Glutaric aciduria type 1
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Glutaric acidemia type 1 (GA1) is an inherited disorder in which the body is unable to completely break down the
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s
lysine Lysine (symbol Lys or K) is an α-amino acid that is a precursor to many proteins. It contains an α-amino group (which is in the protonated form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −C ...
,
hydroxylysine Hydroxylysine (Hyl) is an amino acid with the molecular formula C6H14N2O3. It was first discovered in 1921 by Donald Van Slyke as the 5-hydroxylysine form. It arises from a post-translational hydroxy modification of lysine. It is most widely kno ...
and
tryptophan Tryptophan (symbol Trp or W) is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α- carboxylic acid group, and a side chain indole, making it a polar molecule with a non-polar aromatic ...
. Excessive levels of their intermediate breakdown products (
glutaric acid Glutaric acid is the organic compound with the formula C3H6(COOH)2 . Although the related "linear" dicarboxylic acids adipic and succinic acids are water-soluble only to a few percent at room temperature, the water-solubility of glutaric acid is ...
,
glutaryl-CoA Glutaryl-coenzyme A is an intermediate in the metabolism of lysine and tryptophan Tryptophan (symbol Trp or W) is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α-carboxylic acid gr ...
, 3-hydroxyglutaric acid,
glutaconic acid ''trans''-Glutaconic acid is an organic compound with formula HO2CCH=CHCH2CO2H. This dicarboxylic acid exists as a colorless solid and is related to the saturated chemical glutaric acid, HO2CC(CH2)3CO2H. Esters and salts of glutaconic acid are ...
) can accumulate and cause damage to the
brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...
(and also other organs), but particularly the
basal ganglia The basal ganglia (BG), or basal nuclei, are a group of subcortical nuclei, of varied origin, in the brains of vertebrates. In humans, and some primates, there are some differences, mainly in the division of the globus pallidus into an extern ...
, which are regions that help regulate movement. GA1 causes secondary
carnitine Carnitine is a quaternary ammonium compound involved in metabolism in most mammals, plants, and some bacteria. In support of energy metabolism, carnitine transports long-chain fatty acids into mitochondria to be oxidized for energy production, an ...
deficiency, as
glutaric acid Glutaric acid is the organic compound with the formula C3H6(COOH)2 . Although the related "linear" dicarboxylic acids adipic and succinic acids are water-soluble only to a few percent at room temperature, the water-solubility of glutaric acid is ...
, like other
organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are rel ...
s, is detoxified by
carnitine Carnitine is a quaternary ammonium compound involved in metabolism in most mammals, plants, and some bacteria. In support of energy metabolism, carnitine transports long-chain fatty acids into mitochondria to be oxidized for energy production, an ...
.
Mental retardation Intellectual disability (ID), also known as general learning disability in the United Kingdom and formerly mental retardation,Rosa's Law, Pub. L. 111-256124 Stat. 2643(2010). is a generalized neurodevelopmental disorder characterized by signific ...
may occur. GA1 is an
autosomal recessive In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
disorder caused by deficiency of the enzyme
glutaryl-CoA dehydrogenase Glutaryl-CoA dehydrogenase (GCDH) is an enzyme encoded by the GCDH gene on chromosome 19. The protein belongs to the acyl-CoA dehydrogenase family (ACD). It catalyzes the oxidative decarboxylation of glutaryl-CoA to crotonyl-CoA and carbon dioxid ...
(GCDH), encoded by the ''GCDH'' gene.


Signs and symptoms

The severity of glutaric acidemia type 1 varies widely; some individuals are only mildly affected, while others suffer severe problems. GA1 can be defined as two clinical entities: GA-1 diagnosed at birth or pre-birth and managed through dietary restrictions, and GA-1 diagnosed after an encephalopathic crisis. A crisis may occur under both headings, but the care of individuals diagnosed before a crisis can be managed to avoid most or all injury.


GA1 without encephalopathic crisis


Macrocephaly

Babies with glutaric acidemia type 1 often are born with unusually large heads (
macrocephaly Macrocephaly is a condition in which circumference of the human head is abnormally large. It may be pathological or harmless, and can be a familial genetic characteristic. People diagnosed with macrocephaly will receive further medical tests to ...
). Macrocephaly is amongst the earliest signs of GA1. It is thus important to investigate all cases of macrocephaly of unknown origins for GCDH deficiency, given the importance of the early diagnosis of GA1. Macrocephaly is a pivotal clinical sign of many neurological diseases. Physicians and parents should be aware of the benefits of investigating for an underlying neurological disorder, particularly a neurometabolic one, in children with head circumferences in the highest percentiles.


