The reduced expression of the HADH2 protein causes X-linked mental retardation, choreoathetosis, and abnormal behavior

Aβ-binding with alcohol dehydrogenase drives Alzheimer's disease pathogenesis: A review

Although Alzheimer's disease (AD) characterized with senile plaques and neurofibrillary tangles has been found for over 100 years, its molecular mechanisms are ambiguous. More worsely, the developed medicines targeting amyloid-beta (Aβ) and/or tau hyperphosphorylation did not approach the clinical expectations in patients with moderate or severe AD until now. This review unveils the role of a vicious cycle between Aβ-derived formaldehyde (FA) and FA-induced Aβ aggregation in the onset course of AD. Document evidence has shown that Aβ can bind with alcohol dehydrogenase (ADH) to form the complex of Aβ/ADH (ABAD) and result in the generation of reactive oxygen species (ROS) and aldehydes including malondialdehyde, hydroxynonenal and FA; in turn, ROS-derived H2O2 and FA promotes Aβ self-aggregation; subsequently, this vicious cycle accelerates neuron death and AD occurrence. Especially, FA can directly induce neuron death by stimulating ROS generation and tau hyper hyperphosphorylation, and impair memory by inhibiting NMDA-receptor. Recently, some new therapeutical methods including inhibition of ABAD activity by small molecules/synthetic polypeptides, degradation of FA by phototherapy or FA scavengers, have been developed and achieved positive effects in AD transgenic models. Thus, breaking the vicious loop may be promising interventions for halting AD progression.

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.

3-Hydroxyacyl-CoA and Alcohol Dehydrogenase Activities of Mitochondrial Type 10 17β-Hydroxysteroid Dehydrogenase in Neurodegeneration Study

Background:

Mitochondrial 17β-hydroxysteroid dehydrogenase type 10 (17β-HSD10) is necessary for brain cognitive function, but its studies were confounded by reports of Aβ-peptide binding alcohol dehydrogenase (ABAD), formerly endoplasmic reticulum-associated Aβ-peptide binding protein (ERAB), for two decades so long as ABAD serves as the alternative term of 17β-HSD10.

Objective:

To determine whether those ABAD reports are true or false, even if they were published in prestigious journals.

Methods:

6xHis-tagged 17β-HSD10 was prepared and characterized by well-established experimental procedures.

Results:

The N-terminal 6xHis tag did not significantly interfere with the dehydrogenase activities of 17β-HSD10, but the kinetic constants of its 3-hydroxyacyl-CoA dehydrogenase activity are drastically distinct from those of ABAD, and it was not involved in ketone body metabolism as previously reported for ABAD. Furthermore, it was impossible to measure its generalized alcohol dehydrogenase activities underlying the concept of ABAD because the experimental procedures described in ABAD reports violated basic chemical and/or biochemical principles. More incredibly, both authors and journals had not yet agreed to make any corrigenda of ABAD reports.

Conclusion:

Brain 17β-HSD10 plays a key role in neurosteroid metabolism and further studies in this area may lead to potential treatments of neurodegeneration including AD.

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.

17β-Hydroxysteroid dehydrogenases and neurosteroid metabolism in the central nervous system

17β-Hydroxysteroid dehydrogenases are indispensable for downstream enzyme steps of the neurosteroidogenesis. Neurosteroids are synthesized de novo in neurons and glia from cholesterol transported into mitochondria, or by conversion from proneurosteroids, e. g. dehydroepiandrosterone (DHEA) and pregnenolone, through the same metabolic pathway as revealed in the de novo neurosteroidogenesis. Hormonal steroids generated from endocrine glands are transported into brain from the circulation to exert neuronal activity via genomic pathway, whereas neurosteroids produced in brain cells without genomic targets identified could bind to cell surface targets, e.g., GABA_A or NMDA receptors and elicit antidepressant, anxiolytic, anticonvulsant and anesthetic effects by regulating neuroexcitability. In a broad sense, neurosteroids include hormonal steroids in brain, and they are irrespective of origin playing important roles in brain development including neuroprotection, neurogenesis and neural plasticity. They are also a critical element in cognitive and memory functions. Mitochondrial 17β-HSD10, encoded by the HSD17B10 gene mapping to Xp11.2, is found in various brain regions, essential for the maintenance of neurosteroid homeostasis. Mutations identified in this gene resulted in two distinct brain diseases, namely HSD10 deficiency and MRXS10, of which clinical presentations and pathogenetic mechanisms are quite different. Since elevated levels of 17β-HSD10 was found in brains of Alzheimer's disease patients and AD mouse model, it may also act as an adverse factor in the AD pathogenesis due to an imbalance of neurosteroid metabolism.

