Krabbe disease, Globoid cell leukodystrophy, GLD, GCL, Globoid cell leukoencephalopathy, Galactosylceramide beta-galactosidase deficiency, Galactocerebrosidase deficiency, GALC deficiency
Krabbe disease, also known as globoid cell leukodystrophy, is a rare autosomal recessive lysosomal disorder affecting the white matter of the central and peripheral nervous systems. Krabbe disease is characterized by a severe and progressive neurodegeneration caused by deficiencies in the lysosomal enzyme galactocerebrosidase (GALC) 1, 2. Krabbe disease occurs in approximately 1 in 100,000 births 1, 2 and the carrier frequency in individuals with no family history is approximately 1 in 150 1.
Krabbe disease is characterized by infantile-onset progressive neurologic deterioration and death before age two years 1, 2. Children with the infantile form appear to be normal for the first few months of life but show extreme irritability, spasticity, and developmental delay before age six months; psychomotor regression progresses to a decerebrate state with no voluntary movement. The onset and progression in the late-onset forms can be quite variable. Individuals can be clinically normal until weakness, vision loss, and intellectual regression become evident. The onset of symptoms and clinical course can be variable even among siblings.
Krabbe disease is caused by defective functioning of lysosomal enzyme β-galactocerebrosidase (β-GALC), encoded from the GALC gene located on 14q31.3 1-3. β-GALC is responsible for accumulation of galactosyl ceramide, resulting in production of toxic compound psychosine, which damages the white matter of the peripheral and central nervous system 3, 4.
There are four forms of Krabbe disease, based on the age of onset: infantile, late infantile, juvenile, and adult form (see Table 1).
Table 1. Clinical forms of Krabbe disease based on age of onset.
|Clinical forms of Krabbe disease||Age of diagnosis||Major clinical signs and symptoms|
|Infantile form||<6 months||Irritability, Muscular hypertonia, Peripheral neuropathy, Hyperpyrexia, Failure to thrive, Vomiting, Developmental delay, Optic atrophy, Blindness, Seizures, MRI detects demyelination, Deficient GALC enzyme activity (0-5% of normal activity)|
|Late infantile form||6 months – 3 years||Normal early development, Paresthesias, Decreased muscle strength, Spasticity, ataxia, paresis, Psychomotor arrest, Optic atrophy, Visual loss, Macular cherry red spots, Macrocephaly, Seizures|
|Juvenile form||2-10 years||Normal early development, Irritability, Rapid psychomotor failure, Spasticity, Ataxia, Seizures|
|Adult form||>10 years||Peripheral neuropathy, Cerebellar dysfunction, Irritability, Spasticity, Ataxia, Seizures, Impaired higher cortical functioning,|
Approximately 85-90% of individuals with Krabbe disease have the infantile form presenting with extreme irritability, spasticity, and developmental delay before age six months 1. The remaining 10-15% have onset between age six months and the seventh decade 1.
Individuals with the infantile form of Krabbe disease can present with any or all of the following features 1:
- Muscle hypertonicity
- Progressive neurologic deterioration
- Peripheral neuropathy
- Evidence of white matter disease on neuroimaging
- Elevation of cerebrospinal fluid (CSF) protein concentration
Individuals with late-onset forms can be clinically asymptomatic before presentations of symptoms such as weakness, vision loss, and intellectual regression.
GALC encodes enzyme galactocerebrosidase. All individuals with Krabbe disease have very low GALC enzyme activity (0-5% of normal activity) in leukocytes isolated from whole heparinized blood and cultured skin fibroblasts 1. Measurement of GALC enzyme activity is best done using the radiolabeled natural substrate galactosylceramide (gal-cer) 1, 5. With improvements in treatment options for presymptomatic individuals, efforts to develop newborn screening methods are underway. One method using dried blood spots and tandem mass spectrometry to measure GALC enzyme activity has been widely accepted for early diagnostics of Krabbe disease 6, 7.
