J Inherit Metab Dis (2007) 30:279–294
`DOI 10.1007/s10545-007-0574-2
`
`REVIEW
`
`Therapeutic concepts in succinate semialdehyde
`dehydrogenase (SSADH; ALDH5a1) deficiency
`(g-hydroxybutyric aciduria). Hypotheses evolved
`from 25 years of patient evaluation, studies
`j/j
`in Aldh5a1
`mice and characterization
`of g-hydroxybutyric acid pharmacology
`
`I. Knerr & P. L. Pearl & T. Bottiglieri & O. Carter Snead &
`C. Jakobs & K. M. Gibson
`
`Received: 2 February 2007 / Submitted in revised form: 2 February 2007 / Accepted: 14 February 2007 / Published online: 24 April 2007
`# SSIEM and Springer 2007
`
`Summary We overview the pathophysiological bases,
`clinical approaches and potential therapeutic options for
`succinate semialdehyde dehydrogenase (SSADH;
`EC1.2.1.24) deficiency (g-hydroxybutyric aciduria,
`OMIM 271980, 610045) in relation to studies on SSADH
`gene-deleted mice, outcome data developed from 25
`years of patient evaluation, and characterization of
`g-hydroxybutyric acid (GHB) pharmacology in different
`
`species. The clinical picture of this disorder encompasses
`a wide spectrum of neurological and psychiatric dys-
`function, such as psychomotor retardation, delayed
`speech development, epileptic seizures and behavioural
`disturbances, emphasizing the multifactorial pathophys-
`j/j
`iology of SSADH deficiency. The murine SSADH
`j/j
`(e.g. Aldh5a1
`) mouse model suffers from epileptic
`j/j
`seizures and succumbs to early lethality. Aldh5a1
`
`Communicating editor: Jaak Jaeken
`
`Competing interests: None declared
`
`References to electronic databases: g-Hydroxybutyric aciduria,
`OMIM 271980, 610045. Succinate-semialdehyde dehydrogenase,
`EC 1.2.1.24.
`
`_
`
`I. Knerr
`Hospital,
`s and Adolescents
`Children
`University of Erlangen-Nuremberg,
`Erlangen, Germany
`
`_
`
`P. L. Pearl
`Department of Neurology,
`_
`Children
`s National Medical Center,
`George Washington University School of Medicine,
`Washington, DC, USA
`
`P. L. Pearl
`Clinical Epilepsy Branch, NINDS, NIH,
`Bethesda, Maryland, USA
`
`T. Bottiglieri
`Baylor University Medical Center, Institute of Metabolic Disease,
`Dallas, Texas, USA
`
`O. Carter Snead
`Division of Neurology,
`The Hospital for Sick Children,
`Toronto, Ontario, Canada
`
`C. Jakobs
`VU University Medical Center,
`Amsterdam, The Netherlands
`I. Knerr : K. M. Gibson
`Biochemical Genetics Laboratory,
`_
`Children
`s Hospital Pittsburgh,
`Departments of Pediatrics,
`Pathology and Human Genetics,
`Division of Medical Genetics,
`University of Pittsburgh School of Medicine,
`Pittsburgh, Pennsylvania, USA
`
`K. M. Gibson (*)
`Division of Medical Genetics,
`Rangos Research Building Room 2111,
`_
`Children
`s Hospital of Pittsburgh,
`3460 Fifth Avenue,
`Pittsburgh, PA 15213, USA
`e-mail: michael.gibson@chp.edu
`
`Ranbaxy Ex. 1018
`IPR Petition - USP 9,050,302
`
`

`
`280
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`J Inherit Metab Dis (2007) 30:279–294
`
`mice accumulate GHB and g-aminobutyric acid
`(GABA) in the central nervous system, exhibit alter-
`ations of amino acids such as glutamine (Gln), alanine
`(Ala) and arginine (Arg), and manifest disturbances in
`other systems including dopamine, neurosteroids and
`antioxidant status. Therapeutic concepts in patients with
`SSADH deficiency and preclinical therapeutic experi-
`ments are discussed in light of data collected from
`j/j
`research in Aldh5a1
`mice and animal studies of
`GHB pharmacology; these studies are the foundation
`for novel working approaches, including pharmacolog-
`ical and dietary trials, which are presented for future
`evaluation in this disease.