X-linked intellectual disability update 2017

The X-chromosome comprises only about 5% of the human genome but accounts for about 15% of the genes currently known to be associated with intellectual disability. The early progress in identifying the X-linked intellectual disability (XLID)-associated genes through linkage analysis and candidate gene sequencing has been accelerated with the use of high-throughput technologies. In the 10 years since the last update, the number of genes associated with XLID has increased by 96% from 72 to 141 and duplications of all 141 XLID genes have been described, primarily through the application of high-resolution microarrays and next generation sequencing. The progress in identifying genetic and genomic alterations associated with XLID has not been matched with insights that improve the clinician's ability to form differential diagnoses, that bring into view the possibility of curative therapies for patients, or that inform scientists of the impact of the genetic alterations on cell organization and function.

Ocular manifestations in the X-linked intellectual disability syndromes

Intellectual disability (ID), a common neurodevelopmental disorder characterized by limitations of both intellectual functioning and adaptive behavior, affects an estimated 1-2% of children. Genetic causes of ID are often accompanied by recognizable syndromal patterns. The vision apparatus is a sensory extension of the brain, and individuals with intellectual disabilities frequently have coexisting abnormalities of ocular structures and the visual pathway system. About one-third of the X-linked intellectual disability (XLID) syndromes have significant eye or ocular adnexa abnormalities that provide important diagnostic clues. Some XLID syndromes (e.g. Aicardi, cerebrooculogenital, Graham anophthalmia, Lenz, Lowe, MIDAS) are widely known for their characteristic ocular manifestations. Nystagmus, optic atrophy, and strabismus are among the more common, nonspecific, ocular manifestations that contribute to neuro-ophthalmological morbidity. Common dysmorphic oculofacial findings include anophthalmia, microphthalmia, hypertelorism, and abnormalities in the configuration or orientation of the palpebral fissures. Four XLID syndromes with major ocular manifestations (incontinentia pigmenti, Goltz, MIDAS, and Aicardi syndromes) are notable because of male lethality and expression occurring predominantly in females. The majority of the genes associated with XLID and ocular manifestations have now been identified.

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.

A novel HSD17B10 mutation impairing the activities of the mitochondrial RNase P complex causes X-linked intractable epilepsy and neurodevelopmental regression

We report a Caucasian boy with intractable epilepsy and global developmental delay. Whole-exome sequencing identified the likely genetic etiology as a novel p.K212E mutation in the X-linked gene HSD17B10 for mitochondrial short-chain dehydrogenase/reductase SDR5C1. Mutations in HSD17B10 cause the HSD10 disease, traditionally classified as a metabolic disorder due to the role of SDR5C1 in fatty and amino acid metabolism. However, SDR5C1 is also an essential subunit of human mitochondrial RNase P, the enzyme responsible for 5'-processing and methylation of purine-9 of mitochondrial tRNAs. Here we show that the p.K212E mutation impairs the SDR5C1-dependent mitochondrial RNase P activities, and suggest that the pathogenicity of p.K212E is due to a general mitochondrial dysfunction caused by reduction in SDR5C1-dependent maturation of mitochondrial tRNAs.

Investigation of sex differences in the expression of RORA and its transcriptional targets in the brain as a potential contributor to the sex bias in autism

Background

Autism spectrum disorder (ASD) is a neurodevelopmental condition characterized by significant impairment in reciprocal social interactions and communication coupled with stereotyped, repetitive behaviors and restricted interests. Although genomic and functional studies are beginning to reveal some of the genetic complexity and underlying pathobiology of ASD, the consistently reported male bias of ASD remains an enigma. We have recently proposed that retinoic acid-related orphan receptor alpha (RORA), which is reduced in the brain and lymphoblastoid cell lines of multiple cohorts of individuals with ASD and oppositely regulated by male and female hormones, might contribute to the sex bias in autism by differentially regulating target genes, including CYP19A1 (aromatase), in a sex-dependent manner that can also lead to elevated testosterone levels, a proposed risk factor for autism.

