Clinical variability in 3-hydroxy-2-methylbutyryl-CoA dehydrogenase deficiency

Involvement of Type 10 17β-Hydroxysteroid Dehydrogenase in the Pathogenesis of Infantile Neurodegeneration and Alzheimer's Disease

Type 10 17β-hydroxysteroid dehydrogenase (17β-HSD10) is the HSD17B10 gene product playing an appreciable role in cognitive functions. It is the main hub of exercise-upregulated mitochondrial proteins and is involved in a variety of metabolic pathways including neurosteroid metabolism to regulate allopregnanolone homeostasis. Deacetylation of 17β-HSD10 by sirtuins helps regulate its catalytic activities. 17β-HSD10 may also play a critical role in the control of mitochondrial structure, morphology and dynamics by acting as a member of the Parkin/PINK1 pathway, and by binding to cyclophilin D to open mitochondrial permeability pore. 17β-HSD10 also serves as a component of RNase P necessary for mitochondrial tRNA maturation. This dehydrogenase can bind with the Aβ peptide thereby enhancing neurotoxicity to brain cells. Even in the absence of Aβ, its quantitative and qualitative variations can result in neurodegeneration. Since elevated levels of 17β-HSD10 were found in brain cells of Alzheimer's disease (AD) patients and mouse AD models, it is considered to be a key factor in AD pathogenesis. Since data underlying Aβ-binding-alcohol dehydrogenase (ABAD) were not secured from reported experiments, ABAD appears to be a fabricated alternative term for the HSD17B10 gene product. Results of this study would encourage researchers to solve the question why elevated levels of 17β-HSD10 are present in brains of AD patients and mouse AD models. Searching specific inhibitors of 17β-HSD10 may find candidates to reduce senile neurodegeneration and open new approaches for the treatment of AD.

Infantile Neurodegeneration Results from Mutants of 17β-Hydroxysteroid Dehydrogenase Type 10 Rather Than Aβ-Binding Alcohol Dehydrogenase

Type 10 17β-hydroxysteroid dehydrogenase (17β-HSD10), a homo-tetrameric multifunctional protein with 1044 residues encoded by the HSD17B10 gene, is necessary for brain cognitive function. Missense mutations result in infantile neurodegeneration, an inborn error in isoleucine metabolism. A 5-methylcytosine hotspot underlying a 388-T transition leads to the HSD10 (p.R130C) mutant to be responsible for approximately half of all cases suffering with this mitochondrial disease. Fewer females suffer with this disease due to X-inactivation. The binding capability of this dehydrogenase to Aβ-peptide may play a role in Alzheimer's disease, but it appears unrelated to infantile neurodegeneration. Research on this enzyme was complicated by reports of a purported Aβ-peptide-binding alcohol dehydrogenase (ABAD), formerly referred to as endoplasmic-reticulum-associated Aβ-binding protein (ERAB). Reports concerning both ABAD and ERAB in the literature reflect features inconsistent with the known functions of 17β-HSD10. It is clarified here that ERAB is reportedly a longer subunit of 17β-HSD10 (262 residues). 17β-HSD10 exhibits L-3-hydroxyacyl-CoA dehydrogenase activity and is thus also referred to in the literature as short-chain 3-hydorxyacyl-CoA dehydrogenase or type II 3-hydorxyacyl-CoA dehydrogenase. However, 17β-HSD10 is not involved in ketone body metabolism, as reported in the literature for ABAD. Reports in the literature referring to ABAD (i.e., 17β-HSD10) as a generalized alcohol dehydrogenase, relying on data underlying ABAD's activities, were found to be unreproducible. Furthermore, the rediscovery of ABAD/ERAB's mitochondrial localization did not cite any published research on 17β-HSD10. Clarification of the purported ABAD/ERAB function derived from these reports on ABAD/ERAB may invigorate this research field and encourage new approaches to the understanding and treatment of HSD17B10-gene-related disorders. We establish here that infantile neurodegeneration is caused by mutants of 17β-HSD10 but not ABAD, and so we conclude that ABAD represents a misnomer employed in high-impact journals.

HSD10 disease in a female: A case report and review of literature

HSD10 disease is a rare X-linked mitochondrial disorder caused by pathogenic variants in the HSD17B10 gene. The phenotype results from impaired 17β-hydroxysteroid dehydrogenase 10 (17β-HSD10) protein structure and function. HSD10 is a multifunctional protein involved in enzymatic degradation of isoleucine and branched-chain fatty acids, the metabolism of sex hormones and neurosteroids, as well as in regulating mitochondrial RNA maturation. HSD10 disease is characterised by progressive neurologic impairment. Disease onset is varied and includes neonatal-onset, infantile-onset and late-onset in males. Females can also be affected. Our index case is a 45-month-old female, who initially presented at 11 months of age with global developmental delay. She subsequently began to lose previously acquired cognitive and motor skills starting around 29 months of age. Brain MRI showed abnormalities in the basal ganglia indicative of possible mitochondrial disease. Urine organic acid analysis revealed elevations of 2-methyl-3-hydroxybutyric acid and tiglyglycine. HSD17B10 gene sequencing revealed a likely pathogenic variant, NM_001037811.2:c.439C>T (p.Arg147Cys) inherited from her mother, expected to be causative of HSD10 disease. Her X-chromosome inactivation study is consistent with a skewed X-inactivation pattern. We report a female patient with HSD10 disease caused by a missense pathogenic variant, Arg147Cys in the HSD17B10 gene. The patient is the fifth severely affected female with this disease. This case adds to the small number of known affected families with this highly variable disease in the literature. These findings support the possibility of X-inactivation patterns influencing the penetrance of HSD10 disease in females.

Friend or enemy? Review of 17β-HSD10 and its role in human health or disease

17β-hydroxysteroid dehydrogenase (17β-HSD10) is a multifunctional human enzyme with important roles both as a structural component and also as a catalyst of many metabolic pathways. This mitochondrial enzyme has important functions in the metabolism, development and aging of the neural system, where it is involved in the homeostasis of neurosteroids, especially in regard to estradiol, changes in which make it an essential part of neurodegenerative pathology. These roles therefore, indicate that 17β-HSD10 may be a possible druggable target for neurodegenerative diseases including Alzheimer's disease (AD), and in hormone-dependent cancer. The objective of this review was to provide a summary about physiological functions and pathological roles of 17β-HSD10 and the modulators of its activity.

17B-hydroxysteroid dehydrogenases as acyl thioester metabolizing enzymes

17β-Hydroxysteroid dehydrogenases (HSD17B) catalyze the oxidation/reduction of 17β-hydroxy/keto group in position C17 in C18- and C19 steroids. Most HSD17Bs are also catalytically active with substrates other than steroids. A subset of these enzymes is able to process thioesters of carboxylic acids. This group of enzymes includes HSD17B4, HSD17B8, HSD17B10 and HSD17B12, which execute reactions in intermediary metabolism, participating in peroxisomal β-oxidation of fatty acids, mitochondrial oxidation of 3R-hydroxyacyl-groups, breakdown of isoleucine and fatty acid chain elongation in endoplasmic reticulum. Divergent substrate acceptance capabilities exemplify acquirement of catalytic site adaptiveness during evolution. As an additional common feature these HSD17Bs are multifunctional enzymes that arose either via gene fusions (HSD17B4) or are incorporated as subunits into multifunctional protein complexes (HSD17B8 and HSD17B10). Crystal structures of HSD17B4, HSD17B8 and HSD17B10 give insight into their structure-function relationships. Thus far, deficiencies of HSD17B4 and HSD17B10 have been assigned to inborn errors in humans, underlining their significance as enzymes of metabolism.