`
`Abbreviations
`5-HIAA
`ALLO
`
`AMPA
`
`5-hydroxyindoleacetic acid
`allopregnanolone; 3!-hydroxy-5!-
`tetrahydroprogesterone
`!-amino-3-hydroxy-5-methylisoxazole-
`4-propionic acid
`blood–brain barrier
`BBB
`"-hydroxybutyrate
`BHB
`D-2-HG
`D-2-hydroxyglutarate
`docosahexaenoic acid
`DHA
`dihydroxyhexanoic acid
`DHHA
`g-aminobutyric acid
`GABA
`GABA(B)R GABA(B) receptor
`GABA-T
`GABA transaminase
`g-butyrolactone
`GBL
`g-hydroxybutyric acid
`GHB
`GHBR
`GHB receptor
`HMG-CoA 3-hydroxy-3-methylglutaryl-CoA
`synthase
`hydroxyacid–oxoacid transhydrogenase
`homovanillic avid
`monoamine oxidase
`mitogen-activated protein
`N-methyl-D-aspartate
`peroxisome proliferator-activated
`receptor
`succinic semialdehyde
`succinate semialdehyde dehydrogenase
`trans-4-hydroxycrotonic acid
`
`HOT
`HVA
`MAO
`MAP
`NMDA
`PPAR
`
`SSA
`SSADH
`T-HCA
`
`Introduction
`
`SSADH (EC 1.2.1.24) deficiency (g-hydroxybutyric
`aciduria; OMIM 271980, 610045) is an autosomal-
`recessively inherited neurometabolic disorder of
`GABA metabolism impairing the major oxidative
`conversion of succinic semialdehyde (SSA) to succinic
`acid (Fig. 1). The genetic block leads to accumulation
`
`of SSA, which is converted to GHB. Patients present
`to a variable extent with retardation of speech,
`intellectual and also motor development.
`The genetic basis resides in the SSADH gene
`(i.e. aldehyde dehydrogenase 5 family, member A1:
`ALDH5A1), which maps to chromosome 6p22. Jakobs
`and co-workers described the first patient with g-
`hydroxybutyric aciduria, who was identified by striking
`accumulation of GHB in body fluids (Jakobs et al
`1981). Enzyme deficiency was demonstrated in blood
`lymphocytes (Gibson et al 1983, 1985, 1991). Since the
`first description, numerous patients have been identi-
`fied (Pearl et al 2003).
`To examine pathophysiological mechanisms and
`pharmacotherapeutic approaches, Hogema and co-
`j/j
`workers developed a mouse model of Aldh5a1
`mice
`j/j
`(Hogema et al 2001). Aldh5a1
`mice progressively
`undergo generalized absence seizures starting in the
`neonatal period, followed by tonic-clonic status epilep-
`ticus by the 3rd to 4th week of life and, eventually, early
`j/j
`death (Gupta et al 2004). Aldh5a1
`mice are charac-
`terized by disturbances of neural transmission and
`amino acid homeostasis along with imbalance in
`excitatory/inhibitory signals. These imbalances predom-
`inantly involve GHB and GABA, but also glutamine
`(Gln), arginine (Arg), dopamine and other central-acting
`systems (Jansen et al 2006). GHB exhibits physiological
`and neuropharmacological properties acting on high-
`and low-affinity GHB receptors (GHBR) in the CNS
`and, at higher concentrations, on the GABA(B) re-
`ceptor (GABA(B)R) (Gibson et al 2002; Lingenhoehl
`et al 1999). GABA predominantly exerts an inhibitory
`role in the brain but functions dually in the developing
`neuronal network where it provides trophic functions
`and excitatory input for immature neurons (Kirmse
`and Kirischuk 2006). In SSADH deficiency other
`transmitter systems also appear disturbed, such as
`glutamate (Glu), which is derived from Gln using an
`intercellular shunt from glial cells to neurons and
`serves as the major excitatory transmitter in the CNS
`(Lebon et al 2002).
`Pharmacologically, GHB readily passes the blood–
`brain barrier (BBB) to enter the CNS after exogenous
`administration; it represents a mood-affecting sedative
`drug and also a treatment option, e.g. for alcohol ad-
`diction (Scharf et al 1985; Shannon and Quang 2000)
`and cataplexy (Fuller et al 2004). Chronic administra-
`tion of GHB, or its precursor g-butyrolactone (GBL),
`produces physical dependence along with activation of
`the GABA(B)R and withdrawal phenomena, as shown
`in baboons (Goodwin et al 2006; Weerts et al 2005).
`Acutely intoxicated patients may present with sedation,
`amnesia, coma or even death, but may also show
`
`

`
`J Inherit Metab Dis (2007) 30:279–294
`
`281
`
`Fig. 1 Putative pathophysiological interrelationships in SSADH
`deficiency. The enzyme defect is marked by the hatched arrow.
