Hepatic mitochondrial 3-hydroxy-3-methylglutaryl-coenzyme a synthase deficiency

Critical sample collection delayed? Urine organic acid analysis can still save the day! A new case of HMG-CoA synthase deficiency

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase (mHS) deficiency is an autosomal recessive disorder of ketone body synthesis caused by biallelic pathogenic variants in HMGCS2. Clinical symptoms are precipitated by prolonged fasting and/or intercurrent illness with onset before the first year of life. Clinically, patients may present with hypo-/ non-ketotic hypoglycemia, metabolic acidosis, hyperammonemia, lethargy, hepatomegaly, and encephalopathy. During periods of decompensation, elevations of 4-hydroxy-6-methyl-2-pyrone (4-HMP), several hydroxylated hexanoic and hexenoic acid species, and medium-chain dicarboxylic acids in the absence of significant ketonuria may be observed in the urine organic acid profile. Abnormalities may also be observed in plasma which includes elevated acetylcarnitine (C2) and 3-hydroxybutyryl/3-hydroxyisobutyryl (C4-OH) carnitine. We report a patient who presented to the ED at 13 months of age with an undetectable point-of-care blood glucose level. Continuous infusion of dextrose-containing intravenous (IV) fluids were required to correct the hypoglycemia and routine chemistries were notable for an anion gap metabolic acidosis, transaminasemia, and elevated creatine kinase and lactate dehydrogenase. Urine and blood ketones were undetectable. Qualitative assessment of urine organic acids collected ∼46 and ∼ 99 h post-admission were significant for mild elevations of 4-HMP and hydroxy-hexanoic and hydroxy-hexenoic acid species with a notable absence of ketones. Previously, biochemical abnormalities in urine have been shown to normalize in as few as 27 h after treatment giving providers a narrow window with which to obtain a critical sample. Direct communication of laboratory findings to the ordering provider guided the molecular testing and assisted in results interpretation to confirm the molecular diagnosis. Our case emphasizes the importance of collecting samples for biochemical analysis even if the critical period has been missed and acute metabolic decompensation seems to be resolved, as residual abnormalities observed in our patient greatly narrowed the differential diagnosis.

Hypoglycaemia Metabolic Gene Panel Testing

A large number of inborn errors of metabolism present with hypoglycemia. Impairment of glucose homeostasis may arise from different biochemical pathways involving insulin secretion, fatty acid oxidation, ketone bodies formation and degradation, glycogen metabolism, fructose and galactose metabolism, branched chain aminoacids and tyrosine metabolism, mitochondrial function and glycosylation proteins mechanisms. Historically, genetic analysis consisted of highly detailed molecular testing of nominated single genes. However, more recently, the genetic heterogeneity of these conditions imposed to perform extensive molecular testing within a useful timeframe via new generation sequencing technology. Indeed, the establishment of a rapid diagnosis drives specific nutritional and medical therapies. The biochemical and clinical phenotypes are critical to guide the molecular analysis toward those clusters of genes involved in specific pathways, and address data interpretation regarding the finding of possible disease-causing variants at first reported as variants of uncertain significance in known genes or the discovery of new disease genes. Also, the trio's analysis allows genetic counseling for recurrence risk in further pregnancies. Besides, this approach is allowing to expand the phenotypic characterization of a disease when pathogenic variants give raise to unexpected clinical pictures. Multidisciplinary input and collaboration are increasingly key for addressing the analysis and interpreting the significance of the genetic results, allowing rapidly their translation from bench to bedside.

Hypoglycemia is not a defining feature of metabolic crisis in mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency: Further evidence of specific biochemical markers which may aid diagnosis

