LEUKODYSTROPHIES

Leukodystrophies (LD’s) are genetic disorders caused by mutations of genes whose products are involved in generation, turnover or maintenance of myelin
Myelin is composed of about 70% lipids and 30% proteins.
Central nervous system (CNS) and peripheral nervous system (PNS) myelin have similar lipids but significantly different proteins. Therefore LD’s involving myelin lipids tend to affect CNS and PNS, while LD’s caused by myelin protein abnormalities affect CNS only.
The progressive and cumulative damage to myelinated fibres has devastating consequences.
Leukodystrophies typically present with an insidious and progressive loss of cerebral function, often at younger ages.
Many of leukodystrophies are caused by single enzyme defects resulting in
- altered metabolism of myelin associated lipids
- other forms are caused by several other mechanisms including
  - mutations in genes that encode proteins required for myelin formation or intermediate filament proteins such as glial fibrillary acidic protein (GFAP) or genes for various subunits of translation initiation factor eukaryotic Translation initiation factor 2B (eIF2B)
  - Some LD’s affect cells like astrocytes resulting in indirect damage to myelin.
The following table shows the summary of most commonly known leukodystrophies

Krabbe disease/Globoid cell leukodystrophy
- It is an autosomal recessive leukodystrophy resulting from deficiency of galactocerebroside β-galactosidase, the enzyme required for the catabolism of galactocerebroside to ceramide and galactose
- As a consequence of impaired catabolism of galactocerebroside in the brain, an alternative catabolic pathway shunts galactocerebroside to galactosylphingosine
- Elevated levels of galactosylphinosine are cytotoxic and causes injury to oligodendrocytes
- The clinical course is rapidly progressive, with onset of symptoms often between the ages of 3 and 6 months
- Survival beyond 2 years of age is uncommon
- The clinical symptoms are dominated by motor signs such as stiffness and weakness
- The brain shows loss of myelin and oligodendrocytes in the CNS and PNS
- Neurons and axons are relatively spared
- A unique and diagnostic feature of Krabbe disease is aggregation of engorged macrophage – Globoid cells, in the brain parenchyma and around blood vessels.
- Hematopoietic stem cell transplantation, which allows repopulation of the CNS with enzymatically competent microglia, benefits the patient, particularly when performed before the neurological deficits appear. (This is also true for metachromatic leukodystrophy).

Metachromatic leukocystrophy
- It is an autosomal recessive disease that results from deficiency of the lysosomal enzyme aryl sulfatase A
- This enzyme cleaves the sulfate from sulfate containing lipids (sulfatides) as the first step in their degradation
- Therefore ASA deficiency leads to accumulation of sulfatides, especially cerebroside sulfate, which have a range of biological actions that can contribute to white matter injury, including inhibiting differentiation of oligodendrocytes and eliciting a proinflammatory response from microglia and astrocytes
- The most striking histologic finding is demyelination with resulting gliosis-
  - Macrophages with vacuolated cytoplasm are scattered throughout the white matter
  - The membrane bound vacuoles contain complex crystalloid structures composed of sulfatides.
  - When bound to certain dyes such as toluidine blue, sulfatides shift the absorbance spectrum of the dye, a property called metachromasia. Similar metachromatic material can be detected in peripheral nerves and in the urine, the later being a sensitive method of establishing the diagnosis.

Adrenoleukodystrophy
- It is an x-linked recessive disease associated with mutations in a member of the ATP-binding cassette transporter family of proteins (ABCD1) which is involved in the transport of molecules into the peroxisome
- An allelic disorder presents in adults as a slowly progressive predominantly peripheral nerve disorder known as Adrenomyeloneuropathy
- In the typical form of the disease, young males present with behavioural changes and adrenal insufficiency
- The disease is characterised by the inability to catabolize very long chain fatty acids (VLCFAs) within peroxisomes, resulting in elevated levels of VLCFAs in serum.
- The symptoms result from progressive loss of myelin in the CNS and PNS, as well as adrenal insufficiency
- In the white matter, there is loss of myelin accompanied by gliosis and extensive lymphocytic infiltration
- Atrophy of adrenal cortex is present and VLCFA accumulation can be seen in remaining cells.

