Difference between revisions of "Canavan Disease"

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<figure id="ASPA">
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== Summary ==
  
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[[Image:ASPA_CANAVAN_2O4H.png|thumb|450px|'''<caption>'''Crystal structure of aspartoacylase (2O4H - PDB-file).</caption>]]
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</figure>
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'''Canavan Disease''' ([http://apps.who.int/classifications/icd10/browse/2010/en#/E75.2 ICD-10 E75.2]) is an autosomal recessive disorder, in which a dysfunctional enzyme causes severe brain damage. It is also known under a variety of other names describing the chemical basis or phenotype of the disease. Examples are "Spongy Degeneration Of Central Nervous System", "Aspartoacylase (ASPA) Deficiency", or "Aminoacylase 2 (ACY2) Deficiency"[http://omim.org/entry/271900]. The trivial name, Canavan Disease, originates from the name of Myrtelle Canavan (1879 – 1953)[http://en.wikipedia.org/wiki/Myrtelle_Canavan], an American physician, who first described the disease in 1931.
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There is no cure and almost all patients die within the first decade of their life. The mild / juvenile type is less severe. The treatment is based on the symptoms and is supportive.
   
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== Inheritance ==
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For the disease that we have assigned to you, look up the diesease in Wikipedia, OMIM, HGMD and any other resource you find.
  
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Canavan Disease is an autosomal recessive genetic defect of the ASPA (aspartoacyclase) gene on chromosome 17 (for the crystal structure of the ASPA protein see '''<xr id="ASPA">Figure</xr>'''). With this pattern of heritage a newborn of a couple where both parents are carriers of the defective genome has a 25% chance neither being born suffering from Canavan Disease nor being born a carrier. For some time children born of Ashkenazi Jewish ancestry had a higher prevalence of having Canavan Disease while in the last years this prevalence is sinking due to ongoing prenatal screening programs. Other ethnic groups where Canavan Disease has a higher penetrance are for example populations of Saudi Arabian ancestry. <br>
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Document the resources you have found in this Wiki (resources section).
  
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According to [http://ghr.nlm.nih.gov/condition/canavan-disease ''Genetics Home''] about one in 6400 to 13500 of the Ashkenazi Jewish are affected. No further information about prevalences in other populations was found. However the different populations have also different frequencies regarding the mutation they are based on. For further information see section [[Canavan_Disease#Disease_Causing_Mutations| ''Disease Causing Mutations'']].
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Produce a 5 min presentation for our first practical session.
  
   
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== Phenotype ==
   
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Canavan Disease has a variety of different phenotypes ranging across all body parts.
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NAMES
  
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Here is a short overview:
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* named after Myrtelle Canavan, the experts who first described the disorder in 1931.
  
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* Head
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* aspartoacylase deficiency
 
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** macrocephaly (increased head circumference)
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* aminoacylase 2 deficiency
  
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** mental retardation and impairment (losing mental skills)
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* Canavan-Van Bogaert Bertrand disease
  
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** losing ability to move head
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* SPONGY DEGENERATION OF CENTRAL NERVOUS SYSTEM ASPARTOACYLASE DEFICIENCY
  
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* Eyes
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* ASPA DEFICIENCY
  
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** becoming blind
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* ASP DEFICIENCY
  
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** nystagmus (greek: νυσταζω ''nytaxoo'' "sleep, nod", german: "Augenzittern")
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* ACY2 DEFICIENCY
  
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* Ears
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** becoming deaf
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* Mouth
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** problems with swallowing
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** losing communicational abilities (cannot talk, stay quiet)
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* Body
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** paralysis
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** seizures
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** problems moving the muscles
   
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Children suffering from Canavan Disease usually die within the first decade.
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In the mild/juvenile form of Canavan Disease, the children usually have some developmental delay and some speech problems.
   
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PHENOTYPIC DESCRIPTION OF THE DISEASE.
  
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HEAD
  
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* incresing head circumference
  
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* lack of head control
  
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* Over time they may become developmentally delayed
  
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* Mental impairment
  
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* Megalocephaly / Macrocephaly or a big head
  
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* Mental retardation and mental impairment
  
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EYE
  
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* reduced visual responsiveness
  
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* Over time they may become blind and have trouble swallowing
  
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* Blindness
  
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* Optic Atrophy
  
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* Nystagmus
  
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EAR
  
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* Deafness, loss of hearing
  
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SPEECH
  
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* children with canavan disease cannot talk.
  
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* Difficulties in feeding
  
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* the children are very quiet,
  
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BODY, MUSCLES
  
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* Over time they may suffer seizures, become paralyzed,
 
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* Seizures
  
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* paralysis
  
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* lacking energy, sluggish and indifferent.
  
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* convulsions and seizures
  
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* listless, lethargic and indifferent
  
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* Hypotonia eventually changes to spasticity.
 
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* abnormal muscle tone - stiffness / floppyness
  
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* Children with Canavan disease cannot crawl, walk, sit
  
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* Poor muscle tone, i.e., droopiness / limpness, or extreme stiffness and rigidity
  
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* Loss of formerly attained motor skills
  
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* The patient shows droopiness and limpness or rigidity and stiffness of the body
  
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NEUROLOGICAL
  
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* Demyelination with white matter disease in internal capsule, external capsule, genu of corpus callosum, subcortical white matter, and posterior fossa Brain atrophy
  
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LIFE
  
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* death within first decade (some up two first decades)
  
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* Mild/juvenile Canavan disease is characterized by mild developmental delay that can go unrecognized. Head circumference may be normal.
  