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG Co-A) synthase (mHS) deficiency is an autosomal recessive disorder of ketone body synthesis which has traditionally been associated with hypoketotic hypoglycemia, hepatomegaly and encephalopathy, presenting in early childhood following a period of fasting. We report the third case of mHS deficiency presenting in the absence of hypoglycemia, with profound biochemical abnormalities and further evidence of potential specific diagnostic biomarkers. A previously well, 20-month old, unvaccinated male, of nonconsanguineous Polish heritage, presented with encephalopathy, hepatomegaly, severe metabolic acidosis, and mild hyperammonemia following a brief intercurrent illness. The patient was reported to have taken colloidal silver prior to presentation, posing a further diagnostic challenge. Additionally, he developed features suggestive of hemophagocytic lymphohistiocytosis during treatment. While the patient was normoglycemic prior to dextrose administration, the sample was markedly lipemic, with significant hypertriglyceridemia detected. Urine organic acid analysis revealed dicarboxylic aciduria with 4-hydroxy-6-methyl-2-pyrone (4HMP) and the presence of three other previously reported putative biomarkers for mHS deficiency. Glutarate was markedly elevated in the initial chromatogram, with a mild increase in 3-hydroxyglutarate (3HG) persisting. Raised acetylcarnitine was detected on acylcarnitine profile. Molecular genetic analysis of the HMGCS2 gene identified compound heterozygosity for known pathogenic mutations c.634G>A and c.1016+1G>A, confirming the diagnosis of mHS deficiency. This case provides further evidence that hypoglycemia is not invariably present in symptomatic mHS deficiency. We propose that elevated acetylcarnitine, triglycerides, and 3HG are additional biochemical features during acute presentations. With the expansion of novel biomarkers, further cases of this rare disorder may emerge.

Japanese patients with mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency: In vitro functional analysis of five novel HMGCS2 mutations

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (HMGCS2) deficiency is a metabolic disorder caused by mutations in the HMGCS2 gene. The present study describes the identification of four cases of HMGCS2 deficiency in Japan. Hepatomegaly and severe metabolic acidosis were observed in all cases. Fatty liver was identified in three cases, which suggested the unavailability of fatty acids. All patients presented with a high C2/C0 ratio, suggesting that the fatty acid oxidation pathway was normal during metabolic crisis. Genetic analyses revealed five rare, novel variants (p.G219E, p.M235T, p.V253A, p.S392L and p.R500C) in HMGCS2. To confirm their pathogenicity, a eukaryotic expression system and a bacterial expression system was adopted that was successfully used to obtain affinity-purified HMGCS2 protein with measurable activity. Purified M235T, S392L and R500C proteins did not retain any residual activity, whilst the V253A variant showed some residual enzymatic activity. Judging from the transient expression experiment in 293T cells, the G219E variant appeared to be unstable. In conclusion, the present study identified five novel variants of HMGCS2 that were indicated to be pathogenic in four patients affected by HMGCS2 deficiency.

Clinical, biochemical, molecular and therapeutic characteristics of four new patients of mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency

Thirty patients with mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (HMGCS) deficiency, which is a rare autosomal recessive disorder caused by HMGCS2 gene mutation are known. Here, we present four new patients with this disease. The characteristics including several metabolites of patients were recorded. Next-generation targeted sequencing and multiple sequence alignment of PCR amplified products allowed for mutational analysis of HMGCS2. Minigene assay transcript analysis confirmed pathogenicity of a splice site mutation. All cases had recurrent episodes with infections while they had no symptoms during intermissions. Patient 1, a girl, showed recurrent severe metabolic acidosis after infections from 8 months old and presented with weakness, vomiting and lethargy but had normal blood glucose. After treatment, she revived completely. Patients 2, 3 and 4 were boys who showed episodes of hypoglycemia since 8, 27 and 10 months of age, respectively. Glucose infusion reversed the symptoms. All four patients had hepatomegaly and abdominal imaging showed fatty livers. Serum free fatty acid increased. Urinary dicarboxylic acids and urinary 4-hydroxy-6-methyl-2pyrone presented. Diagnosis was confirmed by HMGCS2 gene analysis and 7 mutations (p.R188H, p.F420S, p.R206C, IVS2 + 1G > T, p.E401*, p.A450Pfs*7 and p.Q427*) of this gene were found. Here we report on the characteristics and genetics of four new patients with HMGCS deficiency. This study will enrich our knowledge of this rare autosomal recessive disorder.