Pelizaeus – Merzbacher disease (PMD)
- PMD is x-linked leukodystrophy caused by abnormalities of the major CNS myelin protein, proteolipid protein 1 (PLP1) and its spliced isoform DM2O
- It is caused by duplications, deletions and point mutations of PLP1 gene located an X_q 21-22
- PMD was subtyped on a morphological and clinical basis into
  - Connatal
  - Classic variant
  - Mild form (which is allelic with x-linked spastic paraplegic type 2)
- The essential pathological process is similar in all cases. The clinical onset is almost always in first year of life with variable rate of progression
- Boys present with nystagmus, hypotonia, progressive spastic paraparesis and movement disorder without evidence of peripheral demyelination
- Thereafter, the disorder is variable with progressive motor disorder, mental retardation and often dementia
- Female carriers of the disease are usually normal. Some females with point mutations may have mild symptoms
- Histologic features shows
  - preserved grey matter with either predominant granule or purkinje cell degeneration
  - In absence of myelin, to the classicial trigoid or discontinuous pattern with preserve perivascular islets
  - Oligodendrocytes are markedly reduced or absent
  - There is astrocytosis, fibrillary gliosis and usually only sparse sudanophilic lipid in perivascular macrophages
  - Axons are preserved, although most are naked, myelin discontinuities can be observed
- Spinal and cranial nerve roots, which have a different myelin structural protein (PMP-22) are normally myelinated.
Alexander disease
- Alexander disease (AD) is caused by heterozygous mutations in the GFAP gene (Glial fibrillary acidic protein) located an 17q21
- GFAP is the protein of intermediate astrocytic filaments
- In this disease GFAP is overproduced and conjugated with abnormally phospharylated and partly ubiquitinated heat shock protein, α-β-crystallin
- This GFAP gets deposited in the astrocytic cytoplasm and processes as inclusions called Rosenthal fibres (RF)
- Thus, Alexander disease appears to be a disorder of astrocytes, primarily. This disease is clinically recognized as three forms
  - Infantile: It is typical form presenting before 2 years of age with hydrocephalus, megalencephaly and seizures ultimately progressing to quadriparesis
  - Juvenile: It shows a more bulbar and cerebellar presentation with later cognitive impairment
  - Adult form: It shows slow progression with ataxia, quadriparesis, an eye movement disorder, palatal myoclonus and late cognitive decline. Finally, the later onset form may present with a posterior fossa mass.

- The principal microscopic feature is
  - extensive accumulation of Rosenthal fibres throughout the brain
  - Demyelination is variable
  - Fat laden macrophages, cyst formation, occasional perivascular lymphocytic cuffs and calcospherites may also occur
  - Under electron microscopy, RF’s appear as amorphous osmophilic granular material surrounded by 10nm filaments
  - With conventional stains, they appear as rod and club shaped hyaline eosinophilic structures
  - RF’s are most numerous in subependymal, subpial and perivascular locations with marked involvement of cerebral white matter, thalamus and basal ganglia
  - RF’s formation is more intense in brain stem nuclei and tracts, cerebellar white matter, thalamus and basal ganglia, proximal part of optic nerves and grey and white matter of spinal cord
  - Cerebral cortex and cerebellar cortex are usually spared
  - In general neurons are preserved, even in severely affected areas
  - Immunohistochemistry demonstrates peripheral staining with antibodies to
    - GFAP
    - α-β-crystallin
    - 27kDa heat shock protein and ubiquitin

Canavan disease/Spongy leukodystrophy/Van Bogaert – Bertrand disease:
- Canavan disease is autosomal recessive disorder caused by deficiency of ASPA, an enzyme necessary for catabolism of N-acetyl aspartate (NAA) and N-acetylaspartylglutamate (NAAG)
- This leads to accumulation of these substrates in neurons and oligodendroglial precursors, which leads to neuronal dysfunction and myelin deficicney
- Onset of disease is normally in the first 6 months of life
- First symptoms include poor visual fixation, irritability and poor sucking
- Developmental stagnation, nystagmus and macrocephaly develop during 1^st year of life
- Epilepsy usually manifests later
- In the typical infantile form, brain weight is usually 50% heavier in the first 2 years of life, but later decreases to normal levels as cerebral atrophy progresses
- Histologically a fine vacuolation with myriad empty
- Histochemically negative spaces, extensively involves white matter of cerebral hemispheres, including corpus callosum, capsules, fornix, optic nerves, cerebellum and long tracts of brian stem and spinal cord. Myelin staining is not seen, but IHC demonstrate myelin basic proteins lining the vacuoles
- The central cerebral and cerebellar white matter is replaced by a loose mass of astrocytes, macrophages and capillaries vacuole formation is particularly intense at cortical grey white junction and spreads into deeper layers of cortex, with reactive astrocytes and Alzheimer II glia
- Another zone of prominent spongy change is boundary between cerebellar molecular and granular cell layers
- Ultrastructural studies show that vacuoles in white matter are large electron lucent spaces surrounded by myelin leaflets, which appear to be formed by splitting of myelin lamellar at intraperiod line
- In cortex, cell bodies and processes of astrocytes are markedly swollen and their watery cytoplasm contains extremely elongated mitochondria
- The diagnosis of canavan disease can be made by measuring NAA levels in urine, blood and CSF, by assaying aspartoacyclase activity in cultured fibroblasts and molecular methods.

