Leukodystrophies are a group of rare and inherited white matter disorders that target the brain's myelin.¹ Myelin is a protective sheath that covers nerve fibres to ensure rapid transmission of signals in the nerves. If this myelin sheath is damaged, it can lead to various neurological symptoms such as numbness, vision problems and issues with balance. There are over 50 different leukodystrophies that can all target the myelin in unique ways. Canavan Disease is a specific leukodystrophy. Canavan disease is caused by a gene mutation for the aspartoacylase (ASPA) gene.² Canavan disease differs from other leukodystrophies such as Krabbe Disease, Metachromatic Leukodystrophy (MLD), Adrenoleukodystrophy (ALD) and Pelizaeus-Merzbacher Disease (PMD) in terms of its genetic basis, mechanism of action, clinical presentation and diagnostic markers. 

Pathophysiology and genetics

Canavan disease is caused by a mutation in the ASPA gene, which leads to a deficiency of aspartoacylase. Aspartoacylase would usually lead to N-acetylaspartic acid (NAA) to be broken down into aspartate and acetate, which are used for neurotransmission and myelin lipid synthesis, respectively.³ However, due to the aspartoacylase deficiency, NAA levels rise. This leads to the spongy degeneration of the white matter of the brain. Spongy degeneration is when fluid-filled vacuoles occur within the white matter of the brain, giving it a sponge-like appearance.⁴ NAA leads to spongy degeneration as it draws water into cells and extracellular spaces. This leads to vacuolation, creating the spongy appearance.⁵ Given that acetate is no longer being produced, there is also a loss of myelin lipid synthesis, which results in demyelination and the structural collapse of white matter.  Canavan disease is also autosomal recessive, meaning that the condition is not sex linked and needs two copies of the mutation in order to be present. A person might have only one copy of the mutation and therefore not exhibit symptoms but be a carrier. If two carriers have children, there is a 1 in 4 chance that their child will be affected by the disease. 

This differs from the other leukodystrophies previously mentioned. Krabbe disease is a lysosomal storage disorder that is caused by galactocerebrosidase (GALC)  deficiency.  This leads to a toxic psychosis accumulation. Psychosine is a toxic lipid that starts to accumulate and is usually broken down by the GALC enzyme. Psychosine is toxic to oligodendrocytes, which are what form the myelin sheaths for the central nervous system. It is also toxic to Schwann cells, which form the myelin sheath for the peripheral nervous system. It is toxic as it induces apoptosis and increases inflammation, which can trigger demyelination.⁶ Metachromatic leukodystrophy involves arylsulfatase A (ASA) deficiency due to a mutation in the ARSA gene. This then leads to the accumulation of sulfatides. Sulfatides are glycolipids that have a sulphur molecule added. When in high concentrations, sulfatides can disrupt cell membranes, impairing the function of oligodendrocytes and Schwann cells.⁷ X-linked adrenoleukodystrophy is caused by an ABCD1 mutation that affects peroxisomal function, as well as having adrenal involvement.  This is because the ABCD1 gene, when unmutated, encodes for a transporter that can move long-chain fatty acids into peroxisomes, where they can then be broken down. Without the transporter, there is a toxic accumulation of these long-chain fatty acids, resulting in demyelination as well as adrenal insufficiency. The mutation is X-linked, meaning it is more likely to appear in males, as they only have one copy of the X chromosome. This differs from Canavan disease, which is not sex linked. The adrenal involvement often appears in the form of Addison's disease, which can produce symptoms such as hypotension, fatigue and weight loss. It is because the cortisol and aldosterone production are impaired due to the accumulation of long-chain fatty acids.⁸ Pelizaeus-Merzbacher disease is caused by a mutation in myelin structural proteins, leading to hypomyelination. This mutation is in the PLP1 gene, which encodes a key structural protein of the CNS myelin. PMD is also X-linked, meaning it predominantly affects males.  PlPD1 is essential for myelin sheath stability, and if it is overexpressed due to gene duplications, it can lead to oligodendrocyte stress. Alternatively, the PLP1 can have a missense mutation, leading to a misfolded protein being formed, which can lead to endoplasmic reticulum stress, which also causes disrupted myelin. 

This shows how leukodystrophies can vary in the way that they are caused, as each one can occur from a different mutation, as well as how their inheritance differs. Whilst these are all recessive, two of them are sex linked, whilst the other two are not. 

Clinical presentation 

Canavan disease typically appears in infancy, at around 3-6 months. The more common symptoms are macrocephaly (enlarged head circumference), hypotonia (floppiness), which becomes spasticity (stiffness), seizures, blindness and developmental delay. Krabbe disease presents in infancy, and key symptoms include limb stiffness, irrationality, and developmental regression. This differs from Canavan, as there is a developmental progression that occurs, but then is reversed. Spasticity occurs as the rapid disruption of the motor pathways due to the demyelination of both the central nervous system and the peripheral nervous system. This makes it distinct from the other leukodystrophies, as the others tend not to affect the PNS or have a slower disease onset, making the symptoms seem less pronounced and sudden.  In MLD, it typically presents later on in life, either in childhood, between the ages of 4-12, but can appear as late as one's 40s.⁸ This might be because a mutation only happens later in life to the ARSA gene. Symptoms can include ataxia and a cognitive and behavioural decline. This is due to the accumulation of sulfatides, which can affect the cerebellum and frontal lobe, leading to this loss of coordination and behavioural changes. ALD is also commonly found in childhood or early adulthood, and its key defining symptoms are adrenal insufficiency, as well as a loss of eyesight. This is because the toxic build-up occurs in both the CNS and the adrenal cortex. The vision loss is due to the occipital white matter being affected first. The other leukodystrophies do not follow this demyelination pattern, making this early vision loss unique to ALD.  PMD often occurs in infancy. Its key symptoms are nystagmus (involuntary rhythmic movement of the eyes) and hypotonia. This is because there is early-onset cerebellar and brain stem dysfunction due to hypomyelination. This then affects the eyes, leading to nystagmus, as well as resulting in poor muscle tone. Other leukodystrophies involve demyelination rather than hypomyelination, meaning that the myelin can form normally and is then destroyed. With PMD, the myelin never forms correctly, resulting in symptoms that occur earlier in life. 

