Lysosomal storage disease

Lysosomal storage diseases (LSDs; /ˌlsəˈsməl/) are a group of over 70 rare inherited metabolic disorders that result from defects in lysosomal function.[1][2]Lysosomes are sacs of enzymes within cells that digest large molecules and pass the fragments on to other parts of the cell for recycling. This process requires several critical enzymes. If one of these enzymes is defective due to a mutation, the large molecules accumulate within the cell, eventually killing it.[3]

Medical condition
Lysosomal storage disease
Micrograph of Gaucher disease, with cells that have the characteristic crumpled tissue paper-like cytoplasm. H&E stain.
Specialty Endocrinology 

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.[4][5] Most of these disorders are autosomal recessively inherited such as Niemann–Pick disease, type C, but 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 use. Lysosomes break down this unwanted matter by enzymes, highly specialized proteins essential for survival. Lysosomal disorders are usually 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 does not function normally, excess products destined for breakdown and recycling are stored in the cell.

Like other genetic disorders, 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.

LSDs affect mostly children and they often die at a young age, many within a few months or years of birth.

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The LSDs 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)

Also, glycogen storage disease type II (Pompe disease) is a defect in lysosomal metabolism as well,[6] although it is otherwise classified into E74.0 in ICD-10. Cystinosis is an LSD characterized by the abnormal accumulation of the amino acid cystine.

Alternatively to the protein targets, LSDs may be classified by the type of protein that is deficient and is causing buildup.

Type of defect protein Disease examples Deficient protein
Lysosomal enzymes primarily Tay–Sachs disease, I-cell disease,[7]Sphingolipidoses (e.g., Krabbe disease, gangliosidosis: Gaucher, Niemann–Pick disease and glycolipids: Metachromatic leukodystrophy), Lysosomal acid lipase deficiency Various
Posttranslational modification of enzymes Multiple sulfatase deficiency Multiple sulfatases
Membrane transport proteins Mucolipidosis type II and IIIA N-acetylglucosamine-1-phosphate transferase
Enzyme protecting proteins Galactosialidosis Cathepsin A
Soluble nonenzymatic proteins GM2-AP deficiency, variant AB, Niemann–Pick disease, type C2 GM2-AP, NPC2
Transmembrane proteins SAP deficiency Sphingolipid activator proteins
Niemann–Pick disease, type C1 NPC1
Salla disease Sialin
Unless else specified in boxes, then the applicable reference is:[8]

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