   
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== Disease Mechanism ==
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<figure id="KEGG">
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[[Image:Canavan disease pathway KEGG.png|thumb|750px|'''<caption>'''Alanine, Aspartate and Glutamate Metabolism (source: [http://www.kegg.jp/kegg-bin/show_pathway?hsadd00250+443 KEGG]) highlighting disease associated enzymes of Canavan Disease.</caption>]]
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</figure>
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Canavan Disease belongs to the group of leukodystrophies. The etymological origin are the greek words: λευκος ''leukos'' "white", δυς ''dys'' "bad, wrong" and τροφη ''trophae'' "feeding, growth". This is a genetic induced metabolic disorder, which affects the white matter of the nervous system. If the white matter is not properly grown, the myelin, which surrounds the nerve cells for protection, is degraded. This is especially true for Canavan Disease. The visible phenotypes are a result of a genetic defect that negatively affects the growth of the myelin sheath covering the nerve fibers. An improperly build myelin sheath, results in a reduced ability to transmit the electric signal along the nerve fibers, eventually losing it completely and finally the degradation of whole nerve cells. <br>
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The cause for the malfunctioning myelin sheath growth is a genetic defect of the aspartoacylase (ASPA) gene. The product of the gene, the enzyme aspartoacylase is crucial in the degradation process of N-acetyl-L-aspartate (NAA) which is present at much higher levels than normal in patients suffering from Canavan Disease. Normally ASPA would degrade NAA into smaller fragments which are required prerequisites for the production of the myelin sheath (see '''<xr id="KEGG">Figure</xr>''' for an overview where APSA is located in the metabolic map). Therefore the missing / defective ASPA is reason for the defective build up process of myelin. The degradation of the nerve cells / white brain matter has the consequence that empty spaces are arising which are filled with brain fluid leading to even more degradation of nerve cells and signal transduction problems.
   
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== Diagnosis ==
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INHERITANCE
  
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* autosomal recessive
  
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* Each pregnancy of a couple, both heterozygous, for a disease-causing mutation in ASPA has a 25% chance of resulting in a child with Canavan disease, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.
  
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* often in persons of Ashkenazi (German and Eastern European) Jewish ancestry. It is estimated that 1 in 40 Ashkenazi Jews is a carrier of the Canavan gene. It is also found in other ethnic groups. In fact, thanks to the success of ongoing screening programs in the Jewish population, most of the children born with Canavan disease today have no known Jewish heritage.
  
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* Ashkenazi Jews from Lithuania, eastern Poland, and Russia, and amongst the Saudi Arabians.
  
   
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There are a couple of possibilities how and when an affected patient is diagnosed with Canavan Disease. The time points are prenatal, postnatal, and when a mild or juvenile form of Canavan Disease is already present. Nevertheless one of the most important things to know before is if both parents carry one copy of the disease causing gene. This can simply be done by DNA testing.
   
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==== Prenatal Diagnosis ====
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DIAGNOSIS
  
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PRENATAL
  
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* due to screening programs.
  
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* Canavan disease is inherited in an autosomal recessive manner. Each pregnancy of a couple in which both partners are heterozygous for a disease-causing mutation in ASPA has a 25% chance of resulting in a child with Canavan disease,
  
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* DNA testing tells you whether you are a carrier of the Canavan Disease
  
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* Carrier testing is available on a population basis for individuals of Ashkenazi Jewish heritage. Carrier testing for at-risk relatives and prenatal testing for pregnancies at increased risk are possible when the disease-causing mutations in the family are known. For couples in which one partner is known to be a carrier and the carrier status of the other is unknown, prenatal testing can be performed by measuring the concentration of NAA in amniotic fluid at 16 to 18 weeks’ gestation.
  
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* Molecular genetic testing. Prenatal testing for pregnancies at 25% risk is possible by analysis of DNA extracted from fetal cells obtained by chorionic villus sampling (CVS) at approximately ten to 12 weeks' gestation or by amniocentesis usually performed at approximately 15 to 18 weeks' gestation. Both disease- causing alleles present in the family must be identified before prenatal testing can be performed. Note: Gestational age is expressed as menstrual weeks calculated either from the first day of the last normal menstrual period or by ultrasound measurements.
  
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* Biochemical genetic testing. For couples in which one partner is known to be a carrier and the mutation or carrier status of the other is unknown, prenatal testing can be performed by measuring the level of NAA in amniotic fluid at 15 to 18 weeks’ gestation [Bennett et al 1993, Al-Dirbashi et al 2009].
  
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*Preimplantation genetic diagnosis (PGD) may be available for families in which the disease-causing mutations have been identified [Yaron et al 2005]. For laboratories offering PGD, see. Note: It is the policy of GeneReviews to include clinical uses of testing available from laboratories listed in the GeneTests Laboratory Directory; inclusion does not necessarily reflect the endorsement of such uses by the author(s), editor(s), or reviewer(s).
  
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POSTNATAL
  
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* Increased N-acetyl-L-aspartic acid (NAA) in urine, CSF, and blood.
  
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* Symptoms usually become apparent when the infant is three to nine months old.
  
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* Neuroimaging
  
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* Reduced aspartoacylase activity in cultured skin fibroblasts
  
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* Spongy degeneration of brain on histology
  
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* deficiency of the enzyme aspartoacylase in skin cells or by testing the gene for Canavan disease in the blood.
  
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MILD / JUVENILE
  
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* In mild/juvenile Canavan disease NAA may only be slightly elevated; thus, the diagnosis relies on molecular genetic testing of ASPA, the gene encoding the enzyme aspartoacylase. Such testing is available on a clinical basis.
  
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* In individuals with mild/juvenile Canavan disease, neuroimaging may not be helpful.
  
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* Mild/juvenile Canavan disease is characterized by mild developmental delay that can go unrecognized. Head circumference may be normal.
  
   
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There are several types of prenatal testing possibilities depending on whether the carrier status of both parents is known or not. For couples where it is only known that one of the parents is a carrier and the remaining parents status is not known, normally testing is done by measuring the concentration of N-acetyl-L-aspartic acid (NAA) in the amniotic fluid within the time between the 16th and 18th week of pregnancy.
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Another possibility is molecular genetic testing. Following this method an analysis of DNA extracted from fetal cells is done. These fetal cells are obtained either between the tenth to 12th week of pregnancy by chorionic villus (“proto-”placental tissue that has the same genetic material as the fetus) sampling or between the 15th and 18th week by amniocentesis, also known as amniotic fluid testing (AFT). However for the molecular genetic testing both disease causing genes of the parents have to be identified first.
   
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==== Neonatal / Infantile Diagnosis ====
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TESTING.
  
