3-Hydroxy-3-methylglutaryl-coenzyme a lyase deficiency: a review

Treatment of HMG-CoA Lyase Deficiency-Longitudinal Data on Clinical and Nutritional Management of 10 Australian Cases

3-Hydroxy-3-Methylglutaryl-CoA Lyase (HMGCL) deficiency can be a very severe disorder that typically presents with acute metabolic decompensation with features of hypoketotic hypoglycemia, hyperammonemia, and metabolic acidosis. A retrospective chart and literature review of Australian patients over their lifespan, incorporating acute and long-term dietary management, was performed. Data from 10 patients contributed to this study. The index case of this disorder was lost to follow-up, but there is 100% survival in the remainder of the cases despite several having experienced life-threatening episodes. In the acute setting, five of nine patients have used 900 mg/kg/day of sodium D,L 3-hydroxybutyrate in combination with intravenous dextrose-containing fluids (delivering glucose above estimated basal utilization requirements). All patients have been on long-term protein restriction, and those diagnosed more recently have had additional fat restriction. Most patients take L-carnitine. Three children and none of the adults take nocturnal uncooked cornstarch. Of the cohort, there were two patients that presented atypically-one with fulminant liver failure and the other with isolated developmental delay. Dietary management in patients with HMGCL deficiency is well tolerated, and rapid institution of acute supportive metabolic treatment is imperative to optimizing survival and improve outcomes in this disorder.

Machine learning approaches to predict gestational age in normal and complicated pregnancies via urinary metabolomics analysis

The elucidation of dynamic metabolomic changes during gestation is particularly important for the development of methods to evaluate pregnancy status or achieve earlier detection of pregnancy-related complications. Some studies have constructed models to evaluate pregnancy status and predict gestational age using omics data from blood biospecimens; however, less invasive methods are desired. Here we propose a model to predict gestational age, using urinary metabolite information. In our prospective cohort study, we collected 2741 urine samples from 187 healthy pregnant women, 23 patients with hypertensive disorders of pregnancy, and 14 patients with spontaneous preterm birth. Using gas chromatography-tandem mass spectrometry, we identified 184 urinary metabolites that showed dynamic systematic changes in healthy pregnant women according to gestational age. A model to predict gestational age during normal pregnancy progression was constructed; the correlation coefficient between actual and predicted weeks of gestation was 0.86. The predicted gestational ages of cases with hypertensive disorders of pregnancy exhibited significant progression, compared with actual gestational ages. This is the first study to predict gestational age in normal and complicated pregnancies by using urinary metabolite information. Minimally invasive urinary metabolomics might facilitate changes in the prediction of gestational age in various clinical settings.

3-hydroxy-3-methylglutaryl-coenzyme A lyase deficiency: one disease - many faces

Background

3-hydroxy-3-methylglutaryl-coenzyme A lyase deficiency (HMGCLD) is an autosomal recessive disorder of ketogenesis and leucine degradation due to mutations in HMGCL.

Method

We performed a systematic literature search to identify all published cases. Two hundred eleven patients of whom relevant clinical data were available were included in this analysis. Clinical course, biochemical findings and mutation data are highlighted and discussed. An overview on all published HMGCL variants is provided.

Results

More than 95% of patients presented with acute metabolic decompensation. Most patients manifested within the first year of life, 42.4% already neonatally. Very few individuals remained asymptomatic. The neurologic long-term outcome was favorable with 62.6% of patients showing normal development.

Conclusion

This comprehensive data analysis provides a systematic overview on all published cases with HMGCLD including a list of all known HMGCL mutations.

The specific molecular architecture of plant 3-hydroxy-3-methylglutaryl-CoA lyase

3-Hydroxy-3-methylglutaryl-CoA (HMG-CoA) lyase (HMGL) is involved in branched-chain amino acid catabolism leading to acetyl-CoA production. Here, using bioinformatics analyses and protein sequence alignments, we found that in Arabidopsis thaliana a single gene encodes two HMGL isoforms differing in size (51 kDa, HMGL51 and 46 kDa, HMGL46). Similar to animal HMGLs, both isoforms comprised a C-terminal type 1 peroxisomal retention motif, and HMGL51 contained a mitochondrial leader peptide. We observed that only a shortened HMGL (35 kDa, HMGL35) is conserved across all kingdoms of life. Most notably, all plant HMGLs also contained a specific N-terminal extension (P100) that is located between the N-terminal mitochondrial targeting sequence TP35 and HMGL35 and is absent in bacteria and other eukaryotes. Interestingly, using HMGL enzyme assays, we found that rather than HMGL46, homodimeric recombinant HMGL35 is the active enzyme catalyzing acetyl-CoA and acetoacetate synthesis when incubated with (S)-HMG-CoA. This suggested that the plant-specific P100 peptide may inactivate HMGL according to specific physiological requirements. Therefore, we investigated whether the P100 peptide in HMGL46 alters its activity, possibly by modifying the HMGL46 structure. We found that induced expression of a cytosolic HMGL35 version in A. thaliana delays germination and leads to rapid wilting and chlorosis in mature plants. Our results suggest that in plants, P100-mediated HMGL inactivation outside of peroxisomes or mitochondria is crucial, protecting against potentially cytotoxic effects of HMGL activity while it transits to these organelles.