Methods

In this study, we examine sex differences in RORA and aromatase protein levels in cortical tissues of unaffected and affected males and females by re-analyzing pre-existing confocal immunofluorescence data from our laboratory. We further investigated the expression of RORA and its correlation with several of its validated transcriptional targets in the orbital frontal cortex and cerebellum as a function of development using RNAseq data from the BrainSpan Atlas of the Developing Human Brain. In a pilot study, we also analyzed the expression of Rora and the same transcriptional targets in the cortex and cerebellum of adult wild-type male and female C57BL/6 mice.

Results

Our findings suggest that Rora/RORA and several of its transcriptional targets may exhibit sexually dimorphic expression in certain regions of the brain of both mice and humans. Interestingly, the correlation coefficients between Rora expression and that of its targets are much higher in the cortex of male mice relative to that of female mice. A strong positive correlation between the levels of RORA and aromatase proteins is also seen in the cortex of control human males and females as well as ASD males, but not ASD females.

Conclusions

Based on these studies, we suggest that disruption of Rora/RORA expression may have a greater impact on males, since sex differences in the correlation of RORA and target gene expression indicate that RORA-deficient males may experience greater dysregulation of genes relevant to ASD in certain brain regions during development.

Electronic supplementary material

The online version of this article (doi:10.1186/2040-2392-6-7) contains supplementary material, which is available to authorized users.

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.

Pure interstitial dup(6)(q22.31q22.31) - a case report

‘Pure’ interstitial duplication of chr6q is rare. The varying size of duplication encompassing 6q22.31 is associated with the expressivity of dysmorphism and autism. Here, we report a unique case with facial dysmorphism, developmental delay, complex neurological impairment and spasticity unrelated to autism. Genetic analysis by aCGH exhibited a 627–971 kb dup(6)(q22.31q22.31) encompassing TRDN and NKAIN2 genes. The presence of the duplication was confirmed by quantitative PCR in the proband and phenotypically normal parents. With the current techniques, we cannot exclude presence of a deleterious homozygous point mutation in the proband where each copy would have been inherited from both parents.

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.

Roles of 17β-hydroxysteroid dehydrogenase type 10 in neurodegenerative disorders

17β-Hydroxysteroid dehydrogenase type 10 (17β-HSD10) is encoded by the HSD17B10 gene mapping at Xp11.2. This homotetrameric mitochondrial multifunctional enzyme catalyzes the oxidation of neuroactive steroids and the degradation of isoleucine. This enzyme is capable of binding to other peptides, such as estrogen receptor α, amyloid-β, and tRNA methyltransferase 10C. Missense mutations of the HSD17B10 gene result in 17β-HSD10 deficiency, an infantile neurodegeneration characterized by progressive psychomotor regression and alteration of mitochondria morphology. 17β-HSD10 exhibits only a negligible alcohol dehydrogenase activity, and is not localized in the endoplasmic reticulum or plasma membrane. Its alternate name - Aβ binding alcohol dehydrogenase (ABAD) - is a misnomer predicated on the mistaken belief that this enzyme is an alcohol dehydrogenase. Misconceptions about the localization and function of 17β-HSD10 abound. 17β-HSD10's proven location and function must be accurately identified to properly assess this enzyme's important role in brain metabolism, especially the metabolism of allopregnanolone. The brains of individuals with Alzheimer's disease (AD) and of animals in an AD mouse model exhibit abnormally elevated levels of 17β-HSD10. Abnormal expression, as well as mutations of the HSD17B10 gene leads to impairment of the structure, function, and dynamics of mitochondria. This may underlie the pathogenesis of the synaptic and neuronal deficiency exhibited in 17β-HSD10 related diseases, including 17β-HSD10 deficiency and AD. Restoration of steroid homeostasis could be achieved by the supplementation of neuroactive steroids with a proper dosing and treatment regimen or by the adjustment of 17β-HSD10 activity to protect neurons. The discovery of this enzyme's true function has opened a new therapeutic avenue for treating AD.