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N-Acetylaspartic acid (NAA)
  
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Urine:
  
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The concentration of NAA in the urine can be measured using gas chromatography-mass spectrometry (GC-MS) [Michals & Matalon 2011]. Control values in one series (n=48) were 23.5±16.1 μmol/mmol creatinine. In neonatal/infantile (severe) Canavan disease the mean concentration of NAA (n=117) was 1440.5±873.3 μmol/mmol creatinine. In mild/juvenile Canavan disease, mild elevation of NAA may be found, (n=2) 106 μmol/mmol creatine
  
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Amniotic fluid:
  
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The concentration of NAA can be measured in the amniotic fluid by stable-isotope dilution and GC-MS or by liquid chromatography tandem mass spectrometry [Bennett et al 1993, Al Dirbashi et al 2009]. NAA concentration in amniotic fluid was 0.30-2.55 μmol/L in controls and 8.68 μmol/L in an affected pregnancy [Bennett et al 1993].
  
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Aspartoacylase enzyme activity
  
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Skin fibroblasts
  
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Although aspartoacylase enzyme activity can be assayed in cultured skin fibroblasts, it may not be reliable because the activity varies with culture conditions. Individuals with severe Canavan disease often have unmeasurable enzyme activity. Carriers of alleles associated with severe Canavan disease have about one-half normal enzyme activity [Matalon et al 1993].
  
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White blood cells and thrombocytes.
 
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Aspartoacylase enzyme activity is not detectable in white blood cells or platelets.
  
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Amniocytes/CVS
  
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Aspartoacylase enzyme activity is extremely low in normal amniocytes and chorionic villus sampling (CVS). Enzyme activity cannot be relied upon for prenatal testing [Bennett et al 1993].
  
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Clinical testing:
  
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Targeted mutation analysis (A)
  
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Two mutations, p.Glu285Ala and p.Tyr231X, account for 98% of disease-causing alleles in the Ashkenazi Jewish population and 3% of alleles in non-Ashkenazi Jewish populations [Michals & Matalon 2011]. One mutation, p.Ala305Glu, accounts for 30%-60% of disease-causing alleles in non-Ashkenazi Jewish populations and approximately 1% of alleles in the Ashkenazi Jewish populations [Kaul et al 1994b, Elpeleg & Shaag 1999].
  
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Sequence analysis (B)
  
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of the ASPA coding region is available for individuals in whom mutations were notidentified by targeted mutation analysis. Note: (1) The c.433-2A>G splice site mutation found in a single Ashkenazi Jewish family should not be considered a typical Ashkenazi Jewish mutation. (2) More than 50 other mutations have been reported in non- Ashkenazi Jewish populations [Kaul et al 1994b, Elpeleg & Shaag 1999, Olsen et al 2002, Zeng et al 2002, Michals & Matalon 2011].
  
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Deletion/duplication analysis. (C)
  
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Deletions and duplications are not detectable by sequence analysis; other methods need to be used. Deleted segments of various sizes of cDNA have been reported [Zeng et al 2006, Kaya et al 2008].The authors encountered two individuals with complete deletion of ASPA and two with partial deletions [Matalon, unpublished data].
  
   
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Postnatal testing for Canavan Disease can be done in several ways. One possibility is to test for a raised N-acetyl-L-aspartic acid (NAA) concentration in urine, blood and cerebrospinal fluid (CSF) (comparable to prenatal testing with the carrier status of one parent unknown).
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Method Mutations Detected Mutation Mutation
  
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Other possibilities may be cultivating skin fibroblasts and test them for reduced aspartoacylase activity, perform neuroimaging of the brain and look for spongy degeneration, or test the gene itself for a defect in the newborn child. However it takes between three to nine months after birth until most of the symptoms become apparent.
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Frequency in Frequency in NON
  
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(A) p.Glu285Ala, p.Tyr231X 98 3
  
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p.Ala305Glu 1 30-60
  
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(B) Sequence variants N/A 87
  
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(C) Large, comprising one or more exons N/A (<10)
  
   
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==== Mild / Juvenile Diagnosis ====
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Note:
 
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Although NAA concentration is also elevated in the blood and CSF of children with neonatal/infantile (severe) Canavan disease, the number of affected individuals evaluated to date is small [Michals & Matalon 2011].
  
   
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Diagnosing a patient with Canavan Disease if he or she is suffering from a mild or juvenile form, is a bit more challenging, as the postnatal diagnosis methods, except testing the gene itself, will not yield in a satisfactory result or may even overlook the disease completely. The concentration of NAA may be elevated only slightly and not as significant such that a proper diagnosis can be made. The same being true for the results of neuroimaging, and the mild developmental delay that is a result of Canavan Disease which can simply be unrecognized.
   
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BIOCHEMICAL DISEASE MECHANISM
  
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* KEGG
  
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* belongs to a group of inherited diseases called the leuko-dystrophies.
 
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These disorders cause defective development of the myelin sheath, (the casing that functions like an insulator / a protective cover for the nerves and also conducts nerve impulses) in the brain. Myelin is a substance that comprises of 10 special chemicals. Each category or type of leukodystrophy has an impact on only one of these chemicals.
  
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* Canavan disease causes defective development of the myelin sheath that covers the nerve fibers. The primary cause for the disorder is a defective / malfunctioning ASPA gene. The ASPA gene is in charge for the synthesis of enzyme aspartoacyclase. Aspartoacyclase decomposes the brain molecule N acetyl aspartate. Thus, impaired functioning of the aspartoacylase does not allow normal decomposition of N-acetyl aspartate, and as a result, a lack of break-down hampers the normal synthesis and the development of myelin.
  
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* the white matter of the brain degenerates in to soft tissue and has tiny fluid filled spaces
  
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* One of the chief Canavan Disease causes is a malfunctioning / faulty ASPA gene. This gene is in charge of the synthesis of the enzyme aspartoacyclase. This enzyme breaks down the concentrated brain molecule N acetyl aspartate. Reduced activity and functioning of the aspartoacylase puts a stop to the normal decomposition of N-acetyl aspartate, and consequently, a lack of break down hinders the normal production and the development of the myelin sheath of the nerves.
  
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* The components of N-acetylaspartate acid (NAA) are involved in forming the myelin sheathe. Normally, aspartoacyclase, decomposes NAA in to the units that are required. However, in Canavan Disease, mutation affects the manufacture of this enzyme (experts say this is the primary Canavan disease cause). NAA build up to unsafe levels, obstructing the brain's transmission network.
  