Coupled brain and urine spectroscopy - in vivo metabolomic characterization of HMG-CoA lyase deficiency in 5 patients

BACKGROUND

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.

METHODS

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.

RESULTS

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.

CONCLUSION

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.

NMDA Receptors and Oxidative Stress Induced by the Major Metabolites Accumulating in HMG Lyase Deficiency Mediate Hypophosphorylation of Cytoskeletal Proteins in Brain From Adolescent Rats: Potential Mechanisms Contributing to the Neuropathology of This Disease

Neurological symptoms and cerebral abnormalities are commonly observed in patients with 3-hydroxy-3-methylglutaryl-CoA lyase (HMG lyase) deficiency, which is biochemically characterized by predominant tissue accumulation of 3-hydroxy-3-methylglutaric (HMG), 3-methylglutaric (MGA), and 3-methylglutaconic (MGT) acids. Since the pathogenesis of this disease is poorly known, the present study evaluated the effects of these compounds on the cytoskeleton phosphorylating system in rat brain. HMG, MGA, and MGT caused hypophosphorylation of glial fibrillary acidic protein (GFAP) and of the neurofilament subunits NFL, NFM, and NFH. HMG-induced hypophosphorylation was mediated by inhibiting the cAMP-dependent protein kinase (PKA) on Ser55 residue of NFL and c-Jun kinase (JNK) by acting on KSP repeats of NFM and NFH subunits. We also evidenced that the subunit NR2B of NMDA receptor and Ca(2+) was involved in HMG-elicited hypophosphorylation of cytoskeletal proteins. Furthermore, the antioxidants L-NAME and TROLOX fully prevented both the hypophosphorylation and the inhibition of PKA and JNK caused by HMG, suggesting that oxidative damage may underlie these effects. These findings indicate that the main metabolites accumulating in HMG lyase deficiency provoke hypophosphorylation of cytoskeleton neural proteins with the involvement of NMDA receptors, Ca(2+), and reactive species. It is presumed that these alterations may contribute to the neuropathology of this disease.

Induction of a Proinflammatory Response in Cortical Astrocytes by the Major Metabolites Accumulating in HMG-CoA Lyase Deficiency: the Role of ERK Signaling Pathway in Cytokine Release

3-Hydroxy-3-methylglutaric aciduria (HMGA) is an inherited metabolic disorder caused by 3-hydroxy-3-methylglutaryl-CoA lyase deficiency. It is biochemically characterized by predominant tissue accumulation and high urinary excretion of 3-hydroxy-3-methylglutarate (HMG) and 3-methylglutarate (MGA). Affected patients commonly present acute symptoms during metabolic decompensation, including vomiting, seizures, and lethargy/coma accompanied by metabolic acidosis and hypoketotic hypoglycemia. Although neurological manifestations are common, the pathogenesis of brain injury in this disease is poorly known. Astrocytes are important for neuronal protection and are susceptible to damage by neurotoxins. In the present study, we investigated the effects of HMG and MGA on important parameters of redox homeostasis and cytokine production in cortical cultured astrocytes. The role of the metabolites on astrocyte mitochondrial function (thiazolyl blue tetrazolium bromide (MTT) reduction) and viability (propidium iodide incorporation) was also studied. Both organic acids decreased astrocytic mitochondrial function and the concentrations of reduced glutathione without altering cell viability. In contrast, they increased reactive species formation (2'-7'-dichlorofluorescein diacetate (DCFHDA) oxidation), as well as IL-1β, IL-6, and TNF α release through the ERK signaling pathway. Taken together, the data indicate that the principal compounds accumulating in HMGA induce a proinflammatory response in cultured astrocytes that may possibly be involved in the neuropathology of this disease.

Analysis of aberrant splicing and nonsense-mediated decay of the stop codon mutations c.109G>T and c.504_505delCT in 7 patients with HMG-CoA lyase deficiency

Eukaryotic cells can be protected against mutations that generate stop codons by nonsense-mediated mRNA decay (NMD) and/or nonsense-associated altered splicing (NAS). However, the processes are only partially understood and do not always occur. In this work, we study these phenomena in the stop codon mutations c.109G>T (p.Glu37*) and c.504_505delCT; the second and third most frequent mutations in HMG-CoA lyase deficiency (MIM #246450). The deficiency affects the synthesis of ketone bodies and produces severe disorders during early childhood. We used a minigene approach, real-time quantitative PCR and the inhibition of NMD by puromycin treatment, to study the effect of stop codons on splicing (NAS) and NMD in seven patients. Surprisingly, none of the stop codons studied appears to be the direct cause of aberrant splicing. In the mutation c.109G>T, the splicing is due to the base change G>T at position 109, which is critical and cannot be explained by disruption of exonic splicing enhancer (ESE) elements, by the appearance of exonic splicing silencer (ESS) elements which were predicted by bioinformatic tools or by the stop codons. Moreover, the mutation c.504_505delCT produces two mRNA transcripts both with stop codons that generate simultaneous NMD phenomena. The effects of the mutations studied on splicing seemed to be similar in all the patients. Furthermore, we report a Spanish patient with 3-hydroxy-3-methylglutaric aciduria and a novel missense mutation: c.825C>G (p.Asn275Lys).