Neurocognitive assessments and long-term outcome in an adult with 2-methyl-3-hydroxybutyryl-CoA dehydrogenase deficiency

Background

2-Methyl-3-hydroxybutyryl-CoA dehydrogenase deficiency (MHBDD) is a rare X-linked disorder associated with the accumulation of 2-methyl-3-hydroxybutyric acid in body fluids as a consequence of a disruption in isoleucine metabolism. The clinical presentation is heterogeneous, including a neurodegenerative course with retinopathy and cardiomyopathy leading to death in early childhood and a slowly progressive disease associated with learning disability and survival into adulthood. The condition is often diagnosed in childhood.

Results

This paper outlines the long-term neurocognitive outcomes in a 38-year old man with MHBDD. Several psychometric tests were used to assess his cognitive ability and adaptive functioning in childhood during an acute illness and in adulthood when the patient showed deterioration in the ability to walk or speak.

Conclusions

There is an increasing demand for an accurate and objective measure of cognitive functioning that can be used to follow the natural progression of MHBDD. Psychological assessment may enable the identification of organic problems. The application and interpretation of psychometric tests used in children may vary from those used in adults.

BRANCHED CHAIN AMINO ACIDS AT THE EDGE BETWEEN MENDELIAN AND COMPLEX DISORDERS

Branched chained amino acids (BCAA) are essential components of the human diet and important nutrient signals, which regain particular interest in recent years with the avenue of metabolomics studies suggesting their potential role as biomarkers. There is now compelling evidence for predictive role of BCAA in progression of diabetes, but causality relationship is still debated concerning insulin resistance and genetic versus non-genetic pathogenesis. Mendelian randomization studies in large cohorts of diabetes indicated pathogenic role of PPM1K (protein phosphatase Mg2+/Mn2+ dependent 1K) on Chr 4q22.1 gene, encoding for a phosphatase that activates BCKDH (branched chain keto acid dehydrogenase) complex. Recent studies indicated that insulin rapidly and dose-dependently regulates gene expression of the same complex, but the relationship with systemic insulin resistance and glucose levels is complex. Rare genetic syndromes due to Mendelian mutations in key genes in BCAA catabolism may be good models to understand potential role of gene of BCAA catabolism. However, in studying complex disorders geneticists are faced to complete new aspects of metabolic regulation complicating understanding genetics of obesity, diabetes or metabolic syndrome. A review of genetic syndromes of BCAA metabolism suggests that insulin resistance is not present, except rare cases of methylmalonic aciduria due to MUT (methylmalonyl-coA mutase) gene on Chr 6p12.3. Another aspect that complicates understanding is the new role of central nervous system (CNS) in insulin resistance. For a long time the hypothalamic hunger/satiety neuronal system was considered a key site of nutrient regulation. Genes may also affect the brain rewarding system (BRS) that would regulate food intake by modulating the motivation to obtain food and considering hedonic properties. Nutrigenomic and nutrigenetic investigations taking into account concurrently BCAA intake, metabolic regulation and gene variation have large perspectives to merge genetic and nutritional understanding in complex disorders.

Japanese Male Siblings with 2-Methyl-3-Hydroxybutyryl-CoA Dehydrogenase Deficiency (HSD10 Disease) Without Neurological Regression

2-Methyl-3-hydroxybutyryl-CoA dehydrogenase deficiency (HSD10 disease) is a rare X-linked disorder caused by a mutation in the HSD17B10 gene. Fewer than 30 patients with this disorder have been reported worldwide. The classical infantile form of HSD10 disease is characterized by a progressive neurodegenerative course with retinopathy and cardiomyopathy, although HSD10 disease has broad clinical heterogeneity. However, several male patients have not shown neurological regression. Here, we describe two Japanese siblings with HSD10 disease without neurological regression. A 4-year-old boy presented with unconsciousness due to severe hypoglycemia. Laboratory testing on admission showed mild metabolic acidosis and mild hyperammonemia. Urinary organic acid analysis in the acute phase showed elevated excretion of 2-methyl-3-hydroxybutyric acid, tiglylglycine, and ketones. However, 2-methylacetoacetate was not elevated. HSD10 disease was suspected based on urinary organic acid data. The patient had a novel hemizygous c.470C>T (p.A157V) mutation in the HSD17B10 gene. His mother was a heterozygous carrier of this mutation. The patient's older brother also had the c.470C>T (p.A157V) mutation. Neurological development was normal at the time of evaluation. The pilot newborn screening results using tandem mass spectrometry of the proband were reevaluated retrospectively and showed a high C5:1 carnitine level of 0.070 nmol/mL (upper cutoff limit, 0.05 nmol/mL) and a normal C5-OH carnitine level of 0.290 nmol/mL (upper cutoff limit, 1.0 nmol/mL). His affected brother and another patient with the atypical form of HSD10 disease having p.A154T also showed elevated C5:1 but not C5-OH in serum acylcarnitine analysis. Thus, these data suggested that some patients with this disorder may be identified using newborn screening.

Hydroxysteroid 17-Beta Dehydrogenase Type 10 Disease in Siblings

Hydroxysteroid 17-beta dehydrogenase type 10 (HSD10) deficiency (HSD10 disease) is a rare X-linked neurodegenerative condition caused by abnormalities in the HSD17B10 gene. A total of 10 mutations have been reported in the literature since 2000. Described phenotypes include a severe neonatal or progressive infantile form with hypotonia, choreoathetosis, seizures, cardiomyopathy, neurodegeneration, and death, as well as an attenuated form with variable regression. Here we present the second report of a c.194T>C (p.V65A) mutation in two half-brothers with a clinical phenotype characterized by neurodevelopmental delay, choreoathetosis, visual loss, cardiac findings, and behavioral abnormalities, with regressions now noted in the older sibling. Neither has experienced a metabolic crisis. Both of the siblings had normal tandem mass spectroscopy analysis of their newborn screening samples. The older brother's phenotype may be complicated by the presence of a 3q29 microduplication. Diagnosis requires a high index of suspicion, as the characteristic urine organic acid pattern may escape detection. The exact pathogenic mechanism of disease remains to be elucidated, but may involve the non-dehydrogenase functionalities of the HSD10 protein. Our report highlights clinical features of two patients with the less fulminant phenotype associated with a V65A mutation, compares the reported phenotypes to date, and reviews recent findings regarding the potential pathophysiology of this condition.Summary Sentence Hydroxysteroid 17-beta dehydrogenase type 10 (HSD10) disease (HSD10 disease) is a rare X-linked neurodegenerative condition with a variable clinical phenotype; diagnosis requires a high index of suspicion.