Expanding the clinical spectrum of mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency with Turkish cases harboring novel HMGCS2 gene mutations and literature review

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (mHS) deficiency is a very rare autosomal recessive inborn error of ketone body synthesis and presents with hypoketotic hypoglycemia, metabolic acidosis, lethargy, encephalopathy, and hepatomegaly with fatty liver precipitated by catabolic stress. We report acute presentation of two patients from unrelated two families with novel homozygous c.862C>T and c.725-2A>C mutations, respectively, in HMGCS2 gene. Affected patients had severe hypoketotic hypoglycemia, lethargy, encephalopathy, severe metabolic and lactic acidosis and hepatomegaly after infections. Surprisingly, molecular screening of the second family showed more affected patients without clinical findings. These cases expand the clinic spectrum of this extremely rare disease.

Severe clinical manifestation of mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency associated with two novel mutations: a case report

Background

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (mHS) deficiency is an autosomal recessive inborn error of metabolism, which will give rise to failure of ketogenesis in liver during illness or fasting. It is a very rare disease with only a few patients reported worldwide, most of which had a good prognosis after proper therapies.

Case presentation

We report a 9-month-old boy with mHS deficiency presenting with unusually severe and persistent acidosis after diarrhea and reduced oral food intake. The metabolic acidosis persisted even after supplementation with sugar and alkaline solution. Blood purification and assisted respiration alleviated symptoms, but a second onset induced by respiratory infection several days later led to multiple organ failure and death. Urine organic acid analysis during the acute episode revealed a complex pattern of ketogenic dicarboxylic and 3-hydroxydicarboxylic aciduria with prominent elevation of glutaric acid and adipic acid, which seem to be specific to mHS deficiency. Plasma acylcarnitine analysis revealed elevated 3-hydroxybutyrylcarnitine and acetylcarnitine. This is the first report of elevated 3-hydroxybutyrylcarnitine in mHS deficiency. Whole exome sequencing revealed a novel compound heterozygous mutation in HMGCS2 (c.100C > T and c.1465delA).

Conclusion

This severe case suggests the need for patients with mHS deficiency to avoid recurrent illness because it can induce severe metabolic crisis, possibly leading to death. Such patients may also require special treatment, such as blood purification. Urine organic acid profile during the acute episode may give a hint to the disease.

A Japanese case of mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency who presented with severe metabolic acidosis and fatty liver without hypoglycemia

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency (mHS deficiency) is a rare autosomal recessive inborn error of ketogenesis caused by a mutation in the HMGCS2 gene, which is characterized by non-(hypo)-ketotic hypoglycemia, lethargy, and hepatomegaly during acute infection and/or prolonged fasting. Clinical presentations are similar to fatty acid oxidation defects; however, diagnosis of mHS deficiency is difficult because of poor biochemical markers. We report the case of a 12-month-old Japanese boy with mHS deficiency who presented with a coma, and hepatomegaly, but no hypoglycemia after a febrile episode and poor oral intake. Metabolic acidosis and severe fatty liver were observed. Serum acylcarnitine analysis revealed a slightly decreased free carnitine (C0) level and an increased acetylcarnitine (C2) level. Urinary organic acid analysis revealed hypoketotic dicarboxylic aciduria, and increased excretions of glutarate, and, retrospectively, 4-hydroxy-6-methyl-2-pyrone. Although the patient did not present with hypoglycemia, the severe fatty liver and elevated free fatty acids to total ketone bodies ratio strongly suggested an inborn error of ketogenesis. In the analysis of the HMGCS2 gene, compound heterozygous mutations of c.130_131ins C (L44PfsX29) and c.1156_1157insC (L386PfsX73) were identified, which led to the diagnosis of mHS deficiency. He had recovered without any complication by the therapy, including intravenous glucose infusion. Unlike the previously reported cases of mHS deficiency, our case did not present with hypoglycemia and the fatty liver lasted over several months. mHS deficiency should be taken into consideration when a patient has severe metabolic acidosis and fatty liver with no or subtle ketosis, even without hypoglycemia.

Novel HMGCS2 pathogenic variants in a Chinese family with mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency

IMPORTANCE

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase deficiency is a rare and underdiagnosed disorder with fewer than 30 patients reported worldwide. The application of whole-exome sequencing in patients could improve our understanding of this disorder.

OBJECTIVE

To identify the genetic causes and evaluate the phenotype of mitochondrial HMG-CoA synthase deficiency in a pediatric patient with uncommon features that included ketosis and elevated lactate and ammonia.

METHODS

The proband was referred to the pediatric intensive care unit of Beijing Children's Hospital and selected for molecular testing with whole-exome sequencing. Her parents and sibling also underwent sequencing for segregation information.

