Krabbe disease: increased incidence in a highly inbred community

Nanomedicines to treat rare neurological disorders: The case of Krabbe disease

The brain remains one of the most challenging therapeutic targets due to the low and selective permeability of the blood-brain barrier and complex architecture of the brain tissue. Nanomedicines, despite their relatively large size compared to small molecules and nucleic acids, are being heavily investigated as vehicles to delivery therapeutics into the brain. Here we elaborate on how nanomedicines may be used to treat rare neurodevelopmental disorders, using Krabbe disease (globoid cell leukodystrophy) to frame the discussion. As a monogenetic disorder and lysosomal storage disease affecting the nervous system, the lessons learned from examining nanoparticle delivery to the brain in the context of Krabbe disease can have a broader impact on the treatment of various other neurodevelopmental and neurodegenerative disorders. In this review, we introduce the epidemiology and genetic basis of Krabbe disease, discuss current in vitro and in vivo models of the disease, as well as current therapeutic approaches either approved or at different stage of clinical developments. We then elaborate on challenges in particle delivery to the brain, with a specific emphasis on methods to transport nanomedicines across the blood-brain barrier. We highlight nanoparticles for delivering therapeutics for the treatment of lysosomal storage diseases, classified by the therapeutic payload, including gene therapy, enzyme replacement therapy, and small molecule delivery. Finally, we provide some useful hints on the design of nanomedicines for the treatment of rare neurological disorders.

Galactocerebrosidase activity by liquid-chromatography tandem mass spectrometry for clinical diagnosis of Krabbe disease

BACKGROUND

Deficiency of galactosylcerebrosidase (GALC) causes Krabbe disease. Historically, a diagnosis is made by measuring GALC enzymatic activity with a radioisotope assay. To improve the workflow and performance, we developed and clinically validated a leukocyte enzymatic assay using liquid chromatography tandem mass spectrometry (LC-MS/MS).

MATERIALS

Extracted cell lysates were quantified and incubated with commercially available multiplexed substrates and internal standards. Liquid-liquid extraction was performed, and pre-analytical and analytical variability were evaluated and validated following clinical laboratory regulation guidelines.

RESULTS

Enzymatic reaction products were resolved from substrate breakdown products by a 3.5-minute column separation. Intra- and inter- assay imprecision were less than 15%. No matrix effects or carryover were observed. ACD anticoagulant tubes provide the best sample stability. Detection of product was linear with an R² of 0.99. Small differences in GALC activity were measurable near the anticipated disease range. Confirmed cases of Krabbe disease were well differentiated from carriers and non-Krabbe individuals (normal reference range).

CONCLUSION

An LC-MS/MS assay was developed, which can measure trace residual GALC activity in leukocytes and aid in the diagnosis of Krabbe disease. The multiplexed mixture allows for built-in sample quality control and enables a streamlined workflow for evaluation of multiple lysosomal storage diseases.

Screening for Krabbe disease: The first 2 years' experience

OBJECTIVES

Globoid cell leukodystrophy or Krabbe disease is an autosomal recessive lysosomal storage disorder characterized by a deficiency in galactosylceramidase (GALC) which hydrolyses galactosylceramide and galactosylsphingosine (psychosine). The accumulation of psychosine results in the apoptosis of myelin-forming cells. The goals of this research were to identify the heterozygous carriers of Krabbe disease in Sicily (Italy), to prevent the birth of foetuses affected by this disease, and eventually in the presence of positive embryos to direct them towards a treatment before symptoms occur when it is too late to receive a useful therapy.

METHODS

Since more than 100 mutations have been reported as a cause of Krabbe disease, we started to screen relatives of the affected patients, whose mutation was known. We used a fast, sensitive and painless assay extracting genomic DNA from buccal swabs. The genotypes of single-nucleotide polymorphisms (SNPs) were analysed to identify the carriers of the selected mutations.

RESULTS

In the last 2 years, we conducted the analysis of almost 100 subjects and individuated 40 heterozygotes carriers of Krabbe disease. One of the women examined was pregnant.

CONCLUSIONS

The knowledge obtained from our investigations provided and will provide notable practical benefit to families in which the disease is manifested and to researchers who deal with this rare pathology. Finally, the results of our study will be useful to know the real incidence of Krabbe disease in a large territory where it is particularly present and to start a Krabbe's register, which at present does not exist.

Phenotypic variability of Krabbe disease across the lifespan

Krabbe disease (galactocerebrosidase deficiency) is an inherited leukodystrophy that results in severe neurological defects due to altered myelination. Classically, disease onset is within the first year of life. Juvenile and adult-onset cases may have less classic presentations, making diagnosis difficult and often delayed. Here, we review the literature to demonstrate the hetereogeneity of presenting symptoms across all age groups. We also discuss diagnostic approach, emphasizing variation in biochemical, functional, and genetic results among Krabbe phenotypes. Better understanding of the various Krabbe disease phenotypes is critical to facilitate timely diagnosis and appropriate treatment of this clinically heterogeneous disorder.