Metabolic annotation of 2-ethylhydracrylic acid

Increased levels of the organic acid, 2-ethylhydracrylic acid (2-EHA) occur in urine of subjects with impaired L(+)-isoleucine metabolism. Chiral intermediates formed during isoleucine degradation are (S) enantiomers. Blockage of (S) pathway flux drives racemization of (2S, 3S) L(+)-isoleucine and its (2S, 3R) stereoisomer, L(+)-alloisoleucine. This non-protein amino acid is metabolized to (R)-2-methylbutyryl CoA via enzymes common to branched chain amino acid degradation. Subsequently, (R) intermediates serve as alternate substrates for three valine metabolic enzymes, generating 2-EHA. Once formed, 2-EHA accumulates because it is poorly recognized by distal valine pathway enzymes. Thus, urinary 2-EHA represents a biomarker of isoleucine pathway defects. 2-EHA levels are also increased in rats exposed to the industrial solvent, ethylene glycol monomethyl ether or the neurotoxin precursor, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. In these cases, a block in (S) pathway isoleucine catabolism occurs at the level of (S)-2-methylbutyryl CoA conversion to tiglyl CoA via inhibition of electron transferring flavoprotein/ubiquinone oxidoreductase dependent reactions. Elevated urinary 2-EHA in propionyl CoA carboxylase deficiency and methylmalonic aciduria results from a buildup of distal intermediates in the (S) pathway of isoleucine degradation. In Barth syndrome and dilated cardiomyopathy with ataxia syndrome, 2-EHA is a byproduct of impeded propionyl CoA entry into the Krebs cycle.

Molecular insights into HSD10 disease: impact of SDR5C1 mutations on the human mitochondrial RNase P complex

SDR5C1 is an amino and fatty acid dehydrogenase/reductase, moonlighting as a component of human mitochondrial RNase P, which is the enzyme removing 5′-extensions of tRNAs, an early and crucial step in tRNA maturation. Moreover, a subcomplex of mitochondrial RNase P catalyzes the N¹-methylation of purines at position 9, a modification found in most mitochondrial tRNAs and thought to stabilize their structure. Missense mutations in SDR5C1 cause a disease characterized by progressive neurodegeneration and cardiomyopathy, called HSD10 disease. We have investigated the effect of selected mutations on SDR5C1's functions. We show that pathogenic mutations impair SDR5C1-dependent dehydrogenation, tRNA processing and methylation. Some mutations disrupt the homotetramerization of SDR5C1 and/or impair its interaction with TRMT10C, the methyltransferase subunit of the mitochondrial RNase P complex. We propose that the structural and functional alterations of SDR5C1 impair mitochondrial RNA processing and modification, leading to the mitochondrial dysfunction observed in HSD10 patients.

Mitochondrial energy failure in HSD10 disease is due to defective mtDNA transcript processing

Muscle, heart and liver were analyzed in a male subject who succumbed to HSD10 disease. Respiratory chain enzyme analysis and BN-PAGE showed reduced activities and assembly of complexes I, III, IV, and V. The mRNAs of all RNase P subunits were preserved in heart and overexpressed in muscle, but MRPP2 protein was severely decreased. RNase P upregulation correlated with increased expression of mitochondrial biogenesis factors and preserved mitochondrial enzymes in muscle, but not in heart where this compensatory mechanism was incomplete. We demonstrate elevated amounts of unprocessed pre-tRNAs and mRNA transcripts encoding mitochondrial subunits indicating deficient RNase P activity. This study provides evidence of abnormal mitochondrial RNA processing causing mitochondrial energy failure in HSD10 disease.

Inborn errors of ketone body utilization

Succinyl-CoA:3-ketoacid CoA transferase (SCOT) deficiency and mitochondrial acetoacetyl-CoA thiolase (beta-ketothiolase or T2) deficiency are classified as autosomal recessive disorders of ketone body utilization characterized by intermittent ketoacidosis. Patients with mutations retaining no residual activity on analysis of expression of mutant cDNA are designated as severe genotype, and patients with at least one mutation retaining significant residual activity, as mild genotype. Permanent ketosis is a pathognomonic characteristic of SCOT-deficient patients with severe genotype. Patients with mild genotype, however, may not have permanent ketosis, although they may develop severe ketoacidotic episodes similar to patients with severe genotype. Permanent ketosis has not been reported in T2 deficiency. In T2-deficient patients with severe genotype, biochemical diagnosis is done on urinary organic acid analysis and blood acylcarnitine analysis to observe characteristic findings during both ketoacidosis and non-episodic conditions. In Japan, however, it was found that T2-deficient patients with mild genotype are common, and typical profiles were not identified on these analyses. Based on a clinical study of ketone body utilization disorders both in Japan and worldwide, we have developed guidelines for disease diagnosis and treatment. These diseases are treatable by avoiding fasting and by providing early infusion of glucose, which enable the patients to grow without sequelae.

The first case in Asia of 2-methyl-3-hydroxybutyryl-CoA dehydrogenase deficiency (HSD10 disease) with atypical presentation

2-Methyl-3-hydroxybutyryl-CoA dehydrogenase (2M3HBD) deficiency (HSD10 disease) is a rare inborn error of metabolism, and <30 cases have been reported worldwide. This disorder is typically characterized by progressive neurodegenerative disease from 6 to 18 months of age. Here, we report the first patient with this disorder in Asia, with atypical clinical presentation. A 6-year-old boy, who had been well, presented with severe ketoacidosis following a 5-day history of gastroenteritis. Urinary organic acid analysis showed elevated excretion of 2-methyl-3-hydroxybutyrate and tiglylglycine. He was tentatively diagnosed with β-ketothiolase (T2) deficiency. However, repeated enzyme assays using lymphocytes showed normal T2 activity and no T2 mutation was found. Instead, a hemizygous c.460G>A (p.A154T) mutation was identified in the HSD17B10 gene. This mutation was not found in 258 alleles from Japanese subjects (controls). A normal level of the HSD17B10 protein was found by immunoblot analysis but no 2M3HBD enzyme activity was detected in enzyme assays using the patient's fibroblasts. These data confirmed that this patient was affected with HSD10 disease. He has had no neurological regression until now. His fibroblasts showed punctate and fragmented mitochondrial organization by MitoTracker staining and had relatively low respiratory chain complex IV activity to those of other complexes.

First report of 3-oxothiolase deficiency in iran

INTRODUCTION

Mitochondrial acetoacetyl-CoA thiolase (3-oxothiolase) deficiency is a rare metabolic disorder involving ketone body metabolism characterized by acute attacks of vomiting, acidosis, ketosis, and lethargy along with some laboratory criteria including excessive excretion of 2-methyl-3-hydroxybutyric acid in urine.

CASE PRESENTATION

This is a case report of 3-oxothiolase deficiency in a young Iranian boy with presentation of intractable vomiting and severe metabolic acidosis following a common cold in six months of age with abundant urinary 2-methyl-3- hydroxybutyric acid.

DISCUSSION

This is the first Iranian 3-oxothiolase deficiency case report as searched in the literature. Because of the high rate of consanguineous marriages in Iran, physicians should consider the 3-oxothiolase deficiency in the differential diagnosis of any patient with intractable vomiting and severe metabolic acidosis.

Transcription start sites and epigenetic analysis of the HSD17B10 proximal promoter

BACKGROUND

Hydroxysteroid (17beta) dehydrogenase X (HSD10) is a multifunctional protein encoded by the HSD17B10 gene at Xp11.2. In response to stress or hypoxia-ischemia its levels increase rapidly. Expression of this gene is also elevated significantly in colonic mucosa of the inactive ulcerative colitis patients. However, accurate information about its several transcripts is still lacking, and additional evidence for its escape from X-chromosome inactivation remains to be obtained in order to help settle a debate (He XY, Dobkin C, Yang SY: Does the HSD17B10 gene escape from X-inactivation? Eur J Hum Genet 2011, 19: 123-124).