RESULTS

We identified two novel mutations (c.1347_1351delAGCCT/p.Ala450Profs*7 and c.1201G>T/ p.Glu401*) in the HMG-CoA synthase-2 gene (HMGCS2, NM_005518.3) in the proband and her brother. Both variants were classified as pathogenic variants according to the American College of Medical Genetics and Genomics/ Association for Molecular Pathology guidelines. Metabolic acidosis in the proband was corrected with continuous renal replacement therapy and she left hospital after 21 days of treatment.

INTERPRETATION

Our results extend the genotypic and phenotypic spectrum of HMGCS2 mutation in mitochondrial HMG-CoA synthase deficiency patients and serve as a reminder for physicians to consider mitochondrial HMG-CoA synthase deficiency in newborns and children with coma and hypoketotic hypoglycemia after fasting.

Recurrent loss of HMGCS2 shows that ketogenesis is not essential for the evolution of large mammalian brains

Apart from glucose, fatty acid-derived ketone bodies provide metabolic energy for the brain during fasting and neonatal development. We investigated the evolution of HMGCS2, the key enzyme required for ketone body biosynthesis (ketogenesis). Unexpectedly, we found that three mammalian lineages, comprising cetaceans (dolphins and whales), elephants and mastodons, and Old World fruit bats have lost this gene. Remarkably, many of these species have exceptionally large brains and signs of intelligent behavior. While fruit bats are sensitive to starvation, cetaceans and elephants can still withstand periods of fasting. This suggests that alternative strategies to fuel large brains during fasting evolved repeatedly and reveals flexibility in mammalian energy metabolism. Furthermore, we show that HMGCS2 loss preceded brain size expansion in toothed whales and elephants. Thus, while ketogenesis was likely important for brain size expansion in modern humans, ketogenesis is not a universal precondition for the evolution of large mammalian brains.

Our brain requires a lot of energy to work properly. Sugars are usually the main type of fuel for the body, but when they run low – for example during a food shortage – fat, in the form of fatty acids, can be used instead. However, the brain cannot directly process these molecules; instead, fatty acids need to go through ketogenesis, a process that turns fat into ketone bodies, which the organ can then burn. Scientists believe that the ability to create ketone bodies was essential for us to evolve large brains. Yet, it is still unclear if all mammals can transform fatty acids into ketone bodies. One way to look into this question is to track whether other species have HMGCS2, the main enzyme that drives ketogenesis.

Jebb and Hiller examined the genomes of 70 different species of mammals for the gene that codes for HMGCS2. The comparisons revealed that cetaceans (whales, dolphins and porpoises), Old World fruit bats and the African savanna elephant have all independently lost their working version of HMGCS2. Yet, many members of these three groups have evolved brains that are large for their body size. The genetic analyses showed that dolphins and elephants developed big brains after the enzyme became inactive, challenging the idea that HMGCS2 – and by extension ketogenesis – is always required for the evolution of large brains.

These results may also be useful for conservation efforts. Many fruit bats across the world are severely threatened, and their lack of ketogenesis could explain why these animals are highly sensitive to starvation and quickly die when food becomes scarce.

Mitochondrial 3-Hydroxy-3-Methylglutaryl-CoA Synthase Deficiency: Unique Presenting Laboratory Values and a Review of Biochemical and Clinical Features