Vanishing white matter disease (VWM)/childhood ataxia with central hypomyelination/ myelinopathia centralis diffusa
- VWM is a autosomal recessive disorder caused by mutations of genes encoding any of the 5 subunits of translation initiation factor EIF2B (EIF2B1-EIF2B5)
- EIF2B regulates the rate limiting step for the initiation of protein translation. However, this disease is not caused by an overall effect on protein translation
- An alternative mechanism is that EIF2B has role in regulation of unfolded protein response (UPR) a cellular stress mechanism that responds to misfolded or denatured proteins
- The UPR system is activated in glia of patients with VWM
- On further activation during cellular stress may lead to an exaggerated stress response, ultimately leading to cell death
- Early development is normal, first symptoms usually develops between 18 months and 5 years, presenting with cerebellar-ataxia, progressive ataxic spastic disorder, later optic atrophy and occasionally epilepsy
- Adult onset and adolescent onset presents with epileptic seizures, complicated migraine, cognitive impairment, psychiatric symptoms and ovarian failure (in women)
- The MRI shows progressive water accumulation in the white matter leading to cyst formation
- The brain looks normal externally
- In cut section, the cerebral cortex is normal. The white matter has a gelatinous consistency and is cystic. In advanced stages, there is severe loss of white matter
- Microscopic examination shows loss of myelin and axons, limited number of reactive astrocytes with atypical cytological features and no inflammation. There is a peculiar increase of oligodendrocytes around cavitated areas and some of them having foamy vacuolated cytoplasm. Inspite of severe white matter loss, the cortex and grey matter structures are normal

By-

LEUKODYSTROPHIES

Leukodystrophies (LD’s) are genetic disorders caused by mutations of genes whose products are involved in generation, turnover or maintenance of myelin

Myelin is composed of about 70% lipids and 30% proteins.

Central nervous system (CNS) and peripheral nervous system (PNS) myelin have similar lipids but significantly different proteins. Therefore LD’s involving myelin lipids tend to affect CNS and PNS, while LD’s caused by myelin protein abnormalities affect CNS only.

The progressive and cumulative damage to myelinated fibres has devastating consequences.

Leukodystrophies typically present with an insidious and progressive loss of cerebral function, often at younger ages.

Many of leukodystrophies are caused by single enzyme defects resulting in

altered metabolism of myelin associated lipids

other forms are caused by several other mechanisms including

mutations in genes that encode proteins required for myelin formation or intermediate filament proteins such as glial fibrillary acidic protein (GFAP) or genes for various subunits of translation initiation factor eukaryotic Translation initiation factor 2B (eIF2B)

Some LD’s affect cells like astrocytes resulting in indirect damage to myelin.

The following table shows the summary of most commonly known leukodystrophies

Krabbe disease/Globoid cell leukodystrophy

It is an autosomal recessive leukodystrophy resulting from deficiency of galactocerebroside β-galactosidase, the enzyme required for the catabolism of galactocerebroside to ceramide and galactose

As a consequence of impaired catabolism of galactocerebroside in the brain, an alternative catabolic pathway shunts galactocerebroside to galactosylphingosine

Elevated levels of galactosylphinosine are cytotoxic and causes injury to oligodendrocytes

The clinical course is rapidly progressive, with onset of symptoms often between the ages of 3 and 6 months

Survival beyond 2 years of age is uncommon

The clinical symptoms are dominated by motor signs such as stiffness and weakness

The brain shows loss of myelin and oligodendrocytes in the CNS and PNS

Neurons and axons are relatively spared

A unique and diagnostic feature of Krabbe disease is aggregation of engorged macrophage – Globoid cells, in the brain parenchyma and around blood vessels.

Hematopoietic stem cell transplantation, which allows repopulation of the CNS with enzymatically competent microglia, benefits the patient, particularly when performed before the neurological deficits appear. (This is also true for metachromatic leukodystrophy).

Metachromatic leukocystrophy

It is an autosomal recessive disease that results from deficiency of the lysosomal enzyme aryl sulfatase A

This enzyme cleaves the sulfate from sulfate containing lipids (sulfatides) as the first step in their degradation

The most striking histologic finding is demyelination with resulting gliosis-

Macrophages with vacuolated cytoplasm are scattered throughout the white matter

The membrane bound vacuoles contain complex crystalloid structures composed of sulfatides.