RESULTS

Two major HSD17B10 transcription start sites were identified by primer extension at -37 and -6 as well as a minor start site at -12 nucleotides from the initiation codon ATG. Epigenetic analysis of the 5'-flanking region of the HSD17B10 gene showed that there was little 5-methylcytosine (< 3%) in a normal male, and that none of CpG dinucleotides in the CpG island approached 50% methylation in females.

CONCLUSION

The actual length of first exon of the HSD17B10 gene was found to be about a quarter larger than that originally reported. Its transcripts result from a slippery transcription complex. The hypomethylation of the CpG island provides additional evidence for the variable escape of the HSD17B10 gene from X-chromosome inactivation which could influence the range of severity of HSD10 deficiency, an inherited error in isoleucine metabolism, in heterozygous females.

A 5-methylcytosine hotspot responsible for the prevalent HSD17B10 mutation

Approximately half of the cases of hydroxysteroid (17β) dehydrogenase X (HSD10) deficiency are due to a missense C>T mutation in exon 4 of the HSD17B10 gene. The resulting HSD10 (p.R130C) loses most or all catalytic functions, and the males with this mutation have a much more severe clinical phenotype than those carrying p.V65A, p.L122V, or p.E249Q mutations. We found that the mutated cytosine which is +2259 nucleotide from the ATG of the gene, is >90% methylated in both the active and inactive X chromosomes in two normal females as well as in the X chromosome of a normal male. Since 5-methylcytosine is prone to conversion to thymine by deamination, the methylation of this cytosine in normal X chromosomes provides an explanation for the prevalence of the p.R130C mutation among patients with HSD10 deficiency. The substitution of arginine for cysteine eliminates several hydrogen bonds and reduces the van der Waals interaction between HSD10 subunits. The resulting disruption of protein structure impairs some if not all of the catalytic and non-enzymatic functions of HSD10. A meta-analysis of residual HSD10 activity in eight patients with the p.R130C mutation showed an average 2-methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) activity of only 6 (±5) % of the normal control level. This is significantly lower than in cells of patients with other, clinically milder mutations and suggests that the loss of HSD10/MHBD activity is a marker for the disorder.

HSD10 disease: clinical consequences of mutations in the HSD17B10 gene

The HSD17B10 gene is located on chromosome Xp11.2 and codes for a multifunctional protein called 17β-hydroxysteroid dehydrogenase type 10 (HSD10). This protein catalyzes the 2-methyl-3-hydroxybutyryl-CoA dehydrogenation (MHBD) reaction in isoleucine metabolism and is an essential component of mitochondrial RNase P required for the processing of mtDNA transcripts. HSD10 is required for normal mitochondrial maintenance, and complete loss of HSD10 is incompatible with life. Mutations in the HSD17B10 gene have been reported in 19 families. The classical infantile form of what is best named HSD10 disease is characterized by a period of more or less normal development in the first 6-18 months of life. Some patients showed transient metabolic derangement in the neonatal period, with good clinical recovery but often persistent lactate elevation. Usually from age 6-18 months affected boys show a progressive neurodegenerative disease course in conjunction with retinopathy and cardiomyopathy leading to death at age 2-4 years or later. A more severe presentation in the neonatal period with little neurological development, severe progressive cardiomyopathy, and early death, is denoted neonatal form. Juvenile and atypical/asymptomatic forms of HSD10 disease have been recognized. Heterozygous females often show non-progressive developmental delay and intellectual disability but may also be clinically normal. The pathogenesis is poorly understood but is unrelated to MHBD function. Diagnosis is based on typical abnormalities in urinary organic acid analysis and molecular studies. The same de novo mutation p.R130C was found in over half of patient families; it is associated with the infantile disease form. There is no effective treatment.

A novel mutation in the HSD17B10 gene of a 10-year-old boy with refractory epilepsy, choreoathetosis and learning disability

Hydroxysteroid (17beta) dehydrogenase 10 (HSD10) is a mitochondrial multifunctional enzyme encoded by the HSD17B10 gene. Missense mutations in this gene result in HSD10 deficiency, whereas a silent mutation results in mental retardation, X-linked, syndromic 10 (MRXS10). Here we report a novel missense mutation found in the HSD17B10 gene, namely c.194T>C transition (rs104886492), brought about by the loss of two forked methyl groups of valine 65 in the HSD10 active site. The affected boy, who possesses mutant HSD10 (p.V65A), has a neurological syndrome with metabolic derangements, choreoathetosis, refractory epilepsy and learning disability. He has no history of acute decompensation or metabolic acidosis whereas his urine organic acid profile, showing elevated levels of 2-methyl-3-hydroxybutyrate and tiglylglycine, is characteristic of HSD10 deficiency. His HSD10 activity was much lower than the normal control level, with normal β-ketothiolase activity. The c.194T>C mutation in HSD17B10 can be identified by the restriction fragment polymorphism analysis, thereby facilitating the screening of this novel mutation in individuals with intellectual disability of unknown etiology and their family members much easier. The patient's mother is an asymptomatic carrier, and has a mixed ancestry (Hawaiian, Japanese and Chinese). This demonstrates that HSD10 deficiency patients are not confined to a particular ethnicity although previously reported cases were either Spanish or German descendants.

Hydroxysteroid (17β) dehydrogenase X in human health and disease

Hydroxysteroid (17β) dehydrogenase 10 (HSD10), the HSD17B10 gene product, is a mitochondrial NAD(+)-dependent dehydrogenase. There are two outstanding features of this vital enzyme: (a) the versatility of its catalytic endowment is attributed to the flexibility of its active site to accommodate diverse substrates such as steroids, fatty acids, bile acid, and xenobiotics; (b) its capacity to bind other proteins and peptides. For example, it tightly binds with three identical subunits to compose a homotetramer. The homotetramer then binds with two other proteins, namely, RNA (guanine-9-)methyl-transferase domain containing-1 and KIAA0391, to form mitochondrial RNase P. Furthermore, various HSD10 functions are inhibited when the enzyme is bound by amyloid-β peptide or estrogen receptor alpha. Missense mutations of HSD10 may cause neurodegeneration related to HSD10 deficiency, whereas a silent mutation of HSD10 results in mental retardation, choreoathetosis and abnormal behavior (MRXS10). The clinical condition of some HSD10 patients mimics mitochondrial disorders. Since normal HSD10 function is essential for brain cognitive activity, elevated levels of HSD10 found in brains of Alzheimer disease (AD) patients and mouse AD model might counterbalance the inhibition of HSD10 by amyloid-β peptide. The investigation of HSD10 may lead to a better understanding of AD pathogenesis.