Variabilité phénotypique dans la maladie de Krabbe au cours de la vie du patient. La maladie de Krabbe (déficit en galactocérébrosidase) est une leukodystrophie héréditaire qui donne lieu à des déficits neurologiques sévères dus à un trouble de la myélinisation. Chez les cas dont la présentation est classique, la maladie débute au cours de la première année de vie. Si la maladie commence chez un adolescent ou un adulte, le mode de présentation peut-ětre moins classique, ce qui rend le diagnostic difficile et souvent tardif. Nous analysons les articles traitant du sujet pour démontrer l'hétérogénéité des symptômes au moment de la première consultation et ceci dans tous les groupes d'âge. Nous discutons également de l'approche diagnostique en mettant l'emphase sur la variation des résultats biochimiques, fonctionnels et génétiques des différents phénotypes dans la maladie de Krabbe. Une meilleure compréhension des différents phénotypes est cruciale pour faciliter un diagnostic précoce et un traitement approprié de cette maladie dont le mode de présentation clinique est hétérogène.

Ethical and policy issues in newborn screening of children for neurologic and developmental disorders

Genetic testing for neurologic and developmental disorders spans the spectrum from universal newborn screening for conditions like phenylketonuria to diagnostic testing for suspected genetic conditions, to predictive genetic testing for childhood-onset conditions. Given that virtually all children in the United States undergo genetic screening in the newborn period, this article focuses on 3 actual case studies of neurologic and developmental disorders that have been included or proposed for inclusion in newborn screening programs: Duchenne muscular dystrophy (a neuromuscular disorder), Krabbe disease (a neurodegenerative disorder), and fragile X syndrome (a neurodevelopmental disorder).

Krabbe Disease in the Arab World

The autosomal recessive inherited Krabbe disease (KD) is a devastating pediatric lysosomal storage disorder affecting white matter of the brain. It is caused by mutations in the gene coding for the lysosomal enzyme galactocerebrosidase. While most patients present with symptoms within the first 6 months of life, others present later in life throughout adulthood. The early infantile form of KD (EIKD) is frequent in the Muslim Arab population in Israel, with a very high prevalence of approximately 1/100 to 1/150 live births. The homozygous variant c.1582G > A (p.D528N) was found to be responsible for EIKD in Palestinian Arab patients. KD was reported in different Arab countries with much lower frequency. While most Arab patients presented with EIKD, late infantile and late onset KD forms were also reported. Most Arab patients presented with variable symptoms ranging from EIKD to late onset KD, with variable clinical findings. Based on literature studies, this review focuses on the clinical and molecular findings of KD patients with Arab ancestry, and highlights the need for developing universal genetic screening programs to overcome the under-reported status of KD prevalence in Arabia. This is expected to improve the prognosis of the disease and promote targeted molecular diagnostics to the Arab patients.

A review of the diverse genetic disorders in the Lebanese population: highlighting the urgency for community genetic services

The review lists the genetic diseases reported in Lebanese individuals, surveys genetic programs and services, and highlights the absence of basic genetic health services at the individual and community level. The incidence of individual diseases is not determined, yet the variety of genetic diseases reported is tremendous, most of which follow autosomal recessive inheritance reflecting the social norms in the population, including high rates of consanguinity, which favor the increase in incidence of these diseases. Genetic services including all activities for the diagnosis, care, and prevention of genetic diseases at community level are extremely inadequate. Services are limited to some clinical and laboratory diagnostic services with no genetic counseling. These services are localized within the capital thus preventing their accessibility to high-risk communities. Screening programs, which are at the core of public health prevention services, are minimal and not nationally mandated. The absence of adequate genetic services is attributed to many factors undermining the importance of genetic diseases and their burden on society, the most important of which is genetic illiteracy at all levels of the population, including high-risk families, the general public, and most importantly health care providers and public health officials. Thus, a country like Lebanon, where genetic diseases are expected to be highly prevalent, is in utmost need for community genetics services. Strategies need to be developed to familiarize public health officials and medical professionals with medical genetics leading to a public health infrastructure that delivers community genetics services for the prevention and care of genetic disorders at community level.

Neuronopathic lysosomal storage diseases: clinical and pathologic findings

BACKGROUND

The lysosomal-autophagocytic system diseases (LASDs) affect multiple body systems including the central nervous system (CNS). The progressive CNS pathology has its onset at different ages, leading to neurodegeneration and early death.

METHODS

Literature review provided insight into the current clinical neurological findings, phenotypic spectrum, and pathogenic mechanisms of LASDs with primary neurological involvement.

CONCLUSIONS

CNS signs and symptoms are variable and related to the disease-specific underlying pathogenesis. LAS dysfunction leads to diverse global cellular consequences in the CNS ranging from specific axonal and dendritic abnormalities to neuronal death. Pathogenic mechanisms for disease progression vary from impaired autophagy, massive storage, regional involvement, to end-stage inflammation. Some of these features are also found in adult neurodegenerative disorders, for example, Parkinson's and Alzheimer's diseases. Lack of effective therapies is a significant unmet medical need.