   
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== Treatment ==
   
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Right now there is no cure for Canavan Disease, but there are treatments depending on the symptoms, which work in a supportive manner.
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GENE
  
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* The gene that is linked to the disease is situated on chromosome 17.
  
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* 17p13.2
  
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* ASPA - ASPARTOACYLASE
  
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* Normal allelic variants. The gene comprises 29 kb with six exons and five introns. The exons vary in size from 94 bp (exon 3) to 514 bp (exon 6).
  
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* Pathologic allelic variants. See Table 2. The major disease-causing allelic variants are p.Glu285Ala, p.Tyr231X, and p.Ala305Glu. (For more information, see Table A.)
  
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* Normal gene product. Aspartoacylase is a protein of 313 amino acids, suggesting a molecular weight of 36 kd [Kaul et al 1993]. The 93% homology of the amino acid and nucleotide sequence of human and bovine aspartoacylase suggest a high degree of conservation of this enzyme in mammals [Kaul et al 1994a]. The protein is observed in most tissues. Recent studies have shown that ASPA is a dimer with zinc at the catalytic site analogous to other carboxypeptidases. Mutations caused conformational changes that affect the activity of the enzyme [Bitto et al 2007].
  
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* Abnormal gene product. Aspartoacylase is responsible for hydrolyzing N-acetylaspartic acid (NAA) into aspartic acid and acetate. The abnormal alleles include null mutations, which make no aspartoacylase, and missense mutations, which make less active forms of aspartoacylase. Although aspartoacylase is expressed widely throughout the body, its absence in the CNS leads to the specific build-up of NAA in the brain that causes demyelinization and other signs of the disease.
  
   
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==== Prenatal Treatment ====
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Gene Symbol Chromosomal Locus Protein name Locus specific HGMD
  
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ASPA 17p13​.2 Aspartoacylase ASPA @ LOVD ASPA
  
   
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There is a possibility of prenatal screening to check whether or not someone is a carrier of the disease (as described in the section before). Other prenatal treatments are under investigation and depend on animal models.
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DNA Nucleotide Change Protein Amino Acid Change
  
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c.433-2A>G --
  
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c.693C>A p.Tyr231X
  
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c.854A>C p.Glu285Ala
  
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c.863A>G p.Tyr288Cys
  
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c.914C>A p.Ala305Glu
  
   
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==== Neonatal / Infantile Treatment ====
   
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Since Canavan Disease also affects the metabolism there is need to control the nutrition and hydration. This includes specialized food to compensate missing metabolites and nutrients as well as different ways of feeding / providing nutrition to the child to prevent problems arising from swallowing difficulties and other physical disabilities. To improve those physical disabilities and muscle problems, it is recommended that children need physical therapy. Additionally there are anti-epileptic drugs against seizures and spastic behavior.
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HOW DOES THE TREATMENT IMPROVE PHENOTYPE?
  
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CURE
  
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* At the present time there is no cure for Canavan disease.
  
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TREATMENT
  
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NOTE
  
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* treatment for Canavan disease is supportive and depends on symptom management.
  
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* the Canavan disease treatment is supportive and symptomatic.
 
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* Neonatal/infantile Canavan disease: Treatment is supportive and directed to providing adequate nutrition and hydration, managing infectious diseases, and protecting the airway. Physical therapy minimizes contractures and maximizes motor abilities and seating posture; special education programs enhance communication skills. Seizures are treated with antiepileptic drugs. Gastrostomy may be needed to maintain adequate food intake and hydration when swallowing difficulties exist.
  
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PRENATAL
  
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* Several laboratories proffer pre- natal screening for the disease to populations that are at risk. Researchers have devised animal models and are using them to assess possible treatment strategies. Studies to comprehend how the brain normally develops and functions and how it gets affected by genetic mutation are carried out. These studies help better understand Canavan disease; and thus offer greater likelihood to new possibilities of treatment.
  
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NEONATAL / INFANTILE
  
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* Treatment is supportive and directed to providing adequate nutrition and hydration, managing infectious diseases, and protecting the airway.
  
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* Children benefit from physical therapy to minimize contractures and to maximize abilities and seating posture, from other therapies to enhance communication skills (especially in those with a more gradual clinical course), and from early intervention and special education programs.
  
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* Seizures may be treated with antiepileptic drugs (AEDs).
  
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* A feeding gastrostomy may be required to maintain adequate intake and hydration in the presence of swallowing difficulties.
  
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* Diamox® seems to reduce intracranial pressure.
  
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* Botox® injections may be used to relieve spasticity.
  
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* Convulsions and seizures are handled suitably using anti-convulsants.
  
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JUVENILE / MILD
  
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* These individuals may require speech therapy or tutoring but require no special medical care.
  
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FUTURE WORK
  
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HUMAN
  
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* Furthermore, there are investigational clinical trials of gene therapy. In the study a normal gene is cloned to work instead of the faulty one that is known to result in Canavan disease.
  
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* Gene transfer to the brains of two children with Canavan disease using a nonviral vector was well tolerated [Leone et al 2000, Janson et al 2002]. Some biochemical, radiologic, and clinical changes may have occurred; however, the children continued to follow the course of those with untreated Canavan disease.
  
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* In ten children with Canavan disease, multisite injection with AAV2 as the vector for ASPA was well tolerated [McPhee et al 2006]. Three developed AAV2 neutralizing antibodies. No children improved.
  
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* Lithium citrate may reduce N-acetylaspartic acid (NAA) concentration in the brain. Six persons with Canavan disease given lithium citrate for 60 days were reported to have reduced NAA in the basal ganglia and mild improvement in frontal white matter [Assadi et al 2010]. The clinical significance of use of lithium citrate is not known.
  
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* The enzyme defect in Canavan disease leads to decreased levels of acetate in the brain. In a clinical trial with glycerol triacetate in two persons with Canavan disease the compound was well tolerated; however, there was no clinical improvement [Madhavarao et al 2009]. It is speculated that higher doses of glycerol triacetate may be needed.
  
