There are three human enzymes with HMG-CoA lyase activity that are able to synthesize ketone bodies in different subcellular compartments. The mitochondrial HMG-CoA lyase was the first to be described, and catalyzes the cleavage of 3-hydroxy-3-methylglutaryl CoA to acetoacetate and acetyl-CoA, the common final step in ketogenesis and leucine catabolism. This protein is mainly expressed in the liver and its function is metabolic, since it produces ketone bodies as energetic fuels when glucose levels are low. Another isoform is encoded by the same gene for the mitochondrial HMG-CoA lyase (HMGCL), but it is located in peroxisomes. The last HMG-CoA lyase to be described is encoded by a different gene, HMGCLL1, and is located in the cytosolic side of the endoplasmic reticulum membrane. Some activity assays and tissue distribution of this enzyme have shown the brain and lung as key tissues for studying its function. Although the roles of the peroxisomal and cytosolic HMG-CoA lyases remain unknown, recent studies highlight the role of ketone bodies in metabolic remodeling, homeostasis, and signaling, providing new insights into the molecular and cellular function of these enzymes.

3-hydroxy-3-methylglutaric aciduria (HMGA) is an inherited disorder of the leucine catabolic pathway in which occurs a deficiency of the 3-hydroxy-3-methylglutaryl-CoA lyase enzyme. Therefore, the organic acids 3-hydroxy-3-methylglutaric (HMG) and 3-methylglutaric (MGA), mainly, accumulate in tissues of affected patients. Lately, much attention has been focused on free radicals as mediators of tissue damage in human diseases, causing lipid peroxidation, protein oxidation and DNA damage. The treatment of this disease is based in a restricted protein ingest and supplementation with l-carnitine (LC), an antioxidant and detoxifying agent. In the present work, we investigated the in vitro oxidative damage to DNA induced by the accumulation of organic acids and oxidative stress parameters in vivo of patients with 3-HMG, as well as the effect of the recommended therapy. The in vitro DNA damage was analyzed by the alkaline comet assay in leukocytes incubated with HMG and MGA (1 mM, 2.5 mM and 5 mM) and co-incubated with LC (90 μM and 150 μM). The in vivo urinary 15-F2t-isoprostane levels and urinary oxidized guanine species were measured by ELISA kits in patient's urine before and after the treatment with LC. HMG and MGA induced a DNA damage index (DI) significantly higher than that of the control group. The DI was significantly reduced in the presence of LC. It was also verified a significant increase of oxidized guanine species and urinary isoprostane levels, biomarker of oxidative DNA damage and lipid peroxidation respectively, in patients before treatment. After the treatment and supplementation with LC, patients presented significantly lower levels of those biomarkers. Analyzing the data together, we can conclude that HMGA patients present oxidative lipid and DNA damage, which is induced by HMG and MGA, and the antioxidant therapy with LC can prevent that kind of injuries.

A sensitive and selective method for the separation and quantification of the three organic acids 3-hydroxy-3-methylglutaric acid, 3-methylglutaric acid, and glutaric acid in human urine samples by CE with mass spectrometry detection has been developed. This methodology is faster, simpler and less time-consuming, than other methodologies previously described, and requires of reduced amounts of reagents as well. Samples are first filtered and then diluted in water. For the electrophoretic separation, a 20mM ammonium acetate and 10% methanol solution at pH 9.1 was selected as the running electrolyte. With 5-s hydrodynamic injection, detection limits ranging from 15.5 to 39.3μM and linear responses ranging from the LOQ calculated for each analyte to more than 400μM were obtained for the analysis of the different organic acids in less than 13min. Remarkable selectivity is achieved by mass spectrometry detection using 0.25% of formic acid in 50% v/v 2-propanol-water solution as sheath liquid, and enough sensitivity without interferences from the matrices was obtained as well. This methodology has revealed as an efficient approach to help the 3-hydroxy-3-methylglutaric aciduria diagnoses in order to discard or confirm the occurrence of the disease as of the presence or absence of the expected increased levels of these analytes in samples of potential patients.

3-Hydroxy-3-Methylglutaryl-Coenzyme A (HMG-CoA) lyase deficiency is a rare inborn error of leucine metabolism and ketogenesis. Despite recurrent hypoglycemia and metabolic decompensations, most patients have a good clinical and neurological outcome contrasting with abnormal brain magnetic resonance imaging (MRI) signals and consistent abnormal brain proton magnetic resonance spectroscopy (1H-MRS) metabolite peaks. Identifying these metabolites could provide surrogate markers of the disease and improve understanding of MRI-clinical discrepancy and follow-up of affected patients. Urine samples, brain MRI and 1H-MRS in 5 patients with HMG-CoA lyase deficiency (4 boys and 1 girl aged from 25days to 10years) were, for each patient, obtained on the same day. Brain and urine spectroscopy were performed at the same pH by studying urine at pH 7.4. Due to pH-induced modifications in chemical shifts and because reference 1H NMR spectra are obtained at pH 2.5, spectroscopy of normal urine added with the suspected metabolite was further performed at this pH to validate the correct identification of compounds. Mild to extended abnormal white matter MRI signals were observed in all cases. Brain spectroscopy abnormal peaks at 0.8-1.1ppm, 1.2-1.4ppm and 2.4ppm were also detected by urine spectroscopy at pH 7.4. Taking into account pH-induced changes in chemical shifts, brain abnormal peaks in patients were formally identified to be those of 3-hydroxyisovaleric, 3-methylglutaconic, 3-methylglutaric and 3-hydroxy-3-methylglutaric acids. 3-Methylglutaric, 3-hydroxyisovaleric and 3-hydroxy-3-methylglutaric acids identified on urine 1H-NMR spectra of 5 patients with HMG-CoA lyase deficiency are responsible for the cerebral spectroscopy signature seen in these patients, validating their local involvement in brain and putative contribution to brain neuropathology.

3-Hydroxy-3-methylglutaric aciduria (3-HMG, OMIN 246450) is a rare autosomal recessive metabolic disorder caused by a deficiency of 3-hydroxy-3-methylglutaryl-CoA lyase, a key enzyme in leucine metabolism and ketone body synthesis. Acute episodes of 3-HMG may be triggered by fasting or infection, and symptoms include vomiting, diarrhea, lethargy and hypotonia. If left untreated, prolonged hypoglycemia and metabolic acidosis may cause breathing problems, seizures, and coma. In addition, 3-HMG is associated with damage to the central nervous system, and there are several reports of white matter abnormality or cerebral atrophy. The presence of bilateral basal ganglia damage in 3-HMG has been rarely reported. Here, we present a case report of a 20-month old male with severe 3-HMG and prominent bilateral lesions in the basal ganglia.