`Pathophysiologically, GABA and GHB downregulate GABA
`receptors. GABA, acting via the GABA(A)R, may result in
`decreased production of neuroactive steroids by lowering the
`activity of 3-b-hydroxysteroid dehydrogenase activity; alterations
`in neurosteroid levels are likely to have an allosteric effect on
`
`the GABA(A)R. GHB, acting via both GHB and GABA(B)R
`activation, reduces signalling via a decrease in mitogen-activated
`protein kinase phosphorylation; MAPK is important in myelin
`basic protein (MBP) expression, a major component of myelin.
`Neurosteroids such as allopregnanolone are able to regulate
`myelination in a concerted manner through steroid receptors
`and also through GABA(A)R (Donarum et al 2006)
`
`paradoxical agitation; as an illicit drug, GHB is also used
`F
`_
`for drug-facilitated sexual assault (
`date rape
`) owing to
`its rapid action (Drasbek et al 2006). We discuss
`empirical and theoretical therapeutic approaches based
`on clinical experiences gleaned from over two decades
`j/j
`of patient evaluation, studies of Aldh5a1
`mice and
`more than 40 years of evaluation of GHB pharmacol-
`ogy in different species (Wong et al 2004).
`
`Clinical phenotype of human SSADH deficiency
`
`SSADH deficiency is a disorder that manifests pre-
`dominantly with neurological findings but has consid-
`erable phenotypic heterogeneity (Gibson et al 1997;
`Pearl et al 2003). The clinical picture encompasses a
`wide spectrum of neurological manifestations and
`psychiatric dysfunction, and predominantly leads to a
`neurodevelopmental disorder with language deficits.
`The patients usually present with mild to severe
`developmental delay, predominantly involving expres-
`sive language. Other typical clinical signs and symp-
`toms include hypotonia,
`truncal or appendicular
`
`ataxia, and hyporeflexia. Patients often develop neu-
`ropsychiatric symptoms such as inattention, anxiety,
`hyperkinesis, sleep disturbances and excessive daytime
`somnolence (Gibson et al 1997, 2003; Pearl et al 2003,
`2005a,b; Philippe et al 2004).
`In contrast to the animal model, the clinical course
`in patients may be static, typically with improvement
`in gait ataxia over time. In contrast, there is a minority
`of patients (approximately 10%) with a progressive
`course featuring developmental regression and sub-
`stantial extrapyramidal manifestations, including dys-
`tonia, choreoathetosis and myoclonus (Pearl et al
`2005a). A larger series of older patients indicates that
`ataxia and language disabilities may show improve-
`ment with age; however,
`language dysfunction and
`psychiatric symptoms remain the predominant handi-
`caps in adult patients (Pearl et al 2005a).
`About half of the patients with SSADH deficiency
`suffer from epilepsy, usually absence, myoclonic epi-
`lepsy or generalized tonic-clonic seizures (Pearl et al
`j/j
`2005b). In contrast to the Aldh5a1
`mice, epileptic
`seizures only occasionally progress to refractory epi-
`lepsy or generalized convulsive status epilepticus.
`
`

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`
`Electroencephalographic (EEG) recordings in af-
`fected patients may exhibit background slowing and
`disorganization and epileptiform discharges (Pearl et al
`2006). The latter are usually generalized spike-and-
`wave between 2 and 3 Hz; photosensitivity and elec-
`trographic status epilepticus of slow-wave sleep have
`been noted rarely. Neuroimaging with magnetic res-
`onance techniques (MRI) reveals cerebral and cer-
`ebellar (especially vermian) atrophy and enhanced
`T2-weighted signal
`intensities involving the globus
`pallidus, subcortical white matter, dentate nucleus
`and brainstem including substantia nigra (Pearl and
`Gibson 2004). By means of 3H MRI spectroscopy
`(MRS) up to threefold elevation of GABA concen-
`trations have been described in the brain parenchyma
`of SSADH-deficient patients (Novotny et al 2003).
`Positron emission tomography (PET) using fluoro-
`deoxyglucose has demonstrated cerebellar hypome-
`tabolism in patients with known cerebellar atrophy,
`without other parenchymal findings (Pearl et al 2003).
`SSADH deficiency has rarely been detected in asso-
`ciation with a second genetic disorder, e.g. in a patient
`_
`with WAGRO syndrome (Wilms
`tumour, aniridia,
`genital abnormalities, mental retardation, and obesity)
`(Jung et al 2006) and Williams–Beuren syndrome
`(Knerr et al 2007).
`
`Metabolic phenotype
`
`In contrast to other inborn errors of metabolism such
`F
`_
`as the
`classical
`organic acidopathies, patients with
`SSADH deficiency, while presenting with considerable
`accumulation of GHB in biological fluids, do not
`present with metabolic acidosis, hyperammonaemia
`or hypoglycaemia but may have significant medium-
`chain dicarboxylic aciduria (Gibson et al 1989b).