We report an 8-month-old infant with decreased consciousness after a febrile episode and reduced oral intake. He was profoundly acidotic but his lactate was normal. Serum triglycerides were markedly elevated and HDL cholesterol was very low. The urine organic acid analysis during the acute episode revealed a complex pattern of relative hypoketotic dicarboxylic aciduria, suggestive of a potential fatty acid oxidation disorder. MRI showed extensive brain abnormalities concerning for a primary energy deficiency. Whole exome sequencing revealed heterozygotic HMGCS2 variants. HMGCS2 encodes mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase-2 (HMGCS2), which catalyzes the irreversible and rate-limiting reaction of ketogenesis in the mitochondrial matrix. Autosomal recessive HMG-CoA synthase deficiency (HMGCS2D) is characterized by hypoketotic hypoglycemia, vomiting, lethargy, and hepatomegaly after periods of prolonged fasting or illness. A retrospective analysis of the urine organic acid analysis identified 4-hydrox-6-methyl-2-pyrone, a recently reported putative biomarker of HMGCS2D. There was also a relative elevation of plasma acetylcarnitine as previously reported in one case. Our patient highlights a unique presentation of HMGCS2D caused by novel variants in HMGCS2. This is the first report of HMGCS2D with a significantly elevated triglyceride level and decreased HDL cholesterol level at presentation. Given this, we suggest that HMGCS2D should be considered in the differential diagnosis when hypertriglyceridemia, or low HDL cholesterol levels are seen in a child who presents with acidosis, mild ketosis, and mental status changes after illness or prolonged fasting. Although HMGCS2D is a rare disorder with nonspecific symptoms, with the advent of next-generation sequencing, and the recognition of novel biochemical biomarkers, the incidence of this condition may become better understood.

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency: urinary organic acid profiles and expanded spectrum of mutations

Mitochondrial 3-hydroxy-3-methylglutaryl CoA synthase (HMCS2) deficiency results in episodes of hypoglycemia and increases in fatty acid metabolites. Metabolite abnormalities described to date in HMCS2 deficiency are nonspecific and overlap with other inborn errors of metabolism, making the biochemical diagnosis of HMCS2 deficiency difficult. Urinary organic acid profiles from periods of metabolic decompensation were studied in detail in HMCS2-deficient patients from four families. An additional six unrelated patients were identified from clinical presentation and/or qualitative identification of abnormal organic acids. The diagnosis was confirmed by sequencing and deletion/duplication analysis of the HMGCS2 gene. Seven related novel organic acids were identified in urine profiles. Five of them (3,5-dihydroxyhexanoic 1,5 lactone; trans-5-hydroxyhex-2-enoate; 4-hydroxy-6-methyl-2-pyrone; 5-hydroxy-3-ketohexanoate; 3,5-dihydroxyhexanoate) were identified by comparison with synthesized or commercial authentic compounds. We provisionally identified trans-3-hydroxyhex-4-enoate and 3-hydroxy-5-ketohexanoate by their mass spectral characteristics. These metabolites were found in samples taken during periods of decompensation and normalized when patients recovered. When cutoffs of adipic >200 and 4-hydroxy-6-methyl-2-pyrone >20 μmol/mmol creatinine were applied, all eight samples taken from five HMCS2-deficient patients during episodes of decompensation were flagged with a positive predictive value of 80% (95% confidence interval 35-100%). Some ketotic patients had increased 4-hydroxy-6-methyl-2-pyrone. Molecular studies identified a total of 12 novel mutations, including a large deletion of HMGCS2 exon 1 in two families, highlighting the need to perform quantitative gene analyses. There are now 26 known HMGCS2 mutations, which are reviewed in the text. 4-Hydroxy-6-methyl-2-pyrone and related metabolites are markers for HMCS2 deficiency. Detection of these metabolites will streamline the biochemical diagnosis of this disorder.

Ketogenesis prevents diet-induced fatty liver injury and hyperglycemia

Nonalcoholic fatty liver disease (NAFLD) spectrum disorders affect approximately 1 billion individuals worldwide. However, the drivers of progressive steatohepatitis remain incompletely defined. Ketogenesis can dispose of much of the fat that enters the liver, and dysfunction in this pathway could promote the development of NAFLD. Here, we evaluated mice lacking mitochondrial 3-hydroxymethylglutaryl CoA synthase (HMGCS2) to determine the role of ketogenesis in preventing diet-induced steatohepatitis. Antisense oligonucleotide-induced loss of HMGCS2 in chow-fed adult mice caused mild hyperglycemia, increased hepatic gluconeogenesis from pyruvate, and augmented production of hundreds of hepatic metabolites, a suite of which indicated activation of the de novo lipogenesis pathway. High-fat diet feeding of mice with insufficient ketogenesis resulted in extensive hepatocyte injury and inflammation, decreased glycemia, deranged hepatic TCA cycle intermediate concentrations, and impaired hepatic gluconeogenesis due to sequestration of free coenzyme A (CoASH). Supplementation of the CoASH precursors pantothenic acid and cysteine normalized TCA intermediates and gluconeogenesis in the livers of ketogenesis-insufficient animals. Together, these findings indicate that ketogenesis is a critical regulator of hepatic acyl-CoA metabolism, glucose metabolism, and TCA cycle function in the absorptive state and suggest that ketogenesis may modulate fatty liver disease.