GA1 after an encephalopathic crisis


Neuromotor aspects

Affected individuals may have difficulty moving and may experience
spasm A spasm is a sudden involuntary contraction of a muscle, a group of muscles, or a hollow organ such as the bladder. A spasmodic muscle contraction may be caused by many medical conditions, including dystonia. Most commonly, it is a muscle c ...
s, jerking, rigidity or decreased muscle tone and muscle weakness (which may be the result of secondary carnitine deficiency). GA, in patients who have suffered a crisis, can be defined as a
cerebral palsy Cerebral palsy (CP) is a group of movement disorders that appear in early childhood. Signs and symptoms vary among people and over time, but include poor coordination, stiff muscles, weak muscles, and tremors. There may be problems with sensa ...
of genetic origins.


=Occupational therapy

= A common way to manage striatal necrosis is to provide special seating. These special wheelchairs are designed to limit abnormal movements. However,
spasticity Spasticity () is a feature of altered skeletal muscle performance with a combination of paralysis, increased tendon reflex activity, and hypertonia. It is also colloquially referred to as an unusual "tightness", stiffness, or "pull" of muscles. ...
can be worsened by constraint. Parents and caregivers can provide a more interactive occupational therapy by enabling the child to use his or her own excessive postural muscle tone to his or her own advantage (see picture; note the care with which minimal pressure is applied while ensuring safety). The excessive tone can also be managed with "jolly jumpers" and other aids to the upright stance that do not constrain the child but help him or her gradually tone down the rigidity.


Bleeding abnormalities

Some individuals with glutaric acidemia have developed bleeding in the brain or eyes that could be mistaken for the effects of
child abuse Child abuse (also called child endangerment or child maltreatment) is physical, sexual, and/or psychological maltreatment or neglect of a child or children, especially by a parent or a caregiver. Child abuse may include any act or failure to a ...
.


Genetics

The condition is inherited in an autosomal recessive pattern: mutated copies of the gene ''GCDH'' must be provided by both parents to cause GA1. The ''GCDH'' gene encodes the enzyme glutaryl-CoA dehydrogenase. This enzyme is involved in degrading the amino acids lysine, hydroxylysine and tryptophan. Mutations in the ''GCDH'' gene prevent production of the enzyme or result in the production of a defective enzyme with very low residual activity, or an enzyme with relatively high residual activity but still phenotypic consequences. This enzyme deficiency allows glutaric acid, 3-hydroxyglutaric acid and to a lesser extent glutaconic acid to build up to abnormal levels, especially at times when the body is under stress. These intermediate breakdown products are particularly prone to affect the basal ganglia, causing many of the signs and symptoms of GA1. GA1 occurs in approximately 1 of every 30,000 to 40,000 births. As a result of
founder effect In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1942, using ...
, it is much more common in the
Amish The Amish (; pdc, Amisch; german: link=no, Amische), formally the Old Order Amish, are a group of traditionalist Anabaptist Christian church fellowships with Swiss German and Alsatian origins. They are closely related to Mennonite churches ...
community and in the
Ojibway The Ojibwe, Ojibwa, Chippewa, or Saulteaux are an Anishinaabe people in what is currently southern Canada, the northern Midwestern United States, and Northern Plains. According to the U.S. census, in the United States Ojibwe people are one of ...
population of
Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...
, where up to 1 in 300 newborns may be affected. Relatives of children with GA1 can have low GCDH activity: in an early study of GA1, GCDH activity was found to be 38%, 42%, and 42% in three of the four unaffected relatives tested, a pattern consistent with the 50% level that would be expected in heterozygous carriers. Those levels are close to those found in some heavily symptomatic GA1-affected children.


Diagnosis

Normally, magnetic resonance imaging shows the Sylvian fissure to be operculated, but in GA1-associated encephalopathy, operculation is absent. In many jurisdictions, GA1 is included in
newborn screening Newborn screening (NBS) is a public health program of screening in infants shortly after birth for conditions that are treatable, but not clinically evident in the newborn period. The goal is to identify infants at risk for these conditions earl ...
panels. Elevated glutarylcarnitine can be detected by
mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...
in a dried blood spot collected shortly after birth. After a positive screening result, confirmatory testing is performed. This includes urine organic acid analysis, looking for glutaric acid and 3-hydroxyglutaric acid. Plasma and urine acylcarnitine analysis can also be informative. Molecular analysis, including gene sequencing and copy number analysis of ''GCDH'', can be performed to confirm the diagnosis. Molecular testing can also provide information for family planning and prenatal testing, if desired.


