mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (mHGGCS, HMGCS2 ) deficiency. New patient series refined clinical knowledge of 3-hydroxy-3-methylglutaryl-CoA lyase (HGGCL, HMGCL ) deficiency. Although affected humans have not been described, two animal model phenotypes are pertinent: zebrafish deficient in monocarboxylate transporter 7 (MCT7, slc16a6 ) (decreased ketone body exit from hepatocytes) or mice lacking D-3-hydroxy-n-butyrate dehydrogenase (BDH1, BDH1 ) (isolated hyperacetoacetatemia; fatty liver). Ketolysis Deficiencies: Monocarboxylate transporter 1 (MCT1, SLC16A1 ) deficiency is a newly described defect of ketone body transport, joining deficiencies of succinyl-CoA:3-oxoacid CoA transferase (SCOT, OXCT1 ) and methylacetoacetyl-CoA thiolase (MAT, ACAT1 ). Some heterozygotes for MCT1 or SCOT deficiency develop ketoacidosis.

Succinyl-CoA:3-oxoacid CoA transferase (SCOT) deficiency is an autosomal recessive disorder of ketone body utilization that is clinically characterized with intermittent ketoacidosis crises. We report here the second Turkish case with SCOT deficiency. She experienced 3 ketoacidotic episodes: The first ketoacidotic crisis mimicked diabetic ketoacidosis because of the associated hyperglycemia. Among patients with SCOT deficiency, the blood glucose levels at the first crises were variable, and this case had the highest ever reported blood glucose level. She is a compound heterozygote with 2 novel mutations, c.517A>G (K173E) and c.1543A>G (M515V), in exons 5 and 17 of the OXCT1 gene, respectively. In patient’s fibroblasts, SCOT activity was deficient and, by immunoblot analysis, SCOT protein was much reduced. The patient attained normal development and had no permanent ketosis. The accurate diagnosis of SCOT deficiency in this case had a vital impact on the management strategy and outcome.

Beta-ketothiolase deficiency is an inherited disorder of ketone body metabolism and isoleucine catabolism. It typically manifests as recurrent ketoacidotic episodes with characteristic abnormalities in the urinary organic acid profile. However, several challenges in the diagnosis of beta-ketothiolase deficiency have been encountered: atypical presentations have been reported and some other disorders, such as succinyl-CoA:3-oxoacid CoA transferase and 2-methyl-3-hydroxybutyryl-CoA dehydrogenase deficiencies, can mimic the clinical and/or biochemical signs of beta-ketothiolase deficiency. A final diagnosis of beta-ketothiolase deficiency requires an enzymatic assay and/or a molecular analysis, but some caveats must be considered. Despite the reported missed cases, screening programs have successfully identified an increasing number of patients with beta-ketothiolase deficiency. Early diagnosis and management of beta-ketothiolase deficiency will enable prevention of its serious acute and chronic complications and ultimately improve the prognosis.