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* There is an investigational therapy using lithium citrate. A person diagnosed with Canavan Disease demonstrated elevated levels of N-acetyl aspartate. Lithium citrate has been used for experimentation in a rat genetic model of Canavan Disease; and results showed it appreciably reduced levels of N-acetyl aspartate. When the dug was administered to a human, the patinet reversed during the 2 week wash-out period, after the removal of lithium. Evidence suggests that a larger controlled clinical study and research of lithium is necessary to use it as supportive therapy for children having Canavan disease. and lithium citrate, used in a rat model of Canavan Disease showed that levels of N-acetyl aspartate were greatly decreased. A larger clinical study and research of lithium is essential to manage children diagnosed with Canavan disease.
  
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ANIMAL
  
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* A knock-out mouse has been created, with a phenotype similar to that of human Canavan disease [Matalon et al 2000]. This model is being used to investigate pathophysiology [Surendran et al 2004] gene therapy and other modes of treatment [Matalon et al 2003].
  
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* Gene therapy with AAV2 was given to knock-out Canavan disease mice: localized improvement did not spread to the entire brain [Matalon et al 2003].
  
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* Stem cell therapy in knock-out Canavan disease mice was done in collaboration with Genzyme Corporation: The stem cells produced some oligodendrocytes but not enough to make myelin [Surendran et al 2004].
  
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* Enzyme replacement therapy using native ASPA and pegylated ASPA (i.e., ASPA in which covalent attachment of polyethylene glycol polymer chains masks the enzyme from the host allowing for longer circulation and less renal clearance) were injected into the peritoneum of Canavan disease mice. Preliminary results show that the enzyme passed the blood brain barrier and there was a decrease of NAA in the brain. These experiments were short term; longitudinal studies are being planned [Zano et al 2011].
  
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* A mouse that seems to have a Canavan disease phenotype was produced by using the mutagen ethylenenitrosourea (ENU) to change the residue Gln193 to a termination codon in exon 4 (577C>T) [Traka et al 2008].
  
   
  +
==== Mild / Juvenile Treatment ====
   
  +
Since mild and juvenile Canavan patients only have some delays in the development and speech, a speech therapy may be useful. Further deep medical care is not necessary.
   
  +
== Future Work ==
   
  +
There are some clinical trials and animal models under investigation to find a cure for Canavan Disease.
−
== Phenotype ==
  
  +
−
Phenotypic description of the disease.
 
  +
==== Gene Therapy ====
  +
  +
There were several studies in the gene therapy, using viral and non viral vectors to transfer genes into the patients that were thought to improve the course of the disease. However none of the children showed an improvement and the disease showed a development similar to an untreated patient.
  +
  +
==== Lithium Citrate as Pharmaceutical ====
  +
  +
Since N-acetyl-L-aspartate (NAA) is one important factor in the biochemical background of Canavan Disease, where the NAA level is too high, lithium citrate may be able to reduce the NAA concentration. Rat models have shown that treating a rat with lithium citrate resulted in a reduced level of NAA. Furthermore if the drug is administered to a human the same effect can be observed with a return to elevated NAA concentration when the lithium citrated is washed out of the body after roughly 2 weeks. However so far no larger controlled clinical studies have been conducted, but lithium citrate shows a potential treatment that is worth pursuing.
  +
  +
==== Animal Models ====
   
  +
Several gene models in knockout mice and rats have been studied, with lithium citrate and an enzyme replacement therapy showing the best result so far and therefore being the most promising at the moment.
−
(Describe this in your own words, avoid plagiarism. Summarise the information from different sources.)
  
   
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=== Cross-references ===
 
−
See also description of this disease in
  
−
* specific link to Wikipedia
  
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* specific link to HGMD
  
−
* specific link to OMIM
  
−
... (see [[Resource data|databases in "resources"]])
  
   
  +
== Aspartoacylase (ASPA) ==
−
== Biochemical disease mechanism ==
  
  +
<figure id="AspaKegg">
−
The example protein is involved in the example pathway...
  
  +
[[Image:NAA hydrolyzation.gif|thumb|450px|'''<caption>'''The hydrolyzation of N-acetyl-L-aspartate (C01042) catalyzed by aspartoacylase to acetyl (C00033) and aspartate (C00049). (source: [http://www.kegg.jp/dbget-bin/www_bget?R00488 KEGG])</caption>]]
  +
</figure>
   
  +
==== Summary ====
−
Ideally, include a graphical pathway representation like this one: [[Image:SphingolipidMetabolism.png|frame|Sphingolipid Metabolism (source: KEGG) highlighting disease associated enzymes]]
  
  +
Aspartoacylase is the enzyme that hydrolyzes N-acetyl-L-aspartate into acetate and L-aspartate, which are essential for the build-up process of the myelin sheath (chemical reaction displayed in '''<xr id="AspaKegg">Figure</xr>'''). Crystallized ASPA exists as a homodimer however it is assumed that the in-vivo form only works as a monomer. The active site of ASPA contains a zinc ion which acts catalytic in the hydrolyzation process and is only accessible through a channel like surface fold of the protein. This channel like structure serves two purposes. On the one hand it hinders polypeptides to enter and bind at the active site, therefore ASPA does not function as protease. On the other hand and more importantly it is assumed, that the positive electrostatic potential that is present on the channel serves as a form of transport mechanism to properly carry the negatively charged substrate (NAA) to the hydrolyzing site. Furthermore, the binding pocket is highly specific to N-acetyl-L-aspartate with a far lower hydrolyzing activity towards other N-acetyl-amino complexes like N-acetylglutamate.
   
  +
==== Gene Position and Mutations ====
−
(see above: own words, no plagiarism)
  
   
  +
The ASPA gene is located on chromosome 17 on the p-arm (upper part, short arm) band 1 subband 3 subsubband 2 (short 17p13.2) (see '''<xr id="Location">Figure</xr>''').
−
<br style="clear: both" />
  
  +
<figure id="Location">
−
=== Cross-references ===
  
  +
[[Image:ASPA gene location.png|thumb|centre|750px|'''<caption>'''Chromosome 17 with highlighted position of ASPA-gene. (source: [http://www.genecards.org/cgi-bin/carddisp.pl?gene=ASPA Genecards])</caption>]]
−
* link to KEGG
  
  +
</figure>
−
* link to MetaCyc
  
−
... see [[Resource data|databases in "resources"]]
  
   
  +
===== Reference Sequence =====
−
== Mutations ==
  
  +
*[[ASPA#Genomic Sequence|Reference sequence (genomic) of ASPA]]
−
Current knowledge about mutations associated with the disease. - Separate into disease causing and neutral mutations. -- These sequence pages will be the starting point for collecting prediction results and result discussions.
  