Mutation or knock-down of 17β-hydroxysteroid dehydrogenase type 10 cause loss of MRPP1 and impaired processing of mitochondrial heavy strand transcripts

17β-Hydroxysteroid dehydrogenase type 10 (HSD10) is multifunctional protein coded by the X-chromosomal HSD17B10 gene. Mutations in this gene cause HSD10 disease characterized by progressive neurological abnormalities and cardiomyopathy. Disease progression and severity of symptoms is unrelated to the protein's dehydrogenase activity. Recently, it was shown that HSD10 is an essential component of mitochondrial Ribonuclease P (RNase P), an enzyme required for mitochondrial tRNA processing, but little is known about the role of HSD10 in RNase P function. RNase P consists of three different proteins MRPP1, MRPP2 (HSD10) and MRPP3, each of which is essential for RNase P function. Here, we show that HSD10 protein levels are significantly reduced in fibroblasts from patients carrying the HSD17B10 mutation p.R130C. A reduction in HSD10 levels was accompanied by a reduction in MRPP1 protein but not MRPP3 protein. In HSD10 knock-down cells, MRPP1 protein content was also reduced, indicating that HSD10 is important for the maintenance of normal MRPP1 protein levels. Ectopic expression of HSD10 partially restored RNA processing in HSD10 knock-down cells and fibroblasts, and also expression of MRPP1 protein was restored to values comparable to controls. In both, patient fibroblasts and HSD10 knock-down cells, there was evidence of impaired processing of precursor tRNA transcripts of the mitochondrial heavy strand but not the light strand compared with controls. Our findings indicate that HSD10 is important for the maintenance of the MRPP1-HSD10 subcomplex of RNase P and that loss of HSD10 causes impaired mitochondrial precursor transcript processing which may explain mitochondrial dysfunction observed in HSD10 disease.

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.

The mitochondrial solute carrier SLC25A5 at Xq24 is a novel candidate gene for non-syndromic intellectual disability

Loss-of-function mutations in several different neuronal pathways have been related to intellectual disability (ID). Such mutations often are found on the X chromosome in males since they result in functional null alleles. So far, microdeletions at Xq24 reported in males always have been associated with a syndromic form of ID due to the loss of UBE2A. Here, we report on overlapping microdeletions at Xq24 that do not include UBE2A or affect its expression, in patients with non-syndromic ID plus some additional features from three unrelated families. The smallest region of overlap, confirmed by junction sequencing, harbors two members of the mitochondrial solute carrier family 25, SLC25A5 and SLC25A43. However, identification of an intragenic microdeletion including SLC25A43 but not SLC25A5 in a healthy boy excluded a role for SLC25A43 in cognition. Therefore, our findings point to SLC25A5 as a novel gene for non-syndromic ID. This highly conserved gene is expressed ubiquitously with high levels in cortex and hippocampus, and a presumed role in mitochondrial exchange of ADP/ATP. Our data indicate that SLC25A5 is involved in memory formation or establishment, which could add mitochondrial processes to the wide array of pathways that regulate normal cognitive functions.

Genome-wide identification of transcriptional targets of RORA reveals direct regulation of multiple genes associated with autism spectrum disorder

BACKGROUND

We have recently identified the nuclear hormone receptor RORA (retinoic acid-related orphan receptor-alpha) as a novel candidate gene for autism spectrum disorder (ASD). Our independent cohort studies have consistently demonstrated the reduction of RORA transcript and/or protein levels in blood-derived lymphoblasts as well as in the postmortem prefrontal cortex and cerebellum of individuals with ASD. Moreover, we have also shown that RORA has the potential to be under negative and positive regulation by androgen and estrogen, respectively, suggesting the possibility that RORA may contribute to the male bias of ASD. However, little is known about transcriptional targets of this nuclear receptor, particularly in humans.