A series of pregnancies in women with inherited metabolic disease

In this case series we report 12 pregnancies, in women treated at four centres, illustrating some of the issues that may be encountered during pregnancy by women with inherited metabolic disease. We discuss how specific pregnancy, labour and delivery issues for mothers with methylmalonic acidemia, homocystinuria, propionic acidemia, glutaric acidemia type 1, ornithine transcarbamylase (OTC) deficiency and 3-hydroxy-3-methylglutaric(HMG)-CoA lyase deficiency were managed and the outcome for the mother and child in each case. Eight of the 12 pregnancies resulted in the successful delivery of a liveborn infant. Several women experienced decompensation of their condition during pregnancy or the post-partum period. There was one maternal death in a women with 3-hydroxy-3-methylglutaric(HMG)-CoA lyase deficiency. Pre-pregnancy counselling and co-management of high risk medical patients by obstetricians and specialist physicians with an understanding of the relationship between pregnancy and inherited metabolic disease is essential.

Alternative routes of acetyl-CoA synthesis identified by comparative genomic analysis: involvement in the lipid production of oleaginous yeast and fungi

For a bio-based economy, microbial lipids offer a potential solution as alternative feedstocks in the oleochemical industry. The existing genome data for the promising strains, oleaginous yeasts and fungi, allowed us to investigate candidate orthologous sequences that participate in their oleaginicity. Comparative genome analysis of the non-oleaginous (Saccharomyces cerevisiae, Candida albicans and Ashbya gossypii) and oleaginous strains (Yarrowia lipolytica, Rhizopus oryzae, Aspergillus oryzae and Mucor circinelloides) showed that 209 orthologous protein sequences of the oleaginous microbes were distributed over several processes of the cells. Based on the 41 sequences categorized by metabolism, putative routes potentially involved in the generation of precursors for fatty acid and lipid synthesis, particularly acetyl-CoA, were then identified that were not present in the non-oleaginous strains. We found a set of the orthologous oleaginous proteins that was responsible for the biosynthesis of this key two-carbon metabolite through citrate catabolism, fatty acid β-oxidation, leucine metabolism and lysine degradation. Our findings suggest a relationship between carbohydrate, lipid and amino acid metabolism in the biosynthesis of acetyl-CoA, which contributes to the lipid production of oleaginous microbes.

Differential HMG-CoA lyase expression in human tissues provides clues about 3-hydroxy-3-methylglutaric aciduria

3-Hydroxy-3-methylglutaric aciduria is a rare human autosomal recessive disorder caused by deficiency of 3-hydroxy-3-methylglutaryl CoA lyase (HL). This mitochondrial enzyme catalyzes the common final step of leucine degradation and ketogenesis. Acute symptoms include vomiting, seizures and lethargy, accompanied by metabolic acidosis and hypoketotic hypoglycaemia. Such organs as the liver, brain, pancreas, and heart can also be involved. However, the pathophysiology of this disease is only partially understood. We measured mRNA levels, protein expression and enzyme activity of human HMG-CoA lyase from liver, kidney, pancreas, testis, heart, skeletal muscle, and brain. Surprisingly, the pancreas is, after the liver, the tissue with most HL activity. However, in heart and adult brain, HL activity was not detected in the mitochondrial fraction. These findings contribute to our understanding of the enzyme function and the consequences of its deficiency and suggest the need for assessment of pancreatic damage in these patients.

3-Hydroxy-3-methylglutaryl-coenzyme A lyase deficiency: initial presentation in a young adult

3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) lyase deficiency is a rare inborn error affecting leucine catabolism and ketogenesis, usually presenting in the neonatal period. Late forms of the disease have been detected in infancy and childhood, but not in adults. We report a case of HMG-CoA lyase deficiency with initial presentation in a 29-year-old adult with no prior history of the disease, which to our knowledge is the first case described with presentation at this age.