`Carnitine status should be tested during metabolic
`work-up, although there are only a few cases present-
`ing with low carnitine concentrations and muscular
`hypotonia (Gibson et al 1997). Interestingly,
`in
`j/j
`mice, free carnitine and acylcarnitine
`Aldh5a1
`concentrations in serum, quantified by tandem mass
`spectrometry (MS/MS), do not exhibit any alteration
`compared to wild-type mice and there is no difference
`in C4-OH carnitine levels, the expected GHB carnitine
`ester, between genotypes (Struys et al 2006b).
`Concentrations of GHB can be routinely deter-
`mined by combined gas chromatography and mass
`spectrometry (GC-MS), optimally using a quantitative
`stable-isotope dilution assay (Gibson et al 1990).
`However, accuracy of detection varies considerably
`between different laboratories (Bonham et al 1994).
`
`Variable excretion of GHB in urine may hamper
`detection using routine organic acid analyses. A
`diagnostic pitfall is the application of GHB as sedative,
`e.g. prior to invasive examination of developmental
`delay in young children (Wolf et al 2004).
`Quantitation of urinary GHB excretion for the
`clinical course is not useful for prognostic purposes.
`Other compounds may be detectable in SSADH
`deficiency, owing to alternative metabolism, such as
`3,4-dihydroxybutyric acid, 3-oxo-4-hydroxybutyric ac-
`id, glycolic acid, 2,4-dihydroxybutyric acid and, to a
`lesser extent, threo- and erythro-4,5-dihydroxyhexanoic
`acids and their lactones, the latter quite specific for
`this disorder. None the less, the white blood cell
`enzyme assay remains the gold standard for diagnostic
`confirmation (Gibson et al 1983, 1985, 1991). The
`frequently observed dicarboxylic aciduria in patients
`with SSADH deficiency may suggest a secondary
`inhibition of mitochondrial fatty acid b-oxidation, but
`there is evidence against this theory (Gibson and
`Nyhan 1989). Propionyl-CoA metabolism may also
`be affected as shown by an enhanced excretion of 3-
`hydroxypropionic acid in patients with SSADH defi-
`ciency (Brown et al 1987).
`CSF GHB is up to 230-fold elevated in patients,
`whereas GABA is up to three times higher and Gln is
`decreased, suggestive of a disruption of the Gln/Glu/
`GABA shuttle between glial cells and neurons (Gibson
`et al 2003). Interestingly, homovanillic avid (HVA)
`and 5-hydroxyindoleacetic acid (5-HIAA) appear to
`exhibit a positive correlation with GHB concentra-
`tion, perhaps suggesting enhanced dopamine and
`serotonin turnover and progressive deterioration of
`neuronal function (Gibson et al 2003). However, this
`observation requires further evaluation with respect to
`patient age.
`
`Enzyme phenotype and superfamily
`
`NAD(+)-dependent SSADH is a mitochondrial matrix
`enzyme that has high specificity for SSA. SSADH
`has been purified from rat and human brain and liver,
`respectively, and its activity can also be determined in
`peripheral
`lymphocytes and cultured lymphoblasts
`using radiometric or spectrofluorometric assays
`(Chambliss et al 1995; Gibson et al 1985, 1991; Nguyen
`and Picklo 2003). Prenatal diagnosis has been success-
`fully performed using isotope-dilution MS to assess
`quantities of GHB in amniotic fluid, determination of
`SSADH activity in chorionic villus samples, and
`molecular genetic analysis (Akaboshi et al 2003;
`Gibson et al 1990). SSADH activity may be a target
`
`

`
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`
`283
`
`of inactivation in instances of elevated oxidative stress
`and lipid peroxidation (Nguyen and Picklo 2003).
`Other enzymes are involved in the metabolism of
`SSA. The aldo-keto reductases, in particular, make up
`a superfamily of enzymes that can reduce a variety of
`aldehydes and ketones to their corresponding alcohols.
`Each family displays distinct preferences for certain
`substrates, presumably reflecting their role within the
`cell. The AKR7A subfamily shows higher affinities for
`SSA than does AKR7A1 (Zhu et al 2006). The SSA
`reductase (SSAR) is also a member of the aldo-keto
`reductase family 7A2 (AKR7A2; Hinshelwood et al
`2002, 2003). In contrast to the mitochondrial localiza-
`tion of SSADH, SSAR is located within the cytoplasm.