Treatment


Correction of secondary carnitine depletion

Like many other organic acidemias, GA1 causes
carnitine Carnitine is a quaternary ammonium compound involved in metabolism in most mammals, plants, and some bacteria. In support of energy metabolism, carnitine transports long-chain fatty acids into mitochondria to be oxidized for energy production, an ...
depletion. Whole-blood carnitine can be raised by oral supplementation. However, this does not significantly change blood concentrations of glutarylcarnitine or esterified carnitine, suggesting that oral supplementation is suboptimal in raising tissue levels of carnitine. Clinical nutrition researchers have likewise concluded that oral carnitine raises plasma levels but does not affect those in muscles, where most of it is stored and used. In contrast, regular intravenous infusions of carnitine cause distinct clinical improvements: "decreased frequency of decompensations, improved growth, improved muscle strength and decreased reliance on medical foods with liberalization of protein intake." Choline increases carnitine uptake and retention. Choline supplements are inexpensive, are safe (probably even in children requiring
anticholinergics Anticholinergics (anticholinergic agents) are substances that block the action of the neurotransmitter called acetylcholine (ACh) at synapses in the central and peripheral nervous system. These agents inhibit the parasympathetic nervous system ...
) and can increase exercise tolerance, truncal tone and general well-being, providing evidence of the suboptimal efficiency of carnitine supplementation alone.


Selective precursor restriction

Dietary control may help limit progression of the neurological damage.


Lysine

Lysine restriction, as well as carnitine supplementation, are considered the best predictors of a good prognosis for GA1. This excludes, however, patients who already suffered an encephalopathic crisis, for whom the prognosis is more related to the treatment of their acquired disorder (striatal necrosis, frontotemporal atrophy).


Protein restriction

Vegetarian diets and, for younger children, breastfeeding are common ways to limit protein intake without endangering tryptophan transport to the brain.


Tryptophan

Formulas such as XLys, XTrp Analog, XLys, XTrp Maxamaid, XLys, XTrp Maxamum or Glutarex 1 are designed to provide amino acids other than lysine and tryptophan, to help prevent protein malnutrition. The entry of tryptophan into the brain is crucial in the proper synthesis of the neurotransmitter
serotonin Serotonin () or 5-hydroxytryptamine (5-HT) is a monoamine neurotransmitter. Its biological function is complex and multifaceted, modulating mood, cognition, reward, learning, memory, and numerous physiological processes such as vomiting and vas ...
in the brain. One way to acutely cause depression, bulimia or anxiety in humans, in order to assess an individual's vulnerability to those disorders, is to supplement with a formula with all or most amino acids except tryptophan. Acute tryptophan depletion is a diagnostic procedure, not a treatment for GA1. The protein synthesis elicited by the amino acids leads circulating amino acids, including tryptophan, to be incorporated into proteins. Tryptophan is thus lowered in the brain as a result of the protein synthesis enhancement, causing circulating tryptophan to drop more than other amino acids. A relative excess of other large neutral amino acids may also compete with tryptophan for transport across the
blood–brain barrier The blood–brain barrier (BBB) is a highly selective semipermeable membrane, semipermeable border of endothelium, endothelial cells that prevents solutes in the circulating blood from ''non-selectively'' crossing into the extracellular fluid of ...
through the large neutral amino acid transporter 1. The consequence is
acute tryptophan depletion Acute tryptophan depletion (ATD) is a technique used extensively to study the effect of low serotonin in the brain. This experimental approach reduces the availability of tryptophan, an amino acid Amino acids are organic compounds that contai ...
in the brain and a consequent decrease in
serotonin Serotonin () or 5-hydroxytryptamine (5-HT) is a monoamine neurotransmitter. Its biological function is complex and multifaceted, modulating mood, cognition, reward, learning, memory, and numerous physiological processes such as vomiting and vas ...
synthesis. 5-Hydroxytryptophan, a precursor of serotonin that is not metabolized to glutaryl-CoA, glutaric acid and secondary metabolites, could be used as an adjunct to selective tryptophan restriction, although it has risks. However, the evidence in favour of selective tryptophan restriction remains insufficient and the consensus is evolving towards the restriction of lysine only. In the
Amish The Amish (; pdc, Amisch; german: link=no, Amische), formally the Old Order Amish, are a group of traditionalist Anabaptist Christian church fellowships with Swiss German and Alsatian origins. They are closely related to Mennonite churches ...
community, where GA1 is overrepresented, patients with GA1 typically do not receive tryptophan-free formulas, either as the sole source of amino acids or as a supplement to protein restriction.