Lifetime risk estimators in epidemiological studies of Krabbe Disease: Review and Monte Carlo comparison

This review addresses difficulties arising in estimating epidemiological parameters of leukodystrophies and lysosomal storage disorders, with special focus on Krabbe disease. Although multiple epidemiological studies of Krabbe disease have been published, these studies are difficult to reconcile since they have used different study populations and varying methods of calculation. Confusion exists regarding which epidemiological parameters have been estimated; the current review shows that most previous estimates can be properly interpreted as lifetime risk at birth. One of the most common estimation methods is shown to be inaccurate, while two other methods are shown to be approximately accurate. Based on the results of the current paper, recommendations are made that are expected to improve the quality of future studies of Krabbe disease. It is anticipated that these recommendations will be applicable to epidemiological studies of other lysosomal storage disorders, as well as any other rare diseases diagnosed with enzymatic screening.

Plasmalogens the neglected regulatory and scavenging lipid species

Plasmalogens are a class of phospholipids carrying a vinyl ether bond in sn-1 and an ester bond in sn-2 position of the glycerol backbone. Although they are widespread in all tissues and represent up to 18% of the total phospholipid mass in humans, their physiological function is still poorly understood. The aim of this review is to give an overview over the current knowledge in plasmalogen biology and pathology with an emphasis on neglected aspects of their involvement in neurological and metabolic diseases. Furthermore a better understanding of plasmalogen biology in health and disease could also lead to the development of better diagnostic and prognostic biomarkers for vascular and metabolic diseases such as obesity and diabetes mellitus, inflammation, neuro-degeneration and cancer.

Molecular genetics of Krabbe disease (globoid cell leukodystrophy): diagnostic and clinical implications

Galactocerebrosidase (GALC) is a lysosomal beta-galactosidase responsible for the hydrolysis of the galactosyl moiety from several galactolipids, including galactosylceramide and psychosine. The deficiency of this enzyme results in the autosomal recessive disorder called Krabbe disease. It is also called globoid cell leukodystrophy (GLD), because of the characteristic storage cells found around cerebral blood vessels in the white matter of affected human patients and animal models. Although most patients present with clinical symptoms before 6 months of age, older patients, including adults, have been diagnosed by their severe deficiency of GALC activity. More than 40 mutations have been identified in patients with all clinical types of GLD. While some mutations clearly result in the infantile type if found homozygous or with another severe mutation, it is difficult to predict the phenotype of novel mutations or when mutations are found in the heterozygous state. A high incidence of polymorphic changes on apparent disease-causing alleles also complicates the interpretation of the effects of mutations. The detection of mutations has greatly improved carrier identification among family members and will permit preimplantation diagnosis for some families. The molecular characterization of the naturally occurring mouse, dog, and monkey models will permit their use in trials to evaluate different modes of therapy.

Two different mutations are responsible for Krabbe disease in the Druze and Moslem Arab populations in Israel

Infantile Krabbe disease is a severe, fatal autosomal recessive disorder resulting from the deficiency of galactocerebrosidase (GALC) activity. It is relatively common in two separate inbred communities in Israel. In the Druze community in Northern Israel and two Moslem Arab villages located near Jerusalem the incidence of Krabbe disease is about 1 in 100-150 live births. With our cloning of the GALC gene, mutation analysis of these populations was undertaken. The Moslem Arabs were homozygous for two mutations in the GALC gene; a T-to-C transition at CDNA position 1637 (counting from the A of the initiation codon), which is considered a polymorphism and a G-to-A transition at position 1582, which changes the codon for aspartic acid to one for asparagine. The Druze patients are homozygous for a T-to-G transversion at position 1748, which changes the codon for isoleucine to one for serine. Expression studies confirmed the deleterious nature of these mutations. The development of a simple polymerase chain reaction (PCR) amplification and restriction enzyme digestion method to identify these alleles will lead to accurate carrier testing and improved genetic counseling for interested individuals in these communities.

Intrafamilial variability in fucosidosis

Two families with five patients affected with fucosidosis are described. Within each family, both type I and type II fucosidosis are present. This suggests that environmental factors or "modifying genes" different from the fucosidase structural gene contribute to the phenotype of a fucosidosis patient.

Intrafamilial variability in lysosomal storage diseases

In most lysosomal storage diseases clinical variability is found and affects age-of-onset, severity, and the degree of neurological involvement. Very often the variability is due to the existence of different mutations leading to the same enzyme deficiency. In most of the families with more than one person affected, the clinical picture is very similar. Intrafamilial variation has been reported in the lysosomal storage diseases related or not related to genetic heterogeneity. In families in which different affected persons have the same genotype, as in X-linked disorders, or autosomal recessive diseases in which both parents of the affected sibs are carriers and healthy, the variability must be due to factors not related to the genotype. On the other hand, when different mutations are present in the same family, the variability may be related to the differences in the genotype of the affected persons. Knowledge of the possibility of intrafamilial variability and its genetic basis is essential in clinical and prenatal diagnosis of the lysosomal storage diseases.