Ketone body metabolism and its defects

Acetoacetate (AcAc) and 3-hydroxybutyrate (3HB), the two main ketone bodies of humans, are important vectors of energy transport from the liver to extrahepatic tissues, especially during fasting, when glucose supply is low. Blood total ketone body (TKB) levels should be evaluated in the context of clinical history, such as fasting time and ketogenic stresses. Blood TKB should also be evaluated in parallel with blood glucose and free fatty acids (FFA). The FFA/TKB ratio is especially useful for evaluation of ketone body metabolism. Defects in ketogenesis include mitochondrial HMG-CoA synthase (mHS) deficiency and HMG-CoA lyase (HL) deficiency. mHS deficiency should be considered in non-ketotic hypoglycemia if a fatty acid beta-oxidation defect is suspected, but cannot be confirmed. Patients with HL deficiency can develop hypoglycemic crises and neurological symptoms even in adolescents and adults. Succinyl-CoA-3-oxoacid CoA transferase (SCOT) deficiency and beta-ketothiolase (T2) deficiency are two defects in ketolysis. Permanent ketosis is pathognomonic for SCOT deficiency. However, patients with "mild" SCOT mutations may have nonketotic periods. T2-deficient patients with "mild" mutations may have normal blood acylcarnitine profiles even in ketoacidotic crises. T2 deficient patients cannot be detected in a reliable manner by newborn screening using acylcarnitines. We review recent data on clinical presentation, metabolite profiles and the course of these diseases in adults, including in pregnancy.

New case of mitochondrial HMG-CoA synthase deficiency. Functional analysis of eight mutations

Mitochondrial HMG-CoA synthase deficiency is a rare inherited metabolic disorder that affects ketone-body synthesis. Acute episodes include vomiting, lethargy, hepatomegaly, hypoglycaemia, dicarboxylic aciduria, and in severe cases, coma. This deficiency may have been under-diagnosed owing to the absence of specific clinical and biochemical markers, limitations in liver biopsy and the lack of an effective method of expression and enzyme assay for verifying the mutations found. To date, eight patients have been reported with nine allelic variants of the HMGCS2 gene. We present a new method of enzyme expression and a modification of the activity assay that allows, for first time, the functional study of missense mutations found in patients with this deficiency. Four of the missense mutations (p.V54M, p.R188H, p.G212R and p.G388R) did not produce proteins that could have been detected in soluble form by western blot; three produced a total loss of activity (p.Y167C, p.M307T and p.R500H) and one, variant p.F174L, gave an enzyme with a catalytic efficiency of 11.5%. This indicates that the deficiency may occur with partial loss of activity of enzyme. In addition, we describe a new patient with this deficiency, in which we detected the missense allelic variant, c.1162G>A (p.G388R) and the nonsense variant c.1270C>T (p.R424X).

Inhibition of mitochondrial fatty acid oxidation in vivo only slightly suppresses gluconeogenesis but enhances clearance of glucose in mice

Mitochondrial fatty acid oxidation (mFAO) is considered to be essential for driving gluconeogenesis (GNG) during fasting. However, quantitative in vivo data on de novo synthesis of glucose-6-phosphate upon acute inhibition of mFAO are lacking. We assessed hepatic glucose metabolism in vivo after acute inhibition of mFAO by 30 mg kg(-1) 2-tetradecylglycidic acid (TDGA) in hypoketotic hypoglycemic male C57BL/6J mice by the infusion of [U-(13)C]glucose, [2-(13)C]glycerol, [1-(2)H]galactose, and paracetamol for 6 hours, which was followed by mass isotopomer distribution analysis in blood glucose and urinary paracetamol-glucuronide. During TDGA treatment, endogenous glucose production was unaffected (127 +/- 10 versus 118 +/- 7 micromol kg(-1) minute(-1), control versus TDGA, not significant), but the metabolic clearance rate of glucose was significantly enhanced (15.9 +/- 0.9 versus 26.3 +/- 1.1 mL kg(-1) minute(-1), control versus TDGA,P < 0.05). In comparison with control mice, de novo synthesis of glucose-6-phosphate (G6P) was slightly decreased in TDGA-treated mice (108 +/- 19 versus 85 +/- 6 micromol kg(-1) minute(-1), control versus TDGA, P < 0.05). Recycling of glucose was decreased upon TDGA treatment (26 +/- 14 versus 12 +/- 4 micromol kg(-1) minute(-1), control versus TDGA, P < 0.05). Hepatic messenger RNA (mRNA) levels of genes encoding enzymes involved in de novo G6P synthesis were unaltered, whereas glucose-6-phosphate hydrolase mRNA expressions were increased in TDGA-treated mice. Glucokinase and pyruvate kinase mRNA levels were significantly decreased, whereas pyruvate dehydrogenase kinase isozyme 4 expression was increased 30-fold; this suggested decreased glycolytic activity.