Enhancement of precursor anabolic pathways


Lysine and hydroxylysine anabolic pathway enhancement

A possible way to prevent the build-up of metabolites is to limit lysine and hydroxylysine degradation, as lysine is one of the most abundant amino acids and tryptophan is one of the least abundant amino acids.


Interaction of GCDH deficiency with vitamin C levels

Humans lack the enzyme L-gulonolactone oxidase, which is necessary for the synthesis of
ascorbic acid Vitamin C (also known as ascorbic acid and ascorbate) is a water-soluble vitamin found in citrus and other fruits and vegetables, also sold as a dietary supplement and as a topical 'serum' ingredient to treat melasma (dark pigment spots) an ...
(vitamin C), leaving them dependent on dietary sources of this vitamin. Vitamin C is a necessary cofactor for the utilization of lysine in collagen synthesis. Collagen, the most abundant protein in the human body, requires great amounts of lysine, the most abundant amino acid in proteins. Ascorbic acid, the main hydroxyl radical quencher, works as the cofactor providing the hydroxyl radical required for collagen cross-linking; lysine thus becomes hydroxylysine. GA1 worsens during stresses and catabolic episodes, such as fasts and infections. Endogenous catabolism of proteins could be an important route for glutaric acid production. It follows that collagen breakdown (and protein breakdown in general) should be prevented by all possible means. Ascorbic acid is used to prevent multiple organ failure and to lessen mortality and morbidity in intensive care units. It thus appears reasonable to add sufficient doses of ascorbic acid to the treatment protocol during stresses and other challenges to growth in order to stimulate collagen synthesis and thus prevent lysine breakdown.


Tryptophan anabolic pathway enhancement

The conversion of tryptophan to serotonin and other metabolites depends on vitamin B6. If tryptophan catabolism has any impact on brain glutaric acid and other catabolite levels, vitamin B6 levels should be routinely assayed and normalized in the course of the treatment of GA1.


Management of intercurrent illnesses

Stress caused by infection, fever or other demands on the body may lead to worsening of the signs and symptoms, with only partial recovery.


Prognosis

A 2006 study of 279 patients found that of those with symptoms (185, 66%), 95% had suffered an encephalopathic crises, usually with following brain damage. Of the participants in the study, 49 children died and the median age of death was 6.6 years. A Kaplan–Meier analysis of the data estimated that about 50% of symptomatic people would die by the age of 25. More recent studies provide an updated prognosis whereby individuals affected can, through proper dietary management and carnitine supplementation, manage the disease with a much improved prognosis. Newborn screening has allowed affected patients to avoid crises and live full lives without any injury to the brain. It is essential that patients with the disease be diagnosed at or before birth and that all variables be strictly managed in order to maintain quality of life. When suspected and in the absence of confirmed diagnosis (through genetic sequencing), it is critical that the individual maintain a diet restrictive of all proteins and that blood sugars be monitored rigorously. The WHO now considers this disease entirely manageable.