Incidence of 3-hydroxy-3-methylglutaryl-coenzyme A lyase (HL) deficiency in Brazil, South America

3-Hydroxy-3-methylglutaryl-CoA lyase (HL) deficiency (3-hydroxy-3-methylglutaric aciduria, 3-HMG) is a rare autosomal recessive inborn error of metabolism involving the final step of leucine degradation. HL is the key enzyme for the production of glucose-sparing ketone bodies for brain. Positive biochemical findings are metabolic acidosis, hyperammonaemia, and hypoketotic hypoglycaemia in the neonatal period or infancy. In the present study we report 15 Brazilian patients with HL deficiency and present their clinical and biochemical findings. Urine from all patients contained large amounts of 3-hydroxy-3-methylglutaric, 3-methylglutaconic, 3-hydroxyisovaleric and 3-methylglutaric acids, and 3-methylcrotonylglycine was also observed in 13 patients. The main features at clinical presentation were hypoglycaemia (12 patients), seizures (10 patients), metabolic acidosis (9 patients), vomiting (6 patients), and hepatomegaly (5 patients). All but two patients were of Portuguese ancestry. HL deficiency comprised 7.3% of total organic acidurias detected in our laboratory during a 13-year time span, indicating a high incidence of this disorder in Brazil. Limited molecular characterization (4/15 patients only) revealed two mutations common for individuals of Portuguese/Spanish (Iberian Peninsula) ancestry (E37X and V168fs(-2)). Our findings increase the number of HL-deficient patients and reinforce the characteristic phenotypic picture of the disease. Effective dietary interventions based on mild protein restriction and avoidance of fasting and possibly alternative C5 ketone body generating therapy for this disorder may provide further impetus and rationale for expanded newborn screening of HL deficiency.

Molecular analysis of Taiwanese patients with 3-hydroxy-3-methylglutaryl CoA lyase deficiency

BACKGROUND

3-Hydroxy-3-methylglutaryl CoA lyase deficiency (HL deficiency) is a rare autosomal recessive mitochondrial disease characterized by a deficiency in the enzyme 3-Hydroxy-3-methylglutaryl CoA lyase (HMGCL). Here, we report on novel mutations identified in the HMGCL gene in 2 Taiwanese patients with HL deficiency.

METHODS

Analysis of organic acids in urine was performed using gas chromatography-mass spectrometry to confirm HL deficiency in the two subjects. The mutations in their HMGCL genes then were determined by direct sequencing. In addition, the effect of a splice site mutation was determined using reverse transcription-polymerase chain reactions (RT-PCR).

RESULTS

A total of 3 novel mutations in the HMGCL gene were revealed by molecular analysis: one missense mutation (c.494G>T, p.Arg165Gln) and 2 splice site mutations (IVS3+1G>A, IVS6-1G>A). The results of RT-PCR revealed that an IVS3+1G>A mutation leads to skipping of exon3. We also calculated that the incidence of HL deficiency in Taiwan is <1 per 1,000,000 live births.

CONCLUSIONS

The results of this study suggest that unique HMGCL gene mutations exist in Taiwanese HL deficiency patients. Therefore, HMGCL gene profiling may be useful in genetic counseling for families affected by HL deficiency.

Mutations underlying 3-hydroxy-3-methylglutaryl CoA lyase deficiency in the Saudi population

BACKGROUND

3-hydroxy-3-methylglutaric aciduria (3HMG, McKusick: 246450) is an autosomal recessive branched chain organic aciduria caused by deficiency of the enzyme 3-Hydroxy-3-Methylglutaryl CoA lyase (HL, HMGCL, EC 4.1.3.4). HL is encoded by HMGCL gene and many mutations have been reported. 3HMG is commonly observed in Saudi Arabia.

METHODS

We utilized Whole Genome Amplification (WGA), PCR and direct sequencing to identify mutations underlying 3HMG in the Saudi population. Two patients from two unrelated families and thirty-four 3HMG positive dried blood spots (DBS) were included.

RESULTS

We detected the common missense mutation R41Q in 89% of the tested alleles (64 alleles). 2 alleles carried the frame shift mutation F305fs (-2) and the last two alleles had a novel splice site donor IVS6+1G>A mutation which was confirmed by its absence in more than 100 chromosomes from the normal population. All mutations were present in a homozygous state, reflecting extensive consanguinity. The high frequency of R41Q is consistent with a founder effect. Together the three mutations described account for >94% of the pathogenic mutations underlying 3HMG in Saudi Arabia.

CONCLUSION

Our study provides the most extensive genotype analysis on 3HMG patients from Saudi Arabia. Our findings have direct implications on rapid molecular diagnosis, prenatal and pre-implantation diagnosis and population based prevention programs directed towards 3HMG.

3-hydroxy-3-methylglutaryl-CoA lyase deficiency in an adult with leukoencephalopathy

3-Hydroxy-3-methylglutaryl-CoA lyase deficiency is a disorder of leucine metabolism that usually presents with recurrent episodes of life-threatening hypoglycemia during early childhood. We report on a 36-year-old woman with seizures, recurrent metabolic disturbances, and severe leukoencephalopathy. The diagnosis was made by analysis of amino acids in urine and serum and was confirmed by demonstration of the deficient enzyme in cultured skin fibroblasts. The patient improved clinically on oral L-carnitine substitution. This treatable condition can remain unrecognized in adults and should be considered a potential cause of leukoencephalopathy.