METHODS

Here we identify transcriptional targets of RORA in human neuronal cells on a genome-wide level using chromatin immunoprecipitation (ChIP) with an anti-RORA antibody followed by whole-genome promoter array (chip) analysis. Selected potential targets of RORA were then validated by an independent ChIP followed by quantitative PCR analysis. To further demonstrate that reduced RORA expression results in reduced transcription of RORA targets, we determined the expression levels of the selected transcriptional targets in RORA-deficient human neuronal cells, as well as in postmortem brain tissues from individuals with ASD who exhibit reduced RORA expression.

RESULTS

The ChIP-on-chip analysis reveals that RORA1, a major isoform of RORA protein in human brain, can be recruited to as many as 2,764 genomic locations corresponding to promoter regions of 2,544 genes across the human genome. Gene ontology analysis of this dataset of genes that are potentially directly regulated by RORA1 reveals statistically significant enrichment in biological functions negatively impacted in individuals with ASD, including neuronal differentiation, adhesion and survival, synaptogenesis, synaptic transmission and plasticity, and axonogenesis, as well as higher level functions such as development of the cortex and cerebellum, cognition, memory, and spatial learning. Independent ChIP-quantitative PCR analyses confirm binding of RORA1 to promoter regions of selected ASD-associated genes, including A2BP1, CYP19A1, ITPR1, NLGN1, and NTRK2, whose expression levels (in addition to HSD17B10) are also decreased in RORA1-repressed human neuronal cells and in prefrontal cortex tissues from individuals with ASD.

CONCLUSIONS

Findings from this study indicate that RORA transcriptionally regulates A2BP1, CYP19A1, HSD17B10, ITPR1, NLGN1, and NTRK2, and strongly suggest that reduction of this sex hormone-sensitive nuclear receptor in the brain causes dysregulated expression of these ASD-relevant genes as well as their associated pathways and functions which, in turn, may contribute to the underlying pathobiology of ASD.

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.

Copy-number gains of HUWE1 due to replication- and recombination-based rearrangements

We previously reported on nonrecurrent overlapping duplications at Xp11.22 in individuals with nonsyndromic intellectual disability (ID) harboring HSD17B10, HUWE1, and the microRNAs miR-98 and let-7f-2 in the smallest region of overlap. Here, we describe six additional individuals with nonsyndromic ID and overlapping microduplications that segregate in the families. High-resolution mapping of the 12 copy-number gains reduced the minimal duplicated region to the HUWE1 locus only. Consequently, increased mRNA levels were detected for HUWE1, but not HSD17B10. Marker and SNP analysis, together with identification of two de novo events, suggested a paternally derived intrachromosomal duplication event. In four independent families, we report on a polymorphic 70 kb recurrent copy-number gain, which harbors part of HUWE1 (exon 28 to 3' untranslated region), including miR-98 and let-7f-2. Our findings thus demonstrate that HUWE1 is the only remaining dosage-sensitive gene associated with the ID phenotype. Junction and in silico analysis of breakpoint regions demonstrated simple microhomology-mediated rearrangements suggestive of replication-based duplication events. Intriguingly, in a single family, the duplication was generated through nonallelic homologous recombination (NAHR) with the use of HUWE1-flanking imperfect low-copy repeats, which drive this infrequent NAHR event. The recurrent partial HUWE1 copy-number gain was also generated through NAHR, but here, the homologous sequences used were identified as TcMAR-Tigger DNA elements, a template that has not yet been reported for NAHR. In summary, we showed that an increased dosage of HUWE1 causes nonsyndromic ID and demonstrated that the Xp11.22 region is prone to recombination- and replication-based rearrangements.

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.

[Copy number variations in the human genome: their mutational mechanisms and roles in diseases]

Copy number variation (CNV) is the main type of structure variation (SV) caused by genomic rearrangement, which mainly includes deletion and duplication of sub-microscopic but large (>1 kb) genomic segments. CNV has been recognized as one of the main genetic factors underlying human diseases. The mutation rate (per locus) of CNV is much higher than that of single nucleotide polymorphism (SNP). The genome-wide assays for CNV study include array-based comparative genomic hybridization (aCGH), SNP genotyping microarrays, and next-generation sequencing techniques. Various molecular mechanisms are involved in CNV formation, which can be divided into two main categories, DNA recombination-based and DNA replication-based mechanisms. CNVs can be associated with Mendelian diseases, sporadic diseases, and susceptibility to complex diseases. CNVs can convey clinical phenotypes by gene dosage, gene disruption, gene fusion, and position effects. Further studies on CNVs will shed new light on human genome structure, genetic variations between individuals, and missing heritability of human diseases.