Though some symptoms overlap, and through the progression of the disease, more and more of the symptoms will overlap, there are still some differences in the initial presentation of each leukodystrophy. 

Neuroimaging and diagnostic differences 

In order to diagnose Canavan disease, an MRI scan is often used, in order to visualise the white matter. In Canavan disease, this white matter will be bright and swollen on both sides of the brain. Small nerve fibres that are located just under the outer layer of the brain are called subcortical U fibres, and are usually affected early on in Canavan disease.⁹

This helps clinicians to distinguish Canavan disease from other conditions in which U-fibres are not damaged. An MR spectroscopy might also be run, in which elevated levels of NAA will be detected, as the body is unable to break NAA down properly.   High choline levels might also be present, indicating active demyelination. Finally, genetic testing can confirm the presence of an ASPA mutation. Krabbe disease is identified on an MRI scan with T2 hyperintensity, which is bright areas. This shows areas in which demyelination and inflammation are occurring. This is because the damaged white matter holds onto more water, which means it shows up bright on a scan, as a T2-weighted MRI is sensitive to water.¹⁰ The periventricular regions of the brain are the first ones to show signs of damage due to the buildup of psychosine, and therefore, these are the areas which will appear brighter in the MRI.  A GALC enzyme assay can also be carried out in order to diagnose Krabbe disease. This test measures the levels of the GALC enzyme in the blood, with low levels confirming Krabbe disease.  MLD will have bright spots, especially in the frontal lobe, and will often form a striped (tigroid) pattern. This pattern appears because some of the myelin is spared, meaning that not all areas will be damaged to the same extent. A urine test in order to measure the levels of excreted sulfatides using mass spectrometry may be used in order to confirm MLD. As the level of sulfatides rises, some will be present in the urine.¹¹ ALD will have brightness around the occipital lobes, towards the back of the brain. This is useful to distinguish ALD from the other leukodystrophies, as they often have damage at the front of the brain first. Tests via gas chromatography in order to test for the presence of these long-chain fatty acids. A positive result is when there is an elevated level of C24 and C26 fatty acids.¹² PMD will have the most distinct results on an MRI scan compared to the other leukodystrophies. There will be less of a contrast between the white and grey matter, due to the hypomyelination. Whilst the white matter will appear bright, there will be a blurred edge between the white and grey matter, with no distinct edges.  Genetic testing might also be carried out in order to detect any missense mutation, duplication or deletion of the PLP1 gene.¹³

Prognosis 

For Canavan disease, there is no curative treatment, and life expectancy rarely exceeds adolescence.¹⁴ Symptoms such as seizures can be managed with medication, and physiotherapy can be given in order to manage symptoms such as loss of muscle tone. There is a milder juvenile form that exists, which has a longer survival rate, but these are rare. Death typically occurs around the ages of 5 to 10 years old, though supportive care can extend the lifespan. Unfortunately, Krabbe disease is similar with it is rapidly fatal in its infantile form. Hematopoietic stem cell (HSC) transplantation can slow down the progression of the disease if treatment is started early. This is because these transplanted cells can supply the GALC enzyme, and clear the build up of psychosine, as the galactosylceramide is metabolised. Prognosis is very poor without treatment, with death often occurring at around 3 years of age.¹⁵ MLD is a progressive disease, with symptoms that worsen over time. However, there are gene therapy options that are emerging that could reduce the severity of the disease and potentially prolong one's life.  For example, Libmeldy¹⁶ is an autologous HSC gene therapy that is specialised for each patient using their own blood cells. It is currently used for early-onset MLD to correct the patient's stem cells by inserting a functioning copy of the ARSA gene.¹⁷ ALD  is most severe in its cerebral childhood form, in which death often occurs within 2-5 years of diagnosis without treatment of HSC transplantation. If ALD onset occurs in adulthood, there is a much longer lifespan, with slower progression of the disease. Patients will often need adrenal hormone replacement due to adrenal insufficiency.  PMD has a slow disease progression, with cognition being preserved in milder cases. If the disease is caused by a missense mutation, then often the prognosis is worse. 

Conclusion 

Overall, Canavan disease is distinct from other leukodystrophies, with its early onset macrocephaly, elevated NAA levels and lack of current treatments. It is important to be able to differentiate between the different leukodystrophies in order to supply appropriate treatments, as well as provide genetic counselling. As new advancements are made in gene therapy, there will hopefully be a treatment that slows down the progression of Canavan disease. 

Summary table