Disruption of mitochondrial homeostasis in organic acidurias: insights from human and animal studies

Organic acidurias or organic acidemias constitute a group of inherited disorders caused by deficient activity of specific enzymes of amino acids, carbohydrates or lipids catabolism, leading to large accumulation and excretion of one or more carboxylic (organic) acids. Affected patients usually present neurologic symptoms and abnormalities, sometimes accompanied by cardiac and skeletal muscle alterations, whose pathogenesis is poorly known. However, in recent years growing evidence has emerged indicating that mitochondrial dysfunction is directly or indirectly involved in the pathology of various organic acidemias. Mitochondrial impairment in some of these diseases are generally due to mutations in nuclear genes of the tricarboxylic acid cycle or oxidative phosphorylation, while in others it seems to result from toxic influences of the endogenous organic acids to the mitochondrion. In this minireview, we will briefly summarize the present knowledge obtained from human and animal studies showing that disruption of mitochondrial homeostasis may represent a relevant pathomechanism of tissue damage in selective organic acidemias. The discussion will focus on mitochondrial alterations found in patients affected by organic acidemias and by the deleterious effects of the accumulating organic acids on mitochondrial pathways that are crucial for ATP formation and transfer. The elucidation of the mechanisms of toxicity of these acidic compounds offers new perspectives for potential novel adjuvant therapeutic strategies in selected disorders of this group.

Evidence that 2-methylacetoacetate induces oxidative stress in rat brain

In the present study we investigated the effects of 2-methylacetoacetate (MAA) and 2-methyl-3-hydroxybutyrate (MHB), the major metabolites accumulating in mitochondrial 2-methylacetoacetyl-CoA thiolase (KT) and 2-methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) deficiencies, on important parameters of oxidative stress in cerebral cortex from young rats. We verified that MAA induced lipid peroxidation (increase of thiobarbituric acid-reactive substances (TBA-RS) and chemiluminescence values), whereas MHB did not alter these parameters. MAA-induced increase of TBA-RS levels was fully prevented by free radical scavengers, indicating that free radicals were involved in this effect. Furthermore, MAA, but not MHB, significantly induced sulfhydryl oxidation, implying that this organic acid provokes protein oxidative damage. It was also observed that MAA reduced GSH, a naturally-occurring brain antioxidant, whereas MHB did not change this parameter. Furthermore, the decrease of GSH levels caused by MAA was not due to a direct oxidative action, since this organic acid did not alter the sulfhydryl content of a commercial solution of GSH in a cell free medium. Finally, MAA and MHB did not raise nitric oxide production. The data indicate that MAA induces oxidative stress in vitro in cerebral cortex. It is presumed that this pathomechanism may be involved in the brain damage found in patients affected by KT deficiency.

X-inactivation of HSD17B10 revealed by cDNA analysis in two female patients with 17β-hydroxysteroid dehydrogenase 10 deficiency

17β-Hydroxysteroid dehydrogenase 10 (HSD10) is a mitochondrial enzyme involved in the degradation pathway of isoleucine and branched-chain fatty acids. The gene encoding HSD10, HSD17B10, has been reported as one of the few genes that escapes X-inactivation. We previously studied two female patients with HSD10 deficiency, one of them was severely affected and the other presented a mild phenotype. To elucidate as to why these two carriers were so differently affected, cDNA analyses were performed. The HSD17B10 cDNA of eight control cell lines, two hemizygous patients and two carriers was obtained from cultured fibroblasts, amplified by PCR and sequenced by standard methods. All HSD17B10 cDNAs were quantified by real-time PCR. In the fibroblasts of the female patient who presented with the severe phenotype, only the mutant allele was identified in the cDNA sequence, which was further confirmed by relative quantification (RQ) of HSD17B10 cDNA. This is in agreement with an unfavourable X-inactivation. The other female patient, with slight clinical affectation, showed the presence of both mutant and wild-type alleles in the cDNA sequence, which was confirmed by RQ of HSD17B10 cDNA in fibroblasts. This is in line with normal X-inactivation and the expression of both alleles in different cells (functional mosaicism). RQ results of HSD17B10 cDNA did not differ significantly between male and female controls, which indicate that the genetic doses of mRNA of HSD17B10 was the same in both sexes. In conclusion, these results suggest that the HSD17B10 gene does not escape X-inactivation as has been reported previously.

Mental retardation linked to mutations in the HSD17B10 gene interfering with neurosteroid and isoleucine metabolism

Mutations in the HSD17B10 gene were identified in two previously described mentally retarded males. A point mutation c.776G>C was found from a survivor (SV), whereas a potent mutation, c.419C>T, was identified in another deceased case (SF) with undetectable hydroxysteroid (17beta) dehydrogenase 10 (HSD10) activity. Protein levels of mutant HSD10(R130C) in patient SF and HSD10(E249Q) in patient SV were about half that of HSD10 in normal controls. The E249Q mutation appears to affect HSD10 subunit interactions, resulting in an allosteric regulatory enzyme. For catalyzing the oxidation of allopregnanolone by NAD+ the Hill coefficient of the mutant enzyme is approximately 1.3. HSD10(E249Q) was unable to catalyze the dehydrogenation of 2-methyl-3-hydroxybutyryl-CoA and the oxidation of allopregnanolone, a positive modulator of the gamma-aminobutyric acid type A receptor, at low substrate concentrations. Neurosteroid homeostasis is critical for normal cognitive development, and there is increasing evidence that a blockade of isoleucine catabolism alone does not commonly cause developmental disabilities. The results support the theory that an imbalance in neurosteroid metabolism could be a major cause of the neurological handicap associated with hydroxysteroid (17beta) dehydrogenase 10 deficiency.

Mapping gene associations in human mitochondria using clinical disease phenotypes

Nuclear genes encode most mitochondrial proteins, and their mutations cause diverse and debilitating clinical disorders. To date, 1,200 of these mitochondrial genes have been recorded, while no standardized catalog exists of the associated clinical phenotypes. Such a catalog would be useful to develop methods to analyze human phenotypic data, to determine genotype-phenotype relations among many genes and diseases, and to support the clinical diagnosis of mitochondrial disorders. Here we establish a clinical phenotype catalog of 174 mitochondrial disease genes and study associations of diseases and genes. Phenotypic features such as clinical signs and symptoms were manually annotated from full-text medical articles and classified based on the hierarchical MeSH ontology. This classification of phenotypic features of each gene allowed for the comparison of diseases between different genes. In turn, we were then able to measure the phenotypic associations of disease genes for which we calculated a quantitative value that is based on their shared phenotypic features. The results showed that genes sharing more similar phenotypes have a stronger tendency for functional interactions, proving the usefulness of phenotype similarity values in disease gene network analysis. We then constructed a functional network of mitochondrial genes and discovered a higher connectivity for non-disease than for disease genes, and a tendency of disease genes to interact with each other. Utilizing these differences, we propose 168 candidate genes that resemble the characteristic interaction patterns of mitochondrial disease genes. Through their network associations, the candidates are further prioritized for the study of specific disorders such as optic neuropathies and Parkinson disease. Most mitochondrial disease phenotypes involve several clinical categories including neurologic, metabolic, and gastrointestinal disorders, which might indicate the effects of gene defects within the mitochondrial system. The accompanying knowledgebase (http://www.mitophenome.org/) supports the study of clinical diseases and associated genes.