`Additionally, a mammalian hydroxyacid–oxoacid
`transhydrogenase (HOT) which was recently cloned
`and resides on human chromosome 8q 13.1, catalyses
`the a-ketoglutarate-dependent oxidation of GHB to
`SSA (Kaufman and Nelson 1991; Kaufman et al 1988;
`Kardon et al 2006). HOT could be a therapeutic target
`in SSADH deficiency. Unfortunately, despite the
`capacity to reduce GHB levels, HOT stoichiometrical-
`ly generates a potentially neuroactive by-product, D-2-
`hydroxyglutarate (D-2-HG; Struys et al 2005, 2006a).
`Elevated D-2-HG is found in physiological fluids
`derived from patients with SSADH deficiency (Struys
`et al 2005, 2006b), and baboons produce D-2-HG fol-
`lowing administration of GHB (Struys et al 2006a).
`These data provide evidence that HOT is active in a
`number of mammalian species.
`
`Molecular genotype
`
`The human ALDH5A1 gene maps to chromosome 6p22
`and consists of 10 exons covering 38 kb of DNA
`(Chambliss et al 1998). Over 40 mutations have been
`identified thus far including missense, nonsense and
`splicing errors; however, no hotspots have been identi-
`fied (Akaboshi et al 2003). Consanguinity is frequent,
`suggesting the occurrence of rare disease-causing alleles
`in the general population. There is no apparent
`correlation between phenotype and underlying geno-
`type, and, additionally, most mutations reported are
`private. Heterozygote carriers are apparently asymp-
`tomatic, yet one report suggests absence epilepsy could
`be associated with the heterozygous state (Dervent et al
`j/j
`2004). In Aldh5a1
`mice the heterozygous animals
`are apparently normal (Gibson et al 2002, 2005). Com-
`parative studies on polymorphisms between humans
`and baboons suggest a putative role for the SSADH
`gene in the evolution of cognitive capabilities unique to
`humans (Blasi et al 2006).
`
`Pathophysiological aspects: effects of GHB,
`GABA, and other metabolites
`
`Deficiency of SSADH is associated with a significant
`elevation of GHB and total GABA in knockout mice,
`similar to alterations detected in patients (Gibson et al
`2002; Hogema et al 2001). At physiological concen-
`trations, GHB primarily acts at the GHBR, which is
`located presynaptically and functions as a G protein-
`coupled receptor. The pharmacological actions of high
`GHB concentrations are likely mediated through
`activation of the GABA(B)R. GHB has no affinity
`for the GABA(A) receptor (GABA(A)R; Castelli
`j/j
`et al 2003; Mathivet et al 1997). In Aldh5a1
`mice,
`increased levels of GABA and GHB alter
`GABA(B)R and GABA(A)R function, which may
`play a role in pathogenesis (Buzzi et al 2006; Chan et
`al 2006; Gibson et al 2005; Wu et al 2004a, 2006).
`Acute administration of GABA yields an increase
`in the phosphorylation level of the cAMP-responsive
`element-binding protein in murine hippocampus, but
`this phenomenon is abolished after repetitive GABA
`exposure, suggesting desensitization of the signalling
`pathways and GABA-induced neuroadaptive pro-
`cesses (Ren and Mody 2006). GABA likely plays a
`key role in synaptic plasticity during development,
`and GABA in this period may be excitatory (Gibson
`et al 2006, Owens et al 1996, 1999). GHB may me-
`tabolize to GABA and trans-4-hydroxycrotonic acid
`(T-HCA), which is also pharmacologically active at the
`GABA(B)R and GHBR, respectively (Quang et al
`2002). In terms of intracellular signalling, GHB in-
`hibits mitogen-activated protein (MAP) kinase activa-
`tion via a GABA(B)R-mediated mechanism (Fig. 1).
`Since MAP kinases mediate numerous physiological
`changes (e.g. regulation of cell division and differenti-
`ation), downregulation of this pathway might occur
`during GHB intoxication. Additionally, MAP kinases
`function in long-term neuroadaptive changes follow-
`ing repeated exposure to GHB (Ren and Mody 2003)
`and perhaps also in myelin expression (Donarum et al
`2006).
`Glutamine metabolism may also play a role in the
`pathophysiology of SSADH deficiency (Gibson et al
`2003). The major ionotropic Glu receptors are the N-
`methyl-D-aspartate (NMDA) receptor and a-amino-3-
`hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/
`kainate receptor. High levels of GHB depress both
`NMDA and AMPA/kainate receptor-mediated func-
`tion and may accordingly alter glutamatergic excit-
`atory synaptic transmission (Berton et al 1999). The
`NMDA receptor antagonist dizocilpine enhances GHB-
`induced catalepsy in diabetic rats treated with GHB
`
`

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`
`F
`
`_
`
`down-and-up
`(Sevak et al 2005). An age-dependent
`regulation of the AMPA-type subunits GluR1, GluR2
`j/j
`and GluR3 was found in the brain of Aldh5a1
`mice (Wu et al 2004b). Immunoblotting studies in cortex,
`hippocampus and cerebellum showed a decrease in the
`j/j
`expression of all three subunits in the brain of Aldh5a1
`mice at day 7, with increase of GluR3 and normalization
`of GluR1 and GluR2 expression at day 20. Aberrant
`regulation of AMPA-type Glu receptors may be
`j/j
`involved in the evolution of convulsions in Aldh5a1
`mice (Wu et al 2004b).