CONCLUSION

Acute pharmacological inhibition of mFAO using TDGA had no effect on endogenous glucose production and only a marginal effect on de novo G6P synthesis. Hence, fully active mFAO is not essential for maintenance of hepatic GNG in vivo in fasted mice.

Refining the diagnosis of mitochondrial HMG-CoA synthase deficiency

Mitochondrial HMG-CoA synthase deficiency is an inherited metabolic disorder caused by a defect in the enzyme that regulates the formation of ketone bodies. Patients present with hypoketotic hypoglycaemia, encephalopathy and hepatomegaly, usually precipitated by an intercurrent infection or prolonged fasting. The diagnosis may easily be missed as previously reported results of routine metabolic investigations, urinary organic acids and plasma acylcarnitines may be nonspecific or normal, and a high index of suspicion is required to proceed to further confirmatory tests. We describe a further acute case in which the combination of urinary organic acids, low free carnitine and changes in the plasma acylcarnitine profile on carnitine supplementation were very suggestive of a defect in ketone synthesis. The diagnosis of mitochondrial HMG-CoA synthase deficiency was confirmed on genotyping, revealing two novel mutations: c.614G > A (R188H) and c.971T > C (M307T). A further sibling, in whom the diagnosis had not been made acutely, was also found to be affected. The possible effects of these mutations on enzyme activity are discussed.

Implications of impaired ketogenesis in fatty acid oxidation disorders

Long-chain fatty acids are important sources of respiratory fuel for many tissues and during fasting the rate of hepatic production of ketone bodies is markedly increased. Many extra hepatic tissues utilize ketone bodies in the fasted state with the advantage that glucose is "spared" for more vital tissues like the brain. This glucose sparing effect of ketones is especially important in infants where there is a high proportional glucose utilization in cerebral tissue. The first reported inherited defect affecting fatty acid oxidation was described in 1973 and to date about 15 separate disorders have been described. Although individually rare, cumulatively fatty acid oxidation defects are relatively common, have major consequences for affected individuals and their families, and carry significant health care implications. The major biochemical consequence of fatty acid oxidation defects is an inability of extra hepatic tissues to utilize fatty acids as an energy source with absent or limited hepatic capacity to generate ketones. Clinically patients usually present in infancy with acute life-threatening hypoketotic hypoglycaemia, liver disease, hyperammonaemia and cerebral oedema, with or without cardiac involvement, usually following a period of catabolic stress. Chronically there may be muscle involvement with hypotonia or exercise intolerance with or without cardiomyopathy. Treatment is generally by the avoidance of fasting, frequent carbohydrate rich feeds and for long-chain defects, the replacement of long-chain dietary fats with medium-chain formulae. Novel approaches to treatment include the use of d,l-3-hydoxybutyrate or heptanoate as an alternative energy source.

