Niemann-Pick Disease, What is It? Why is it a Child Killer?

Niemann-Pick disease is an inherited condition involving lipid metabolism (the breakdown and use of fats and cholesterol in the body) in which harmful amounts of lipids accumulate in the spleen, liver, lungs, bone marrow, and brain.

There are three variants of Niemann-Pick disease based on the genetic cause and the symptoms exhibited by the patient. All variants are inherited in an autosomal recessive pattern.

Mutations in the NPC1, NPC2, and SMPD1 genes cause Niemann-Pick disease.

This condition is inherited in an autosomal recessive pattern, which means two copies of the gene must be altered for a person to be affected by the disorder. Most often, the parents of a child with an autosomal recessive disorder are not affected but are carriers of one copy of the altered gene. If both parents are carriers, there is a one in four, or 25%, chance with each pregnancy for an affected child. Genetic counseling and genetic testing is recommended for families who may be carriers of Niemann-Pick.

Type C is characterized by onset in childhood, although infant and adult onsets are possible. Other signs include severe liver disease, breathing difficulties, developmental delay, seizures, increased muscle tone (dystonia), lack of coordination, problems with feeding, and an inability to move the eyes vertically. People with this disorder can survive into adulthood. The incidence of Niemann-Pick disease, type C is estimated to be 1 in 150,000 people. The disease occurs more frequently in people of French-Acadian descent in Nova Scotia.

Mutations in either the NPC1 or NPC2 gene cause Niemann-Pick disease, type C. The NPC1 gene produces a protein that is located in membranes inside the cell and is involved in the movement of cholesterol and lipids within cells. A deficiency of this protein leads to the abnormal build up of lipids and cholesterol within cell membranes.

The molecular basis for this disease is extremely complex due to the role that endosome formation has on affected patients. Recently, three theories have attempted to explain the buildup of cholesterol in the lysosomes of affected patients of Niemann-Pick Disease Type C due to the malfunction of the protein NPC-1.

* The contention by Neufel et al is that the buildup of mannose 6-phosphate receptors (MPRs) in the late endosome suggests that the retrograde breakdown of cholesterol via the Trans Golgi Network cannot occur.[1]

* Another theory suggests that the blockage of retrograde cholesterol breakdown in the late endosome is due to decreased membrane elasticity and thus the return vesicles of cholesterol to the Trans Golgi Network cannot bud and form.

The support of these theories has considerable evidence using mutant proteins in vitro to determine the buildup of cholesterol in the lysosomes. Researchers have also discovered that the NPC-1 protein may function as a pump of cholesterol.[2]

The overall effect of a malfunction in NPC-1 is that low levels or an absence of the protein lead to the abnormal accumulation of lipids and cholesterol in the cells of people with this condition.