An important prerequisite for successful disease gene identification is the assessment, with minimal ambiguity, of a particular clinical trait or phenotype. Even with years of experience, recognizing and diagnosing mitochondrial diseases is still a major hurdle in clinical medicine. Computational tools supporting clinicians not only help identify affected individuals, but also guide studies of the genetic and biological causes of these disorders. In this study we dissect and categorize individual clinical features, signs, and symptoms of 174 disease genes and then identify gene similarities based on their shared phenotypic features. We demonstrate that genes sharing more similar phenotypes have a stronger tendency for functional interactions, proving the usefulness of phenotype similarity values in disease gene network analysis. Our study of a large functional network of mitochondrial genes revealed distinct properties that differentiate disease and non-disease genes. Disease genes showed a lower average total connectivity but a tendency to interact with each other; a finding that we used to predict 168 high-probability disease candidates. The accompanying knowledgebase allows for easy navigation between disease and gene information. We believe the open source format will support and encourage further research that will benefit this and other human phenome projects.

Study of patients and carriers with 2-methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) deficiency: difficulties in the diagnosis

OBJECTIVES

To search for biochemical and molecular markers for the diagnosis of patients and carriers with 2-Methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) deficiency.

DESIGN AND METHODS

Organic acids in urine, MHBD activity in fibroblasts, immunoblotting and molecular studies were performed in seven patients. Seven carriers were also studied.

RESULTS

Under low protein diet or poor feeding all the patients showed only a slightly altered organic acid profile. Measurement of 2-methyl-3-hydroxybutyric acid and tiglylglycine after an isoleucine loading test, failed to demonstrate the carrier status of one patient. However, measurement of 2-ethylhydracrylic acid (EHA) was positive in all the carriers tested. MHBD activity was clearly deficient in males and in one female patient. We identified four missense mutations, two of them were novel.

CONCLUSIONS

Quantification of EHA may be of help for the diagnosis of the heterozygous condition. The carrier females showed the classical biochemical variability of X-linked diseases due to random X-chromosome inactivation.

2-Methylbutyryl-coenzyme A dehydrogenase deficiency: functional and molecular studies on a defect in isoleucine catabolism

2-Methylbutyryl-CoA dehydrogenase (MBD; coded by the ACADSB gene) catalyzes the step in isoleucine metabolism that corresponds to the isovaleryl-CoA dehydrogenase reaction in the degradation of leucine. Deficiencies of both enzymes may be detected by expanded neonatal screening with tandem-mass spectrometry due to elevated pentanoylcarnitine (C5 acylcarnitine) in blood, but little information is available on the clinical relevance of MBD deficiency. We biochemically and genetically characterize six individuals with MBD deficiency from four families of different ethnic backgrounds. None of the six individuals showed clinical symptoms attributable to MBD deficiency although the defect in isoleucine catabolism was demonstrated both in vivo and in vitro. Several mutations in the ACADSB gene were identified, including a novel one. MBD deficiency may be a harmless metabolic variant although significant impairment of valproic acid metabolism cannot be excluded and further study is required to assess the long-term outcome of individuals with this condition. The relatively high prevalence of ACADSB gene mutations in control subjects suggests that MBD deficiency may be more common than previously thought but is not detected because of its usually benign nature.

Amyloid-beta-induced mitochondrial dysfunction

As an important molecule in the pathogenesis of Alzheimer's disease (AD), amyloid-beta (Abeta) interferes with multiple aspects of mitochondrial function, including energy metabolism failure, production of reactive oxygen species (ROS) and permeability transition pore formation. Recent studies have demonstrated that Abeta progressively accumulates within mitochondrial matrix, providing a direct link to mitochondrial toxicity. Abeta-binding alcohol dehydrogenase (ABAD) is localized to the mitochondrial matrix and binds to mitochondrial Abeta. Interaction of ABAD with Abeta exaggerates Abeta-mediated mitochondrial and neuronal perturbation, leading to impaired synaptic function, and dysfunctional spatial learning/memory. Thus, blockade of ABAD/Abeta interaction may be a potential therapeutic strategy for AD.

2-methylbutyryl-CoA dehydrogenase deficiency associated with autism and mental retardation: a case report

Background

2-methylbutyryl-CoA dehydrogenase deficiency or short/branched chain acyl-CoA dehydrogenase deficiency (SBCADD) is caused by a defect in the degradation pathway of the amino acid L-isoleucine.

Methods

We report a four-year-old mentally retarded Somali boy with autism and a history of seizures, who was found to excrete increased amounts of 2-methylbutyryl glycine in the urine. The SBCAD gene was examined with sequence analysis. His development was assessed with psychometric testing before and after a trial with low protein diet.

Results

We found homozygosity for A > G changing the +3 position of intron 3 (c.303+3A > G) in the SBCAD gene. Psychometric testing showed moderate mental retardation and behavioral scores within the autistic spectrum. No beneficial effect was detected after 5 months with a low protein diet.

Conclusion

This mutation was also found in two previously reported cases with SBCADD, both originating from Somalia and Eritrea, indicating that it is relatively prevalent in this population. Autism has not previously been described with mutations in this gene, thus expanding the clinical spectrum of SBCADD.

HSD17B10: a gene involved in cognitive function through metabolism of isoleucine and neuroactive steroids

The HSD17B10 gene maps on chromosome Xp11.2, a region highly associated with X-linked mental retardation. This gene encodes HSD10, a mitochondrial multifunctional enzyme that plays a significant part in the metabolism of neuroactive steroids and the degradation of isoleucine. The HSD17B10 gene is composed of six exons and five introns. Its exon 5 is an alternative exon such that there are several HSD17B10 mRNA isoforms in brain. A silent mutation (c.605C-->A) and three missense mutations (c.395C-->G; c.419C-->T; c.771A-->G), respectively, cause the X-linked mental retardation, choreoathetosis, and abnormal behavior (MRXS10) and the hydroxyacyl-CoA dehydrogenase II deficiency. The latter condition seems to be a multifactorial disease due to the disturbance of more than one metabolic pathway by the HSD10 deficiency. HSD10 inactivates the positive modulators of GABAA receptors, and plays a role in the maintenance of GABAergic neuronal function. This working model may account for the mental retardation of these patients. The dehydrogenase activity is slightly inhibited by the binding of amyloid-beta peptide to the loop D of HSD10. Elevated levels of HSD10 were observed in hippocampi of Alzheimer disease patients so this multifunctional enzyme may be related to Alzheimer disease pathogenesis; however, the molecular mechanism of its involvement remains to be ascertained.

Neuroimage findings in 2-methyl-3-hydroxybutyryl-CoA dehydrogenase deficiency

A case of 2-methyl-3-hydroxybutyryl-coenzyme A dehydrogenase deficiency, an X-linked defect of isoleucine degradation, is reported. A 10-month-old male infant with developmental regression, visual impairment, movement disorder, and seizures, he suffered acute deterioration with multiorganic failure after a respiratory infection. Laboratory studies revealed hyperlactacidemia and increased excretion of 2-methyl-3-hydroxybutyric acid (2M3HBA) and tiglylglycine (TG). The diagnosis was established by molecular genetic analysis of the involved X-chromosome gene HADH2. The patient was hemizygous for the mutation R130C (c. 388C>T). Magnetic resonance imaging disclosed frontotemporal atrophy and bilateral signal abnormalities in the putamina. The presence of basal ganglia abnormalities and lactic acidemia, also shared by mitochondrial disorders, suggests a common pathophysiologic mechanism of damage.