Strategies for the diagnosis of mitochondrial fatty acid beta-oxidation disorders

Mitochondrial fatty acid beta-oxidation disorders (FAOD) are a group of clinically and biochemically heterogeneous inherited metabolic defects. The spectrum of phenotypes has expanded from hepatic encephalopathy to encompass myopathy, cardiomyopathy, peripheral neuropathy, sudden death and pregnancy complicated by fetal FAOD. Pre-symptomatic diagnosis is important to prevent morbidity and this is now achievable through newborn screening using tandem mass spectrometry (MS/MS). Moreover, most of the diagnosed defects are treatable and the prognosis is generally favourable. This article reviews the features of FAOD, critically evaluates methods of investigation including metabolite analyses in body fluids, in vitro oxidation rates and acylcarnitine profiling studies, enzymatic and mutational tests, and discusses genotype-phenotype correlation, treatment and monitoring options. Based on this knowledge, strategies for the biochemical investigation and differential diagnosis of patients presenting clinically, asymptomatic neonates detected by newborn screening, infants born after complications during late pregnancy, and cases of sudden death with suspected FAOD are presented. Laboratory investigation commonly begins with a search for diagnostic metabolites in physiological fluids, followed by in vitro functional studies if the initial findings are inconclusive, and confirmation by enzymology and molecular analyses. Occasionally a stress test in vivo may be required. At other times there may be no firm diagnosis achieved.

The diagnosis of mitochondrial HMG-CoA synthase deficiency

Deficiency of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase, the only disorder exclusively affecting hepatic ketogenesis, is a cause of hypoglycemic coma. We report that the diagnosis can be made by typical laboratory findings (hypoketosis, elevated free fatty acids, normal acylcarnitines, specific urinary organic acids) during acute episodes.

Fatty acid oxidation disorders

Genetic disorders of mitochondrial fatty acid beta-oxidation have been recognized within the last 20 years as important causes of morbidity and mortality, highlighting the physiological significance of fatty acids as an energy source. Although the mammalian mitochondrial fatty acid-oxidizing system was recognized at the beginning of the last century, our understanding of its exact nature remains incomplete, and new components are being identified frequently. Originally described as a four-step enzymatic process located exclusively in the mitochondrial matrix, we now recognize that long-chain-specific enzymes are bound to the inner mitochondrial membrane, and some enzymes are expressed in a tissue-specific manner. Much of our new knowledge of fatty acid metabolism has come from the study of patients who were diagnosed with single-gene autosomal recessive defects, a situation that seems to be further evolving with the emergence of phenotypes determined by combinations of multiple genetic and environmental factors. This review addresses the normal process of mitochondrial fatty acid beta-oxidation and discusses the clinical, metabolic, and molecular aspects of more than 20 known inherited diseases of this pathway that have been described to date.

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency: clinical course and description of causal mutations in two patients

Hereditary deficiency of mitochondrial HMG-CoA synthase (mHS, OMIM 600234) is a poorly defined, treatable, probably underdiagnosed condition that can cause episodes of severe hypoketotic hypoglycemia. We present clinical follow-up and molecular analysis of the two known mHS-deficient patients. The diagnosis of mHS deficiency is challenging because the symptoms and metabolite pattern are not specific. Moreover, enzyme analysis is technically difficult and requires sampling of an expressing organ such as liver. The patients, now aged 16 and 6 y, have normal development and have had no further decompensations since diagnosis. Patient 1 is homozygous for a phenylalanine-to-leucine substitution at codon 174 (F174L). Interestingly, although the F174 residue is conserved in vertebrate mHS and cytoplasmic HS isozymes, a Leu residue is predicted in the corresponding position of HS-like sequences from Caenorhabditis elegans, Arabidopsis thaliana, and Brassica juncea. Bacterial expression of human F174L-mHS produces a low level of mHS polypeptide with no detectable activity. Similarly, in purified cytoplasmic HS, which in contrast to purified human mHS is stable and can be studied in detail, the corresponding F-->L substitution causes a 10,000-fold decrease in V(max) and a 5-fold reduction in thermal stability. Patient 2 is a genetic compound of a premature termination mutation, R424X, and an as-yet uncharacterized mutant allele that is distinguishable by intragenic single nucleotide polymorphisms that we describe. Molecular studies of mHS are useful in patients with a suggestive clinical presentation.

Fatty acid oxidation defects in muscle

Fatty acid oxidation defects can cause recurrent rhabdomyolysis or chronic progressive muscle weakness. Diagnosis is often possible on blood using tandem mass spectrometry or molecular genetic techniques. Riboflavin and carnitine are effective in some cases of multiple acyl-CoA dehydrogenase deficiency and primary carnitine deficiency, respectively. Controlled trials are needed to evaluate other proposed forms of treatment.