Molecular basis of 3-hydroxy-3-methylglutaric aciduria

3-Hydroxy-3-methylglutaric aciduria is a human autosomal recessive metabolic disorder that usually appears within the first year of life. The causes of this aciduria are lethal mutations in the gene encoding for 3-hydroxy-3-methylglutaryl coenzyme A lyase (HL). HL is a mitochondrial matrix enzyme that catalyzes the last step of ketogenesis and leucine catabolism. This gene has been mapped to chromosome 1 at locus 1pter-p33 and its genomic organisation comprises 9 exons whose sizes vary between 64-678 bp. The human cDNA sequence was reported in 1993 with the first genetic study of two Acadian-French Canadian siblings. To date, 24 mutations in 36 patients have been described; most of them are single-base substitutions causing amino acid replacements and a variety of splicing defects. In the population studied two mutations appear predominant: g.122GA (8 patients and 15 alleles) frequent in Saudi Arabia, and g.109GT (6 patients and 12 alleles), prevalent in Spain. At least seven mutations are clustered in the second half of exon 2 affecting aminoacids E37, R41 and D42 and conforming a possible hot spot. The genotype-phenotype correlation is difficult to establish since the probands received different treatments, and the onset of an acute episode frequently depends on external factors such as fasting or acute illness.

Structural (betaalpha)8 TIM barrel model of 3-hydroxy-3-methylglutaryl-coenzyme A lyase

This study describes three novel homozygous missense mutations (S75R, S201Y, and D204N) in the 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) lyase gene, which caused 3-hydroxy-3-methylglutaric aciduria in patients from Germany, England, and Argentina. Expression studies in Escherichia coli show that S75R and S201Y substitutions completely abolished the HMG-CoA lyase activity, whereas D204N reduced catalytic efficiency to 6.6% of the wild type. We also propose a three-dimensional model for human HMG-CoA lyase containing a (betaalpha)8 (TIM) barrel structure. The model is supported by the similarity with analogous TIM barrel structures of functionally related proteins, by the localization of catalytic amino acids at the active site, and by the coincidence between the shape of the substrate (HMG-CoA) and the predicted inner cavity. The three novel mutations explain the lack of HMG-CoA lyase activity on the basis of the proposed structure: in S75R and S201Y because the new amino acid residues occlude the substrate cavity, and in D204N because the mutation alters the electrochemical environment of the active site. We also report the localization of all missense mutations reported to date and show that these mutations are located in the beta-sheets around the substrate cavity.

Biochemical and molecular analyses in three patients with 3-hydroxy-3-methylglutaric aciduria

Two methods, spectrophotometry and HPLC, were compared in the analyses of 3-hydroxy-3-methylglutaryl-CoA lyase (HL) activity in three unrelated Czech patients with 3-hydroxy-3-methylglutaric (HMG) aciduria and their family members. The HL activities in cultured fibroblasts and/or isolated lymphocytes of probands were below the detection limits of the methods used. Both methods were also suitable for recognition of all heterozygotes in affected families. We searched for pathogenic mutations in the HL gene. Molecular analyses revealed that two patients are homozygous for known mutation H233R and R41Q, respectively, whereas the third patient is a compound heterozygote for the mutation H233R and a novel mutation Pro9fs(-1). This study expands the knowledge of the genotypic variability of the HMG aciduria.

Two missense point mutations in different alleles in the 3-hydroxy-3-methylglutaryl coenzyme A lyase gene produce 3-hydroxy-3-methylglutaric aciduria in a French patient

Two novel point mutations in the 3-hydroxy-3-methylglutaryl coenzyme A lyase gene were found in a French patient with double heterozygous 3-hydroxy-3-methylglutaric aciduria. Amplification by reverse transcriptase-polymerase chain reaction of the mRNA using five different pairs of oligonucleotides produced no differences in the fragments amplified with respect to the control. Single-strand conformation polymorphism analysis showed that only one amplified fragment was different in the patient vs. control. Sequencing of the amplified fragments showed two missense point mutations, A698G and T788C, each of them mixed with the wild-type sequence. These mutations produced the changes H233R and L263P, leading to changes in the enzyme activity, which was largely abolished. The father and one brother of the proband were heterozygous for the L263P mutation and the mother and one daughter were heterozygous for the H233R mutation, which confirms the double-heterozygous character of the patient. Another sibling was free of the mutations. An enzymatic restriction analysis has been proposed to screen the occurrence of these two mutations in future patients.

