Clinical manifestations

Presentation and course

Clinical symptoms caused by mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency are usually those of acute hypoglycemia. Patients show unexpected deterioration of consciousness and rapidly deepening coma leading to respiratory arrest during prolonged periods of poor food intake or fasting. Hepatomegaly with liver steatosis, severe metabolic acidosis, hyperammonemia, and dyslipidemia sometimes without hypoglycemia have been reported in several children (08; 06; 16). Very severe cases may be associated with complete loss of enzyme function due to HGCS2 null variants (09; 16). The condition may present at any age but the first symptomatic episode is usually in late infancy due to greater intervals between meals in conjunction with a higher incidence of common infections with fever, vomiting, and reduced food intake. Variable manifestations were documented in one highly consanguineous family from Turkey with several affected individuals homozygous for a pathogenic splice variant c.725-2A>C (07). One child presented in infancy and has been well since, and one child died in a metabolic decompensation at two years of age, whereas four individuals including an adult were asymptomatic, with a history of hepatosteatosis in two individuals.

Prognosis and complications

Irreversible brain damage after an acute decompensation, prolonged metabolic acidosis, and death have been reported. Unrecognized lethal manifestations may have occurred in some individuals. The prognosis after diagnosis is usually excellent if fasting is avoided.

Clinical vignette

An affected boy was the first child of unrelated German parents (01). After an unremarkable neonatal and early infantile period, at 11 months of age, he developed gastroenteritis with vomiting and poor feeding. On the third day of this illness, he suddenly deteriorated with coma and respiratory arrest. Blood glucose after resuscitation was 1.2 mmol/l (norm: > 3 mmol/l). Apart from elevated transaminases (aspartate aminotransferase 283 U/l, norm: 10-27 U/l; alanine aminotransferase 138 U/l, norm: 5-23 U/l) and lactate dehydrogenase (1502 U/l, norm: 200-500 U/l), there were no abnormalities in routine clinical chemical analysis. Urinary organic acid analysis revealed massive dicarboxylic aciduria without adequate ketonuria. Acylcarnitine analyses in dried blood spots were normal. The patient recovered well with intravenous glucose infusion.

Enzyme studies in fibroblasts showed normal beta-oxidation capacity. Unfortunately, free fatty acids and ketone bodies had not been measured at the time of the acute decompensation, and it was felt necessary to perform a monitored fasting test under controlled conditions in a metabolic hospital unit. During this test the boy developed hypoglycemia (blood glucose 2.3 mmol/l) much earlier than envisaged, 12 hours after the last meal. At this time there was massive elevation of plasma free fatty acids (3290 µmol/l, norm: < 300) without concomitant elevation of total plasma ketones (174 µmol/l, norm: < 150, in prolonged fasting: > 1500). Blood lactate, insulin and transaminases were within normal ranges. Urinary organic acids again showed massive dicarboxylic aciduria without adequate ketonuria. Acylcarnitines in dried blood spots were normal.

The diagnosis was later confirmed by molecular studies that revealed compound heterozygosity for the pathogenic variants p.G212R (c.634G> A) in exon 3 and p.R500H (c.1499G> A) in exon 9 of the HMGCS2 gene. The family was advised to avoid fasting periods of more than 6 to 8 hours. There were no subsequent hypoglycemic episodes, and the child has developed normally with no residual neurologic impairment.

Biological basis

Etiology and pathogenesis

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency is an autosomal recessive disorder caused by pathogenic variants in the HMGCS2 gene. This gene stretches over 20 kb on chromosome 1p13-p12 and contains 10 exons. Greater than 30 pathogenic and likely pathogenic variants and numerous variants of uncertain significance are listed in ClinVar (http://www.ncbi.nlm.nih.gov/clinvar); the majority of predicted pathogenic variants are missense. Functional analyses showed complete loss of protein or protein function in most pathogenic variants studied (12). Correct maintenance of the dimerization surface is crucial for retaining the structure of the active center and, therefore, the activity of the enzyme (11). Most pathogenic variants reported so far are rare. Exceptions are p.Phe174Leu (c. 520T> C), which was reported in an affected boy of Chinese descent and has an allele frequency of approximately one of 700 in East Asians, and p.G212R (c.634G> A, exon 3), which was reported in three European patient families and has an allele frequency of approximately one of 2000 in Europeans. Variant p.Arg501Pro (c.1502G> C) may be prevalent in Thai patients (13). A stop variant p.Tyr242Ter has an allele frequency of one in 1560 in Africans/African Americans but has not yet been reported in patients. Frequency data can be accessed at the following database: http://gnomad.broadinstitute.org. In line with the fasting-dependent clinical presentation and the diagnostic challenges, the available allele frequency data indicate that some individuals with biallelic pathogenic HMGCS2 variants may stay asymptomatic, or they may remain undiagnosed after manifestation.