`The synthesis and presynaptic release of dopamine are
`inhibited by GHB, whereas dopamine turnover may be
`increased (Gibson et al 2003; Gupta et al 2003; Maitre
`1997). GHB may increase dopamine levels within the
`CNS under certain conditions (Mamelak 1989). The
`substantia nigra contains the cell bodies of GABAergic
`and dopaminergic neurons, giving rise to various
`mesolimbic and nigrostriatal projections. GHB modu-
`lates dopamine release in this network, which may be
`involved in addiction-related behaviours (Brancucci
`et al 2004). Therefore, GHB may induce effects such
`as mild euphoria and altered vigilance directly by acting
`at the GHB receptor and as an agonist for GABA(B)R
`(Brown 2007), but also indirectly by affecting other
`neurotransmitters within the serotoninergic and dopa-
`minergic system (Brancucci et al 2004).
`j/j
`Amino acid studies in Aldh5a1
`mice have shown
`elevations of Ala and decreases in Arg in different
`brain regions (Gupta et al 2004). Similarly, SSA,
`homocarnosine (the GABA–histidine conjugate) and
`guanidinobutyrate (derived from GABA conjugation
`with the guanidino moiety of Arg) are also elevated
`j/j
`in Aldh5a1
`brain tissue (Gibson et al 2006; Gupta
`et al 2004; Jansen et al 2006). GHB alters monoamine
`neurotransmitters such as dopamine (Gupta et al 2003).
`Additional mitochondrial processes may be affected in
`SSADH deficiency. SSA is a reactive carbonyl and
`may lead to increased oxidative stress. 4,5-Dihydrox-
`yhexanoic acid (DHHA), a metabolite believed to
`result from interaction of SSA with the pyruvate
`dehydrogenase reaction (Brown et al 1987; Schorken
`and Sprenger 1998; Shaw and Westerfeld 1968),
`is
`j/j
`significantly elevated in Aldh5a1
`mouse brain tissue
`(Gupta et al 2003). Free radicals contribute to seizure-
`induced neurodegeneration (Bellissimo et al 2001) and
`j/j
`oxidative stress occurs in brain tissue of Aldh5a1
`mice, potentially resulting in secondary cell damage
`(Gibson et al 2006; Gupta et al 2003). In relation to
`this, oxidative stress may be responsible for loss of
`striatal dopamine (Visser et al 2002). The conversion
`of dopamine to HVA results in generation of hydro-
`gen peroxide, suggesting that oxidative stress and
`
`dopamine turnover contribute to pathophysiology
`(Gibson et al 2006; Gupta et al 2003; Trabace and
`Kendrick 2000).
`Neurosteroids are neuromodulatory intermediates
`that may be involved in the pathogenesis of SSADH
`deficiency. Neuroactive steroids are synthesized in
`brain and are potent allosteric modulators of the
`GABA(A)R. Allopregnanolone (3a-hydroxy-5a-tetra-
`hydroprogesterone, ALLO) is a potent positive mod-
`ulator of GABA-induced chloride influx through the
`GABA(A)R ionophore and thereby exerts anticon-
`vulsant,
`locomotor stimulant and muscle relaxant
`properties (Donarum et al 2006; Ford et al 2005).
`There is also a growing body of evidence that neuro-
`steroids, by regulating GABA(A)R function and
`expression, may play a role in certain pathophysiolog-
`ical conditions, e.g. ethanol tolerance (Morrow et al
`2001). GABA, conversely, has been found to reduce
`the production of neuroactive steroids (Fig. 1) by
`lowering the activity of 3-hydroxysteroid dehydroge-
`nase via the GABA(A)R (Do-Rego et al 2000).
`
`Phenotype in Aldh5a1
`
`j/j
`
`mice
`
`j/j
`
`mice present with a phenotype reminis-
`Aldh5a1
`cent of the human disorder. Their clinical features are
`progressive neurological impairment, ataxia and seiz-
`ures. In the third week of life, approximately from day
`16 on, mutant mice manifest a transition from absence
`seizures to tonic-clonic seizures and eventually con-
`vulsive status epilepticus resulting in high mortality
`(Gibson et al 2004, 2005). GHB induces slow rhythmic
`3–5 Hz spike-and-wave activity consistent with ab-
`sence epilepsy or petit-mal seizures with involvement
`of
`thalamocortical pathways and thalamic nuclei
`(Banerjee and Snead 1994, 1995).