A nonsense mutation in the exon 2 of the 3-hydroxy-3-methylglutaryl coenzyme A lyase (HL) gene producing three mature mRNAs is the main cause of 3-hydroxy-3-methylglutaric aciduria in European Mediterranean patients

3-Hydroxy-3-methylglutaric aciduria is a rare recessive monogenic disorder that affects ketogenesis and the catabolism of L-leucine. We report the biochemical and molecular characterization of a mutation in the 3-hydroxy-3-methylglutaryl coenzyme A lyase gene in four new probands, three Spanish and one Turkish, affected by 3-hydroxy-3-methylglutaric aciduria, all homozygous for the nonsense mutation Glu37Ter, which was reported by our group in two probands of Portuguese and Moroccan origin (15). In addition to the aberrant mRNAs found in the two previous probands, a novel species of mature HL mRNA was observed in the patients studied here, since a new cDNA, skipped in exons 2 and 3, was obtained from the mRNAs by reverse-transcription PCR (RT-PCR). Thus, three mRNA species were produced in aberrant splicings as a result of this nonsense mutation: (i) one of the expected size that contains the premature stop codon UAA, (ii) another with a deletion of 84 bp corresponding to the whole of exon 2, and (iii) a new species found now, with a deletion of 192 bp corresponding to skipping of the whole of exons 2 and 3, whose translation product led to the loss of seven amino acids in the leader peptide and 57 amino acids in the terminal domain of the mature enzyme. The association of a nonsense mutation with the skipping of the exon that contains it, plus the following exon, is an unusual finding not seen previously in HL deficiencies. The mutation described here shows the highest incidence (> 37%) of total HL deficiencies reported.

A nonsense mutation in the 3-hydroxy-3-methylglutaryl-CoA lyase gene produces exon skipping in two patients of different origin with 3-hydroxy-3-methylglutaryl-CoA lyase deficiency

A novel nonsense mutation associated with the skipping of constitutive exon 2 of the 3-hydroxy-3-methylglutaryl-CoA lyase gene was found in two patients, from Portugal and Morocco, with 3-hydroxy-3-methylglutaric acidemia. By reverse transcriptase PCR and single-strand conformational polymorphism a G-T transversion was located, at nucleotide 109, of the 3-hydroxy-3-methylglutaryl-CoA lyase cDNA, within exon 2. Two mRNAs were produced as a result of this nonsense mutation: one of the expected size that contains the premature stop codon UAA, and the other with a deletion of 84 bp corresponding to the whole of exon 2. This deletion produced the loss of the last seven amino acids of the leader peptide and the first 21 amino acids of the mature protein. The nonsense mutation was found in a purine-rich GGAAG sequence, which is equal to, or similar to, others reported to be exonic splicing enhancers (ESE). We suggest that the nonsense mutation may affect a possible ESE on exon 2, which would hinder the splice site selection and facilitate an aberrant splice with the skipping of this exon. Determination by quantitative PCR shows that the ratio of mRNA with the nonsense mutation to the mRNA with the deletion is approx. 3:1.

A 3-hydroxy-3-methylglutaryl-CoA lyase gene in the photosynthetic bacterium Rhodospirillum rubrum

A 1.2 kb long DNA segment from Rhodospirillum rubrum has been sequenced (EMBL/GenBank accession number: U41280). This DNA segment includes the first sequenced gene for a putative 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) lyase, termed hmgL, from a photosynthetic organism. The sequenced segment also contains a ribosome-binding site and two clusters of possible-35 and -10 promotor sequences preceding the hmgL gene. Translation of the gene would yield a 303 amino-acid-long protein with a calculated molecular weight of 31.1 kDa. This protein shows 55-60% identity and approx. 75% similarity, including conservative substitutions, with the three eukaryotic and the single prokaryotic HMG-CoA lyases which previously have been sequenced. The R. rubrum enzyme showed stronger homology to the chicken HMG-CoA lyase than to the other bacterial protein. Significant sequence similarity was also found with homocitrate synthases from nitrogen-fixing prokaryotes. In contrast to the other sequenced prokaryotic HMG-CoA lyase (from Pseudomonas mevalonii), the R. rubrum hmgL does not seem to appear in an operon together with a HMG-CoA reductase. The hmgL gene was transcribed in photosynthetically grown cells as judged by amplification of cDNAs synthesised from DNA-free total RNA. In addition, HMG-CoA lyase activity was found in R. rubrum cells grown anaerobically in the light with leucine as the carbon source.

Organic acidurias and related abnormalities

Organic acid analysis is a powerful technique in the diagnosis of inborn errors of metabolism. Since the development of the technique over twenty-five years ago, it has evolved into a sophisticated and powerful method and is an essential tool in the diagnosis of the organic acidurias. The chemistry and biochemistry of organic acids, as well as sample preparation, instrumentation, and many aspects of the more commonly used methods for the analysis of these compounds, are reviewed. The biochemical and clinical characteristics of each of the primary organic acidurias are described. In addition, the various noninherited causes of secondary organic acidurias that lead to the excretion of abnormal organic acids are also described, and ways of differentiating primary from secondary causes are discussed.