  +
*[[ASPA#Protein Sequence|Reference sequence (protein) of ASPA]]
   
  +
===== Disease Causing Mutations =====
−
'''Note:''' Until further notice you only ''need to'' care about the reference sequence pages. -- At a later stage we will assign mutations we expect you to work on. Then, it will make sense to create on page per mutation that is assigned to you.
  
  +
The disease causing mutations can be found in '''<xr id="DisCausMut">Table</xr>''' and '''<xr id="AllelicVar">Table</xr>''' below. Very interesting in this Table is the frequency of some mutations across different populations.
  +
<figtable id="DisCausMut">
  +
{| border="1" cellpadding="5" cellspacing="0" align="center"
  +
|-
  +
! colspan="7" style="background:#87cefa;" | Summary of Molecular Genetic Testing Used in Canavan Disease
  +
|-
  +
! style="background:#BFBFBF;" align="center" | Gene Symbol
  +
! style="background:#BFBFBF;" | Test Method
  +
! colspan="2" style="background:#BFBFBF;" | Mutations Detected
  +
! colspan="2" style="background:#BFBFBF;" | Mutation Detection Frequency by Test Method
  +
! style="background:#BFBFBF;" | Test Availability
  +
|-
  +
! style="background:#E5E5E5;" align="center" |
  +
! style="background:#E5E5E5;" align="center" |
  +
! colspan="2" style="background:#E5E5E5;" align="center" |
  +
! style="background:#E5E5E5;" align="center" | Ashkenazi Jewish
  +
! style="background:#E5E5E5;" align="center" | Non-Ashkenazi Jewish
  +
! style="background:#E5E5E5;" align="center" |
  +
|-
  +
|rowspan="4"| ASPA
  +
|rowspan="2"| Targeted mutation analysis
  +
|rowspan="2"| Panel
  +
|| p.Glu282Ala, p.Tyr231X
  +
|| 98%
  +
|| 3%
  +
|rowspan="4"| Clinical Testing
  +
|-
  +
|| p.Ala305Glu
  +
|| 1%
  +
|| 30%-60%
  +
|-
  +
|| Sequence analysis
  +
|colspan="2"| Sequence variants
  +
|| N/A
  +
|| 87%
  +
|-
  +
|| Deletion / duplication analysis
  +
|colspan="2"| Large genomic deletions/duplications <br> comprising one or more exons
  +
|| N/A
  +
|| Unknown (<10%)
  +
|-
  +
|}
  +
<center><small>'''<caption>''' Disease causing Mutations in Canavan Disease. (source: [http://www.ncbi.nlm.nih.gov/books/NBK1234/ NCBI]) </caption></small></center>
  +
</figtable>
   
  +
<figtable id="AllelicVar">
−
=== Reference sequence ===
  
  +
{| border="1" cellpadding="5" cellspacing="0" align="center"
  +
|-
  +
! colspan="3" style="background:#87cefa;" | Selected ASPA Pathologic Allelic Variants
  +
|-
  +
! style="background:#BFBFBF;" align="center" | DNA Nucleotide Change
  +
! style="background:#BFBFBF;" align="center" | Protein Amino Acid Change
  +
! style="background:#BFBFBF;" align="center" | Reference Sequence
  +
|-
  +
|| c.433-2A>G
  +
|| -
  +
|rowspan="5"| NM_000049.2 <br> NP_000040.1
  +
|-
  +
|| c.693C>A
  +
|| p.Tyr231X
  +
|-
  +
|| c.854A>C
  +
|| p.Glu285Ala
  +
|-
  +
|| c.863A>G
  +
|| p.Tyr288Cys
  +
|-
  +
|| c.914C>A
  +
|| p.Ala305Glu
  +
|-
  +
|}
  +
<center><small>'''<caption>''' Disease causing Mutations in Canavan Disease. (source: [http://www.ncbi.nlm.nih.gov/books/NBK1234/ NCBI]) </caption></small></center>
  +
</figtable>
   
  +
== References ==
−
Which sequence does not cause the disease and is most often found in the population.
 
  +
The written text is based on a summary of different sources: <br>
−
* [[example_sequence|Create a page for the reference sequence.]]
 
  +
Canavan Disease:
  +
* [http://en.wikipedia.org/wiki/Myrtelle_Canavan Wikipedia: Mytelle Canavan]
  +
* [http://ghr.nlm.nih.gov/condition/canavan-disease Genetics Home Reference]
  +
* [http://www.pnas.org/content/104/2/456.short PNAS]
  +
* [https://www.counsyl.com/diseases/canavan-disease/ Counsyl]
  +
* [http://omim.org/entry/271900 OMIM]
  +
* [http://www.canavanfoundation.org Canavan Foundation]
  +
* [http://www.canavandisease.net CanavanDisease.net]
  +
* [http://www.nlm.nih.gov/medlineplus/ency/article/001586.htm Medline Plus]
  +
* [http://www.ninds.nih.gov/disorders/canavan/canavan.htm National Institute of Neurological Disorders]
  +
* [http://www.ncbi.nlm.nih.gov/books/NBK1234/ NCBI bookshelf]
  +
* [http://www.kegg.jp/kegg-bin/get_htext?htext=br08402.keg&query=canavan KEGG]
  +
* [http://rarediseases.info.nih.gov/gard/5984/canavan-disease/resources/1 US Department of Health and Human Services - Genetic Rare Disease Information Center]
   
  +
ASPA:
−
=== Neutral mutations ===
  
  +
* [http://omim.org/entry/608034 OMIM]
−
* [[example_sequence|Create one page per mutated sequence]].
  
  +
* [http://www.uniprot.org/uniprot/P45381 Uniprot]
   
  +
== Tasks ==
−
=== Disease causing mutations ===
  
  +
* Link to Task 01: [[Canavan_Disease|Canavan Disease]]
−
* [[example_sequence|Create one page per mutated sequence]].
  
