Lysosomal storage disease

Lysosomal storage diseases (LSDs; ) are a group of approximately 50 rare inherited metabolic disorders that result from defects in lysosomal function. Lysosomal storage diseases result when a specific organelle in the body's cells – the lysosome – malfunctions.

Lysosomal storage disorders are caused by lysosomal dysfunction usually as a consequence of deficiency of a single enzyme required for the metabolism of lipids, glycoproteins (sugar containing proteins) or so-called mucopolysaccharides. Individually, LSDs occur with incidences of less than 1:100,000; however, as a group the incidence is about 1:5,000 - 1:10,000. Most of these disorders are autosomal recessively inherited, however a few are X-linked recessively inherited, such as Fabry disease and Hunter syndrome (MPS II).

The lysosome is commonly referred to as the cell’s recycling center because it processes unwanted material into substances that the cell can utilize. Lysosomes break down this unwanted matter via enzymes, highly specialized proteins essential for survival. Lysosomal disorders are triggered when a particular enzyme exists in too small an amount or is missing altogether. When this happens, substances accumulate in the cell. In other words, when the lysosome doesn’t function normally, excess products destined for breakdown and recycling are stored in the cell.

Like other genetic diseases, individuals inherit lysosomal storage diseases from their parents. Although each disorder results from different gene mutations that translate into a deficiency in enzyme activity, they all share a common biochemical characteristic – all lysosomal disorders originate from an abnormal accumulation of substances inside the lysosome.

Lysosomal storage diseases affect mostly children and they often die at a young and unpredictable age, many within a few months or years of birth. Many other children die of this disease following years of suffering from various symptoms of their particular disorder.

Symptoms
The symptoms of lysosomal storage disease vary, depending on the particular disorder and other variables like the age of onset, and can be mild to severe. They can include developmental delay, movement disorders, seizures, dementia, deafness and/or blindness. Some people with Lysosomal storage disease have enlarged livers (hepatomegaly) and enlarged spleens (splenomegaly), pulmonary and cardiac problems, and bones that grow abnormally.

Diagnosis
The majority of patients are initially screened by enzyme assay, which is the most efficient method to arrive at a definitive diagnosis. In some families where the disease-causing mutation(s) is known and in certain genetic isolates, mutation analysis may be performed. In addition, after a diagnosis is made by biochemical means, mutation analysis may be performed for certain disorders.

Treatment
There are no cures for lysosomal storage diseases and treatment is mostly symptomatic, although bone marrow transplantation and enzyme replacement therapy (ERT) have been tried with some success. In addition, umbilical cord blood transplantation is being performed at specialized centers for a number of these diseases. In addition, substrate reduction therapy, a method used to decrease the accumulation of storage material, is currently being evaluated for some of these diseases. Furthermore, chaperone therapy, a technique used to stabilize the defective enzymes produced by patients, is being examined for certain of these disorders. The experimental technique of gene therapy may offer cures in the future.

History
Tay-Sachs disease was the first of these disorders to be described, in 1881, followed by Gaucher disease in 1882. In the late 1950s and early 1960s, de Duve and colleagues, using cell fractionation techniques, cytological studies and biochemical analyses, identified and characterized the lysosome as a cellular organelle responsible for intracellular digestion and recycling of macromolecules. This was the scientific breakthrough that would lead to the understanding of the physiological basis of the Lysosomal Storage Diseases. Pompe disease was the first disease to be identified as an LSD in 1963, with L. Hers reporting the cause as a deficiency of α-glucosidase. Hers also suggested that other diseases, such as the Mucopolysaccharidosis, might be due to enzyme deficiencies.

Standard classification
The lysosomal storage diseases are generally classified by the nature of the primary stored material involved, and can be broadly broken into the following: (ICD-10 codes are provided where available)
 * (E75) lipid storage disorders, mainly sphingolipidoses (including Gaucher's and Niemann-Pick diseases)
 * (E75.0-E75.1) gangliosidosis (including Tay-Sachs disease)
 * (E75.2) leukodystrophies
 * (E76.0) mucopolysaccharidoses (including Hunter syndrome and Hurler disease)
 * (E77) glycoprotein storage disorders
 * (E77.0-E77.1) mucolipidoses

Also, Glycogen storage disease type II (Pompe disease) is also a defect in lysosomal metabolism, although it is otherwise classified into E74.0 in ICD-10.

By type of defect protein
Alternatively to the protein targets, lysosomal storage diseases may be classified by the type of protein that is deficient and is causing buildup.

Alphabetical list
Following are lysosomal storage diseases in alphabetical order:

Activator Deficiency/GM2 Gangliosidosis

Alpha-mannosidosis

Aspartylglucosaminuria

Cholesteryl ester storage disease

Chronic Hexosaminidase A Deficiency

Cystinosis

Danon disease

Fabry disease

Farber disease

Fucosidosis

Galactosialidosis

Gaucher Disease
 * Type I
 * Type II
 * Type III

GM1 gangliosidosis
 * Infantile
 * Late infantile/Juvenile
 * Adult/Chronic

I-Cell disease/Mucolipidosis II

Infantile Free Sialic Acid Storage Disease/ISSD

Juvenile Hexosaminidase A Deficiency

Krabbe disease
 * Infantile Onset
 * Late Onset

Lysosomal acid lipase deficiency
 * Early onset
 * Late onset

Metachromatic Leukodystrophy

Mucopolysaccharidoses disorders
 * Pseudo-Hurler polydystrophy/Mucolipidosis IIIA
 * MPSI Hurler Syndrome
 * MPSI Scheie Syndrome
 * MPS I Hurler-Scheie Syndrome
 * MPS II Hunter syndrome
 * Sanfilippo syndrome Type A/MPS III A
 * Sanfilippo syndrome Type B/MPS III B
 * Sanfilippo syndrome Type C/MPS III C
 * Sanfilippo syndrome Type D/MPS III D
 * Morquio Type A/MPS IVA
 * Morquio Type B/MPS IVB
 * MPS IX Hyaluronidase Deficiency
 * MPS VI Maroteaux-Lamy
 * MPS VII Sly Syndrome
 * Mucolipidosis I/Sialidosis
 * Mucolipidosis IIIC
 * Mucolipidosis type IV

Multiple sulfatase deficiency

Niemann-Pick Disease
 * Type A
 * Type B
 * Type C

Neuronal Ceroid Lipofuscinoses
 * CLN6 disease - Atypical Late Infantile, Late Onset variant, Early Juvenile
 * Batten-Spielmeyer-Vogt/Juvenile NCL/CLN3 disease
 * Finnish Variant Late Infantile CLN5
 * Jansky-Bielschowsky disease/Late infantile CLN2/TPP1 Disease
 * Kufs/Adult-onset NCL/CLN4 disease
 * Northern Epilepsy/variant late infantile CLN8
 * Santavuori-Haltia/Infantile CLN1/PPT disease
 * Beta-mannosidosis

Pompe disease/Glycogen storage disease type II Pycnodysostosis Sandhoff disease/Adult Onset/GM2 Gangliosidosis Sandhoff disease/GM2 gangliosidosis - Infantile Sandhoff disease/GM2 gangliosidosis - Juvenile Schindler disease Salla disease/Sialic Acid Storage Disease Tay-Sachs/GM2 gangliosidosis Wolman disease