Multiple respiratory chain abnormalities associated with hypertrophic cardiomyopathy and 3-methylglutaconic aciduria

In a 4.5-month-old boy presenting with marked muscular hypotonia in the neonatal period, hepatomegaly, cardiac hypertrophy, recurrent hypoglycemia, metabolic acidosis, and secondary carnitine deficiency, there was a considerable urinary excretion of 3-methylglutaconic and 3-methylglutaric acid. Estimation of 3-methylglutaconyl-CoA hydratase, 3-hydroxy-3-methylglutaryl-CoA lyase and initial enzymatic steps of cholesterol biosynthesis in cultured fibroblasts and in different tissues postmortem revealed no enzyme deficiency. Analyses of the respiratory chain in postmortem tissues demonstrated severe impairment of complex I (NADH ubiquinone oxidoreductase) and complex IV (cytochrome c oxidase) activities in skeletal muscle and reduced complex IV activity in heart.

Organic acidurias: a review. Part 1

Organic acidemias are disorders of intermediary metabolism that lead to accumulation of organic acids in biologic fluids, disturb acid-base balance, and derange intracellular biochemical pathways. Their clinical presentation reflects the resultant systemic disease and progressive encephalopathy. While in some organic acidemias, disturbed acid-base metabolism is the predominant presenting feature, in others it is less prominent or even absent. The etiologies of the more than 50 different phenotypes include impaired metabolism of branched-chain amino acids, vitamins, glucose, lipids, glutathione, and gamma-aminobutyric acid and defects of oxidative phosphorylation. Most organic acidemias present with neurologic manifestations, which include acutely or subacutely progressive encephalopathy that involves different parts of the nervous system. The age of presentation and the associated systemic, hematologic, and immune findings provide additional guidelines for differential diagnosis. We summarize major organic acidemias, while emphasizing their usual and unusual neurologic presentations.

3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) lyase deficiency in Saudi Arabia

Deficiency of 3-hydroxy-3-methylglutaryl-CoA lyase has been studied in 11 Saudi infants. The diagnosis was established by the measurement of enzyme activity in lymphocytes, in fibroblasts and, in seven patients, by the gas chromatography/mass spectrometer pattern of excreted organic acids in the urine. In seven infants the disease caused a devastating acidotic attack within the first day of life, while in two the crisis occurred by the third day of life. In two infants from one family the disease appeared later in infancy. The clinical presentation of an acidotic attack is lethargy, hyperpnoea, tachypnoea and seizures, either at birth (two infants), following first feeding (in five infants), or following vomiting or refusal of food in later infancy. The acidotic attacks recurred later in life following minor illness or refusal to eat. The acidosis of this enzyme deficiency progresses rapidly, leading to cardiopulmonary arrest and death within hours of onset unless treated promptly. In four surviving infants diagnosed and treated early, development is normal. Magnetic resonance and computerized tomography brain scans in these infants, however, show white matter lesions and mild atrophy.

Rapid diagnosis of 3-hydroxy-3-methylglutaryl-coenzyme A lyase deficiency via enzyme activity measurements in leukocytes or platelets using a simple spectrophotometric method

Patients with 3-hydroxy-3-methylglutaric aciduria due to a deficiency of 3-hydroxy-3-methylglutaryl Coenzyme A lyase usually present with a life-threatening crisis of hypoglycemia, metabolic acidosis and hyperammonemia. Diagnosis of this inborn error of leucine degradation is usually based upon gas-chromatographic analysis of organic acids in a patient's urine. In this paper we describe a simple spectrophotometric method allowing the activity of HMG-CoA lyase to be measured in leukocytes or platelets within a few hours, thus contributing to a rapid, unequivocal diagnosis and subsequent treatment. The validity of the method was established by demonstrating a deficient activity of HMG-CoA lyase in two patients with 3-hydroxy-3-methylglutaric aciduria. Furthermore, using this method, heterozygote detection can be done with great reliability.

The contribution of protein catabolism to metabolic decompensation in 3-hydroxy-3-methylglutaric aciduria

Leucine and protein metabolism were studied using stable isotope techniques in 6-year-old twins with 3-hydroxy-3-methylglutaric aciduria during acute metabolic decompensation. The decompensation was preceded by prolonged fasting in twin 1 and by an upper respiratory infection in twin 2. Twin 2 was also studied when well (control study). During infection, leucine oxidation (36 mumol/kg per hour), protein catabolism (6.0 g/kg per day) and urinary excretion of major leucine metabolites (104 mumol/kg per hour) were all increased compared with the control study (16 mumol/kg per hour, 4.7 g/kg per day and 28 mumol/kg per hour respectively). During fasting, leucine oxidation (18 mumol/kg per hour) was unchanged and protein catabolism (4.1 g/kg per day) was decreased despite substantially increased urinary metabolite excretion (87 mumol/kg per hour) compared with the control study. These results indicate that protein mobilisation and leucine oxidation played important roles in metabolic decompensation during infection but not during fasting. It is likely that the increased metabolite excretion during fasting arose primarily from fatty acid catabolism, indicating the importance of this substrate in metabolic decompensation in 3-hydroxy-3-methylglutaric aciduria.