  +
* Link to Task 02: [[Canavan_Disease:_Task_02_-_Alignments|Alignments]]
  +
* Link to Task 03: [[Canavan_Disease:_Task_03_-_Sequence-based_Predictions|Sequence-based Predictions]]
  +
* Link to Task 04: [[Canavan_Disease:_Task_04_-_Structural_Alignments|Structural Alignments]]
  +
* Link to Task 05: [[Canavan_Disease:_Task_05_-_Homology_Modelling|Homology Modelling]]
  +
* Link to Task 06: [[Canavan_Disease:_Task_06_-_Protein_Structure_Prediction|Protein Structure Prediction from Evolutionary Sequence Variation]]
  +
* Link to Task 07: [[Canavan_Disease:_Task_07_-_Researching_SNPs|Researching SNPs]]
  +
* Link to Task 08: [[Canavan_Disease:_Task_08_-_Sequence-based_Mutation_Analysis|Sequence-based Mutation Analysis]]
  +
* Link to Task 09: [[Canavan_Disease:_Task_09_-_Structure-based_Mutation_Analysis|Structure-based Mutation Analysis]]
  +
* Link to Task 10: [[Canavan_Disease:_Task_10_-_Normal_Mode_Analysis|Normal Mode Analysis]]

Latest revision as of 10:50, 5 September 2013

<figure id="ASPA">

Crystal structure of aspartoacylase (2O4H - PDB-file).

</figure> Canavan Disease (ICD-10 E75.2) is an autosomal recessive disorder, in which a dysfunctional enzyme causes severe brain damage. It is also known under a variety of other names describing the chemical basis or phenotype of the disease. Examples are "Spongy Degeneration Of Central Nervous System", "Aspartoacylase (ASPA) Deficiency", or "Aminoacylase 2 (ACY2) Deficiency"[1]. The trivial name, Canavan Disease, originates from the name of Myrtelle Canavan (1879 – 1953)[2], an American physician, who first described the disease in 1931. There is no cure and almost all patients die within the first decade of their life. The mild / juvenile type is less severe. The treatment is based on the symptoms and is supportive.

Inheritance

Canavan Disease is an autosomal recessive genetic defect of the ASPA (aspartoacyclase) gene on chromosome 17 (for the crystal structure of the ASPA protein see <xr id="ASPA">Figure</xr>). With this pattern of heritage a newborn of a couple where both parents are carriers of the defective genome has a 25% chance neither being born suffering from Canavan Disease nor being born a carrier. For some time children born of Ashkenazi Jewish ancestry had a higher prevalence of having Canavan Disease while in the last years this prevalence is sinking due to ongoing prenatal screening programs. Other ethnic groups where Canavan Disease has a higher penetrance are for example populations of Saudi Arabian ancestry.
According to Genetics Home about one in 6400 to 13500 of the Ashkenazi Jewish are affected. No further information about prevalences in other populations was found. However the different populations have also different frequencies regarding the mutation they are based on. For further information see section Disease Causing Mutations.

Phenotype

Canavan Disease has a variety of different phenotypes ranging across all body parts. Here is a short overview:

  • Head
    • macrocephaly (increased head circumference)
    • mental retardation and impairment (losing mental skills)
    • losing ability to move head
  • Eyes
    • becoming blind
    • nystagmus (greek: νυσταζω nytaxoo "sleep, nod", german: "Augenzittern")
  • Ears
    • becoming deaf
  • Mouth
    • problems with swallowing
    • losing communicational abilities (cannot talk, stay quiet)
  • Body
    • paralysis
    • seizures
    • problems moving the muscles

Children suffering from Canavan Disease usually die within the first decade. In the mild/juvenile form of Canavan Disease, the children usually have some developmental delay and some speech problems.


Disease Mechanism

<figure id="KEGG">

Alanine, Aspartate and Glutamate Metabolism (source: KEGG) highlighting disease associated enzymes of Canavan Disease.

</figure> Canavan Disease belongs to the group of leukodystrophies. The etymological origin are the greek words: λευκος leukos "white", δυς dys "bad, wrong" and τροφη trophae "feeding, growth". This is a genetic induced metabolic disorder, which affects the white matter of the nervous system. If the white matter is not properly grown, the myelin, which surrounds the nerve cells for protection, is degraded. This is especially true for Canavan Disease. The visible phenotypes are a result of a genetic defect that negatively affects the growth of the myelin sheath covering the nerve fibers. An improperly build myelin sheath, results in a reduced ability to transmit the electric signal along the nerve fibers, eventually losing it completely and finally the degradation of whole nerve cells.
The cause for the malfunctioning myelin sheath growth is a genetic defect of the aspartoacylase (ASPA) gene. The product of the gene, the enzyme aspartoacylase is crucial in the degradation process of N-acetyl-L-aspartate (NAA) which is present at much higher levels than normal in patients suffering from Canavan Disease. Normally ASPA would degrade NAA into smaller fragments which are required prerequisites for the production of the myelin sheath (see <xr id="KEGG">Figure</xr> for an overview where APSA is located in the metabolic map). Therefore the missing / defective ASPA is reason for the defective build up process of myelin. The degradation of the nerve cells / white brain matter has the consequence that empty spaces are arising which are filled with brain fluid leading to even more degradation of nerve cells and signal transduction problems.

Diagnosis

There are a couple of possibilities how and when an affected patient is diagnosed with Canavan Disease. The time points are prenatal, postnatal, and when a mild or juvenile form of Canavan Disease is already present. Nevertheless one of the most important things to know before is if both parents carry one copy of the disease causing gene. This can simply be done by DNA testing.

Prenatal Diagnosis

There are several types of prenatal testing possibilities depending on whether the carrier status of both parents is known or not. For couples where it is only known that one of the parents is a carrier and the remaining parents status is not known, normally testing is done by measuring the concentration of N-acetyl-L-aspartic acid (NAA) in the amniotic fluid within the time between the 16th and 18th week of pregnancy. Another possibility is molecular genetic testing. Following this method an analysis of DNA extracted from fetal cells is done. These fetal cells are obtained either between the tenth to 12th week of pregnancy by chorionic villus (“proto-”placental tissue that has the same genetic material as the fetus) sampling or between the 15th and 18th week by amniocentesis, also known as amniotic fluid testing (AFT). However for the molecular genetic testing both disease causing genes of the parents have to be identified first.