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.

SMN2 exon 7 splicing is inhibited by binding of hnRNP A1 to a common ESS motif that spans the 3' splice site

Spinal Muscular Atrophy is caused by homozygous loss of SMN1 with phenotypic modulation by SMN2. SMN2 expresses only limited amounts of full-length transcript due to skipping of exon 7 caused by disruption of an SF2/ASF binding ESE. Additionally, hnRNP A1 has been reported to inhibit inclusion of SMN2 exon 7. We previously reported high similarity between the sequence spanning the 3' ss of SMN1 and SMN2 exon 7 and an hnRNP A1 binding ESS, which regulates MCAD exon 5 splicing. We show here that this 3' ss motif indeed functions as a crucial hnRNP A1 binding ESS, which inhibits inclusion of SMN1/2 exon 7 and is antagonized by the SMN1 ESE, but not by the inactive SMN2 sequence. Pull-down experiments revealed a specific interaction between hnRNP A1 and the 3' ss AG-dinucleotide, which could be disrupted by mutations shown to improve splicing in reporter minigenes. Genomic analyses revealed that in the human genome, 3' ss matching the SMN1/2 ESS motif region are much less abundant than 3' ss with a disrupted ESS motif. This indicates that this ESS may be a general splicing inhibitory motif, which binds hnRNP A1 and inhibits exon inclusion by binding to 3' ss harboring this ESS motif.

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.

[Screening and identification of interactive proteins of SH2D4A]

SH2D4A is a member of SH2 signaling protein family, which is involved in the signal transduction mediated by protein tyrosine kinase-related receptor, cell growth, proliferation, differentiation, and thereby affects the development of human disorders. To determine the role of SH2D4A in the cell signal transduction pathway, SH2D4A interactive proteins were screened using yeast two-hybrid system, and yeast mating and GST pull-down assays were carried out to further confirm the interaction. We successfully generated a bait protein expression construct-pGBKT7-SH2D4A, screened the human kidney cDNA library, and obtained 46 positive yeast clones. After isolation of positive colonies, DNA sequencing, and sequence alignment analysis with BLAST software, we obtained 5 potential SH2D4A interactive proteins, AZGP1, DAD1, HSD17B10, KAT5, and PKM2, which were predicted by NetPhos 2.0 Server software and were all shown to be phosphorylated tyrosine (pY)-containing proteins except for HSD17B10. KAT5 and HSD17B10 were selected to perform yeast mating and GST pull-down experiments, indicating their direct binding to SH2D4A.

A compendium of human mitochondrial gene expression machinery with links to disease

Mammalian mitochondrial DNA encodes 37 essential genes required for ATP production via oxidative phosphorylation, instability or misregulation of which is associated with human diseases and aging. Other than the mtDNA-encoded RNA species (13 mRNAs, 12S and 16S rRNAs, and 22 tRNAs), the remaining factors needed for mitochondrial gene expression (i.e., transcription, RNA processing/modification, and translation), including a dedicated set of mitochondrial ribosomal proteins, are products of nuclear genes that are imported into the mitochondrial matrix. Herein, we inventory the human mitochondrial gene expression machinery, and, while doing so, we highlight specific associations of these regulatory factors with human disease. Major new breakthroughs have been made recently in this burgeoning area that set the stage for exciting future studies on the key outstanding issue of how mitochondrial gene expression is regulated differentially in vivo. This should promote a greater understanding of why mtDNA mutations and dysfunction cause the complex and tissue-specific pathology characteristic of mitochondrial disease states and how mitochondrial dysfunction contributes to more common human pathology and aging.