`Alterations of both GABA(A)R and GABA(B)R
`j/j
`occur early in life in Aldh5a1
`mice, all in the presence
`of high GHB and enhanced GABA release. Besides
`GHBR- and GABA(B)R-mediated effects, a develop-
`mental downregulation of GABA(A)R-mediated neu-
`j/j
`rotransmission in Aldh5a1
`mice likely contributes to
`the progression of generalized convulsive seizures in the
`mutant animals (Buzzi et al 2006; Wu et al 2006).
`Analysis of regional brain homogenates from
`j/j
`mice revealed downregulation of genes
`Aldh5a1
`associated with myelin biogenesis and maintenance,
`predominantly in hippocampus and cortex, associated
`with astrogliosis and progressive neurodegeneration
`(Donarum et al 2006; Hogema et al 2001). Elevated
`GHB and GABA may decrease progesterone and
`allopregnanolone levels, as well as phosphorylation of
`
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`J Inherit Metab Dis (2007) 30:279–294
`
`285
`
`j/j
`
`MAP kinase, contributing to profound myelin abnor-
`malities (Gupta et al 2003; Donarum et al 2006).
`Somatic development is also impaired in Aldh5a1
`mice (Gupta et al 2003). Although birth weight is
`normal, there is stagnation of weight gain, especially in
`the critical period from day 16 to day 22, with an
`absence of body fat (Gupta et al 2002; Hogema et al
`2001). This observation is intriguing since GHB
`appears to decrease oxidative metabolism during sleep
`or hibernation, functioning naturally as a physiological
`agent in central and peripheral tissues when energy
`supply is limited (Mamelak 1989).
`
`Therapeutic interventions
`
`Pharmacotherapeutics
`
`Vigabatrin, valproate, ethosuximide and other
`antiepileptic drugs
`
`One therapeutic objective is reduction of GHB levels via
`inhibition of GABA transaminase (GABA-T) (Fig. 1).
`Vigabatrin is an irreversible inhibitor of GABA trans-
`aminases (Fig. 1) which leads to decrease of GHB levels
`
`and elevation of GABA. Vigabatrin is the most widely
`used therapy for human SSADH deficiency, although it
`is expected to exacerbate the hyperGABAergic status
`(Gibson et al 1989a, 1995, 1997; Gropman 2003; Pearl
`et al 2005b; Table 1). In CSF obtained from patients
`with SSADH deficiency, GHB concentrations are
`either static or slightly lower after vigabatrin interven-
`tion (Ergezinger et al 2003). Clinical results are diverse,
`ranging from improvement in ataxia and speech in some
`patients to worsening of symptoms (Matern et al 1996).
`Lower doses (30–50 mg/kg per day) divided into two
`daily doses in conjunction with monitoring for side-
`effects including visual field disturbances (Ergezinger
`et al 2003; Gibson et al 1997; Gordon 2004) are as-
`sociated with fewer side effects than high dosage in-
`tervention (Matern et al 1996). Although vigabatrin has
`not been consistently successful in SSADH deficiency,
`intervention with vigabatrin enhances survival of the
`j/j
`mouse at very high doses (Gupta et al 2002;
`Aldh5a1
`Hogema et al 2001).