3-Hydroxy-3-methylglutaryl-coenzyme A lyase deficiency: biochemical studies and family investigation of four generations

3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) lyase activity was determined by the recently described spectrophotometric method of Wanders et al. (1988a) in polymorphonuclear leukocytes and lymphocytes obtained from 33 members of a highly consanguineous Arab-Bedouin family belonging to four generations. Seven subjects were obligatory heterozygotes (parents and grandparents of three propositi); in seven additional subjects enzyme activity in both cell types was in the heterozygote range. No asymptomatic homozygotes were found. The results support the proposed autosomal recessive mode of inheritance of this disorder.

Hypoglycemia: a pathophysiologic approach

An exploration of the factors that sustain glucose levels in the normal fasting subject reveals that the single major component is conservation of glucose rather than gluconeogenesis. Conservation is achieved by recycling of glucose carbon as lactate, pyruvate and alanine, and a profound decrease in the oxidation of glucose by the brain brought about by the provision and use of ketones. What glucose continues to be oxidized is for the most part formed from glycerol. Gluconeogenesis from protein plays little part in the process. Fasting hypoglycemia results from disorders affecting either one of the two critical sustaining factors--the recycling process or the availability and use of ketones. Individual hypoglycemic entities are examined against this background.

Prenatal diagnosis of 3-hydroxy-3-methylglutaric aciduria by GC-MS and enzymology on cultured amniocytes and chorionic villi

This paper reports the prenatal diagnosis of HMG CoA lyase deficiency at 16 weeks' gestation by direct chemical analysis of cell-free amniotic fluid and by measurement of HMG CoA lyase activity in cultured amniocytes. Termination of an affected fetus allowed study of chorionic villus tissue, the results providing the basis for future first trimester prenatal diagnoses of this condition. An abstract report of this work has appeared elsewhere.

Clinical approach to inherited metabolic diseases in the neonatal period: a 20-year survey

Every newborn with unexplained neurological deterioration, ketosis, metabolic acidosis or hypoglycaemia should be suspected of having an inherited error of intermediary metabolism. Many of these conditions can be diagnosed clinically with the aid of simple laboratory investigations. Since a substantial number of these diseases respond well to treatment but may otherwise be fatal, and in order to assure adequate prenatal diagnosis in subsequent pregnancies, a high index of suspicion and rapid diagnosis are necessary in the face of the clinical presentations described. According to three major clinical presentations observed in 218 neonates with inborn errors of intermediary metabolism (neurological distress 'intoxication' type, neurological distress 'energy-deficiency' type and hypoglycaemia with liver dysfunction) and according to the proper use of few laboratory investigations, we propose a method of diagnosis which groups these children into five categories. Initial therapy, and sophisticated investigations can be planned on the basis of this grouping.

The biochemical basis of mitochondrial diseases

Dysfunctioning of human mitochondria is found in a rapidly increasing number of patients. The mitochondrial system for energy transduction is very vulnerable to damage by genetic and environmental factors. A primary mitochondrial disease is caused by a genetic defect in a mitochondrial enzyme or translocator. More than 60 mitochondrial enzyme deficiencies have been reported. Secondary mitochondrial defects are caused by lack of compounds to enable a proper mitochondrial function or by inhibition of that function. This may result from malnutrition, circulatory or hormonal disturbances, viral infection, poisoning, or an extramitochondrial error of metabolism. Once mitochondrial ATP synthesis decreases, secondary mitochondrial lesions may be generated further, due to changes in synthesis and degradation of mitochondrial phospholipids and proteins, to mitochondrial antibody formation following massive degradation, to accumulation of toxic products as excess acyl-CoA, to the depletion of Krebs cycle intermediates, and to the increase of free radical formation and lipid peroxidation.

3-Hydroxy-3-methylglutaryl-CoA lyase in human skin fibroblasts: study of its properties and deficient activity in 3-hydroxy-3-methylglutaric aciduria patients using a simple spectrophotometric method

In this paper we studied the properties of 3-hydroxy-3-methylglutaryl-CoA lyase (HMG-CoA lyase) in human skin fibroblasts. The enzyme was found to exhibit an absolute requirement for divalent cations such as magnesium. Furthermore, dithiothreitol was necessary for full activity. The enzyme was found to be maximally active at pH 9.25. When measured at this pH in the presence of magnesium and dithiothreitol enzyme activity was high enough to be determined by simple spectrophotometry by measuring the amounts of acetoacetate produced. The results obtained suggest that the large variation in the values reported in literature for the activity of HMG-CoA lyase in human skin fibroblasts is due to the fact that the enzyme shows little activity at pH values below 8.

3-Hydroxy-3-methylglutaryl-coenzyme A lyase deficiency: report of five new patients

Five new patients are reported and the pathogenesis of the hypoglycaemia without ketogenesis is discussed. This report extends a recent review.