Neonatal / Infantile Diagnosis

Postnatal testing for Canavan Disease can be done in several ways. One possibility is to test for a raised N-acetyl-L-aspartic acid (NAA) concentration in urine, blood and cerebrospinal fluid (CSF) (comparable to prenatal testing with the carrier status of one parent unknown). Other possibilities may be cultivating skin fibroblasts and test them for reduced aspartoacylase activity, perform neuroimaging of the brain and look for spongy degeneration, or test the gene itself for a defect in the newborn child. However it takes between three to nine months after birth until most of the symptoms become apparent.

Mild / Juvenile Diagnosis

Diagnosing a patient with Canavan Disease if he or she is suffering from a mild or juvenile form, is a bit more challenging, as the postnatal diagnosis methods, except testing the gene itself, will not yield in a satisfactory result or may even overlook the disease completely. The concentration of NAA may be elevated only slightly and not as significant such that a proper diagnosis can be made. The same being true for the results of neuroimaging, and the mild developmental delay that is a result of Canavan Disease which can simply be unrecognized.


Treatment

Right now there is no cure for Canavan Disease, but there are treatments depending on the symptoms, which work in a supportive manner.

Prenatal Treatment

There is a possibility of prenatal screening to check whether or not someone is a carrier of the disease (as described in the section before). Other prenatal treatments are under investigation and depend on animal models.

Neonatal / Infantile Treatment

Since Canavan Disease also affects the metabolism there is need to control the nutrition and hydration. This includes specialized food to compensate missing metabolites and nutrients as well as different ways of feeding / providing nutrition to the child to prevent problems arising from swallowing difficulties and other physical disabilities. To improve those physical disabilities and muscle problems, it is recommended that children need physical therapy. Additionally there are anti-epileptic drugs against seizures and spastic behavior.

Mild / Juvenile Treatment

Since mild and juvenile Canavan patients only have some delays in the development and speech, a speech therapy may be useful. Further deep medical care is not necessary.

Future Work

There are some clinical trials and animal models under investigation to find a cure for Canavan Disease.

Gene Therapy

There were several studies in the gene therapy, using viral and non viral vectors to transfer genes into the patients that were thought to improve the course of the disease. However none of the children showed an improvement and the disease showed a development similar to an untreated patient.

Lithium Citrate as Pharmaceutical

Since N-acetyl-L-aspartate (NAA) is one important factor in the biochemical background of Canavan Disease, where the NAA level is too high, lithium citrate may be able to reduce the NAA concentration. Rat models have shown that treating a rat with lithium citrate resulted in a reduced level of NAA. Furthermore if the drug is administered to a human the same effect can be observed with a return to elevated NAA concentration when the lithium citrated is washed out of the body after roughly 2 weeks. However so far no larger controlled clinical studies have been conducted, but lithium citrate shows a potential treatment that is worth pursuing.

Animal Models

Several gene models in knockout mice and rats have been studied, with lithium citrate and an enzyme replacement therapy showing the best result so far and therefore being the most promising at the moment.


Aspartoacylase (ASPA)

<figure id="AspaKegg">

The hydrolyzation of N-acetyl-L-aspartate (C01042) catalyzed by aspartoacylase to acetyl (C00033) and aspartate (C00049). (source: KEGG)

</figure>

Summary

Aspartoacylase is the enzyme that hydrolyzes N-acetyl-L-aspartate into acetate and L-aspartate, which are essential for the build-up process of the myelin sheath (chemical reaction displayed in <xr id="AspaKegg">Figure</xr>). Crystallized ASPA exists as a homodimer however it is assumed that the in-vivo form only works as a monomer. The active site of ASPA contains a zinc ion which acts catalytic in the hydrolyzation process and is only accessible through a channel like surface fold of the protein. This channel like structure serves two purposes. On the one hand it hinders polypeptides to enter and bind at the active site, therefore ASPA does not function as protease. On the other hand and more importantly it is assumed, that the positive electrostatic potential that is present on the channel serves as a form of transport mechanism to properly carry the negatively charged substrate (NAA) to the hydrolyzing site. Furthermore, the binding pocket is highly specific to N-acetyl-L-aspartate with a far lower hydrolyzing activity towards other N-acetyl-amino complexes like N-acetylglutamate.

Gene Position and Mutations

The ASPA gene is located on chromosome 17 on the p-arm (upper part, short arm) band 1 subband 3 subsubband 2 (short 17p13.2) (see <xr id="Location">Figure</xr>). <figure id="Location">

Chromosome 17 with highlighted position of ASPA-gene. (source: Genecards)

</figure>

Reference Sequence
Disease Causing Mutations

The disease causing mutations can be found in <xr id="DisCausMut">Table</xr> and <xr id="AllelicVar">Table</xr> below. Very interesting in this Table is the frequency of some mutations across different populations. <figtable id="DisCausMut">

Summary of Molecular Genetic Testing Used in Canavan Disease
Gene Symbol Test Method Mutations Detected Mutation Detection Frequency by Test Method Test Availability
Ashkenazi Jewish Non-Ashkenazi Jewish
ASPA Targeted mutation analysis Panel p.Glu282Ala, p.Tyr231X 98% 3% Clinical Testing
p.Ala305Glu 1% 30%-60%
Sequence analysis Sequence variants N/A 87%
Deletion / duplication analysis Large genomic deletions/duplications
comprising one or more exons 		N/A Unknown (<10%)
Disease causing Mutations in Canavan Disease. (source: NCBI)

</figtable>

<figtable id="AllelicVar">

Selected ASPA Pathologic Allelic Variants
DNA Nucleotide Change Protein Amino Acid Change Reference Sequence
c.433-2A>G - NM_000049.2
NP_000040.1
c.693C>A p.Tyr231X
c.854A>C p.Glu285Ala
c.863A>G p.Tyr288Cys
c.914C>A p.Ala305Glu
Disease causing Mutations in Canavan Disease. (source: NCBI)

</figtable>

References

The written text is based on a summary of different sources:
Canavan Disease:

ASPA:

Tasks

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