Inborn errors of isoleucine degradation: a review

Three inborn errors have been identified in the pathway of isoleucine degradation. Deficiency of beta-ketothiolase (beta-KT, also known as T2, mitochondrial acetoacetyl-CoA thiolase and acetyl-CoA acetyltransferase 1) is a well-described disorder which presents with acute episodic ketoacidosis. In contrast, short/branched-chain acyl-CoA dehydrogenase (SBCAD) and 2-methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) deficiencies are recently described and relatively rare defects which present with predominantly neurological manifestations, although acute metabolic decompensation may occur in the early newborn period. Careful examination of urine organic acids is required for identification and differential diagnosis of these disorders, with awareness that the abnormalities may be subtle and variable. Tandem MS analysis of acylcarnitines may reveal elevated C5 (SBCAD) or C5:1 and/or OH-C5 species (MHBD and beta-KT deficiencies) but the abnormalities are non-diagnostic and may be intermittent or absent. Confirmation of diagnosis is therefore advisable by specific enzyme assay and/or mutation analysis of the ACAT1 (beta-KT), ACADSB (SBCAD) or HADH2 (MHBD) genes. The latter is located on the X chromosome, accounting for the milder clinical phenotype in females. If beta-KT deficiency is diagnosed early and treated by fasting avoidance and modest protein restriction, ketoacidosis episodes can be prevented and the prognosis is excellent. The role of treatment in SBCAD deficiency remains unclear pending further delineation of its clinical phenotype and pathogenicity, particularly regarding asymptomatic individuals detected by expanded newborn screening. The ineffectiveness of isoleucine restriction in MHBD deficiency is consistent with the additional roles of this multifunctional enzyme in sex steroid and neurosteroid metabolism and its interaction with amyloid-beta peptide.

Biochemical findings in common inborn errors of metabolism

The application of tandem mass spectrometry (MS/MS) to newborn screening has led to the detection of patients with a wider spectrum of inborn errors of metabolism. A definitive diagnosis can often be established early enough to start treatment before symptoms appear. Here, we review common biochemical findings in disorders caused by deficiency of 3-methylcrotonyl-CoA carboxylase, isobutyryl-CoA dehydrogenase, 2-methyl-3-hydroxybutyryl-CoA dehydrogenase, 3-ketothiolase, 2-methylbutyryl-CoA dehydrogenase, and medium chain acyl CoA dehydrogenase. The diagnosis of these disorders requires biochemical confirmation by measurement of plasma acylcarnitine profile, urine organic acids, and urine acylglycine profiles followed by measurement of enzyme activity or detection of causative mutations. Early treatment can improve the outcome of these disorders.

X-linked mental retardation: many genes for a complex disorder

X-linked mental retardation (XLMR) is a common cause of moderate to severe intellectual disability in males. XLMR is very heterogeneous, and about two-thirds of patients have clinically indistinguishable non-syndromic (NS-XLMR) forms, which has greatly hampered their molecular elucidation. A few years ago, international consortia overcame this impasse by collecting DNA and cell lines from large cohorts of XLMR families, thereby paving the way for the systematic study of the molecular causes of XLMR. Mutations in known genes might already account for 50% of the families with NS-XLMR, and various genes have been pinpointed that seem to be of particular diagnostic importance. Eventually, even therapy of XLMR might become possible, as suggested by the unexpected plasticity of the neuronal wiring in the brain, and the recent successful drug treatment of a fly model for fragile X syndrome.

Short/branched-chain acyl-CoA dehydrogenase deficiency due to an IVS3+3A>G mutation that causes exon skipping

Short/branched-chain acyl-CoA dehydrogenase deficiency (SBCADD) is an autosomal recessive disorder of L: -isoleucine catabolism. Little is known about the clinical presentation associated with this enzyme defect, as it has been reported in only a limited number of patients. Because the presence of C5-carnitine in blood may indicate SBCADD, the disorder may be detected by MS/MS-based routine newborn screening. It is, therefore, important to gain more knowledge about the clinical presentation and the mutational spectrum of SBCADD. In the present study, we have studied two unrelated families with SBCADD, both with seizures and psychomotor delay as the main clinical features. One family illustrates the fact that affected individuals may also remain asymptomatic. In addition, the normal level of newborn blood spot C5-acylcarnitine in one patient underscores the fact that newborn screening by MS/MS currently lacks sensitivity in detecting SBCADD. Until now, seven mutations in the SBCAD gene have been reported, but only three have been tested experimentally. Here, we identify and characterize an IVS3+3A>G mutation (c.303+3A>G) in the SBCAD gene, and provide evidence that this mutation is disease-causing in both families. Using a minigene approach, we show that the IVS3+3A>G mutation causes exon 3 skipping, despite the fact that it does not appear to disrupt the consensus sequence of the 5' splice site. Based on these results and numerous literature examples, we suggest that this type of mutation (IVS+3A>G) induces missplicing only when in the context of non-consensus (weak) 5' splice sites. Statistical analysis of the sequences shows that the wild-type versions of 5' splice sites in which +3A>G mutations cause exon skipping and disease are weaker on average than a random set of 5' splice sites. This finding is relevant to the interpretation of the functional consequences of this type of mutation in other disease genes.

3-Hydroxyacyl-CoA dehydrogenase and short chain 3-hydroxyacyl-CoA dehydrogenase in human health and disease

3-Hydroxyacyl-CoA dehydrogenase (HAD) functions in mitochondrial fatty acid beta-oxidation by catalyzing the oxidation of straight chain 3-hydroxyacyl-CoAs. HAD has a preference for medium chain substrates, whereas short chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD) acts on a wide spectrum of substrates, including steroids, cholic acids, and fatty acids, with a preference for short chain methyl-branched acyl-CoAs. Therefore, HAD should not be referred to as SCHAD. SCHAD is not a member of the HAD family, but instead, belongs to the short chain dehydrogenase/reductase superfamily. Previously reported cases of SCHAD deficiency are due to an inherited HAD deficiency. SCHAD, also known as 17beta-hydroxysteroid dehydrogenase type 10, is important in brain development and aging. Abnormal levels of SCHAD in certain brain regions may contribute to the pathogenesis of some neural disorders. The human SCHAD gene and its protein product, SCHAD, are potential targets for intervention in conditions, such as Alzheimer's disease, Parkinson's disease, and an X-linked mental retardation, that may arise from the impaired degradation of branched chain fatty acid and isoleucine.

2-Methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) deficiency: an X-linked inborn error of isoleucine metabolism that may mimic a mitochondrial disease

We describe three patients, from two Spanish families, with 2-methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) deficiency, a recently described X-linked neurodegenerative inborn error of isoleucine metabolism. Two of them are males with severe lactic acidosis suggestive of a mitochondrial encephalopathy, and the third is a female who was less severely affected, suggesting skewed X-inactivation. Molecular studies revealed a new missense mutation, 740A-->G, in one family and a previously described mutation, 388C-->T, in the other, causing the amino acid substitutions N247S and R130C, respectively. Both male patients died, one of them despite treatment with an isoleucine-restricted diet, but the disease has remained stable in the female patient after 1 y of treatment.