`Sodium valproate (di-n-propyl acetate) is employed
`for treatment of generalized and partial seizures in
`humans. Valproate enhances GABA synthesis and
`release leading to augmented GABAergic functions
`in some areas of the brain, such as substantia nigra
`
`Table 1 Therapeutic concepts in patients with SSADH deficiency or in Aldh5a1
`
`j/j
`
`mice
`
`Intervention
`
`Rationale (possible mode of action; selected references)
`
`Anticonvulsive drugs
`Vigabatrin
`
`Valproate
`
`Ethosuximide
`GHB antagonists
`NCS-382
`GABA(B)R antagonists
`CGP 35348, CGP 36742
`
`Inhibits GABA transaminase; variable effects in patients with SSADH deficiency; efficacious in
`j/j
`mice (Gibson et al 1989a, 1995, 1997, 2002; Gropman 2003; Hogema et al 2001;
`Aldh5a1
`Pearl et al 2005a,b)
`Augments GABAergic functions; of questionable use in patients with SSADH deficiency (Gibson
`et al 1997; Gordon 2004; Pattarelli et al 1988; Shinka et al 2003)
`Inhibits Ca2+ channels; clinically useful for absence seizures (Cortez et al 2004; Gibson et al 1997)
`
`Blocks GHB at GHBR (Gobaille et al 2002; Gupta et al 2002; Mehta et al 2001, 2006)
`
`May reduce absences and improve motor function; could worsen convulsive seizures (Gobaille
`et al 2002; Goodwin et al 2005; Gupta et al 2003; Vergnes et al 1997)
`
`GABA(B)R agonist properties (Vacher and Bettler 2003)
`
`Increases GABAergic effects (Gibson et al 2003)
`
`GABA(B)R agonists
`Baclofen
`GABA(A)R modulators
`Diazepam
`Pharmacological modulators
`Antiepileptic modulator, GABAergic properties (Guarneri et al 1985; Zhao et al 2006a)
`Uridine
`Neuromodulator, increases activity of aldehyde dehydrogenases (Ward et al 2001)
`Taurine
`Acamprosate (homotaurinate) Glu antagonistic properties, putative GABA analogue; interaction with taurine (Berton et al 1998;
`Heilig and Egli 2006)
`Improvement of extrapyramidal symptoms (Gibson et al 2003)
`NMDA receptor blockade (Laroia et al 1997; Lee et al 2003)
`Ketone bodies serve as energy supply for neurons (Freeman et al 2006)
`May reduce oxidant stress and neurodegeneration (Bazan 2006; Gibson et al 2006)
`
`Dopaminergic agents
`Dextromethorphan
`Ketogenic diet
`Antioxidants/miscellaneous
`
`

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`286
`
`J Inherit Metab Dis (2007) 30:279–294
`
`(Loscher 1999). Furthermore, the effects of valproate
`on the NMDA receptor subclass may underlie some of
`its anticonvulsant effects (Loscher 2002). In addition,
`valproate is a peroxisome proliferator-activated recep-
`tor (PPAR) activator which is protective against cellular
`inflammatory reactions and induces the main ketogenic
`enzyme within the mitochondria, 3-hydroxy-3-methyl-
`glutaryl-CoA (HMG-CoA) synthase (Cullingford
`et al 2002). The effect on PPAR may be accompanied
`by a decreased inflammatory response with reduced
`cytotoxic stimuli; stimulation of HMG-CoA synthase
`leads to greater production of ketone bodies and could
`have therapeutic relevance in SSADH deficiency
`comparable to treatment with the ketogenic diet as
`discussed below. There have been published anecdotal
`reports on successful intervention with valproate in
`human SSADH deficiency, but no controlled trials
`have been reported. Valproate is capable of inhibiting
`cytosolic GHB dehydrogenase, thereby preventing
`GHB conversion to GABA. At the cellular level,
`valproate inhibits MAP kinase phosphorylation in
`cortex and hippocampus. However, since acute val-
`proate administration may induce an increase of GHB
`in brain, it is possible that the elevated GHB concen-
`tration following valproate administration is responsi-
`ble for the observation of suppressed MAP kinase
`phosphorylation (Ren and Mody 2003, 2006). Urinary
`GHB concentration increased during valproate thera-
`py in a patient with SSADH deficiency (Shinka et al
`2003). Further, valproate is capable of decreasing
`SSADH activity by up to 68% in cultured lympho-
`blasts (Pattarelli et al 1988),
`leading us to the
`assumption that valproate may be contraindicated in
`affected patients.
`Traditional anticonvulsive drugs that have been used
`to control seizures in patients with SSADH deficiency
`include primidone, carbamazepine and ethosuximide.
`Ethosuximide is a prototypical absence seizure drug,
`capable of blocking T-type Ca2+ channels in neurons
`(Gomora et al 2001). Ethosuximide was effective for
`j/j
`treating absence seizures in Aldh5a1
`mice (Cortez
`et al 2004; Gibson et al 1997). However, other anti-
`convulsants drugs such as phenytoin and phenobarbital
`j/j
`were unsuccessful in rescue of Aldh5a1
`mice from
`status epilepticus (Hogema et al 2001), although de-
`tailed studies were not reported.
`
`Receptor antagonism
`
`GHB receptor antagonist: NCS-382
`
`Binding studies using the GHBR receptor antagonist
`NCS-382 (6,7,8,9-tetrahydro-5-hydroxy-5H-benzo-
`
`cyclohept-6-ylidene-acetic acid) have been employed
`to characterize the pharmacological actions of this
`receptor, and also in SSADH deficiency (Mehta et al
`2001, 2006). NCS-382, a structural analogue of GHB,
`blocks the effects of GHB such as accumulation of Trp
`and 5-HIAA (Gobai