Fasting is accompanied by an increased breakdown of fatty acid stores for provision of energy to peripheral tissues. Humans are unable to convert even-chain fatty acids into glucose, and organs such as the brain and muscle that do not have the enzymes for complete fatty acid oxidation rely on the utilization of the ketone bodies acetoacetate and 3-hydroxybutyrate during fasting. The hepatic biosynthesis of acetoacetate from acetoacetyl-CoA involves a 2-step process catalyzed by the enzymes 3-hydroxy-3-methylglutaryl-CoA synthase and 3-hydroxy-3-methylglutaryl-CoA lyase. The latter enzyme is also required for the breakdown of the amino acid leucine, and 3-hydroxy-3-methylglutaryl-CoA lyase deficiency usually presents as an organic aciduria with acute metabolic (keto-) acidosis, hypoglycemia, liver disease, and sometimes fatal Reye syndrome-like crisis. In contrast, mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase appears to be required mainly for ketogenesis during fasting. A deficiency of this enzyme, therefore, leads to symptoms or metabolic abnormalities during fasting or poor food intake. In mice, Hmgcs2 knockout was reported to cause spontaneous fatty liver during postnatal development, which was rescued by a shift to a low-fat dietary composition via early weaning. Heterozygous adult mice, which exhibited lower ketogenic activity, were more susceptible to diet-induced NAFLD development (02). It remains to be seen whether altered HMGCS2 function influences the manifestation of multifactorial liver disease in humans.

Epidemiology

This disease has been more frequently diagnosed in the last years, mostly due to the availability of exome sequencing, which allows diagnosis in the absence of metabolic alterations. Considering that most affected patients show age-appropriate development and no metabolic abnormalities except during fasting periods, the condition may be underdiagnosed.

Prevention

The enzyme deficiency itself is of genetic origin and cannot be prevented. However, hypoketotic hypoglycemia and its complications may be prevented by avoiding fasting periods greater than 6 to 8 hours in infants and small children. It can be assumed that fasting tolerance is greater in older children, but no systematic data are available to support or refute this assumption.

Differential diagnosis

Confusing conditions

The clinical presentation of mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase with hypoketotic hypoglycemia resembles that of the disorders of mitochondrial fatty acid oxidation, particularly medium-chain acyl-CoA dehydrogenase (MCAD) deficiency. However, these disorders can usually be recognized by metabolic abnormalities in the interval. For example, disorders of long-chain fatty acid oxidation may also show additional features such as cardiomyopathy, muscle disease or liver disease. The fasting tolerance in mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase may be shorter than in medium-chain acyl-CoA dehydrogenase deficiency. In patients with atypically severe presentation, additional other diseases should be excluded based on clinical features.

Diagnostic workup

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency may be easily recognized through examination of appropriate laboratory tests in samples obtained during hypoglycemia (17). Most patients are now identified through undirected massive parallel sequencing. For biochemical diagnosis, it is essential to recognize deficient conversion of free fatty acids to ketones by measuring these substances in serum. 3-hydroxybutyrate is sufficient for the assessment of ketogenesis capacity as it usually accounts for over two-thirds of total ketones and is more stable than acetoacetate. During prolonged fasting, free fatty acids and total ketones typically reach concentrations of 1500 µmol/ml or higher, with ketones usually exceeding free fatty acids. Insufficient ketogenesis in this situation (eg, ratio free fatty acids / ketones > 2) is pathognomonic for a deficiency of mitochondrial beta-oxidation or ketogenesis even if ketones are elevated compared to the fed state. The discrepancy between highly elevated free fatty acids and low ketones is most pronounced in mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency where the ratio may exceed 20. Urinary stix tests may show apparent elevations of ketones; however, the ketone concentration is far less than expected for the fasting state. In line with increased fatty acid mobilization, blood triglyceride concentrations may be elevated (05).

Once hypoketosis is recognized, the combination of normal or nonspecific acylcarnitines in dried blood spots (some compounds may be elevated as is normal during fasting) and a “fasting pattern without ketones” in urinary organic acids during the acute episode is highly suggestive of mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency. Although acylcarnitine profiles in most patients were normal, elevation of C2-carnitine and other carnitine species may be observed (09). Careful examination of the urinary organic acid profile obtained from the acute presentation may show highly increased concentrations of 4-hydroxy-6-methyl-2-pyrone. Although this metabolite may also be elevated in some ketotic patients, it is highly suggestive of mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency. This observation led to the diagnosis of seven additional patients from various parts of the world in a single Australian center (10).

Molecular studies are used for the confirmation of the diagnosis. DNA analysis should involve full sequencing of the coding region as well as genomic quantification as the disease has been caused by larger deletions in several patients (10). No metabolic abnormalities are usually present in the fed state in mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency, and enzyme studies not only require liver biopsy but are hampered by the cytosolic isoenzyme. A fasting test may be helpful in exceptional cases with equivocal genetic findings. This is potentially dangerous and must be carried out in a specialist hospital setting only.

Management

Management rests on avoidance of fasting periods greater than 6 to 8 hours in infants and small children. Fasting tolerance is thought to be greater in older children and adults. No additional measures are usually necessary. Specifically, there is no evidence that administration of carnitine is of benefit.

Outcomes

The outcome is usually excellent provided that acute metabolic decompensation prior to diagnosis did not lead to irreversible brain damage.

Special considerations

Pregnancy

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase deficiency is not thought to be associated with complications of pregnancy providing that prolonged fasting is avoided.

Anesthesia

Great care should be taken that prolonged fasting is avoided prior to, during, and after surgery.