A non-enzymatic function of 17beta-hydroxysteroid dehydrogenase type 10 is required for mitochondrial integrity and cell survival

Deficiency of the mitochondrial enzyme 2-methyl-3-hydroxybutyryl-CoA dehydrogenase involved in isoleucine metabolism causes an organic aciduria with atypical neurodegenerative course. The disease-causing gene is HSD17B10 and encodes 17β-hydroxysteroid dehydrogenase type 10 (HSD10), a protein also implicated in the pathogenesis of Alzheimer's disease. Here we show that clinical symptoms in patients are not correlated with residual enzymatic activity of mutated HSD10. Loss-of-function and rescue experiments in Xenopus embryos and cells derived from conditional Hsd17b10^−/− mice demonstrate that a property of HSD10 independent of its enzymatic activity is essential for structural and functional integrity of mitochondria. Impairment of this function in neural cells causes apoptotic cell death whilst the enzymatic activity of HSD10 is not required for cell survival. This finding indicates that the symptoms in patients with mutations in the HSD17B10 gene are unrelated to accumulation of toxic metabolites in the isoleucine pathway and, rather, related to defects in general mitochondrial function. Therefore alternative therapeutic approaches to an isoleucine-restricted diet are required.

Genomic features defining exonic variants that modulate splicing

A comparative analysis of SNPs and their exonic and intronic environments identifies the features predictive of splice affecting variants.

Background

Single point mutations at both synonymous and non-synonymous positions within exons can have severe effects on gene function through disruption of splicing. Predicting these mutations in silico purely from the genomic sequence is difficult due to an incomplete understanding of the multiple factors that may be responsible. In addition, little is known about which computational prediction approaches, such as those involving exonic splicing enhancers and exonic splicing silencers, are most informative.

Results

We assessed the features of single-nucleotide genomic variants verified to cause exon skipping and compared them to a large set of coding SNPs common in the human population, which are likely to have no effect on splicing. Our findings implicate a number of features important for their ability to discriminate splice-affecting variants, including the naturally occurring density of exonic splicing enhancers and exonic splicing silencers of the exon and intronic environment, extensive changes in the number of predicted exonic splicing enhancers and exonic splicing silencers, proximity to the splice junctions and evolutionary constraint of the region surrounding the variant. By extending this approach to additional datasets, we also identified relevant features of variants that cause increased exon inclusion and ectopic splice site activation.

Conclusions

We identified a number of features that have statistically significant representation among exonic variants that modulate splicing. These analyses highlight putative mechanisms responsible for splicing outcome and emphasize the role of features important for exon definition. We developed a web-tool, Skippy, to score coding variants for these relevant splice-modulating features.

Complex segmental duplications mediate a recurrent dup(X)(p11.22-p11.23) associated with mental retardation, speech delay, and EEG anomalies in males and females

Submicroscopic copy-number variations make a considerable contribution to the genetic etiology of human disease. We have analyzed subjects with idiopathic mental retardation (MR) by using whole-genome oligonucleotide-based array comparative genomic hybridization (aCGH) and identified familial and de novo recurrent Xp11.22-p11.23 duplications in males and females with MR, speech delay, and a peculiar electroencephalographic (EEG) pattern in childhood. The size of the duplications ranges from 0.8-9.2 Mb. Most affected females show preferential activation of the duplicated X chromosome. Carriers of the smallest duplication show X-linked recessive inheritance. All other affected individuals present dominant expression and comparable clinical phenotypes irrespective of sex, duplication size, and X-inactivation pattern. The majority of the rearrangements are mediated by recombination between flanking complex segmental duplications. The identification of common clinical features, including the typical EEG pattern, predisposing genomic structure, and peculiar X-inactivation pattern, suggests that duplication of Xp11.22-p11.23 constitutes a previously undescribed syndrome.

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.

Integrated view on 17beta-hydroxysteroid dehydrogenases

17beta-Hydroxysteroid dehydrogenases (17beta-HSDs) are important enzymes in steroid metabolism. Long known members of the protein family seemed to be well characterised concerning their role in the regulation of the biological potency of steroid hormones, but today more and more evidence points to pivotal contributions of these enzymes in a variety of other metabolic pathways. Therefore, studies on 17beta-HSDs develop towards metabolomic survey. Latest research results give new insights into the complex metabolic interconnectivity of the 17beta-HSDs. In this paper metabolic activities of 17beta-HSDs will be compared, their interplay with endogenous substrates summarised, and interlacing pathways depicted. Strategies on deciphering the physiological role of 17beta-HSDs and the genetic predisposition for associated diseases will be presented.