Multiple functions of type 10 17beta-hydroxysteroid dehydrogenase

Human 17beta-hydroxysteroid dehydrogenase type 10 (17beta-HSD10) is a mitochondrial enzyme encoded by the SCHAD gene, which escapes chromosome X inactivation. 17Beta-HSD10/SCHAD mutations cause a spectrum of clinical conditions, from mild mental retardation to progressive infantile neurodegeneration. 17Beta-HSD10/SCHAD is essential for the metabolism of isoleucine and branched-chain fatty acids. It can inactivate 17beta-estradiol and steroid modulators of GABA(A) receptors, and convert 5alpha-androstanediol into 5alpha-dihydrotestosterone (DHT). Certain malignant prostatic epithelial cells contain high levels of 17beta-HSD10, generating 5alpha-DHT in the absence of testosterone. 17Beta-HSD10 has an affinity for amyloid-beta peptide, and might be linked to the mitochondrial dysfunction seen in Alzheimer's disease. This versatile enzyme might provide a new drug target for neuronal excitability control and for intervention in Alzheimer's disease and certain cancers.

2-ethylhydracrylic aciduria in short/branched-chain acyl-CoA dehydrogenase deficiency: application to diagnosis and implications for the R-pathway of isoleucine oxidation

BACKGROUND

Isolated excretion of 2-methylbutyrylglycine (2-MBG) is the hallmark of short/branched-chain acyl-CoA dehydrogenase deficiency (SBCADD), a recently identified defect in the proximal pathway of L-isoleucine oxidation. SBCADD might be underdiagnosed because detection and recognition of urine acylglycines is problematic. Excretion of 2-ethylhydracrylic acid (2-EHA), an intermediate formed in the normally minor R-pathway of L-isoleucine oxidation, has not previously been described in SBCADD.

METHODS

Samples from four patients with 2-MBG excretion were analyzed by gas chromatography-mass spectrometry for urine organic acids, quantification of 2-MBG, and chiral determination of 2-methylbutyric acid. Blood-spot acylcarnitines were measured by electrospray-tandem mass spectrometry. Mutations in the ACADSB gene encoding SBCAD were identified by direct sequencing.

RESULTS

SBCADD was confirmed in each patient by demonstration of different ACADSB gene mutations. In multiple urine samples, organic acid analysis revealed a prominent 2-EHA peak usually exceeding the size of the 2-MBG peak. Approximately 40-46% of total 2-methylbutyric acid conjugates were in the form of the R-isomer, indicating significant metabolism via the R-pathway.

CONCLUSIONS

If, as generally believed, SBCAD is responsible for R-2-MBG dehydrogenation in the R-pathway, 2-EHA would not be produced in SBCADD. Our observation of 2-ethylhydracrylic aciduria in SBCADD implies that a different or alternative enzyme serves this function. Increased flux through the R-pathway may act as a safety valve for overflow of accumulating S-pathway metabolites and thereby mitigate the severity of SBCADD. Awareness of 2-ethylhydracrylic aciduria as a diagnostic marker could lead to increased detection of SBCADD and improved definition of its clinical phenotype.

Spastic diplegia and periventricular white matter abnormalities in 2-methyl-3-hydroxybutyryl-CoA dehydrogenase deficiency, a defect of isoleucine metabolism: differential diagnosis with hypoxic-ischemic brain diseases

A 19-month-old boy with 2-methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) deficiency, a defect of isoleucine degradation, had cognitive and motor development delay, spastic diplegia, dysmorphism, and occipital periventricular white matter lesions on MRI scan of the brain. The urinary accumulation of the isoleucine metabolites 2-methyl-3-hydroxybutyrate and tiglylglycine was only moderate under basal conditions. These abnormalities became more pronounced after a 100mg/kg oral isoleucine challenge. Enzyme studies showed a markedly decreased activity of MHBD in fibroblasts and lymphocytes. Sequence analysis of the involved X-chromosome gene (HADH2), revealed the presence of 364C -->G mutation in the patient. His mother was heterozygous for the 364C-->G mutation, whereas the mutation was not found in the other members of the family (father, brother, and sister). This report indicates that an enzyme defect in the metabolism of branched-chain fatty acid oxidation and isoleucine may present features resembling sequelae of neonatal hypoxic-ischemic brain injury. All patients with MHBD deficiency identified so far are characterized by a neurologic phenotype rather than ketoacidotic attacks, unlike patients with the related isoleucine defect beta-ketothiolase deficiency.

2-Methyl-3-hydroxybutyryl-CoA dehydrogenase deficiency: impaired catabolism of isoleucine presenting as neurodegenerative disease

We describe a further case of recently reported 2-methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) deficiency, a disorder of isoleucine metabolism. The development of pronounced brain atrophy and symmetrical alterations of the basal ganglia were observed and the importance of specific enzymatic tests is emphasized, which should be performed if urinary metabolites suggest impaired catabolism of isoleucine.

2-Methyl-3-hydroxybutyryl-CoA dehydrogenase deficiency is caused by mutations in the HADH2 gene

2-methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) deficiency is a novel inborn error of isoleucine degradation. In this article, we report the elucidation of the molecular basis of MHBD deficiency. To this end, we purified the enzyme from bovine liver. MALDI-TOF mass spectrometry analysis revealed that the purified protein was identical to bovine 3-hydroxyacyl-CoA dehydrogenase type II. The human homolog of this bovine enzyme is a short-chain 3-hydroxyacyl-CoA dehydrogenase, also known as the "endoplasmic reticulum-associated amyloid-beta binding protein" (ERAB). This led to the identification of the X-chromosomal gene involved, which previously had been denoted "HADH2." Sequence analysis of the HADH2 gene from patients with MHBD deficiency revealed the presence of two missense mutations (R130C and L122V). Heterologous expression of the mutant cDNAs in Escherichia coli showed that both mutations almost completely abolish enzyme activity. This confirms that MHBD deficiency is caused by mutations in the HADH2 gene.

3-Hydroxy-2-methylbutyryl-CoA dehydrogenase deficiency

A boy now 8 years old presented at 21 months of age with developmental arrest, followed by regression, cortical blindness and myoclonic seizures. Urine organic acid analysis revealed 3-hydroxy-2-methylbutyric acid and tiglyglycine; 3-ketothiolase enzyme activity was normal and he was subsequently found to have 3-hydroxy-2-methylbutyryl-CoA dehydrogenase deficiency.

2-methyl-3-hydroxybutyryl-CoA dehydrogenase deficiency in a 23-year-old man

2-Methyl-3-hydroxybutyryl-CoA dehydrogenase (EC 1.1.1.178) deficiency is a recently described defect of isoleucine catabolism. The disorder is characterized by normal early development followed by a progressive loss of mental and motor skills. Deterioration may be rapid or may follow a slower decline with a possible stabilization of the disorder on a low-protein diet and appropriate medication. We report a 23-year-old man with 2-methyl-3-hydroxybutyryl-CoA dehydrogenase deficiency with a very mild clinical course. He had apparently normal early development and remained relatively well until the age of 6 years, when he contracted measles. Following this illness, his motor skills and school progress deteriorated. At 15 years he had significant dysarthria, and generalized rigidity with some dystonic and unusual posturing. He was then treated with a low-protein high-carbohydrate diet with a good response in terms of balance and gait. At 18 years he was given benzhexol (Artane), increased slowly from 2 mg to 6 mg daily, resulting in improvement in tremor and dystonia. At 23 years he can dress himself and works in sheltered employment but remains severely dysarthric.