Epidemiology

GA1 can be described as a
metabolic disorder A metabolic disorder is a disorder that negatively alters the body's processing and distribution of macronutrients, such as proteins, fats, and carbohydrates. Metabolic disorders can happen when abnormal chemical reactions in the body alter the ...
, a neurometabolic disease, a
cerebral palsy Cerebral palsy (CP) is a group of movement disorders that appear in early childhood. Signs and symptoms vary among people and over time, but include poor coordination, stiff muscles, weak muscles, and tremors. There may be problems with sensa ...
or a
basal ganglia The basal ganglia (BG), or basal nuclei, are a group of subcortical nuclei, of varied origin, in the brains of vertebrates. In humans, and some primates, there are some differences, mainly in the division of the globus pallidus into an extern ...
disorder (it may also be misdiagnosed as
shaken baby syndrome Shaken baby syndrome (SBS), also known as abusive head trauma (AHT), is the leading cause of fatal head injuries in children younger than two years. Diagnosing the syndrome has proved both challenging and contentious for medical professionals, ...
). Depending on the paradigm adopted, GA1 will mostly be managed with precursor restriction or with neurorehabilitation. So-called "orphan diseases", such as GA1, can be adopted into wider groups of diseases (such as carnitine deficiency diseases, cerebral palsies of diverse origins, basal ganglia disorders, and others); Morton at al. (2003b) emphasize that acute striatal necrosis is a distinctive pathologic feature of at least 20 other disorders of very different etiologies, including, HIV encephalopathy–
AIDS dementia complex HIV-associated neurocognitive disorders (HAND) are neurological disorders associated with HIV infection and AIDS. It is a syndrome of progressive deterioration of memory, cognition, behavior, and motor function in HIV-infected individuals during ...
, pneumococcal
meningitis Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, headache, and neck stiffness. Other symptoms include confusion or ...
, hypoadrenal crisis,
methylmalonic acidemia Methylmalonic acidemia, also called methylmalonic aciduria, is an autosomal recessive metabolic disorder that disrupts normal amino acid metabolism. It is a classical type of organic acidemia. The result of this condition is the inability to proper ...
,
propionic acidemia Propionic acidemia, also known as propionic aciduria or propionyl-CoA carboxylase deficiency (PCC deficiency), is a rare autosomal recessive metabolic disorder, classified as a branched-chain organic acidemia. The disorder presents in the early ...
,
middle cerebral artery The middle cerebral artery (MCA) is one of the three major paired cerebral artery, cerebral arteries that supply blood to the cerebrum. The MCA arises from the internal carotid artery and continues into the lateral sulcus where it then branches an ...
occlusion, hypertensive vasculopathy, acute ''
Mycoplasma pneumoniae ''Mycoplasma pneumoniae'' is a very small bacterium in the class Mollicutes. It is a human pathogen that causes the disease mycoplasma pneumonia, a form of atypical bacterial pneumonia related to cold agglutinin disease. ''M. pneumoniae'' is char ...
'' infection, 3-nitropropionic acid intoxication, late-onset familial
dystonia Dystonia is a neurological hyperkinetic movement disorder in which sustained or repetitive muscle contractions result in twisting and repetitive movements or abnormal fixed postures. The movements may resemble a tremor. Dystonia is often inten ...
, cerebrovascular abrupt and severe neonatal asphyxia ("selective neuronal necrosis"). In a cohort of 279 patients who had been reported to have GA1, 185 were symptomatic (two-thirds); being symptomatic was seen as an indication of low treatment efficacy. Screening of those known to be at high risk, neonatal population screening and a diagnosis of
macrocephaly Macrocephaly is a condition in which circumference of the human head is abnormally large. It may be pathological or harmless, and can be a familial genetic characteristic. People diagnosed with macrocephaly will receive further medical tests to ...
are the ways to identify bearers of the ''GCDH'' mutation who are not frankly symptomatic.
Macrocephaly Macrocephaly is a condition in which circumference of the human head is abnormally large. It may be pathological or harmless, and can be a familial genetic characteristic. People diagnosed with macrocephaly will receive further medical tests to ...
remains the main sign of GA1 for those who have no relatives with GA1 and have not been included in a population screening program. GA1 is considered a treatable disease. Two-thirds of the patients who have GA1 encephalopathy will receive little benefit from the treatment for GA1 but can benefit from treatments given to victims of middle cerebral artery occlusion, AIDS dementia and other
basal ganglia The basal ganglia (BG), or basal nuclei, are a group of subcortical nuclei, of varied origin, in the brains of vertebrates. In humans, and some primates, there are some differences, mainly in the division of the globus pallidus into an extern ...
disorders:
brain implant Brain implants, often referred to as neural implants, are technological devices that connect directly to a biological subject's brain – usually placed on the surface of the brain, or attached to the brain's cortex. A common purpose of modern brai ...
s,
stem cell In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...
neurorestoration,
growth factor A growth factor is a naturally occurring substance capable of stimulating cell proliferation, wound healing, and occasionally cellular differentiation. Usually it is a secreted protein or a steroid hormone. Growth factors are important for regu ...
s,
monoamine Monoamine neurotransmitters are neurotransmitters and neuromodulators that contain one amino group connected to an aromatic ring by a two-carbon chain (such as -CH2-CH2-). Examples are dopamine, norepinephrine and serotonin. All monoamines are ...
rgic agents, and many other neurorehabilitation strategies.


References


Further reading

* * * see also Part 1 referenced above


External links

* - Type 1 {{DEFAULTSORT:Glutaric Aciduria Type 1 Amino acid metabolism disorders Autosomal recessive disorders