More than 230 pathogenic variants have been identified 1, 2, 11. A large deletion in the GALC gene, a 30-kb deletion, accounts for approximately 45% of the mutant alleles in individuals of European ancestry 1, 8 and 35% of the mutant alleles in individuals of Mexican heritage 1, 2. The 30-kb deletion results in the classic infantile phenotype of Krabbe disease when it is in the homozygous state, or when it is in the compound heterozygous state together with another GALC pathogenic variant known to cause infantile Krabbe disease. In addition to the 30-kb deletion, three other variants are associated with the infantile phenotype and these three represent another 15% of the pathogenic GALC alleles in individuals of European ancestry 1, 9, 10.
Table 2: The most representative and most frequent variants in the GALC gene in individuals of European ancestry 1 (reference transcript numbers: NM_000153.3; NP_000144.2):
|Nucleotide change||Amino acid change||HGMD accession||% of all GALC pathogenic alleles 1|
At CENTOGENE we have analyzed a large number of individuals for GALC gene. 25% of Krabbe disease-suspected individuals had pathogenic variant in GALC gene, while 31% were identified as carriers 13.
Out of all GALC identified pathogenic variants 66% were identified as substitutions, 16% as deletions, 11% as gross/complex rearrangements and 2% as duplications and other type of variants each (Figure 1) 13. GALC classification of variants on protein level identified 48% missense variants, 19% frameshift variants, 11% nonsense, 9% splicing, 7% in-frame and 6% variants with unknown effect (Figure 2) (CentoMD® 4.1) 13.
Figure 1. Types of GALC clinically relevant variants on DNA level (CentoMD® 4.1)
Figure 2. Types of GALC clinically relevant variants on protein level (CentoMD® 4.1)
Unfortunately, there is no cure for Krabbe disease. Preliminary studies and results of clinical trials suggest hematopoietic stem cell transplantation (HSCT) may be an effective treatment in affected presymptomatic newborns and infants. Also, pharmacological chaperone therapy (PCT) is one strategy being explored to overcome defects in galactocerebrosidase enzyme12.
CENTOGENE offers enzymatic testing for galactocerebrosidase enzymatic testing, sequencing and deletion/duplication analysis of the GALC gene. The GALC gene is also part of the following panels:
The differential diagnosis of GALC–related diseases - depending on the major symptoms in the initial case – includes the following diseases:
- Arylsulfatase A deficiency, also known as metachromatic leukodystrophy (ARSA gene)
- GM1 gangliosidosis (GLB1 gene)
- GM2 gangliosidosis (HEXA gene)
- Canavan disease (ASPA gene)
- Saposin A deficiency (PSAP gene)
- X-linked adrenoleukodystrophy (ABCD1 gene)
- Pelizaeus-Merzbacher disease (PLP1 gene)
- Alexander disease (GFAP gene)
To confirm/establish the diagnosis, we offer galactosylceramidase enzymatic testing, GALC gene sequencing and deletion/duplication gene testing. We also offer a broad selection of NGS panels designed for the molecular diagnostics of related conditions/phenotypes.
Thus, CENTOGENE offers the following testing strategy for GALC gene testing:
Step 1: Galactocerebrosidase enzymatic testing.
Step 2: GALC gene sequencing – covers the entire coding region, exon/intron boundaries and 200 bp of the gene promoter
Step 3: Deletion/duplication analysis/variant scanning of GALC
Step 4: If no variant is identified after analysis of the GALC gene, panel testing with related genes or further genetic testing of related genes is recommended.
Step 5: If no variant is identified in any of the panel genes listed, we can offer whole exome sequencing, based on NGS technology.
The following individuals are candidates for GALC gene testing:
- Individuals with a family history of Krabbe disease and presentation of the most common symptoms
- Individuals without a positive family history, but with symptoms resembling Krabbe disease
- Individuals with a negative but suspected family history of Krabbe disease, in order to perform proper genetic counseling (prenatal analyses are recommended in families with affected individuals).
Sequencing, deletion/duplication of the GALC gene should be performed in all individuals suspected of having Krabbe disease. In parallel, other genes reported to be related with this clinical phenotype should also be analyzed for the presence of pathogenic variants due to the overlap in many clinical features caused by those particular genes.
Confirmation of a clinical diagnosis through genetic testing can allow for genetic counseling and may direct medical management.
Genetic counseling can provide a patient and/or family with the natural history of Krabbe disease, identify at-risk family members, provide information about reproductive risks as well as preconception/prenatal options, and allow for appropriate referral for patient support and/or resources.