Copy number variation in human health, disease, and evolution

Copy number variation (CNV) is a source of genetic diversity in humans. Numerous CNVs are being identified with various genome analysis platforms, including array comparative genomic hybridization (aCGH), single nucleotide polymorphism (SNP) genotyping platforms, and next-generation sequencing. CNV formation occurs by both recombination-based and replication-based mechanisms and de novo locus-specific mutation rates appear much higher for CNVs than for SNPs. By various molecular mechanisms, including gene dosage, gene disruption, gene fusion, position effects, etc., CNVs can cause Mendelian or sporadic traits, or be associated with complex diseases. However, CNV can also represent benign polymorphic variants. CNVs, especially gene duplication and exon shuffling, can be a predominant mechanism driving gene and genome evolution.

RNase P without RNA: identification and functional reconstitution of the human mitochondrial tRNA processing enzyme

tRNAs are synthesized as immature precursors, and on their way to functional maturity, extra nucleotides at their 5' ends are removed by an endonuclease called RNase P. All RNase P enzymes characterized so far are composed of an RNA plus one or more proteins, and tRNA 5' end maturation is considered a universal ribozyme-catalyzed process. Using a combinatorial purification/proteomics approach, we identified the components of human mitochondrial RNase P and reconstituted the enzymatic activity from three recombinant proteins. We thereby demonstrate that human mitochondrial RNase P is a protein enzyme that does not require a trans-acting RNA component for catalysis. Moreover, the mitochondrial enzyme turns out to be an unexpected type of patchwork enzyme, composed of a tRNA methyltransferase, a short-chain dehydrogenase/reductase-family member, and a protein of hitherto unknown functional and evolutionary origin, possibly representing the enzyme's metallonuclease moiety. Apparently, animal mitochondria lost the seemingly ubiquitous RNA world remnant after reinventing RNase P from preexisting components.

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.

Submicroscopic duplications of the hydroxysteroid dehydrogenase HSD17B10 and the E3 ubiquitin ligase HUWE1 are associated with mental retardation

Submicroscopic copy-number imbalances contribute significantly to the genetic etiology of human disease. Here, we report a novel microduplication hot spot at Xp11.22 identified in six unrelated families with predominantly nonsyndromic XLMR. All duplications segregate with the disease, including the large families MRX17 and MRX31. The minimal, commonly duplicated region contains three genes: RIBC1, HSD17B10, and HUWE1. RIBC1 could be excluded on the basis of its absence of expression in the brain and because it escapes X inactivation in females. For the other genes, expression array and quantitative PCR analysis in patient cell lines compared to controls showed a significant upregulation of HSD17B10 and HUWE1 as well as several important genes in their molecular pathways. Loss-of-function mutations of HSD17B10 have previously been associated with progressive neurological disease and XLMR. The E3 ubiquitin ligase HUWE1 has been implicated in TP53-associated regulation of the neuronal cell cycle. Here, we also report segregating sequence changes of highly conserved residues in HUWE1 in three XLMR families; these changes are possibly associated with the phenotype. Our findings demonstrate that an increased gene dosage of HSD17B10, HUWE1, or both contribute to the etiology of XLMR and suggest that point mutations in HUWE1 are associated with this disease too.

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.

New perspectives for the elucidation of genetic disorders

For almost 15 years, genome research has focused on the search for major risk factors in common diseases, with disappointing results. Only recently, whole-genome association studies have begun to deliver because of the introduction of high-density single-nucleotide-polymorphism arrays and massive enlargement of cohort sizes, but most of the risk factors detected account for only a small proportion of the total genetic risk, and their diagnostic value is negligible. There is reason to believe that the complexity of many "multifactorial" disorders is primarily due to genetic heterogeneity, with defects of different genes causing the same disease. Moreover, de novo copy-number variation has been identified as a major cause of mental retardation and other complex disorders, suggesting that new mutations are an important, previously overlooked factor in the etiology of complex diseases. These observations support the notion that research into the previously neglected monogenic disorders should become a priority of genome research. Because of the introduction of novel high-throughput, low-cost sequencing methods, sequencing and genotyping will soon converge, with far-reaching implications for the elucidation of genetic disease and health care.