When bound to certain dyes such as toluidine blue, sulfatides shift the absorbance spectrum of the dye, a property called metachromasia. Similar metachromatic material can be detected in peripheral nerves and in the urine, the later being a sensitive method of establishing the diagnosis.

Adrenoleukodystrophy

It is an x-linked recessive disease associated with mutations in a member of the ATP-binding cassette transporter family of proteins (ABCD1) which is involved in the transport of molecules into the peroxisome

An allelic disorder presents in adults as a slowly progressive predominantly peripheral nerve disorder known as Adrenomyeloneuropathy

In the typical form of the disease, young males present with behavioural changes and adrenal insufficiency

The disease is characterised by the inability to catabolize very long chain fatty acids (VLCFAs) within peroxisomes, resulting in elevated levels of VLCFAs in serum.

The symptoms result from progressive loss of myelin in the CNS and PNS, as well as adrenal insufficiency

In the white matter, there is loss of myelin accompanied by gliosis and extensive lymphocytic infiltration

Atrophy of adrenal cortex is present and VLCFA accumulation can be seen in remaining cells.

Pelizaeus – Merzbacher disease (PMD)

PMD is x-linked leukodystrophy caused by abnormalities of the major CNS myelin protein, proteolipid protein 1 (PLP1) and its spliced isoform DM2O

It is caused by duplications, deletions and point mutations of PLP1 gene located an Xq 21-22

PMD was subtyped on a morphological and clinical basis into

Connatal

Classic variant

Mild form (which is allelic with x-linked spastic paraplegic type 2)

The essential pathological process is similar in all cases. The clinical onset is almost always in first year of life with variable rate of progression

Boys present with nystagmus, hypotonia, progressive spastic paraparesis and movement disorder without evidence of peripheral demyelination

Thereafter, the disorder is variable with progressive motor disorder, mental retardation and often dementia

Female carriers of the disease are usually normal. Some females with point mutations may have mild symptoms

Histologic features shows

preserved grey matter with either predominant granule or purkinje cell degeneration

In absence of myelin, to the classicial trigoid or discontinuous pattern with preserve perivascular islets

Oligodendrocytes are markedly reduced or absent

There is astrocytosis, fibrillary gliosis and usually only sparse sudanophilic lipid in perivascular macrophages

Axons are preserved, although most are naked, myelin discontinuities can be observed

Spinal and cranial nerve roots, which have a different myelin structural protein (PMP-22) are normally myelinated.

Alexander disease

Alexander disease (AD) is caused by heterozygous mutations in the GFAP gene (Glial fibrillary acidic protein) located an 17q21

GFAP is the protein of intermediate astrocytic filaments

In this disease GFAP is overproduced and conjugated with abnormally phospharylated and partly ubiquitinated heat shock protein, α-β-crystallin

This GFAP gets deposited in the astrocytic cytoplasm and processes as inclusions called Rosenthal fibres (RF)

Thus, Alexander disease appears to be a disorder of astrocytes, primarily. This disease is clinically recognized as three forms

Infantile: It is typical form presenting before 2 years of age with hydrocephalus, megalencephaly and seizures ultimately progressing to quadriparesis

Juvenile: It shows a more bulbar and cerebellar presentation with later cognitive impairment

Adult form: It shows slow progression with ataxia, quadriparesis, an eye movement disorder, palatal myoclonus and late cognitive decline. Finally, the later onset form may present with a posterior fossa mass.

The principal microscopic feature is

extensive accumulation of Rosenthal fibres throughout the brain

Demyelination is variable

Fat laden macrophages, cyst formation, occasional perivascular lymphocytic cuffs and calcospherites may also occur

Under electron microscopy, RF’s appear as amorphous osmophilic granular material surrounded by 10nm filaments

With conventional stains, they appear as rod and club shaped hyaline eosinophilic structures

RF’s are most numerous in subependymal, subpial and perivascular locations with marked involvement of cerebral white matter, thalamus and basal ganglia

RF’s formation is more intense in brain stem nuclei and tracts, cerebellar white matter, thalamus and basal ganglia, proximal part of optic nerves and grey and white matter of spinal cord

Cerebral cortex and cerebellar cortex are usually spared

In general neurons are preserved, even in severely affected areas

Immunohistochemistry demonstrates peripheral staining with antibodies to

GFAP

α-β-crystallin

27kDa heat shock protein and ubiquitin

Canavan disease/Spongy leukodystrophy/Van Bogaert – Bertrand disease:

It is caused by duplications, deletions and point mutations of PLP1 gene located an X_q 21-22

Developmental stagnation, nystagmus and macrocephaly develop during 1^st year of life